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Vol. 51 - SUPPLEMENT
Bollettino di Geofisica
teorica ed applicata
An International Journal of Earth Sciences
Guest Editors: A. Tassone, E. Lodolo, M. Menichetti, A. Rapalini
International Geological Congress
on the Southern Hemisphere
Scientific Contributions
of the GeoSur2010
22-23 November, 2010
Mar del Plata, Argentina
Istituto Nazionale di Oceanografia
e di Geofisica Sperimentale
ISSN 0006-6729
Responsibility for all statements made in B.G.T.A. lies with the authors
Cover design: Nino Bon, OGS
Printing: ArgenGraphics – Buenos Aires, Argentina
Authorized by the Tribunale di Trieste, n. 242, September 17, 1960
INTERNATIONAL SYMPOSIUM
International Geological Congress
of the Southern Hemisphere
22-23 November 2010
Mar del Plata, Argentina
SCIENTIFIC CONTRIBUTIONS
GEOSUR2010
22-23 NOVEMBER 2010 – MAR DEL PLATA
ORGANIZERS
ISTITUTO NAZIONALE
DI OCEANOGRAFIA E DI GEOFISICA SPERIMENTALE - OGS, TRIESTE, ITALY
INSTITUTO DE GEOFÍSICA “DANIEL A. VALENCIO”,
UNIVERSIDAD DE BUENOS AIRES, ARGENTINA
DIPARTIMENTO DI SCIENZE GEOLOGICHE,
UNIVERSITÀ DI URBINO, ITALY
4
GEOSUR2004
Conference Organizers
ALEJANDRO TASSONE - CONICET. INGEODAV. Universidad de Buenos Aires, Argentina
EMANUELE LODOLO - Istituto Nazionale di Oceanografia e di Geofisica Sperimentale - OGS, Trieste, Italy
MARCO MENICHETTI - Universitá di Urbino, Italy
AUGUSTO RAPALINI - CONICET. INGEODAV. Universidad de Buenos Aires, Argentina
JOSÉ CARCIONE - Istituto Nazionale di Oceanografia e di Geofisica Sperimentale - OGS, Trieste, Italy
Local Organizing Committee
JUAN FRANCISCO VILAS - CONICET. INGEODAV. Universidad de Buenos Aires, Argentina
HORACIO LIPPAI - CONICET. INGEODAV. Universidad de Buenos Aires, Argentina
FEDERICO ISLA - CONICET. Universidad Nacional de Mar del Plata, Argentina
GABRIELE PAPARO - Italian Embassy in Buenos Aires, Argentina
JOSÉ LUÍS HORMAECHEA - Estación Astronómica Rio Grande (CONICET-UNLP), Argentina
FEDERICO ESTEBAN - CONICET, INGEODAV. Universidad de Buenos Aires, Argentina
JAVIER PERONI - CONICET, INGEODAV. Universidad de Buenos Aires, Argentina
MARIA ELENA CERREDO - CONICET. Universidad de Buenos Aires, Argentina
MARÍA PAULA IGLESIA LLANOS - CONICET. INGEODAV. Universidad de Buenos Aires, Argentina
Scientific Committee
ASTINI RICARDO - CONICET. Universidad Nacional de Cordoba, Argentina
BEN-AVRAHAM ZVI - University of Tel Aviv, Israel
CANALS MIQUEL - Universitat de Barcelona, Spain
CAWOOD PETER - Western Australian University
CONCHEIRO ANDREA - CONICET, Dpto. Geologia, Universidad de Buenos Aires, Argentina
CORDANI UMBERTO - Universidade do Sao Paulo, Brazil
DALZIEL IAN - Institute for Geophysics, Jackson School of Geosciences, The University of Texas at Austin, USA
DE BATIST MARC - Renard Centre of Marine Geology, Gent, Belgium
GAMBOA LUIS - PetroBras, Brazil
GHIDELLA MARTA - Instituto Antártico Argentino, Buenos Aires
GOTZE JURGEN-HANS - Universität Otto Hahn, Germany
HARTNADY CHRIS - Umvoto Africa (Pty) Ltd., South Africa
HERNANDEZ-MOLINA JAVIER - Dpto. de Geociencias Marinas, Universidad de Vigo, Spain
HERVÉ FRANCISCO - Universidad de Santiago de Chile
JOKAT WILFRIED - Alfred Wegener Institute, Bremerhaven, Germany
LARTER ROBERT - British Antarctic Survey, UK
LEITCHENKOV GERMAN - Institute for Geology and Mineral Resources of the World Ocean, St. Petersburg, Russia
LÓPEZ DE LUCHI MÓNICA - Instituto de Geología Isotópica. CONICET-UBA, Argentina
MARENSSI SERGIO - Intituto Antártico Argentino, Buenos Aires
PATERLINI MARCELO - Servicio de Hidrografía Naval, Argentina
PERILLO GERARDO - Instituto Argentino de Oceanografía, Argentina
RAMOS VICTOR - Universidad de Buenos Aires, Argentina
RAPELA CARLOS - CONICET. Centro de Investigaciones Geológicas, La Plata, Argentina
RENZULLI ALBERTO - Università di Urbino, Italy
RUSSI MARINO - Istituto Nazionale di Oceanografia e di Geofisica Sperimentale - OGS, Trieste, Italy
RUZZANTE JOSÉ - Comisión Nacional de Energía Atómica. ICES. Argentina
SOMOZA LUÍS - Instituto Geológico y Minero de España, Madrid, Spain
SPALETTI LUÍS - CONICET. Centro de Investigaciones Geológicas, La Plata, Argentina
Conference Secretariat
CAROLINA GAMBA – Fundación Ciencias Exactas y Naturales
MARGHERITA PERSI – Ufficio Comunicazione Istituzionale - OGS, Trieste, Italy
5
CONTENTS
Session 1 - Rodinia in South America
RODINIA ANCESTRIES OF NEOPROTEROZOIC–EARLY PALEOZOIC
SEDIMENTARY ROCKS OF SOUTHERN SOUTH AMERICA,
THE ROSS SEA REGIONS OF ANTARCTICA, ZEALANDIA AND
SOUTHEAST AUSTRALIA: POSSIBLE ORIGINS IN THE SOUTH CHINA BLOCK. . . . . . . . . . .1-01
C.J. Adams
WESTERN PRECORDILLERA OPHIOLITE BELT: CORRELATIONS BETWEEN CORDÓN DEL
PEÑASCO AND CORTADERAS LOCALITIES (MENDOZA PROVINCE, ARGENTINA) . . . . . . . .1-02
L. Boedo Florencia, I. Vujovich, Graciela I.
PAMPIA: A FRAGMENT OF THE AUTHOCHTONOUS
MESOPROTEROZOIC OROGEN OF WESTERN RÍO DE LA PLATA
CRATON. ITS DETACHMENT DURING RODINIA`S BREAK-UP, AND
RE-ACCRETION DURING GONDWANA´S AMALGAMATION . . . . . . . . . . . . . . . . . . . . . . . . . . .1-03
C.J. Chernicoff, E.O. Zappettini, J.O.S. Santos
NEW INSIGHTS ON THE PALEOPROTEROZOIC BASEMENT OF TANDILIA BELT,
RÍO DE LA PLATA CRATON, ARGENTINA: FIRST HF ISOTOPE
STUDIES ON ZIRCON CRYSTALS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1-04
C. Cingolani, J.O.S. Santos, W. Griffin
THE RODINIAN RELICS OF AUSTRALIA-MAWSON, AZANIA,
NEOPROTEROZOIC INDIA AND A GREATER KALAHARI –
DIVINING THEIR EXTENT AND INTERPRETING THEIR EVOLUTION. . . . . . . . . . . . . . . . . . . . .1-05
A.S. Collins, K. Selway, C. Clark, P.D. Kinny, D. Plavsa, U. Amarasinghe
RODINIA: IS A RAPPROCHEMENT OF CURRENT
‘SOUTHERN OCEAN’ AND ‘NORTH ATLANTIC’ MODELS ACHIEVABLE? . . . . . . . . . . . . . . .1-06
Ian.W.D. Dalziel
VOLCANISM IN THE WESTERN OUACHITA-CUYANIA BASIN AND
SEPARATION OF LAURENTIA AND GONDWANA . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1-07
P.W. Dickerson, R.E. Hanson, J.M. Roberts, C.M. Fanning
GEOCHEMISTRY, GEOCHRONOLOGY AND PALEOMAGNETISM OF
PALEOPROTEROZOIC GRANITES OF THE ULKAN MASSIF,
SE SIBERIAN CRATON . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1-08
A.N. Didenko, V.A. Guryanov, A.Yu. Peskov, A.N. Perestoronin, D.V. Avdeev
THE AGE AND SIGNIFICANCE OF THE PUNCOVISCANA
FORMATION WITH RESPECT TO NEOPROTEOZOIC TO CAMBRIAN
TECTONIC EVOLUTION OF THE PROTO-ANDEAN MARGIN OF GONDWANA. . . . . . . . . . . . .1-09
M. Escayola, C. van Staal
THE PUTUMAYO OROGEN OF NORTHWEST SOUTH AMERICA:
IMPLICATIONS FOR RODINIAN CONNECTIONS BETWEEN AMAZONIA,
BALTICA AND THE MIDDLE- AMERICAN OAXAQUIAN TERRANES. . . . . . . . . . . . . . . . . . . . .1-10
M. Ibanez-Mejia, J. Ruiz, V. Valencia, A. Cardona, G. Gehrels, A. Mora, P. DeCelles
COUPLED DELAMINATION AND INDENTOR-ESCAPE TECTONICS
IN THE SOUTHERN PART OF THE C. 650-500 MA
EAST AFRICAN/ANTARCTIC OROGEN . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1-11
J. Jacobs, R.T. Thomas, K. Ueda, I. Kleinhanns, B. Emmel, R. Kumar, B Bingen, A. Engvik
SM-ND ISOTOPIC CONSTRAINTS ON THE NEOPROTEROZOIC –
EARLY PALEOZOIC EVOLUTION OF THE EASTERN SIERRAS PAMPEANAS . . . . . . . . . . . . .1-12
M.G. López de Luchi, A. Steenken, C.I. Martínez Dopic, M. Drobe, K. Wemmer, S. Siegesmund
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CONTENTS
PALEOMAGNETIC POLE FOR THE NEOPROTEROZOIC DIKES
OF THE NICO PÉREZ TERRANE (URUGUAY) AND THE APPARENT
POLAR WANDER PATH (APWP) FOR THE RÍO DE LA PLATA CRATON . . . . . . . . . . . . . . . . . .1-13
A.L. Lossada, A.E. Rapalini, L. Sanchez Bettucci
ORDOVICIAN MAGMATISM IN THE NORTHEASTERN
NORTH PATAGONIAN MASSIF: FURTHER EVIDENCE FOR
THE CONTINUITY OF THE FAMATINIAN OROGEN . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1-14
C.I. Martínez Dopico, M.G. López de Luchi, K. Wemmer, A.E. Rapalini, E. Linares
IAPETAN EVOLUTION OF APPALACHIAN PERI-LAURENTIAN
AND PERI-GONDWANAN ARC COMPLEXES: A NEWFOUNDLAND COMPARISON . . . . . . . .1-15
Brian. H. O’Brien
U- PB ZIRCON GEOCHRONOLOGY OF THE SIERRA VALLE FÉRTIL,
FAMATINIAN ARC, ARGENTINA: PETROLOGICAL AND
GEOLOGICAL IMPLICATIONS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1-16
J.E. Otamendi, M.N. Ducea, G. Bergantz
CERRO LA TUNA MAFIC TO ULTRAMAFIC COMPLEX:
AN OCEAN FLOOR REMNANT . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1-17
E. Peel, M.A.S. Basei, L. Sánchez Bettucci
GRENVILLE-AGE SOURCES IN CUESTA DE RAHUE, NORTHERN PATAGONIA:
CONSTRAINS FROM U/PB SHRIMP AGES FROM DETRITAL ZIRCONS . . . . . . . . . . . . . . . . .1-18
V.A. Ramos, E García Morabito, F. Hervé, C.M. Fanning
WAS THE RIO DE LA PLATA CRATON NEVER PART OF RODINIA?
SOME PALEOMAGNETIC HINTS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1-19
A.E. Rapalini
THE AFRICAN PROVENANCE OF SOUTHERN SOUTH AMERICA TERRANES
A RECORD FROM RODINIA BREAK-UP TO GONDWANA ASSEMBLY . . . . . . . . . . . . . . . . . .1-20
C.W. Rapela, C.M. Fanning, C. Casquet,t R.J. Pankhurst, L.A. Spalletti,
D. Poiré, E.G. Baldo
THE PIEDRA ALTA TERRANE: A PALEOPROTEROZOIC
JUVENILE MAGMATIC ARC, RIO DE LA PLATA CRATON, URUGUAY . . . . . . . . . . . . . . . . . . .1-21
L. Sánchez Bettucci, E. Peel, M.A.S. Basei
FURTHER EVIDENCE FOR MULTIPLE REVERSALS
IN THE NEOPROTEROZOIC ARARAS CAP CARBONATE (BRAZIL) . . . . . . . . . . . . . . . . . . . . .1-22
P. Sansjofre, R.I.F. Trindade, M. Ader , A.C.R. Nogueira, J.L. Soares
PARAGUAY BELT FOLDING AND OROCLINAL BENDING DURING
THE FINAL ASSEMBLY OF WESTERN GONDWANA . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1-23
R.I. Trindade, E. Tohver, A. C. Nogueira, C. Riccomini
EVIDENCE FOR MIDDLE-LATE ORDOVICIAN SUBDUCTION AND A
LOWER PLATE SETTING OF THE CUYANIA TERRANE DURING ITS
ACCRETION TO THE PROTO-ANDEAN MARGIN OF GONDWANA . . . . . . . . . . . . . . . . . . . . .1-24
C. van Staa, G. Vujovich, K. Currie, M. Naipauer.
LOW-PRESSURE ANATEXIS IN FAMATINIAN FORELAND OF ARGENTINA,
SOUTH-WESTERN MARGIN OF GONDWANA: SOURCE HEAT PROBLEM. . . . . . . . . . . . . . .1-25
S.O. Verdecchia, E.G. Baldo
THE MESO-NEOPROTEROZOIC SUBDUCTION-ACCRETION EVENTS
AND MAGMATIC EVOLUTION ALONG THE WESTERN MARGIN
OF THE SIBERIAN CRATON: TO THE PROBLEM OF RODINIA BREAK-UP . . . . . . . . . . . . . . .1-26
V.A. Vernikovsky, A.E. Vernikovskaya
THE SUTURE ZONE BETWEEN CUYANIA AND
CHILENIA TERRANES: A SUBDUCTION CHANNEL AND A-TYPE OROGEN? . . . . . . . . . . . . .1-27
G.I. Vujovich, F.L. Boedo, A.P Willner
8
CONTENTS
Session 2 - Volcanism and Petrology
EXOTIC EXHALATIONS FROM ACTIVE S-ANDES VOLCANOES:
DOMUYO, TROMEN AND COPAHUE VOLCANOES, ARGENTINA . . . . . . . . . . . . . . . . . . . . . .2-01
A. Bermudez, D. Delpino, J. C. Varekamp, T. Kading
CERRO NEGRO DEL GHÍO: MAGMATISM IN-BETWEEN
SOUTHERN PATAGONIAN BATHOLITH AND
LAGO BUENOS AIRES PLATEAU LAVAS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .2-02
J. Castro, A. Sánchez, F. Hervé, M. de Saint Blaquat, M. Polvé
MAGMATIC ACTIVITY AND STRIKE SLIP TECTONICS
IN THE SOUTHERNMOST ANDES: KRANCK PLUTON,
CHARACTERIZATION AND PRELIMINARY AMS SURVEY . . . . . . . . . . . . . . . . . . . . . . . . . . . . .2-03
M.E. Cerredo, M.B. Remesal, A.A. Tassone, J.I. Peroni, M. Menichetti, H. Lippai
CHARACTERIZATION OF AN ULTRABASIC LAMPROPHYRE
(EVOLVED DAMTJERNITE) IN THE TANDILIA BASEMENT,
SOUTHERNMOST RÍO DE LA PLATA CRATON, ARGENTINA. . . . . . . . . . . . . . . . . . . . . . . . . .2-04
J.A. Dristas, J.C. Martínez, H.J. Massonne, K. Wemmer
CONSTRAINTS ON OIB-TYPE PREMA AND EM1 MANTLE SOURCES
FROM TRACE ELEMENT AND PB, SR, AND ND ISOTOPIC RATIOS
OF PRIMITIVE EOCENE TO RECENT BACKARC PATAGONIAN BASALTS . . . . . . . . . . . . . . . .2-05
K.S. Mahlburg, J. Helen, G. Matthew
SERRA GERAL VOLCANISM IN THE PROVINCE OF MISIONES (ARGENTINA):
GEOCHEMICAL ASPECTS AND INTERPRETATION OF ITS GENESIS
IN THE CONTEXT OF THE LARGE IGNEOUS PROVINCE PARANÁ-ETENDEKA-ANGOLA. ITS
RELATION WITH THE ALKALINE VOLCANISM OF CÓRDOBA PROVINCE . . . . . . . . . . . . . . .2-06
S. Leonor Lagorio, H. Vizán
LITHOLOGY AND AGE OF THE CUSHAMEN FORMATION.
DEVONIAN MAGMATISM IN THE WESTERN
NORTH PATAGONIAN MASSIF. ARGENTINA . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .2-07
M.G. López de Luchi, M.E. Cerredo, C. Martínez Dopico
METAMORPHIC EVOLUTION OF THE CINCO CERROS AREA,
SIERRA DE TANDIL, ARGENTINA . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .2-08
H.J. Massonne, J. Dristas, J.C. Martinez
PALEOMAGNETIC STUDIES OF CENOZOIC BASALTS
FROM NORTHERN NEUQUÉN AND SOUTHERN MENDOZA PROVINCES:
STRATIGRAPHIC IMPLICATIONS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .2-09
G.H. Re, J.F. Vilas,
AMPHIBOLE MEGACRYSTS OF THE CERRO JEU-JEPÉN PLUTON:
NEW CONSTRAINTS ON MAGMA SOURCE AND EVOLUTION
(FUEGIAN ANDES, ARGENTINA) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .2-10
F. Ridolfi, A. Renzulli, M.E. Cerredo, R. Oberti, M. Boiocchi, F. Bellatreccia, G. Della Ventura
M. Menichetti, A. Tassone
“CIRCULAR FEATURES” ON OLD SOLIDIFIED LAVA FLOW FIELDS
ASSOCIATED WITH SOME YOUNG SCORIA CONES FROM LLANCANELO
AND PAYÚN MATRU VOLCANIC FIELDS, MENDOZA PROVINCE, ARGENTINA. . . . . . . . . . . .2-11
C. Risso, K. Nemeth, F. Nullo, M. Inbar
THE NEOGENE BARRIL NIYEU VOLCANIC COMPLEX
SOMÚN CURÁ MAGMATIC PROVINCE.
NORTHERN EXTRA ANDEAN PATAGONIA. ARGENTINA . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .2-12
F.M. Salani, M. Remesal, M.E. Cerredo
THE MAGNETIC SUSCEPTIBILITY OF IGNIMBRITES FROM
THE ALTIPLANO- PUNA VOLCANIC COMPLEX, CENTRAL ANDES:
A USEFUL TOOL TO DISTINGUISH LITHOMAGNETIC DOMAINS ACROSS THE ARC . . . . . .2-13
S.E. Singer
9
CONTENTS
FIRST KIMBERLITE PIPE IN CENTRAL YAKUTIA (RUSSIA):
MINERAL COMPOSITION AND THICKNESS OF LITHOSPHERIC MANTLE AND AGE. . . . . .2-14
A.P. Smelov, A.I. Zaitsev, I.V. Ashchepkov
DEPOSITION AND REWORKING OF PRIMARY PYROCLASTIC
DETRITUS IN PUESTO LA PALOMA MEMBER, CRETACEOUS CERRO
BARCINO FORMATION, SOMUNCURÁ-CAÑADÓN ASFALTO BASIN,
PATAGONIA, ARGENTINA . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .2-15
A.M. Umazano
GEODYNAMIC THAT GENERATED THE CRETACEOUS VOLCANISM
OF CÓRDOBA AND THE LARGE IGNEOUS PROVINCE OF PARANÁ,
INCLUDING THE ORIGIN OF THE TRISTAN “PLUME” . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .2-16
H. Vizán, S. Leonor Lagorio
MAFIC MICROGRANULAR ENCLAVE SWARMS IN
GRANITIC PLUTONS OF GASTRE, CENTRAL PATAGONIA . . . . . . . . . . . . . . . . . . . . . . . . . . . .2-17
C.B. Zaffarana, R. Somoza
Session 3 - Geophysical prospecting and Geodesy
ANALYSIS OF A PRECISE REGIONAL GEOID MODEL IN
BUENOS AIRES PROVINCE COMPUTED BY LEAST SQUARES COLLOCATION . . . . . . . . . .3-01
D. Bagú, D. Del Cogliano, M. Scheinert, R. Dietrich, J. Schwabe, L. Mendoza
GEOPHYSICAL SURVEY IN THE NORTHERN REGION OF CUYO BASIN . . . . . . . . . . . . . . . .3-02
M. García, E. Luna, O. Alvarez, S. Spagnotto, S. Nacif, P. Martínez, M. Gimenez
MONITORING AND FORECASTING THE STATE OF THE SOUTH ATLANTIC
MAGNETIC ANOMALY . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .3-03
J.C. Gianibelli
IMPORTANCE AND FUTURE OF THE MAGNETIC
OBSERVATORIES NETWORK IN THE SOUTH AMERICA . . . . . . . . . . . . . . . . . . . . . . . . . . . . .3-04
J.C. Gianibelli, L. Sánchez Bettucci, R.E. García, G.D. Rodriguez, N. Quaglino,
R. Novo, G. Tancredi
IGMAS+ A NEW 3D MODELLING TOOL FOR GRAVMAG FIELDS
AND GRADIOMETRY . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .3-05
H.J. Goetze, S. Schmidt
ADVANCES IN THE DETERMINATION OF A HEIGHT REFERENCE
SURFACE FOR TIERRA DEL FUEGO . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .3-06
M. E. Gomez, R. Perdomo, D. Del Cogliano, J. L. Hormaechea
FIRST MAGNETOMETRIC SURVEY IN THE ZAPICÁN AND NICO PÉREZ AREA (URUGUAY) .3-07
R. Novo, N. Seluchi, I. Suarez, L. Sánchez Bettucci, J. Gianibelli
MAPS OF ABSOLUTE GRAVITY, GRAVITY ANOMALY, AND
TOTAL MAGNETIC FIELD ANOMALY OF VENEZUELA FROM SATELLITE DATA. . . . . . . . . . . .3-08
N.O. Guevara, A.G. Reyes, T. Tabare
GEOPHYSICAL INVESTIGATION OF THE NAVARINO ISLAND PLUTONS
(BEAGLE CHANNEL, CHILE) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .3-09
J.I. Peroni, A. Tassone, H. Lippai, F. Hervé, M. Menichetti, E. Lodolo
GEOPHYSICAL CHARACTERIZATION OF FILLED ZONES ALONG
THE COAST OF BUENOS AIRES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .3-10
C. Prezzi, R. López, C. Vásquez, S. Marcomini, S. Fazzito
BAJADA DEL DIABLO IMPACT CRATER-STREWN FIELD (ARGENTINA):
GROUND MAGNETIC AND ELECTROMAGNETIC SURVEYS . . . . . . . . . . . . . . . . . . . . . . . . . .3-11
C.B. Prezzi, M.J. Orgeira, R. Acevedo, F. Ponce, O. Martínez, C. Vásquez,
H. Corbella, M. González , J. Rabassa
10
CONTENTS
EARTH TIDE OBSERVATIONS IN TIERRA DEL FUEGO (ARGENTINA) . . . . . . . . . . . . . . . . . . .3-12
A. Richter, R. Perdomo, J. L. Hormaechea, L. Mendoza, D. Del Cogliano, M. Fritsche,
M. Scheinert, R. Dietrich
MORPHO-BATHYMETRIC SURVEY OF LAGO ROCA (TIERRA DEL FUEGO) . . . . . . . . . . . . . . 3-13
E. Lodolo, A. Tassone, L. Baradello, H. Lippai, M. Grossi
Session 4 - Tectonic processes and Seismology
NEW STRUCTURAL MAPS AND CROSS-SECTIONS OF THE PATAGONIAN
FOLD-THRUST BELT NEAR SENO OTWAY, SENO MARTÍNEZ AND
PENINSULA BRUNSWICK, SOUTHERN CHILE . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .4-01
P. Betka, K. Klepeis, S. Mosher
TECTONIC EVOLUTION OF THE BERMEJO BASIN FROM
BROKEN PLATE FLUXURAL MODEL (PRELIMINARY STUDY) . . . . . . . . . . . . . . . . . . . . . . . . .4-02
G. Carugati, I.L. Novara, M.E. Gimenez, A. Introcaso
TECTONIC IMPLICATIONS OF A PALEOMAGNETIC STUDY OF MESOZOIC
MAGMATIC ARC ROCKS IN CIERVA POINT, NORTHWEST ANTARCTIC PENINSULA . . . . . .4-03
N.J. Cosentino, A.A. Tassone, H.F. Lippai, J.F.A. Vilas
PALAEOTECTONIC SETTING OF PRECUYANO GROUP.
UPPER TRIASSIC- LOWER JURASSIC VOLCANIC DEPOSITS OF
THE NEUQUEN BASIN (37º- 39º 30´LS). ARGENTINA . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .4-04
D. Delpino, A. Bermudez
PRELIMINARY RESULTS OF A PALEOMAGNETIC STUDY ON THE ORDOVICIAN
CALMAYO GRANITOID, SIERRAS DE CÓRDOBA, ARGENTINA . . . . . . . . . . . . . . . . . . . . . . .4-05
S. Geuna, F. D’Eramo, L. Pinotti, A. Di Marco, D. Mutti, L. Escosteguy
NORTH-SOUTH VARIATIONS IN PROVENANCE IN THE LATE PALEOZOIC
ACCRETIONARY COMPLEX OF CENTRAL CHILE (34º – 40º LAT. S)
AS INDICATED BY SHRIMP DETRITAL ZIRCON U-TH-PB AGES . . . . . . . . . . . . . . . . . . . . . . .4-06
F. Hervé, M.Calderon, C.M. Fanning, E. Godoy
THE ROLE OF TRUE POLAR WANDER IN THE JURASSIC . . . . . . . . . . . . . . . . . . . . . . . . . . .4-07
M.P. Iglesia Llanos, C.B. Prezzi
THE WEDDELL SEA REVISITED . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .4-08
L.A. Lawver, M.E. Ghidella
CRUSTAL STRUCTURE AND TECTONICS OF
THE EAST ANTARCTICA PASSIVE MARGIN . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .4-09
G.L. Leychenkov, G. L. Guseva
STRUCTURE AND TECTONIC DEVELOPMENT OF THE SOUTHERN MARGIN
OF THE SCOTIA SEA . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .4-10
E. Lodolo, D. Civile, A. Tassone
3D DENSITY MODEL OF THE CENTRAL AMERICAN SUBDUCTION ZONE
FROM SATELLITE GRAVITY DATA INTERPRETATION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .4-11
O.H. Lücke, H.J. Götze
ITALY-ARGENTINA COOPERATION IN THE FIELD OF SEISMOLOGY:
THE ASAIN. HISTORICAL REVIEW AND RECENT PROGRESS . . . . . . . . . . . . . . . . . . . . . . . . .4-12
M. Russi, C. Cravos, M. P. Plasencia Linares
OROGENESIS REFLECTED IN THE TRANSITION FROM EXTENSIONAL RIFT
BASIN TO COMPRESSIONAL FORELAND BASIN IN THE SOUTHERNMOST
ANDES (54.5°S): NEW PROVENANCE DATA FROM BAHÍA BROOKES AND SENO OTWAY
J. McAtamney, K. Klepeis, C.r Mehrtens, S. Thomson
.4-13
CRUSTAL DEFORMATIONS ASSOCIATED TO THE M 8.8 MAULE
EARTHQUAKE IN CENTRAL CHILE, 27 FEBRUARY 2010, DETECTED
BY PERMANENT GPS STATIONS IN ARGENTINA . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .4-14
L. Mendoza, J. Vasquez, D. Del Cogliano
11
CONTENTS
STRUCTURAL GEOLOGY OF THE EASTERN TIERRA DEL FUEGO ISLAND . . . . . . . . . . . . . .4-15
M. Menichetti, A. Tassone, H. Lippai
THE PRE-FARELLONES DEFORMATION (PEHUENCHE FASE)
CORDILLERA PRINCIPAL AND FRONTAL (31°45’LS), SAN JUAN PROVINCE,
ARGENTINA . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .4-16
D. José Pérez, J.M. Sanchez Magariños
CORRELATIONS OF TECTONO-MAGMATIC EVENTS IN THE SOUTH
VERKHOYANSK OROGENIC BELT (EASTERN SIBERIA, NORTHEAST ASIA) . . . . . . . . . . . . . .4-17
A.V. Prokopiev
THE LATE OLIGOCENE-MIOCENE ÑIRIHUAU FORMATION INTERPRETED
AS A FORELAND BASIN IN THE NORTHERN PATAGONIAN ANDES . . . . . . . . . . . . . . . . . . . .4-18
M.E. Ramos, D. Orts, F. Calatayud, A. Folguera, V.A. Ramos
A CASE OF PALEOHORIZONTAL RESTORATION OF PLUTONIC BODIES USING
PALEOMAGNETIC DATA: THE SIERRA DE VALLE FÉRTIL MAGMATIC COMPLEX,
WESTERN ARGENTINA . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .4-19
A. Rapalini, L. Pinotti, F. D’Eramo, J. Otamendi, N. Vegas
SEISMICITY AND EARTHQUAKE HAZARD IN TIERRA DEL FUEGO PROVINCE, . . . . . . . . . . . . . .
ARGENTINA. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .4-20
N.C. Sabbione, C. Buffoni, N. Barbosa, G. Badi, G. Connon, J.L. Hormaechea
SYNOROGENIC SEQUENCES ASSOCIATED WITH THE ANDEAN
FRONT AT 37º S AS A CLUE FOR AGE EXHUMATION AND STRUCTURATION
OF THE FORELAND AREA . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .4-21
L. Sagripanti, M. Naipauer, A. Folguera, V.A. Ramos
FURTHER EVIDENCE OF LOWER PERMIAN REMAGNETIZATION
IN THE NORTH PATAGONIAN MASSIF, ARGENTINA . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .4-22
R. Tomezzoli, A. E. Rapalini, M.G. Lopez de Luchi
TECTONIC CONTROL ON THE EVOLUTION OF
MAASTRICHTIAN-PALEOGENE SYNOROGENIC SEQUENCES
OF THE FUEGIAN THRUST FOLD BELT, ARGENTINA . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .4-23
P.J. Torres Carbonell, E.B. Olivero, L.V. Dimieri
Session 5 - Stratigraphy and Sedimentology
PRESERVATION OF TOTAL ORGANIC CARBON AND EVALUATION
OF CORG/NTOT ATOMIC RATIO IN A SEDIMENT OUTCROP LOCATED S-E
OF THE LAGO FAGNANO (TIERRA DEL FUEGO, ARGENTINA) . . . . . . . . . . . . . . . . . . . . . . . .5-01
M. Caffau, C. Comici, M. Zecchin, M. Presti, E. Lodolo, A. Tassone,
H. Lippai, M. Menichetti
STRATIGRAPHIC AND STRUCTURAL REVIEW OF CAÑADÓN ASFALTO BASIN,
CHUBUT PROVINCE, ARGENTINA . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .5-02
E. Figari, V.A. Ramos
THE CONTROVERSY ABOUT MIOCENE MARINE SEDIMENTATION ALONG
THE FORELAND OF ANDES, SOUTH AMERICA: THE CASE
OF SANTA MARIA GROUP, ARGENTINA . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .5-03
I.J.C. Gavriloff, M.N. Arce
GEOLOGY OF THE LAGO FAGNANO AREA (FUEGIAN ANDES,
TIERRA DEL FUEGO ISLAND) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .5-04
M. Menichetti, A. Tassone, H. Lippai, E. Lodolo
THE AGE OF DINOSAURS IN SOUTH AMERICA . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .5-05
F. Novas
SEDIMENTATION ENVIRONMENTS AND FACIES DEVELOPED
DURING THE MIOCENE TO EARLY PLIOCENE IN THE EASTERN BASIN
OF FALCON, WESTERN VENEZUELA. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .5-06
B.F. Romero, P. Bastos, K. Strakos, M. Baquero
12
CONTENTS
AN EXAMPLE OF COMPLEX FLUVIO-AEOLIAN SEDIMENTATION:
THE UPPER MEMBER OF THE MIOCENE-PLIOCENE RÍO NEGRO FORMATION,
NORTHERN PATAGONIA, ARGENTINA . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .5-07
A.M. Umazano, G. Visconti, M. Pérez
THE BRUNHES/MATUYAMA BOUNDARY AND ROCK MAGNETIC
PARAMETERS IN PLEISTOCENE LOESS DEPOSITS OF CAMET,
MAR DEL PLATA (ARGENTINA) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .5-08
J.C. Bidegain, M. Gomez Samus
THE LATE CENOZOIC SEDIMENTARY SEQUENCES IN THE
CHAPADMALAL AREA (BUENOS AIRES). POLARITY CHANGES
AND MAGNETOCLIMATOLOGY . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .5-09
J.C. Bidegain, Yamile Rico
Session 6 - Surface processes and Paleoclimate
RECONSTRUCTION OF LATE-GLACIAL TO HOLOCENE CLIMATE AND
EARTHQUAKE HISTORIES ACROSS SOUTHERN CHILE BASED
ON THE SEDIMENTARY RECORD OF 21 LAKES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .6-01
M. De Batist, J. Moernaut, K. Heirman, M. Van Daele, S. Bertrand
RAPID CRUSTAL UPLIFT IN PATAGONIA AS A CONSEQUENCE
OF INCREASED ICE LOSS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .6-02
R. Dietrich, E.R. Ivins, G. Casassa, H. Lange, J. Wendt, M. Fritsche
IMPLEMENTATION OF AQUIFER PROTECTION ZONING . . . . . . . . . . . . . . . . . . . . . . . . . . . . .6-03
S. Dustay, J. Nel, Y. Xu, H Massone
TERRESTRIAL AND LACUSTRINE EVIDENCE OF HOLOCENE
GLACIER ACTIVITY IN TIERRA DEL FUEGO (SOUTHERNMOST SOUTH AMERICA) . . . . . . . .6-04
M. Maurer, B. Menounos, J.J. Clague, G. Osborn, J. Rabassa, J.F Ponce,
G. Bujalesky, M. Fernández, A. Coronato
CK MAGNETISM STUDY ON SEDIMENTS FROM STREAMS
OF THE PARANÁ DELTA (ARGENTINA) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .6-05
M. Mena, J.L. Dupuy
CHARACTERIZATION OF COMPLEXITY OF FRACTURED ROCK AQUIFERS . . . . . . . . . . . . .6-06
J. Nel, Y. Xu, O. Batelaan
ENVIRONMENTAL MAGNETISM STUDY OF A HOLOCENE EOLIAN
SEDIMENTS AND PALEOSOLS SEQUENCE IN THE NORTH OF
TIERRA DEL FUEGO (ARGENTINA) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .6-07
M.J. Orgeira, A. Coronato, C.A. Vásquez, A. Ponce, A. Moretto, R. Egli, M.R. Onorato
MANAGEMENT AND CONTROL OF THE WATER RESOURCES OF
SAN LUIS PROVINCE (ARGENTINA) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .6-08
O.A. Pedersen
SEDIMENTARY IMPRINT OF THE 2007 AYSÉN EARTHQUAKE AND
TSUNAMI IN AYSÉN FJORD (CHILEAN PATAGONIA) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .6-09
M. Van Daele, M. De Batist, W.. Versteeg, K. De Rycker, V. Cnudde, R. Gieles,
P. Duyck, M. Pino, R. Urrutia
RECONSTRUCTION OF THE EVOLUTIVE STAGES OF LLANCANELO LAKE
AND SURROUNDINGS (SOUTHERN MENDOZA PROVINCE, WESTERN ARGENTINA) . . . . . .6-10
E.I. Rovere, R.A. Violante, A. Osella, M. de la Vega, E. López
ROCK-MAGNETISM CHARACTERIZATION OF A LATE QUATERNARY
SOIL HORIZON (SAN SEBASTIÁN BAY, ISLA GRANDE OF TIERRA DEL FUEGO)
A.M. Walther, M.I.B. Raposo, J.F. Vilas
. . . . . . . . .6-11
13
CONTENTS
Session 7 - Marine geology and geophysics
CHARACTERIZATION OF THE MAGNETIC RESPONSE OF
THE NORTHERN ARGENTINE CONTINENTAL MARGIN (SOUTH ATLANTIC OCEAN) . . . . . . .7-01
D.A. Abraham, M. Ghidella, M. Paterlini, B. Schreckenberger
LINKING SEAFLOOR MORPHOLOGY, HYDROSEDIMENTARY PROCESSES
AND LIVING RESOURCES IN SUBMARINE CANYONS OF
THE NW MEDITERRANEAN SEA: A UNIQUE STUDY CASE . . . . . . . . . . . . . . . . . . . . . . . . . . .7-02
M. Canals
MORPHOSTRUCTURE OF THE WESTERN SECTOR OF
THE NORTH SCOTIA RIDGE. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .7-03
F.D. Esteban, A. Tassone, E. Lodolo, M. Menichetti
GIANT MOUNDED DRIFTS IN THE ARGENTINE CONTINENTAL MARGIN . . . . . . . . . . . . . . . .7-04
F.J. Hernández-Molina, M. Paterlini, L. Somoza, R. Violante, M.A. Arecco,
M. de Isasi, M. Rebesco, G. Uenzelmann-Neben, P. Marshall
PLIOCENE SUBMERGED CRATERS AT THE UPPER CONTINENTAL
SLOPE OF MAR DEL PLATA (ARGENTINA) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .7-05
F. Isla, A. Madirolas
THE 2005-2006 EXPERIMENT IN ANTARCTICA WITH
MABEL SEAFLOOR MULTIDISCIPLINARY OBSERVATORY . . . . . . . . . . . . . . . . . . . . . . . . . . . .7-06
G. Marinaro, G. Falcone, F. Frugoni, P. Favali
COASTAL EROSION IN MAR DE COBO (BUENOS AIRES PROVINCE) . . . . . . . . . . . . . . . . . . . . . .7-07
L. San Martín, S.C. Marcomini, R.A. López
CONDITIONING FACTORS AND RESULTING MORPHOSEDIMENTARY
FEATURES IN THE UPPER-MIDDLE CONTINENTAL SLOPE OFFSHORE EASTERN BUENOS
AIRES PROVINCE, ARGENTINA . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .7-08
R.A. Violante, C.M. Paterlini, F.J. Hernández Molina, G. Bozzano, I.P. Costa, S. Marcolini
CENOZOIC GROWTH PATTERNS AND PALEOCEANOGRAPHY
OF THE OCEAN BASINS NEAR THE SCOTIA-ANTARCTIC PLATE BOUNDARY . . . . . . . . . . . .7-09
A. Maldonado, F. Bohoyo, J. Galindo-Zaldívar, F.J. Hernández-Molina, F.J. Lobo,
Y. Martos-Martin, A.A. Schreider
Session 8 - Oil and Mineral resources
OCCURRENCE OF SHALLOW GAS IN THE EASTERNMOST LAGO FAGNANO
(TIERRA DEL FUEGO) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .8-01
A. Darbo, L. Baradello, E. Lodolo, M. Grossi, A. Tassone, H. Lippai
GEOLOGY OF THE SAN PEDRO MINING DISTRICT,
SAN RAFAEL MASSIF (ARGENTINA) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .8-02
A.l. Gómez, N. Rubinstein
INDUCED POLARIZATION–RESISTIVITY EXPLORATION IN THE POLYMETALLIC
PURÍSIMA-RUMICRUZ DISTRICT, JUJUY PROVINCE (ARGENTINA) . . . . . . . . . . . . . . . . . . . .8-03
L. López, H. Echeveste, M. Tessone
ROCK-MAGNETISM PROPERTIES FROM DRILL CUTTING AND
THEIR RELATION WITH HYDROCARBON PRESENCE AND
PETROPHYSICAL PARAMETERS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .8-04
M. Mena, A.M. Walther
USE OF ASTER IMAGERY TO IDENTIFY MINERALIZATION IN THE ANDEAN
CORDILLERA FRONTAL (31º45´S), SAN JUAN PROVINCE (ARGENTINA) . . . . . . . . . . . . . . . .8-05
D.J. Pérez, P. DÓdorico
SEISMIC EVIDENCE OF A GAS HYDRATE SYSTEM IN
THE WESTERN ROSS SEA (ANTARCTICA) BY TOMOGRAPHY,
AVO ANALYSIS AND PRESTACK DEPTH MIGRATION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .8-06
S. Picotti, R. Geletti, D. Gei, A. Mocnik, J.M. Carcione
14
Session 1
RODINIA IN SOUTH AMERICA
GEOSUR2010
RODINIA ANCESTRIES OF NEOPROTEROZOIC–EARLY PALEOZOIC
SEDIMENTARY ROCKS OF SOUTHERN SOUTH AMERICA,
THE ROSS SEA REGIONS OF ANTARCTICA, ZEALANDIA AND
SOUTHEAST AUSTRALIA: POSSIBLE ORIGINS IN THE SOUTH CHINA BLOCK
1-01
Adams, C.J.*
GNS Science, Private Bag 1930, Dunedin, New Zealand
* Presenting author’s e-mail: argon@gns.cri.nz
Late Neoproterozoic, Cambrian and Early Ordovician sedimentary rocks form extensive basins in Argentina,
West Antarctica (Marie Byrd Land), East Antarctica (North Victoria Land), the greater New Zealand region
(‘Zealandia’) and southeast Australia (Victoria and New South Wales). Their abundant Precambrian detrital
mineral assemblages provide a memory bank of Rodinia ancestries. It is remarkable that detrital zircon age
patterns for all these areas are similar, with important dominant ages groups falling in the late
Mesoproterozoic (1000-1200 Ma) and mid-(750-850 Ma) and late (550-650 Ma) Neoproterozoic. Primary
sources of these zircons are often enigmatic however, since igneous complexes of this age are sometimes
absent (New Zealand and North Victoria Land), or very localised local (southeast Australia), or less extensive
than might be expected to supply the very large sedimentary basins on a >1000 km-scale (Argentina). Of
course, their sources may have been completely eroded during this phase, or may still remain, but now
hidden, beneath younger rocks. Conversely, it is also possible that many of these sources may have
themselves been buried during deposition of Neoproterozoic sediments at the late Rodina continental
margin.
Evidence from the Puncoviscana Formation (late Neoproterozoic-Cambrian) of northwest Argentina suggests
that important sources of late Mesoproterozoic and late Neoproterozoic-Cambrian zircons could have been,
respectively, in the Sunsás and Brasiliano Orogens of the Precambrian Shield to the east, Both Paleozoic
and Mesozoic sedimentary rocks throughout Zealandia show the same dominant late Mesoproterozoic and
late Neoproterozoic zircon sources that must, at least in part, be endemic to that region. In Antarctica, a similar
pattern is recorded in early Paleozoic rocks in Marie Byrd Land and North Victoria Land.
These common features suggest that late Rodinia continental margins in South America, Australia, Zealandia
and Antarctica had similar cratonic hinterlands. Whilst these in part would have included the Brasilian Shield
of South America, it is also possible that the South China Block was an important cratonic neighbour to the
west. Recent Rodinia global syntheses place a South China Block east of Precambrian Australia, in a position
now occupied by Zealandia (Li et al. Precambrian Research 160: 2008). The South China Block has
distinctive Meso- and Neoproterozoic igneous complexes with age patterns similar to Precambrian detrital
ages from Zealandia, Antarctica and southern South America. During late Neoproterozoic - early Paleozoic
break-up of Rodinia, it is possible that a small fragment of this block remained to form a Zealandia basement.
Upon later erosion this has left ‘ghost’ zircon age signatures in Phanerozoic sedimentary rocks of the region.
At Rodinia break-up, the South China Block could then have supplied major sedimentary basins on its
western (Australia), southern (Zealandia, Antarctica) and eastern (South America) margins.
WESTERN PRECORDILLERA OPHIOLITE BELT: CORRELATIONS
BETWEEN CORDÓN DEL PEÑASCO AND CORTADERAS LOCALITIES (MENDOZA
PROVINCE, ARGENTINA)
1-02
Boedo, F.L.1*, Vujovich, G.I.1
(1) Laboratorio de Tectónica Andina, FCEN, Universidad de Buenos Aires/CONICET
* Presenting author’s e-mail: florenciaboedo@gmail.com
Cordón del Peñasco area is located in western Precordillera, northern Mendoza province, Argentina
(Fig. 1). Low grade metamorphic, slope and deep marine metasiltstones and metasandstones of
17
GEOSUR2010
Fig. 2 - Geologic map of southern Precordillera
showing Cordón del Peñasco and Cortaderas
localities (From Davis et al., 1999).
Fig 1 - Geologic map of Precordillera fold-and-thrust
belt, Argentina, showing the main exposures of ophiolite
assemblages (From Ramos et al., 2000)
Eopaleozoic age, associated with mafic and
ultramafic rocks are the most widely
exposed rock types. Mafic and ultramafic
rocks correspond to an ophiolitic belt
composed of discontinuous exposures along
western Precordillera (Ramos et al., 1984,
2000; Haller y Ramos, 1984; Cortés y Kay,
1994; Davis et al., 1999; Fauqué y Villar,
2003). This belt can be recognized, from
north to south at: río Bonete area (La Rioja
province), Rodeo and Calingasta (San Juan
province), Cordón del Peñasco, Sierra de las
Cortaderas and Cordón de Bonilla (Mendoza
province) (Fig. 1). Particularly, Cordón del
Peñasco and Cortaderas localities show great
similarities (Fig. 2 and 3).
Cordón del Peñasco mafic and ultramafic rocks comprise serpentinized peridotites (probably dunites
and harzburgites) and mafic granulites retrograded to greenschist facies, and homogeneous gabbro
dikes or sills, amigdaloid metabasalts and metahialoclastic rocks with greenschist facies
metamorphism. Cortaderas mafic and ultramafic bodies comprise serpentinized ultramafic rocks
(wehrlites, harzburgites, lherzolite websterites and dunites), layered gabbros, gabbros, microgabbros
and diabases. In both areas, these bodies are heavily deformed and in tectonic contact with slope and
deep marine metasedimentary rocks. In addition, the petrography of serpentinites and mafic
granulites are similar. In Cortaderas, Davis et al. (1999) have recognized that a low-grade regional
metamorphism has partially replaced the igneous and high temperature assemblages in mafic and
ultramafic rocks. In Cordón del Peñasco, we have recognized the same process and retrogradation
evidences such as reaction rims on garnets, recristalization and formation of a fine-grained
18
GEOSUR2010
Fig 3 - Geological map of Cordón del Peñasco
area.
clinopiroxene-plagioclase-quartz mosaic and lizarditechrysotile-talc folded veins on an antigorite mesh
texture. In both localities, a similar low-grade
metamorphism has affected metavolcanic and
metasedimentary rocks, where primary textures are
better preserved. Additionally, Davis et al. (1999)
calculated metamorphic P and T conditions on garnets
of mafic granulites obtaining temperatures of
approximately 850-1000ºC and a minimum pressure of
9 kbar. As mafic granulites have very similar
mineralogy and textures in both localities, we assume
that this PT condition, that fall into high pressure
granulite facies, could be similar in our study area.
Geochemical analyses on mafic and ultramafic rocks
of different localities plot in the E-MORB (Enriched
Mid Ocean Ridge Basalt)/within plate basalt field
(Haller and Ramos, 1984; Kay et al., 1984; Cortés and
Kay, 1994; Fauqué and Villar, 2003). Besides, these
rocks show positive values of ÂNd (+6 to +9.3) that
confirm their oceanic character (Kay et al., 2005).
Western Precordillera Ophiolite Belt has been
interpreted as a suture zone between Chilenia and
Cuyania terranes (Ramos et al., 1984). Davis et al.
(1999, 2000) suggested the occurrence of two ophiolite
assemblages along the suture based on different ages
for the mafic and ultramafic rocks. We propose that the
mechanism of exhumation in a eastward dipping
subduction channel could explain the association of
low and high-grade metamorphic rocks, rocks with
evidence of retrogradation and the east and west
structural vergence reported by many authors (Ramos
et al., 1984; Davis et al., 1999; Von Gosen, 1997) along
the ophiolite belt.
REFERENCES:
• Cortés, J. M., Kay, S. M. 1994. Una dorsal oceánica como origen de las lavas almohadilladas del Grupo Ciénaga del Medio
(Silúrico-Devónico) de la Precordillera de Mendoza, Argentina. 7º Congreso Geológico Chileno, Actas 2: 1005- 1009.
• Davis, J., Roeske, S., McClelland, W., Snee, L. 1999. Closing an ocean between the Precordillera terrane and Chilenia: Early
Devonian ophiolite emplacement and deformation in the southwest Precordillera. En Laurentia-Gondwana Connection before
Pangea. Geological Society of America Special Publication 336, 115-138.
• Davis, J., Roeske, S., McClelland, W., Kay, S. 2000. Mafic and ultramafic crustal fragments of the southwestern Precordillera
terrane and their bearing on tectonic models of the early Paleozoic in western Argentina. Geology, 28(2): 171-174.
• Fauqué, L. E., Villar, L. M. 2003. Reinterpretación estratigráfica y petrología de la Formación Chuscho, Precordillera de La Rioja.
Revista de la Asociación Geológica Argentina 58(2): 218-232.
• Haller, M. J., Ramos, V. A. 1984. Las ofiolitas famitinianas (Eopaleozoico) de las provincias de San Juan y Mendoza. 9º Congreso
Geológico Argentino, Actas 3: 66- 83.
• Kay, S. M., Ramos, V. A., Kay, R. 1984. Elementos mayoritarios y trazas de las vulcanitas ordovícicas de la Precordillera occidental;
basaltos de rift oceánico temprano(?) próximo al margen continental. 9º Congreso Geológico Argentino, Actas 2: 48-65.
• Kay, S. M., Boucakis, K. A., Porch, K., Davis, J. S., Roeske, S. M., Ramos, V. A. 2005. E-MORB like mafic magmatic rocks on the
western border of the Cuyania terrane, Argenina. In Pankhurst, R. J. and Veiga, G. D. (Eds.) Gondwana 12 “Geological and
biological heritage of Gondwana”, p. 216, Mendoza.
• Ramos, V., Jordan, T., Allmendinger, R., Kay, S., Cortés, J., Palma, M. 1984. Chilenia: un terreno alóctono en la evolución
paleozoica de los Andes Centrales. 9º Congreso Geológico Argentino, Actas 2: 84-106.
• Ramos, V., Escayola, M., Mutti, D., Vujovich, G., 2000. Proterozoic- Early Paleozoic ophiolites of the Andean basement of southern
South America. Ophiolitic and Oceanic Crust: new insights from field studies and the Ocean Drilling Program. Geological Society
of America Special Paper, 349, 331-349.
• Von Gosen, W. 1997. Early Paleozoic and Andrean structural evolution in the Río Jáchal section of the Argentine Precordillera.
Journal of South American Earth Sciences 10(5/6): 361-388.
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PAMPIA: A FRAGMENT OF THE AUTHOCHTONOUS
MESOPROTEROZOIC OROGEN OF WESTERN RÍO DE LA PLATA
CRATON. ITS DETACHMENT DURING RODINIA`S BREAK-UP, AND
RE-ACCRETION DURING GONDWANA´S AMALGAMATION
1-03
Chernicoff, C.J.1*, Zappettini, E.O.2, Santos, J.O.S.3
(1) Consejo Nacional de Investigaciones Científicas y Técnicas (CONICET), Argentina
(2) Servicio Geológico-Minero Argentino, Argentina
(3) University of Western Australia, Perth, Australia
* Presenting author’s e-mail: jchern@secind.mecon.gov.ar
The Río de la Plata craton (RPC) is the southernmost craton of the South American continent, and is considered to encompass autochthonous Achaean to Palaeoproterozoic basement. We regard the RPC to have
also originally encompassed an òuter´ Mesoproterozoic belt accreted to the old nucleous during the amalgamation of Rodinia, and that this óuter` belt forms part of the RPC.
Increasing evidence points to the existence of an authochtonous Mesoproterozoic orogen (southern continuation of the Sunsás orogen of SW Amazonas) at the western border of the RPC, e.g. i)
Mesoproterozoic Nd model ages of the Cambrian igneous basement of Sierra de la Ventana (i.e. immediately southwest of the RPC basement exposures of Tandilia, ii) identification of abundant
Mesoproterozoic detrital zircon in the Neoproterozoic supracrustal sequences of the core RPC, iii)
increased relative abundance of Mesoproterozoic detrital zircon in the Neoproterozoic RPC metasediments towards the west of the craton, iv) Paleo-Mesoproterozoic Hf model ages obtained by the present
authors both on magmatic zircons from Lower Paleozoic magmatic units and on detrital zircon from
Neoproterozoic-Lower Cambrian cover sequences in La Pampa province. The depositional age of the
RPC supracrustal sequences would preclude Cuyania as a provenance, and indicate that the authochtonous Mesoproterozoic orogen acted partly as a source for these supracrustal sequences (and also partly as
topographic barrier, depending on the depocenters´ locations).
It is in this tectonic context that a fragment of the authochtonous Mesoproterozoic belt of the RPC —
and some RPC Paleoproterozoic crustal material as well—, would have later been rifted away from the
RPC nucleous —sometime prior to 600 Ma—, defining the Pampia Terrane (PT). This extensional event
would have given rise to the Puncoviscana basin, whose late-stage equivalent at the latitude of La Pampa
province is regarded to be represented by the Santa Helena Schists. The detachment of the PT from the
western border of the RPC would, in turn, have been roughly coeval with the inferred westward subduction of Adamastor Ocean lithosphere beneath the eastern border of the RPC. These two coeval events
would coincide with the late stage of the Rodinia break-up which, in turn, partly overlaps the timing of
Western Gondwana amalgamation.
The PT would have been re-accreted to the RPC during the Early Cambrian Pampean orogeny, occurred
during the final stage of Gondwana amalgamation at ca 530-520 Ma. We envisage the PT / RPC collision to have been preceded by west dipping subduction of oceanic crust (i.e. PT = active margin). To the
north of the Sierras Pampeanas, the eastern border of Arequipa-Antofalla/Pampia would have been the
active margin, and the Rio Apa cratonic fragment would have been the passive margin, this scheme being
consistent with the occurrence of Lower Cambrian arc magmatism in northwestern Argentina.
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NEW INSIGHTS ON THE PALEOPROTEROZOIC BASEMENT OF TANDILIA BELT,
RÍO DE LA PLATA CRATON, ARGENTINA: FIRST HF ISOTOPE
STUDIES ON ZIRCON CRYSTALS
1-04
Cingolani, C.1*, Santos, J.O.S.2, Griffin W.3
(1) División Científica de Geología Museo de La Plata and Centro de Investigaciones Geológicas
(CONICET-UNLP), La Plata, Argentina.
(2) Centre for Global Targeting, University of Western Australia, Australia.
orestes.santos@bigpond.com
(3) GEMOC (Centre for Geochemical Evolution and Metallogeny of Continents (GEMOC), MacQuarie
University, Sydney, Australia
* Presenting author’s e-mail: carloscingolani@yahoo.com; ccingola@cig.museo.unlp.edu.ar
Introduction
Following the fragmentation of the Rodinia supercontinent, Archean to Mesoproterozoic cratonic
blocks were amalgamated to form the Gondwana continent during Neoproterozoic-Cambrian times.
One of these continental blocks place at the core of western Gondwana, is the Río de la Plata craton
(Almeida et al., 1973). The southernmost outcrops of this cratonic region are located in the Tandilia
belt (also know as Sierras Septentrionales de Buenos Aires) in eastern Argentina (Fig. 1). It is exposed
as a 350 km long and maximum 60 km wide northwest trending orographic belt, located in the central
part of the Buenos Aires province. The Tandilia belt outcrops are in between the Salado (Mesozoic)
and the Claromecó (Neoproterozoic-Paleozoic) basins. Some reviews on different aspects of Tandilia
basement rocks were published by Dalla Salda et al. (1988), Cingolani and Dalla Salda (2000),
Hartmann et al. (2002a), Pankhurst et al. (2003), Rapela et al. (2007) and Bossi and Cingolani (2010
and references therein). The main purpose of this contribution is to give new Hf isotopic insight on
zircon crystals from the Tandilia Paleo- proterozoic igneous-metamorphic rocks in order to analyze
their magmatic evolution and tectonic interpretation.
Fig. 1 - Geological sketch map
of Tandilia belt from Iñiguez et
al. (1989) and Dalla Salda et
al. (1988) with the studied
sample locations. A relative
location of the Tandilia belt in
the Río de la Plata cratonic
area is shown in the inset.
Geological setting
The geological evolution of Tandilia comprises mainly a juvenile igneous-metamorphic
Paleoproterozoic basement rocks which are covered by thin Neoproterozoic to Early Paleozoic
sedimentary units. The Paleoproterozoic basement called ‘Buenos Aires Complex’ (Fig. 1) consists
mainly of granitic-tonalitic gneisses; migmatites; amphibolites, some ultramafic rocks and granitoid
plutons (Dalla Salda et al., 1988). Subordinate rock types include schists, marbles, and dykes of felsic
and mafic composition. Tandilia was recognized as an important shear belt district with mylonitic
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rocks derived mainly from granitoids. The available geochemical data show that the igneous basement
rocks have a calc-alkaline signature. Crust-derived Sm–Nd model ages (Cingolani et al., 2002;
Hartmann et al., 2002a; Pankhurst et al., 2003) are in between 2.69-2.4 Ga implying that although the
principal rock-forming events were Paleoproterozoic, the Neoarchean derivation could be possible.
After U-Pb zircon crystals SHRIMP dating (Hartmann et al., 2002a; Cingolani et al., 2005) the
tectonic scenario seems related to juvenile accretion (2.25-2.12 Ga) along an active continental
margin, followed by continental collision (2.1-2.08 Ga). A lack of recrystallization or new zircon
growth in the Neoproterozoic, suggests that the Tandilia Paleoproterozoic basement was preserved
from younger orogenies such as those of the Brasiliano cycle. This geological evolution can be
correlated with the Piedra Alta terrane (Uruguay), where Rb-Sr, Sm-Nd and U-Pb data show a similar
signature (Cingolani et al., 1997; Hartmann et al., 2002b and references therein) during
Paleoproterozoic times. After a long weathering process there is a record of Neoproterozoic
sedimentary units and the final marine transgression at the Early Paleozoic.
Sampling and analytical techniques
Samples were taken from tonalitic-monzonitic granitoid and gneissic rocks from the Tandil region
(Ta3; Ta4 and Ta5), and Opx-gneisses from the Balcarce region (Ta9, El Triunfo Hill) during the field
work for U-Pb SHRIMP research (Hartmann et al., 2002a). The same zircon crystals and spots dated
by U-Pb were analyzed for Hf isotope studies (Fig. 2). Samples Ta3 and Ta4 are foliated tonalitic
gneiss and monzogranite (37º22’33”S-59º12’33”W) respectively sampled on the road cut about 10 km
away from the Tandil town, both from the same outcrop. The Ta5 sample, is a granitoid from
Montecristo quarry (37º22’15”S-59º10’42”W) homogeneous medium-grained grey rock. Sample Ta9
from El Triunfo Hill near Balcarce city (37º49’26”S-58º12’15”W) is a mafic to intermediate
orthopyroxene-bearing gneiss.
Fig. 2 - Backscattered electron images of zircons analyzed
by Hartmann et al. (2002a) and Cingolani et al. (2005) by
U-Pb SHRIMP. Same zircon crystals and spots were
analyzed for Hf isotopes by ICP-MS-LA.
Hf-isotope analyses were carried out using a New Wave/ Merchantek UP213 laser-ablation
microprobe, attached to a Nu Plasma multi-collector ICP-MS at MacQuarie University, Sydney. Mud
Tank (MT) zircon was used as reference material which has an average 176Lu/ 177Hf ratio of
0.282522±42 (2SE) (Griffin et al., 2000). Initial 176Hf/177Hf ratios are calculated using measured
176Lu/177Hf ratios, with a typical 2 standard error uncertainty on a single analysis of 176Lu/177Hf
±1–2%. Such error reflects both analytical uncertainties and intragrain variation of Lu/Hf typically
observed in zircon. Chondritic values (Blichert-Toft and Albarède, 1997) of 1.93?10?11 have been used
for the calculation of ÂHf values. Whilst a model of (176Hf/177Hf)i=0.279718 at 4.56 Ga and
176Lu/177Hf=0.0384 has been used to calculate model ages (T
DM) based on a depleted mantle source,
producing a present-day value of 176Hf/177Hf (0.28325). TDM ages, which are calculated using
measured 176Hf/177Hf of the zircon, give only the minimum age for the source material from which
the original magmas were derived. We have therefore also calculated a “crustal” model age (TDM C)
for each zircon which assumes that the parental magma was produced from an average continental
crust (176Lu/177Hf=0.015) that was originally derived from depleted mantle.
Hf isotopes data and discussion:
As it is shown in Fig. 3 the coherent results on zircon crystals from all studied samples suggest that
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the depleted mantle model age (crustal) is Neoarchean 2.65 Ga, older (±350 Ma) than the
crystallization age. Positive ÂHf obtained data show also derivation from juvenile material. An
alternative interpretation could be a mixing with juvenile (2.27 Ga?) and crustal (more than 2.65 Ga)
magmatic components. TDM ages, which are calculated using measured 176Hf/177Hf of the zircon,
give only the minimum age for the source material from which the original magmas were derived. The
average of 28 Hf model-ages (2646 Ma) is almost coincident with the age of the only one inherited
zircon of sample Ta3 (A.14-1, 2657 ± 8 Ma). This is strong evidence supporting the derivation from
a Neoarchean crust (Fig. 3).
Fig. 3 - Plot of 176Hf/177Hf ratios versus ages on dated zircon crystals of samples Ta3, Ta4, Ta 5 (Tandil region) and Ta9
(Balcarce region). The slope of the dashed line uses the ratio of 0.015 for the 176Lu/177Hf ratio.
The present Hf isotope study confirm the Sm-Nd data published by Hartmann et al. (2002a) and
Pankhurst et al. (2003) showing that the constituting material of the source region from the mantle
was Neoarchean (c.2.6 Ga). These results are in agreement with precise U–Pb dating of the craton in
western Uruguay and southernmost Brazil, which also indicate a relatively short-lived
Paleoproterozoic orogeny.
Acknowledgements
This study was enabled by grants from CONICET (PIP 5027) which are gratefully acknowledged. We
thank Prof. Léo A. Hartmann (UFRGS, Brazil) for stimulating discussions and Norberto Uriz and
Mario Campaña (UNLP) for technical support.
REFERENCES
• Almeida, F.F.M., Amaral, G., Cordani, U.G., Kawashita, K. 1973. The Precambrian evolution of the South American cratonic
margin, south of the Amazon River. In: Nairn, A.E., Stehli, F.G. (Eds.), The Ocean Basins and Margins1, Plenum Publishing, New
York, pp. 411–446.
• Blichert-Toft, J., Albarède, F. 1997. The Lu-Hf isotope geochemistry of chondrites and the evolution of the mantle-crust system.
Earth and Planetary Science Letters, 148 (1-2): 243-258.
• Bossi, J., Cingolani, C.A. 2010. Extension and general evolution of the Río de la Plata Craton. In: Gaucher C., Sial A.N, Halverson
G.P, Frimmel H.E. (eds.). Neoproterozoic- Cambrian tectonics, global change and evolution: a focus on southwestern Gondwana.
Developments in Precambrian Geology 16:73-85
• Cingolani, C.A., Varela, R., Dalla Salda, L., Bossi, J., Campal, N., Ferrando, L., Piñeyro, D.,Schipilov, A. 1997. Rb-Sr
geochronology from the Río de la Plata craton of Uruguay. South-American Symposium on Isotope Geology, Brazil, Extended
Abstracts 73-75.
• Cingolani, C.A., Dalla Salda, L. 2000. Buenos Aires cratonic region. In Cordani, U., Milani, E., Thomaz Filho, A., y Campos D.
(eds.) Tectonic evolution of South America. 31° International Geological Congress, 139-146, Río de Janeiro, Brazil.
• Cingolani, C.A., Hartmann, L.A., Santos, J.O.S., McNaughton, N.J. 2002. U–Pb SHRIMP dating of zircons from the Buenos Aires
complex of the Tandilia belt, Río de La Plata cratón, Argentina, Actas CD-ROM, XV Congreso Geológico Argentino (El Calafate,
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Santa Cruz), Asociación Geológica Argentina, Buenos Aires.
• Cingolani, C.A., Santos, J.O.S., McNaughton, N.J., Hartmann, L.A. 2005. Geocronología U-Pb SHRIMP sobre circones del
Granitoide Montecristo, Tandil, Provincia de Buenos Aires, Argentina. 16º Congreso Geológico Argentino, La Plata, 1: pp. 299-302.
• Dalla Salda, L., Bossi, J., Cingolani, C. 1988. The Rio de la Plata cratonic region of southwestern Gondwana. Episodes,11(4):263269.
• Griffin,W.L., Pearson, N.J., Belousova, E.A., Jackson, S.R., van Achterbergh, E., O’Reilly, S.Y., Shee, S.R. 2000. The Hf isotope
composition of cratonic mantle: LAM-MC-ICPMS analysis of zircon megacrysts in kimberlites. Geochimica et Cosmochimica Acta
64, 133–147.
• Hartmann, L.A., Santos, J.O.S., Cingolani, C.A., McNaughton, N.J. 2002a. Two Paleoproterozoic Orogenies in the Evolution of the
Tandilia Belt, Buenos Aires, as evidenced by zircon U-Pb SHRIMP geochronology. International Geology Review, 44: 528-543.
• Hartmann, L.A., Santos, J.O.S., Bossi, J., Campal, N., Schipilov, A., McNaughton, N.J. 2002b. Zircon and titanite U–Pb SHRIMP
geochronology of Neoproterozoic felsic magmatism on the eastern border of the Río de la Plata craton, Uruguay. Journal of South
American Earth Sciences 15, 229–236.
• Iñiguez, A.M., Del Valle, A., Poiré, D., Spalletti, L., Zalba, P. 1989. Cuenca Precámbrica-Paleozoica inferior de Tandilia, Provincia
de Buenos Aires. In: Chebli, G. y L.A. Spalletti (Eds.). Cuencas sedimentarias argentinas. Instituto Superior de Correlación
Geológica, Universidad Nacional de Tucumán, Serie Correlación Geológica, 6:245-263.
• Pankhurst, R.B., Ramos, A., Linares, E. 2003. Antiquity of the Rio de la Plata craton in Tandilia, southern Buenos Aires province,
Argentina. Journal of South American Earth Sciences, 16 (2003) 5–13 10.
• Rapela, C.W., Pankhurst, R.J., Casquet, C., Fanning, C.M., Baldo, E.G., González-Casado, J.M., Galindo, C., Dahlquist, J. 2007.
The Río de la Plata Craton and the assembly of SW Gondwana. Earth Science Reviews, 83, 49-82.
THE RODINIAN RELICS OF AUSTRALIA-MAWSON, AZANIA,
NEOPROTEROZOIC INDIA AND A GREATER KALAHARI –
DIVINING THEIR EXTENT AND INTERPRETING THEIR EVOLUTION
1-05
Collins, A.S.1,2*, Selway, K.1, Clark, C.3, Kinny, P.D.3, Plavsa, D.1, Amarasinghe, U.1
(1) Tectonics Resources and Exploration (TRaX), School of Earth and Environmental Sciences,
University of Adelaide, Adelaide, SA 5005, Australia
(2) Instituto de Geociências and IAG-USP Departamento de Geofisica, Universidade de São Paulo,
Butantã, São Paulo, Brazil)
(3) The Institute for Geoscience Research (TIGeR), Curtin University of Technology, WA, Perth,
Australia
* Presenting author’s e-mail: alan.collins@adelaide.edu.au
Rodinia broke up into numerous Australia-sized continents in the Tonian-Cryogenian that
amalgamated to form an Ediacaran-Cambrian Gondwana. A number of fragments of these
Neoproterozoic continents are now found in East Antarctica and parts of India, Sri Lanka,
Madagascar, Australia and Africa that were contiguous in central Gondwana. Tracing the extent of
these continents and gaining an understanding of the tectonic events that they were part of is
complicated by the dual problem that, a) many of these Neoproterozoic continental margins have been
deformed, and, b) the boundaries in East Antarctica are buried under extensive ice cover. To partially
overcome this latter problem, we undertook a 180 kilometre long magnetotelluric (MT) survey
parallel to the Prydz Bay coast with the joint aims of developing MT methodologies for Antarctic
experiments and investigating the geological history of the Prydz Bay area. Analysis and inversion of
the data has imaged a crustal-scale boundary beneath the Sørsdal Glacier separating two distinct
regions that correlate with the Vestfold Hills and the Rauer Group. The successful imaging of this
feature shows that MT is potentially a useful tool for finding the location of other proposed suture
zones in Antarctica. What geophysical imaging cannot tell us, though, is what the continents were that
now crop out as the Vestfold Hills and the Rauer Islands. For this we undertook a detrital zircon study
of Proterozoic samples from the Vestfold Hills. The results show a similarity with potential sources in
the Singbhum craton of India. Numerous ~2.4 Ga zircons also open the possibility that the Gawler
craton of South Australia was linked to the Vestfold/Singbhum craton in the Palaeoproterozoic.
Detrital zircon studies in Sri Lanka, Southern India and Madagascar are also helping divine the
correlation between various rock units and Neoproterozoic continents. In Madagascar and southern
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India, highly metamorphosed sedimentary rocks are shown to have many Palaeoproterozoic sources.
There is a paucity of Palaeoproterozoic source rocks in the Dharwar craton of India, but considerable
potential in eastern Africa, suggesting that this sedimentary basin was depositionally joined to Africa
in the Proterozoic. Juvenile Neoproterozoic volcanics and sedimentary rocks to the west of this belt
(south western Madagascar and east Africa) suggest that this continental (Azania) rifted off Africa in
the early Neoproterozoic to later recombine in the Cryogenian/Ediacaran.
Sri Lanka joins India to East Antarctica in Gondwana. Here the Wanni Complex has long resisted
attempts to be correlated with adjacent parts of India. New detrital zircon data suggests that quartzites
from the Wanni Complex a) are Neoproterozoic, b) have similar sources to the metasedimentary rocks
found in the Madurai Block of India.
RODINIA: IS A RAPPROCHEMENT OF CURRENT
‘SOUTHERN OCEAN’ AND ‘NORTH ATLANTIC’ MODELS ACHIEVABLE?
1-06
Dalziel, I.W.D.
Institute for Geophysics, Jackson School of Geosciences, The University of Texas at Austin
10100 Burnet Rd, Austin, TX 78758
Models of latest Mesoproterozoic-Neoproterozoic global paleogeography have recently been
developing in two widely separated regions over the past few years. The question is whether they are
compatible. In the ‘Southern Ocean’ model, geochemical data have emerged to support the 1990’s
suggestion that the Laurentian and Mawson cratons were juxtaposed prior to the Neoproterozoic
opening of the Pacific Ocean basin (Goodge et al., Science, 2008), essentially the Southwest United
States-East Antarctica (so-called ‘SWEAT’) reconstruction of Moores, Geology, 1991 and of Dalziel,
Geology, 1991). The Mawson craton could have rifted from the Pacific margin of Laurentia in the
mid-Neoproterozoic and collided with the Weddell Sea margin of the Antarctic craton, the Coats Land
block, in the latest Neoproterozoic along the Pan-African suture through the Shackleton Range. In so
doing, it need not have travelled at a rate higher than plates have been measured to move during the
Mesozoic and Cenozoic (Dalziel, Sp. Pub. Geol. Soc. London, No. 335, 2010). Published geologic,
radiometric and paleomagnetic data together with unpublished Pb isotope data indicate that Coats
Land was part of the Laurentian craton. Hence, according to this model, Laurentia and the newly
amalgamated Gondwanaland formed the geologically emphemeral Pannotia supercontinent prior to
the Early Cambrian opening of the Iapetus Ocean basin between the proto-Appalachian and protoAndean margins. The present northern hemisphere craton of Laurentia would have been juxtaposed
with cratons now bordering the Southern Ocean.
In the North Atlantic region recent geochronologic and geochemical studies have led to a
reconstruction that places Baltica against the Labrador-Scotland-Greenland promontory of Laurentia
in the Neoproterozoic along what is termed the ‘Valhalla orogen’ (Cawood et al., Geology, 2010). This
model also places Baltica against northern Amazonia. The configuration is thought to have existed
until opening of Iapetus between Laurentia on one side and Baltica plus Amazonia on the other. In this
presentation I will address the question of whether these two models are compatible. Study of this
problem using PLATES paleogeographic reconstruction software indicates that it is difficult to
reconcile the ‘northern’ and ‘southern’ models as published. Perhaps, however a rapprochement can
be reached if uncertainties over the position of the Iapetus Ocean rifted margin within Gondwanaland,
a margin extensively altered by Phanerozoic tectonism and magmatism, are taken into account.
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VOLCANISM IN THE WESTERN OUACHITA-CUYANIA BASIN AND
SEPARATION OF LAURENTIA AND GONDWANA
1-07
Dickerson, P.W.1*, Hanson, R.E.2, Roberts, J.M.2, Fanning C.M.3
(1) American Geological Institute and Jackson School of Geosciences, University of Texas, Austin,
TX 78712 USA
(2) Department of Geology, Texas Christian University, Ft. Worth, TX 76129 USA
(3) Research School of Earth Sciences, Australian National University,
Canberra ACT 0200, Australia
* Presenting author’s e-mail: patdickerson@earthlink.net
Neoproterozoic through early Paleozoic times witnessed the sundering of Rodinia and the
amalgamation of several allochthonous and parautochthonous blocks with Western Gondwana;
Cuyania was one such terrane, which originated as an element of the central-southern Laurentian
margin. The western Ouachita basin (Marathon basin, west Texas) and Cuyania (greater Precordillera,
western Argentina) have fundaments of Laurentian Mesoproterozoic (Grenvillian) basement and
evolved together, as evidenced by isotopic, litho-, bio- and chronostratigraphic data, as well as by
recent paleomagnetic determinations. Ages, isotopic and geochemical data correspond well for
Cuyania (Western Sierras Pampeanas, anorthosite massif of Sierra de Umango) and for west-central
Texas crystalline basement rocks (Llano Uplift, Pecos layered mafic complex). Proterozoic through
Eocambrian outcrops around the northern basin rim supplied detrital zircons to Middle Cambrian
sandstones of Cuyania.
Fully correlative Ordovician carbonate successions developed on platforms of both the northern and
southern basin and hosted homologous sponge-algae-stromatoporoid bioherms. Within the off-shelf
calcarenite debris flows and bentonitic shales of the Marathon Fm. (Floian) is an interval of megaolistoliths of shelf carbonate rocks (limestone cobble to boulder conglomerates) that were likely shed
from fault-bounded blocks during extension/transtension in the basin. The olistostrome (Monument
Spring Mbr.) extends more than 20 km along strike and foundered into sediments belonging to a
single graptolite zone (Tetragraptus approximatus; Toomey, 1978). Along with conglomerate the unit
includes abundant megaclasts of lime wackestone, many of which show unusually pervasive
silicification. Immediately beneath those megaclasts, a 0.7-m basalt boulder has been found within a
limestone cobble conglomerate; the basalt has also undergone pervasive silicification. The extensive
silicification of limestone megaclasts is suggestive of low-T hydrothermal processes at shallow levels
beneath the seafloor, which could conceivably be related to volcanism in the source area for both the
boulders and blocks.
At the base of the superjacent Ft. Peña Fm. (Dariwillian ) additional cobbles and boulders up to ~0.5
m across of volcanic rock (basalt to trachyte) and volcaniclastic lithic wackestone have recently been
discovered within an 8.5-m-thick limestone conglomerate layer. Six boulders, including that from the
underlying Marathon Fm., have so far been analyzed for major and trace elements; with these ancient
altered volcanic rocks, emphasis has been on trace elements that are resistant to secondary alteration.
(Analyses of additional boulders are in progress.) Preliminary geochemical data for all six basalt to
trachyte boulders indicate a within-plate setting for the magmatism.
Evidence of explosive volcanism is found in metabentonites of the western Ouachita-Cuyania basin.
Metabentonite intervals of the Marathon (Floian) and Ft. Peña (Dariwillian) Formations are within
identical faunal zones to those for ash beds within the San Juan and Gualcamayo Formations,
respectively, of Cuyania. Precordilleran metabentonites have been dated at 469.5 ± 3.2 to 470.1 ± 3.3
Ma (U-Pb, SHRIMP, zircons; Fanning et al., 2004). Notably, all these pyroclastic deposits belong to
a distinctly older (by ~14 Ma) suite than the well-known Deicke-Millbrig-Kinnekulle metabentonites
of the central Appalachians and Baltica.
New preliminary geochronologic data (U-Pb, SHRIMP) for zoned igneous zircons from a basalt
boulder within the basal Ft. Peña Fm. and from superjacent metabentonite and porcellanite revealed
a strong Neoproterozoic (669 – 740 Ma; Cryogenian) population. A few Grenvillian (1.0 – 1.2 Ga)
grains were present in both, but neither contained Ordovician zircons. However, in southernmost
exposures of the Ft. Peña, initial results indicate the presence of a 470 ± 6 Ma zircon component, as
well as Grenvillian grains (U-Pb, SHRIMP), in a metabentonite containing Dariwillian graptolites. If
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supported by further dating, the Ft. Peña metabentonite would be coeval with San Juan Fm.
metabentonites of the Precordillera.
New paleomagnetically derived plate reconstructions for Neoproterozoic through medial Ordovician
time place Cuyania and Western Gondwana at low southern latitudes (~26º S) and adjacent to southern
Laurentia (Rapalini, 2005; 2008, written comm.). The Western Gondwanan margin trended west – that
is, ~90º clockwise from its present orientation – and faced southern Laurentia.
Western Ouachita-Cuyania deformation, sedimentation, and volcanism are consonant with dextral
transtension in response to north-northeastward translation of Laurentia and counterclockwise
rotation of Gondwana – a regime that evolved from the dextral transpressional situation which had
prevailed in Early Cambrian time, as other Laurentia-derived blocks (Arequipa-Antofalla, Western
Sierras Pampeanas, Amazonia) were accreted to Gondwana. With continued oblique dextral
separation of the two supercontinental masses, the attenuated Laurentian slab broke apart and Cuyania
was severed from Laurentia.
New insight into Laurentia-Gondwana tectonic interactions is emerging from field and
geochronological investigations in the western Ouachita-Cuyania basin. Preliminary
geochronological data from volcanic boulders and ash in Lower to Middle Ordovician deposits of the
Marathon basin reveal the presence of Cryogenian zircons. These zircons are clearly xenocrystic in
the Ordovician volcanic ash, and their significance is uncertain. However, Cryogenian zircons in the
volcanic boulders may provide evidence for a previously unrecognized magmatic episode at that time
in the Marathon segment of the Laurentian margin. Additional geochronological data are needed to
better define the timing of within-plate magmatism recorded by the volcanic boulders.
REFERENCES
• Fanning, C. M., Pankhurst, R. J., Rapela, C. W., Baldo, E. G., Casquet, C., and Galindo, C., 2004, K-bentonites in the Argentine
Precordillera contemporaneous with rhyolite volcanism in the Famatina Arc: London, Journal of the Geological Society, v. 161, p.
747-756.
• Rapalini, A., 2005, The accretional history of southern South America from the late Proterozoic to the late Paleozoic: a
paleomagnetic perspective (abstract): Córdoba, Academia Nacional de Ciencias, Gondwana 12 Conference, Abstracts, p. 305.
• Toomey, D. F., 1978, Observations of the Monument Spring Member of the Lower Ordovician Marathon Formation, Marathon
region, southwest Texas: Midland, Permian Basin SEPM, Publication 78-17, p. 215-221.
GEOCHEMISTRY, GEOCHRONOLOGY AND PALEOMAGNETISM OF
PALEOPROTEROZOIC GRANITES OF THE ULKAN MASSIF,
SE SIBERIAN CRATON
1-08
Didenko, A.N.1,2*, Guryanov, V.A.1, Peskov, A.Yu.1, Perestoronin, A.N.1, Avdeev D.V.1
(1) Yu.A. Kosygin Institute of Tectonics and Geophysics, FEB RAS. 65, Kim Yu Chen St.,
Khabarovsk, 680000, Russia
(2) Geological Institute, RAS; 7, Pyzhevsky lane, Moscow, 119017, Russia
* Presenting author’s email: alexei_didenko@mail.ru
The interpretation of the evolution of the Siberian craton’s SE margin in the Paleoproterozoic involves
the Ulkan massif, one of the key structures filled with sedimentary-volcanogenic assemblages, which
are the stratotype for the Upper Karelian (Ulkanian, according to the regional scale) of the AldanStanovoy province. Three volcano-plutonic complexes are distinguished within the massif: 1)
Ulkachan, mainly gabbro-trachybasaltic, 2) Elgetei, chiefly basalt-trachyrhyolitic, and 3) Ulkan, most
widely represented by granitoids, which are subdivided into three phases.
The figurative points of their composition occupy the fields of alkali granites and granites on the TASdiagram. The Ulkan granites are characterized by significant dominance of Fe over Mg and a high K
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content; they are oversaturated with silica, iron, alkalis, fluorine, and sometimes alumina
accompanied with moderate and low Mg and Ca contents. According to classification diagrams, a)
granitoids belong to ferruginous assemblages, FeOt/(FeOt+MgO) ? 0.9, b) fall into the fields of
alkaline, alkaline-calc and calc-alkaline assemblages, and c) occupy all the three possible areas of the
Frost binary plots (ASI vs. A/NK) – agpaitic, peraluminous, and metaluminous.
All the three phase granites of the Ulkan complex are distinguished by high Rb, Th, U, Nb, Ta, and REE
contents, actual absence of the negative Nb-Ta anomaly and depletion in Ba, Sr, P, Ti, which is peculiar
to anorogenic rare-metal alkali granites. REE contents of the Ulkan granitoids differ substantially; their
sums in the 1st, 2nd and 3rd phase granites equal 466±68, 94±38 Ë 997±455 mg/g, respectively. The 3rd
phase granites are noted for extreme enrichment with LREE: the average (Lan/Ybn) = 25.6; it drops to
7.6 for the 2nd phase granites; and to 5.3, for the 1st phase. The average values of the Lan/Smn ratio for
the 1st, 2nd, and 3rd phase granites are 3.6±0.3, 7.9±1.5 and 8.6±2.8, respectively. All granitoids are
characterized by a deep negative Eu anomaly (Eu/Eu* = 0.04-0.11). The figurative points of the study
rocks occupy areas of distribution of three granite types, the lot, on the Hf–Rb–Ta diagram – intraplate,
postcollisional, and those of volcanic arcs. The figurative points of the study granites occupy two different
fields on the Yb–Ta–Hf diagram – Ä1 and Ä2; the former corresponds to anorogenic intraplate granitoids,
and the latter, to postcollisional (postectonic) granitoids.
For four zircon fractions from the 1st phase granites, the age 1730?2 Ma was derived. For zircons of
two dimensional fractions, and also of two more fractions after their abrasive treatment from the 3rd
phase granites, the age 1725 ?4 Ma was obtained. Positive ÂNd(T) values, +3.5 and +0.7, respectively,
were determined for the 1st and 3rd phase granites.
The first paleomagnetic evidence for pilot granite collections from the Ulkan complex has been
acquired. The direction of the high-temperature component of granite magnetization in the modern
coordinate system is Dec=60.8°, Inc=48.4° (K=5.6, a95=10.3) (we believe that the Ulkan massif did
not experience any rotations about horizontal axis after granite intrusion). This corresponds to the
paleomagnetic pole with the coordinates Plat=-47.4°, Plong=64.4° (dp=8.8°, dm=13.5°) which,
considering the Aldan-Stanovoy province turn correction with respect to the Angara-Anabar province
in the Middle Paleozoic, is close to the paleomagnetic pole by ~1,730Ma, based on postectonic
granitoids of the Angara-Kansk protrusion of the Siberian craton.
The investigations were financially supported by the Russian Basic Research Foundation (Project No.
09-05-00223a).
THE AGE AND SIGNIFICANCE OF THE PUNCOVISCANA
FORMATION WITH RESPECT TO NEOPROTEOZOIC TO CAMBRIAN
TECTONIC EVOLUTION OF THE PROTO-ANDEAN MARGIN OF GONDWANA
1-09
Escayola, M.1*, Van Staal C.2
(1) CONICET (National Research Council of Argentina). Laboratorio de Téctonica Andina,
Universidad de Buenos Aires, Pabellón II, Nunez. Buenos Aires
(2) Geological Survey of Canada. 625 Robson Street,
Vancouver, V6B 5J3 BC, Canada
*Presenting author’s email: mescayola@gl.fcen.uba.ar
We present a new tectonic model for the Pampean-Tilcarian accretion of the Arequipa-Antofalla-Western
Pampia (AA-WP) ribbon continent to the Proto-Andean margin of Gondwana represented by the
Amazonia and Rio de La Plata cratons, based on our studies of the Puncoviscana Formation and adjacent
units in northern and central Argentina. A compilation of existing detrital zircon ages of the Puncoviscana
Formation and correlative units along strike in the Pampean orogenic belt to the south combined with our
new U-Pb SHRIMP zircon ages and conventional single grain TIMS of Puncoviscana Formation, which
are based on recently discovered felsic tuffs and mafic volcaniclastic rocks (~537±1 Ma) in the unit’s type
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locality suggests that the Puncoviscana Formation mainly represents an Early Cambrian arc-trench gap to
foreland basin succession formed during east-directed closure of a late Neoproterozoic oceanic back-arc
basin. The back-arc basin, which probably remained relatively narrow, initially had opened behind an eastfacing ~650-570 Ma island arc (Eastern Pampia arc), built upon the rifted, leading edge of the AA-WP.
The c. 537±5 Ma felsic tuffs are interpreted to represent the products of a new, short-lived Early Cambrian
magmatic arc built upon the composite Proto-Andean margin, following Late Neoproterozoic, softaccretion of the Eastern Pampia arc and a subduction polarity reversal. Puncoviscana Formation
conglomerates and mafic volcanics previously interpreted as early rift-related deposits are better
interpreted as late-orogenic basin fills and/or were deposited after basin closure. Our new U-Pb SHRIMP
and conventional single grain zircon age of the post-collision Canani tonalite (524±1 Ma), which intruded
into Tilcarian deformed Puncoviscana Formation rocks in the north westernmost part of Argentina in the
Puncoviscana type locality, combined with the existing 529-520 Ma zircon ages for post-collision
peraluminous granites and tonalites in the Eastern Pampean Ranges to the south indicates that the
synorogenic Puncoviscana basin formed between 540 and 524Ma, progressively cannibalizing its
orogenic hinterland over time. In addition, the Tilcarian and Pampean orogenies represent the same event.
We suggest that AA-WP rifted-off from Laurentia between 700 and 650 Ma, shortly after Amazonia’s
departure during Rodinia’s break-up. We emphasize that it is the departure of AA-WP, not Amazonia that
opened Iapetus in the Late Neoproterozoic. We also suggest that the Ganderia terrane in the northern
Appalachians, originally formed an extension of the AA-WP, but returned later to Laurentia during
Iapetus’ closure.
THE PUTUMAYO OROGEN OF NORTHWEST SOUTH AMERICA:
IMPLICATIONS FOR RODINIAN CONNECTIONS BETWEEN AMAZONIA,
BALTICA AND THE MIDDLE- AMERICAN OAXAQUIAN TERRANES
1-10
Ibanez-Mejia, M.1*, Ruiz, J.1, Valencia, V., Cardona, A.2, Gehrels, G.1,
Mora, A.3, DeCelles, P1
(1) Department of Geosciences, The University of Arizona, Tucson, Arizona, USA
(2) Smithsonian Tropical Research Institute, Balboa-Ancon, Panama
(3) Instituto Colombiano del Petroleo (ICP) - ECOPETROL, Piedecuesta, Colombia
* Presenting author’s email: ibanezm@email.arizona.edu
The recognition of mobile belts and tectonometamorphic provinces in Precambrian cratons is key for
the study and accurate geological reconstruction of ancient orogenic belts, where sea-floor magnetic
stripes are no longer available and in cases where paleomagnetic data is limited and solutions for the
existent poles become non-unique. Consequently, reconstructions of Proterozoic supercontinents and
connections between different cratons heavily rely on geological correlations drawn upon finely
resolved geochronology and different geochemical and isotopic tracers. The Amazon Craton is the
largest of the Precambrian blocks that constitute the South American continent, and its role in the
amalgamation of the supercontinent Rodinia has for long been inferred. However, there are still
sizable areas that remain completely unknown as a result of dense vegetation cover, heavy tropical
weathering, and the widespread development of Phanerozoic sedimentary basins.
In this communication, we present new zircon U-Pb geochronological results from Proterozoic highgrade metamorphic inliers found in the Colombian Andes, and basement drilling-cores from deep
exploratory wells in the adjacent north Andean foreland basins. Our results constitute the first
geochronological evidence for the presence of a Grenville-age belt (s.l.) in autochthonous NW
Amazonia, and also document the complex Proterozoic accretionary and collisional evolution
experienced by this segment of the craton. Two distinct metamorphic events were recognized, one
represented by migmatite formation under amphibolite-facies conditions at ca. 1.05- 1.02 Ga, and the
other characterized by granulite-facies conditions attained at ca. 0.99 Ga. Detrital zircon U-Pb results
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from metasedimentary units and protolith ages from metaigneous suites suggest a pre-collisional
disconnect between the evolution of (autochthonous) Paleoproterozoic Amazonia’s leading-edge, and
that of the interpreted parautochthonous terranes represented by the Cordilleran inliers, these last ones
to be predominantly built on Mesoproterozoic crust. Close comparisons with the known geologic
evolution of the Sunsas-Aguapei belt in SW Amazonia reveals clear contrasts in terms of the
collisional and pre-collisional records of these two orogenic segments, placing fundamental
differences between them. Based on this, we propose the existence of a distinct Grenville-age belt in
NW South America, herein called the Putumayo Orogen, characterized by a two-stage
tectonometamorphic evolutionary model comprised by 1) early terrane accretions of a periAmazonian fringing-arc system onto the continental margin and 2) final collisional interactions with
the Sveconorwegian province of Baltica and the Oaxaquian terranes at the heart of Rodinia.
COUPLED DELAMINATION AND INDENTOR-ESCAPE TECTONICS
IN THE SOUTHERN PART OF THE C. 650-500 MA
EAST AFRICAN/ANTARCTIC OROGEN
1-11
Jacobs, J.1*, Thomas, R.T.2, Ueda, K.1, Kleinhanns, I.1, Emmel, B.1, Kumar, R.1,
Bingen, B3, Engvik, A.3
(1) Department of Earth Science, University of Bergen, Allegaten 41, N-5007 Bergen, Norway,
(2) British Geological Survey, Kingsley Dunham Centre, Keyworth, Nottingham, United Kingdom
(3) Geological Survey of Norway, Trondheim, Norway
* Presenting author’s e-mail: Joachim.Jacobs@geo.uib.no
The East African/Antarctic Orogen (EAAO) is one of the largest orogenic belts on the planet,
resulting from the collision of various parts of East and West- Protogondwana between 620 and 550
Ma. The central and southern parts of the orogen are typified by high-grade rocks, representing the
overprinted margins of the various colliding continental blocks. The southern third of this Himalayantype orogen can be interpreted in terms of a lateral tectonic escape model, similar to the situation
presently developing in SE-Asia. One of the escape-related shear zones of the EAAO is exposed as
the approximately 20 km wide Heimefront transpression zone in western Dronning Maud Land
(Antarctica). During Gondwana break-up, the southern part of the EAAO broke up into a number of
microplates (Falkland, Ellsworth-Haag and Filchner blocks). These microplates probably represent
shear zone-bound blocks, which were segmented by tectonic translation during lateral tectonic
extrusion. The southern part of the EAAO is also typified by large volumes of late-tectonic A2-type
granitoids that intruded at c. 530-490 Ma, and can constitute up to 50% of the exposed basement. They
are likely the consequence of delamination of the orogenic root and the subsequent influx of hot
asthenospheric mantle during tectonic escape. The intrusion of these voluminous melts into the lower
crust was accompanied by orogenic collapse. The A2-type magmatism appears to terminate along the
Lurio Belt in northern Mozambique. Therefore, the Lurio Belt could represent an accommodation
zone, separating an area to the south in which the orogen underwent delamination of the orogenic root,
and an area to the north, where the orogenic keel is still present. Erosional unroofing of the northern
EAAO is documented by the remnants of originally extensive areas covered by Cambro-Ordovician
molasse-type clastic sedimentary rocks throughout North Africa and Arabia, testifying to the size of
this mega-orogen. Whilst the EAAO molasse in the north covers almost the entire North African
platform, probably resulting from a long lasting high standing mountain range (no delaminated root),
the molasse deposits of the southern EAAO are comparatively smaller, possibly resulting from the
rapid and mechanical thinning of the orogen in the south (delaminated root).
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SM-ND ISOTOPIC CONSTRAINTS ON THE NEOPROTEROZOIC –
EARLY PALEOZOIC EVOLUTION OF THE EASTERN SIERRAS PAMPEANAS
1-12
López de Luchi, M.G.1*, Steenken, A.2, Martínez Dopico, C.I.1, Drobe, M., Wemmer, K.,
Siegesmund, S.
(1) Instituto de Geocronología y Geología Isotópica (INGEIS), Pabellón INGEIS, Ciudad Universitaria,
C1428EHA, Buenos Aires, Argentina
(2) Universität Greifswald, Geologisches und Geographisches Institut, Friedrich-Ludwig-Jahn
Strasse 17a, 17489 Greifswald, Germany
(3) Geoscience Centre of the University of Göttingen (GZG), Goldschmidtstr. 1-3, 37077 Göttingen,
Germany.
* Presenting author’s e-mail: deluchi@ingeis.uba.ar
The aim of this contribution is to present the result of an extensive integration of 276 already published Sm-Nd data of the Sierras de Córdoba, San Luis and Chepes in order to provide an isotopical
background for the crustal average age of the Eastern Sierras Pampeanas (ESP). ESP constitute a
polyphase deformed morphotectonic unit, which was structured by three main events: the Ediacaran
to early Cambrian (580–510 Ma) Pampean, the late Cambrian–Ordovician (500–440 Ma) Famatinian
and the Devonian-Carboniferous (400–350 Ma) Achalian orogenic cycles (among others Aceñolaza et
al. 2000, López de Luchi et al. 2007, Otamedi et al. 2005, Ramos 1988, Rapalini 2005, Rapela et al.
2007, Sims et al. 1999, Siegesmund et al. 2009, Steenken et al. 2006). In the Pampean orogen a collisional stage predates a high grade granulite metamorphism resulting from the collapse of the subducting slab and delamination of the mantle lithosphere. In the Famatinian orogen high grade metamorphism is related to the active subduction with close temporal and spatial relationships between
mid-crustal felsic plutonism, mafic magmatism and zones of high-grade metamorphism. Achalian
cycle magmatism which would be related with a compressional event farther west from the ESP is
mainly represented by extensive granitoid batholiths. This magmatism involves a transient heat anomaly that controlled melts derived from an enriched mantle source which probably mixed with melts
derived from a segment of crust different from the Neoproterozoic to Ordovician continental crust of
the Eastern Sierras Pampeanas.
Sm-Nd results for the metamorphic basement suggest that the TDM (Nd) interval of 1.7-1.8 Ga (Fig.
1), which is associated with the less radiogenic ÂNd(540) of -6 to -8 can be considered as the average
crustal composition for the Eastern Sierras Pampeanas. Increasing metamorphic grade in rocks with
similar detrital sources and metamorphic ages like in the Sierras de Córdoba is associated with a
younger TDM and a more positive ÂNd(540). In metaclastic rocks of different metamorphic age predominance of Grenville detrital components is associated with a relatively more positive ÂNd(540).
Granitoids emplaced pre-peak metamorphism in the Pampean orogen form two clusters, one with TDM
(Nd) between 1.75-2.0 and another between 1.5-1.6 Ga. Pampean post- 540 Ma granitoids exhibit
more homogenous TDM (Nd) ranging 1.75-2.0 Ga. TDM (Nd) for the Ordovician Famatinian granitoids
define a main interval of 1.6-1.8 Ga, except for the Ordovician TTG suites of the Sierras de Córdoba,
which show a younger TDM (Nd) ranging 1.0-1.3 Ga and a variable radiogenic epsilon. Achalian magmatism exhibits more radiogenic epsilon compared to the Pampean and Famatinian Nd parameters
ranging between 0.5 to -4 and TDM (Nd) younger than 1.3 Ga. Two types of Pampean related mafic
rocks are recognized: one with a depleted mantle signature and LREE depleted sources that could
indicate the stage of ocean crust formation and another younger group with an enriched mantle signature, which is associated with the peak of metamorphism. In the Ordovician mafic-ultramafic rocks
processes of mixing/assimilation of depleted mantle signature melts and continental crust are more
likely. Therefore, the geodynamic scenario for the Ordovician mafic rocks would imply a thicker continental crust. On the contrary the emplacement of the Pampean mafic rocks would imply a thinner
continental crust or alternatively a fast extensional process (Pampean extensional collapse) that could
prevent the modification of the mafic melts in their ascent to the emplacement level. Although some
discussion could be addressed concerning the degree of crust-mantle interaction in pre-Devonian
times, crustal recycling is dominant, whereas processes during the Achalian cycle led to different geochemical and isotopic signatures that reflect a major input of juvenile sources to the magmatism.
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Fig. 1- Simplified geological map of the Sierras de Córdoba, the Sierra Norte, the Sierra de Chepes and the Sierra de
San Luis (Steenken et al 2006, Siegesmund et al. 2009) with the approximate location of TDM (Nd) data for
metaclastic and felsic igneous rocks. Detrital zircon spectra are included as another evidence for the source.
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REFERENCES
• Aceñolaza, F.G., Miller, H., Toselli, A.J., 2000. The Pampean and Famatinian cycles –superimposed orogenic events in West
Gondwana. In: Miller, H., Herve, F. (Eds.), Geoscientific Cooperation with Latin America. Zeitschrift für Angewandte Geologie
Sonderheft, vol. 1, pp. 337–344.
• Brodtkorb, M.K. de., Ostera, H., Pezzutti, N., Tassinari, C. 2005. Sm/Nd and K-Ar data from W-bearing amphibolites of Eastern
Pampean Ranges, San Luis and Córdoba, Argentina. 5° South American Symposium on Isotope Geology, 478-482,Punta del Este.
• Drobe M, López de Luchi MG, Steenken A, Frei R, Naumann R, Wemmer, K., Siegesmund S., (2009) Provenance of the Late
Proterozoic to Early Cambrian metaclastic sediments of the Sierra de San Luis (Eastern Sierras Pampeanas) and Cordillera oriental,
Argentina. Journal of South American Earth Sciences, 28:239–262
• Escayola, M.P., Pimentel, M.M., Armstrong, R., 2007. Neoproterozoic back-arc basin: sensitive high-resolution ion microprobe
U–Pb and Sm–Nd isotopic evidence from the Eastern Pampean Ranges, Argentina. Geological Society of America 35 (6): 495–498.
• López de Luchi, M., Siegesmund, S., Wemmer, K., Steenken, A., Naumann, R., 2007.Geochemical constraints on the petrogenesis
of the Palaeozoic granitoids of the Sierra de San Luis, Sierras Pampeanas, Argentina. Journal of South American Earth Sciences 24
(2–4), 138–166.
• Otamendi, J.E., Ribaldi, A.M., Demichelis, A.H., Rabbia, O.M., 2005. Metamorphic evolution of the Río Santa Rosa granulites,
north Sierra de Comechingones, Argentina. Journal of South American Earth Sciences 18:163–181.
• Ramos, V.A., 1988. Late Proterozoic–early Paleozoic of South America: a collisional story. Episodes 11, 168–174.
• Rapalini, A.E., 2005. The accretionary history of southern South America from the latest Proterozoic to the late Paleozoic: some
paleomagnetic constraints. In: Vaughan, A.P.M., Leat, P.T., Pankhurst, R.J. (eds). Terrane processes at the margins of Gondwana,
London, Geological Society of London Special Publication 246, 305-328.
• Rapela, C.W., Pankhurst, R.J., Casquet, C., Fanning, C.M., Baldo, E.G., Gonzáles- Casado, J.M., Galindo, C., Dahlquist, J., 2007.
The Río de la Plata craton and the assembly of Gondwana. Earth-Science Reviews 83, 49–82.
• Schwartz, J.J., Gromet, L.P., 2004. Provenance of a late Proterozoic–early Cambrianbasin, Sierras de Córdoba, Argentina.
Precambrian Research 129, 1–21.
• Siegesmund, S., Steenken, A., Martino, R. Wemmer, K. López de Luchi, M.G., Frei R., Presniakov, S., Guereschi, A., 2009. Time
constraints on the tectonic evolution of the Eastern Sierras Pampeanas (Central Argentina). International Journal of Earth Sciences,
DOI: 10.1007/s00531-009-0471-z
• Sims, J.P., Ireland, T.R., Camacho, A., Lyons, P., Pieters, P.E., Skirrow, R.G., Stuart-Smith, P.G., Miró, R., 1998. U–Pb, Th–Pb and
Ar–Ar geochronology form the southern Sierras Pampeanas: implication for the Palaeozoic tectonic evolution of the western
Gondwana margin. In: Pankhurst, R.J., Rapela, C.W. (Eds.), The Proto-Andean Margin of Gondwana. Geological Society of London
Special Publication, vol. 142, pp. 259–281.
• Steenken, A., Siegesmund, S., López de Luchi, M.G., Frei, R., Wemmer, K., 2006. Neoproterozoic to early Palaeozoic events in the
Sierra de San Luis: implications for the Famatinian geodynamics in the Eastern Sierras Pampeanas (Argentina). Journal of the
Geological Society 163, 965–982.
PALEOMAGNETIC POLE FOR THE NEOPROTEROZOIC DIKES
OF THE NICO PÉREZ TERRANE (URUGUAY) AND THE APPARENT
POLAR WANDER PATH (APWP) FOR THE RÍO DE LA PLATA CRATON
1-13
Lossada, A.L.1, Rapalini, A.E.1, Sanchez Bettucci, L.2
(1) INGEODAV, Depto. Cs. Geológicas, FCEyN, Univ. Buenos Aires, Pabellón 2, Ciudad
Universitaria, C1428EHA, Buenos Aires., Argentina
(2) Departamento de Geología, Área de Geofísica-Geotectónica, Facultad de Ciencias, Universidad
de la República. Iguá 4225, Malvín Norte, CP 11400, Montevideo, Uruguay
Neoproterozoic basic dikes intrude the Paleoproterozoic basement rocks of the Nico Pérez terrane,
in central Uruguay. They are sub-vertical, trend broadly E-W, are up to 10 meters thick and can be
followed in the field for tens to hundreds of meters. The general texture is porphyritic with
phenocrysts of plagioclase, inmerse in an intersertal groundmass consisting of plagioclase,
clinopyroxene and intersticial glass. Opaque minerals represent a minor constituent. Samples are
fresh, with very low grade of alteration and no evidence of metamorphism. The age of the dikes is
poorly established, having been reported one K/Ar data that indicates an age of 581±13 Ma.
One hundred and five (105) oriented samples, distributed into seven (7) sites, were collected in the
surroundings of Zapicán (Lavalleja department, Uruguay, 33º31´S, 55º56´W). Anisotropy of magnetic
susceptibility (AMS) results suggests subhorizontal flow direction, while isothermal remanent
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magnetization (IRM) acquisiton curves indicate a ferrimagnetic phase as the carrier of magnetization.
Samples were submitted to AF and thermal desmagnetization, with typical desmagnetization steps of
0, 3, 6, 9, 12, 15, 20, 25, 30, 35, 40, 50, 60, 80, 100 and 120 mT, and 0, 100, 150, 200, 250, 300, 350,
400, 450, 500 and 550ºC. All samples presented high stability of magnetization with unblocking
temperatures close to 550ºC, which indicates the presence of magnetite or low-Ti titanomagnetite as
the principal mineral carrier of the magnetization. Consistency of the isolated magnetic components
was found in five sites, three of which showed normal and two reverse polarities. The mean of five
virtual geomagnetic poles (VGPs) is located at 9.9ºS, 262.7ºE (A95=14.1º). The position of this
preliminary paleomagnetic pole for the Nico Pérez terrane is consistent with the apparent polar
wander path (APWP) for the Río de la Plata (RP) craton for the Ediacaran-Cambrian interval, and its
location on the curve suggests an age for the intrusion at sometime between 580 and 530 Ma,
consistent with the scarce geochronologic data. This PP is also consistent with coeval poles from other
Gondwanan blocks of ca. 550Ma, suggesting that RP and several other plates were already assembled
in a Gondwana arrangement by that time.
Over one hundred and thirty oriented samples from thirteen (13) sites were recently collected in the
same region for a more detailed paleomagnetic study. These and an Ar-Ar dating of these basalts,
which is under way, will help in confirming or modifying these preliminary paleomagnetic results
ORDOVICIAN MAGMATISM IN THE NORTHEASTERN
NORTH PATAGONIAN MASSIF: FURTHER EVIDENCE FOR
THE CONTINUITY OF THE FAMATINIAN OROGEN
1-14
Martínez Dopico, C.I.1 3*, López de Luchi, M.G.1, Wemmer, K.2,
Rapalini, A.E.3, Linares, E.1
1 Instituto de Geocronología y Geología Isotópica (INGEIS), Pabellón INGEIS, Ciudad Universitaria,
C1428EHA, Buenos Aires, Argentina
2 Geoscience Centre of the University of Göttingen (GZG), Goldschmidtstr. 1-3, 37077 Göttingen,
Germany
3 Instituto de Geofísica Daniel A.Valencio (INGEODAV), Depto. Cs. Geológicas, FCEyN, Universidad
de Buenos Aires
(*) Presenting author’s e-mail: candusky@gmail.com
Different scenarios have been proposed for the tectonic and paleogeographic evolution of Patagonia
during Paleozoic times. Allochthonous hypotheses point out an accretionary event along the
southwestern margin of Gondwana during the Late Paleozoic (Ramos, 1984), whereas (para-)
autochthonous ideas propose that Patagonia belonged to Gondwana since the Early Paleozoic
(Rapalini et al., 2010 and references therein). Pankhurst et al. (2006), on the basis of strong
geochronological data, suggested that whereas since Ordovician times the North Patagonian Massif
(NPM) was already part of Gondwana, southern Patagonia (Deseado Massif, DM) constituted an
allochthonous terrane that might have collided with the NPM during Mid Carboniferous times. Recent
studies (e.g. Rapalini et al., 2010) provide geophysical, isotopic and petrologic evidence on the
autochthony of the NPM considering the continuity of the Pampean basement signature on the NPM,
assuming a short Siluro-Devonian rifting stage –Sierra Grande small ocean- followed by accretion
onto Gondwana in the Late Carboniferous-Early Permian.
Since the first geological studies in the northeastern NPM (Valcheta area, Caminos, 1983) the
existence of an Ordovician magmatism was assumed, but few sound isotopic evidence has come to
light only recently (López de Luchi et al., 2008; Tovher et al., 2008, Gozálvez, 2009 among others).
Further south, in the Sierra Grande area, Varela et al. (1997, 2009), among others, have provided
geochronological constraints on geographically overlapped Ordovician and Permian magmatism. In
this short paper we present new field mapping, petrographic correlation and a comprehensive
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Fig. 1 - Geologic sketch of the pre-Cenozoic geologic units near Valcheta town, northeastern North Patagonian Massif.
Available Ordovician ages for intrusive bodies are pointed out in their respective location.
summary of the available geochronological data which provide robust evidence of the extension of
Ordovician magmatism further west in the northeastern areas of the NPM (Fig. 1). This evidence is
analyzed in the framework of the tectonic evolution of the NPM and its relationship to Gondwana.
Local Geology
The studied magmatic products mainly correspond to undeformed muscovite bearing leucogranites
formerly known as Punta Sierra granitoids (Caminos, 2001) and referred to as the Valcheta pluton by
Gozálvez (2009). These rocks crop out as small and discontinuous elongated bodies between Valcheta
and Nahuel Niyeu towns (Fig. 1) in the Rio Negro province of Argentina. The alignment of these
outcrops might suggest a SW-NE belt which is no longer recognizable further south.
Field relationships indicate that the Valcheta pluton intrudes the Early Cambrian low grade
metamorphic rocks of the Nahuel Niyeu Formation. The host rock is a greenschist facies chloritemuscovite schist. The limited field observations indicate that in the central part granites are medium
grained whereas most of the outcropping facies towards the external part of the belt are finer grained.
Very fine grained dykes related to the main intrusions were observed in all cases. Granitoids vary
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Table 1- Available data for the Ordovician magmatism in the Sierra Grande area. Sources 1) López de Luchi et al.,
2008; 2) Tohver et al., 2008; 3) Gozálvez 2009; 4) INGEIS K-Ar repository [K2O 6.39 wt%; 40Ar rad 51.49E-10
mol/gr.; 40Ar 93.2%]. See map for location of the samples.
from medium to fine grained gray or pinkish equigranular and allotriomorphic leucogranites
composed of weakly zoned oligoclase-microcline-quartz-muscovite, scarce chloritized biotite, with
zircon, apatite and opaques as accessory minerals. Alteration to fine grained white mica and clay
minerals affected principally the plagioclase. In one example, K-feldspar is an orthoclase in transition
to microcline. As muscovite is a primary magmatic phase pressure might have been higher than 3.5
kbar at the moment of emplacement. Only in one case (sample SA110-07, table 1) plagioclase
appeared as subhedral lath. No pervasive planar fabric was observed but some evidence of high
temperature ductile deformation is indicated by myrmekite and scarce chessboard in quartz.
Deformation in plagioclase is shown by tappered and curved twins and kink-bands and bending of the
muscovite crystals. Analogue observations were reported by Gozálvez (2009), who also described
amphibolite lenses and xenolithes with gneissose textures within the granites of the central part of the
belt.
K/Ar-dating method on muscovite and biotite might be used to characterize the post-crystallization
cooling history of granitoid rocks. K/Ar biotite ages indicate the cooling history below 300° ± 50°C
whereas the closure temperature for ‘normal’ fine to coarse-grained rocks white mica is 350° ± 50°C
(Purdy and Jäger, 1976). Same considerations can be applied to the Ar-Ar system.
Cooling Ages Implications and Constraints
A first observation of the K/Ar and Ar-Ar on muscovite dataset (Table 1) indicates either a slow
cooling rate or perhaps different intrusions as suggested by the heterogeneity of cooling ages ranging
from the 470 to 449 Ma.
The closer constraint to the age of the metamorphic peak of the Nahuel Niyeu Fm. in the area of
Valcheta is provided by the younger detrital zircon age of 515Ma (Pankhurst et al., 2006).
Amphibolite-grade metamorphism in the region is dated at 472±5Ma (Pankhurst et al., 2006) in
quartz-feldspathic gneiss from Mina Gonzalito. These gneisses were considered as deeper crustal
equivalents of both the El Jaguelito and Nahuel Niyeu formations based on zircon provenance data
(Pankhurst et al., 2006).
Ordovician granitoids, intruding the very low grade metamorphic rocks of the El Jaguelito Fm, in the
easternmost sector of the NPM near Sierra Grande, yielded U-Pb crystallization ages between 472476 Ma (Pankhurst et al., 2006, González et al., 2008, Varela et al., 1998) and Rb-Sr ages between
483-428 Ma (Varela et al., 1997; Varela et al., 2009). The older Rb-Sr age corresponds to the Punta
Sierra biotite granite whereas the younger ages were calculated for amphibole bearing biotite
granitoids which exhibit mafic microgranular enclaves. The U-Pb age of 472 Ma corresponds to the
El Molino pluton which is a ductile deformed garnet bearing two-mica granite. With the exception of
the latter, for which no indication about the structural state of the K-feldspar is available, granitoids
are characterized by orthoclase instead of microcline. Although most of the above mentioned
granitoids exhibit clear–cut contacts with the low grade El Jaguelito Fm, the inferred age of the
metamorphic peak suggest some overlapping.
The older Ar-Ar ages in the Valcheta area (Table 1) almost overlap with the crystallization ages of the
granodioritic plutons of the Sierra Grande area. Granites are either slightly younger (472 Ma for the
two mica El Molino pluton) or older (483 Ma for the biotite granite of Punta Sierra) than the
granodiorites. Cooling ages as determined by the K-Ar and Ar-Ar ages in the Valcheta area are
broadly consistent with those U-Pb and Rb-Sr from the Sierra Grande area in the east. However,
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compositional differences between the plutons in these two areas deserve further studies. Considering
the youngest available biotite ages of ca 430 Ma in the Sierra Grande area (Weber 1983) and 413 Ma
west of Valcheta area (Table 1), a long-lasting Ordovician thermal event associated with shallow
emplacement due to crustal thinning could be inferred.
In summary, our new field mapping and the geochronological analysis reinforce a wider extension of
the Ordovician magmatic event in the North Patagonian Massif.
REFERENCES
• Caminos, R., 1983. Descripción geológica de las Hojas 39g, Cerro Tapiluke y 39h, Chipauquil, prov. de Río Negro. Servicio
Geológico Nacional (inédito). Buenos Aires.
• Caminos, R. 2001. Hoja Geológica N 4166-I Valcheta, provincia de Río Negro, Boletín 310, Servicio Geológico Minero Argentino,
78 pag.
• González, P.D., Sato, A.M., Varela, R., Llambías, E.J., Naipauer, M., Basei, M:A:S:, Campos, H., Greco, G.A., 2008. El Molino
pluton: a granite with regional metamorphism within El Jagüelito Formation, North Patagonian Massif. VI South American
Symposium on Isotope Geology. Electronic files
• Gozálvez, M.R., 2009. Petrografía y Edad 40Ar-39Ar de granitos peraluminosos al oeste de Valcheta. Macizo Nordpatagónico (Río
Negro). Revista de la Asociación Geológica Argentina 64(2): 285-294.
• López de Luchi, M.G., Wemmer, K., Rapalini, A.E., 2008. The cooling history of the North Patagonian Massif. First results for the
granitoids of the Valcheta area, Río Negro, Argentina. VI South American Symposium on Isotope Geology. Electronic Files, S.C.
de Bariloche
• Rapalini, A.E., López de Luchi, M.G., Martínez Dopico, C., Lince Klinger, F., Giménez, M., Martínez, P., 2010. Did Patagonia
collide against Gondwana in the Late Paleozoic? Some insights from a multidisciplinary study of magmatic units of the North
Patagonian Massif. Geologica Acta (in press).
• Varela, R., Cingolani, C., Sato, A., Dalla Salda, L., Brito Neves, B.B., Basei, M.A.S., Siga Jr., O., Teixeira, W., 1997. Proterozoic
and Paleozoic evolution of the Atlantic area of North-Patagonian Massif, Argentine. South American Symposium on Isotope
Geology Abstracts: 326-329. San Pablo
• Varela, R., Basei, M.A.S., Sato, A.M., Siga Jr., O., Cingolani, C.A., Sato, K., 1998. Edades isotópicas Rb/Sr y U/Pb en rocas de
Mina Gonzalito y Arroyo Salado, Macizo Norpatagónico Atlántico, Río Negro, Argentina. 10° Congreso Latinoamericano de
Geología, Actas I: 71-76, Buenos Aires.
• Varela, R., Sato, K., González, P.D., Sato, A.M., Basei, M.A.S., 2009. Geología y Geocronología Rb-Sr de los granitoides de Sierra
Grande, Provincia de Río Negro. Revista de la Asociación Geológica Argentina 64(2): 275-284.
• Pankhurst, R.J., Rapela, C.W. Fanning, C.M., Márquez, M. 2006. Gondwanide continental collision and the origin of Patagonia
Earth-Sciences Reviews, 76, 235-257.
• Purdy, J.W., Jäger, E., 1976. K-Ar ages on rock forming minerals from the Central Alps. Memorie degli Istituti di Geologia e
Mineralogia dell’ Universita di Padova v. 30,. 1-31.
• Ramos, V.A., 1984. Patagonia: ¿Un continente paleozoico a la deriva? 9th Congreso Geológico Argentino, San Carlos de Bariloche,
Actas 2: 311-325.
• Tohver, E, Cawood, P.A., Rossello, E., López de Luchi, M.G., Rapalini, A., Jourdan; F., 2008. New SHRIMP U-Pb and 40Ar/39Ar
constraints on the crustal stabilization of southern South America, from the margin of the Rio de Plata (Sierra de Ventana) craton
to northern Patagonia. AGU, Fall Meeting, EOS.
• Weber, E.I., 1983. Descripción Geológica de la Hoja 40J, cerro El Fuerte, prov. de Río Negro. Dirección Nacional de Geología y
Minería. Boletín 196:1-69. Buenos Aires.
IAPETAN EVOLUTION OF APPALACHIAN PERI-LAURENTIAN
AND PERI-GONDWANAN ARC COMPLEXES: A NEWFOUNDLAND COMPARISON
1-15
Brian H. O’Brien(*)
Geological Survey of Newfoundland and Labrador, P.O. Box 8700, St. John’s, NL, Canada A1B 466,
* Presenting author’s e-mail: brianobrien@gov.nl.ca
Numerous studies of the well-exposed regional cross section of the Appalachian orogen on the Island
of Newfoundland have provided a basis for the proposal that suprasubduction zone ophiolites
developed in marginal basins on opposing sides of the Iapetus Ocean during gaps in island arc
volcanism. Older extinct oceanic arcs were commonly uplifted and disconformities mainly developed
beneath cover sequences related to younger ensialic arcs. However, certain well-timed tectonic events
differentiate many of the early Paleozoic arc complexes of the peri-Laurentian and peri-Gondwanan
oceanic realms.
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First, the oldest Cambrian arc sequence found in the peri-Gondwanan part of Iapetus is ensialic and
contemporaneous with the first generation of Cambrian ophiolites in the peri-Laurentian part of
Iapetus. Similarly, the main late Cambrian-early Ordovician period of ophiolite generation in periLaurentia is coeval with a renewed phase of peri-Gondwanan island arc volcanism during which the
underlying Ediacaran basement was rejuvenated. Second, the peri-Gondwanan ophiolites were
involved in initial arc collision outboard of the continental margin at an earlier stage than they were
in the peri-Laurentian Iapetan realm. Furthermore, ophiolite obduction and final emplacement also
occurred earlier in the Ordovician on the peri-Gondwanan continental margin than it had done in periLaurentia. Third, though arc complexes generally grew oceanward away from the opposing
continental margins, the youngest Middle Ordovician volcanism dated in the peri-Laurentian ensialic
arc is about 8 My older than the youngest dated Late Ordovician rhyodacite in the peri-Gondwanan
ensialic arc. Thus, at the arc-arc suture, tectonically encroaching peri-Gondwanan rocks were thrust
beneath dormant peri-Laurentian rocks and active mid Caradocian arc magmatism became focused
along old fault lines in the upper and lower plates.
Peri-Gondwana
The peri-Gondwanan ribboned microcontinent that has been argued to comprise the basement to
Appalachian Ganderia has an early Paleozoic evolution governed by its paleogeographic position near
the southern margin of the Iapetus Ocean but also an earlier geological history related to the Avalonian
development of juvenile crust on the Neoproterozoic margin of the Gondwanan supercontinent. In
south-central Newfoundland, the earliest pre-Appalachian phase of orogenesis began with basement
orthogneiss formation in the Cryogenean (ca. 686 Ma), continued with deposition of a
volcanosedimentary cover sequence in the Ediacaran (ca. 585-565 Ma) and culminated with
emplacement of an arc-related suite of Avalonian stitching plutons (ca. 575-560 Ma). Ediacaran
remobilization of Cryogenean crystalline basement was coeval with intrusion of the youngest of these
plutonic rocks; however, it is the earliest Cambrian tectonism (ca. 540-535 Ma) of the linked
basement-cover complexes that is characteristic of the pre-Iapetan rocks included in Newfoundland’s
Ganderia.
Passive continental margin deposition of the quartz-rich turbidite prism that is one of the hallmarks of
Ganderia is postulated to have begun after ca. 535 Ma in the Cambrian and may have persisted in
places until the Early Ordovician (Arenig). In contrast, the oldest subduction-related volcanic strata
preserved within the Gondwanan realm of Iapetus are mineralized Middle and Late Cambrian ensialic
arc sequences (ca. 515-496 Ma). The younger parts of this active arc and the easterly adjacent passive
margin were locally drowned and buried by black shale (Tremadoc to Arenig). However, the oldest
Cambrian arc rocks, which have zircons probably inherited from subjacent Ediacaran intrusions, are
interpreted to have accumulated above an uplift of Cryogenean basement on the open ocean-facing
margin of the peri-Gondwanan microcontinent.
The oldest suprasubduction zone ophiolites on the peri-Gondwanan Iapetan margin (ca. 500-495 Ma)
formed near the ribboned microcontinent during a ca. 5 My hiatus in volcanic arc construction.
Related bimodal arc plutons had crosscut the supracrustal rocks of the Ediacaran cover sequence in
the pre-Appalachian basement complex during this same interval. Fragments of the peri-Gondwanan
ophiolites were mylonitized along with parts of a younger group of latest Cambrian-early Ordovician
ensialic volcanic arc sequences (ca. 491-485 Ma) in a narrow arc collision zone of probable latest
Tremadoc age. Ophiolitic mélange tracts commonly developed where ultramafic rocks were
subsequently obducted onto the passive continental margin in the early Arenig.
The northern portion of the peri-Gondwanan Iapetan realm was composed of basalt-dominated
primitive arcs and oceanic seamounts during the latest Cambrian to early Ordovician period. By
comparison, time equivalent ensialic arcs that had lain farther south near the pre-Appalachian
basement inliers have a significant component of subvolcanic quartz-feldspar porphyry and
mineralized felsic pyroclastic rocks associated with island arc tholeiite. Many of the ensimatic
northern arc sequences became dormant and were rifted in the Arenig (ca. 475 Ma). They were either
uplifted in horst blocks or buried beneath widening grabens infilled by arc-related pyroclastic
turbidites, tholeiite-derived epiclastic turbidites and distal ash tuffs sourced from continental margin
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melts. By comparison, in the region to the south, a more protracted phase of compressional and
extensional Middle Ordovician tectonism resulted in the infrastructure of the peri-Gondwanan Iapetan
realm becoming upwelled. There, the quartz-rich turbidite prism and the crystalline basement of the
southern arc volcanic sequences were syntectonically intruded by mesozonal plutonic rocks in the
Arenig (ca. 474-468 Ma) and they continued to be regionally metamorphosed within Abukuma-type
migmatite domes until the Llanvirn (ca. 465-460 Ma).
The youngest phase of volcanism recorded in the peri-Gondwanan realm of Iapetus occurred during
the late Middle Ordovician and the earliest Late Ordovician. Deposits formed as recently as the early
Llanvirn possess a Celtic paleobiogeographic fauna, indicating deposition in the southern low
latitudes or near islands in the middle part of the Iapetus Ocean. In the northern part of Iapetan
Ganderia, dominantly alkaline mafic volcanic rocks, bioclastic limestone and ribbon chert
accumulated within an oceanic back arc that was invaded by an arc-related suite of gabbro laccoliths
beginning in the late Llanvirn. In places, the back-arc basin infill passed laterally northward and
stratigraphically upward into a relatively thin, rhyodacite-bearing Caradocian ensialic arc sequence
that evolved immediately south of the relict Iapetus Ocean.
In contrast, in the area lying closer to the Arenig-obducted peri-Gondwanan ophiolites, the back-arc
sequence also passed laterally into a black shale-hosted mélange marked by psammitic schist
olistoliths preserving pre-incorporation amphibolite facies shear zones. Here, a temporally equivalent,
unconformity-bounded volcanosedimentary sequence was deposited in a continental back-arc basin
situated above the older ocean-continent suture present in southern Ganderia.
Peri-Laurentia
The Middle Cambrian to Middle Ordovician stratified and intrusive rocks that comprise the periLaurentian Iapetan realm extend from the external Taconic foreland-propagating thrust belt in western
Newfoundland to a subduction-controlled accretionary complex-type of foldbelt in central
Newfoundland. The southern boundary of the peri-Laurentian Iapetan rocks is the 350 km long Red
Indian Line, an arcuate feature broadly equivalent in age to structures developed near the Appalachian
front, but having the opposite tectonic polarity. Peri-Laurentian volcanic rocks occupy an upper plate
position with respect to the structurally underthrust peri-Gondwanan volcanic rocks along this intraIapetus Ocean suture.
Fossil-bearing peri-Laurentian strata near the Red Indian Line represent the Toquima- Table Head
faunal realm which links them biogeographically to the platformal and epeirogenic strata that formed
along the periphery of the North American craton, at least from early Arenig to early Llanvirn time.
Such strata may have never interacted with Newfoundland’s Grenville Province basement inliers and
the overlying late Neoproterozoic to early Paleozoic rift-related sedimentary prism that developed on
the passive margin of ancestral Laurentia. However, some unfossiliferous older Ordovician and
Cambrian magmatic arc rocks and intraoceanic ophiolites belonging to central Newfoundland’s periLaurentian realm are also found in western Newfoundland. There, they were tectonically accreted, at
the structural top of the foreland thrust belt, to the Early Ordovician platformal carbonate bank, the
Middle Ordovician hinterland-derived flysch and the far-travelled allochtons that carry the coeval
Laurentian continental slope deposits. This occurred during a Late Ordovician phase of deformation
and mélange formation, some 15-20 My after the metamorphic soles of the arc ophiolites had
crystallized.
At least three discrete generations of suprasubduction zone ophiolites have been recognized in the
peri-Laurentian Iapetan realm: those formed in the Middle to Late Cambrian (ca. 508-500 Ma), in the
late Cambrian to Early Ordovician (ca. 490-485 Ma) and in the mid Early Ordovician (ca. 481-479
Ma). In places, the mantle section of the oldest ophiolite suites and the overlying primitive arc crust
were sheared near postulated spreading ridges and were then incorporated as enclaves in the
intermediate-age ophiolite suites. Both generations were intruded between ca. 488-484 Ma by
pretectonic suites of granodiorite-granite-tonalite plutons and by mafic dyke swarms of boninitic and
tholeiitic composition. Importantly, however, the mid Cambrian and earliest Ordovician
suprasubduction zone ophiolites were syntectonically intruded by continental arc plutons around 466465 Ma and by posttectonic plutons between ca. 457-455 Ma at relatively deep crustal levels. In other
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locations, the crustal sections of the ophiolites and the coeval arc volcanic rocks were never
tectonically buried or metamorphosed above the lower greenschist facies.
To explain the presence of continental crust lying beneath the Middle to Late Cambrian intraoceanic
arc ophiolites by the time that a latest Cambrian (ca. 494 Ma) suite of lamprophyric dykes were
emplaced, it has been recently proposed that the eastern part of Newfoundland’s peri-Laurentian
Iapetan realm was underlain by an unexposed Precambrian microcontinental block that may have also
acted as a rigid buttress during accretion of the island arc and ophiolite sequences. It would also
account for the continental signature of the post-ophiolite Ordovician arc metaplutonic rocks and the
ubiquitous enclaves and screens of continental margin metasedimentary gneiss.
Within the peri-Laurentian part of the Iapetus Ocean, fossil-bearing Early and Middle Ordovician
volcanosedimentary strata evolved in several island arc and back arc complexes situated above or
tectonically adjacent to the dormant Middle Cambrian primitive oceanic arc. The longest ranging
group (ca. 480-467 Ma) comprises an oceanic arc to oceanic back arc basin succession that was
stratigraphically continuous and deposited directly above the ca. 490-485 Ma suite of ophiolites. In
contrast, those that are coeval with and younger than the ca. 481-479 Ma ophiolite suite constitute the
volcanic fill of ensialic arc-back arc complexes (ca. 480-462 Ma). These peri-Laurentian basins are
generally distinguished from similar peri-Gondwanan depocentres by the lack of a voluminous
sedimentary infill.
One such ensialic arc complex is interpreted to have formed as a cover sequence (ca. 480-464 Ma)
above a basement represented by a dormant Middle Cambrian arc, although an approximate 5 My
hiatus is also recorded in the cover between the late Early Ordovician and the early Middle
Ordovician. In the regional hanging wall sequence of the Red Indian Line, to the east of the
hypothetical microcontinent, the oldest and the youngest peri-Laurentian ophiolites succeed each
other without any evidence of an intervening ca. 490-485 Ma ophiolite suite. There, another grouping
of felsic volcanic-dominant ensialic arc-back arc complexes ranging in age from at least ca. 473-462
Ma may have evolved by reactivating the ca. 479 Ma tonalite-lined magma conduits in the mid Early
Ordovician ophiolites. Their relationship with the partly coeval unconformity-bounded ensialic arc
sequences is uncertain; however, it has been postulated that they are geodynamically unrelated and
were later accreted to the peri-Laurentian margin in the region that had once lain immediately north
of the relict Iapetus Ocean.
U- PB ZIRCON GEOCHRONOLOGY OF THE SIERRA VALLE FÉRTIL,
FAMATINIAN ARC, ARGENTINA: PETROLOGICAL AND
GEOLOGICAL IMPLICATIONS
1-16
Otamendi, J.E.1*, Ducea, M.N.2, Bergantz, G.3
(1) Departamento de Geología, Universidad Nacional de Río Cuarto, X5804 Río Cuarto, Argentina
(2) Department of Geosciences, University of Arizona, Tucson, 85721 AZ, USA
(3) Department of Earth and Space Sciences, University of Washington, Seattle, WA98195, USA
* Presenting author’s e-mail: jotamendi@exa.unrc.edu.ar
We analyse the significance of sixteen U-Pb zircon crystallization ages on igneous plutonic and
metasedimenatry rocks from Sierra Valle Fértil that we have reported elsewhere (Ducea et al., 2010).
U-Pb geochronology of zircons was conducted by laser ablation multicollector inductively coupled
plasma mass spectrometry (LA-MC-ICP-MS) at the Arizona LaserChron Center. These sixteen
plutonic and metasedimentary rocks were collected in an east-west transect across the plutonic section
from the central Sierra Valle Fértil. Sampling encompasses a plutonic suite from diorites to
leucogranites, includes a granulite-facies metasedimentary rock, and covers all the lithological
diversity observed in the crystalline section from Valle Fértil. This igneous plutonic suite formed at
low- to middle-crustal paleo-depths within the Early Ordovician subduction-related magmatic belt
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from central- and northwestern Argentina, which is known as Famatinian arc.
The most relevant outcomes to communicate from our study are:
Eleven out of fifteen samples of igneous plutonic rocks yielded ages between 469 and 477 Ma.
Furthermore, several crystallization ages cluster around 472 ± 2 Ma, reflecting that a large volume of
middle plutonic crust was built within a few million (ca. 5) years.
Based on geological mapping and petrologic constraints we estimated that 15 km of the paleoarc crust
is mainly constituted by intermediate to silicic plutonism (Otamendi et al., 2009). Thus, this 15 km
thick middle- to upper arc crust was created in 20 My (485 to 465 Ma) at an average magmatic growth
rate of 0.75 km3/My per km2 of active arc surface. The magmatic production rate expressed as
km3/My.km would depend on the width of the Famatinian paleoarc active between 485 and 465 My.
For example, a typical hundred-km-wide arc section would have been formed at a magmatic rate of
75 km3/My.km (km3 over strike length times My). Timing of intermediate to silicic plutonism from
Valle Fértil suggests generation at high flux rates that are similar to major high flux events reported
in mature continental arcs (Ducea and Barton, 2007).
3- A statistically robust population of U-Pb spot data measured in plutonic rocks proves that Early
Ordovician magmatism arc in the Valle Fértil section terminated at around 465 Ma. This is consistent
with the idea that the collisional accretion of Cuyania terrane on western Gondwana margin began ca.
465 - 460 Ma (Thomas and Astini, 1996).
4- The inherited core zircon ages for all of the rocks from the Sierra Valle Fértil define mainly late
Mesoproterozoic - early Neoproterozoic (ca. 1150-850 My) and Neoproterozoic - early Cambrian (ca.
720-530 My) populations, reflecting zircon inheritance from Grenvillian-type, Brasiliano-Pan African
and Pampeano orogenic cycles.
5- The existence of inherited zircon cores in the tonalitic and granodioritic rocks require of
widespread partial to nearly complete melting of pelitic and semi-pelitic host rocks and subsequent
assimilation into the evolving magmas. From our previous work the process took place at between 22
and 10 km depths (P < 6.5 kbar).
6- Consistently with the last point, U-Pb zircon ages from the metasedimentary migmatite define
clusters of inherited core ages with peaks at 531, 585 and 913 My; whereas, just two of thirty one spot
analyses yielded Early Ordovician ages.
This study ultimate demonstrates that the full Sierra de Valle Fértil, as currently exposed by Andean
faults, was utterly built up by Early Ordovician arc-related magmatism.
REFERENCES
• Ducea, M. N., and Barton, M. D., 2007. Igniting flare-up events in Cordilleran arcs, Geology 35, 1047-1050.
• Ducea, M.N., Otamendi, J.E., Bergantz, G., Stair, K., Valencia, V., and Gehrels, G., 2010. Timing constraints on building an
intermediate plutonic arc crustal section: U-Pb zircon geochronology of the Sierra Valle Fértil, Famatinian Arc, Argentina.
Tectonics, in press.
• Otamendi, J.E., Vujovich, G.I., de la Rosa, J.D., Tibaldi, A.M., Castro, A., Martino, R.D., and Pinotti, L.P., 2009. Geology and
petrology of a deep crustal zone from the Famatinian paleo-arc, Sierras Valle Fértil-La Huerta, San Juan, Argentina, Journal of South
America Earth Sciences 27, 258-279.
• Thomas, W.A., and Astini, R.A., 1996. The Argentine Precordillera: A traveler from the Ouachita embayment of North American
Laurentia. Science 273, 752-757.
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CERRO LA TUNA MAFIC TO ULTRAMAFIC COMPLEX:
AN OCEAN FLOOR REMNANT
1-17
Peel, E.1, Basei, M.A.S.2, Sánchez Bettucci, L.1*
(1) Instituto de Ciencias Geológicas, Facultad de Ciencias-UdelaR, Montevideo, Uruguay
(2) Instituto de Geociencias, Universidade de São Paulo
* Presenting author’s e-mail: leda@fcien.edu.uy
The Cerro La Tuna Complex is located in the northeastern portion of Uruguay. Based on petrographic
and chemical data (mineral assemblages and mineral compositions) this Complex is defined as a
lithotectonic association composed of mafic and ultramafic rocks. The geology of this region is
separated into three major domains: northern, central and southern. The northern domain is
represented by granitoids with variable deformation, formed by coarse-grained (titanite) biotite
granites with pink feldspar megacrystals, possibly related to the Aigua batholith. In tectonic contact
occurs the central mafic-ultramafic domain. This domain (Cerro La Tuna Complex s.s) is constituted
by a major anticline structure with peridotites (harzburgite) forming the core, surrounded by a
volcano-sedimentary sequence represented by BIFs, metabasalts, quartzites, calc-silicate rocks and
cherts layers. Some levels contain chromite pods, dunite dismembered bodies and mafic–ultramafic
dikes. The entire unit is cut by ultrabasic dikes (tremolite schist) and it is disrupted by W-NNW shear
zones and strike-slip faults. To the south, this mafic-ultramafic sequence is intercalated with banded
migmatites belonging to the southern “basement” domain. The banded migmatites show mesosome
layers composed of biotite. The leucosome is composed of sub-milimetric banded granite-gneiss with
folded quartz-feldspar showing biotitic melanosome borders suggesting in situ melting.
E. Peel is thankful for the financial support given by CAPES (Coordenação de Aperfeiçoamento de
Pessoal de Nível Superior).
GRENVILLE-AGE SOURCES IN CUESTA DE RAHUE, NORTHERN PATAGONIA:
CONSTRAINS FROM U/PB SHRIMP AGES FROM DETRITAL ZIRCONS
1-18
Ramos, V.A.1*, García Morabito, E.1, Hervé, F.2, Fanning, C.M.3
(1) Laboratorio de Tectónica Andina, FCEyN, Universidad de Buenos Aires – CONICET, Argentina
(2) Departamento de Geología, Universidad de Chile
(3) Research School of Earth Sciences, Australian National University, Canberra, Australia
* Presenting author’s e-mail: andes@gl.fcen.uba.ar
The northern part of Patagonia has scarce basement exposures. They are located in the south-eastern
sector of the province of Neuquén along the foothills of the Patagonian Cordillera at 39º16’S latitude
and 70º50’W longitude (see Fig. 1). These exposures were studied by Turner (1965) who mapped them
as part of the Colohuincul Formation. This unit at that time was assigned to the Precambrian to early
Paleozoic, based on regional correlations. Digregorio (1972) considered this basement as
Precambrian, although in a later contribution followed the proposal of Turner (Digregorio and Uliana,
1979).
The outcrops of Cuesta de Rahue, as well as other exposures of the Cordón de la Piedra Santa located
a few kilometers to the east, were included by Franzese (1995) in the Piedra Santa Complex. This
author presents the first K/Ar ages of these metamorphic rocks which yielded values between 372 and
311 Ma with large errors, but that identified a late Paleozoic age for the metamorphism.
The first hard evidence that the rocks of the Colohuincul Formation could be of younger age was
presented by Basei et al. (1999), who found 345 ± 4.3 Ma old zircons in an amphibolite further south
in the Cañadón de la Mosca, near Bariloche. This finding implies that these rocks were
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Fig. 1 - Geological map of the Cuesta de Rahue exposures of the Colohuincul Formation, in the southern Neuquén
province, northern Patagonia (after García Morabito, 2010).
metamorphosed in late Paleozoic times. An age on U-Pb in titanite ca. 360 Ma together with the
previous K-Ar ages, was interpreted as the cooling age of the metamorphic peak.
The objective of this study was to know more about the age of these rocks in the Cuesta de Rahue
section in order to constrain the age of the basement of the Neuquén Basin in its south-western
margin.
Cuesta de Rahue
This magnificent exposure is located in the eastern margin of the Aluminé valley, a few kilometers
down waters of the town of Aluminé. It is approximately 7 km east of the junction of roads 46 and 23,
along a pronounced cuesta where phyllites and fine grained quartz-schists of greenish to gray color
are exposed. There are also mica-schists with fenoblasts of biotite up to 5 mm size. Heavily deformed
schists are exposed in a west facing wall, where the samples were taken in the upper part of the cuesta
(Fig. 1).
Although there is no any visible contact with the late Paleozoic granitoids, nearby there is good
evidence of emplacement of these granites in the metamorphic rocks of Colohuincul Formation
(Turner, 1965; Franzese, 1995). These rocks further west were studied by Dalla Salda et al. (1992)
who obtained K-Ar ages of 354 ± 4 Ma and 324 ± 6 Ma for tonalitic gneisses, and 376 ± 9 Ma for a
biotitic granodiorite in the Lago Lacar area, near San Martín de Los Andes. Recent U/Pb zircon dating
on these granitoids constrained the age of the plutonic rocks between 420-380 Ma (Basei et al., 2005).
More precise geochronological studies were done by Pankhurst et al. (2006) obtaining ages ranging
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a
b
Fig. 2 - a) Wetherill plot and b) relative probability ages of zircons from Colohuincul Formation sampled in the upper
section of Cuesta de Rahue dated by U/Pb SHRIMP.
from 395 ± 4 Ma (U/Pb SHRIMP).
These dates indicate a Late Devonian to Early Carboniferous age for the plutonic rocks.
U/Pb SHRIMP geochronology
The zircons from the Cuesta de Rahue sample appear to be detrital in since they have round to subround shapes consistent with abrasion during surface transport. Some grains are subhedral and under
CL imaging, the zircons have a dominantly zoned igneous structure. Thirty grains have been analysed.
The U-Th-Pb analyses were made using SHRIMP II at the Research School of Earth Sciences. The
Australian National University, Canberra, Australia, following procedures given in Williams (1998),
and references therein. Each analysis consisted of 6 scans through the mass range, with the Temora
reference zircon grains analyzed for every three unknown analyses. The data have been reduced using
the SQUID Excel Macro of Ludwig (2001). The Pb/U ratios have been normalized relative to a value
of 0.0668 for the Temora reference zircon, equivalent to an age of 417 Ma (see Black et al., 2003).
Uncertainty in the U-Pb calibration was 0.72% for the SHRIMP II session. The U and Th
concentrations are relative to the SL13 zircon which has 238 ppm U.
A wide range of ages is recorded (Fig. 2a), with a main peak between ~950 Ma and ~1200 Ma, with
a secondary peak at about ~1490 Ma (Fig. 2b). Two grains yield Ordovician ages. However, one of
these is enriched in common Pb and so not considered to be a significant analysis. The youngest
zircon gave an age of 364 Ma, in accordance with the maximum latest Devonian age for the
Colohuincul Formation.
Discussion
The detrital zircons show a few Paleozoic ages within the rank expected for this unit, in accordance
with previous constrains obtained for the metamorphic and plutonic rocks. The dominant peak
indicates a Grenville source between 950 and 1200 Ma, an age range typical from central Argentina
(see discussion in Rapela et al., 2007). The potential sources for these ages are traditionally assumed
to be derived from either the Sunsas belt in southern Brazil or from the Namaqualand orogen in South
Africa (Bahlburg et al., 2008). However, there are also some local sources as the Cuyania basement
which has a similar range of ages as shown by Naipauer et al. (2010) and Ramos (2010). Besides,
there is an increasing evidence of sources derived from some of the Patagonian massifs or the adjacent
Malvinas plateau. The importance of this dominant peak, plus the late Paleozoic paleogeography favor
a more local source within Patagonia as the most probable provenance of the analyzed detrital zircons.
Thus, more isotopic data are needed in order to differentiate these potential Grenville-age sources, as
the main known source in central-western Argentina has a mantle signature opposed to the more
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recycled sources of southern Patagonia (see Augustsson and Bahlburg, 2008).
REFERENCES
• Augustsson, C.; Bahlburg, H., 2008. Provenance of late Palaeozoic metasediments of the Patagonian proto-Pacific margin
(southernmost Chile and Argentina). International Journal of Earth Sciences. Geologische Rundschau 97:71–88.
• Bahlburg, H.; Vervoort, J.D.; Du Frane, S.A.; Bock, B.; Augustsson, C.; Reimann, C., 2009. Timing of crust formation and recycling
in accretionary orogens: Insights learned from the western margin of South America. Earth-Science Reviews 97(1-4): 215-241.
• Basei, M.A.S.; Brito Neves, B.B.; Varela, R.; Teixeira,W.; Siga Jr.,O., Sato, A.M.; Cingolani, C., 1999. Isotopic dating on the
crystalline basement rocks of the Bariloche region, Río Negro, Argentina. IIº South American Symposium on Isotope Geology,
Anales SEGEMAR 34: 15-18, Villa Carlos Paz.
• Basei, M.A.; Varela, R.; Passarelli, C.; Siga Jr., O.; Cingolani, C.; Sato, A.; Gonzalez, P.D., 2005. The crystalline basement in the
north of Patagonia: isotopic ages and regional characteristics. In: Pankhurst, R., Veiga, G. (Eds.) Gondwana 12: Geological and
Biological Heritage of Gondwana, Abstracts, Academia Nacional de Ciencias, p. 62, Córdoba.
• Black, L.P.; Kamo, S.L.; Allen, C.M.; Aleinikoff, J.N.; Davis, D.W.; Korsch, R.J.; Foudoulis, C., 2003. TEMORA 1: a new zircon
standard for Phanerozoic U–Pb Geochronology. Chemical Geology 200: 155-170.
• Dalla Salda, L.; Cingolani, C.A.; Varela, R., 1992. El basamento cristalino de la región nordpatagónica de los lagos Gutiérrez,
Mascardi y Guillelmo, provincia de Río Negro. Revista de la Asociación Geológica Argentina 46 (3-4): 263-276.
• Digregorio, J., 1972. Neuquén. In: A.F. Leanza (ed.). Geología Regional Argentina. Academia Nacional de Ciencias, pp. 439-505, Córdoba.
• Digregorio, J.H.; Uliana, M.A., 1980. Cuenca Neuquina. In: J.C.M. Turner (ed.) Segundo Simposio de Geología Regional Argentina,
Academia Nacional de Ciencias 2: 985-1032, Córdoba.
• Franzese, J.R., 1995. El Complejo Piedra Santa (Neuquén, Argentina): parte de un cinturón metamórfico neoplaeozoico del
Gondwana suroccidental. Revista Geológica de Chile 22(2): 193-202.
• Ezequiel García Morabito, 2010. Evolución tectónica de la Cordillera de Catán Lil, Neuquén. PhD Thesis, Universidad de Buenos
Aires (unpublished), Buenos Aires.
• Naipauer, M.; Vujovich. G.I.; Cingolani, C.A.; McClelland, W.C., 2010. Detrital zircon analysis from the Neoproterozoic-Cambrian
sedimentary cover (Cuyania terrane), Sierra de Pie de Palo, Argentina: Evidences of a rift and passive margin system? Journal South
American Earth Sciences 29 (2): 306-326.
• Pankhurst, R.J.; Rapela, C.W.; Fanning, C.M.; Márquez, M., 2006. Gondwanide continental collision and the origin of Patagonia.
Earth Science Reviews 76: 235-257.
• Ramos, V.A., 2010. The Grenville-age basement of the Andes. Journal of South American Earth Sciences 29(1): 77-91.
• Rapela, C.W.; Pankhurst, R.J.; Casquet, C.; Fanning, C.M.; Baldo, E.G.; González-Casado, J.M.; Galindo, C.; Dahlquist, J., 2007.
The Río de la Plata craton and the assembly of SW Gondwana. Earth-Science Reviews 83: 49–82.
• Turner, J.C.M., 1965. Estratigrafia de Aluminé y adyacencias (provincia del Neuquén). Asociación Geológica Argentina, Revista 20(2): 153184.
• Williams, I.S., 1998. U-Th-Pb geochronology by ion microprobe. In: McKibben, M.A. et al., (Eds.), Applications of microanalytical
techniques to understanding mineralizing processes. Review Economic Geology 7: 1–35.
WAS THE RIO DE LA PLATA CRATON NEVER PART OF RODINIA?
SOME PALEOMAGNETIC HINTS
1-19
Rapalini, A.E.*
INGEODAV, Depto. Cs. Geológicas, FCEyN, Univ. Buenos Aires, Pabellón 2, Ciudad Universitaria,
C1428EHA, Buenos Aires., Argentina
* Presenting author’s e-mail: rapalini@gl.fcen.uba.ar
The formation and dispersal of the Rodinia supercontinent were long and complex processes in the
global paleogeographic evolution during the Meso- and Neo- Proterozoic. In most paleogeographic
reconstructions of Rodinia the Rio de la Plata craton (RP) is situated attached to eastern Laurentia, in
a similar relative position to Amazonia as in present-day South America. In such classical
reconstructions present-day western RP and eastern North America should have been conjugate
margins during break-up and dispersal of the final remnants of Rodinia. Evidence of such geological
process is widespread along eastern Laurentia and has been accurately dated as occurring between
600 and 550 Ma. Lack of exposed Mesoproterozoic rocks in RP and the possible existence of a large
Neoproterozoic ocean to the west (present-day coordinates) of RP, called the Brasiliano-Pampean
Ocean, have been interpreted by some authors as evidence of RP never forming part of Rodinia. Direct
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paleomagnetic test of the paleogeographic position of RP during Rodinian times (ca. 1.10-0.75 Ga) is
not expected due to the lack of exposed rocks of such ages. However, a more restricted test can be
attempted with the available paleomagnetic information from late Neoproterozoic rocks (ca. 600-540
Ma) from both Laurentia and RP. A long standing controversy on the proper paleogeographic position
of Laurentia between ca. 590 and 560 Ma has not been solved yet. While at least three key
paleomagnetic poles of such age suggest high to polar latitudes for this continent in that interval, some
authors have preferred to ignore such poles and interpolate a very similar paleogeographic position
than that indicated at ca. 600 and at 550 Ma, in order to avoid non-actualistic drift rates for the
continent. This means Laurentia in ecuatorial to low southern hemisphere latitudes throughout the
whole period. On the other hand, systematic paleomagnetic studies in RP in the last decade have
provided a list of 12 paleomagnetic poles or virtual geomagnetic poles from ca. 600 Ma up to ca. 500
Ma. These poles are distributed systematically along a path that starts to merge with other major
Gondwana blocks (e.g. Congo-Sao Francisco) at around 570 Ma. This path permits to determine that
RP remained at intermediate to low latitudes during the whole Ediacaran. Comparison of coeval poles
of 580-570 Ma from RP and Laurentia are highly discordant in the classical configuration of RP
rifting apart from Eastern Laurentia. In the high-latitude option for Laurentia, over 40° of latitude
separates both continental margins. In the low latitude option, however, latitudes are similar, but to
match the polarity of the rifting margins a nearly 180° rotation of RP, incompatible with
paleomagnetic and geologic data, is needed. This suggests that RP was not the crustal block that rifted
apart from eastern Laurentia in the Ediacaran and indirectly supports models that portray it as a “nonRodinian” craton.
THE AFRICAN PROVENANCE OF SOUTHERN SOUTH AMERICA TERRANES:
A RECORD FROM RODINIA BREAK-UP TO GONDWANA ASSEMBLY
1-20
Rapela, C.W.1*, Fanning, C.M.2 Casquet, C.3, Pankhurst, R.J.4, Spalletti, L.A.1,
Poiré, D.1, Baldo, E.G.5
(1) Centro de Investigaciones Geológicas (CONICET-UNLP), 1900 La Plata, Argentina
(2) Research School of Earth Sciences, The Australian National University, Canberra, Australia
(3) Departamento de Petrología y Geoquímica, Universidad Complutense, 28040 Madrid, Spain
(4) British Geological Survey, Keyworth, Nottingham NG12 5GG, United Kingdom
(5) CICTERRA (CONICET-UNC), 5000 Córdoba, Argentina
* Presenting author’s e-mail: crapela@cig.museo.unlp.edu.ar
A remarkable characteristic of southern South America, is that the 2.26-2.02 Ga Palaeoproterozoic
sequences of the Río de la Plata craton that define the oldest southern core of the continent, have not
been affected by the widespread Neoproterozoic deformation and magmatism associated with the
assemblage of Gondwana. In Uruguay, the Sarandí del Yi megashear separates the Paleoproterozoic
basement unaffected by Neoproterozoic events (Piedra Alta and Pando terranes), from the complex
Archean to Mesoproterozoic Nico Pérez terrane, which was reworked during the Mesoproterozoic
(Bossi and Cingolani 2009, Oyhantçabal et al., 2009 and references therein), as well as the collage
of terranes accreted during the Brasiliano-Panafrican orogeny (e.g., Punta del Este terrane and Dom
Feliciano belt). Further south in the Tandilia belt in Argentina, SHRIMP analyses of the 2.23-2.06 Ga
Paleoproterozoic basement do not show a Neoproterozoic overprint (Hartmann et al., 2002b); the
2.19-2.09 Ga samples recovered from deep drill cores in the western side of the craton also show no
such evidence (Rapela et al., 2007).
Another important piece of evidence comes from the zircon provenance patterns of Neoproterozoic
sedimentary and metasedimentary sequences located along the western and southern sides of the Río
de la Plata craton (i.e. the Pampean Belt and the North Patagonian Massif). These sequences are
dominated by a bimodal pattern, with peaks at 1250-960 Ma and 680-570 Ma and a minor peak c.
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1900 Ma, with scarce, if any, Palaeoproterozoic zircons in the Río de la Plata craton range of ages
(e.g. Basei et al., 2005, 2008; Pankhurst et al., 2006; Rapela et al., 2007). To explain these bimodal
patterns, “African” (Namaqua-Natal) and “Brazilian” sources have been postulated (e.g. Schwartz and
Gromet, 2004; Basei et al., 2005; Rapela et al, 2007).
However, it is difficult to make a coherent scenario incorporating the several geodynamic models
recently proposed for the pre-Atlantic (Adamastor ocean), and those for the western side of the Río
de la Plata craton. Another important issue that remains poorly explained is the relative position of the
Kalahari, Congo and Río de la Plata cratons during the early Neoproterozoic. This paper presents new
U-Pb SHRIMP results on drill core samples from close to the present Atlantic coast, at the tip of the
Tandilia belt on the eastern margin of the Río de la Plata craton (Punta Mogotes), as well as from the
Sierra Ancasti and Sierra Brava in the Pampean belt, on the western side of the craton. Zircon
provenance patterns on these critically located samples, together with previous results, allow an
interpretation of the Neoproterozoic rifting-drifting of the Río de la Plata craton, and infer a close
connection with similar processes in southwestern Africa. This process covers the transition from
Rodinia dispersal to Gondwana assembly.
The analyses of 5 samples from the 504 m deep Punta Mogotes borehole (Marchese and Di Paola,
1975) show the expected sharp contrast in zircon age pattern between the Neoproterozoic low-grade
metapelites of the Punta Mogotes Formation and that of the overlying quartzites of the Balcarce
Formation. The patterns of the two samples of the Punta Mogotes Formation are complex but
remarkably similar, suggesting a similar source for at least the upper section of this sequence.
Conspicuous younger peaks at 760-790 Ma defined by concordant zircons are the most important
characteristic of these patterns, with significant populations in the Mesoproterozoic (peaks at 1250
and 1270 Ma respectively), and Upper Paleoproterozoic (peaks at 1735 and 1835 Ma), and minor but
concordant populations at 1420-1560 Ma, 2070-2200 Ma, together with Early Palaeoproterozoic and
Archaean zircons. This pattern and those found in the metasedimentary rocks of the Pampean Belt,
characterized by prominent bi-modal peaks at 560-625 Ma and 1025-1110 Ma, with minor peaks at
730-760 Ma and c. 1900 Ma, have been used to constraint a plate reconstruction for various time
periods involving the Río de la Plata, Congo and Kalahari cratons.
The conspicuous peaks at 760-780 Ma of the Punta Mogotes Formation are unique among the
Neoproterozoic successions, and these dominantly concordant detrital zircons define a minimum age
for the siliciclastic succession. There are no Brasiliano-Panafrican ages (560-680 Ma) in the Punta
Mogotes Formation, although this is a widespread event in southwestern South America, suggesting
that the sequence is older than 680 Ma as well as younger than or coeval with the 760-780 Ma detrital
peaks. Major detrital peaks at 635-660 Ma are otherwise observed in all samples of the overlying
Balcarce Formatiom. Orthogneisses with U-Pb SHRIMP ages of 762 ± 8 and 776 ± 12 Ma have been
described from the Punta del Este Terrane in eastern Uruguay, and inferred to be a portion of the
Coastal Terrane of the Kaoko Belt (Hartmann et al., 2002a; Oyhantçabal et al., 2009). In southwestern
Africa, this period is characterized by the inception of a large alkaline igneous province associated
with rifting that was eventually superseded by drifting and finally by inversion of the basins (Jacobs
et al., 2008 and references therein). A comparison with the Neoproterozoic detrital patterns of
southwestern Africa and southeastern South America suggests that the most suitable source for the
Punta Mogotes Formation was the basement of the Kaoko belt, on the southwestern edge of the
Congo craton. The Piedras de Afilar Formation, a thick siliciclastic and carbonate sequence located
on the edge of the Río de la Plata craton in Uruguay, shows a similar detrital pattern, which however
lacks the 760-790 Ma peak (Gaucher et al., 2008), indicating that both sequences were derived from
similar sources in the Congo craton.
These similarities in detrital patterns strongly suggest that the Río de la Plata craton was a conjugate
rift margin of the Congo craton at the time of the 760-830 Neoproterozoic rifting. The NW-SE branch
of the aulacogenic triple point located at the western end of the Damara orogen (Goscombe et al.,
2005 and references therein), is here considered as initiating separation of the Río de la Plata and the
Congo cratons, resulting in development of the northern Adamastor ocean at the time of Rodinia
break-up. It is also considered that discrete continental terranes might have rifted away from the
Congo craton. The Archaean to Mesoproterozoic Nico Pérez terrane in Uruguay (Bossi and Cingolani,
2009) may have been produced during this episode.
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A second NE-SW branch of the triple point runs along the western edge of the Kalahari craton and
faced an open ocean to the west. Discrete continental “African” terranes may have also rifted away
during the opening of the southern Adamastor ocean. The continental terranes affected by rifting at
the latitude of the Gariep Belt were mostly composed of the Mesoproterozoic complexes of the NatalNamaqua orogen (c. 1000-1100 Ma), and secondly by the 1700-2000 Ma Eburnean age basement,
such as the Richtersveld terrane (Frimmel et al., 2001). Maximum expansion of the northern
Adamastor and Khomas oceans took place at c. 700 Ma, while ocean opening continued in the
southern Adamastor ocean. East-directed subduction started in the northern Adamastor ocean (Gray
et al., 2006) at c. 680 Ma, with the possible formation of intra-oceanic arcs. Closing of the northern
branch took place at c. 640 Ma, involving transpression, e.g. the Kaoko Belt (Goscombe et al., 2005;
Gray et al., 2006) and Punta del Este terrane (Oyhantçabal et al., 2009). West-directed subduction
started in the southern Adamastor ocean with development of 620-580 “Brasiliano” magmatic arcs
preserved in the “African” terranes , now located to the west and southwest of the Río de la Plata
craton (present coordinates). Southward displacement of the Río de la Plata craton with the attached
Nico Pérez terrane led to the highly oblique collision against the southwestern Congo craton,
developing sinistral transpression in the Kaoko and Dom Feliciano belts in the 640-600 Ma time
interval.
Protracted oblique subduction led to closure of the Adamastor ocean at ca. 545 Ma involving collision
between the Río de la Plata and the Kalahari cratons (Frimmel and Frank, 1998). On the west and
southwest, the Río de la Plata craton was involved at ca. 530-520 Ma into right-lateral collision with
a large continental terrane, developing the transpressional Pampean Belt.
REFERENCES
• Basei, M.A.S., Frimmel, H.E., Nutman, A.P., Preciozzi, F., Jacob, J., 2005. A connection between the Neoproterozoic Dom Feliciano
(Brazil/Uruguay) and Gariep (Namibia/South Africa) orogenic belts –evidence from a reconnaissance provenance study.
Precambrian Research 139, 195-221.
• Bossi, J., Cingolani, C., 2009. Extension and general evolution of the Río de la Plata craton. In: Gaucher, G., Sial, A.N., Halverson,
G.P., Frimmel, H.E. (eds.) Neoproterozoic-Cambrian Tectonics, Global Change and Evolution: A Focus on Southwesten Gondwana.
Developments in Precambrian Geology, 16, Elsevier, pp.73-85.
• Basei, M.A.S., Frimmel, H.E., Nutman, A.P., Preciozzi F., 2008. West Gondwana amalgamation based on detrital zircon ages from
Neoproterozoic Ribeira and Dom Feliciano belts of South America and comparison with coeval sequences from SW Africa. In:
Pankhurst, R.J., Trouw, R.A., Brito Neves, B.B. and de Wit, M.J. (eds.) West Gondwana: Pre-Cenozoic Correlations Across the
South Atlantic Region. Geological Society, London, Special Publications, 294, 239-256.
• Frimmel, H.E., Frank, W., 1998. Neoproterozoic tectono-thermal evolution of the Gariep Belt and its basement, Namibia/South
Africa. Precambrian Research 90, 1-28.
• Frimmel, H.E., Zartman, R.E., Späth, A., 2001. The Richstersveld Igneous Complex, South Africa: U-Pb zircon and geochemical
evidence for the beginning of the Neoproterozoic continental break-up. The Journal of Geology 109, 493-508.
• Gaucher, C., Finney, S.C., Poiré, D.G., Valencia, V.A., Grove, M., Blanco, G. Paamoukaghlian, K., Gómez Peral, L., 2008. Detrial
zircon ages of Neoproterozoic sedimentary successions in Uruguay and Argentina: insightsinto the geological evolution of the Río
de la Plata Craton. Precambrian Research 167, 150-170.
• Goscombe, B., Gray, D., Armstrong, R., Foster, D.A., Vogl, J., 2005. Event geochronology of the Pan-African Kaoko Belt, Namibia.
• Gray, D.R., Foster, D.A., Goscombe, B., Passchier, C.W., Trouw, R.A.J., 2006. 40Ar/39Ar thermochronology of the Pan-African
Damara orogen, Namibia, with implications for tectonothermal and geodynamic evolution. Precambrian Research 150, 49-72.
• Hartmann, L.A., Santos, J.O.S., Bossi, J., Campal, N., Schipilov, A., McNaughton, N.J., 2002a. Zircon and titanite U–Pb SHRIMP
geochronology of Neoproterozoic felsic magmatism on the eastern border of the Río de la Plata craton, Uruguay. Journal of South
American Earth Sciences 15, 229-236.
• Hartmann, L.A., Santos, J.O.S., Cingolani, C.A., McNaughton, N.J., 2002b. Two Palaeoproterozoic orogenies in the evolution of the
Tandilia Belt, Buenos Aires, as evidenced by zircon U–Pb SHRIMP geochronology. International Geology Review 44, 528-543.
• Jacobs, J., Pisarevsky, S., Thomas, R.J., Becker, T., 2008. The Kalahari Craton during the assembly and dispersal of Rodinia.
• Marchese, H.G., Di Paola, E.C., 1975. Reinterpretación estratigráfica de la Perforación Punta Mogotes Nº 1, Provincia de Buenos
Aires, República Argentina. Revista de la Asociación Geológica Argentina 30, 44-52.
• Oyhantçabal, P., Siegesmund, S., Wemmer, K., Presnyacov, S., Layer, P., 2009. Geochronological constraintson the evolution of the
southern Dom Feliciano Belt (Uruguay). Journal of the Geological Society, London, 166, 1075-1084.
• Pankhurst, R.J., Rapela, C.W., Fanning, C.M., Márquez, M., 2006. Gondwanide continental collision and the origin of Patagonia.
Earth Science Reviews 76, 235-257.
• Rapela, C.W. , Pankhurst, R.J., Casquet, C., Fanning, C.M., Baldo, E.G., González-Casado, J.M., Galindo, C., Dahlquist, J., 2007.
The Río de la Plata craton and the assembly of SW Gondwana. Earth Science Reviews 83, 49-82.
• Schwartz, J.J., Gromet, L.P., 2004. Provenance of Late Proterozoic-early Cambrian basin, Sierras de Córdoba, Argentina.
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THE PIEDRA ALTA TERRANE: A PALEOPROTEROZOIC
JUVENILE MAGMATIC ARC, RIO DE LA PLATA CRATON, URUGUAY
1-21
Sánchez Bettucci, L.1*, Peel, E.1, Basei, M.A.S.2
(1) Instituto de Ciencias Geológicas, Facultad de Ciencias-UdelaR, Montevideo, Uruguay
(2) Instituto de Geociencias, Universidade de São Paulo
The Río de la Plata craton best exposures occur in the southwest of Uruguay with a north extension
into the Rivera region and Taquarembó block in southern Brazil and in the neighbourhood of the
Tandil hills in Argentina to the south. The Río de la Plata craton is divided in Uruguay into two
tectonostratigraphic terranes: Piedra Alta (PATT) and Nico Pérez (NPTT). The PATT is constituted by
juvenile arc-related granitoids and by volcano-sedimentary belts with E-W structural trend. These
sequences were metamorphosed under low to medium grade conditions. The PATT granitoids
represent an excellent example of the roots of a Palaeoproterozoic magmatic arc, which was cool and
stable since 1.7 Ga without recording the Neoproterozoic orogeny as it is shown by its
Paleoproterozoic K-Ar ages. It is considered as the best preserved Palaeoproterozoic block of the Río
de La Plata craton. The late to post-orogenic magmatism (Albornoz Complex) with calcalkaline,
peraluminous and alkaline granites and gabbros presents ages ca. 2100 Ma. Available data allows to
define two generations of granites, the first event at ca. 2053-2086 Ma and the second one related to
mafic intrusions at ca. 2016-2033 Ma, both intruding the volcano-sedimentary belts. Anorogenic
granites –like the A-type Soca Granite- are emplaced into graphite mica-schist, quartzites, gneisses,
amphibolites and deformed granitoids. This subalkaline, metaluminous or slightly peraluminous Atype granite was emplaced after the Paleoproterozoic orogenic climax presenting xenoliths of
metamorphic rocks (graphite-schist). This body has a U-Pb isotopic age of 2078 ± 8 Ma. The available
geologic and isotopic data do not support the hypothesis that suggest another terrane named Tandilia
for the outcrops of the PATT located at the south of the Santa Lucía rift.
FURTHER EVIDENCE FOR MULTIPLE REVERSALS
IN THE NEOPROTEROZOIC ARARAS CAP CARBONATE (BRAZIL)
1-22
Sansjofre, P. 1, Trindade, R.I.F.1, Ader, M.2, Nogueira, A.C.R.3, Soares, J.L.3
(1) Departamento de Geofisica, Instituto de Astronomia, Geofísica e Ciências Atmosféricas,
Universidade de São Paulo, Rua do Matão 1226, 05508-900 São Paulo, Brazil
(2) Laboratoire de Géochimie des Isotopes Stables, IPGP, Université Paris-Diderot, UMR 7154, 4
place Jussieu, 75252 Paris Cedex 05, France
(3) Faculdade de Geologia, Instituto de Geociências, Universidade Federal do Pará,CP 1611,
66.075-900, Belém, Brazil
The extent and duration of Neoproterozoic glaciations is still a matter of contention. Paleomagnetic
inclination data on glacial deposits and post-glacial cap carbonates are one of the pillars of the
Snowball Earth hypothesis that postulates an ice-covered Earth at the beginning of the Ediacaran
period. Paleomagnetism has also contributed in constraining the time-scale of glaciations itself and
the deglaciation process. Multiple reversals have been reported for the glacial deposits (Elatina
Formation, in Australia) and the cap carbonates (Nuccaleena Formation, Australia, Mirbat Formation,
Oman, and Araras Formation, Brazil) suggesting that the deposition during and after glacial events
spent hundreds of thousands of years at least. These estimates are useful constraints on paleoclimatic
and isotopic models for these extreme climatic scenarios. Here we have revisited one of these
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successions, the Araras Group in western Brazil (Mato Grosso State) in order to study two correlative
sections of cap carbonates located along a platform transect from shallow water (Tangara section) to
moderately deep water platform (Carmelo section). Sampling in Tangara and Carmelo quarries was
stratigraphically controlled and tightly spaced in order to gather the directional variations of the
magnetic field through time. Paleomagnetic results obtained in these sections were combined with
those of Trindade et al. (Terra Nova, vol. 15, p. 441-446, 2003) and Font et al. (Journal of Geophysical
Research, doi.10.1029/2005JB004106, 2006). Magnetic reversals observed in Terconi section were
reproduced in the new sections, and a clear magnetostratigraphic correlation could be drawn between
the two proximal sections (Terconi and Tangara). Samples in the top of Carmelo section show the
same remagnetized magnetic component observed previously at the top of Terconi. These new results
confirm the multiple reversals reported previously for the Araras cap dolostones, and indicate that
transgression and sedimentation after Marinoan ice-ages was long-standing at odds with the snowball
Earth model.
PARAGUAY BELT FOLDING AND OROCLINAL BENDING DURING
THE FINAL ASSEMBLY OF WESTERN GONDWANA
1-23
Trindade, R.I.1, Tohver, E.4, Nogueira, A.C.3, Riccomini, C.2
(1) Geofisica, Universidade de Sao Paulo, Sao Paulo, Brazil.
(2) Geologia Sedimentar e Ambiental, Universidade de Sao Paulo, Sao Paulo, Brazil.
(3) Geociencias, Universidade Federal do Para, Belem, Brazil.
(4) Western Australia University, Perth, WA, Australia.
A paleomagnetic investigation was undertaken along the Paraguai belt (Western Brazil), which marks
the Neoproterozoic limit of the SE Amazon craton. This belt displays ca. 90 degrees of curvature
along its ca. 1200 km extent, where the well-preserved sedimentary cover found over the craton is
tightly folded but not metamorphosed. We have sampled the Araras Group, that consists of dolostones
and limestones of Ediacaran age and the Alto Paraguay Group, which comprises siliciclastic deposits,
starting with fluvial sandstones overlaid by turbidites and lake sediments. Sedimentological evidence
(presented in a companion talk) is compatible with the inversion of the Araras passive margin after the
final collision between the Amazonian Craton and the Central Gondwana. Paleomagnetic data from
the Neoproterozoic Araras Formation shows that it retains a magnetization that is secondary in nature,
as indicated by a negative fold test reported by Trindade et al. (Terra Nova, 2003). However, the
declination of this secondary magnetization varies along strike, suggesting that the curvature of the
belt was generated subsequent to an initial phase of folding and thrusting (Tohver et al., 2010), with
vertical axis rotation being responsible for the E-W trend branch of the Paraguay belt, which served
as a transform zone during the late Cambrian collision between the West Gondwanan elements
Amazonia-Rio Apa-West Africa and the Central Gondwanan cratons: Congo-São Francisco, Rio de
Plata, Kalahari. The same rotation is also observed on the siliciclastic deposits that immediately
overlie the Araras carbonates but is not recorded by the upper pelites at the top of the Alto Paraguay.
These data enable to continuously track the deformation along the belt during the final episodes of
suturing of West Gondwana.
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EVIDENCE FOR MIDDLE-LATE ORDOVICIAN SUBDUCTION AND A
LOWER PLATE SETTING OF THE CUYANIA TERRANE DURING ITS
ACCRETION TO THE PROTO-ANDEAN MARGIN OF GONDWANA
1-24
Van Staal, C.1*, Vujovich, G.2, Currie, K.3, Naipauer, M.2
(1) Geological Survey of Canada. 625 Robson Street, Vancouver, V6B 5J3 BC, Canada
(2) CONICET (National Research Council of Argentina). Laboratorio de Téctonica Andina,
Universidad de Buenos Aires, Pabellón II, Nunez. Buenos Aires 1428
(3) Geological Survey of Canada. 601 Booth Street, Ottawa, K1A 0E8 ON, Canada
* Presenting author’s e-mail: e-mail: cvanstaa@nrcan.gc.ca
Lithostratigraphic and structural analysis of the Caucete Group and the immediately structurally
overlying Pie de Palo Complex in the Sierra de Pie de Palo, Argentina, indicate a basement-cover
relationship, which we suggest was established during late Early Cambrian (~515 Ma) final rifting of
Cuyania from Laurentia, The main deformation (Dm) and associated metamorphism indicate
conditions typical of the blueschist to high-pressure amphibolite facies conditions, which leaves little
doubt that a subduction zone existed between Cuyania and the proto-Andean margin of Gondwana.
This provides strong support for the Laurentia-derived microntinent model of earlier workers. The
main structures involve two phases (F1 and F2) of fold nappe formation and associated thrusting; the
latter forming by shearing-out of the lower limbs of the inclined to recumbent folds. The style of
deformation indicates that strain localization decreased during peak-T metamorphism and imposes a
penetrative non-coaxial flow on the rocks involved in the A-subduction of the Cuyania’s leading edge
beneath the proto-Andean margin. We relate this to widening of the subduction channel after entrance
of Cuyania into the trench (start of A-subduction and collision) associated with thermal weakening
becoming more prevalent than fabric-related softening.
LOW-PRESSURE ANATEXIS IN FAMATINIAN FORELAND OF ARGENTINA,
SOUTH-WESTERN MARGIN OF GONDWANA: SOURCE HEAT PROBLEM
1-25
Verdecchia, S.O.*, Baldo, E.G.
CICTERRA – Consejo Nacional de Investigaciones Científicas y Técnicas (CONICET). Av. Veléz
Sarsfield 1611. Facultad de Ciencias Exactas, Físicas y Naturales. Universidad Nacional de
Córdoba. Argentina
* Presenting author’s e-mail: sverdecchia@efn.uncor.edu
Within the foreland region of the Famatinian belt in central-western Argentina low- to mediumpressure metamorphic complexes crop out, one of them being the Ordovician metasedimentary rocks
from La Cébila. This complex was affected by a main tectono-thermal event (M1) characterized by
low pressure (2-2.5 kbar) and high thermal gradient (~80º C/km) conditions. In this complex, a welldefined nearly isobaric metamorphic zonation was developed. It progrades towards the intrusive
contact with syn- to post-tectonic peraluminous granitic bodies (S-type; 760-780º C crystallization
temperature). The metamorphic grade increases from white mica-chlorite (~300-400º C; Kübler index
values from 0.23 to 0.17 ¢º2ı) to cordierite-K-feldspar (714-740º C; Ti-in-biotite thermometry) zones,
through the andalusite and sillimanite stability fields, reaching anatexis conditions with the
development of a wide migmatitic belt (1-3 km) adjacent to the granites. In the M1 event (~460 Ma),
a well-defined secondary foliation was developed with syn-kynematic blastesis of andalusite and Kfeldspar porphyroblasts, associated with a strongly compressive episode (D1). Granitic magma by
itself, because of its relatively low crystallization temperature, would not be a suitable heat source to
explain the widespread melting in the country rocks, thus ruling out contact metamorphism as the
single cause of metamorphism.
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22-23 NOVEMBER 2010 – BUENOS AIRES
The paleontological record (brachiopods in quartzites metamorphosed under sillimanite zone
conditions) suggests a foreland marine basin in the Early to Middle Ordovician (480-460 Ma). High
heat flow during the extensional regime would explain the low-pressure metamorphism in shallow
crustal levels (<9 km), but it is difficult to produce anatexis without associated mafic intrusion. The
integration of sedimentary, metamorphic and igneous information suggests that heat source of the
main metamorphic event could be related to a combination of mechanisms: high heat flow crustal
regimen related to the extensional period and immediately a plutonic intrusion (S-type granite) related
to the a compressional regime.
THE MESO-NEOPROTEROZOIC SUBDUCTION-ACCRETION EVENTS
AND MAGMATIC EVOLUTION ALONG THE WESTERN MARGIN
OF THE SIBERIAN CRATON: TO THE PROBLEM OF RODINIA BREAK-UP
1-26
Vernikovsky, V.A.*, Vernikovskaya A.E.,
Institute of Petroleum Geology and Geophysics, Koptyuga ave., 3, Novosibirsk, Russia, 630090.
* Presenting author’s e-mail: VernikovskyVA@ipgg.nsc.ru
The Meso-Neoproterozoic subduction-accretion events and magmatic evolution along the western
margin of the Siberian Craton has attracted attention in the context of debatable problems concerning
the formation and break-up of Rodinia supercontinent.
Available on geological, geochronological and paleomagnetic data established that in the
Neoproterozoic a transformation of the Siberian Craton western margin from a passive to an active
one was taking place (Vernikovsky et al., 2003 Tectonophysics, 375, 147–168; 2009 Russian Geology
and Geophysics, 50, 4, 372-387; Metelkin et al., 2007 Russian Geology and Geophysics, 48, 1, 3245.; Pisarevsky et al., 2008 Precambrian Research, 160, 66-76). The Neoproterozoic subductionaccretion processes along the Siberian Craton western margin were not synchronous; they take in a
wide interval of time – from 960 Ma to 630 Ma.
An island arc system started to form at the north-western margin of the Siberian Craton approximately 960 Ma (Vernikovsky et al., 2009, op. cit.). Its fragments were included in the Central Taimyr accretionary belt. Paleomagnetic poles for the 960 Ma acid volcanic rocks in the Three Sisters Lake island
arc from this belt have been determined. These poles are very close to the poles of same age for
Siberia (Pavlov et al., 2002 Geotectonics, 36, 278–292.). These results significantly expand the previously acquired evidences for the forming of island arcs with their subsequent accretion and obduction
onto the Taimyr margin of Siberia in the interval of 750-660 Ma (Vernikovsky and Vernikovskaya,
2001 Precambrian Research, 110, 4, 127-141).
The succession of the forming of Siberia’s western margin, represented by the Yenisey Ridge accretional orogen, was determined. We assume that the genesis of this structure is a result of three events:
a) syn-collisional events (probably outside the Siberian Craton), which resulted in the forming of the
Teya granites with the age 880-860 Ma in the Central Angara terrane; b) the collision between the
Central Angara terrane and the Siberian craton and the formation of the syn- and post-collisional
Ayakhta and Glushikha granites with the age 760-720 Ma; c) the formation of island arcs and ophiolites along the margin of the Siberian Craton, their accretion and obduction onto the continent in the
interval of 700-630 Ma (Vernikovsky and Vernikovskaya, 2006 Russian Geology and Geophysics, 47,
1, 32-50). The last event is of special interest because at the same time in the Tatarka-Ishimba suture
zone of the Yenisey Ridge, which is sub parallel to the continental margin, the forming of intrusive
and volcanic rocks of various composition and heightened alkalinity was taking place, including alkaline syenites as well as carbonatites and A-type granites. They formed synchronously with the rocks
of the island arc complex and their accretion and obduction onto the continental margin of Siberia in
the interval of 700-630 Ma (Vernikovsky et al., 2008 Doklady Earth Sciences, 419, 2, 226-230). It is
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quite probable that their formation in the back-arc suprasubduction zone was taking place at the same
time that the oceanic plate was subducting below the continent from the western margin of the Siberian
Craton and reached the asthenospheric layer. The obtained data and the developed models uncover the
geodynamic evolution of the formation of accretional orogens in the western margin of the Siberian
Craton in the Neoproterozoic, and allow us to discuss the questions of the mutual location of the
Siberian and North-American Cratons within the Rodinia supercontinent and Rodinia break-up.
THE SUTURE ZONE BETWEEN CUYANIA AND
CHILENIA TERRANES: A SUBDUCTION CHANNEL AND A-TYPE OROGEN?
1-27
Vujovich, G.I.1*, Boedo, F.L.1, Willner, A.P.2
(1) Laboratorio de Tectónica Andina, FCEN, Universidad de Buenos Aires / CONICET, Pabellón II, C.
Universitaria, Buenos Aires, Argentina.
(2) Institut für Mineralogie und Kristallchemie, Universität Stuttgart, Azenbergstr. 18, D-70174
Stuttgart, Germany
* Presenting author’s e-mail: graciela@gl.fcen.uba.ar
A discontinuous belt of Paleozoic mafic-ultramafic rocks outcrops along the Western Precordillera
Argentina and Cordillera Frontal and constitute part of the Central Andean basement between La
Rioja and Mendoza provinces, Argentina (Fig. 1). This belt has been interpreted from several authors
as the limit between the Cuyania and Chilenia terranes accreted to the proto-Gondwana margin during
Paleozoic times.
Western Precordillera area: the most important outcrops of the mafic-ultramafic rocks are
represented by a massive sequence of basaltic pillow lavas and columnar-jointed flows, sills of mafic
and ultramafic composition (Jachal – Rodeo and Calingasta – 114 km2) intruding a metasedimentary
sequence mainly composed by metagraywackes and shales. At Río Jachal section the mafic and
ultramafic belt and metasediments have been steeply folded with a westward vergence (Ramos et al.,
1984; von Gosen, 1997).
A thick sequence of basaltic pillow lavas are interbedded with shales and others slope-deposits
outcrops along the San Juan River (Km 114) and in Calingasta area. Upper Ordovician graptolites
indicate a Late Ordovician age for the sequence.
Serpentinized peridotites, ultramafic cumulates, coarse grained gabbros to microgabbros, diabase,
and layered gabbros (garnet granulites) outcrops at Cordón del Peñasco and Cortaderas – Bonilla
areas and constitute the dominant rock types. They are in tectonic contact with mafic submarine flows
(hyaloclastites), tuffs and pillow lavas interlayered with metasandstones, metasilstones and scarce
chert (Davis et al., 1999; Boedo, 2010).
Based on field observations, petrological, geochemical and isotopic data the basaltic pillow lavas, lava
flows and mafic-ultramafic sills are interpreted as evolved tholeiites (E-MORB) related to an oceanic
ridge environment (see Ramos et al., 2000 and references therein; Kay et al., 2005). On geochemical
grounds the garnet granulites (layered gabbros) display a weak arc magmatic signal (Davis et al.
1999). Upper Proterozoic to Ordovician ages (U-Pb zircon) for diabases and mafic sills, and minimum
Silurian ages for the layered gabbros were reported by Davis et al. (2000).
Low-grade regional metamorphism has partially replaced the igneous and high temperature
assemblages in the mafic and ultramafic rocks and layered gabbros. Metamorphic P-T conditions for
the metabasites at Rodeo and Calingasta areas are constrained to a low-temperature/low pressure
setting at ca. 250-350°C and 2-3 kbar (Robinson et al., 2005). Petrologic studies of the ultramafic
cumulates and layered gabbros indicate a recristallization at granulite facies (T= 850 – 1000°C) and
P= 9 – 11 kbars (Davis et al., 1999). Later retrograde metamorphism occurred under greenschist
facies conditions.
At Cortaderas area in the low grade units Davis et al. (1999) yielded Ar-Ar plateau ages of 384±0.5
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Fig. 1 - Mafic-ultramafic belt of Western Precordillera
(based on Vujovich and Ramos, 1999).
Ma and 377±0.5 Ma on undefined white mica concentrates. This is interpreted as the timing of low
grade peak metamorphism.
Cordillera Frontal: A medium grade unit of micaschist with intercalated amphibolite, marble and
serpentinite outcrops at Río de Las Tunas area (Bjerg et al., 1990; López and Gregori 2004; López et
al., 2009) and it is known as the Guarguaráz Complex. Metasedimentary sequences are interpreted as
metagraywackes derived from a mature cratonic continental basement. The mafic-ultramafic rocks are
represented by lenses of partly garnet-bearing amphibolite with an N- or E-MORB geochemical
signature, serpentinite and talc-bearing wall-rocks. Marbles and calc-silicate rocks deposited in a
platform environment are part of the sedimentary sequence. Metapelitic rocks achieved the peak of
the medium grade metamorphism at 13.5 kbar, 500°C followed by a decompression to mid-crustal
conditions at 8 kbar, 565°C (Massonne and Calderón, 2008).
The maximum depositional age of 563 Ma for the metagraywackes of Guarguaráz complex has been
estimated by Willner et al. (2008), which is consistent with the Vendian-Cambrian age estimated by
López et al. (2009) based on microfossils, and a whole-rock Sm-Nd isochron age of 655±76 Ma for
the intercalated metabasite interpreted as the probable crystallization age of the protolith (López et al.,
2009).
The age of metamorphism is weakly defined by an Rb/Sr whole rock isochron of the Guarguaráz
micaschist at 375±34 Ma (Basei et al., 1998) and a K/Ar whole rock age of 370±18 Ma by Caminos
et al. (1979). Willner et al. (2010) yielded a 390.0±2.2 Ma age (Lu-Hf isochrons on mineral separates
from metapelites and metabasites) for the peak of high pressure metamorphism, and Ar-Ar plateau
age of white mica at 353±1 Ma for the late decompression event.
Most of these mafic-ultramafic rocks formed in different regions are related to a marine setting and
represent the upper part of an ophiolite sequence. There is not conclusive evidence to define the
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Fig. 2 - Ordovician to Late Devonian evolution of Cuyania and Chilenia terranes.
tectonic setting of the layered gabbros. They could be part of a magmatic arc or represent the lower
section of the ophiolite assemblage. Based on the PT conditions, the tectonic contacts between low
grade metamorphic units and high grade rocks retrograded to greenschist facies, and the dominant
western vergence, we proposed the existence of an east-dipping subduction channel. This channel was
responsible of the up-ward rock flux and exhumation during the A-type collisional orogeny between
Chilenia and Cuyania terranes developed during the Late Devonian times (Fig. 2).
REFERENCES
• Basei, M., Ramos, V.A., Vujovich, G.I., Poma, S., 1998. El basamento metamorfico de la Cordillera Frontal de Mendoza: nuevos
datos geocronologicos e isotopicos. Actas X Congreso Latinoamericano de Geología y VI Congreso Nacional de Geología
Económica II: 412-417.
• Bjerg, E.A., Gregori, D.A., Losada Calderón, A,, Labadía, C.H., 1990. Las metamorfitas del faldeo oriental de la Cuchilla de
Guarguaráz, Cordillera Frontal, Provincia de Mendoza: Revista de la Asociación Geológica Argentina, 45: 234-245.
• Boedo, F.L., 2010. Geología del área del Cordón del Peñasco, Precordillera occidental, provincia de Mendoza. Trabajo Final
Licenciatura, FCEN, Univ. Buenos Aires, 143 pp.
• Caminos, R., Cordani, U., Linares, E., 1979. Geología y geocronología de las rocas metamórficas y eruptivas de la Precordillera y
Cordillera Frontal de Mendoza. Actas 2. Congreso Geológico Chileno Santiago 1: 43-61.
• Davis, J., Roeske, S., McClelland, W., Snee, L., 1999. Closing the ocean between the Precordillera terrane and Chilenia: Early
Devonian ophiolite emplacement and deformation in the southwest Precordillera. In: Ramos, V.A., Keppie, J. (Eds.) Laurentia and
Gondwana Connections before Pangea. Geological Society of America, Special Papers 336: 115-138.
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Precordillera terrane and their bearing on tectonic models of the early Paleozoic in western Argentina. Geology 28: 171-174.
• Gerbi, C., Roeske, S.M., Davis, J.S., 2002. Geology and structural history of the southwest Precordillera margin, northern Mendoza
Province, Argentina. Journal of South American Earth Sciences 14: 821-835.
• Kay, S.M., Boucakis, K.A., Porch, K., Davis, J.S., Roeske, S.M., Ramos, V.A., 2005. E-MORB-like mafic magmatic rocks on the
western border of the Cuyania terrane Argentina. Gondwana 12, Abstracts, p. 216. Academia Nacional de Ciencias, Córdoba.
• López, V.L., Gregori D.A., 2004. Provenance and evolution of the Guarguaráz Complex, Cordillera Frontal, Argentina. Gondwana
Research 7: 1197-1208.
• López, V.L., Escayola, M., Azarevich, M.B., Pimentel, M.M., Tassinari, C., 2009. The Guarguaráz Complex and the Neoproterozoic-
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•
•
•
•
•
•
•
•
•
Cambrian evolution of southwestern Gondwana: Geochemical signatures and geochronological constraints. Journal of South
American Earth Sciences, 28: 333-344.
Massonne, H.J., Calderón, M., 2008. P-T evolution of metapelites from the Guarguaráz Complex, Argentina: evidence for Devonian
crustal thickening close to the western Gondwana margin. Revista Geológica de Chile, 35: 215-231.
Ramos, V.A,, Jordan, T., Allmendinger, R., Kay, S., Cortés, J., Palma, M., 1984. Chilenia: un terreno alóctono en la evolución
paleozoica de los Andes Centrales. Actas IX Congreso Geológico Argentino 2: 84-106.
Ramos, V.A., Escayola, M., Mutti,D.I., Vujovich, G.I., 2000. Proterozoic-early Paleozoic ophiolites of the Andean basement of
southern South America. Geologival Society of America. Special Paper 349: 331-349.
Robinson, D., Bevins, R.E., Rubinstein, N., 2005. Subgreenschist facies metamorphism of metabasites from the Precordillera
terrane of western Argentina: constraints on la later stages of accretion onto Gondwana. European Journal of Mineralogy, 17: 441452.
Von Gosen, W., 1997. Early Paleozoic and Andean structural evolution in the Río Jáchal section of the Argentine Precordillera.
Journal of South American Earth Sciences, 10: 361-388.
Vujovich, G.I., Ramos, V.A., 1999. Mapa geotectónico de la República Argentina (1: 2.500.000), Subsecretaría de Minería de la
Nación, Buenos Aires, Servicio Geológico Minero Argentino.
Willner, A.P., Gerdes, A., Massonne, H.J., 2008. History of crustal growth and recycling at the Pacific convergent margin of South
America at latitudes 29°-36°S revealed by a U-Pb and Lu-Hf isotope study of detrital zircon from late Paleozoic accretionary
systems. Chemical Geology, 253: 114-129.
Willner, A.P., Massonne, H.J., Gerdes, A., Hervé, F., Sudo, M., Thomson, S., 2009. The contrasting evolution of collisional and
coastal accretionary systems between the latitudes 30°S and 35°S: evidence for the existence of a Chilenia microplate. Abstracts
XII Congreso Geológico Chileno Santiago S9-099: 223.
Willner, A.P., Gerdes, A., Massonne, H., Schmidt, A, Sudo, M., Thomson, S., Vujovich, G.I. 2010. Pressure-temperature-time
evolution of a collisional belt (Guarguaráz Complex, W-Argentina): Evidence for the accretion of the Chilenia microplate. AGU
Joint Assembly “Meeting of the Americas”, Foz do Iguazu, Session V2.
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Session 2
VOLCANISM AND
PETROLOGY
GEOSUR2010
EXOTIC EXHALATIONS FROM ACTIVE S-ANDES VOLCANOES:
DOMUYO, TROMEN AND COPAHUE VOLCANOES, ARGENTINA
2-01
Bermudez, A.1*, Delpino, D.2, Varekamp, J. C.3, Kading, T.3
(1) CONICET, National University of Comahue, Neuquén, Argentina
(2) REPSOL-YPF, Dirección General de Exploración, Talero 360 – (8300) Neuquén, Argentina
(3) Department of Earth and Environmental Sciences, Wesleyan University, Middletown, CT 06459,
USA
* Presenting author’s e-mail: delpinus3@speedy.com.ar
Introduction
Active volcanoes carry some of the most extreme fluids to the surface of the earth, either very acid
and/or very concentrated brines. Highly acid fluids have been described from crater lakes and hot
springs on active volcanoes, which are condensed volcanic gases, rich in three strong volcanic acids
H2SO4, HCl, and HF (Type 1 fluids). Sulphur is the dominant volcanogenic element in these acid fluids with up to 7% sulphate. These fluids acquire their cation loading from water-rock interaction at
elevated temperatures at depth. Another class of acid fluids results from the boiling of deep volcanic
aquifers, where boiled-off H2S is oxidized to sulphuric acid in the surface environment. These fluids
react with surrounding surface rocks, usually at temperatures not far from above the local boiling
point. The first type of acid fluids are rich in both SO4 and Cl-, may have extremely low pH values
(0 or below) and very high concentrations of the rock forming elements (RFE). The second type of
acid fluids have more modest pH levels (1 and up), are poor in Cl and F, and have lower RFE concentrations (type 2 fluids). Hypersaline neutral brines are rare at the surface of the earth, and may form
through immiscibility from early formed magmatic vapours creating Na-K-Cl brines together with
aqueous, sulphur and CO2 rich volcanic vapours that commonly escape through fumarolic fields in
craters. Monitoring and analyzing these various types of fluids provides insights into volcanic
degassing (volcano monitoring) and water-rock interaction processes in the underlying geothermal
reservoirs. Several hot springs and crater lakes of active volcanoes in Argentina have been investigated in this study: Copahue (37.45oS, 71.17oW); Tromen (37º S -70º W, 3979 m) and Domuyo (36o40’S,
70o40’W, 5400m) (Fig. 1).
The Tromen Volcanic Field (Fig. 1) is formed from andesitic volcanic products, domes and basaltic
cinder cones, and developed in several cycles of igneous activity, some with multiple eruptions. Two
small, cool volcanic lakes, one red and one green, occur in the Tromen volcano summit region
(Bermúdez et al, 2006) and contain acid fluids, with pH values ~1. The Domuyo volcanic complex
(Fig. 1) consist of an Upper Miocene-Pliocene subvolcanic granodioritic dome surrounded by numerous Quaternary acid extrusive domes associated with emissions of pyroclastic flows, ash fallout and
lava flows. Domuyo has an active hydrothermal field on its western flanks, with hot springs at about
90 oC. The reservoir temperature of a vapour-dominated system was estimated at >200 oC whereas the
liquid-dominated fluids had temperatures of 160-200 oC. The hot spring area is strongly altered into
silica with Smectite and Kaolinite and rare zeolites. Copahue volcano (Fig. 1) occurs at the rim of a
2 Ma mega caldera (19x15km) and during the Pleistocene a composite large cone was formed. Both
Pliocene and Pleistocene edifices are dissected when a large ice sheet totally filled the caldera depression. Post - glacial Copahue active volcano summit area is formed by eight cinder cones aligned along
a N60ºE fissure of 2.5 km length. Craters are partially covered by glaciers and inside the active cone,
a small glacier lobe covers its western wall and supplies melt water to a circular (250 m diameter)
crater lake. Copahue Volcano had minor phreato-magmatic eruptions in 1992-1995 and a magmatic
eruption in 2000 (Delpino D. and Bermúdez, 2002 and Bermúdez et al., 2002).
Chemical composition of the hydrothermal fluids
The chemical compositions of water samples from Tromen, Domuyo and Copahue are listed in Table
1. Copahue fluids are characterized by extremely low pH values, and very high anion and cation concentrations. The Domuyo fluids on the other hand are pH-neutral with very high Cl and Na contents.
The Tromen lake has a low pH, low Cl contents and modest cation concentrations. The active volcanohydrothermal system at depth in Copahue (pH~0, T~300 oC) scrubs all volcanic volatiles, and SO2
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disproportionates into bi-sulphate and
liquid elemental Sulphur (Varekamp et
al., 2001). These acid fluids are injected
into a crater lake after modest water rock
interaction, and emerge as acid hot
springs after more extensive water-rock
interaction. The hot springs feed a glacial
meltwater river (Upper Rio Agrio) that
drains into the large glacial Lake
Caviahue. We measured the water fluxes
and river water compositions of the
Upper Rio Agrio just before it enters
Lake Caviahue from 1997 to 2009, which
includes the 2000 eruptive period. This
has provided a flux record of volcanic
elements (F, Cl, S, B, As) and major RFE
(Al, Fe, Mg, Ca, K, Na). The annual river
flux measurements are complemented
with analytical data from vertical water
profiles through Lake Caviahue, which
Fig. 1 - Localization of active S-Andes volcanoes: Domuyo,
through its ~3.5 years water residence
Tromen and Copahue.
time has a ‘chemical memory’ of past
element fluxes. The latter can be used to
reconstruct mean element fluxes over the years based on changes in the element contents of the lake.
The mean, time-weighted measured volcanogenic fluxes (tonnes/month) were 1510 S, 1180 Cl and
104 F, with peak values in 1999-2000 and very low yields in 2001-2002 after the eruption. The F-ClS concentrations in Lake Caviahue waters have dropped from their high values in 2000 as a result of
the more modest element fluxes in the last few years (Varekamp, 2008). The RFE fluxes also peaked
during the 2000 eruption, and went through a minimum in 2001-2002. Precipitation of Jarosite /
Alunite in the hydrothermal reservoir since 2000 has strongly reduced the flux of K, Al and SO4 into
the lake. The S flux rate can be translated into a volumetric magma degassing rate of ~ 3.5 108
m3/decade (using a high estimate of 200 ppm S released from the melt, based on glass inclusion and
matrix glass analyses (Goss, 2003), whereas the mean RFE flux provides a rock dissolution rate of ~2
105 m3/decade (Varekamp et al., 2001). The rock dissolution processes together with the summit fractures with active cones probably led to periodic flank collapses, which, both with glacial processes,
has given Copahue its rounded shape and modest elevation of 3000 m.
Discussion
The origin of the hyper-acid brines is fairly well agreed upon and best explained by the capture of
magmatic waters, SO2, HCl, and HF in a meteoric cell of water above the degassing magma at ~ 1500
m depth below the summit. The resulting acid brine reacts with surrounding rock, creating high concentrations of the common rock-forming elements (Varekamp et al., 2009). Precipitation of secondary minerals like Alunite, Jarosite, Anhydrite and Silica fractionate the fluids over time, and dissolution of these mineral phases during later stages may enhance the element fluxes again. The Tromen
lakes are most likely fed by high temperature steam with H2S, and the sulphuric acid that forms from
the oxidation of H2S then dissolves the surrounding rocks. The neutral Cl brines at Domuyo are relatively uncommon volcanic fluids, and their origin is enigmatic. Similar fluids occur as fluid inclusions
in minerals associated with porphyry copper deposits (Heinrich, 2007). The high density of these fluids prevents them from rising into the surface environments where they could mix with shallow
waters. Intrusive igneous masses may retain the exsolved brines as small blebs of NaCl-rich fluids,
which may be incorporated into circulating meteoric fluids. Stable isotope systematic of the Domuyo
fluids suggests a mixing line between a possible saline magmatic end member with ~ 2 % NaCl and
local meteoric water (Unpublished data).This Cl-rich fluid may also be a residual fluid from a boiling
geothermal aquifer associated with the vapour dominated geothermal system at Domuyo. A third pos60
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Table 1 - Chemical composition of hydrothermal fluids (ppm).
Sample
pH
F
Cl
SO4
HCO3
Al
Fe
Mg
Ca
K
Na
Domuyo
Agua Blanca 6
7.4
2.2
915
93
96
2
22.6
40
550
Los Tachos 90C
8.1
4.8
1834
180
155
1
52.3
69
1055
Los Tachos 1
8.0
4.8
1786
182
138
1
53.1
69
1051
Covunco 3A
7.9
1.5
512
215
95
6
84.1
36
320
Las Olletas 5
8.1
4.0
1515
154
106
1
40.5
57
931
Aguas Calientes
8.0
2.7
1011
101
78
1
25.5
39
601
3.0
4.1
4
3246
130
2.2
162
520
0.5
51
Hot spring 2006
1.2
601
6770
14,213
2022
1111
867
805
240
921
Crater Lake 2006
0.7
541
6967
15,766
1135
515
402
965
156
376
Tromen
Laguna Rojiza
Copahue
sibility is that these saline fluids have absorbed salts from Jurassic evaporites in the subsoil of
Domuyo, and then would be non-volcanic in origin.
Clear differences exist in major element chemistry between the different fluid types: the Na/K ratios
are fixed in the Domuyo hot springs (Na=20 K) whereas the Copahue fluids have highly variable Na/K
values, with large K excesses relative to the neutral saline fluids. The variations in Na/K may stem
from the precipitation and redissolution of Alunite/Jarosite together with the dissolution of volcanic
glass inside the volcano. The hyper-acid fluids have high Mg concentrations from olivine and pyroxene dissolution, whereas the neutral saline fluids lack Mg and have much higher alkalinities (mainly
HCO3-). The Ca concentrations in the acid fluids (several 100 ppm of Ca) are possibly buffered by
CaSO4 precipitation. The acid fluids carry relative high concentrations of Al and Fe, elements that are
almost insoluble in the neutral brines. The concentrations of toxic elements in the acid fluids are up
to 14 ppm As, 3 ppm Pb, and 0.16 ppm Cd. Such element concentrations are all very low in the neutral and more dilute acid fluids, possibly the result of adsorption on precipitated hydrous Fe-oxides.
The Copahue acid fluids are mixtures between acid volcanic brines (condensed volcanic gases) and
local meteoric waters, modified by evaporation for the crater lake (Varekamp et al., 2004). The
Tromen lake consist largely of meteoric water modified by evaporation at temperatures slightly above
ambient. The Domuyo brines have isotopic compositions very close to local meteoric waters despite
their high Cl contents, which suggest that they are mixtures of very saline source fluids and meteoric
waters.
Conclusions
Volcanic fluids emitted at the surface in the southern Andes thus cover the full range from acid sulphate brines to neutral saline fluids, with the Tromen crater lake fluids as intermediate type-2 acid fluids.
REFERENCES
• Bermúdez A Delpino D., and López Escobar. L., 2002. Caracterización geoquímica de lavas y piroclastos holocenos del volcán
Copahue, incluyendo los originados en la erupción del año 2000. Actas de XV Congreso Geológico Argentino. El Calafate, Tomo
I, pp.: 377 – 382.
• Bermúdez A., Delpino D. and Loscerbo C., 2006. Anomalía termal en la cima del Volcán Tromen (37ºS - 70ºO), Provincia del
Neuquén, Argentina Geoacta (Revista de la Asociación Argentina de Geofísicos y Geodestas) Vol. 31:pp133 – 141.
• Delpino D. and Bermúdez A 2002. La Erupción del Volcán Copahue del año 2000. Impacto social y al medio natural. Neuquen.
Argentina, Actas de XV Congreso Geológico Argentino. El Calafate, Tomo III, pp: 365 - 370 Heinrich, C.H., 2007, Fluid-Fluid
Interactions in Magmatic-Hydrothermal Ore Formation, Mineralogical Society of America, Reviews in Mineralogy &
Geochemistry, 65, pp: 363-387
• Varekamp, J.C., Ouimette, A.P., Herman, S.W., Bermudez, A., and Delpino, D., 2001. Hydrothermal element fluxes from Copahue,
Argentina: a “beehive” volcano in turmoil. Geology: 29, pp1059-1062.
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Varekamp, J.C., Ouimette, A. and Kreulen. R., 2004. The magmato-hydrothermal system of Copahue volcano, Argentina.
Proceedings of the 11th Water Rock Interaction Symposium, (Eds Wanty, R.B. and Seal, R.R.) V.1, pp. 215-218, Balkema
Publishers, Leiden.
• Varekamp, J. C., 2008, The acidification of glacial Lake Caviahue, Province of Neuquén, Argentina. Special Issue Volcanic Lakes,
J, Volcanology and Geothermal Research, 178, pp.184-196.
• Varekamp, J.C., Herman, S., Ouimette, A., Flynn, K., Bermudez, A., and Delpino, D., 2009, Naturally acid waters from Copahue
volcano, Argentina. Applied Geochemistry, Spec. Issue on ‘Geogenic acid fluids’, 24 pp. 208–220
CERRO NEGRO DEL GHÍO: MAGMATISM IN-BETWEEN
SOUTHERN PATAGONIAN BATHOLITH AND
LAGO BUENOS AIRES PLATEAU LAVAS
2-02
Castro,J.1*, Sánchez, A.1 , Hervé, F.1, De Saint Blaquat M.2, Polvé M.2
(1) Universidad de Chile,Plaza Ercilla #803, Santiago Centro
(2) LMTG/Observatoire Midi-Pyrénées, Université de Toulouse, 14 av. Edouard-Belin, 31400
Toulouse, France
* Presenting author’s e-mail: jcioro@gmail.com
Introduction
In Patagonia, east of the South America-Nazca-Antarctica triple joint, there are several magmatic
units, being the Patagonian batholith, and plateau lavas the most important of them. Nevertheless, in
the region south of the Lago General Carrera/Buenos Aires, there are several satellite intrusive bodies,
Fig. 1 - Regional geological map. After Espinoza 2007 and Lagabrielle et al., 2007.
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with Pliocene to Miocene radiometric ages (Ramos, 2002). Several
works relate the Pliocene magmatism to an astenospheric window
generated by the subduction of the
Chile Rise beneath the South
American plate (e.g. Gorring et al.,
1997).
We present new petrographic and
geochemical data of Cerro Negro
del Ghío diorites, located between
the Chile Chico and Lago Buenos
Aires plateaus (10 km westward
meseta del Lago Buenos Aires
(MLBA), emplaced in volcanic
rocks of Jurassic Ibañez Group
(Fig. 1). It has a K/Ar determination in hornblende of 15.8 ± 0.6
Ma (Ramos, 2002).
The outcrops of the Miocene
Cerro Negro diorite are split in
two areas, separated by a little valley. The larger outcrop is round
and flat of 2 km in diameter and
with variable thickness, with a
maximum of ca. 100 m, it has a
tabular sill shape. The other outcrop is in the top off a cliff just in
face the sill and it has a lesser outcrop area and is highly affected
by weathering.
Fig. 2 - Geochemical diagrams. Showing in A: TAS classification (after
Cox et al., 1979, subalkaline-alkaline divide after Irvine and Baragar
(1971)) open symbols correspond to sill samples, filled symbol
correspond to the cliff sample.; B: trace element spidergram (Sun and
McDonough, 1989) and C: 87Sr/86Sr vs ÂNd diagram. In all diagrams
Southern Patagonian Batholith (SPB) projection data is shown (from
Hervé et al., 2007) for comparison. In A and B the analysed samples
clearly differ from the SPB by their more alkaline nature.
Petrographic features
The igneous bodies consist of
grey porphyritic rocks mainly of
pyroxene diorite. Four samples
were studied. The samples have
porphyritic textures with a 40% of
microphaneritic mass composed
of plagioclase, biotite, piroxene,
hornblende, magnetite, calcite and
glass.
The phenocrysts are mainly of
euhedral zoned plagiocase with 2
mm mean size. Some of them
have inclusions of fundamental
mass and re-absorption textures
with glass inclusions in the core
of the crystal. Also clinopyroxene
is a common mineral, they range
in size being bigger northward up
to 4 mm. They have a reaction
border composed of magnetite
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GEOSUR2010
and biotite. Locally they have ophitic texture. Biotite occurs as crystals less than 0.5 mm.
Cumulophyric texture is usual for biotite and plagioclase. Also green hornblende with size up to 5 mm
are common. They are zoned and/or in cumulophyric textures. Most of them have a biotite rim as a
reaction border. The hornblende together with the plagioclase present in some areas glomeroporphyric
textures.
Chemical features
The same samples described before, were used for geochemical analyses. They are chemically classified as syenodiorites (classification of Cox et al., 1979). They range in SiO2 from 58 to 60 wt.% they
are in the limit of alkaline/subalkaline fields (Fig. 2A)
Trace element diagrams (Fig. 2B) in general terms show similar features with the SPB including
negative anomaly in Nb and Ti, although not so strong. Furthermore Cerro Negro is richer in LILE
and LREE than SPB; the Ba/La ratio is >10 which is a characteristic signature of intraplate magmatism.
Isotopic ratios show positive ÂNd (+2-+3), near the mantle array, similar to MLBA transitional
basalts (Gorring et al., 1997) and also similar to the projection area of the Neogene SPB samples (Fig.
2C).
Discussion
As seen before trace elements pattern of Cerro Negro syenodiorites shows some differences with both:
those of MLBA main plateau lavas, which have OIB signature. (Guivel et al., 2006) and with BSP
which are arc derived (Hervé et al., 2007). Nevertheless they share chemical properties with both
magmatic units (e.g. Nb and Ti anomaly, La/Nb >1 TiO2 wt% <2 and positive ÂNd –SPB shows a
Jurassic to Neogene trend of <0 to >0 ÂNd). This allows us to interpret the Cerro Negro magmatism
as originated in a transition magmatic event, so much geochemical as temporarily, between the two
main igneous units.
Acknowledgements
We thank “Becas de Doctorado” Conicyt (A.Sánchez), Anillo Project ARTG-04, ECOS-CONICYT
C05U02 grant and the Ea. Sol de Mayo owner. Bruno Scalabrino field assistance is also appreciated.
REFERENCES
• Espinoza, F.; 2007: Evolución Magmática de la Región de Trasarco de Patagonia Central (47º S) durante el Mio-Plioceno. Ph.D
Thesis, Universidad de Chile, 195 p.
• Gorring. M., Kay, S., Zeitler, P., Ramos, V., Rubiolo, D., Fernandez, M., Panza, J.; 1997: Neogene Patagonian plateau lavas:
Continental magmas associated with ridge collision at the Chile triple junction. Tectonics, 16, 1-17.
• Guivel, C., Morata, D., Pelleter, E., Espinoza, F., Maury, R., Lagabrielle, Y., Polve, M., Bellon, H., Cotten, J., Benoit, M., Suárez,
M. and de la Cruz, R.; 2006: Miocene to Late Quaternary Patagonian basalts (46-47º S): Geochronometric and geochemical
evidence for slab tearing due to active spreading ridge subduction. Journal of Volcanology and Geothermal Research 149, 346-370
• Hervé, F., Pankhurst, R.J., Fanning, C.M., Calderón, M. and Yaxley, G.M.; 2007: The South Patagonian batholith: 150 my of granite
magmatism on a plate margin. Lithos, 97, 373-394
• Irvine, T. N. and Baragar, W. R. A.; 1971: A guide to the chemical classification of the common volcanic rocks. Canadian Journal
of Earth Sciences 8, 523–548.
• Lagabrielle, Y., et al.; 2007: Pliocene extensional tectonics in the eastern central Patagonian Cordillera: Geochronological
constraints and new field evidence. Terra Nova, 19, 413 – 424.
• Ramos, V.A.; 2002: El magmatismo neógeno de la Cordillera Patagónica. In M.J. Haller (ed.) Geología y recursos naturales de Santa
Cruz. XV Congreso Geológico Argentino (El Calafate) Relatorio I (13): 187-200, Buenos Aires.
• Sun, S. and McDonough, W.F.; 1989: Chemical and isotopic systematics of oceanic basalts; implication for mantle composition and
processes. Magmatism in the ocean basins, Spec Pub. Geol. Soc. London, 42, 313-45.
64
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MAGMATIC ACTIVITY AND STRIKE SLIP TECTONICS
IN THE SOUTHERNMOST ANDES: KRANCK PLUTON,
CHARACTERIZATION AND PRELIMINARY AMS SURVEY
2-03
Cerredo, M.E.1, Remesal, M. B.1, Tassone, A. A.2, Peroni, J. I.2, Menichetti, M.3, Lippai, H.2
(1) CONICET-Dpto. Geología, FCEN, UBA, Ciudad Universitaria, C1428EHA, Bs Aires, Argentina
(2) CONICET-INGEODAV,Dpto. Geología, FCEN, UBA, Ciudad Universitaria, C1428EHA, Bs Aires,
Argentina
(3) Istituto di Scienze della Terra, Università di Urbino, Campus Scientifico Universitario, 61029,
Urbino, Italy
Three major strike-slip structures of dominant WNW-ESE trend characterize the southernmost Andes,
from N to S: the Magallanes-Fagnano Fault System (which represents the onland boundary between
the South America and Scotia plates), the Carbajal Valley Fault System in central Tierra del Fuego and
the Beagle Channel Fault System which separates de Tierra del Fuego Island from the southern archipelago. The Neogene cinematic evolution of the Magallanes-Fagnano Fault System (MFS) is characterized by its transtensive nature with associated pull-apart basins both in onland and onshore areas
(Lodolo et al., 2003).
These major fault systems are near parallel to the main lineaments of the Late Cretaceous to Tertiary
Fuegian fold and thrust belt which suggests that the transtensional structures may have developed
along preexisting zones of weakness formed by crustal shortening (Klepeis and Austin, 1997).
The Jeujepen and Kranck plutons are located along the strike of MFS (Fig. 1A), the former at the
western termination of the Río Turbio fault segment and the latter at the eastern tip of the M. HopeCatamarca-fault segment (Fig. 1B). The alignment of intrusive bodies along the several transforming
Fig. 1 - A) General map showing the tract of Magallanes-Fagnano Fault System (MFS) in central Tierra del Fuego; stars
indicate the plutonic bodies located along the strike of MFS. B) Geological sketch depicting the main structures and
units in the area of Kranck Pluton (KP) within the fold-and-thrust Fuegian belt overprinted by strike-slip structures.
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structures of the southernmost Andes has already been reported as a causal relationship (i.e. the
Palaeocene Yamana Granite Suite in Chilean Tierra del Fuego, Cunningham 1993; the Ushuaia Pluton,
Peroni et al., 2009; the Jeu Jepen pluton, Cerredo et al., 2000).
The Jeujepen and Kranck plutons are hosted in the pelite/mudstone sequence of the Lower Cretaceous
Beauvoir Formation which was involved in the Andean compression. Presently available radiometric
data indicate a Late Cretaceous age (93 ± 4 Ma, K/Ar whole rock age) for the Jeujepen Pluton
(Acevedo et al. 2000).
The Kranck Pluton (KP) is located along the WNW-ESE left-lateral M. Hope-Catamarca fault, the
western strand of Magallanes Fagnano Fault system in Argentine Tierra del Fuego (Menichetti et al.,
2008). KP is affected by a set of transtensive faults which have down-thrown (with offsets of several
hundreds meters) the southern outcrops the intrusive body. These transtensional structures exhibit
many fault planes dipping at low angle and are superimposed on the north-verging thrust slices.
Although KP outcrops are restricted and largely covered by forest, aeromagnetic surveys have
revealed an outstanding anomaly with a subcircular pattern and average diameter of around 6 km.
Modeling of the aeromagnetic anomaly related to KP yielded a laccolithic body (Peroni et al., 2008 a
and b).
The KP is an epizonal intrusive body with a large compositional variation (cumular ultrabasic facies, gabbros, monzodiorite and monzonite facies and late stage syenite veins and dykes). Monzodiorite to monzonite facies are generally heterogeneous, they host mafic microgranular enclaves (several cm to dm in
size) either with typical crenulate outlines or as diffuse ghosts parallel to magmatic banding.
Preliminary chemical data indicates that KP shares with other Fuegian intrusions a dominant shoshonitic
nature. The magmatic series of amphibole ± clinopyroxene ± biotite-gabbro to syenite is metaluminous,
evolved from a highly hydrous melt. Characteristic Nb-Ta troughs on multi-element plots point to a subduction-related component in the petrogenesis of KP. The anticorrelation (Dy/Yb)N vs silica attests to significant amphibole fractionation along evolution. Accessory phases also played a role in liquid evolution,
i.e. apatite fractionation resulted in typical P troughs which is associated with a slight decrease in LREE
content along evolution. The characteristic Sr spyke on multi-element plots along with lack of significant
Eu anomaly for all lithologies suggests that no plagioclase fractionation has controlled significantly the
series evolution. This agrees with the H2O-rich nature of liquids which precludes plagioclase stability, and
restricts its crystallization to shallow upper crustal levels.
Deformation/crystallization relationships indicate a dynamic scenario for the emplacement and cooling evolution; synmagmatic foliations were recognized both at the meso- and microscales; not penetrative subsolidus medium- T microstructures parallel the submagmatic ones and both are variably
overprinted by brittle deformation.
A pilot AMS (anisotropy of magnetic susceptibility) survey of the different petrographic facies of KP
and its host was carried out. It comprised 11 sampling sites (eighty three cores), 7 within the central
part of the intrusive body and the remainder in the sedimentary host. The intrusive rocks show magnetic susceptibility (k) values falling in the ferromagnetic field (Tarling and Hrouda, 1993), between
2.7* 10-4 and 3.7 * 10-1 SI units, whereas
the host rocks have k values in the range
9.1* 10-5 to 4.5* 10-4 SI units, in the paramagnetic field.
Both groups display distinct trends in the k
vs. P´ plot (Fig. 2). The intrusive rocks
show fairly well clustered k values over a
restricted P´ range (1.03-1.15) and the host
rocks display a trend toward lower k values
within a similar range of anisotropy degree
(1.03-1.13). The AMS ellipsoid is oblate
(T>0) to neutral both in the intrusive as in
the host rocks.
Regardless of the involved lithology, both
in the marginal areas of the intrusion as
Fig. 2 - Î bulk (site average mean bulk susceptibility) vs P´
well as in the thermal aureole within the
(anisotropy degree) plot for each AMS sampling site.
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host the magnetic fabric is dominated by WNW/NW-ESE/SE vertical to steeply dipping (either to the
NE or the SW) foliations. When magnetic lineations are considered two populations emerge. One
shows K1 clusters along foliation strike with characteristic horizontal to very shallow plunge. The
other, restricted to the KP sites, displays magnetic lineations of high N-NE plunge (50-70º) clustered
in the down dip direction of foliation planes.
Within the central areas of KP a distinct magnetic fabric was acquired, it is characterized by subhorizontal foliation and no clustering of magnetic lineations. This fabric pattern corresponds to a monzodiorite facies rich in mafic (ultramafic) enclaves.
Preliminary AMS data on KP suggest a probable layered nature for the central part of the intrusion.
The prevailing WNW-ESE oriented, vertical magnetic foliations mimic the extensional component
inferred for the transtentive movements along the M. Hope-Catamarca fault, with horizontal magnetic lineations pointing to the stretching direction whereas the steep lineations might signal upsurges of
fresh magma. We preliminary infer that there would be some competence between the internal dynamics of pluton building and assembly and the externally imposed stress field.
REFERENCES
• Acevedo, R D., Roig, C.E., Linares, E., Ostera, H, Valín-Alberdi, M. and Queiroga-Mafra, Z.M. 2000 La intrusión plutónica del
Cerro Jeu-Jepén. Isla Grande de Tierra del Fuego, República Argentina. Cadernos Laboratorio Xeolóxico de Laxe, 25: 357-359
• Cerredo; M. E., Tassone, A. A., Coren, F., Lodolo, E. y Lippai, H. 2000. Postorogenic, alkaline magmatism in the Fuegian Andes:
The Hewhoepen intrusive (Tierra del Fuego Island). IX Congreso Geológico Chileno, Actas, (2): 192-196, Puerto Varas.
• Cunningham, W. D., (1993) Strike-slip faults in the southernmost Andes and the development the Patagonian orocline. Tectonics, v.
12 (1), 169-186
• Klepeis, K. A., and J. A. Austin Jr., 1997. Contrasting styles of superposed deformation in the southernmost Andes, Tectonics, 16:
755 – 776,
• Lodolo E., Menichetti M., Bartole R., Ben Avram Z., Tassone A. and Lippai H. 2003. Magallanes-Fagnano continental transform
fault (Tierra del Fuego, Southernmost South America). Tectonics 22 (6), 1076, doi:10.1029/2003TC001500
• Menichetti, M., Lodolo, E., Tassone, A. 2008. Structural geology of the Fuegian Andes and Magallanes fold and thrust belt: a
reappraisal. GeoSur Special Issue. Geologica Acta. Vol. 6 (1), 19-42.
• Peroni, J. I.; Tassone, A.; Lippai, H.; Menichetti, M.; Lodolo, E.; Vilas, J. F. 2008a. Estudio Geofísico Del Plutón Kranck. Tierra Del
Fuego. Argentina . Actas del XVII Congreso Geológico Argentino
• Peroni, J.; Tassone, A.; Menichetti, M; Lippai, H.; Lodolo, E.; Vilas, J.F. 2008b. Geologia e geofisica del plutone Kranck.
Rendiconti online della Societá Geologica Italiana. Roma: Societá Geologica Italiana, vol. 1: 132-136
• Peroni, J: I.; Tassone; A. A., Menichetti,M. and Cerredo, M. E. 2009. Geophysical modeling and structure of Ushuaia Pluton,
Fuegian Andes, Argentina. Tectonophysics, 476,(3-4), 25:436-449
• Tarling, D.H.; Hrouda, F. 1993. The Magnetic Anisotropy of Rocks. Chapman and Hall: 217 p. London.
CHARACTERIZATION OF AN ULTRABASIC LAMPROPHYRE
(EVOLVED DAMTJERNITE) IN THE TANDILIA BASEMENT,
SOUTHERNMOST RÍO DE LA PLATA CRATON, ARGENTINA
2-04
Dristas, J.A.1,2*, Martínez, J.C.1, Massonne, H.J.3, Wemmer K.4
(1) Universidad Nacional del Sur, Departamento de Geología and Ingeosur-CONICET
(2) CIC de la provincia de Buenos Aires, Argentina
(3) Institut für Mineralogie und Kristallchemie, Universität Stuttgart
(4) Geozentrum Göttingen, Universität Göttingen
* Presenting author’s e-mail: jdristas@criba.edu.ar
A small dyke of an ultrabasic lamprophyre was recently found in the Sierra Alta de Vela (SAV), east
of the town of Tandil, where basement of the southernmost Río de la Plata Craton is exposed. The wall
rock of this dyke is a variably deformed meta-igneous rock of granodioritic composition. The ultrabasic dyke, which is also partially deformed, consists mainly of zoned clinopyroxene (Cr-free diopside) and phlogopite (Cr-free, Ti-poor and Al-rich) phenochrysts. A scarce oxide phase is chromite.
The three mentioned minerals are considered to be primary. The groundmass is also dominated by Cr67
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free diopside, which occurs as micro-phenochrysts with aegerine-augite rims, and subordinate interstitial phlogopite altered to chlorite. Additional phases are calcite, chlorite, sericite, andradite-rich garnet, pure albite, epidote and apatite forming a partial “ocellar” interstitial texture. According to the
bulk rock chemistry, the dyke is a damtjernite (SiO2: 44.70- 46.45 wt. %). The Cr (550 ppm) and Ni
(160 ppm) contents as well the Mg number (64) point to a moderately evolved rock, though the Ce to
Pb ratio was found to be 44 (>25), but the contents of this rather mobile elements could have been
changed during the secondary processes. The Ti and K contents of the SAV damtjernite are low compared to typical damtjernites, but CO2, Ca, Al and LaN/YbN fit well world-wide occurrences of damtjernites (see Tappe et al., 2006 Journal of Petrology, 47 (7): 1261-1315). Also the Zr/Nb ratio of 7 satisfies the higher values for typical damtjernites compared to other ultrabasic dykes. The positive
anomaly of Dy in the REE pattern of the SAV-damtjernite may be due to the dominant presence of
clinopyroxene in the rock, which enhances the fractionation (higher mineral/melt partition coefficient)
of Dy from basic melts according to Fujimaki et al. (1984, J. Geophys. Res., 89, suppl. B662-B672).
The oscillatory zoning of the andradite-rich garnets reflects mainly the Fe3+-Al3+ substitution. This
phase probably formed at upper crustal levels. The corrosion of garnet (atoll texture) is also due to
late-stage processes. The absence of large quantities of Ti in andradite also points to a secondary origin of this phase.
We will continue our work in the Sierra Alta de Vela area by studying the probable presence of a other
lithological variety of this type of diatremites outcropping there and in other areas of Tandilia.
CONSTRAINTS ON OIB-TYPE PREMA AND EM1 MANTLE SOURCES
FROM TRACE ELEMENT AND PB, SR, AND ND ISOTOPIC RATIOS
OF PRIMITIVE EOCENE TO RECENT BACKARC PATAGONIAN BASALTS
2-05
Mahlburg K.S.1*, Jones, H,1, Gorring, M.L.2
(1) Dept of Earth Atmospheric Sciences, Cornell University, Ithaca, NY, 14853 USA
(2) Earth Studies, Montclair State Univ., Upper Montclair, NJ, 07043, USA
* Presenting author’s e-mail: smk16@cornell.edu
The South America Patagonian region between 36° and 52°S is the host to extensive and voluminous
Eocene to Recent backarc mafic volcanic flows that have erupted under neutral to mildly extensional
stress conditions. This widespread mafic magmatism is best attributed to a mantle that has been continuously on the verge of melting since before the Eocene. Major backarc melting events can be correlated with tectonic perturbations that vary in space and time. These perturbations include late
Miocene to Pliocene steepening of a shallow subduction zone in the north, Oligocene to early
Miocene trench roll-back in the middle and asthenospheric windows related to the Miocene to Recent
collision of the Chile Ridge with the Chile Trench in the south.
Clues to the general nature of the backarc Patagonian mantle come from trace element and isotopic
signatures on Eocene to Recent primitive to near primitive alkaline to sub-alkaline lavas (44-53%
SiO2) chosen from across the region based on their high Cr (>200 ppm), Ni (>130 ppm) and MgO
(>7%) contents. Evidence that crustal contamination plays little or no role in perturbing their mantle
signatures comes from trace element analyses, as plots of Th/La versus Ta/U and Ce/Pb versus Nb/U
show the samples falling in or near fields for MORB and OIB lavas. Overall, their trace elements
show a generally OIB-like character with little to no slab influence as indicated by relatively low ratios
of La/Ta (7.7 and 21.5), Sr/Ta (180 to 800), Ba/Ta (60-560) and Th/Ta (0.8-3.6).
Comparisons of Nd, Sr and Pb isotopic measurements of the most primitive samples with globally
defined mantle reservoirs on diagrams from the W.M. White website (Cornell Univ.) show that
Patagonian basalts dominantly have OIB type signatures that generally fall between those of PREMA
(prevalent mantle) and EMI (enriched mantle with lithospheric and lower crust type tendencies) man68
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tle reservoirs. Measurements on 30 samples yield 87Sr/86Sr from 0.7031 to 0.7049 and 143Nd/144Nd
from 0.51288 to 0.51254 (?Nd = -0.2 to +5) with measurements on 19 yielding 206Pb/204Pb from 18.2
to 19.1, 207Pb/204Pb from 15.5 to 15.7 and 208Pb/204Pb from 38.2 to 39.3. The ratios plot between
PREMA and EMI mantle reservoirs on ?Nd-87Sr/86Sr and 87Sr/86Sr-206Pb/204Pb diagrams, falling on
the low 87Sr/86Sr side of South Atlantic Tristan/Walvus/Gough and Indian Kerguelen oceanic basalt
fields. Pb ratios plot in the regions of these same oceanic islands, overlapping Indian MORB in the
PREMA 206Pb/204Pb range. Both 207Pb/204Pb and 208Pb/204Pb generally plot above the Pacific MORB
field consistent with a role for recycled continental material.
Overall, a general PREMA-like mantle source with variable tendencies towards EM1 in the
Patagonian mantle is consistent with the consequences of ridge collision, contributions from depleted backarc mantle above a subducting slab, recycled oceanic slabs from the long subduction history
along the Patagonian margin and recycling of continental lithosphere at the South American margin
by subduction erosion. The same processes have played a role in shaping the chemical signatures of
South Atlantic and Indian Ocean hotspot magma mantle sources.
SERRA GERAL VOLCANISM IN THE PROVINCE OF MISIONES (ARGENTINA):
2-06
GEOCHEMICAL ASPECTS AND INTERPRETATION OF ITS GENESIS
IN THE CONTEXT OF THE LARGE IGNEOUS PROVINCE PARANÁ-ETENDEKA-ANGOLA.
ITS RELATION WITH THE ALKALINE VOLCANISM OF CÓRDOBA PROVINCE
Lagorio, S.L.1*, Vizán, H.2
(1) Servicio Geológico Minero Argentino (SEGEMAR). J. Roca 651, piso 10 – Ciudad Autónoma de
Buenos Aires
(2) Departamento de Ciencias Geológicas – Facultad de Ciencias Exactas y Naturales (UBA –
CONICET). Ciudad Universitaria, Pab. II. Ciudad Autónoma de Buenos Aires
(*) Presenting author’s e-mail: slagor@minplan.gov.ar
New geochemical data from Serra Geral basalts of Misiones Province (Argentina) are complementary of
the already published data of the Large Igneous Province (LIP) Paraná-Etendeka-Angola (PEA; e.g.
Piccirillo and Melfi, 1988; Peate, 1997; Marzoli et al., 1999; see Fig. 1a and b), displaying the typical
tholeiitic nature. The volcanic rocks from Misiones belong either to the high- or low-Ti varieties; the coexistence of both types in the present sampling, agrees with the fact that this area belongs to the central and
southern regions of the Paraná Magmatic Province. Paranapanema, Ribeira, Gramado, Pitanga and
Urubici varieties were recognized, being the former the most abundant in the collected rocks. Urubici type
sample, from San Ignacio area, is the westernmost occurrence of this variety at this latitude. Chemical
data point out that magmas of high- and low-Ti were originated from different sources, and evolved
through fractional crystallization under low pressure conditions, involving significant crustal contamination only in the Gramado magma type.
Heterogeneity in the magma, on small and large scales, is in agreement with a subcontinental lithospheric mantle source. Geochemical features, particularly the Nb-Ta negative anomaly in the multi-elemental
diagram normalized to primitive mantle as well as some ratios (e.g. La/Nb and Zr/Ta) point out significant differences in relation to alkaline volcanic rocks from the called plume Tristan da Cunha, as mentioned also by other authors (e.g. Ernesto et al., 2002).
Geochemical data, particularly the Nb-Ta negative anomaly shown by tholeiites from Misiones and from
the whole PEA LIP, would be related to ancient subduction processes (e.g. Transamazonian and Brasiliano
Precambrian Events) that metasomatized the mantle source as mentioned by other authors also considering isotopic data (e.g. Iacumin et al., 2003). This is also supported by Nd model ages (e.g. Comin
Chiaramonti and Gomes, 2005) and Hf model ages (Santos et al., 2008) also involving Grenvillian Events.
The great heat budget involved in mantle melting might correspond to a thermal blanketing caused by
69
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Fig. 1 - a) Sketch-map showing the Early Cretaceous tholeiitic magmatism of Large Igneous Province (LIP) ParanáEtendeka-Angola Province (PEA). b) Sketch-map of PEA and Sierra Chica de Córdoba (SCC) alkaline locality in a
reconstruction at nearly 130 Ma (after Marzoli et al., 1999, Lagorio, 2008 and therein references). RPC = Río de la
Plata Craton, SFC = San Francisco Craton, AC = Amazonia Craton, CC = Congo Craton, RAC = Río Apa Craton, MB
CB = Córdoba Mobile Belt, MB FM = Dom Feliciano Mobile Belt, MB R-SI = Rondonia-San Ignacio Mobile Belt.
ancient Pangaea. The location of the main melting zone in the lithospheric mantle must have been
determined by the effects of the hot uprising limb of a possible large scale convection that affected
zones of crustal weakness (e.g. sutures between former cratons).
On the other hand, volcanism of the sierra Chica de Córdoba (SCC, Fig. 1a and b), nearly coeval with
that of the PEA LIP, is alkaline of high-Ti (Lagorio, 2008) and display a peripheral location in relation with the great heat source mentioned above. This is consistent with the involved lower melting
degrees of the volcanism of SCC in relation to those of PEA LIP. Melting could have been triggered
by a small scale process like edge-driven convection resulting from the thickness contrast between Rio
de la Plata Craton and Pampia terrane, as is shown in Fig. 2 of Vizán and Lagorio, this volume.
REFERENCES
• Comin-Chiaramonti, P. and C.B. Gomes (Eds), 2005. Mesozoic to Cenozoic alkaline magmatism in the Brazilian Platform.
Edusp/Fapesp, San Pablo. pp 750.
• Ernesto, M., L.S. Marques, E.M. Piccirillo, E.C. Molina, N. Ussami, P. Comin-Chiaramonti and G. Bellieni, 2002. Paraná Magmatic
Province-Tristan da Cunha plume system: fixed versus mobile plume, petrogenetic considerations and alternative heat sources.
Journal of Volcanology and Geothermal Research, 118: 15-36.
• Iacumin, M., A. De Min, E.M. Piccirillo and G. Bellieni, 2003. Source mantle heterogeneity and its role in the genesis of Late
Archean-Proterozoic (2.7-1.0 Ga) and Meozoic (200 and 130 Ma) tholeiitic magmatism in the South American Platform. EarthScience Reviews, 62: 365-397.
• Lagorio, S.L. 2008. Early Cretacous alkaline volcanism of the Sierra Chica de Córdoba (Argentina): Mineralogy, gochemistry and
petrogenesis. Journal of South American Earth Sciences 26(2): 152-171.
• Marzoli, A., L. Melluso, V. Morra, P.R. Renne, I. Sgrosso, M. D’Antonio, L. Duarte Morais, E.A.A. Morais and G. Ricci, 1999.
Geochronology and petrology of Cretaceous basaltic magmatism in the Kwanza basin (western Angola), and relationships with the
Paraná-Etendeka continental flood basalt province. Journal of Geodynamics, 28: 341-356.
• Peate, D.W., 1997. The Paraná-Etendeka Province. In: Large Igneous Provinces: Continental Oceanic and Planetary Flood
Volcanism. Mahoney, J.J. and M.F. Coffin (Eds). Geophysical Monograph, 100. American Geophysical Union. 215-245.
• Piccirillo, E.M. and A.J. Melfi (Eds), 1988. The Mesozoic Flood Volcanism from the Paraná Basin (Brazil): Petrogenetic and
Geophysical Aspects. Universidad de San Pablo, San Pablo. pp 600.
• Santos, J.O.S., W. Wildner, L.A. Hartmann, W.L. Griffin y N.J. McNauthton, 2008. Lower Cretaceous U-Pb age and Grenvillian Hf
model-age of the large Serra Geral magmatism of Paraná basin, South America. 6° South American Symposium on Isotope Geology.
Abstracts: 90, Bariloche.
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LITHOLOGY AND AGE OF THE CUSHAMEN FORMATION.
DEVONIAN MAGMATISM IN THE WESTERN
NORTH PATAGONIAN MASSIF. ARGENTINA
2-07
López de Luchi, M.G.1*, Cerredo, M.E.2, Martínez Dopico, C.1
(1) CONICET-INGEIS, Pabellón INGEIS, Ciudad Universitaria, C1428EHA, Bs Aires, Argentina
(2) CONICET-Dpto. Geología, FCEN, UBA, Ciudad Universitaria, C1428EHA, Bs Aires, Argentina
* Presenting author’s e-mail: deluchi@ingeis.uba.ar
Introduction
Accurate temporal constraints for the metamorphic peak conditions, granitoid emplacement, deformational events and cooling paths are a key issue for unraveling the tectono-metamorphic history of
basement complexes. One of the greatest difficulties in Rb-Sr dating on metamorphic rocks is the lack
of evidence of complete re-homogenization and the assumption of a common initial Sr ratio.
Magmatic units that were emplaced at different stages of the metamorphic evolution as indicated by
deformation features could be satisfactorily dated with this method.
The pre Jurassic metamorphic basement of the eastern foothills of the Main Cordillera along the western border of the North Patagonian Massif (NPM) is composed of metamorphic and igneous complexes. The former is represented by the Cushamen Formation (CF) which forms a poorly exposed N-S
trending belt from Catan-Lil River (39º 45´S/ 70º 36´W, Neuquén province) in the North down to
Leleque in the South. The general tracks of the regional metamorphism in CF had been reported for
some areas (Volkheimer 1964, Varela et al. 1991, Cerredo 1997, Franzese et al. 1992, Cerredo and
López de Luchi 1998, Giacosa et al. 2004, Lucassen et al. 2004, López de Luchi et al. 2005, Von
Gosen 2009). Ostera et al. (2001 and references therein) proposed an Early- Middle Devonian metamorphic event probably associated with a collisional-accretional episode for rocks located west of Río
Chico and in Colonia Cushamen (Fig. 1) whereas Hervé et al. (2005) proposed a 335 Ma -earliest
Permian metamorphism based on U-Pb SHRIMP dating of magmatic rims in zircon from an inferred
medium grade metaclastic unit located in the Puesto Miranda area (Fig. 1). U-Pb SHRIMP dating of
amphibolite grade paragneiss from south of El Maitén showed zircons with metamorphic overgrowths
at 330, 340 and 365 Ma, and zircon cores with a major provenance at 440 Ma (Pankhurst et al. 2006).
These authors considered that the protoliths were continental margin sediments that hosted their
inferred Carboniferous arc. In more regional perspective a migmatite from San Martín de los Andes
(40º09´S-71º21´W) yielded an internal Rb–Sr isochron of 368± 9 Ma whereas U–Pb age determinations of four concordant fractions of titanite of a calc-silicate rock from a gneiss/migmatite sequence
near Piedra del Aguila (40º02´S-70º04´W) indicate 375 ± 15 Ma (207Pb/235U) and 380± 2 Ma
(206Pb/238U) (Lucassen et al. 2004). This age of ca. 380 Ma is interpreted as the age of crystallization
of titanite, close to the metamorphic peak.
This paper focuses on the recalculation of whole rock Rb/Sr isochrons based on lithological and
chemical analysis of the different lithologies that make up the CF at the Río Chico-Cushamen and El
Maitén-Leleque areas (Fig. 1). These data are used to constrain the metamorphic evolution and the
emplacement of the granitoids in relation with deformational phases at the SW corner of the North
Patagonian Massif.
Geological background
The metamorphic series of CF is made up of metasedimentary and metaigneous rocks; the former
includes metapelitic, metagreywackes, minor quartz-rich sandstones, metaconglomerates, calcsilicates and sparsely interbedded tourmaline bearing schists. Magmatic additions accompanied the
entire CF evolution since the pre-metamorphic stage. Near Río Chico (Fig. 1) bimodal volcanism
emplaced in the CF sedimentary basin is represented by rare thin dikes and layers of dacitic and
basaltic composition (López de Luchi et al. 2002). Variably foliated tonalitic to leucogranitic rocks
(ranging from pegmatite through aplite textures) occurring as sheets, veins and dykes within the metamorphic series have been reported for the whole CF belt. The metamorphic series display a complex
structural evolution characterized by several deformation phases (Franzese et al., 1992; Cerredo,
1997; Cerredo et al., 2002; Giacosa et al., 2004; von Gosen, 2009). The two older deformations, D1
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Table 1 - Lithologies and representative mineral assemblages of Cushamen Formation (summary from Cerredo, 1997;
Giacosa et al., 2004; von Gosen, 2009). Mineral symbols after Kretz, 1983.
Mineral assemblagles
Metapsammites/m
etapelites
Chlorite zone
Chlorite-Garnet
zone
Muscovite-Garnet
zone
AluminosilicatePlagioclase zone
Calcsilicate
TourmalineMetavolcanic Metagranitoid
bearing schists
rocks
s
Chl-Ms-Ab
Chl-Ms-Ab
Chl-Ms-Bt-Grt-Olig
Tr-Ep-Chl
Ms-Bt-Grt-Olig
Cc-Scp-Di-Qtz
Tur-Bt-Pl
Pl-Bt
Sil-Pl-Tur
Hb-Bt-Sph
Bt-And-Qtz
and D2, produced foliations and have been recognized in the whole CF belt and interpreted as part of
the prograde metamorphic evolution, whereas the later D3 and D4 phases have only been reported for
the northern areas (from Comallo to Río Chico, Fig. 1) and interpreted to accompany the retrograde,
uplift path of CF (Cerredo and López de Luchi, 1998; von Gosen, 2009). Several episodes of synkynematic and intertectonic magmatic additions accompanied the D1-D3 evolution of CF (Giacosa et al.
2004, López de Luchi and Cerredo, 2007, Von Gosen, 2009).
The metamorphic series of CF underwent a medium-pressure regional metamorphism ranging from
low-greenschist facies to upper-amphibolite facies. Table 1 summarizes the mineral assemblages.
Major element chemical data point to an active continental margin as a likely setting for the sedimentary protoliths of CF, which bear compositional signatures suggesting their provenance from felsicacid plutonic and volcanic detritus as well as recycled mature polycyclic quartzose detritus (unpublished data). Metagranitoids are calc-alkaline, peraluminous Bt-tonalites to Ms (± Bt ±Grt)-granites
to leucogranites.
Geochronological constraints
Metasedimentary rocks (Ms-Grt schists) did not produce isochrons since MSWDs are higher than 10
and errors than 20% which indicate the lack of re-homogenization for the Rb-Sr system.
Metagranitoids were separated in groups on the basis of their chemical composition and relationship
with the prograde deformation phases D1-D2.
A main group mostly represented at Cañadón La Angostura and at Pto. Miranda areas (Fig. 1) is made
up by foliated Bt-tonalite to granite which underwent D1-D2 and are characterized by 87Sr/86Sri > 0.710
either at 370 or 330 Ma. These rocks yielded a Rb-Sr WR isochron of 373±15 Ma, 87Sr/86Sri= 0.71130,
MSWD = 4.6. If only the isotopic results for Cañadón La Angostura area are considered a Rb-Sr WR
isochron of 374.4±7.3 Ma, 87Sr/86Sri=0.71129, MSWD = 0.76 is obtained. If granites are separated
from tonalite-granodiorite the error increase and no isochron is obtained.
Rocks with calculated 87Sr/86Sri < 0.710 either at 370 or 330 Ma from Leleque and Pto. Miranda area
yielded a Rb-Sr WR isochron of 323±26 Ma, 87Sr/86Sri=0.71079, MSWD=2.
Discussion
The obtained isochrons correspond to igneous rocks therefore the age of the metamorphic peak has
to be interpreted based on the analysis of the timing of the emplacement of the magmatic units in relation with the metamorphic evolution.
A minimum age for the metamorphism ca 370 Ma could be inferred based on the isochron for the
granitoids the area Río-Chico-Cushamen which underwent the D1-D2 deformation. In the area of
Laguna del Toro the 372 Ma Cáceres orthogneiss (Pankhurst et al., 2006) is emplaced in a metamorphic pile which exhibit similar lithologies than the CF. (Fig. 1). Recalculation of the U-Pb data for
metamorphic rims of zircons of El Maitén paragneiss (Pankhurst et al., 2006) indicate a peak between
360-380 Ma and subordinate peak at 330-340 Ma. Results extracted from the analysis of the relativi72
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Fig. 1 - Geological sketch of the main outcrops of the Cushamen Formation
ty probability vs. age plot of the zircons of the Pto. Miranda area (Hervé et al., 2005) indicate that
only three data have 206Pb/238U ages younger than 350 Ma. In this connection, we want to highlight
that our new isochron enlarges the group of Late Devonian intrusions occurring along the western border of the NPM. From San Martin de los Andes in the north to the area of Colonia Cushamen in the
south, several ~400-360 Ma (Varela et al., 2005; Pankhurst et al., 2006;) deformed tonalitic to granitic
intrusions show isotopic signatures indicating to crustal sources (Sr/Sri > 0.710, eNd < -4). Such a
widespread episode of crustal melting might result either within a thickened chemically closed crust
or within a crust of normal thickness with advective heat contributions in the form of mantle derived
melts. Current scenarios for the Devonian in the western NPM propose a magmatic arc setting
(Pankhurst et al., 2006) whereas the Devonian tectonomagmatic evolution has been also related to the
Chanic movements of Precordillera (Varela et al 2005). The lack of complete chemical data is a severe
restriction to properly constrain the meaning of this magmatism.
Our younger Rb-Sr WR isochron age ca 320 Ma is close to the reported U-Pb SHRIMP 329 Ma age
for intermediate rocks of the Tonalite El Platero and Cordón El Serrucho (Pankhurst et al. 2006).
Tonalite El Platero intrudes CF south of Pto Miranda. Giacosa et al. (2004) mentioned that the NW
planar fabric of the tonalite is subvertical whereas the remaining igneous and clastic rocks of CF
exhibit a shallower dipping parallel NW fabric. Rocks at Cordón el Serrucho belong to a belt of foliated calc-alkaline rocks which were interpreted as syn to late tectonic regarding the deformation of its
host metamorphic rocks. On the contrary Pankhurst et al. (2006) proposed that the ca. 330 Ma represent a subduction event developed along the southwestern margin of the NPM.
Furthermore in the area of Chacay Huarruca, immediately north of Cañadón La Angostura (Fig. 1)
Varela et al. (2005) obtained a 206Pb/238U lower intercept at 302±39 Ma for a leucogranite which
intrudes the CF. Pankhurst et al. (2006) reported U-Pb SHRIMP ages for S-type peraluminous Grtbearing leucogranites, 314±2Ma from Paso del Sapo and 318± 2Ma from Sierra de Pichiñanes south
of the studied areas. Both were interpreted as crustal granitoids emplaced in a syn-collisional setting
after the ca. 330 Ma subduction.
Therefore we propose that at least a part of CF underwent a Devonian metamorphic event and was
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intruded by a series of granitoids with different chemical signatures probably emplaced in different
tectonic settings. Accurate interpretation of the timing and meaning of penetrative fabric in these
granitoids needs further study.
REFERENCES
• Cerredo, M.E., López de Luchi, M.G., 1998. Mamil Choique Granitoids, southwestern North Patagonian Massif, Argentina:
magmatism and metamorphism associated with a polyphasic evolution. J. South Amer Earth Sciences, 11(5): 499-515
• Cerredo, M.E., 1997. The metamorphism of Cushamen Formation, Río Chico area. North Patagonian Massif. Argentina. 8ºCong.
Geol. Chileno, Actas 2:1236-1240, Antofagasta.
• Cerredo, M E.; Parica, C.A., Remesal, M.B. 2002. Facies de alto grado de la Formación Cushamen en Aguada del Pajarito, Macizo
Norpatagónico. Chubut. 6º MinMet, Actas: 85-88, Buenos Aires
• Dalla Salda, L., Varela, R., Cingolani, C., Aragón, E., 1994. The Río Chico Paleozoic Crystalline Complex and the evolution of
Northern Patagonia. J. South American Earth Sciences 7 (3-4), 377-386
• Franzese, J., Días, G., Dalla Salda, L., 1992. Las estructuras de las Metamorfitas Cushamen, Provincia de Chubut. VI Reunión
Microtectónica. Acad. Nac. Cs Ex, Físicas y Naturales. Monografías, 8: 27-30.
• Giacosa, R., Márquez, M., Nillni, A., Fernández, M., Fracchia, D., Parisi, C., Afonso, J., Paredes, J., Sciutto, J. 2004. Litología y
estructura del basamento ígneo-metamórfico del borde SO del Macizo Nordpatagónico al oeste del río Chico. Revista de la
Asociación Geológica Argentina, 59 (4): 569-577
• Hervé, F., Haller, M.J., Duhart, P., Fanning, C.M. 2005. SHRIMP U-Pb ages of detrital zircons from Cushamen and Esquel Fms,
North Patagonian Massif, Argentina, 16º Cong Geol Arg, Actas I: 309-314
• Kretz, R. 1983. Symbols for rock-forming minerals. American Mineralogist, 68: 277-279
• López de Luchi, M.G., Cerredo, M.E., 1997. Paleozoic basement of the southwestern corner of the North Patagonian Massif: an
overview, 8º Cong. Geol. Chileno, Actas 3: 1674-1678, Antofagasta
• López de Luchi, M.G., Ostera, H., Cerredo, M.E., Cagnoni, M., Linares, E. 2000. Permian magmatism in Sierra de Mamil Choique,
North Patagonian Massif. 9º Cong. Geol. Chileno, Actas 2: 750-754, P Varas.
• López de Luchi, M.G, Ostera, H., Cagnoni, M., Cerredo, M.E., Linares, E., 2002. Geodynamic setting for the western border of the
North Patagonian Massif: Cushamen Formation at Río Chico, Río Negro. In: Cabaleri, N., Linares, E., López de Luchi, M.G.,
Ostera, H., Panarello, H. (Eds). 15º Cong. Geol. Arg., Actas 2: 210-216.
• López de Luchi, M.G., Cerredo, M.E., Wemmer, K. 2006. Time constraints for the tectonic evolution of the SW corner of the North
Patagonian Massif. 5º S Amer Symp Isotope Geol, Actas: 114-118, Pta del Este.
• López de Luchi, M. G., Cerredo, M. E. 2008. Geochemistry of the Mamil Choique granitoids at Rio Chico, Río Negro, Argentina:
Late Paleozoic crustal melting in the North Patagonian Massif, J. South Amer Earth Sciences, 25, (4): 526-546
• Lucassen, F., Trumbull, R., Franz, G., Creixel, C., Vázquez, P., Romer, R., Figueroa, O., 2004. Distinguishing crustal recycling and
juvenile additions at active continental margins: the Paleozoic to Recent evolution of the Chilean Pacific margin (36º-41ºS). J. South
Amer Earth Sciences 17, 103–119.
• Ostera, H.A., Linares E., Haller, M.J., Cagnoni, M.C., López de Luchi, M.G., 2001. A widespread Devonian metamorphic episode
in Northern Patagonian. In: Tomlinson, A. (Ed.), 3º S Amer Symp. Isotope Geol., Abb. Abstr. Vol, Edición Especial Revista
Comunicaciones, 52 and CD edition. Pucon
• Pankhurst, R.J.; Rapela, C.W., Fanning, C.M., Márquez, M. 2006. Gondwanide continental collision and the origin of Patagonia,
Earth-Science Reviews 76: 235–257
• Varela, R., Dalla Salda, L.H., Cingolani, C., Gómez, V. 1991. Estructura, petrología y geocronología del basamento de la Región
del Limay, provincias de Río Negro y Neuquén. Rev Geol. Chile, 18: 147-163.
• Varela, R., Basei, M., Cingolani, C.A., Siga, O., Passarelli, C., 2005. El basamento cristalino de los Andes Norpatagónicos en
Argentina: Geocronología e interpretación tectónica. Rev Geol. Chile 32: 167–187.
• Volkheimer, W., 1964. Estratigrafía de la zona extrandina del Dpto. Cushamen (Chubut) entre los paralelos 42º y 42º 30´ y los
meridianos 70º y 71º. Revista de la Asociación Geológica Argentina, 19 (2): 85-107
• von Gosen, W. 2009. Stages of Late Paleozoic deformation and intrusive activity in the western part of the North Patagonian Massif
(S. Argentina) and their geotectonic implication. Geol Mag. 146: 48-71
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METAMORPHIC EVOLUTION OF THE CINCO CERROS AREA,
SIERRA DE TANDIL, ARGENTINA
2-08
Massonne, H.J.1*, Dristas, J.2, Martinez J.C.2
(1) Institut für Mineralogie und Kristallchemie, Universität Stuttgart, Azenbergstr. 18, D-70174
Stuttgart, Germany
(2) Departamento de Geología, Universidad Nacional del Sur, Bahia Blanca, Argentina
* Presenting author’s e-mail: h-j.massonne@mineralogie.uni-stuttgart.de
The metamorphic basement of the WNW-ESE-trending Tandilia range in the Buenos Aires Province
represents the southernmost extension of the Rio de la Plata craton and consists mainly of metaigneous rocks. However, garnet-bearing metapelites occasionally crop out as well. These rocks have a
much higher potential for the derivation of metamorphic P-T conditions than the garnet-absent, metaigneous rocks. For this reason, we have studied metapelites of the Cinco Cerros area in the northeastern Tandilia range. In this area various lithologies occur which partially show migmatitic structures.
However, gneiss sample 06181109 does not show any evidence for partial melting. Garnet in this sample is slightly zoned with core and rim compositions of pyrop17gross6spess2alm75 and
pyrop13gross5.5spess1.5alm80, respectively. The Mg to Mg+Fe ratio of biotite is around 0.49. The plagioclase composition is Ab65An34Kf1. No cordierite, staurolite, and Al2SiO5-phase were noted to
occur. We used the PERPLE_X computer software package to calculate a P-T pseudosection for
metapelite 06181109. The calculated pseudosection was contoured by isopleths of various parameters
especially molar fractions of garnet components. According to the latter calculation results we derived
P-T conditions of 6.5 kbar and 670°C for an early metamorphic stage. Subsequently, a pressure release
occurred at decreasing temperatures. The final metamorphic P-T conditions recorded by the studied
rock are 4.5 kbar and 600°C. The corresponding P-T path is compatible with the observed mineral
assemblage and the fact that in rocks adjacent to that of sample 06181109 partial melting occurred.
We achieved 16 analyses of monazite in this sample with the electron microprobe for age dating. The
obtained ages from 11 analyses of Th-bearing monazite scatter around 2.02 Ga, whereas 5 almost Thfree monazites gave ages around 1.82 Ga. Thus, the metamorphic event can be in any case related to
the Transamazonian cycle.
As our study area is close to the margin of the Rio de la Plata Craton, where abundant magmatic-arc
derived plutonic rocks outcrop, we interpret the derived P-T data as follows: An early metamorphic
event (6.5 kbar, 670°C) of the Transamazonian cycle could have resulted from underplating of magmas of a magmatic arc. This event was followed by relatively slow exhumation, as we observed a significant cooling and, thus, thermal relaxation during a pressure release of about 2 kbar. Possibly, only
superficial erosion caused the exhumation of the studied rocks at this metamorphic stage. On the basis
of a study of metamorphic rocks from an area nearby our study area, Delpino and Dristas proposed
thinning of the crust possibly by the formation of a marginal back-arc basin. However, these authors
deduced a somewhat different P-T path starting at 750-800°C and 5-6 kbar and ending at 450-500°C
and 5.5-6.5 kbar. This demonstrates the necessity to derive P-T paths as precise as possible in order
to relate metamorphic processes of a study area to the right geodynamic model. For this reason, we
will continue our work in the Cinco Cerros area by studying the complete lithological variety outcropping there.
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PALEOMAGNETIC STUDIES OF CENOZOIC BASALTS
FROM NORTHERN NEUQUÉN AND SOUTHERN MENDOZA PROVINCES:
STRATIGRAPHIC IMPLICATIONS
2-09
Re, G.H(*)1, Vilas, J.F.2
(1) INGEODAV (Instituto de Geofísica Daniel A. Valencio) Dpto. Ciencias Geológicas - FCEyN (UBA).
(2) CONICET - INGEODAV (Instituto de Geofísica Daniel A. Valencio) Dpto. Ciencias Geológicas FCEyN (UBA.
*Presenting author’s e-mail: indio@gl.fcen.uba.ar
Introduction
The Cenozoic basaltic sequences of Neuquén have been thoroughly studied, in terms of their regional
geological, paleomagnetic and geochronological features. The distribution and chemical composition
of the Paleogene and Neogene volcanism in north western Neuquén and southern Mendoza are
controlled by several factors, such as: i- changes in the speed and direction of convergence between
the oceanic plate (the Farallón and later Nazca plate) and the continental plate (South American plate);
ii- the inclination change in the Wadati-Benioff zone; iii- the age of the subducted oceanic plate; and
iv- the thickness of the overriding plate (Kay et al., 1988; Ramos and Barbieri, 1988). Noteworthy, the
variation of such parameters produced different magmatic arcs, which formed during the following
time intervals: Paleocene-Eocene, Oligocene, middle to late Miocene, late Miocene - Pliocene, and
Fig. 1 - a) Left: satellite image, b) Right: map, northwestern Mendoza and southern Neuquén map with the location of
the sites that were studied.
A
B
C
Fig. 2 - Photographs of chalcographic polished: A) titanomagnetite with exsolution to ilmenite, B) titanomagnetite
altered to hematite, C) a: titanomagnetite to hematite at the edges, b: and c: hematite to pseudobrookite
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Pleistocene-Holocene.
The volcanic centers are located
between the precordilleran blocks
which are separated by structures
feature an extensional intra-arc setting
during the Pliocene and Quaternary.
The volcanism in the back-arc area, on
the other hand, consists of extensive
basaltic mantels, with volcanic cones
that set either along fractures or
dispersed with no apparent structural
control (Bermudez et al., 1993). In
general, they present low grade of
chemical differentiation and significant
compositional homogeneity.
Fig. 3 - Results of the IRM studies and back-field sample locations
of Arroyo Rucachoroy (Ar), Junin de los Andes (Ja) and Puesto
Quiroga (Pq).
Studies of basalt in northern
Neuquen and southern Mendoza
Systematic paleomagnetic samplings
were carried out in various volcanic
units cropping in north western
Neuquén and southern Mendoza (Fig.
1), in order to establish a detailed
chronology of the volcanic activity
from the late Oligocene to the
Pleistocene. The study area is located
to the east of the current volcanic front,
characterized by intra-arc and back-arc
volcanism. Geochronological and
paleomagnetic results from 11
localities are presented, which
comprise the Rancahué Michacheo and
Hueyeltué Formations, the informally
“Basalto Zapala”, Basalto Macho
Viejo, Basalto Los Mellizos and the
“Chapualitense inferior”. These
correspond to alkaline-type basalts,
conformed
by
olivine
and
clinopyroxene phenocrysts immersed
in a groundmass composed by
plagioclase, clinopyroxene, olivine and
opaque minerals (e.g. magnetite and
hematite) microlites. In general, they
present porphyric to sub-ophytic
textures in intergranular groundmass,
with olivine phenocrysts. The
groundmass is mainly composed by
euhedral
plagioclase
microlites
arranged in a fluid way, together with
small crystals of clinopyroxene
somewhat altered to chlorites and
opaque minerals, and scarce olivine
crystals. Occasionally, plagioclase
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Fig. 4 - Hysteresis loops of samples from the localities of Arroyo
Rucachoroy (Ar), Junín de los Andes (Ja) and Puesto Quiroga (Pq).
HYS = without eliminating the contribution of paramagnetic minerals,
SLO = with elimination of the contribution of paramagnetic minerals
cores look propylitized altered
to chlorites and oxides.
Polished sections studies
allowed
establishing
the
occurrence of titanomagnetites
with exsolution to illmenite and
in some cases to hematite.
Likewise, hematite can be
occasionally dismissal to
illmenite, pseudobrookite and
titanohematite (Fig. 2).
From studies of acquisition
of Isothermal Remanent
Magnetization (IRM) and
back-field of the IRM (Fig. 3), it
was interpreted that the main
carrier would be mainly
magnetite
and
secondly,
hematite. Hysteresis loops
(Fig. 4) in ambient temperature
and low temperatures (190°C) established on the
other hand, that occasionally,
samples yield Bcr (magnetic
coercivity remanence) / Bc
(coercivity field) ratios of ~ 4
that is indicative of mixtures,
and relatively high Jr/Js rations
=0.3, suggestive of a distributed
mixture of MD (multi-domain)
and PSD (pseudo-singledomain). In other cases, the
Fig. 5 - Theoretical Day graph,
(modified by Dunlop, 2002) for
magnetite.
References:
SD = single domain,
MD = Multiple Domain,
SP = Super-paramagnetic.
Mrs = saturation remanent
magnetization,
Ms = saturation magnetization;
Hcr = remnant coercivity force,
Hc = coercivity force.
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Fig. 6 - Magnetostratigraphic scheme, and geochronological
correlation, between the studied area and the Geomagnetic Time Scale
(Cande and Kent, 1995)
Jr/Js increases to 0,4 and up to 0,6
at low temperatures, which could
indicate that particles would be in
the PSD size range or correspond
to a mixture of
SD with
superparamagnetic particles (Fig. 5).
On the basis of magnetostratigraphic and geochronological
correlations (Fig. 6), we interpret
that the Rancahué Formation that
yield a radiometric age of 13±1 Ma
(39Ar/40Ar), recorded normal and
reverse polarities (Re, 2008; Re et
al, 2000). The phonobasalts of the
Michacheo Fm on the other hand,
bear a radiometric age of 17.7 ± 0.8
Ma,. The Tipilihuque Formation
has a wide regional distribution.
However, the Lonco Luan and
Rahue (6.2±0.3Ma) sequences are
not coeval with the Zapala Basalt
that bears only normal polarity
with a radiometric of 4.8±0.7 Ma
in Cañadón Santo Domingo (Re,
2008; Re, et al. 2000).
The Chapua Formation, assigned to
the lower Chapualitense like the
Zapala Basalt, bear a radiometric
age of 2.8±0.6 Ma in the Barranca
River, southern Mendoza (Re,
2008).
The basalts cropping out in Cerro
Bandera and Primeros Pinos that
are assigned to the informal
“Basalto Macho Viejo” unit,
traditionally assigned to the
Chapualitense cycle, are however
upper Pliocene (1.7±0.3Ma) (Re, 2008).
On the other hand, the basalts in Arroyo Covunco, are assigned to the Pleistocene based on their
position between the Macho Viejo Basalt and the Los Mellizos Basalt cropping out in Portada
Covunco and in Estancia Llamuco.
Therefore, our results allow interpreting that the Michacheo and Rancahué Formations constituted the
middle to late Miocene volcanic arc, and are the result of a change in the convergence angle with the
South American margin after carrying from significantly oblique to practically orthogonal to it. This
convergence style continued, with minor changes, in the Mio-Pliocene. During this time interval,
many basaltic extrusions took place as part of this arc, such as the Tipilihuque Formation, Zapala,
Macho Viejo and Los Mellizos Basalts.
REFERENCES
• Bermudez, A.; Delpino, D.; Frey, F.; Saal, A.; 1993. Los Basaltos de Retorarco Extrandinos. XII Cong. Geol. Arg. Geología y
Recursos Naturales de Mendoza V. Ramos (Ed.), Relatorio, 1(13):161 172.
• Dunlop, J.; Özdemir, Ö; 2002. Rock Magnetism: Fundamentals and forntiers. Cambridge University Press. , pp:573. UK.
• Kay, S. M.; Maksaev, V.; Mpodozis, C.; Moscoso, R.; Nasi, D. C.; Gordillo, C. E.; 1988. Tertiary Andean magmatism in Argentina
y Chile between 28-33 S: Correlation of magmatic chemistry with a changing Benioff zone. J. South American Earth Sciences 1:2138.
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• Leanza, H.A.; Hugo, C., Repol, D.; González, R.; Daniela, J.; Lizuain, A.; 2005. Hoja Geológica 3969-I, Zapala. Instituto de
Geología y Recursos Naturales, SEGEMAR, Bol. 275:1 128. Buenos Aires.
• Ramos, V. A.: Barbieri, M.; 1988. El volcanismo Cenozoico de Huantraico: edad y dataciones isotópicas iniciales, provincia del
Neuquén. Asociación Geológica Argentina, Rev. 43(2): 210-223. Buenos Aires.
• Re, G. H.; 2008. Magnetoestratigrafías del NO argentino (entre 27° y 31°s) aplicadas al análisis de la deformación andina, y su
relación con la subducción de la placa de nazca durante el cenozoico tardio). Doctoral Thesis (FCEyN-UBA) pp:278.
• Re, G.H.; Geuna, S.E.; Lopez Martinez, M.; 2000. Geoquímica y geocronología de los basaltos neógenos de la región de Aluminé
(Neuquén- Argentina). XI Congreso Geológico Chileno, Actas 2:62-66. Puerto Varas - Chile.
AMPHIBOLE MEGACRYSTS OF THE CERRO JEU-JEPÉN PLUTON:
NEW CONSTRAINTS ON MAGMA SOURCE AND EVOLUTION
(FUEGIAN ANDES, ARGENTINA)
2-10
Ridolfi, F.1*, Renzulli, A.1, Cerredo, M.E.2, Oberti, R.3, Boiocchi, M.4, Bellatreccia, F.5, Della
Ventura, G.5, Menichetti, M.1, Tassone, A.2
(1) Università degli Studi di Urbino “Carlo Bo”, Dipartimento di Scienze Geologiche, Tecnologie
Chimiche e Ambientali, 61029 Urbino (PU), Italy
(2) Universidad de Buenos Aires, Dpto. de Ciencias Geológicas, Facultad de Ciencias Exactas y
Naturales. Ciudad Universitaria. Pabellón 2. CP - C1428EHA- Buenos Aires. Argentina
(3) CNR-Istituto di Geoscienze e Georisorse, UOS Pavia, 27100 Pavia, Italy
(4) Centro Grandi Strumenti, Università di Pavia, 27100 Pavia, Italy
(5) Università degli Studi Roma Tre, Dipartimento di Scienze Geologiche, 00146 Roma, Italy.
* Presenting author’s e-mail: filippo.ridolfi@uniurb.it
The Cretaceous plutons of Argentine Tierra del Fuego Island are characterized by mildly alkaline
(shoshonitic) to high-K affinity (hereafter Fuegian Potassic Magmatism; González Guillot et al.,
2009), with igneous intrusives ranging from ultramafic rocks to monzogabbro and monzonite/syenite (Cerredo et al., 2000, 2005, 2007; Peroni et al., 2009). By contrast the coeval intrusions of the
South Patagonian Batholith (Nelson et al., 1988; Hervé et al., 2007) and the Beagle Channel Plutonic
Group of the southern archipelago (Hervé et al., 1984) are characterized by a dominant subalkaline
gabbro to granite trend.
Among the intrusive bodies belonging to the Fuegian Potassic Magmatism, the Cerro Jeu-Jepén (CJJ)
pluton is a small (< 10 km2), Late Cretaceous (93±4 Ma, Acevedo et al., 2000) intrusive body locat-
Fig. 1 - (a) Pseudo-secondary fluid inclusions filling cracks and cleavage and (b) a multi-phase primary inclusion
within amphibole megacrysts.
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Fig. 2 - (a) K2O-SiO2 diagram showing the composition (and uncertainty bars) of the estimated melt in equilibrium
with amphibole megacrysts (filled circles) and the shoshonitic trend of the CJJ whole-rocks (red arrow). (b) P-T plots
for the various thermobarometric constraints (see text); uncertainty bars of amphibole (black) and clinopyroxene-liquid
(red) thermobarometric methods are also given.
ed at the SE tip of the Lago Fagnano which is stretched out along the inferred strike of the MagallanesFagnano Fault (MFF) system (Cerredo et al., 2005). Magnetic and gravimetric data suggest a prominent crystalline body in the subsurface (roots at depth of about 8 km; Cerredo et al. 2000), partially
exposed at CJJ (ca. 55.6°S - 67.2°W). The shape and position of the CJJ pluton suggest that its
emplacement was localized in a releasing bend of the MFF system (Tassone et al., 2005). CJJ is a
composite intrusive plug made of basic (monzogabbro) to intermediate (monzodiorite, monzonite and
syenite) rocks showing a shoshonitic affinity (Cerredo et al., 2000, 2005). Monzonite is the dominant
lithology and shows poikilitic and patchy (perthitic) alkali feldspars (Or 8-88%) enclosing highlyzoned plagioclases (An 56-23%) and Fe-diopside clinopyroxenes (with aegirine molecula up to 4% at
their rims), phlogopite and minor amounts of magnetite, titanite and apatite. A remarkable peculiarity of the studied monzonite is the widespread occurrence of amphibole megacrysts (up to 5 cm long)
often showing corona reactions (made of phlogopite and clinopyroxene crystals) and cracks filled
with crystalline materials. The megacryst core displays abundant pseudo-secondary and primary fluid
inclusions (Fig. 1a and 1b, respectively), which also occur within larger Fe-diospide crystals.
In order to unravel the physical-chemical conditions of crystallization of the CJJ amphibole megacryst
and the whole evolution of
the monzonite body, a comprehensive study based on
EMP, single-crystal XRD,
SREF, FTIR and thermobarometry, was carried out.
A compositional and thermobarometric picture of the
genesis and evolution of CJJ
magmas was obtained with
the application of several
phase equilibria methods.
EMP analyses and structure
refinement of a portion of
an amphibole megacryst
show a magnesio-hastingsite composition, with a
partial but significant oxocomponent (0.45 O2- apfu),
which is completely balanced by the occurrence of
Fig. 3 - Fourier transform infrared (FTIR) spectrum for a fluid inclusion in an
Ti at M(1), a typical feature
amphibole megacryst, showing the occurrence of CO2.
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of mantle-megacrysts (e.g. Oberti et al., 2007; King et al., 1999). Single-crystal amphibole thermobarometry (Ridolfi et al., 2010) has been extended to high P-T with the supplemental collection of literature experimental amphiboles in equilibrium with both subalkaline and alkaline melts, at conditions up to 2200 MPa and 1100°C. A series of formulations to estimate P, T, H2Omelt (total content of
H2O and CO2 dissolved in the melt), fO2 and the major anhydrous element oxide composition (i.e.
SiO2, TiO2, Al2O3, FeO, MnO, MgO, CaO, Na2O, K2O) of the melt in equilibrium with amphibole,
gives reasonably low uncertainties: 11% for P, 24°C for T, 0.5 wt% for H2Omelt, 0.17 log units for NNO
(i.e. logfO2 – Ni-NiO buffer) and errors within 2.0-0.05 wt% for all the major oxides.
Fig. 2 shows (a) the K2O-SiO2 diagram for the calculated basic liquids in equilibrium with the amphiboles and (b) the P-T conditions of amphibole crystallization. The H2Omelt was possibly very high and
preliminary FTIR, EMP and textural analyses of the primary fluid inclusions indicate the presence of
CO2 species (Fig. 3) and tiny (<3 Ìm) magnetite crystals. Oxygen fugacity was slightly above the NNO
buffer and the application of GFluid (Zhang and Duan 2010) to the P-T-fO2 conditions obtained for
the amphibole megacrysts (i.e. 959-1029°C, 1.2-2.5 GPa, ∆NNO 0-0.8), confirms the presence of
CO2 and strongly indicates the stability of graphite which, together with magnetite, has been possibly
entrapped within the tiny (<30 Ìm; Fig. 1) amphibole fluid inclusions. Figure 2b suggests that the crystallization of amphibole megacrysts occurred at mantle conditions, at the upper experimental limit of
amphibole stability, during the uprising of the shoshonitic magma. The equilibrium between the melt
(estimated from amphibole thermobarometry) and the core composition of fluid inclusion-bearing
clinopyroxenes was tested by the thermobarometric equations of Putirka (2008). P-T conditions indicated by the clinopyroxene-liquid pairs (955-1019°C, 1.7-2.7 GPa) are consistent with those obtained
by amphibole thermobarometry (Fig. 2b). Petrogenesis of CJJ intrusives involved fractional crystallization and magma mixing and EMP results of plagioclase and alkali feldspar pairs allowed to estimate, according to the two-feldspar thermometer of Putirka et al. (2008), the late-stage magmatic temperature of crystallization (665-750°C; Fig. 2b) of the shallow pluton.
This study emphasizes the mantle P-T condition of crystallization of the CJJ magnesio-hastingsite
megacrysts, most likely after small degree of partial melting of the peridotite source. In addition, the
strong-zoning of monzonite plagioclase and clinopyroxene phenocrysts, and the disequilibrium corona textures around amphibole megacrysts in the studied CJJ monzonite suggest that the crystallization of the shoshonitic magma (which finally gave rise to the pluton at shallow crustal levels) followed
a decreasing P-T path just above the upper limit of amphibole stability, probably promoted by
transtensive movements along the MFF system.
REFERENCES
• Acevedo, R D., Roig, C.E., Linares, E., Ostera, H, Valín-Alberdi, M., Queiroga-Mafra, Z.M. 2000 La intrusión plutónica del Cerro
Jeu-Jepén. Isla Grande de Tierra del Fuego, República Argentina. Cadernos Laboratorio Xeolóxico de Laxe, 25: 357-359.
• Cerredo, M.E., Remesal, M.B., Tassone, A., Menichetti, M., Peroni, J.I., 2007. Ushuaia pluton: petrographic facies and geochemical
signature. Tierra del Fuego Andes. International Geological Congress on the Southern Hemisfere (Geosur 2). Santiago. Chile. Libro
de Resúmenes, p. 31.
• Cerredo, M.E., Remesal, M.B., Tassone, A., Lippai, H. (2005). The shoshonitic suite of Hewhoepen pluton, Tierra del Fuego,
Argentina. In: XVI Congreso Geológico Argentino, La Plata, Actas I, 539-544.
• Cerredo, M.E., Tassone, A., Coren, F., Lodolo, E., Lippai, H. (2000). Postorogenic, alkaline magmatism in the Fuegian Andes: the
Hewhoepen intrusive (Tierra del Fuego Island). In: IX Congreso Geológico Chileno, Puerto Varas, Actas 2, Simposio Nacional 2,
192-196.
• González Guillot, M., Escayola, M., Acevedo, R., Pimentel, M., Seraphim, G., Proenza, J., Schalamuk, I. (2009). The Plutón
Diorítico Moat: Mildly alkaline monzonitic magmatism in the Fuegian Andes of Argentina. Journal of South American Earth
Sciences 28, 345-359.
• Hervé F., Pankhurst R.J., Fanning C.M., Calderón M., Yaxley G.M.; 2007. The South Patagonian Batholith: 150 My of Granite
Magmatism on a plate margin. Lithos 97, 373–394.
• Hervé, M, Suárez, M. and Puig, A. 1984 The Patagonian Batholith S of Tierra del Fuego, Chile: timing and tectonic implications.
Journal Geological Society, London, 141: 909-917
• King, P.L., Hervig, R.L., Holloway, J.R., Vennemann, T.W., Righter, K. (1999). Oxy-substitution and dehydrogenation in mantlederived amphibole megacrysts. Geochimica et Cosmochimica Acta 63, 3635-3651.
• Nelson, E., Bruce, B., Elthon, D., Kammer, D., Weaver, S., 1988. Regional lithologic variations in the Patagonian Batholith. Journal
of South American Earth Sciences 1, 239-247.
• Oberti, R., Hawthorne, F.C., Cannillo, E., Cámara, F. (2007). Long-Range Order in Amphiboles. Reviews in Mineralogy and
Geochemistry 67, 125-171.
• Peroni, J: I.; Tassone; A. A., Menichetti,M., Cerredo, M. E. 2009. Geophysical modeling and structure of Ushuaia Pluton, Fuegian
Andes, Argentina. Tectonophysics, 476,(3-4), 25:436-449
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• Putirka, K.D. (2008). Thermometers and Barometers for Volcanic Systems. Reviews in Mineralogy and Geochemistry 69, 61-120.
• Ridolfi, F., Renzulli, A., Puerini, M. (2010). Stability and chemical equilibrium of amphibole in calc-alkaline magmas: an overview,
new thermobarometric formulations and application to subduction-related volcanoes. Contributions to Mineralogy and Petrology
160, 45-66.
• Tassone, A.,. Lippai, H., Lodolo, E., Menichetti, M, Comba, A., Hormaechea, J.L., Vilas, J.F. (2005). A geological and geophysical
crustal section across the Magallanes–Fagnano fault in Tierra del Fuego. Journal of South American Earth Sciences 19, 99-109.
• Zhang, C., Duan, Z. (2010). GFluid: An Excel spreadsheet for investigating C–O–H fluid composition under high temperatures and
pressures. Computers and Geosciences 36, 569-572.
“CIRCULAR FEATURES” ON OLD SOLIDIFIED LAVA FLOW FIELDS
ASSOCIATED WITH SOME YOUNG SCORIA CONES FROM LLANCANELO
AND PAYÚN MATRU VOLCANIC FIELDS, MENDOZA PROVINCE, ARGENTINA
2-11
Risso, C.1*, Nemeth, K.2, Nullo, F. 3, Inbar, M.4
(1) Dept. de Ciencias Geológicas-Universidad de Buenos Aires
(2) Massey University, New Zealand
(3) CONICET-SEGEMAR
(4) Dept. of Geography, University of Haifa, Israel
* Presenting author’s e-mail: corina@gl.fcen.uba.ar
The Plio-Pleistocene Llancanelo and Payún Matru Volcanic Fields (LPMVF) are two broad back-arc
lava plateau with hundreds (more than 800) of monogenetic scoria cones that cover an extensive area
behind the active Andean volcanic arc.
Volcanic activity of these fields started at the beginning of the Pliocene, and probably continued until
the last millennium with three main peaks of volcanic activity occurring at 3.6-1.7 Ma, c. 450 Ka, and
in the Holocene; equivalents to the Chapúa, Puente and Tromen Formations/Groups.
Basalts of Chapúa Formation (Nullo, 1985), or Chapúa Group (Bermúdez et al., 1993) are of a glossy
black color, with texture that goes from vesicular on top to dense, non-vesicular at it’s base. Extensive,
fresh-looking lava flows are predominantly pahoehoe type with subordinate “aa” type, and in the
western part of the volcanic field with tumuli, ropy lava and occasional lava tubes and skylights. The
source of these extensive lava flows is unknown. They have been formed by successive overlapping
(37-40 meters depth) of individual flow units and form the depositional and palaeo surface upon
which most of the new and younger volcanoes (Pencoso, Colorado, etc.) formed and deposited their
eruptive products.
Overlying the previous rock units is the Puente Formation (Nullo, 1985) or Puente Group (Bermúdez
et al., 1993), which also consists of extensive lava flows related to volcanic cones still recognizable.
Volcanic activity in the LPMVF was primarily of Strombolian and Hawaiian type, resulting in scoria
and/or lava spatter cones. Cone deposits are coarse-grained and commonly consist of red, scoriaceous
lapilli beds with meter-sized ballistic bombs and blocks. Large vesicular, spindle shaped lava bombs
and blocks as well as bread crusted bombs and blocks up to 3.5 m in diameter, are common.
In both lava fields, we observe some special “circular pattern” on the present surface. It seems they
formed in the previous lava flows and surrounding the younger volcanic cones (Fig. 1a,b,c,d,e,f, and
g in white) like: Colorado, las Bombas, Pencoso, etc. The diameter of these “circular feature” is
around 1000 m (Table 1).
A possible explanation about this curious feature is given in Fig. 2. In Fig. 2.1 we can presume the
Hawaiian-Strombolian style explosive eruptions that occurred between 3.6-1.7 Ma with the formation
of pyroclastic cones and successive lava flows. During a long period of erosion (Fig. 2.2) intense
eolian and fluvial processes removed significant portion of these cones.
Circa 450.000 years BP a new batch of magma was rising, causing the fracturation of the older
solidified lava succession (Fig. 2.3). Fig. 2.4 and 5 mark the explosive disruption that we infer to have
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Fig. 1 - Circular feature in different volcanoes of
LPMVF. a: unnamed volcano with “circular feature” in
Payun Matru Volcanic Field. b: Pencoso volcano in
Llancanelo Volcanic Field. c: unnamde volcano feature
south of Colorado volcano in Llancanelo Volcanic
Field. d: Colorado volcano in Llancanelo Volcanic
Field. e: Las Bombas volcano in Llancanelo Volcanic
Field. f: Same as c. g: Front view of oldest lava flows in
Colorado volcano.
been occurred due to the sudden degassing of
the rising magma in near surface which
potentially opened up new fractures and
gravitationally destabilised the older solidified
lava flows. Perhaps we cannot rule out an initial
phreatomagmatic explosive event triggered by
the explosive interaction between magma and
water that was trapped between solidified lava
flow units. Such process has been interpreted to
be the mechanism of the formation of few
phreatomagmatic volcanoes like Carapacho
volcano in the LPMVF (Risso et al., 2008).
Unfortunately there is no direct evidence such as
preserved phreatomagmatic pyroclastic deposit
that could support to constrain better such
model.
When the fracturing, decompression and sudden
discharge of fractured blocks due to explosion
ended, the new magma has built a new cone in
the newly formed depression on the solidified
lava surface (Fig. 2.6). The deposits produced
prior the magmatic eruptions are not preserved
in the circular zones around the cone and the old
solid flows, perhaps due to extreme arid and
windy conditions. A new period of degradation
of the cone by climatic forces take place (Fig.
2.7) leading to the present day scenario with a
scoria cone with a reduced size of cone heights
range between 20 and 100 meters and slope
angles much lower (Table 1) than young scoria
cones of 32-35º. The modification of the cone
geometry (height and width) over time opened a
gap between the cone and the old lava flows
allowing to function as a small sedimentary
basin around the cone collecting aeolian as well
Fig. 2 - Geological sketch (out of scale) about development of “circular features” during time.
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Name
Alco
(m)
Hco
(m)
Wco
(m)
Hco/ Wco
Slope
(degrees)
Dcr
(m)
Circular feature
(m)
Pencoso
1505
53
552
0.09
19
235
1123
Las Bombas
1485
33
330
0.1
10
129
1120
Colorado
1550
38
520
0.07
12
—
1341
Jarilloso
1558
97
1059
0.09
20
240
2670
186-15
1575
60
550
0.11
19
125
1134
186-40
1810
113
700
0.16
10
164
1227
n/n 1
1524
21
280
0.07
11
—
570
n/n 2
1475
20
350
0.05
12
170
1180
as sheet wash deposits from the new cone (Fig. 2.8). From the air, these depressions look circular and
surrounding relatively young scoria cones as it can be seen in Fig. 1.
REFERENCES
• Nullo, F., (1985). Hoja Geológica Cerro Campanario, SEGEMAR. 1:250.000. Unpublished.
• Bermúdez, A., Delpino, D., Frey, F. and Saal, A., (1993). Los basaltos de retroarco extraandinos. In:Ramos, V., (Ed.) Geología y
Recursos Naturales de Mendoza. XIIº Congreso Geológico Argentino y IIº Congreso de Exploración de Hidrocarburos, Mendoza.
Relatorio, I (13), 161-172.
• Risso,C., Németh, K., Combina, A.M., Nullo and F., Drosina, M., (2008). The role of phreatomagmatism in a Plio-Pleistocene highdensity scoria cone field: Llancanelo Volcanic Field (Mendoza), Argentina. Journal of Volcanology and Geothermal Research, 169
(1-2): 61-86.
THE NEOGENE BARRIL NIYEU VOLCANIC COMPLEX.
SOMÚN CURÁ MAGMATIC PROVINCE. NORTHERN EXTRA ANDEAN PATAGONIA.
ARGENTINA
2-12
Salani, F.M.*, Remesal, M., Cerredo, M.E.
CONICET – Universidad de Buenos Aires
* Presenting author’s e-mail: ms@gl.fcen.uba.ar
The mainly Oligocene (25-28Ma) Somún Curá basaltic plateau covers about 25000 km2 in the north
of Extra Andean Patagonia (Argentina, Fig. 1). Several eruptive centers of different complexities were
built over the basaltic plateau during Late Oligocene-(Early) Miocene which, according to the wide
compositional and eruptive diversity may be grouped in two main types: a) large bimodal complexes
and b) monogenetic eruptive centers. The former are characterized by their alkaline nature associated
with a typical gap in the ~ 53-60% SiO2 range.
Most of the large bimodal complexes are aligned along a WNW-ESE trending belt (Fig. 1). A
prominent transcurrent fault of roughly WNW-ESE orientation was identified offshore the northern
Extra Andean Patagonia (Urien and Zambrano, 1996), where it bounds the Cenozoic Valdez and
Rawson basins. On land, the Telsen fault (Ciciarelli, 1990), and further W the alignment of large
bimodal complexes, might represent the continuation of this major onshore-offshore wrench structure.
From NW to SE: Agua de la Piedra, Pire Mahuida, Barril Niyeu, Talagapa, Apas and Telsen volcanic
complexes (Salani et al, 2008) form a fairly well defined mountain chain which cuts across the main
Somún Curá plateau (Fig. 1). We interpret that the emplacement of these bimodal complexes was
controlled by transtensive movements along the Telsen-Valdez fault system.
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Fig. 1 - Location map and geological sketch displaying Somún Curá plateau and post-plateau bimodal complexes; box
highlights the Barril Niyeu Volcanic Complex.
The most voluminous post-plateau volcanic field is the Lower Miocene (20-18 Ma) Barril Niyeu
Volcanic Complex (BNVC; Remesal et al, 2001), which was built through several stages of both
effusive and explosive activity outpoured from at least five emission centres (presently cauldrons,
ranging from 2 to 5 km in diameter) and of distinct trachyte/rhyolite and basaltic compositions. The
final basaltic stage is dominated by lavas, although minor breccia and clastogenic facies and spatter
cones also occur. Local variations from this general stratigraphic scheme exist, which suggest a
certain synchronism of different composition events related to distinct vents.
The bulk of the complex is formed by porphyric trachyte lavas (bearing sanidine and aegirine,
aegirine/augite phenocrysts set in a trachytic groundmass) and comendite or quartz-trachyte lava
domes within the cauldrons. Although several trachyte groups were dintinguished (Salani et al., 2006),
all share a peralkaline character, as is common in other post- Somun Curá plateau Neogene
complexes. They show > 10% alkali content (Fig. 2), high Nb/Y = 2.6 -4.2, (La/Yb)N (~15-16) and Eu
negative anomaly mirroring feldspar fractionation.
The explosive volcanic facies is represented by two mesosiliceous to acid, and subordinate basic
pyroclastic episodes. The earlier one includes fall-out mainly plinian (lesser strombolian) and
pyroclastic flow deposits. This episode was partially contemporaneous with the deposition of primary
and reworked Andean-related pyroclastics of the Miocene Sarmiento Group. Sometimes these
deposits are interlayered with trachyte and basaltic flows.
The later pyroclastic stage is characterized dominantly by ignimbrites that commonly overlay trachyte
lavas and, to a smaller extent, the older pyroclastics flows. Rocks are composed of crystals (30-20%)
of sanidine, embayed quartz, with subordinated zircon and opaque minerals; lithic fragments (<10%)
of trachyte and basaltic volcanics. Vitroclastic components are pumice and devitrified glass shards.
Trydimite occurs as vapour phase crystallization. The deposits show different welding degrees which,
in many cases, produce eutaxitic textures. Strombolian
deposits are lapilli and block size, mainly composed of
brown glassy patches, porfiritic pumices with olivine
crystals, and basaltic lithic fragments. The pyroclastic
rocks show alkaline and subalkalic affinities, (La/Yb)N
ratios (6-13) and strong negative Ba, Sr, P, Eu and Ti
anomalies.
Basaltic lavas mainly cover the northern area of the
Complex and a minor facies appear in the southwest
side. Petrographic and geochemical characteristics
distinguish three main basaltic groups: transitional
basalts, alkaline basalts and trachybasalts (Fig. 2); the
Fig. 2 - SiO2 vs. total alcalis (TAS) Le Maitre et
former correspond to porphyric lavas with olivine
al. (1989). Data from Remesal et al (submitted),
and plagioclase phenocrysts which display the lowest
Salani et al. (2006) and Remesal et al. (2008).
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(La/Yb)N ratio (9.6). The alkaline
basalts are mainly porphyritic
lavas bearing plagioclase, olivine
and
titanoaugite
phenocrysts;
intermediate (La/Yb)N ratios (12-14)
characterize this group. The last
eruptive stage of the BNVC
corresponds to the trachybasalts
which appear either as lava flows or
as spatter cones; rocks display
aphanitic textures and the highest
(La/Yb)N ratios (14-18) of the basic
rocks.
According to Th/Yb vs. Ta/Yb ratios,
Fig. 3 - 143Nd/144Nd vs. 87Sr/86Sr of CVBN. DM and EM1
the basaltic rocks show an intraplate
components are from Faure (2001); solid squares: new data for
character (Pearce, 1982, 1983). The
BNVC, hollow squares: data of trachybasalts likely forming part of
differentiated rocks also fit within
BNVC (from Kay et al. 2007).
plate field in the Gorton and Schandl
(2000) diagram with low ratios Th/Ta
(1-6). Nevertheless, some interelemental ratios (i.e. high Ba/La > 20) in the basaltic rocks suggest the
contribution of a subduction zone component.
Simple mixing models that combine 90–85% of a depleted mantle end-member (DM) with 5–10% of
a type 1 enriched mantle (EM1) approach the 87Sr/86Sr 20 Ma ratios (0.704085-0.704384) and
143Nd/144Nd
20 Ma (0.512733-0.512652), eNd 20 Ma (0.8 to 2.4) of the BNVC basalts.
A recent seismic tomography survey carried on across the North Patagonian Massif revealed a
Paleogene subduction gap due to Aluk plate breakoff (Aragón et al., 2009). The pacific margin of the
South American plate was dominated by transcurrent motion during Paleogene times (Somoza and
Ghidella, 2005) until ~25 Ma when the breakup of the Farallón plate caused a major change from
highly oblique to near-normal convergence along the Andean margin (Somoza, 1998).The Somún
Curá magmatic province evolved within this regional geodynamic scenario. The BNVC (as well as
other Early Miocene bimodal complexes) was built when a dramatic plate reorganization process had
taken place along the pacific margin. Although the subduction regime was restablished by Late
Oligocene times, the tectono-thermal flux in the supra-subduction zone was active only from Middle
Miocene onwards as indicated by magmatic activity in the Patagonian Batholith (Aragón et al., 2009).
We propose that the partial melting of the Patagonian asthenosphere was triggered both by this drastic
geodynamic change as well as by the dehydration/partial melting processes underwent by foundered
Aluk plate. Therefore, the arc-like signatures of some inter-elemental ratios would be related to the
detached slab and not to the influence of a contemporaneous arc.
This contribution was possible due to the financial support of Universidad de Buenos Aires
(UBACYT-X185).
REFERENCES
• Aragón, E., Spakman, W. Brunelli, D., Rivalenti, G., DÈramo G. DÈramo, F., Pinotti, L., Rabbia , O. Cavarozzi, C., Aguilera, and
Ribot, A., Mazzucchelli, M., 2009. El gap de subducción Paleógeno del segmento patagónico 35º-44º. XIV Reunión de tectónica.
Abstracts: 53, Río Cuarto. Córdoba, Argentina.
• Ciciarelli, M. 1990. Análisis estructural del sector oriental del Macizo Nordpatagónico y sus significado metalogenético. Tesis
doctoral, Facultad de Ciencias Naturales y Museo. Universidad Nacional de La Plata. 155 pp.
• Faure, G. 2001. Origin of Igneous Rocks. The isotopic Evidence. Springer. 496 p.
• Gorton, M. P., Schandl, E. S., 2000. From Continents to Island Arcs: A Geochemical Index of Tectonic Setting for Arc-Related and
Within-Plate Felsic to Intermediate Volcanic Rocks. The Canadian Mineralogist, 38: 1065-1073.
• Kay, S. M., Ardolino, A. A., Gorring, M.L. and Ramos, V. 2007. The Somuncura Large Igneous Province in Patagonia: interaction
of a transient mantle thermal anomaly with a subducting slab. Journal of Petrology, 48 (1): 43-77.
• Le Maitre, R.W, Bateman, P., Dudek, A., Keller. J., Lameyre, J., Le Bas, M.J., Sabine, P.A., Schmid, R., Sorensen, H., Strekeisen
A., Woolley, A.R, Zanettin, B. 1989. A classification of igneous rocks and glossary of terms. Blackwell, Oxford.
• Pearce, J.A. 1982. Trace element characteristics of lavas from destructive plate boundaries. En Andesites: Orogenic Andesites and
Related Rocks (R.S. Thorpe, ed.). John Wiley and Sons, Chichester, U.K, 525-548.
• Pearce, J.A. 1983. Role of the sub-continental lithosphere in magma genesis at active continental margins. En Continental Basalts
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and Mantle Xenoliths (C.J. Hawkesworth and M.J. Norry, eds.). Shiva Press, Nantwich, U.K, 230-249.
• Somoza, R. 1998. Updated Nazca (Farallón)–South America relative motions during the last 49 m.y.; implications for mountain
building in the Central Andean region. Journal of South American Earth Sciences 11, 3211–3215.
• Somoza, R., M.E. Ghidella, 2005. Convergencia en el margen occidental de América del Sur durante el Cenozoico: subducción de
las placas de Nazca, Farallón y Aluk, Revista de la Asociación Geológica Argentina, 60 (4): 797-809.
• Remesal, M., F. Salani, M. Franchi and A. Ardolino, 2001, Hoja Geológica 4169-IV, Maquinchao. Provincia de Río Negro. Instituto
de Geología y Recursos Minerales, Servicio Geológico Minero Argentino. Boletín Nº 312: 1- 72. Buenos Aires.
• Remesal, M, Salani, F. M. and Cerredo, M. E., 2008. Los basaltos del Complejo Volcánico Barril Niyeu, en 9 º Congreso de
Mineralogía y Metalogenia, San Salvador de Jujuy, Argentina, Asociación Mineralógica Argentina, 265-270.
• Remesal, M. , Salani, F. and Cerredo, M.E. Submitted. Petrología y Evolución Del Complejo Volcánico Barril Niyeu, Patagonia
Argentina. Revista Mexicana de Ciencias Geológicas .
• Salani, F.; M. Remesal and M. E. Cerredo. 2008. Somun Curá post- plateau stage: large bimodal complexes, northern Patagonia.
Argentina. IAVCEI. Reykjavík, Iceland.
• Salani, F. M.; M. B Remesal and M. E. Cerredo, 2006. Las Rocas Traquíticas del Complejo Volcánico Barril Niyeu. . 8º Congreso
de Mineralogía y Metalogenia. Buenos Aires, Acta: 427-434.
• Urien, C. M. and Zambrano, J. J. 1996. Estructura del margen continental. XIII Congreso Geológico Argentino and III Congreso de
Exploración de Hidrocarburos. Geología y Recursos Naturales de la Plataforma Continental Argentina. Ramos, V. and Turic, M.
eds.: 29-65.
THE MAGNETIC SUSCEPTIBILITY OF IGNIMBRITES FROM
2-13
THE ALTIPLANO- PUNA VOLCANIC COMPLEX, CENTRAL ANDES:
A USEFUL TOOL TO DISTINGUISH LITHOMAGNETIC DOMAINS ACROSS THE ARC
Singer S.E.*
INGEODAV (Instituto de Geofísica Daniel A. Valencio), Dpto. Ciencias Geológicas, FCEyN
(UBA) Ciudad Universitaria, Pab. II.1428 Buenos Aires, Argentina.
* Presenting author´s e-mail: singer@gl.fcen.uba.ar
Silicic volcanism in the Andean Central Volcanic Zone originated one of the world´s largest Neogene
ignimbrite provinces. Between 21° and 24° S, the Altiplano - Puna Plateau shows a concentration of
predominantly dacitic ignimbrites that constitute the Altiplano- Puna Volcanic Complex. Their
compositions and huge erupted volumes suggest they originated from large scale crustal melting.
Moreover, geophysical evidence presently indicates the occurrence of partial melting zones in the
middle crust beneath the plateau and very high heat flow values.
On the other hand, the bulk susceptibility of a rock represents the addition of the susceptibilities of all
minerals that are present in a sample, although the magnetite is present even in small amounts it will
control the magnetic properties due to its high magnetic susceptibility. As a consequence, bulk
susceptibility of a rock gives us a very first idea of the amount of magnetite which is in that rock. In
agreement, susceptibilities of different rock types give rise to the concept of lithomagnetic domains.
For our study, measurements of magnetic susceptibility were carried out, together with microscopic
observations of Fe-Ti oxides of Late Miocene ignimbrites in the Central Andes (22°S - 23°S) located
in the forearc and retroarc. The main goal of this project is thus to study the variations in
susceptibilitiy values as well as the creation, alteration and breakdown of the magnetite in these rocks
and subsequently, to establish a probable link between susceptibility and process.
The analysis of the data shows two clearly defined lithomagnetic domains, one paramagnetic located
in the retroarc and the other ferromagnetic situated in the forearc whose susceptibility values vary in
an order of magnitude. These results indicate different contents of magnetite in the ignimbrites that,
combined with the different mineralogical assemblages, suggest that such variations could be
explained by two types of magmas.
On the basis of these results, some interesting challenges arise:
Study the contribution from the serpentinized forearc mantle to the composition of the magmas in the
forearc, taking into account that quantitative models based on gravity and aeromagnetic anomalies
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suggest that the magnetic mantle may be common in forearc settings.
Although it is still hot matter of debate, arc basin magmas are produced by partial melting of an
asthenospheric mantle source that has been metasomatized by the addition of one or more slab
components. The key topics are the nature of the metasomatizing agent(s), their detailed chemical
composition and mass fraction, and the relative proportion of subducted sediment and altered oceanic
crust in the source of the metasomatizing agent. On the other hand, models of the thermal structure of
the mantle wedge and subducting slab generally conclude that slab-surface temperatures are too low
for melting to occur, except in cases where the subducting slab is exceptionally young and warm.
However, new models predict in slab-surface, temperatures that are 100°C – 300°C hotter, with the
implication of slab melting. Thus the main challenge that remains is to test the contributions from the
slab, particularly if we consider the anomalous thermal state of the region.
Finally, to integrate the “exotic” iron-oxide rich lava flows at El Laco (23°48’S, 67°30’W) within the
regional tectonic framework moreover taking also into account the temporal coincidence with the
dacitic ignimbrite Cerro Galán that heralds the beginning of a new flare-up in the Southern Puna. In
other words, we argue that these exotic lava flows could actually be one of the key issues to the
understanding of the magnetic magmatic processes in the region.
FIRST KIMBERLITE PIPE IN CENTRAL YAKUTIA (RUSSIA):
2-14
MINERAL COMPOSITION AND THICKNESS OF LITHOSPHERIC MANTLE AND AGE
Smelov, A.P.1*, Zaitsev, A.I.2, Ashchepkov, I.V.3
(1) Diamond and Precious Metal Geology Institute, Siberian Branch, Russian Academy of Sciences,
Yakutsk, Russian Federation
Yakutsk, Russian Federation
(3) Institute of Geology and Mineralogy, Siberian Branch, Russian Academy of Sciences,
Novosibirsk, Russian Federation
* Presenting author´s e-mail: a.p.smelov@diamond.ysn.ru
Introduction
Active diamond prospecting, which was initiated by scientific forecasts (Sobolev, 1951) and done
within the Siberian platform in the late 1940s and 1950s, revealed commercial diamondiferous
kimberlites in Western Yakutia. Comprehensive studies of kimberlites, their mantle xenoliths and
diamonds, yielded important information on the origin of kimberlites, the structure and composition
of the lithospheric mantle parageneses and formation conditions of natural diamonds. In Central
Yakutia, kimberlite and diamond prospecting has become more active since 2000. That year chromian
pyropes were found in a locality with known “pipelike” geochemical anomalies (Protopopov, 1993) in
the modern alluvium of the Kengkeme and Chakyya Rivers (Lena Basin) and high-chromium
spinellids of diamond association, with up to 68 wt % Cr2O3 (Izbekov et al., 2006; Okrugin et al.,
2007), were found in the Menda Basin. These findings, combined with structural data, allowed to
suggest kimberlite magmatism in that area. Drilling of the geophysical anomalies carried out by
geologists from Yakutskgeologiya in 2007 – 2008 revealed the first kimberlite pipe (Fig. 1).
We studied the composition of the rocks that conform the Manchary pipe and the mantle-derived
minerals contained in them (Smelov et al., 2009, 2010). The focus of study was to determine the
characteristics of the kimberlites, to estimate their diamond potential and to obtain preliminary
information on the composition of the lithospheric mantle substrate. Indicator minerals composition
from the pipe under study were compared with those of alluvium samples from some of streams of
the area. This comparative analysis provides reasons to estimate objectively the possibility of the
occurrence of other kimberlites in the area.
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Fig. 1 - Tectonic sketch of the North Asian craton (Smelov and Timofeev, 2007) showing the kimberlite magmatism.
1- Siberian platform; 2 - subsided craton margin—fold and thrust belts (ST, South Taimyr; EA, East Angara; BP,
Baikal–Patom; VR, Verkhoyansk); 3 - Precambrian basement (shields and uplifts); 4 - Mesozoic volcano-plutonic belt;
5 - kimberlite fields; 6 - Manchary kimberlite pipe; 7 - sites with kimberlites minerals (Izbekov et al., 2006; Okrugin
et al., 2007); 8 - “pipelike” geophysical anomalies (Protopopov, 1993). Close up of Yakutsk area. 1-2 deposits: 1 Cenozoic; 2 - Mesozoic.
Geological setting
The Manchary pipe is located in the Tamma basin (the right tributary of the Lena) 100 km southward
of Yakutsk. It was explored by three drill holes up to 150 - 170 m depth. It breaks through Upper
Cambrian carbonate deposits and is overlain by Jurassic terrigenous masses.
Mineral composition
The pipe consists of a greenish-gray kimberlite breccia with a massive serpentine-micaceous cement.
The rocks in the upper parts are mudded to varying degrees. Kimberlite breccia typically hosts
serpentinites, micaites and micaceous and garnet serpentinites up to 25 cm in size. They display
porphyric texture due to the presence of serpentinized olivine, phlogopite, picroilmenite, and garnet
phenocrysts (macrocrystals). Serpentinite inclusions and macrocrystals are highly resorbed.
Phlogopite and picroilmenite macrocrystals often occur as fragments with fine-grained serpentinemagnetite rims. The rock texture in these areas resembles autolithic texture. Both the chemical
analysis of the least altered rocks from the pipe without xenogenic matter and the petrographic and
mineralogical results confirm their kimberlite nature. Most of the samples are typical
noncontaminated kimberlites, according to their SiO2 (20-35 wt.%) and Al2O3 (5 wt.%) contents.
Based on the ratio of CaO/(CaO + MgO) to SiO2/MgO and depending on the degree of secondary
alteration, the pipe rocks fall into three petrochemical categories: magnesian kimberlites, kimberlites
s.s., and carbonatite kimberlites.
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The
main
kimberlite-forming
minerals are serpentine, carbonates,
and phlogopite. The groundmass has
a significant content of ore minerals –
aggregates of ferro- and chromian
spinels, perovskite, magnetite and
minor
magnesian
chromian
magnetite.
Serpentine forms greenish-gray
ribbon-like lamellar aggregates or
fibrous foliated aggregates after
olivine. Relictic olivine is recognized
in the least altered kimberlite at 141146 m depth. Porphyry segregations
of serpentinized olivine content vary
from 20% in fine-grained kimberlites
up to 45% in coarse-grained ones.
Phlogopite macrocrystals (8x?6 mm),
which conform up to 5-10% of the
rock occur as nodules or xenomorphic
Fig. 2 - TP estimates for the mantle culumn beneath the Manchary
segregations with traces of deformation
kimberlite pipe. 1-2 Gr thermobarometry (1 - Ashepkov et al., 2008;
and resorption. Phlogopite crystals are
Krogh, 1998. 2 - Ashepkov et al., 2008; McCammon et al., 2001); 3 chloritized and surrounded by
Ilm thermobarometry (Taylor et al., 1998); 4 - Sp thermobarometry
reaction rims composed by fine
(Ashepkov et al., 2008; O’Neill and Wall,1987).
aggregates of ore minerals. They host
idiomorphic grains of perovskite and
titanomagnetite. It is rich in MgO and Al2O3, and almost depleted in TiO2. According to the Fe-Ti
ratio, the macrocrystals and groundmass phlogopites belong to the reaction type of micas from
metasomatized garnet-spinel peridotites, as well as from metasomatized ilmenite peridotites and
pyroxenites. The micas of the binder mass show BaO contents up to 8.0 wt.%.
Spinelloid macrocrystals include both high-titanium (TiO2 >1.0 wt.%) and low-titanium (TiO2 < 1.0
wt.%) types. According to the ratio of Fe2/(Fe2 + Mg) to Cr/(Cr + Al), two groups of spinelloids form
a kimberlite trend.
Picroilmenite macrocrystals, 2 to 5 mm in size, have irregular, baylike boundaries. They have high
MgO contents (8.2 to 11.5 wt.%) and reaction rims consisting of fine grains of perovskite,
ferrospinels, and magnetite. According to their MgO/TiO2 ratios they plot along the “kimberlite trend”
of ilmenites.
Pyropes show uneven distribution in kimberlite breccia. Most pyrope grains are sharp-edged
fragments. Chemically, they are of lherzolite, wehrlite, or nondiamondiferous dunite-harzburgite
parageneses. We did not find pyropes typical of deep-seated xenoliths of deformed peridotites from
the Udachnaya pipe. Neither we did find high-chromium subcalcic pyropes analogous to diamond
inclusions, which could show the diamond potential of those kimberlites. Garnets corresponding to
lherzolites of anomalous composition (Tychkov et al., 2008) conform up to 8% of the rock; this is
close to the garnet content of Middle Paleozoic kimberlites from the Yakutian province. However,
eclogitic garnets were not found in the Manchary pipe, and this is not typical of the kimberlite
breccias.
Thickness of lithospheric mantle
P-T crystallization parameters were estimated using monomineralic garnet thermometers as well as
choromite and ilmenite thermobarometers (Fig. 2). From chemical composition of garnets we
established a conductive geotherm (35 mw/m2) typical of the mantle beneath the Paleozoic pipes in
Yakutia, and a layered structure of the mantle column to a depth of 230 km (70 kbar) consisting of 8
intervals. In the middle part, they are separated by a 50-40 km thick horizon (35-65 kbar) with little
garnet. The horizon is supposed to have a pyroxenite composition. That structure of lithosphere is
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typical of the Arkhangelsk diamondiferous province and the Nakyn kimberlite field. P-T parameters
estimated by chromite and ilmenite geothermobarometry display a high-temperature line which is due
to the heating of mantle rocks by protokimberlitic melts. In general, the estimated P-T parameters and
geotherms show that lithosphere in Central Yakutia has thicknesses of a mantle root corresponding to
the diamond stability field.
Conclusions
Petrographic, mineralogical, and geochemical studies confirm the presence of a kimberlite breccia
pipe near Yakutsk. It resembles Middle Paleozoic kimberlite breccias from the Yakutian province in
geological position and the composition of pyropes. Chemical composition of pyropes and chrome
spinellids from Manchary pipe kimberlites is similar to that from marginally diamond - bearing
Middle Paleozoic kimberlites in the north of the Yakutian province, for example, in Middle Olenek
area (Smelov et al., 2010). The first Rb-Sr isochrone of unaltered kimberlites yielded an age of
359±50 Ma (Io=0.7052), which is close to that of diamond - bearing kimberlite pipes from the
Yakutian province. A comparative analysis revealed a significant compositional difference between
pyropes from the Manchary pipe and those from modern fluvial alluvium. Consequently, the rocks of
the pipe could not be a source of pyropes for this alluvium. Many “pipelike” geophysical anomalies
allow forecasting a new kimberlite field in Central Yakutia.
REFERENCES.
• Ashchepkov I. V., Pokhilenko N. P., Vladykin N.V. et al.; 2008: Reconstruction of mantle sections beneath Yakutian kimberlite pipes
using monomineral thermobarometry. In: Geological Society, London, Special Publications, 293, pp.335-352.
• Izbekov E.D., Pod’yachev B.P. and Afanas’ev V.P.; 2006: Signs of symmetric diamond concentration in the eastern Siberian Platform
(relative to the Vilyui syneclise axis) - Dokl. Earth Sci., 411A (9), 1339-1340.
• Krogh E.J.; 1988: The garnet-clinopyroxene Fe-Mg geothermometer a reinterpretation of existing experimental data - Contrib.
Minera. Petrol., 99, 44-48.
• McCammon C.A., Griffin W.L., Shee S.R. et al.; 2001: Oxidation during metasomatism in ultramafic xenoliths from the Wesselton
kimberlite, South Africa: implications for the survival of diamond - Contrib. Mineral. Petrol., 141( 3), 287-296.
• Okrugin A.V., Belolyubskii I.N., Oleinikov O.B., et al.; 2007: Kimberlitic indicator minerals from Kengkeme River alluvium within
the Yakut uplift - Nauka i Obrazovanie, 4, 17-23.
• O’Neill, H. St. C. and Wall, V. J.; 1987: The olivine orthopyroxene-spinel oxygen geobarometer, the nickel precipitation curve, and
the oxygen fugacity of the Earth’s upper mantle - Journal of Petrology, 28, 1169-1191.
• Protopopov Yu.Kh.; 1993: Tectonic Complexes of the Platform Cover of the Vilyui Syneclise [in Russian]. Izd. Yakut. Nauchn.
Tsentr, Sib. Otd. Ross. Akad. Nauk, Yakutsk.
• Smelov A.P., Timofeev V.F.; 2007: The age of the North Asian Cratonic basement: An overview - Gondwana Res., 12, 279-288.
• Smelov A.P., Ashchepkov I.V., Oleinikov O.B. et al.; 2009: Chemical composition and P-T conditions of the formation of barophilic
minerals from Manchary kimberlitic pipe (Central Yakutia) - Otechestvennaya Geologiya, 5, 27-30.
• Smelov A.P., Andreev A.P., Altukhova Z.A. et al.; 2010: Kimberlites of the Manchary pipe: a new kimberlite field in Central Yakutia
- Russian Geology and Geophysics (Geologiya i Geofizika) 51 (1), 121-126.
• Sobolev V.S.; 1951: Geology of Diamond Deposits of Africa, Australia, Borneo Island, and North America [in Russian].
Gosgeoltekhizdat, Moscow.
• Taylor W.L., Kamperman M. and Hamilton R.;1998: New thermometer and oxygen fugacity sensor calibration for ilmenite amd Crspinel- bearing peridotite assemblage. In: 7th IKC Extended abstracts. pp. 891.
• Tychkov N.S., Pokhilenko N.P., Kuligin S.S. et al.; 2008: Composition and origin of peculiar pyropes from lherzolites: evidence for
the evolution of the lithospheric mantle of the Siberian Platform - Russian Geology and Geophysics (Geologiya i Geofizika) 49 (4),
pp. 225-239 (302-318).
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DEPOSITION AND REWORKING OF PRIMARY PYROCLASTIC
DETRITUS IN PUESTO LA PALOMA MEMBER, CRETACEOUS CERRO
BARCINO FORMATION, SOMUNCURÁ-CAÑADÓN ASFALTO BASIN, PATAGONIA,
ARGENTINA
2-15
Umazano, A.M.*
INCITAP (CONICET-UNLPam); Av. Uruguay 151, 6300 Santa Rosa, La Pampa, Argentina
* Presenting author’s e-mail: amumazano@exactas.unlpam.edu.ar
The Puesto La Paloma Member (PLPM) constitutes the basal section of the Cretaceous Cerro Barcino
Formation (Chubut Group), which was accumulated during sag conditions in the Somuncurá-Cañadón
Asfalto Basin, Patagonia, Argentina. The PLPM overlies Los Adobes Formation and is covered by
Cerro Castaño Member. It is composed of greenish, pyroclastic-rich strata, which were interpreted as
continental deposits without further details. The aim of this contribution is to analyze the sedimentary
paleoenvironments of the unit with emphasis in the recognition of reworking processes of the primary
pyroclastic deposits.
The PLPM was studied in four localities of the western sector of the basin within the Chubut province;
from NW to SE they are: Cerro Los Chivos (43º13’10’’ S, 68º56’43’’ W; 42 m thick), Estancia La
Payanca (43º29’43’’ S, 68º52’31’’ W; 20 m thick), Estancia La Madrugada (43º35’20’’S, 68º56’32’’W;
19 m thick) and Estancia La Juanita (43º48’2’’ S, 68º52’15’’W; 18 m thick). In all localities, the PLPM
is mainly constituted of fine-grained tuffs and tuffaceous sandstones with minor amounts of
tuffaceous mudstones, conglomerates and chert.
The recognized facies associations (FA) include sub-aerial primary pyroclastic deposits (FA1) and
epiclastic deposits partially originated by reworking of the former (FA2 to FA5). FA1 (ash-falls)
comprises massive or plane parallel laminated, accretionary lapilli-rich, fine-grained tuffs with mantle
bedding and non erosive bases; intercalations of apedal paleosoils and burrowing levels are common.
FA2 (unconfined fluvial flows) includes stacked and amalgamated sheet-like bodies composed of
fine-grained tuffaceous sandstones; individual bodies have irregular bases and commonly show
massive aspect in the base and lamination and roots in the top. FA3 (permanent fluvial channel belts)
is mainly composed of tuffaceous sandstones with irregular and erosive bases and plane tops;
internally, the bodies present laterally stacked large-scale inclined surfaces. FA4 (aeolian system) is
represented by a single body composed of medium-grained tuffaceous sandstones; the lower part
(dunes) shows planar tabular cross-beddings with average foresets dip 25°; the upper sector
(interdunes) exhibits low-angle cross-bedding with intralamina inverse grading. FA5 (ponds) is
formed by tabular tuffaceous mudstones with massive aspect in the base then passing upward to levels
with plane parallel lamination or asymmetrical ripples; coarse to fine-grained chert intercalations are
relatively common.
FA1 and FA2 occur in all localities, the first with less occurrence (<10%) and the later constituting
always more than 50% of the sections. FA3 only occurred in Estancia La Payanca (? 40%) and FA4
and FA5 in Cerro Los Chivos (7% and 8%, respectively). Facies relationship suggests accumulation
of sub-aerial ash-falls (FA1) and frequent reworking of the tephra by unconfined fluvial flows (FA2);
locally, primary pyroclastic substrates were also reworked by channelled fluvial flows (FA3), aeolian
processes (FA4) and shallow lacustrine sedimentation (FA5).
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GEODYNAMIC THAT GENERATED THE CRETACEOUS VOLCANISM
OF CÓRDOBA AND THE LARGE IGNEOUS PROVINCE OF PARANÁ,
INCLUDING THE ORIGIN OF THE TRISTAN “PLUME”
2-16
Vizán, H.1*, Lagorio, S.L.2
(1) Departamento de Ciencias Geológicas – Facultad de Ciencias Exactas y Naturales (UBA –
CONICET). Ciudad Universitaria, Pab. II. Ciudad Autónoma de Buenos Aires
(2) Servicio Geológico Minero Argentino (SEGEMAR). J. Roca 651, piso 10 – Ciudad Autónoma de
Buenos Aires
* Presenting author e-mail: haroldo@gl.fcen.uba.ar
The aim of this work is to present a geodynamic model coherent with geochemical data pertaining to
two contemporaneous volcanic events occurred in Córdoba and in Misiones (Argentina). In this model
Fig. 1 - Palaeoreconstruction of Pangaea
for 130 Ma. Cratons and terranes: AM
(Amazonia), RA (Río Apa), CN (Congo),
K (Kalahari), OAC (Oriental Africa
Craton), A (Arequipa), PA (Pampia), C
(Cuyania). The black point inside a circle
represents the main centre of the eruptive
pipe of the Large Igneous Province
Paraná-Etendeka-Angola. The other black
points represent the location of Misiones
and Córdoba Cretaceous volcanisms. In
colours: seismic velocity anomalies (S
wave) in the lower mantle. High (low)
velocities anomalies are interpreted as
high (low) temperatures.
Fig. 2 - East-West section of Western
Gondwana (South America and Africa) for
130 Ma. The thikness of the cratons are
assumed according to the thikness
determined for San Francisco craton (200300 km). (1) Large-scale convection roll
induced by subduction lateral cooling. (2)
Edge-driven convection determined between
Río de la Plata Craton and the terrane
Pampia.
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Fig. 3a and b - Temporal evolution of the different geodynamic processes that occurred between 125 Ma and 80 Ma.
The break-up of Western Gondwana and the drift of South America respect to Africa are represented. Astenospheremesosphere boundary at about 650 km; core-mantle boundary at about 2,900 km. Orange triangles: volcanic sea
mounts caused by a conduit of magma outpouring that evacuated energy insulated by Pangaea; grey triangle: central
oceanic ridge; dark blue: subducted slab; different orange tones: Pangaea thermal blanketing; black closed circuits:
edge-driven convection; purple closed circuit: large-scale lateral convection induced by subduction; green closed
circuit: convective cell that cause the South America drift; curve arrows in blue: downwelling currents in the lower
mantle; curve arrows in red: upwelling currents in the lower mantle. a) 125 Ma; b) 80 Ma.
Fig. 3c and d - Temporal evolution of the different geodynamic processes that occurred between 40 Ma and 10 Ma. See
references of Fig. 3a and b. c) 40 Ma; d) 10 Ma.
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it is considered what is known for the convection processes that happen in the mantle. It is suggested
that the alkaline Cretaceous volcanism of Córdoba was basically generated by edge-driven convection
(King and Anderson, 1998) between the Río de La Plata craton and the Pampia terrane or the ancient
mobile belt (suture zone) that separates both geologic elements. On the other hand, it is considered
that the origin of the tolheiitic basalts of the Large Igneous Province of Paraná and its African counterpart was controlled by a large-scale lateral convection induced by subduction (Nataf et al., 1981),
stimulated also by the energy insulated by Pangaea (Anderson, 1982), (Fig. 1 and Fig. 2). It is
suggested, in addition, that the Tristán “plume” was actually a conduit of magma outpouring caused
by lithospheric discontinuities due to the distribution of mobile belts surrounded by old cratons. The
pipe for carrying the magma would have being set up by the mobile belts where the lithosphere was
weaker (Fig. 2). This pipe would have evacuated the caloric energy insulated by Pangaea and would
not correspond to a plume generated by a thermal discontinuity at the core-mantle boundary (Fig. 3a,
b, c and d). This conduit could have acted as a guide for deep currents that transported chemical
elements to the surface, previously carried to deep zones of the mantle in former subducted slabs
(Hofman, 1997). A geodynamic evolution from 125 Ma to 10 Ma is shown in Fig. 3a, b, c and d. It is
suggested that the processes of convection in the mantle would be determined by the architecture and
dynamics of the lithosphere.
REFERENCES
•
•
•
•
Anderson, D.L., 1982. Hotspots, polar wander, Mesozoic convection and the geoid. Nature, 297: 391-393.
Hofman, A.W., 1997. Mantle geochemistry: the message from oceanic volcanism. Nature, 385: 219-229.
King, S.D. and D.L.Anderson, 1998. Edge-driven convection. Earth and Planetary Science Letters, 160: 289-296.
Nataf, H.C., C. Froidevaux, J.L. Levrat and M. Rabinowicz, 1981. Laboratory convection experiments: Effect of lateral cooling and
generation of instabilities in the horizontal boundary layers. Journal of Geophysical Research, 86: 6143-6154.
MAFIC MICROGRANULAR ENCLAVE SWARMS IN
GRANITIC PLUTONS OF GASTRE, CENTRAL PATAGONIA
2-17
Zaffarana C.B.1*, Somoza R,1
(1) Departamento de Ciencias Geológicas, Universidad de Buenos Aires. Intendente Güiraldes
2160. Buenos Aires C1428EGA
* Presenting author’s e-mail: zaffarana@gl.fcen.uba.ar
In the Gastre region, Central Patagonia, the Late Triassic-Early Jurassic Central Patagonian Batholith,
(Rapela et al., 1991) crops out. These authors recognized two superunits, according to field and petrographic characteristics: the Gastre Superunit, composed of biotite-hornblende granitoids, porphyritic and
equigranular biotite granitoids and the younger Lipetrén Superunit, mostly composed of biotitic granitoids.
One outstanding difference between them is that the Gastre Superunit granitoids host mafic microgranular
enclaves (MME), mafic dikes and mafic stocks. In particular, in one of the localities where the porphyritic
biotitic granitoids crop out, spectacular occurrences of homogeneous enclave swarms have been found.
Mafic microgranular enclaves are interpreted as hybrids of acidic and basic magmas (Didier and Barbarin,
1991). In this contribution we describe field and petrographic characteristics of enclave swarms recognized
in the Gastre superunit.
Field appearance of the enclave swarms
Enclave swarms have a NW-SE strike and extend along a 2,5 km corridor (Fig. 1A). The area covered by the
enclave swarms has an approximated surface of 1, 24 km2. The geometry of the swarms is mostly tabular
(Fig. 1C). Host granitoid is a coarse-grained porphyritic amphibole-biotitic granodiorite with up to 2-3 cm
long microcline megacrysts.
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Fig. 1 - Field appearance of
the enclave swarms of the
Gastre area A) General view
of a swarm with a NW-SE
strike
B)
Microcline
megacrysts mechanically
intruded in an MME C)
Ovoid to ellipsoidal MME
shapes defining a planar
enclave swarm geometry D)
MME
appearing
as
dismembered mafic dikes
E) MME appearing with
irregular, angular shapes F)
More irregular, lenticular
shapes G) Diffuse borders
in an MME. Note that this
MME has microcline
megacrysts inside.
MME are ovoidal to ellipsoidal-lenticular (Fig. 1C), but also irregular (subangular to angular) shapes can be
observed (Fig. 1D, F). The MME size is variable, they are within the common reported size range of 1 cm
to 1 m in diameter (Vernon, 1983). MME have sharp boundaries with the granitoid, although sometimes the
borders may look more diffuse (Fig. 1G). In several places, microcline megacrysts of the granodiorite
impinge the enclave border (Fig. 1B). Sometime a dike appears dismembered in several MME (Fig. 1E).
The fabric of individual MME inside the swarms is magmatic and parallel to the NW-SE strike – 30/40º SW
dip magmatic fabric of the hosting granitoid and parallel to the strike of the swarm.
Although in many plutons the presence of MME swarms increases towards contacts with wall rocks (Vernon,
1983), in the Gastre area this relationship can not be controlled because plutons are nested in plutons, and
their contacts are difficult to identify.
Petrography
Petrographic analysis was made on the base of 10 thin sections. The MME have magmatic fabric (Fig.
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Fig. 2 - Petrography of the MME A) Magmatic texture defined mostly by plagioclase and biotite, and a plagioclase
xenocryst with numerous inclusions of hornblende, opaque minerals and apatite B) Plagioclase nuclei amalgamated and
surrounded by acidic borders. Note the difference in grain size with the groundmass material composed of smaller
plagioclase and biotite C) Anhedral poikilitic amphibole with pyroxne nueclei and anhedral to subhedral biotite, photograph
taken with parallel nichols D) Anhedral microcline poikilitically enclosing hornblende, biotite and plagioclase (oikocrysts)
2A) and are composed of plagioclase (60-15%), hornblende (35-10%), biotite (25-10%), quartz, microcline,
apatite, titanite and opaque minerals. They are classified as quartzose diorites transitional to monzonites and
quartzose monzonites.
Plagioclase is euhedral and has calcic nuclei and acidic borders. Some euhedral, early-formed nuclei are
amalgamated and surrounded by plagioclase of late crystallization (Fig. 2B). Some plagioclase individuals
have inclusions of hornblende, acicular apatite, titanite and opaque minerals (Fig. 2A).
Hornblende is subhedral and can occasionally have pyroxene nuclei (Fig. 2C). Acicular apatite presence indicates a magmatic quench origin (Vernon, 1983, Wyllie et al., 1962) when hotter mafic magma gets in contact with the cooler felsic magma.
Quartz and microcline content is generally scarce and interstitial, except where they poikilitically enclose
plagioclase and mafic grains conforming oikocrysts, (Fig. 2D) common in monzodioritic enclaves
(Janousek et al., 2000).
The Gastre enclave swarms are a field evidence that juvenile, mafic magmas have been added to the central
Patagonian crust in late Triassic - early Jurassic times.
REFERENCES
• Didier, J. and Barbarin, B.; 1991: The different types of enclaves in granites - Nomenclature. In: B.B. J. Didier (ed), Enclaves and Granite
Petrology, Orsay, France, pp. 19-23.
• Janousek, V., Bowes, D. R., Braithwaite, C. J. R. and Rogers, G.; 2000: Microstructural and mineralogical evidence for limited involvement
of magma mixing in the petrogenesis of a Hercynian high-K calc-alkaline intrusion: the Kozárovice granodiorite, Central Bohemian Pluton,
Czech Republic. Edinb. Roy. Soc. Trans., Earth Sciences, 91, 15-26.
• Rapela, C. W., Dias, G. F., Franzese, J. R., Alonso, G. and Benvenuto, A. R.; 1991: El Batolito de la Patagonia central: evidencias de un
magmatismo triásico-jurásico asociado a fallas transcurrentes. Rev. Geol. Chile, 18, 121-138.
• Vernon, R. H.; 1983: Restite, xenoliths and microgranitoid enclaves in granites. Journal and Proceedings, Roy. Soc. N. S. Wales, 116, 77103.
• Wyllie, P. J., Cox, K. G. and Biggar, G. M.; 1962: The habit of apatite in synthetic systems and igneous rocks. J. Petrol., 3, 238-243.
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Session 3
GEOPHYSICAL PROSPECTING
AND GEODESY
GEOSUR2010
ANALYSIS OF A PRECISE REGIONAL GEOID MODEL IN
BUENOS AIRES PROVINCE COMPUTED BY LEAST SQUARES COLLOCATION
3-01
Bagú, D.1*, Del Cogliano, D.1,2, Scheinert, M.3, Dietrich, R.3, Schwabe, J.3, Mendoza, L.1,2
(1) Grupo Geodesia Espacial, Facultad de Ciencias Astronómicas y Geofísicas, Universidad
Nacional de La Plata, La Plata, Argentina
(3) Institut fur Planetare Geodasie, Technische Universitat Dresden, Dresden, Germany
* Presenting author’s e-mail: dbagu@fcaglp.unlp.edu.ar
An analysis of a precise regional geoid model in Buenos Aires province computed by Least Squares
Collocation (LSC) is shown. The study zone (lat 35°S to 38.5°S, long 63°W to 58.5°W) is mainly flat
with two zones to the south with rough topography.
To achieve the goal, terrestrial and satellite data were combined in the well known Remove-ComputeRestore technique. The terrestrial data consisted in gravity observations, levelling heights and GPS on
benchmarks.
The long-wavelengths components of the Earth’s gravity field were modelled by the EGM2008 global
gravity model. The SRTM3v.4 was used to compute the topographic effects in the Helmert’s second
method of condensation scheme (short-wavelengths). Taking into account this reduction, gravity
anomalies were computed.
After removing the long and short wavelengths from the gravity anomalies, the residual data (over
9462 gravity stations) was used to compute 156 residual geoid heights by LSC (Nlsc) using the
GravSoft package. The differences with respect to the GPS/levelling heights observations (Nobs) were
estimated (“only gravity solution”). The results show that 47% of the absolute differences between
Nlsc and Nobs are below 5cm, and 83% below 10cm.
Other solutions using the 9462 residual gravity anomalies but now also GPS/levelling as input
observations were computed, showing improvements with respect to the “only gravity solution”.
GEOPHYSICAL SURVEY IN THE NORTHERN REGION OF CUYO BASIN
3-02
García, M.1*, Luna, E.1, Alvarez, O.1, Spagnotto, S.2, Nacif, S.2, Martínez, P.3, Gimenez, M.3
(1) Instituto Geofísico Sismológico Volponi – Univ. Nac. de San Juan. Ignacio de La Roza y Meglioli
S/N. Rivadavia, San Juan (5400)
(2) CONICET
(3) CONICET. Instituto Geofísico Sismológico Volponi – Univ. Nac. de San Juan
* Presenting author’s e-mail: maximiliano.a.garcia@gmail.com
A seismic-gravimetric survey was made in the northern area of the Cuyo basin, between the cities of
San Juan and Mendoza.
From the processing of gravimetric data a Bouguer map of anomalies was obtained, which was
adequately filtered to the purpose of obtaining residual anomalies linked to the upper crust. The
analysis of these Bouguer residual anomalies allowed to identify the Jocolí graben on the base of the
fold and thrust belt of Precordillera. Anomaly enhancement techniques such as analytical signal, tilt
and phase of tilt were done to highlight different wavelength anomalies. These gravimetric gradients
match with the eastern edge of the suture area of the Pie de Palo range. Once the morphology of
serious anomalies was determined, the depths and distribution of generating sources of said anomalies
through the Werner and Euler deconvolutions were analyzed as well as the average of power
spectrum for the structures of the survey area.
From the analysis of seismic data, combined with well logs and gravimetric results, it was possi101
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ble to interpret three horizons: Paleozoic, Triassic and Tertiary-Quaternary. With the obtained results,
one upper crust models were done in two sections where there is more seismic coverage. The models
proved the Bouguer residual anomaly and the interpretation of the seismic horizons. In the model
located to the South of the survey area, a sedimentary body of around 7 kilometers deep filled by
Triassic and Tertiary-Quaternary sediments is interpreted. This indicates one tectonic clash area of
thin skinned and thick-skinned corresponding to Cuyania and Pampia terrane.
In the section located to the north of the survey area, a small inverted depocenter was interpreted
through seismic and below it the Paleozoic lies unconformably. The sedimentary layers that were confirmed by gravity and seismic, generate prospective expectation of hydrocarbon in the area.
MONITORING AND FORECASTING THE STATE OF THE SOUTH ATLANTIC
MAGNETIC ANOMALY
3-03
Gianibelli, J.C.*
Departamento de Geomagnetismo y Aeronomía, Facultad de ciencias Astronómicas y Geofísicas,
Universidad Nacional de La Plata, Argentina
* Presenting author’s e-mail: jcg@fcaglp.unlp.edu.ar.
Introduction
The models of Geomagnetic Field of the Earth (IGRF: International Geomagnetic Reference Field),
are published from 1900 up to 2010 at intervals of 5 years. Barraclough (1978) published and
normalized all available models coefficients of the spherical harmonics analysis for the magnetic field
of the earth from 1550 up to 1978. From this models, Gianibelli (2006) calculated the energy
evolution for dipole and cuadrupole effects at the surface of the Earth, from 1550 to 2005 and made
a forecast for the period 2010-2500. The energy of cuadrupole with respect of the dipole energy is
less than 2% at the present. The evaluation of the energies from IGRF coefficients, corresponding to
the orders 1,2,3 and 4, and the prediction of them up to 2500, is presented in Fig. 1. This result was
Fig. 1 - Energy
relationship
versus
dipole energy W1.
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Table 1 - Results obtained from annual mean values analysis.
OBS:
YEAR
HUA
F:nT
LQA
F:nT
VSS
F:nT
PIL
F:nT
HER
F:nT
LAS
F:nT
TRW
F:nT
ORC
F:nT
AIA
F:nT
DRV
F:nT
2000
26290
23790
23300
23470
26230
23840
26480
33090
39430
69380
2050
23850
21150
21820
20620
20870
21150
23080
27370
34480
67200
2100
21100
18860
20050
17770
15450
18910
19680
21650
29530
64980
¢F 1:
-5190
-4930
-3250
-5700
-10780
-4930
-6800
-11440
-9900
-4400
calculated by a linear model of each coefficient of spherical harmonics analysis (SHA) of orders 1,2,3
and 4, and the most important result is that order 2 of the SHA represents the effects of magnetic field
at the surface of the Earth and the evolution of the South Atlantic Magnetic Anomaly (SAMA).
The Magnetic Observatories in the SAMA region, and in the South Hemisphere are (Fig. 2):
Huancayo (HUA), La Quiaca (LQA), Pilar (PIL), Vassouras (VSS), Hermanus (HER), Las Acacias
(LAS), Trelew (TRW), Islas Orcadas del Sur (ORC), and Vernadsky (AIA). The Magnetic
Observatory of Dumond D’Urville (DRV) is also plotted because is close to the Geomagnetic South
Pole region and is monitoring the change of intensity.
The annual mean values of HUA are important because they allow to monitor the possible future
Fig. 2 - Distribution of Magnetic Observatories.
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Table 2 - Results obtained from the IGRF model.
OBS:
YEAR
HUA
F:nT
LQA
F:nT
VSS
F:nT
PIL
F:nT
HER
F:nT
LAS
F:nT
TRW
F:nT
ORC
F:nT
AIA
F:nT
DRV
F:nT
2000
26069
23507
23431
23622
26358
23593
26844
33630
39889
66693
2050
24138
21382
22388
20962
21273
20990
22906
28526
34385
65587
2100
22208
19257
21345
18301
16188
18387
18968
23421
28882
64480
F 2:
-3861
-4250
-2086
-5321
-10171
-5207
-7875
-10209
-11007
-2213
Table 3 - Discrepancies of estimated values for 2100
OBS:
HUA
F:nT
LQA
F:nT
VSS
F:nT
PIL
F:nT
HER
F:nT
LAS
F:nT
TRW
F:nT
ORC
F:nT
AIA
F:nT
DRV
F:nT
F 1:
-5190
4930
-3250
-5700
-10780
-4930
-6800
-11440
-9900
-4400
F 2:
3861
4250
-2086
5321
10171
5207
7875
10209
11007
-2213
DIFF:
-1329
-679
-1164
-379
-609
277
1075
-1231
1107
-2188
changes of magnetic equator position.
Data analysis and results
The annual mean values of total magnetic intensity (F) of each observatory is analized and the
tendence through time is calculated.
The objetive of this paper is to obtain the values of F from this linear time series model for 2000 and
to predict the tendency for 2050 and 2100 for each observatory and compare the results obtained by
the IGRF model for the same observatories in the same date. From this comparison it is possible to
predict the state of SAMA in the future from two different elementary methods.
The results starting from annual mean values analysis of each observatory is presented in Table 1.
Table 2 shows the values obtained from the IGRF model. The ¢F value changes from 2000 to 2100 is
shown in each table.
In Tables 1 and 2, ¢F values are different, and in the case of DRV Observatory, ¢F is 50 % greater than
the IGRF estimation from the annual mean values.
Table 3 is a resume of the discrepancy, wich is the difference DIFF=¢F1 – ¢F2 calculated for each
observatory.
Conclusion
With this two methods, a continuous magnification of the total intensity of magnetic field depression
of the SAMA region is predicted. The estimation of the annual mean changes for the time interval
2000-2100 in some observatories is less than the estimation from the IGRF change for the same epoch
(LAS, TRW and AIA).
The values of DIFF show a change of sign and magnitude in nT. A great difference occurs in the
region of south pole (DRV), and in the region of magnetic equator (HUA), and this is a possible
situation is the fitting with a cut up of degree 10 in the model of the IGRF. In this scenario, changes
in the solar wind that interacts with the magnetic field of the Earth in the SAMA region are expected,
with an important deformation of the inner radiation belts to ionospheric levels according with SAMA
evolution, the total magnetic field intensity reduction and the effects in the ionospheric pattern
equivalent current systems. This possible situation is evaluated by the estimation of changes of the
energy of the dipole and cuadrupole fields effects at different ionospheric heights for 2000, 2050 and
2100.
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Fig. 3 - Energy variations for year 2000 and predicted for years 2050 and 2100.
This result is presented in Fig. 3.
In conclusion, the cuadrupole energy from 2050 to 2100 is increasing with a magnitude greater than
the increment from 2000 to 2050. Possibly, a change of solar wind-Earth magnetic field interactions
may account for this trend.
The relatively low number of magnetic observatories in the South Hemisphere for monitoring the
evolution of SAMA is a problem that should be considered in the near future. It is suggested to build
in the future an improved network of observatories that monitor the absolute values of the
geomagnetic elements at the surface of the Earth to model the IGRF with greater detail.
REFERENCES
• Barraclough, D. R. 1978, Spherical Harmonic Models of the Geomagnetic Field. Institute of Geological Sciences. Geomagnetic
Bulletin 8. 1-66.
• Gianibelli, J. C. 2006. Sobre la Evolución temporal del Dipolo y Cuadrupolo del Campo Geomagnético. Geoacta, vol 31, 175-181.
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IMPORTANCE AND FUTURE OF THE MAGNETIC
OBSERVATORY NETWORK IN SOUTH AMERICA
3-04
Gianibelli, J.C.1*, Sánchez Bettucci, L.2, García, R.E.3, Rodriguez, G.D.3, Quaglino, N.1,
Novo, R.4, Tancredi, G.5
(1) Departamento de Geomagnetismo y Aeronomía, Facultad de Ciencias Astronómicas y
Geofísicas, Universidad Nacional de La Plata
Paseo del Bosque S/N. 1900 La Plata, Argentina.TE: (054)0221-4236594 ext 132
(2) Área Geofísica-Geotectónica, Departamento de Geología, Facultad de Ciencias, Universidad de
la República, Uruguay
(3) Departamento de Electrónica. Facultad de Ciencias Astronomicas y Geofísicas
Universidad Nacional de La Plata. Paseo del Bosque S/N 1900 La Plata Argentina
(4) Instituto de Ciencias Geológicas, Facultad de Ciencias, Universidad de la República, Uruguay
(5) Instituto de Ciencias Físicas, Universidad de la República, Uruguay
* Presenting author’s e-mail: jcg@fcaglp.unlp.edu.ar
Introduction
The Earth’s magnetic field is known in South America since the beginning of twentieth century. The
first Magnetic Observatory was Pilar (PIL), and started operations in 1904 with the absolute determination of Declination, Horizontal and Vertical components and their variations with a photographic
recorder. Following PIL, the other observatories were Huancayo (HUA, started in 1922), Vassouras
(VSS, started in 1915) La Quiaca (LQA, started in 1920 and stopped in 1992), Islas Orcadas del Sur
(ORC, started in 1905, before the International Geophysical Year (IGY) in 1957). After the IGY, followed the installations of the observatories of Tatuoca (TTB, started in 1957), Trelew (TRW, started
in 1957), Las Acacias (LAS, started in 1961) and Port Stanley (PST, started in 1994). Now, the
Goverment of Uruguay is constructing the non-magnetic houses to install a new observatory at a place
named “Casa de Aigua” (Fig. 1).
The future Observatory of Aigua (ODA), and the others observatories locations are shown in Fig. 2.
The objective of this paper is to give a brief overview of the state of the Total Magnetic Intensity for
ODA, and comparatively for the others observatories, using the IGRF-11 model.
Fig. 1 - Residence for the magnetic installation at “Casa de Aigua”.
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Fig. 2 - Location of the Magnetic observatoires in South America.
Data analysis and results
The proton precession magnetometer installed at the “Casa de Aigua” was designed in the Faculty of
Astronomical and Geophysical Sciences (UNLP) by Ing. Ezequiel Garcia and Ing. Guillermo
Fig. 3 - Comparative IGRF values of Total Magnetic Intensity F for PIL, ODA, LAS and VSS time series.
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Fig. 4 - The total magnetic intensity changes with latitude determined for HUA, LQA, PIL, TRW, PST and ORC
Rodriguez. The system is similar to the instrument wich operates at LAS Magnetic Observatory
(Garcia et al., 2007). For analyzing the site of ODA, we evaluated and compared all the elements of
the geomagnetic field, obtained by the model of the International Geomagnetic Reference Field
(IGRF-11) from 1900 to 2015. This model is based in the Spherical Harmonic Analysis up to degree
10 of the all absolute values obtained by the World Magnetic Observatory Network (Lanza and
Meloni, 2006; Merrill et al., 1996).
The results of IGRF evaluation for each observatories show important values for the magnetic total
Fig. 5 - Secular variations characteristic for HUA, LQA, PIL, TRW, PST and ORC.
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intensity secular change in the time interval 1900-2010:
TTB=-44.3 nT/year; HUA=-41.4 nT/year; LQA=-43.2 nT/year; VSS=-20.1 nT/year; PIL=-52.7
nT/year; ODA=-46.2 nT/year; LAS=-50.9 nT/year; TRW=-77.1 nT/year; PST=-87.5 nT/year; ORC=99.5 nT/year.
These values demonstrate that this region has the minimum change of the secular variation in the
region of the VSS Observatory. LAS, PIL, and ODA sites have similar values.
The comparative IGRF values of Total Magnetic Intensity F for PIL, ODA, LAS and VSS time series
is shown in Fig. 3.
The total magnetic intensity changes with latitude determined for HUA, LQA, PIL, TRW, PST and
ORC, is shown in Fig. 4. The minimum of F is seen in the region of PIL.
The secular variations characteristic for HUA, LQA, PIL, corresponding to continental environment
aspects, and TRW, PST and LQA, corresponding to oceanic environment, are shown in Fig. 5.
REFERENCES.
• García R. E., Gianibelli J. C., Solans J. H. y Quaglino N., 2007. Ampliación de la capacidad de memoria en los magnetómetros de
precesión protónica. Geoacta 32: pp.207-212.
• IGRF-11 (2010). http://ngdc.noaa.gov/geomagmodels/IGRFWMM.jsp
• Lanza R. and Meloni A (2006). The Earth’s Magnetism, pp 1-66. Springer, Berlin.
• Merrill R.T., McElhinny M.W.and McFadden P.L. (1996). The magnetic field of the Earth: Paleomagnetism, the core and the deep
mantle. Academic Press, San Diego, California, 531 pp.•
IGMAS+ A NEW 3D MODELLING TOOL FOR GRAVMAG FIELDS
AND GRADIOMETRY
3-05
Goetze, H.J.*, Schmidt S.,
Institute of Earth Sciences, University Kiel, Otto-Hahn-Platz 1, 24118 Kiel, Germany
* Presenting author’s e-mail: hajo@geophysik. uni-kiel.de
It is well known that 3D gravity and magnetic modelling appreciably improves the results of distinct
depth imaging projects. This regards especially to areas of strong lateral velocity and density contrasts
and corresponding imaging problems. Typical areas where grav/mag modelling has been successfully
used are sub-salt and sub-basalt. The interactive 3D gravity and magnetic application IGMAS+
(Interactive Gravity and Magnetic Application System) has been around for more than 20 years. Being
initially developed on a mainframe and then transferred to the first DOS PCs, it was in the 90s adapted
to Linux PCs. The program has proven to be very fast, accurate and easy to use once a model has been
established.
The analytical solution of the volume integral for the gravity and magnetic effect of a homogeneous
body is based on the reduction of the volume integral to an integral over the bounding polyhedrons;
triangles in the case of IGMAS+. Later the algorithm has been extended to cover all elements of the
gravity tensor as well. In the modelling interface, after geometry changes the gravity effect of the
model can quickly be updated because only the changed triangles have to be recalculated. Optimized
storage enables very fast inversion of densities. Changes of the model geometry are restricted to
predefined parallel vertical sections. This is a small restriction to the flexibility but makes geometry
changes easy. No complex 3D editor is needed. The vertical sections are displayed together with the
measured and calculated gravity fields.
The geometry is updated and the gravity recalculated immediately after each change. Because of the
triangular model structure IGMAS can handle complex structures (multi Z surfaces) like the
overhangs of salt domes very well. Drawbacks are the lack of integration with seismic interpretation
systems and the fairly complex and slow model building of the start model. The software development
was directed towards scientific usage at universities with frequent, often experimental changes.
The integration of the workflow and the tools is important to meet the needs of today’s more
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interactive and interpretative depth imaging workflows. For the integration of gravity and magnetics
IGMAS+ can play an important role in this workflow.
ADVANCES IN THE DETERMINATION OF A HEIGHT REFERENCE
SURFACE FOR TIERRA DEL FUEGO
3-06
Gomez,M.E.1,2*, Perdomo, R.1,2, Del Cogliano, D.1,2, Hormaechea, J.L.1,2,3
(1) Facultad de Ciencias Astronómicas y Geofísicas, Universidad Nacional de La Plata, La Plata,
Argentina
(3) Estación Astronómica Río Grande, Acceso Aeropuerto, V9420EAR Río Grande, Argentina
* Presenting author’s e-mail: megomez@fcaglp.unlp.edu.ar
From 1998 to the present, several efforts from different scientific institutions have contributed to
collect gravity data, as well as the establishment of leveling lines including GPS coordinates in Tierra
del Fuego province.
This work is devoted to the use of the Equivalence Source Technique (EST) as a tool for combining
different kind of data, in order to obtain a geoid model for Tierra del Fuego.
The first attempts to build a geoid model in the region were purely numeric, interpolating the geoid
undulations from a grid of measured differences between ellipsoidal and orthometric heights
Many experiences have been done using EST in order to estimate the impact of gravity contribution
in contrast with these numeric models, obtaining successful results from the combined data.
Besides, the possibility of including GPS measurements on the surface of the Lago Fagnano
(assuming that it is a geopotential surface) is also analyzed.
The main results show that it is possible to get a geoid model with a 1Û error of the order of 0.07 m
for a big portion of the island. This represents a significant improvement for the region where the
recent global geopotential models do not fit satisfactorily.
FIRST MAGNETOMETRIC SURVEY IN THE ZAPICÁN AND NICO PÉREZ AREA
(URUGUAY)
3-07
Novo, R.1, Seluchi, N.1, Suarez, I.1, Sánchez Bettucci, L.1*, Gianibelli, J.2
(1) Area Geofísica-Geotectónica, Instituto de Ciencias Geológicas, Facultad de Ciencias-UdelaR,
Montevideo, Uruguay
(2) Departamento de Geomagnetismo y Aeronomía de la Facultad de Ciencias Astronómicas y
Geofísicas, Universidad Nacional de La Plata. Paseo del Bosque S/N, CP 1900, La Plata.
Argentina
A magnetometer survey in the Zapicán and Nico Pérez region was performed, Department of
Lavalleja, Uruguay. The area is characterized by a deformed granitic basement intruded by tholeiitic
basaltic and basalts andesite dikes (age 581 ± 13 Ma). These dikes have an approximate E-W trend in
this area specifically, with few outcrops. These dikes present porphyric texture with plagioclase and
clynopiroxene. Recent studies suggest an important contribution of magnetite and/or titanomagnetite.
A transect in a N-S direction was realized (6 kilometers long) with the purpose of corroborate the
existence of intrusive bodies and locate other buried bodies. The instrument used is a proton
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precession magnetometer from Geometrics brand, model G-856. The magnetic profiles obtained show
evidence of significant magnetic anomalies that testify the presence of these dikes.
MAPS OF ABSOLUTE GRAVITY, GRAVITY ANOMALY, AND
TOTAL MAGNETIC FIELD ANOMALY OF VENEZUELA FROM SATELLITE DATA
3-08
Guevara, N.O.1*, Reyes, A.G.2, Tabare T.2
(1) Universidad Central de Venezuela, Escuela de Geología, Minas y Geofísica. Caracas, Venezuela
(2) Agencia Bolivariana para Actividades Espaciales, MCTI. Caracas, Venezuela
* Presenting author’s e-mail: nurisorihuela@gmail.com
Abstract
We present three maps (scale 1:2.000.000) of absolute gravity, Bouguer anomaly and total magnetic
field anomaly of Venezuela, in the geographic window 0º to 13º latitude N and 58º to 74º longitude
W. The absolute gravity and Bouguer anomaly are projected on the mean sea level, while the total
magnetic field anomaly is projected to 4000 meters above the mean sea level. The database used for
the construction of each map has 188.072 sampling points with spacing of 3.7 km. Absolute gravity
was obtained from the Venezuelan gravity satellital database (Orihuela and Garcia, 2009). The calculation of the Bouguer anomaly was made from the absolute gravity and ETOPO2v2 terrain model
(NGDC, 2006). The total magnetic field anomaly was calculated using the combined model EMAG2
(Maus et al., 2009) which was developed from satellite data of the CHAMP mission and land and
marine data from the global network. This paper presents a general review of the absolute gravity,
Bouguer anomaly and total magnetic field anomalies of the major geological structural features of
Venezuela and it classifies the Venezuelan territory gravitationally and magnetically.
Introduction
There are relatively few direct measurements of absolute gravity and total magnetic field at the surface of the Earth due to the difficulty of transportation and handling of instruments for this purpose.
However, a large amount of absolute gravity data and total magnetic field data from satellites are now
available. Regular grids of absolute gravity and total magnetic field measurements represent the necessary condition to generate gravity anomaly and total magnetic field anomalies maps that can represent the distribution of changes in density and magnetization of the subsurface.
In addition to the existing studies, this work presents total magnetic field anomaly map of Venezuela
derived from satellite (magnetic anomaly EMAG2 model) projected 4000 m above the mean sea level,
allowing regional geological interpretations of the subsurface of our territory without disturbance for
cultural noise or superficial geological features.
Methodology
The absolute gravity values available in the Venezuelan gravity satellital database (Orihuela and
García, 2009) were reduced to estimate Bouguer anomaly, the digital terrain model selected was
ETOPO2v2. The density of reduction was 2.67 g/cm3 and the theoretical gravity estimation was made
from the 1967 International gravity equation. This allowed the comparison with the terrestrial data.
The topographic correction was applied using the Oasis Montaj algorithm (Geosoft, 2007). The free
air correction was applied with the assumption that gravity varies 0.3086 mGal/m.
The total magnetic field anomaly was calculated from the Earth Magnetic Anomaly Grid (EMAG2).
The magnetic anomalies are associated with magnetic highs and lows that complement positively or
negatively depending on the lateral contrast of magnetic susceptibility and the relative position of the
bodies with respect to the orientation of the total field vector in the study area.
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Fig. 1 - Venezuelan absolute gravity map, units in mGals (a); Venezuelan gravity anomaly map, units in mGals (b);
Venezuelan total magnetic field anomaly map, units in nT (c); Venezuela geographical ubication (d). For all these maps
the projection system is WGS84. Longitude and latitude are in degrees.
Results
Absolute gravity map (Fig. 1a). From north to south: Gravity high at the southern end of the Lesser
Antilles and the extinct Aves arcs, with a clear continuity of the Lesser Antilles Arc with the Margarita
Island platform, and of the Aves Arc with the Blanquilla Island platform (A), the gravity highs associated with the Netherlands and Venezuelan islands (B), a relative gravity low associated with Bonaire
Basin (C) with a high gravimetric value to the east that seems to have continuity with the gravity high
associated with the previously mentioned arcs. In the Maracaibo Lake Basin the map highlights a pronounced gravity low, which is sub-parallel to the Andean Chain (D) and the expression of Icotea Fault
(E). Among the Barinas-Apure basin and Eastern basin, the map shows the gravitational expression
of the Baúl High (F), which is evidenced by a significant gravity high that breaks the continuity
between the basins cited above, it extends to the northeast in the basement of Anzoátegui State (G)
marking the northern flank of the Espino Graben (H). In the Maturín Subbasin, there is a gravity low
associated with the depocenter of the basin (I) which is expressed as a discontinuous low, that represents the expression of the faults present in the zone (Urica, San Francisco, etc), as well as the prolongation in the basin (J). At the eastern end of the map, a gravity high in the south limit of the
Orinoco Delta is present (K), with a NW-SE direction, defining the southwestern termination of a
wide channel of low gravity that extends sub-parallel to the northern South America Atlantic coast (L)
and changes direction between the parallels 10° and 11° N to join the low gravity belt and associated
to the accretionary prism of the Lesser Antilles (M).
Gravity anomaly map (Fig. 1b). From north to south: Positive gravity anomalies range from 269 to
86 mGal in the area, bounded by the southern end of the Lesser Antilles Arc and Venezuelan and
Netherlands Antilles, the southern end of Venezuela and Grenada Basins, the eastern end of the northcentral coastal platform. The distribution of the contours reflect a semicircular geometry, with concavity to the north, and may indicate the expression of a tectonic stress pattern (transpressional plate
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boundary), with a component of convergence at the extremities in the western and eastern boundaries.
The gravity anomalies in the physiographic provinces are distributed, in general, with negative intervals, as is characteristic of continental areas. This trend is broken in the central plains region where a
strong positive anomaly is present. In the low gravity belt of the Andean region, the contours are interrupted by a short wavelength gravity signatures associated with high density outcrops. In the eastern
part of Anzoátegui State and the Monagas State, a negative gravity anomaly is associated with the
Venezuela eastern basin with a minimum of -269 mGal. The Guiana Shield is characterized by mostly negative gravity anomalies, distributed between 0 and -120 mGal, with smaller areas of sparse positive gravity anomalies. The northeast side of the shield and the Orinoco delta front is characterized
by a positive gravity anomaly that extends east to reach the domain of positive anomalies characteristic of the shelf in the Atlantic Ocean.
Total magnetic field anomaly map (Fig. 1c). Magnetic anomalies are associated to the presence of
magnetic susceptibility contrasts and the relative position of the bodies with respect to the total field
vector orientation in the study area. The islands present magnetic anomalies in the range of -132 to
175 nT. Magnetic anomalies to the north of the Aves Island and the Los Roques Archipelago present
values which range from -130 to -35 nT with preferential direction N78°W, sub-parallel to the
Curaçao high. The center of this anomaly is slightly shifted southward with respect to the main axis
of the Curaçao high. The Coastal Range is characterized by the presence of values in the range -22 to
36 nT, with preferred orientation N94°W. The Interior Ridge is characterized by magnetic contours
that range from -3 to 34 nT, with preferred orientation E-W. The southern limit of the Araya Peninsula
has very well-defined anomaly contours with a broad E-W direction; its range is between 49 to 60 nT
and follows the preferential orientation of the El Pilar fault. The eastern part of the Cariaco Trench is
distinguished by a high magnetic anomaly of the Araya Peninsula; the length of this high is about 40
km and presents values in the range of 18 to -28 nT. The geographic window between the parallels 4º
and 6º N and the meridians 61º and 64º W has no magnetic data available. The magnetic response is
mainly positive in the Guiana Craton. Along the Orinoco River, stands a magnetic corridor, with a
preferential direction N59ºE, whose values range from -371 to -56 nT, representing the area of most
relevant and extensive low magnetic anomalies in the Venezuelan territory. In the Barinas-Apure
Basin and its eastern alignment, a low magnetic anomaly in the direction N70°E with values in the
range of -56 to -155 nT is found. This area represents a strip of magnetic anomalies of great importance that divides the territory in a NE-SW direction. No information are available to extend the analysis of the magnetic anomalies in the region of the Andean mountain system.
Acknowledgements
The authors appreciate the facilities of access to geophysical information in the databases of the
NOAA and ICGEM service, and the support provided by Franz Barthelmes, Nikolaos Pavlis, and
Stefan Maus.
REFERENCES
• Maus, S., Barckhausen, U., Berkenbosch, H., Bournas, N., Brozena, J., Childers, V., Dostaler, F., Fairhead, J., Finn, C., Von Frese,
R., Gaina, C., Golynsky, S., Kucks, R., Lühr, H., Milligan, P., Mogren, S., Müller, R., Olesen, O., Pilkington, M., Saltus, R.,
Schreckenberger, B., Thébault, E. AND F. Caratori. (2009) EMAG2: A 2-arc-minute resolution Earth Magnetic Anomaly Grid
compiled from satellite, airborne and marine measurements. Journal of Geophysical Research. Estados Unidos. DOI: 10.1029, 30
pp.
• National Geophysical Data Center (2006) ETOPO2 V.2 2-Minute Gridded Global Relief Data. Nacional Geophysical Data Center.
Estados Unidos.
• ORIHUELA, N & A. GARCÍA (2009) Mapas de anomalías gravimétricas y magnéticas de Venezuela generados a partir de datos
satelitales. Thesis. Universidad Central de Venezuela. Facultad de Ingeniería. Escuela de Geología, Minas y Geofísica. Caracas.
Venezuela. 205 pp.
• Pavlis, N., Holmes, S., Kenyou, S. and J. Factor (2008) An Earth Gravitational Model to Degree 2160. EGM2008. National
Geospatial Intelligence Agency. EGU General Assembly, Vienna, Austria.
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GEOPHYSICAL INVESTIGATION OF THE NAVARINO ISLAND PLUTONS
(BEAGLE CHANNEL, CHILE)
3-09
Peroni, J.I.1*, Tassone, A.1, Lippai, H.1, Hervé, F.2, Menichetti, M.3, Lodolo, E.4
(1) CONICET-INGEODAV. Dpto. de Ciencias Geológicas. Facultad de Ciencias Exactas y Naturales.
Universidad de Buenos Aires. Argentina
(2) Departamento de Geología, Universidad de Chile, Casilla 13518, Correo 21, Santiago, Chile
(3) Istituto di Scienze della Terra. Università di Urbino. Campus Scientifico Universitario.- 61029 Urbino. Italy
(4) Istituto Nazionale di Oceanografia e di Geofisica Sperimentale (OGS), Trieste, Italy
* Presenting author’s e-mail: peroni@gl.fcen.uba.ar
Geological setting
The Canal de Beagle Plutonic Group (BCPG) is a complex of granitoids exposed along the margins
of the Beagle channel, in the Gordon, Londonderry, Hoste and Navarino islands (Suárez et al., 1985a).
The BCPG belongs to the Fuegian Batholith (Figure 1c), the southernmost segment of the three that
compose the Patagonian Batholith (Hervé et al. 2007).
Along the northern shore of the Beagle Channel, in the Argentine part of Isla Grande de Tierra del
Fuego, there are small outcrops, less than 20 km2 in size (Fig. 1b) of the Ushuaia Pluton (Menichetti
et al., 2007; Tassone et al., 2007; Peroni et al., 2008; Peroni et al., 2009a) and Trapecio Pluton (Peroni
et al., 2009b), while in the southern sector of the channel, in the Navarino island, larger plutonic
bodies outcrop (Fig. 1b). The upper Cretaceous (83-93 Ma, hornblende K/Ar age, Suárez et al., 1985a)
Castores and Santa Rosa plutons plutons hosted in the Yahgán Formation are mainly composed of
tonalite/granodiorite facies, with common dismembered melanocratic synplutonic dykes (Suárez et
al., 1985b). The Castores Pluton shows a system of radial and concentric fractures, with an unfoliated
granitoid core, interpreted as a diapiric intrusion (Suárez et al., 1985a).
The Beagle Channel area and the Navarino island are dominated by a system of E-W sinistral strikeslip faults (Figure 1b) which belong to the Beagle Chanel fault system (Cuningham 1993) and are
associated with several normal faults (Menichetti et al., 2008). These transtensional structures show
many fault planes dipping at low angle and are superimposed on the north-verging thrust slices.
Sampling and data processing
In order to determine the geometry in depth of the Castores and Santa Rosa plutons, three magnetic,
lithologic, and structural data campaigns were carried out between 2008 and 2010 in the area of
Navarino and Hoste islands and in the Beagle Channel. Magnetic measurements were acquired with
two magnetometer (EG&G Geometrics and a Scintrex Envi Grad). 72 hand-oriented samples were
obtained in 13 sites for lithologic, paleomagnetic, magnetic susceptibility and AMS studies.
A DC-squid cryogenic magnetometer (2G-750R) was used in the laboratory to verify the absence of
remanent magnetization. This information is essential to generate the magnetic model presented in
this work. The Koenigsberger coefficient (Q), which compares the value of the remanent
magnetization (Jr) and the induced magnetization (Ji), is Q << 1 for the studied intrusive bodies. The
susceptibility values used for the model are shown in Table 1.
Modeling
The first step to make de model was to generate a grid from the 4070 measured stations which cover
an area of 430 km2, including both onland (along the northern margins of Hoste and Navarino islands)
and offshore (in the Beagle channel) data.
Based on this grid, an E-W-trending, 21-km-long magnetic profile (profile A-B in Fig. 1) was
generated which contains the highest density of measured data both in the Navarino Island as in
Beagle Channel offshore. The profile cuts across the central part of the Santa Rosa Pluton and a small
part of the northern sector of the Castores Pluton.
The magnetic profile (Fig. 2) shows two main anomalies. The easternmost one corresponds to the
outcrops of the Castores Pluton and reaches a value of +1020 nT, with a secondary maximum of +840
nT located 1 km to the west. The westernmost maximum (+1090 nT) occurs in correspondence with
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Fig. 1 - A) Location of the area of Fig. 1B within the southern tip of South America. B) Geologic map of the Navarino
and Hoste islands (modified from Suárez et al., 1985a). BCFS: Beagle Channel Fault System. Segment A-B: Magnetic
profile employed for the model. 1: Quaternary sediments 2: Yahgán Fm. 3: Ushuaia Pluton 4: Castores Pluton 5: Santa
Rosa Pluton 6: Other plutons belonging to the Canal Beagle Plutonic Group 7: Tortuga Complex. C) The three
segments that compose the Patagonian Batholith. BPN: North Patagonian Batholith. BPA: South Patagonian Batholith.
BF: Fuegian Batholith
outcrops of ultrabasic rocks on Calete Fique and Segura islands (Fig. 3C), which are very similar to
those observed in the Ushuaia pluton in the northern margin of the Beagle Channel (Fig. 1 of Peroni,
et al., 2009a). Except for this maximum, the value of magnetic anomaly remain around +400 nT for
the profile segment corresponding to the outcrops of Santa Rosa Pluton.
Geological and geophysical interpretation
Two intrusive bodies were modeled along the magnetic profile using the Encom ModelVision Pro 7.0
software (Encom Technology, 2002). The obtained model for Castores Pluton (Body 1, Figure 2)
yielded an ellipsoidal body with a 2 km. vertical axis, E-W horizontal axis of 4.4 km. and N-S
horizontal axis of 8 km. Given that the magnetic profile cuts across the northern margin of the
Castores Pluton, the obtained modeled body represents only a lateral image of the whole intrusive
body. The magnetic modeling of the Santa Rosa Pluton (Body 2, Figure 2) produced an overall oblated
body with an average thickness of 1 km., with horizontal axis of 7.1 (E-W) and 4 km. (N-S).
Table 1 - Values of the magnetic susceptibility
measured in laboratory, for the different lithologies
employed for the magnetic modeling. (*)
susceptibility from Peroni et al. (2009).
Unit
Susceptibility [SI]
Santa Rosa Pluton
30 x 10-3
Ultrabasic Facies SR
62 x 10-3 (*)
Castores Pluton
45 x 10-3
Yahgán Formation
6 x 10-5
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Fig. 2 - Magnetic model profile A-B. Location in Fig. 1b.
Fig. 3 - A: Block diagram composed by combining a MrSid satellite image , a digital elevation model (DEM), and the
magnetic model profile A-B located in Fig. 1; view from the north. Dashed lines indicate the limits of the outcrops of
both plutons. B: Panoramic view of the Castores pluton outcrops. C: Photography of the outcrops of the ultrabasic
rocks of the Caleta Fique and Segura island, where a magnetic anomaly maximum (1090 nT) was measured.
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The A-B magnetic section with the obtained modeled plutonic bodies was combined with a DEM
(SRTM30) and a MrSid satellite image (GeoCover Landsat – NASA) in Figure 3a which clearly
shows the topography and the outcrops of the studied plutons. The obtained modeled section provides
a 2D picture of the uppermost 2 km of the crust of Navarino island viewed from the north.
REFERENCES
• Cuningham; W. D. 1993. Strike-slip faults in the Southernmost Andes and the development of the Patagonian Orocline. Tectonics
12(1): 169-186
• Encom Technology, 2002. ModelVision Pro v.7.0. Encom Technology, Sydney, Australia.
• GeoCover Landsat – NASA <https://zulu.ssc.nasa.gov/mrsid/>
• Hervé F., Pankhurst R.J., Fanning C.M., Calderón M., Yaxley G.M.; 2007. The South Patagonian Batholith: 150 My of Granite
Magmatism on a plate margin. Lithos 97, 373–394.
• Menichetti, M., Tassone, A., Peroni, J.I., Gonzàlez Guillot, M., Cerredo M.E.; 2007: Assetto strutturale, petrografia e geofisica della
Bahía Ushuaia – Argentina. Rend. Soc. Geol. It., 4 (2007), Nuova Serie, 259-262, 3 ff.
• Menichetti M., Lodolo, E., Tassone A.; 2008: Structural geology of the Fuegian Andes and Magallanes fold-and-thrust belt – Tierra
del Fuego Island. Geologica Acta, 6, 1.
• Peroni, J. I., Tassone, A., Cerredo, M., Lippai, H., Menichetti, M., Lodolo, E., Esteban, F.,Vilas, J. F.; 2008: 3D Geophysic model
of Ushuaia Pluton. Tierra del Fuego. Argentina. GeoMod 2008 Bollettino di Geofísica teorica ed applicata. Nº2 supplement.
Extended Abstract. pp: 263-267
• Peroni, J.I., Tassone, A., Menichetti, M., Cerredo M.E.; 2009a: Geophysical modeling and structure of Ushuaia Pluton, Fuegian
Andes, Argentina, Tectonophysics doi:10.1016/j.tecto.2009.07.016
• Peroni, J. I., Tassone, A., Menichetti, M., Lippai, H., F.,Vilas, J. F.; 2009b: Geologia e geofisica del plutone del Cerro Trapecio Tierra del Fuego – Argentina. Rendiconti online Soc. Geol. It., Vol. 5: 160-163
• Suárez, M., Hervé, M. and Puig, A.; 1985a: Hoja Isla Hoste e islas adyacentes, XII Región. Carta Geológica de Chile No. 65, 113
pp. Servicio Nacional de Geología y Minería de Chile.113 pp
• Suárez, M., Hervé, M. and Puig, A.; 1985b: Plutonismo diapírico del Cretácico en Isla Navarino. IV Congreso Geológico Chileno,
Actas (4): 549-563
• SRTM30 Digital Elevation Map (DEM) <http://dds.cr.usgs.gov/srtm/version2_1/SRTM30/>
• Tassone, A., Peroni, J. I., Cerredo, M. E., Lippai, H., Vilas, J. F.; 2006: Estudio geofísico del cuerpo intrusivo Ushuaia. Margen norte
del Canal de Beagle, Argentina. XI Congreso Geológico Chileno. 7-11 de agosto, Antofagasta, Chile. Actas.
GEOPHYSICAL CHARACTERIZATION OF FILLED ZONES ALONG
THE COAST OF BUENOS AIRES
3-10
Prezzi, C.1*, López, R.2, Vásquez, C.1, Marcomini, S.2, Fazzito S.1
(1) CONICET – Universidad de Buenos Aires. INGEODAV, Dpto. Cs. Geológicas, FCEyN, UBA,
Ciudad Universitaria, Pabellón 2, Buenos Aires, Argentina
(2) Universidad de Buenos Aires. Dpto. Cs. Geológicas, FCEyN, UBA, Ciudad Universitaria,
Pabellón 2, Buenos Aires, Argentina
* Presenting author’s e-mail: prezzi@gl.fcen.uba.ar
The coast of Buenos Aires (Argentina) is located on the southern margin of La Plata river estuary. The
original morphology of this area was completely changed by fill works. Such works began in 1836
and continue until today. However, fill was carried out mostly between 1964 and 1991. The total filled
surface is of approximately 2054 hectares, with a coast advance ranging between 400 and 1000 m
(Marcomini and López, 2004) (Fig. 1). The anthropogenic changes imposed on the coast line
configuration have generated a great variety of problems in building foundations due to the
heterogeneous composition of the fill materials.
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Fig. 1 - Buenos Aires coastline in 1836 (Natural coast line), in 1964 (Artificial coast line) and nowadays. Modified
from Marcomini and López (2004).
Standard drilling methods do not provide the level of stratigraphic detail necessary to accurately
describe the geology and hydrogeology of a site and to evaluate the true distribution of fill materials.
Such lack of detail may result in construction worker health and safety issues, difficult excavation
conditions, and off-site waste disposal requirements. Consequently, it could generate project delays,
cost overruns and, in some cases, the termination of the project (Green et al., 1999; Byer and Mundell,
2004). On the other hand, near surface geophysical surveys allow for a very detailed and rapid
characterization of subsurface materials at relatively low cost (Byer and Mundell, 2004; Prezzi et al.,
2005).
Taking into account the ongoing urban development and the absence of information about the nature
of the filled zones in La Plata river, we try to determine the variation in the subsurface materials by
identifying areas of similar and dissimilar properties. The type, homogeneity, spatial distribution and
thickness of the fill materials will be investigated by means of ground geophysical surveys. Such
information is vital for the adequate evaluation of the coastal sectors’ development options, for the
corresponding environmental impact assessment and for a proper planning of urban expansion.
Our approach consists of a multi-faceted geophysical survey conducted applying three types of
geophysical methods: magnetic, ground penetrating radar (GPR) and electrical resistivity imaging
(ERI). These methods are sensitive to: 1) metallic/conductive objects such as reinforced concrete,
structural steel, and metal-bearing fill materials, and: 2) variations in soil and fill types based on
subtle changes in soil moisture, porosity, and chemistry across the site. We surveyed different filled
sectors (A and B) in Ciudad Universitaria (Fig. 2) with the aim of calibrate and test the suitability of
the distinct geophysical methods for the characterization of the diverse fill materials.
In sector A short wave-length, high amplitude, conspicuous and localized circular magnetic anomalies
ranging between -700 and 500 nT were detected (Fig. 3). Such anomalies suggest the presence of
demolition materials (beams, concrete blocks with iron rods, etc.) at shallow depths (fill). Maximum
fill depths of approximately 10 m were estimated applying Euler deconvolution. In sector B, the
magnetic survey detected a pattern which indicated the presence of an underground canal. Such
pattern showed an elongated positive anomaly of 1900 nT. In this sector, fill depths of approximately
5 m were calculated through Euler deconvolution. GPR profiles did not generate good results due to
high signal attenuation. The presence of clayey wet soil would be responsible of the very limited GPR
signal penetration. Only a couple of diffraction hyperbolae were observed. In sector A, two ERI
surveys were carried out using different electrode spacing and spread lengths. At the top of both
sections a 5m thick layer was observed, which showed a patchy resistivity pattern with values ranging
between 35 and 65 ohm.m (Fig. 4). This layer would indicate the existence of demolition material fill.
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A
Fig. 2 - Satellite images showing: A) the location of
Buenos Aires and Ciudad Universitaria, B) the location
of the sectors (A and B) surveyed in Ciudad
Universitaria. Continuous white lines: magnetic
profiles, dashed white lines: GPR profiles, black line:
ERIs.
B
Fig. 3 - Magnetic anomalies detected in sector A. Open circles: measured magnetic stations. Short wavelength, high
amplitude, circular anomalies ranging between -700 and 500 nT were detected.
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Fig. 4 - One of the ERI surveys conducted in sector A. Electrode spacing: 3m, spread length: 141 m.
Below this layer, an important resistivity reduction is registered (down to 7 ohm.m), probably related
to the phreatic zone. Below 20 m depth, resistivity increases up to 50 ohm.m (Fig. 4).
Although GPR profiles did not provide good results, ERI and magnetic methods proved to be useful
techniques, appropriate to characterize the filled zones along the coast of Buenos Aires. The obtained
results allowed the determination of the thickness, homogeneity and type of the fill material in the
studied areas. To further investigate the coastal filled zones in Buenos Aires, new magnetic, GPR, ERI
and microgravity surveys will be conducted.
REFERENCES
• Byer, G. B. and Mundell, J. A., 2004. Use of Geophysical Surveys for Fill Characterization and Quantity Estimation at Brownfield
Sites – A Case History. Proceedings: SAGEEP 2004, Environmental and Engineering Geophysical Society, Colorado Springs,
Colorado, United States.
• Green, A., Lanz, E., Maurer, H., and Boerner, D., 1999. A template for geophysical investigations of small landfills. The Leading
Edge, 2: 248-254.
• Marcomini, S.C. and López, R.A., 2004. Generación de nuevos ecosistemas por albardones de relleno en la costa de la ciudad de
Buenos Aires. Revista de la Asociación Geológica Argentina, 59(2):261-272.
• Prezzi, C., Orgeira, M.J., Ostera, H. and Vásquez, C.A., 2005. Ground magnetic survey of a municipal solid waste landfill: pilot
study in Argentina. Environmental Geology, 47(7): 889-897. doi: 10.1007/s00254-004-1198-6.
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BAJADA DEL DIABLO IMPACT CRATER-STREWN FIELD (ARGENTINA):
GROUND MAGNETIC AND ELECTROMAGNETIC SURVEYS
3-11
Prezzi, C.B.1*, Orgeira, M.J.1, Acevedo, R.2, Ponce, F.2, Martínez, O.3, Vásquez, C.1,
Corbella, H.4, González, M.2, Rabassa, J.2
(1) CONICET – Universidad de Buenos Aires. INGEODAV, Dpto. Cs. Geológicas, FCEyN, UBA,
Ciudad Universitaria, Pabellón 2, Buenos Aires, Argentina
(2) CADIC – CONICET. Ushuaia, Tierra del Fuego, Argentina
(3) Universidad Nacional de la Patagonia San Juan Bosco. Esquel, Chubut, Argentina
(4) CONICET – Museo Argentino de Ciencias Naturales Bernardino Rivadavia. Buenos Aires,
Argentina
* Presenting author’s e-mail: prezzi@gl.fcen.uba.ar
Bajada del Diablo impact crater field is located in the Northern Patagonian Massif, Chubut, Argentina
(Fig. 1). Impact craters have been identified on two rock types: the Quiñelaf Eruptive Complex and
Pampa Sastre Formation (Acevedo et al., 2009). Most of the rocks forming the Quiñelaf Eruptive
Complex have been classified as trachytes, but other rocks are present, such as rhyolites,
trachyandesites, trachybasalts, and pyroclastic rocks. Pampa Sastre Formation corresponds to
conglomerate layers with basalt clasts boulder and blocks in size (up to 50 cm in diameter) in a coarse
sandy matrix. The study area (Fig. 1) includes at least 66 impact craters found in Miocene olivine
basalts of the Quiñelaf Eruptive Complex and in the Late Pliocene/Early Pleistocene Pampa Sastre
conglomerate (Acevedo et al., 2009).
It is widely accepted that a key tool in the initial recognition and characterization of terrestrial impact
craters is geophysics (e.g. Pilkington and Grieve, 1992; Hawke, 2004). The magnetic signature of craters
varies considerably (Pilkington and Grieve, 1992), but an overall circular magnetic low due to
demagnetization of the target rocks and reduction in susceptibility is expected (Pilkington and Grieve,
1992; Hawke, 2004). Pilkington and Grieve (1992) established a set of general criteria that correspond to
the geophysical signature of impact craters. These criteria can be used to evaluate the hypothesis of impact
origin of circular structures. However, such origin can only be confirmed on the basis of geologic
evidence.
With the aim of further investigate the proposed impact origin of the circular structures identified in
Fig. 1 - Location map. The inset shows a satellite image of the study area. Modified from Acevedo et al. (2009).
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Fig.
2
Detailed
topography of crater 8.
Black dots: magnetic
stations measured in and
out crater 8.
Bajada del Diablo (Acevedo et al.,
2009), we carried out detailed
topographic,
magnetic
and
electromagnetic ground surveys in two
craters (8 and A) found in Pampa Sastre
conglomerates. Both craters are simple,
bowl-shaped structures with rim
diameters of 300 m and maximum
depths of 10 m (Figs. 2 and 3). They
have been partially filled in by debris
flows from the rims and wind-blown
sands (Acevedo et al., 2009).
Total magnetic field was measured at
1563 stations located in and out of craters
A and 8, using a Geometrics 856 proton
precession magnetometer (Figs. 2 and 3).
The obtained data were corrected for the
diurnal variations in the Earth’s magnetic
field and the IGRF value was subtracted.
Basalts boulders, sandy matrix and
infilling sediments were collected, and the
corresponding magnetic susceptibilities
were measured; the intensity of the
Fig. 3 - Detailed topography of crater A. Black dots: magnetic
remanent magnetization of basalt
stations measured in and out crater A.
boulders was also measured. 20 profiles
were surveyed at crater 8 with a GEM-2
small broadband electromagnetic sensor using 5 different frequencies. Detailed crater topography was
determined using a total station. 726 topographic points were surveyed in craters A and 8.
The magnetic anomalies show a circular pattern with magnetic lows (-100 to -200 nT) in the crater’s
floors, characteristic of impact structures. Furthermore, in the crater’s rims, high-amplitude,
conspicuous and localized (short wavelength) anomalies, ranging between 2000 and -1500 nT, are
observed (Figs. 4 and 5). Such large amplitude and short wavelength anomalies are not detected out
of the craters. Euler’s deconvolution was applied in order to estimate the depth of the sources. The first
and the second vertical derivatives, the analytic signal and the curvature attributes of the residual
magnetic field, were also calculated with the aim of sharpening and further analyse the detected
anomalies. 2.5 and 3D modelling were carried out, considering the existence of induced and remanent
magnetizations. The parameters used for each modeled body (i.e. susceptibility and remanent
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Fig. 4 - Magnetic
anomalies detected in and
out crater 8. Diamonds:
magnetic stations.
magnetization intensity) were measured in
the laboratory and/or estimated taking into
account previously published data.
For
all
used
frequencies,
the
electromagnetic profiles show lower
apparent electrical conductivities in the
crater’s floor, while the rims present
notably higher values (Fig. 6). Our results
suggest that in the crater’s floors Pampa
Sastre conglomerate would be absent or
deeply buried. On the contrary, the crater’s
rims exhibit high-amplitude, localized
magnetic anomalies and higher apparent
electrical conductivities, which would be
related to the anomalous accumulation of
basalt boulders and blocks remanently
magnetized (probably due to shock and
heat effects). The fact that such highamplitude anomalies are not present out of
Fig. 5 - Magnetic anomalies detected in and out
crater A. Diamonds: magnetic stations.
Fig. 6 - Apparent
electrical
conductivity
registered in crater 8 using
3950 Hz. Grey dashed
lines:
contour
lines
showing crater 8’s detailed
topography.
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the surveyed craters, supports this hypothesis. The morphological, geological and geophysical features
of the studied circular structures could only be satisfactorily explained assuming an extra-terrestrial
projectile impact.
REFERENCES
• Acevedo, R., Ponce, J., Rocca, M., Rabassa, J. and Corbella, H., 2009. Bajada del Diablo impact crater-strewn field: The largest
crater field in the Southern Hemisphere. Geomorphology, 110: 58-67.
• Hawke, P., 2004. The geophysical signatures and exploration potential of Australia’s meteorite impact structures. PhD Thesis, The
University of Western Australia, 314 pp.
• Pilkington, M. and Grieve, R., 1992. The geophysical signature of terrestrial impact craters. Reviews of Geophysics, 30: 161-181.
EARTH TIDE OBSERVATIONS IN TIERRA DEL FUEGO (ARGENTINA)
A.1*,
R.1,
Richter,
Perdomo,
Hormaechea, J.
Fritsche, M.3, Scheinert, M.3, Dietrich, R.3
L.2,
Mendoza,
L.1,
Del Cogliano,
3-12
D.1,
(1) Facultad de Ciencias Astronómicas y Geofísicas, Universidad Nacional de La Plata, Paseo del
Bosque, 1900 La Plata, Argentina
(2) Estación Astronómica Río Grande, Acceso Aeropuerto, V9420EAR Río Grande, Argentina
(3) Institut für Planetare Geodäsie, Technische Universität Dresden, 01062 Dresden, Germany
* Presenting author’s e-mail: richter.a@daad-alumni.de
The solid earth tides, originating from luni-solar forcing, manifest themselves in different effects,
such as gravity variations, deformations of the earth crust, and variations in the tilt of the earth surface
with respect to an equipotential surface. The ocean tidal loading generates an additional contribution
to these effects. The load tides depend on the ocean tide signal and the elastic-rheological properties
of the earth crust.
These tidal effects are contained in geodetic (e.g. GPS), astronomical (e.g. zenith tube) and
geophysical (e.g. gravity metre) observations and exceed nowadays achievable measurement
uncertainties. They must therefore be accounted for in high-accuracy observations of different
geophysical phenomena. On the other hand, such observations provide the opportunity to gain new
insights into the response of the solid earth to the tidal forcing.
The first systematic investigation of tidal effects in Tierra del Fuego was based on lake-level
observations in Lago Fagnano. Based on pressure tide gauge records at three sites in the lake the
amplitudes, phase angles, and circulation patterns for the four main tidal waves were determined. In
this way we utilized the lake as a 100 km long tidal tilt sensor. The comparison of the observed lake
tide signal with a model accounting for both solid earth tides and load tides revealed a significant
deviation of the observations from the theoretical prediction. It suggests that this difference is due to
an anomalous amplification of the load tides in the order of 20%. One possible explanation of this
anomaly consists in a deviation of the elastic crustal properties in the Lago Fagnano region from the
global earth model assumed in our load tide model.
With the aim to shed light onto the cause of the detected anomaly additional earth tide observations
were commenced at a number of sites in the Argentine part of Tierra del Fuego main island in
November 2009. Here we present the continuous tidal gravity metre and GPS records obtained so far.
The results of a preliminary tidal analysis of these records are compared to model predictions. They
are discussed with an emphasis on their possible implications for the effective elastic crustal rheology
in the region under investigation.
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MORPHO-BATHYMETRIC SURVEY OF LAGO ROCA (TIERRA DEL FUEGO)
3-13
Lodolo, E.1*, Tassone, A.2, Baradello, L.1, Lippai, H.2, Grossi, M.1
Universidad de Buenos Aires, Argentina
* Presenting Author’s email: elodolo@ogs.trieste.it
Lago Roca occupies a secondary tectonic lineament pertaining to the main Beagle Channel fault
system (Tassone et al., 2005). It is located within the Lapataia Natural Park, 25 km to the west of the
city of Ushuaia (Fig. 1). The lake, trending broadly NW-SE, is about 10.5-km-long, with average
width of about 0.7 km. In order to analyze the main morphological and shallow structural setting of
this basin, an extensive high-resolution seismic survey was carried out on November 2009, in the
frame of an Italian-Argentinean cooperative research study funded by the Italian Foreign Ministry.
These data have permitted to derive for the first time a bathymetric map of the lake (through the
conversion of seismic arrival times in water depths), and image morphologies and depositional
architecture of the glacial and glacio-lacustrine deposits filling the basin. These information,
combined with analysis of structural and geomorphological features of the surrounding areas, will aid
to reconstruct the Roca basin origin and identify the Late Quaternary inter-glacial episodes
responsible of the deposition of the sedimentary sequences.
The glacial activity, in combination with the sea level variations and tectonic activity, has played an
important role in shaping the morphology of the Lago Roca basin. In fact, the area lies at the foot of
the Cordillera Darwin, where a large ice-sheet is still present above the higher peaks of this mountain
chain (Gordillo et al., 1993). Morphological and sedimentological evidence, mostly represented by
raised beaches and deposits rich in marine organisms, testify that the northern coast of the Beagle
Channel has undergone a general drop of the sea level during Holocene (Rabassa et al., 1986). This
progressive marine regression has modified the coastal landscape of the area and in some cases has
severely changed the morphological environment of the surrounding areas (Borromei and
Quattrocchio, 2007). Moreover, the general morphology of Lago Roca is clearly controlled by the
tectonic activity along the Beagle Channel fault system and its geometry most probably reflects its
sub-bottom structure.
Fig. 1 - Left: Bathymetric map of Lago Roca (maximum water depth is 85 m), and Digital Elevation Model of the
surrounding areas. Box indicates the location (light grey star points to Lago Roca). Right: Position map of the highresolution seismic profiles acquired in Lago Roca.
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REFERENCES
• Borromei, A. M. and Quattrocchio, M. (2007). Holocene sea-level change inferred from palynological data in the Beagle Channel,
southern Tierra del Fuego, Argentina. Ameghiniana, 44, (1). Buenos Aires. Jan./Mar. Print version.
• Gordillo, S., Coronato, A. and Rabassa, J. (1993). Late Quaternary evolution of a sub-antarctic paleofjord, Tierra del Fuego. Quat.
Sci. Rev., 12, 889-897.
• Rabassa, J., Heusser, C. and Stuckenrath, R. (1986). New data on Holocene sea transgression in the Beagle Channel: Tierra del
Fuego, Argentina. Quat. S. Am. Ant. Pen., 4, 291-309.
• Tassone, A., Lippai, H., Lodolo, E., Menichetti, M., Comba, A., Hormaechea, J. L. and Vilas, J.F. (2005). A geological and
geophysical crustal section across the Magallanes-Fagnano fault in Tierra del Fuego and associated asymmetric basins formation.
Jour. South Am. Earth Sci., 19, 99-109.
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Session 4
TECTONIC PROCESSES AND
SEISMOLOGY
GEOSUR2010
NEW STRUCTURAL MAPS AND CROSS-SECTIONS OF THE PATAGONIAN
FOLD-THRUST BELT NEAR SENO OTWAY, SENO MARTÍNEZ AND PENINSULA
BRUNSWICK, SOUTHERN CHILE
4-01
Betka, P.1*, Klepeis, K.2, Mosher, S.1
(1) Jackson School of Geosciences, The University of Texas at Austin, 1 University Station C1100,
Austin, Texas 78712, USA
(2) Geology Department, The University of Vermont, 180 Colchester Ave, Burlington, VT 05405,
USA
* Presenting author’s e-mail: pmbetka@mail.utexas.edu
We present preliminary results, maps and cross-sections from two strike-perpendicular transects
across the Patagonian retroarc fold-thrust belt (FTB) near: A) Seno Otway; B) Peninsula Brunswick
and Seno Martínez. The onset of the Andean orogeny between 50°-56°S is marked by the LateCretaceous closure and inversion of the Rocas Verdes back-arc basin and the development of the
Patagonian retroarc FTB. The nearly 90° bend in the trend of the orogen in this location is commonly
attributed to significant along-strike variation in structural-style and tectonic shortening during the
development of the FTB. Despite studies elsewhere in southern Patagonian, relatively little is known
about the FTB in a ~150 km2 area south of 53.0°S and west of 70.5°W.
Near the southwestern regions of Seno Otway chlorite-grade pelitic schists (basement) are imbricated
with massive basalt, gabbro, chert and quartzite of the Rocas Verdes basin floor and mudstone (Zapata
Fm.) representing the basin fill. Farther to the north, basement involved thrust sheets are structurally
above the Zapata Fm. and foreland basin strata (Punta Barrosa and Cerro Torro Fms.) along an outof-sequence thrust (OOST). Below the OOST overturned-to-the north, northwest plunging tight folds
thicken the Zapata Fm. Foreland basin strata display decameter-scale overturned folds. At least one
main thrust imbricates the foreland basin strata. Several late, brittle strike slip faults cross cut
contractional structures in this area.
Near Seno Martínez, south of the Magallanes-Fangano fault zone (MFFZ) mafic, chloritic schsits
interpreted as Rocas Verdes infill are structurally above a ~25 km exposure of chorlite- and garnetgrade basement schists of the Darwin Complex. An ~8 km wide ductile shear zone characterized by
a SW-plunging down-dip quartz lineation, quartz rods up to 50 cm long, and sheath folds that have
southwest-plunging long-axes internally thickens the basement schists. Structurally below this shear
zone, chlorite-grade phyllitic shists of the Darwin Complex are in startigraphic contact with the
Tobifera Formation. North of the MFFZ, the Tobifera Fm. outcrops structurally above turbiditic rocks
of the Zapata Fm. that are folded by tight overturned folds with a top-to-the-northeast vergence. The
Zapata Fm. is cut by a south-dipping OOST that places deformed rocks from the Zapata Fm., and
Tobifera Fm. above weakly deformed foreland basin strata (Punta Barrosa, Cerro Torro and Tres Pasos
Fms.). Deformation propagated at least 25 km northward into the foreland and at least three thrust
faults cut up-section into foreland basin strata. Two large (>5m wide) strike-slip fault zones cut
contractional structures along the eastern shore of Peninsula Brunswick. Our preliminary results
provide new structural constraints on the development of the Patagonian FTB and help to link
structures previously described in the Ultima Esperanza and Tierra del Fuego regions of southern
Patagonia.
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TECTONIC EVOLUTION OF THE BERMEJO BASIN FROM
BROKEN PLATE FLUXURAL MODEL (PRELIMINARY STUDY)
4-02
Carugati, G.1*, Novara, I.L.1, Gimenez, M.E.2, Introcaso, A.1
(1) CONICET. Grupo de geofísica del Instituto de Física Rosario – UNR, Av. Pellegrini 250. Rosario.
(2) CONICET. Instituto Geofísico Sismológico “Ing. Fernando Séptimo Volponi”. Facultad de
Ciencias Exactas, Físicas y Naturales, UNSJ, Av. Jose I. de la Rosa y Meglioli. San Juan. 5400
* Presenting author's e-mail: guillermo.carugati@gmail.com
In this paper an isostatic flexural elastic model is proposed as the main mechanism in Bermejo
foreland basin structures formation. Two periods can be described since the structures formation, or
at least since the Neogene’s time, in which tectonic phase has canghed. A first one includes an interval
since the formation of Andean foreland until the beginning of Western Pampean Ranges uplift. In this
period the main mechanism of elastic flexure in continuous plate was suggested. A second and later
period, includes the Desaguadero-Bermejo megafault reactivation when the continuous plate elastic
mechanism ends, giving place to a broken plate flexural mechanism. Flexural responses have been
obtained by considering a 7 km equivalent elastic thickness crust. Sedimentary thickness and shallow
basin features obtained from residual Bouguer anomalies is consistent with previous models
TECTONIC IMPLICATIONS OF A PALEOMAGNETIC STUDY OF MESOZOIC
4-03
MAGMATIC ARC ROCKS IN CIERVA POINT, NORTHWEST ANTARCTIC PENINSULA
Cosentino, N.J.1*, Tassone, A.A.1,2, Lippai, H.F.1,2, Vilas, J.F.A.1,2
(1) Departamento de Ciencias Geológicas, Facultad de Ciencias Exactas y Naturales, Universidad
de Buenos Aires - Pabellón II, Ciudad Universitaria, Ciudad de Buenos Aires (Argentina)
(2) Consejo Nacional de Investigaciones Científicas y Técnicas (CONICET)
* Presenting author’s e-mail: pinpochis@yahoo.com.ar
Introduction
Antarctic Peninsula’s paleogeographic evolution since Gondwana’s fragmentation is still a subject of
debate. This is so for two main reasons: the fact that the Scotia
plate got in the way between the South America and Antarctica
plates many millions of years after Gondwana’s break-up,
destroying the ocean floor’s magnetic anomalies between these two
plates in the process, and the fact that only a reduced amount of
paleomagnetic data exists for Antarctic Peninsula (AP). A
paleomagnetic sampling of Cierva Point’s Late Jurassic – Early
Cretaceous magmatic arc rocks has been carried out; the studied
area is located in the Danco coast northwest of the peninsula, at
coordinates 64°09’S and 60°57’W (Fig. 1), within a protected area
(ZAEP 134) which also includes the argentinian base
Primavera.
The outcrops consist of plutonic rocks (granodiorites, tonalites
and granites) intruded in acid volcaniclastic host rocks
(cristaline and vitreous tuff). This predominantly acidic
volcanism belongs to the Antarctic Peninsula Volcanic Group
(APVG), defined in other parts of northern AP. The age of the
Fig. 1 - (a) Regional map
volcaniclastic rocks is assigned to the interval 162-153 My,
showing the sampling area in the
based on a correlation with similar outcrops of that age, in an
context of Antarctic Peninsula.
area close to Cierva Point (Pankhurst et al., 2000). Available
(b) A local map showing the
K/Ar whole rock dating of the intrusive rocks yielded a Late
sampling location in detail
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a
b
Fig. 2 - (a) IRM adquisition curve showing a quick saturation attained at values of ~ 200 mT. (b) curves numerically
fitted according to two mineralogical phases with defined coercitivity intervals. The number of phases is iteratively
calculated using the maximization-expectation algorithm.
Cretaceous age (95 Ma, Codignotto et
al., 1978).
Magnetic mineralogy
The magnetic mineralogy of the sampled
rocks (including the plutonic body and
its host) was studied with different
techniques.
Isothermal
remanent
magnetization (IRM) was acquired by
samples, obtaining curves which were
then modelled into their individual
components of coercitivity using the
maximization-expectation algorithm
(Fig. 2). Also, Lowrie demagnetization
experiments were carried out in order to
analyse different magnetic mineralogy
according to coercitivity (Fig. 3). Finally,
thermorremanent experiments allowed
determination of magnetic domains (Fig.
4). These studies were carried out with
the objective of defining the magnetic
remanence carriers. The results show a
generalized dominance of MD or PSD
magnetite with coercitivities in the range
of 15-54 mT among the magnetic
mineralogy.
Fig. 3 - Lowrie thermal demagnetization curves. The x, y and z
curves represent mineralogical populations with coercitivities of
0-120, 120-400 and 400-1000 mT, respectively. The most
important contributor is the first of these populations, which is in
accordance with the IRM acquisition curves (Fig. 2 (a)). Plotting
normalized values of IRM allow a ~ 580°C visual estimation of
the Curie temperature for all populations.
Paleomagnetism
Samples were demagnetized by the AF
method in almost all cases, successfully
Fig. 4 - Thermorremanent curves showing the relationship
between mass-normalized magnetic susceptibility and
defining characteristic remanence
temperature. The continuous curve shows the heating cycle (in
magnetizations by principal component
Ar atmosphere) and the discontinuous one the cooling cycle. The
analysis (Fig. 5). Two populations were
Verwey transition is identified, as well as a drastic Km drop at ~
defined according to the strength of the
580°C. Also, an irreversible local maximum at ~ 300°C in Km is
remanence, measured by the field by
observed during the heating cycle.
which half of the original NRM vanished
(Fig. 6).
The hard and soft remanence populations strongly coincide with the volcaniclastic host rocks and the
plutonic rocks, respectively. Previous sampling of the former in this same area (Valencio et al., 1979)
defined five sites with similar magnetic behaviour which group with high precision parameter with
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a
b
Fig. 5 - NRM demagnetization curve and vector components diagrams of a sample corresponding to the (a) hard and
(b) soft magnetization populations. Solid and open data points indicate vector end points projected onto the horizontal
and vertical plane, respectively.
the hard population defined in this study. Hence, a
unique population was defined in this case.
Two average poles were calculated from these two
populations: 76.5°S, 49.2°E, dp = 7.6°, dm =8.7°, N =
11 (hard population) and 82.4°S, 148.3°E, dp = 3.6°,
dm = 4.1°, N = 19 (soft population).
VGP dispersion values of S ~ 14.3° and ~ 9.6° for the
hard and soft populations, respectively, indicate an
adequate paleosecular variation sampling in the first
case. In the second case, the extremely low value of S
can be explained either by an inadequate sampling of
the paleosecular variation or by an adequate sampling
of it at in intra-site level.
All of the sites corresponding to the soft population
have normal polarity directions, while some of the hard
population sites are of reverse polarity. However, the
Fig. 6 - Equal-area projection of the 26 sitemean directions. The directions in black
correspond to the hard remanence directions,
while the white ones correspond to the soft
remanence directions. The latter’s average is
distinct from the GAD direction in the area to
the 95% significance.
latter consist of too few specimens to
define
well-grouped
site-mean
directions.
Fig. 7 - South America’s reference paleomagnetic poles (Somoza
& Zaffarana, 2008; Besse & Courtillot, 2002) were rotated using
the SAM with respect to AP Euler pole (of appropriate age) and
marked with their respective ages. Also shown are the average
poles obtained in this study (1: hard population, 2: soft
population) and Antarctica in today’s coordinates.
132
Discussion
A number of different rigid-plate
cinematic models have been proposed
for Antarctica since Gondwana’s breakup. One of these (Ghidella et al., 2002)
calculates its rotation poles from four
interval poles between 160 and 83 My,
which are obtained from magnetic and
gravimetric lineations in western
Weddell under the assumption that they
represented
movement
between
Antarctica and South America (SAM).
This model does not take into
consideration
relative
movement
between AP and SAM, and does not
GEOSUR2010
Fig. 8 - Equal-area projection showing the average poles
obtained in this study (black: hard population, grey: soft
population) and the EANT reference paleopoles between 150
and 90 Ma (Torsvik et al., 2008).on, 2: soft population) and
Antarctica in today’s coordinates.
generate any overlapping between these
continental masses. Considering this
model to be correct, the comparison
between SAM’s reference poles and this
study’s average poles (Fig. 7) suggests a
net local counter-clockwise rotation with
respect to a vertical axis of 61-47°
between 150-120 My and 100-90 My. If,
on the other hand, the poles obtained in
this study are considered trustful
paleomagnetic poles, the comparison
between these and the reference East
Antarctica (EANT) paleopoles of the
same age (Fig. 8) suggests a regional
counter-clockwise rotation of AP with
respect to EANT between 150-120 My
and 100 My, after which no more relative
motion takes place. Paleogeographic
reconstructions according to the Konig
and Jokat (2006) plate model give
credence to these results.
REFERENCES
• Besse, J., Courtillot, V. 2002. Apparent and true polar wander and the geometry of the geomagnetic field over the last 200 Myr.
Journal of Geophysical Research, vol. 107, no. B11.
• Codignotto, J. O., Llorente, R. A., Mendía, J. E., Olivero, E., Spikermann, J. P. 1978. Geología del Cabo Spring y de las islas
Leopardo, Pingüino y César. Contribución del Instituto Antártico Argentino Nº 216, Buenos Aires
• Ghidella, M. E., Yaniez, G., LaBrecque, J. L. 2002. Revised tectonic implications for the magnetic anomalies of the western Weddell
Sea. Tectonophysics 6528.
• Konig, M., Jokat, W. 2006. The Mesozoic breakup of the Weddell Sea. Journal Geophysical Research 111 (B12): 102, doi:
10.1029/2005JB004035.
• Pankhurst, R.J., Riley, R. R., Fanning, C.M., Kelley, S. P. 2000. Episodic silicic volcanism in Patagonia and the Antarctic Peninsula:
Chronology of magmatism associated with the break-up of Gondwana. Journal of Petrology: 41(5): 605-625.
• Somoza, R., Zaffarana, C. B. 2008. Mid-Cretaceous polar standstill of South America, motion of the Atlantic hotspots and the birth
of the Andean cordillera. Earth and Planetary Science Letters 271, 167-277.
• Torsvik, T., Gaina, C., Redfield, T. 2008. Antarctica and Global Paleogeography: From Rodinia, Through Gondwanaland and
Pangea, to the Birth of the Southern Ocean and the Opening of Gateways.
• Valencio, D. A., Mendía, J. E., Vilas, J. F. 1979. Palaeomagnetism and K-Ar Age of Mesozoic and Cenozoic Igneous Rocks From
Antarctica Earth and Planetary Science Letters, 45 61-68.
PALAEOTECTONIC SETTING OF PRECUYANO GROUP.
UPPER TRIASSIC- LOWER JURASSIC VOLCANIC DEPOSITS OF
THE NEUQUEN BASIN (37º- 39º 30´LS). ARGENTINA
4-04
Delpino, D.1*, Bermudez A.2
(1) YPF, E and D, Talero 360, Neuquén, Argentina
(2) CONICET, National University of Comahue, Neuquén, Argentina
* Presenting author’s e-mail: dhdelpinos@ypf.com
The Upper Triassic–Lower Jurassic volcanic continental sedimentary sequences contained within the
Precuyano Group are considered to be related to the beginning of the structural evolution of the
Neuquén Basin, one of the most productive hydrocarbon basins of Argentina.
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Fig. 1 - Plate reconstruction
for Pangaea after Vaughan
and Storey (2007). Areas
affected by deformation (dark
grey). Precuyano depocentres: L (Lapa), CB (C°
Bandera),
CM
(Cupen
Mahuida), LN-LY (Loma
Negra-La Yesera), M-PA
(Medanito - Portezuelo Alto),
LA (Loma Amarilla), CC
(Cara Cura), Z (Zapala).
The purpose of this paper is to define the
palaeotectonic setting of this particular
unit using a geochemical data set of 150
samples obtained from different
depocentres within the Neuquen Basin,
(Fig 1). The Basin is located adjacent to
the western palaeomargin of the Pangaea
supercontinent, and developed with a
regional arc/backarc extensional event
that generated dozens of half-graben on
Paleozoic and lower Triassic age “basement”. The typical half-graben has an
elongate shape with an axial length from
15 to 30 km. Today some half-grabens
Fig. 2 - Winchester and Floyd (1977) Nb/Y vs Zr/Ti diagram.
are now partially or totally exposed due
Field A,120 acid rocks. Field B 30 intermediate and basic rocks.
to Andean orogenesis, while others
remain covered by Mesozoic and
Tertiary sequences. The half-graben evolution coeval with the development of a
Upper Triassic-Lower Jurassic eruptive
period
are
referred
to
as
“Precuyanolitense”. A large volume of
volcanic rocks was erupted within a
short geologic time period (213-198
Ma). Tectonic-volcanic activities generated sedimentary-volcanic sequences up
to 2000 m thick.
Spatial and volumetric thickness distributions were controlled primarily by
Fig. 3 - Sun and McDonough (1989) chondrite
Normalized diagram. Open boxes average acid rocks pattern.
faults, and associated local syn-deposiDotted line basic rocks pattern.
tional structures. As a consequence, volcanic and sedimentary deposits show
abrupt thickness changes and local discordant relationships. Typical Precuyano deposits are formed
by alternating sequences of acid pyroclastic flow, tuff fallout, volcaniclastic and continental sedimentary rocks, as well as basic and intermediate lava flows. Dacitic and Rhyolitic (SiO2 66-74%) pyro134
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clastic rocks generally predominate.
Acid tuffs have crystal-rich and vitriclastic porphyritic textures with
phenocrysts mineralogy: plagioclase + quartz + anfíbol +/- biotite
+/- opaque minerals. Intermediate
rocks (SiO2 56-60 %) have porphyritic textures with phenocrysts
of plagioclase and amphibole.
Porphyritic texture, with plagioclase as phenocrystal, characterize
basic rocks (SiO2 47- 53 %).
Using the Nb/Y vs Zr/Ti diagram
for identifying rock type samples
classified as Dacites, Rhyodacites,
Andesites, Basaltic Andesites and
Fig. 4 - Basic (dotted line) and acid (open squares) rocks patterns on
Basalts. Nb/Y ratios are typical
MORB-normalized diagram with trace elements placed in order of
calc-alkaline series (Fig. 2). Rare
increasing incompatibilty from Sr to Yb.
Earth Elements abundances normalized to Chondrite values,
shows different patterns in basic and acid rocks.
Basaltic rocks normalized to Chondrite show patterns with low La/Yb(n)=4.4, La/Sm(n)=1.87
Sm/Yb(n)= 2.35 and lack Eu anomaly (Fig. 3). Low La/Yb and Sm/Yb ratios indicate relative high
percentage melts without residual garnet in the source. Dacites and Rhyolites rocks pattern are distinguished by higher La/Yb(n)= 10, La/Sm (n) = 5, but lower Sm/Yb(n)= 1.8 and negative
Eu/Eu*= 0.57 anomalies (Fig. 3). High La/Yb ratios in silicic magmas can be a sign of magmas
equilibrated with amphibole or accessory mineral-bearing residual mineral assemblages.
Incompatible LREEs enrichment and negative Eu anomaly indicate plagioclase subtraction or
residual plagioclase in a crustal source region or both. HREE straight pattern shows that amphibole could have played an important part during the crystallization processes. Basic and acid rocks
MORB-normalized show patterns of calc-alkaline series and the key feature of volcanic arc rocks
which is Nb-Ta negative anomaly. (Fig. 4). Another feature of basic calc-alkaline rocks formed in
continental arcs is the similar abundances of Ti and Y than N-MORB and the relatively higher than
MORB Nb and Zr contents. Acid rocks patterns on MORB-normalized diagrams show negative
anomalies of Ti, P and Ba and low values of Sr.
These anomalies were probably controlled by fractionation of Titanite (Ti), Apatite (P), feldspar (Ba)
a
b
Fig. 5 - 5a. Discrimination diagram (Wood, 1980). Field A Basic lavas. Field B Acid lavas. 5b. Discrimination diagram
for acid rocks (after Pearce et al, 1984). All samples plott in volcanic arc field.
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Fig. 6 - Schematic cartoon showing the proposed tectonic setting of the Precuyano Group.
and plagioclase (Relative low Sr and Eu negative anomaly) crystallizing phases.
Ratios of basic rocks La/Ta=44, Ba/La=33 and Ba/Ta=726 reflect slab related processes. Low
Ta/Hf=0.11 ratio indicate depleted MORB or arc mantle sources and Th/Hf=0.61 are typical of calcalkaline arc sources with components from a subducting oceanic slab. Acid rocks ratios La/Ta=34,
Ba/Ta=906 and Ba/La=27 are also typical of arc magmas. Low Ta/Hf=0.16 are characteristic of
depleted mantle and low Th/Hf=0.25 is distinctive of source components from a subducting slab.
On Th-Hf-Nb discrimination diagram that can be applied to intermediate and silicic lavas as well as
to basalts and it is particularly good at identifying volcanic-arc basalts. Basic and acid rocks plot in
the field of calc-alkaline arc rocks (Figs. 5a and b). On tectonic discrimination diagram based on Ta
vs Yb, Precuyano samples plot within the field of acid rocks originated from an igneous source (Type
I) characteristic of rocks originated in active Andean Type convergent margins.
Conclusions
The interpretation of the geological and geochemical data indicates that between latidudes 37°-39°LS
the western margin of the Pangaea supercontinent during the Upper Triassic-Low Jurassic times was
strongly influenced by the subduction of an oceanic plate underneath a continental plate (Fig. 6). This
collisional zone has an evident continuity with similar structures postulated by different authors for
the same times for Pangea margin south of 40° LS extending the paleosubduction continuity northward to 37º LS.
According to rock geochemical characteristics this active continental margin could be classified as an
“Andean” type, with patterns displaying negative anomalies of Nb-Ta, normalized values of some
trace elements La, Ta, Hf, Th and low Y values. This indicates that these rocks belong to the calc-alkaline series. Basic rocks do not show evidence of important fractionation processes. On the other hand,
acid rocks show evidence of magmatic disequilibrium or fractional crystallization processes developed inside the magmatic chambers. Half-graben volcanic sequences have notable regional compositional homogeneity suggesting that its formation was controlled by major geotectonic processes,
influenced by subducting oceanic plate with associated intra-arc to back arc extension. Two principal
features strongly influenced depocenters fill histories: 1) volume of volcanic rocks, generate during
the maxima volcanic activity and 2) coeval tectonic evolution of the half-grabens.
REFERENCES
• Pearce J., Harris N. and Tindle A., 1984. Trace element discrimination diagrams for the tectonic interpretacion of granitic rocks.
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Journal of Petrology, V. 25, pp. 956-983.
• Sun,S. and McDonough,W.,1989. Chemical and isotopic systematics of oceanic basalts: implications for mantle composition and
processes. Geological Society of London, Special Publications, V.42, pp. 313-345.
• Vaughan,A and Storey,B, 2007. A new supercontinet self-destruct mechanism: evidence from the Late Triassic-Early Jurassic.
Journal of the Geological Society of London, V.164, pp. 383-392.
• Winchester, J. A. and Floyd, P. A., 1977. Geochemical discrimination of different magma series and their differentiation products
using immobile elements. Chemical Geology, V 20, pp. 325–343.
• Wood, D., 1980. The application of a Th–Hf–Ta diagram to problems of tectonomagmatic classification and to establishing the
nature of crustal contamination of basaltic lavas of the British Tertiary volcanic province. Earth and Planetary Science Letters V. 50,
pp. 11–30.
PRELIMINARY RESULTS OF A PALEOMAGNETIC STUDY ON THE ORDOVICIAN
CALMAYO GRANITOID, SIERRAS DE CÓRDOBA, ARGENTINA
4-05
Geuna, S.1,2*, D’Eramo, F.1,3, Pinotti, L.1,3, Di Marco, A.2, Mutti, D.2, Escosteguy, L.4
(1) CONICET
(2) Departamento de Ciencias Geológicas, FCEyN, Universidad de Buenos Aires. Ciudad
Universitaria Pab. 2, 1874 CABA, Argentina
(3) Departamento de Geología, FCEFQyN, Universidad Nacional de Río Cuarto
(4) Instituto de Geología y Recursos Minerales, Servicio Geológico Minero Argentino
* Presenting author’s e-mail: geuna@gl.fcen.uba.ar
Introduction
The South American margin is characterized by the successive accretion of terranes (Pampia,
Precordillera-Cuyania, and Chilenia) during the Early-Middle Palaeozoic; the evolution of the
associated magmatic arcs and the collisional settings can be studied through the presently outcropping
related granitoids. The Sierras de Córdoba are an outstanding place to study the Pampean, Famatinian
and Achalian magmatism, linked to the aforementioned accretions in the Cambrian, Ordovician and
Devonian, respectively.
This contribution presents preliminary results of the paleomagnetic study of the Calmayo tonalite, an
Ordovician pluton outcropping in the Sierra Chica de Córdoba (Fig. 1). The results allowed
establishing several working hypotheses involving the Pampean terrane history, and the way it was
affected by later deformation events.
Geological Background
The Ordovician magmatism in the Eastern Sierras de Córdoba is coeval with Famatinian arc
magmatism located to the west. They are a dozen of small, discordant plutons, interpreted as emplaced
at shallow depths in a rigid basement (Bonalumi and Baldo, 2002). The Calmayo tonalitetrondhjemite (32o S, 64o30’ W, Córdoba, Argentina) is an elliptical pluton that trends NE, with a
maximum extension of 4.5 x 2.5 km (Fig. 1). It is a zoned leucotonalite which locally shows fragile
deformation overprinted. A crystallization age of 490 Ma was obtained by D’Eramo (2003).
The north-eastern corner of the tonalite is affected by the Soconcho shear belt, imposing a mylonitic
foliation to the area (Martino 2003). The belt has inverse cinematic with a dextral component, and it
was reactivated later in a fragile regime. The age for the shear belt is unknown, though it was
interpreted as a contractional Achalian (Devonian) belt related to the collision of Cuyania/Chilenia
(Martino, 2003).
Paleomagnetic Study
Palaeomagnetic sampling was carried out on twenty-eight sites widely distributed in the Calmayo
tonalite (Fig. 1).
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Fig. 1 - Geological map of Calmayo area in the Sierra Chica de Córdoba. Location of other trondhjemite plutons of similar
age is also shown.
Fig. 2 - Typical magnetic
behavior. (left) Vector end-point
diagrams
and
(right)
normalized intensity plots for
thermal demagnetization from 0
to 680oC. A) Most of the sites
show hematite with a small
content of ilmenite as the main
magnetic carrier. The high
coercivity shown by AF
demagnetization (inner inset) is
characteristic of hematite, while
the unblocking temperature
(higher than magnetite -580oCand lower than pure hematite 680oC-) indicates a small
amount of Ti in the structure.
B) The magnetic component
erased at lower temperatures
and lower alternate fields points
to magnetite accompanying
hematite in a few sites (an AF of
20 mT was applied previous to
thermal demagnetization). On
the demagnetization diagram
open (solid) symbols indicate
projection onto the vertical
(horizontal) plane. Insets in
normalized intensity plots show
normalized alternating field
demagnetization on the same
samples.
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Magnetite is present only in the north-eastern area of
the pluton, the rest of it being weakly magnetic, with
a mean magnetic susceptibility of 25 x 10-5 (SI),
mainly due to the content of ilmeno-hematite and
biotite.
Five sites were discarded due either to intra-site
inconsistency or very weak magnetization. The
remaining 23 sites have ilmeno-hematite as the main
magnetic carrier of a stable remanence, as identified
by thermal and alternating field (AF)
demagnetization procedures (Fig. 2). Magnetite may
appear in subordinate proportions. The ilmenohematite is an abundant accessory mineral, which
shows as exsolved intergrowths with (hemo)ilmenite.
The hematite-rich member is usually the host of discshaped rods of exsolved ilmenite-rich member.
Thermal demagnetization up to 620-640oC isolated
steeply dipping, normal polarity remanence
directions (Fig. 3). The preliminary mean direction is
Dec. 270o, Inc. -73o, a95 6.4o. The palaeomagnetic
pole is Lat. 27oS, Long. 330oE (Fig. 4).
Fig. 3 - Mean directions for the magnetic
remanence in Calmayo sites. All directions with
negative (up) inclination. The 95% confidence
cone for the mean is drawn around the cross.
Discussion
The eutectoid of the hematite-ilmenite system has been estimated at 390oC (Robinson et al., 2004) or
520oC (McEnroe et al., 2007). It means the magnetic carrier of the remanence was formed at high
temperature in the pluton, and the remanence became locked at the moment of ilmeno-hematite
exsolution. It is expected that the remanence direction records the paleomagnetic field at the moment
the pluton was still at high temperatures, possibly near the crystallisation age.
The paleomagnetic pole calculated from the in situ remanence directions carried by the Calmayo
tonalite is not consistent with the 500-470 Ma segment of the Gondwana apparent polar wander path
(APWP) proposed by McElhinny et al. (2003), but it coincides with an approximate 350 Ma-position
(Fig. 4). The discrepancy of the pole with the APWP accepts several hypotheses which must be
Fig. 4 - Paleomagnetic poles in the
southern hemisphere, Schmidt projection,
in present African coordinates (parameter
reconstructions after Lawver and Scotese,
1987). The Pampia terrane poles (ACH
Achala batholith, Late Devonian, Geuna et
al. 2008; CMY Calmayo tonalite, Early
Ordovician, this work; CMP Campanario
Fm., Late Cambrian, Spagnuolo et al.
2008a) are compared with the Gondwana
APWP proposed by McElhinny et al.
(2003). Numbers note age of the mean
APWP poles in Ma.
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explored in the light of further geological-petrological evidence:
a) Hematite-ilmenite is a magmatic mineral which retains a primary magnetisation acquired on
cooling, at ~490 Ma. The discrepancy is due to not having restored the pluton to its palaeohorizontal
position in the Ordovician, and it could be accounted if the pluton axis, supposed originally vertical,
would now plunge 50o to the SW.
b) Hematite-ilmenite formed (or recrystallized) in response to high-temperature deformation affecting
the pluton. The coincidence of the paleomagnetic pole with the 350 Ma- segment of the APWP would
imply that high-temperature deformation along the Soconcho shear belt continued until the Devonian
(Achalian orogeny). Calmayo would not have experienced significant net tilting since the Devonian.
c) Should the pluton acquired the remanence at its present position, and the remanence was primary
(i.e. Calmayo pluton shows no significant net tilting since the Ordovician), then the discrepancy can
be analized together with discrepancies noted for other Pampean poles (Spagnuolo et al., 2008 a, b,
Geuna et al., 2008; see Fig. 4). All these poles could be reconciled with the Gondwana APWP by
moving Pampia in a way consistent with current geological models of dextral displacement relative to
Gondwana (Rapela et al., 2007, Spagnuolo et al., 2008 b).
Further petrological – geophysical – structural studies will allow establishing the preference for a), b)
or c), either of them having important implications to elucidate the mechanisms of Pampia accretion
to the South American margin and its late history of deformation.
Acknowledgements
This work was partially financed with research grants by UBACyT (X156, X442), ANPCyT (PICT
1074 y 02266/06), and CONICET (PIP 1502). SuperIAPD and GMAP programs were utilized in the
analysis of palaeomagnetic data and palaeoreconstructions, respectively.
REFERENCES
• Bonalumi, A., Baldo, E., 2002. Ordovician magmatism in the Sierras Pampeanas of Córdoba. En: Aceñolaza, F.G. (Ed.): Aspects of
the Ordovician System in Argentina. INSUGEO, Serie Correlación Geológica 16, p. 243-256. San Miguel de Tucumán.
• D’Eramo, F., 2003. Petrología y emplazamiento de los plutones El Hongo y Calmayo, y su relación con la evolución de la Sierra
Chica de Córdoba. Tesis Doctoral, UNRC, 200 pp.
• Geuna, S.E., Escosteguy, L.D., Miró, R. 2008. Palaeomagnetism of the Late Devonian - Early Carboniferous Achala Batholith,
Córdoba, central Argentina: implications for the apparent polar wander path of Gondwana. Gondwana Research, Special Issue “The
Western Gondwana Margin: Proterozoic to Mesozoic”, 13: 227-237.
• Lawver, L.A., Scotese, C.R., 1987. A revised reconstruction of Gondwana. In: McKenzie, G.D. (Ed.), Gondwana Six: structure,
tectonics, and geophysics. American Geophysical Union, Monographs, 40: 17-23.
• Martino, R.D., 2003. Las fajas de deformación dúctil de las Sierras Pampeanas de Córdoba: Una reseña general. Revista de la
Asociación Geológica Argentina, 58 (4): 549-571.
• McElhinny, M.W., Powell, Ch.McA., Pisarevsky, S.A., 2003. Paleozoic terranes of eastern Australia and the drift history of
Gondwana. Tectonophysics, 362: 41-65.
• McEnroe, S.A., Robinson, P., Langenhorst, F., Frandsen, C., Terry, M.P., Boffa Ballaran, T., 2007. Magnetization of exsolution
intergrowths of hematite and ilmenite: Mineral chemistry, phase relations, and magnetic properties of hemo-ilmenite ores with
micron- to nanometer-scale lamellae from Allard Lake, Quebec. Journal of Geophysical Research 112 (B10103),
doi:10.1029/2007JB004973.
• Rapela, C.W., Pankhurst, R.J., Casquet, C., Fanning, C.M., Baldo, E.G., González-Casado, J.M., Galindo, G., Dahlquist, J., 2007.
The Río de la Plata craton and the assembly of SW Gondwana. Earth-Science Reviews, 83: 49-82.
• Robinson, P., Harrison, R.J., McEnroe, S.A., Hargraves, R.B., 2004. Nature and origin of lamellar magnetism in the hematiteilmenite series. American Mineralogist 89, 725-747.
• Spagnuolo, C.M., Rapalini, A.E., Astini, R.A., 2008 a. Paleogeographic and tectonic implications of the first paleomagnetic results
from the Middle–Late Cambrian Mesón Group: NW Argentina. Journal of South American Earth Sciences, 25: 86-99.
• Spagnuolo, C.M., Rapalini, A.E., Astini, R.A., 2008 b. Palaeomagnetic confirmation of Palaeozoic clockwise rotation of the
Famatina Ranges (NW Argentina): implications for the evolution of the SW margin of Gondwana. Geophysical Journal
International, 173: 63-78.
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NORTH-SOUTH VARIATIONS IN PROVENANCE IN THE LATE PALEOZOIC
ACCRETIONARY COMPLEX OF CENTRAL CHILE (34º – 40º LAT. S)
AS INDICATED BY SHRIMP DETRITAL ZIRCON U-TH-PB AGES
4-06
.Hervé,F1*, Calderon, M.1, Fanning, C.M.2, Godoy, E.1
(1) Departamento de Geología, Universidad de Chile, Casilla 13518, Correo 21, Santiago, Chile
(2) Research School of Earth Sciences, Australian National University, Canberra, Australia
* Presenting author’s e-mail: fherve@cec.uchile.cl
A late Paleozoic to Early Mesozoic fossil accretionary complex extends from 34º S. Lat to the extreme
south of the continent along coastal Chile. Previous detrital zircon studies in the southern part of this
complex (44º - 55º S) have shown that Permian to Jurassic detrital rocks have been incorporated into
the complex where they have been metamorphosed under high P/T conditions.
New detrital zircon ages have been obtained from the northern (34 – 42º S) segment of the
accretionary complex, area in which a well established paired metamorphic belt system with the high
P/T Western Series (WS) - interpreted to represent basally accreted rocks - lies outboard of the Eastern
Series (ES) - interpreted to represent frontally accreted rocks. The data reveals that they have Permian
or older maximum possible sedimentation ages, with no Mesozoic detrital zircons present. In the
northernmost section, (34 – 36 ºS) the detrital zircon age spectra show more than 50 % of Proterozoic
zircon grains in contrast with less than 10% further south. Also, the younger igneous detrital zircons,
in both the WS (330 Ma) and the ES (345 Ma) are older than the Pennsylvanian (310 Ma) intrusion
of the Coast Range Batholith. In contrast, detrital zircons of Early Permian (285 – 295 Ma) age are
present in the Western Series of the southernmost (40º S) exposures, together with even younger
zircons of late Permian ages.
These results suggest that the cratonic source areas of South America, probably the Pampia and maybe
even the Rio de la Plata cratons, were able to shed detritus to the paleo-Pacific coast prior to the
inception of subduction as indicated by the establishment of the Pennsylvanian magmatic arc near the
continental margin. It is speculated here that the sedimentary protolith of the northern portion of the
accretionary complex was probably deposited in a passive margin setting, and that they were
incorporated later into de accretionary wedge. The data also suggest that the accretionary processes,
or at least the preservation of the subducted rocks, were diachronous along the considered segment of
the belt, as the sedimentary protoliths become younger southwards. This research is supported by
FONDECYT Project 1095099.
THE ROLE OF TRUE POLAR WANDER IN THE JURASSIC
Iglesia Llanos,
M.P.1*,
Prezzi,
4-07
C.B.1
(1) INGEODAV, Depto. Ciencias Geológicas, Fac. Ciencias Exactas y Naturales, Universidad de
Buenos Aires, Pab. 2, Ciudad Universitaria, C1428EHA, Buenos Aires, Argentina
* Presenting author’s e-mail: mpiglesia@gl.fcen.uba.ar
It is generally accepted that Gondwana remained in a present-day latitudes during most of the
Mesozoic and even part of the Palaeozoic. This “stationary” geodynamic model is recurrently shown
in the literature, and is based on the fact that the South American Jurassic palaeomagnetic poles
clustered around the southern geographic pole (e.g. Valencio et al., 1983; Oviedo and Vilas, 1984;
Rapalini et al., 1993; Beck, 1999). In more recent years however, it has been demonstrated that some
of the palaeopoles of this age fell far away from the geographic pole, particularly during the Early
Jurassic. As a consequence, it was mandatory to test the former geodynamical model, which is what
we present here, i.e. a substantially different model for Pangea during the Jurassic.
We used other continents that by then were part of Pangea yielding reliable palaeomagnetic data, i.e.
Eurasia, North America and Africa, and constructed the corresponding apparent polar wander (APW)
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Fig. 1 - Master Jurassic APW
path derived from those of
North America, Eurasia,
Africa and South America in
South American present-day
coordinates.
paths, which we compared with the one we obtained in South America (Iglesia Llanos et al., 2006).
We observed that all four curves were fully consistent (Fig. 1, in South American present-day
coordinates). The rotation to South America was performed after testing several well-known
palaeogeographical reconstructions for Pangea, and decided on those which provided the best
palaeomagnetic fit. We established that such fit varied with pole ages, and thus used different
kinematic models for palaeopoles bearing ages between ~215 and 193 Ma, and those between ~ 192155 Ma. According to our palaeomagnetic data, some sort of geodynamic event might have taken
place around 192 Ma.
The cusp in the path implies a substantial change in pole positions between 197 and 185 Ma (Early
Jurassic). Typically, such change can be attributed to some sort of geodynamical phenomenon, such
as lithospheric motion and/or true polar wander (TPW), which is defined as the drift of the spin axis
relative to the rigid Earth. Precisely, one of the goals of this study is throwing some light regarding
the primary cause of the polar shift.
On the basis of palaeomagnetic and hotspots data, we performed palaeolongitude-controlled or
“absolute” palaeogeographical reconstructions. We used Morgan’s (1983) “fixed” grid of hotspots
(HS) that goes back to 200 Ma to compensate for the motion of South America in relation with the
Atlantic Ocean hotspots. From the resulting palaeoreconstructions, changes in latitude and orientation
in the Early Jurassic look more conspicuous. Given that these palaeolatitudinal shifts might have
occured at a global scale, we investigated whether there was a correlation with reported geological
and/or palaeocological major changes/turnovers during this time from both hemispheres, and found
that indeed there was. The applicability of completely different disciplines is maybe the most
convincing argument to sustain or discard the methodologies used.
REFERENCES
• Beck Jr., M.E., 1999. Jurassic and Cretaceous apparent polar wander relative to South America: Some tectonic implications. J.
Geophys. Res. 104: 5063-5067.
• Creer, K. M., Irving, E., and Runcorn, S. K., 1954. The direction of the geomagnetic field in remote epochs in Great Britain. J.
Geomag. and Geoelect. 6 163-168.
• Iglesia Llanos, Riccardi, A.C., Singer, S.E., 2006. Palaeomagnetic study of Lower Jurassic marine strata from the Neuquén Basin,
Argentina: A new Jurassic Apparent Polar Wander Path for South America. Earth Planet. Sci. Lett. 252: 379-397.
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• Morgan, W.J., 1983. Hotspot tracks and the early rifting of the Atlantic. Tectonophysics 94: 123-139.
Oviedo, E., Vilas, J.F., 1984. Movimientos recurrentes en el Permo-Triásico entre el Gondwana Occidental y el Oriental. Actas 9º
Congreso Geológico Argentino 3, Buenos Aires, 97-114.
• Rapalini, A.E., Abdeldayem, A.L., Tarling, D.H., 1993. Intracontinental movements in Western Gondwanaland: a palaeomagnetic
test. Tectonophysics 220: 127-139.
• Valencio, D.A., Vilas, J.F., Pacca, I.G., 1983. The significance of the palaeomagnetism of Jurassic-Cretaceous rocks from South
America: predrift movements, hairpins and Magnetostratigraphy. Geophys. J. R. Astron. Soc. 73: 135-151.
THE WEDDELL SEA REVISITED
4-08
Lawver, L.A.1*, Ghidella, M.E.2
(1) The Jackson School of Geosciences, Institute for Geophysics, University of Texas at Austin,
10100 Burnet Rd. – R2200, Austin, TX 78758-4445, U.S.A.
(2) Instituto Antártico Argentino, Cerrito 1248, C1010AAZ, Buenos Aires, Argentina
* Presenting author’s e-mail: lawver@ig.utexas.edu
The satellite derived gravity map of the Weddell Sea allows new interpretations of the early tectonic
evolution of the area. Many presentations of identified magnetic anomalies in the Weddell Sea have
been made with new data being recently added by König and Jokat (2006). Unfortunately, most all
interpretations showing magnetic anomaly identifications in the Weddell Sea fail when the isochrons
are put into a system that utilizes major plate motions. It is known that there was virtually no motion
between South America and Africa prior to 132 Ma, the time of the eruption of the Parana-Edenteka
mantle plume at 132 Ma, or about magnetic anomaly M10. The earliest identified magnetic anomalies
in the South Atlantic are variously M9 or M7 depending on the anomalies used. Consequently any
anomalies older than M10 found in the Weddell Sea need to be the product of a simple two-plate
breakup between East (East Antarctica, India, Australia and Madagascar) and West (Africa and South
America) Gondwana. Also, any identified anomalies in the Weddell Sea need to be able to be
reconstructed with room for their conjugate half available. In addition, the Cretaceous Normal
Superchron or Cretaceous magnetic quiet zone is exceptionally noisy in the Weddell Sea with even
the identification by many authors of an “isochron” at 93 Ma.
All of these problems will be examined and a tectonic evolution scenario will be presented that may
reduce many of these discrepancies.
CRUSTAL STRUCTURE AND TECTONICS OF
THE EAST ANTARCTIC PASSIVE MARGIN
Leychenkov,
G.L.1*,
Guseva,
4-09
G.L.2
(1) Institute for Geology and Mineral Resources of the World Ocean. 1, Angliysky Ave. 190121, St.Petersburg, Russia
(2) Polar Marine Geosurvey Expedition. 24, Pobeda St., 189510, Lomonosov,
St.-Petersburg, Russia
* Presenting author’s e-mail: german_l@mail.ru
The East Antarctic margin (EAM) formed as a result of Gondwana break-up and is of a rifted (passive)
type. Its different sectors developed due as a consequence of the successive separation of Africa, India
and Australia from the Antarctic. The best studied part of the EAM is the 6500 km-long southern
Indian Ocean between 5E and 150E (which is mostly non-volcanic margin) where c. 150 000 km of
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multichannel seismic data, c. 300
refraction seismic data and more than
250 000 km of potential field data
have been acquired by different
countries and organizations (Fig. 1).
All these numerous data sets were
integrated with use of the international
ADMAP (Antarctic Digital Magnetic
Anomaly Map), SDLS (Seismic Data
Library System) and other data bases.
Conducted investigations enabled us to
define crustal structure/characteristics
of this region, to map principal
tectonic provinces and features and to
develop models of its geodynamic
evolution.
The
most
complicated
and
controversial scientific problem in
study of the EAM tectonics (as well
as other passive margins) is the
identification of the boundary
between rifted continental and
oceanic crust (continent-to-ocean
boundary, COB) that is the locus of
continental break-up. The rifted
continental crust is generally
characterized by a complex structure
with development of extensional
features while the oceanic crust is
usually undisturbed. However these
differences are rarely recorded in
seismic data especially close to COB
where crustal parameters of stretched
continental and oceanic domains are
very similar.
Various geophysical criteria are
applied to recognize the COB on the
EAM. Magnetic data are generally
reliable source of information to
identify the oceanic crust by
appearance of spreading-related linear
anomalies however in Antarctica
magnetic measurements are still sparse
enough to map magnetic lineations.
Gravity information has a limited
capability in COB identification but
nevertheless, free-air anomaly field
Fig. 1 - Tectonic sketch of the EAM (Indian Ocean part)
derived from satellite altimetry data
1 - magnetic leniation (with number of chron polarity) and fracture
shows in many places position of
zones; 2 - extinct ridges; 3 – COB; 4 – volcanic margins; 5 - volcanic
fracture zones (paleotransform
edifices; 6 - zones of mantle unroofing; 7 - thickness of sediments.
faults) which are recognized only
within the oceanic crust and fades
close to the COB. Using the different geophysical and structural approaches the COB has been
mapped (with different reliability) around most of the APM and this study shows that the width the
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rifted (stretched) continental crust around East Antarctica ranges from 200 to 400 km. One of the
basic criteria for identification of crustal types on the ACM is believed to be the differences in
basement velocities which are changed across the COB from 5.9–6.2 km/s (typical to continental
crust) to 4.9-5.8 km/s (typical to oceanic crust; layers 2A or 2B). In many cases seismic reflection
pattern and morphology of a basement surface show marked changes across the margin/abyssal plane
and this information is often decisive for identification and areal mapping of the COB. Following are
brief overview of three sectors of the EAM: eastern Dronning Maud Land margin, Enderby–Queen
Mary Land margin and Wilkes Land–George V Land margin, which are conjugate to south-east
Africa/Madagascar, India and Australia, respectively (Fig. 1).
The Dronning Maud Land margin is the oldest one. Rifting and sea-floor spreading started here about
170–180 Ma and 160 Ma ago respectively (Fig. 1). This margin has contrasting basement
morphologies and crustal thicknesses. The crust ranges in thickness from about 35 km under the shelf,
26–28 km under the Gunnerus Ridge (500 km-long projection of unstretched continental crust toward
the ocean), 12–17 km under the Astrid Ridge, and 9.5–10 km under the deep-water basin. A 50-kmwide block with increased density and magnetization is modeled from potential field data in the upper
crust of the inshore zone and is interpreted as being representative of mafic intrusions. The succession
of linear magnetic anomalies from M0 to M24 is identified in the western part of this area and from
M2 to M16 in the eastern part. Within the southwestern continental rise, a symmetric succession from
M24B to 24n with the central anomaly M23 is recognized. This succession is obliquely truncated by
younger lineation M22–M22n. It is proposed that seafloor spreading stopped at about M23 time and
reoriented to the M22 opening direction.
The Enderby Land–Queen Mary Land margin is characterized by complex Late Jurassic to Early
Cretaceous extensional evolution. The eastern part of this area (between 65E and 95E) has been
studied in more details owing to the joint IPY 2007-2008 Russian-German Project (Fig. 1). The COB
in the study area is defined by a set of structural and geophysical features, the major of which are
distinct differences in seismic pattern of crustal structure and peculiar high-amplitude positive linear
magnetic anomalies. The outer part of the marginal rift is proposed to be saturated by mantle rocks
and its differentiates. A sequence of E-W oriented magnetic lineations from M11An to M2 is well
identified to the west and east of Kerguelen Plateau. Oceanic crust around the southern Kerguelen
Plateau is thicker than normal due to excessive magmatic production in spreading ridges (as a
response to Kerguelen Plume). The southern Kerguelen Plateau itself is thought to be underlain by a
block of stretched and modified continental crust. This block was probably isolated from the India
margin at about 129 Ma due to ridge jump(s).
The Wilkes Land–George V Land margin developed as a result of extreme crustal extension and synrift mantle unroofing culminating in the formation of peridotites/ gabbro ridges. The COB is defined
mostly by the changes in acoustic basement morphology and crustal reflection pattern and is
interpreted to be located at a distance of 300–500 km from the middle shelf where the inboard rift
boundary is suggested. The oceanic crust of the study region is characterized by sea-floor spreading
lineations from 33 to 18 with a spreading half-rate ranging between 2.5 and 11 mm/yr. Revised
identification suggests that break-up between Australia and Antarctica commenced within anomaly
33, i.e. at about 79–81 Ma ago. Previously recognized anomaly 34 is situated within the zone of
mantle unroofing (Fig. 1).
Our research shows that the width of marginal rift (zone of crustal stretching) varies significantly
along the studied margin and amounts c. 250 km on the eastern Droning Maud Land, 200–350 km on
the Enderby Land–Queen Mary Land and 300–500 km on the Wilkes Land–George V Land.
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STRUCTURE AND TECTONIC DEVELOPMENT OF THE SOUTHERN MARGIN
OF THE SCOTIA SEA
4-10
Lodolo, E.1*, Civile, D.1, Tassone, A.2
(2) CONICET - Instituto de Geofísica “Daniel Valencio”, Univ. de Buenos Aires, Argentina
* Presenting author’s e-mail: elodolo@ogs.trieste.it
The southern margin of the Scotia Sea corresponds to the Scotia-Antarctica plate boundary, along
which the relative motion between the two plates is mostly left-lateral. Available multichannel seismic
reflection profiles (see Fig. 1), integrated with earthquake data, and literature information, have
allowed to define the general structural architecture of the margin, and propose a tectonic evolutionary
history.
Three main tectonic segments have been identified along the margin, from the Elephant Island to the
Herdman Bank. Along the arcuate western segment (from the Elephant Island to the South Orkney
microcontinent), seismic data have shown the presence of a buried, scarcely developed accretionary
body, and an evident deepening of the oceanic crust beneath the crustal blocks forming the South
Scotia Ridge. Along this segment of the margin, the transition between the continental elevated blocks
and the oceanic crust is abrupt. The central part of the south Scotia margin is occupied by the northern
margin of the South Orkney microcontinent, where a quite developed S-verging subduction zone of
the Scotia sea oceanic crust beneath the continental block, is present. The sector to the east of the
South Orkney microcontinent till the Herdman Bank shows a very complex structural assemblage, due
to the presence of several bathymetric continental highs separated by deep troughs and restricted
oceanic basins. An ENE-oriented basin (the Bruce Deep) was found to the E of the South Orkney
microcontinent. To the south of the Bruce Deep, a wide deformation zone with N-verging folds and
thrusts (here named Jane Thrust Belt), has been identified. The easternmost segment of the plate
boundary is structurally the less constrained, and may be composed by a series of tectonic lineaments
of different lengths.
Analyzed data in general have shown that in the western sector of the southern Scotia margin, the
Scotia oceanic crust seems to have subducted beneath the Antarctic plate, whereas the Weddell Sea
subducted beneath Scotia plate, in the eastern sector. The activation of left-lateral transtensional
strike–slip lineaments generated narrow pull-apart basins in the eastern sector in correspondence of
Fig. 1 - Seismic profiles position map for the southern margin of the Scotia Sea (bathymetry based on satellite-derived
data). Data are available at the web site: http://snap.ogs.trieste.it/SDLS/. It works under the auspices of Scientific
Committee on Antarctic Research (SCAR) to provide open access to all multichannel seismic reflection data collected
south of 60°S. SSR = South Scotia Ridge; EI = Elephant Island; SOM = South Orkney microcontinent.
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the fore-arc of the convergent zones. These evidence suggest that the southern Scotia margin may
represent an example of an opposite subduction polarity environment, for many aspects similar to the
tectonic setting of the central sector of the northern Carribbean margin where a double subduction
with N- and S-vergence is documented in correspondence of the Hispaniola Island.
Three main tectonic phases have been recognized in the deformational history of the Scotia-Antarctica
plate boundary. The first phase (Lower Miocene) was characterized by a north-directed convergence
of the Weddell Sea beneath the series of bathymetric highs now distributed along the south-eastern
part of the Scotia Sea. A second phase (about 12 Ma) was characterized by the presence of the two
possibly coeval and opposite-vergence subduction zones of the Scotia Sea and Weddell Sea oceanic
crust. The subduction of the Scotia Sea is testified by the presence of some small and scarcely
developed accretionary prisms. In the eastern zone the system of NNE-trending dextral transform
faults was active and separated zones with different direction or rate of movement. These faults
dismembered and partly deactivated the subduction zone and facilitated the process of fragmentation
and dispersion of the crustal blocks. During this phase, an accretionary prism (Jane Thrust Belt),
formed to the south of the Bruce Bank due to a presence of a S-verging subduction zone. The final
phase was characterized by the deactivation of the subduction zones and by the activation of leftlateral strike-slip regional lineaments. These structures generated several narrow pull-apart basins in
the eastern sector of the Scotia margin, while an abrupt contact between continental and oceanic crust,
without the presence of a transitional crust, is observed along the western margin.
3D DENSITY MODEL OF THE CENTRAL AMERICAN SUBDUCTION ZONE
FROM SATELLITE GRAVITY DATA INTERPRETATION
4-11
Lücke, O.H.1*, Götze H.J.1
(1) Christian-Albrechts-Universität, Kiel Germany.
* Presenting author’s e-mail: osluecke@geophysik.uni-kiel.de
The Central American convergent margin poses several challenges regarding the availability and
quality of potential field data. Satellite gravity data and subsequent combined geopotential models
provide a homogenous database with global coverage which is suitable for geophysical applications.
The aim of this work is to model in 3D the lithospheric structure along the Middle American trench
and assess the scale in which available satellite derived gravity data provides input for the modelling
of the solid Earth. The EGM2008 combined geopotential model is being used for the density
modelling on the regional scale and the density model was constrained by available and previously
published seismic velocity models, magnetotelluric profiles and receiver function data. The 3D
density model shows the overall geometry and density distribution of the subducting Cocos Plate and
the overriding Caribbean Plate. Thickening of the oceanic crust by the influence of the Galapagos hotspot was also modelled and the structure was carried into the slab outlining the effects of ridge
subduction. The structure of the overriding plate is heavily influenced by the tectonic evolution of the
Caribbean region and its crust presents a patchwork of tectonic blocks with crustal basements of
contrasting densities such as the continental Chortis Block, the mainly ultramafic Mesquito
Composite Oceanic Terrain and a basaltic unit part of the Caribbean Large Igneous Province.
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ITALY-ARGENTINA COOPERATION IN THE FIELD OF SEISMOLOGY:
THE ASAIN. HISTORICAL REVIEW AND RECENT PROGRESS
4-12
Russi, M.*, Cravos, C., Plasencia Linares, M.P.
Istituto nazionale di oceanografia e di Geofisica Sperimentale OGS. Borgo Grotta Gigante 42/c,
34010 Sgonico (TRIESTE), Italia
* Presenting author’s e-mail: mrussi@ogs.trieste.it
On August 4th, 2003 a major earthquake of 7.5 Ms known as “Centenary Earthquake”, struck the
Scotia Sea region. The earthquake epicentre was located on the ocean bottom in the proximity of the
South Orkney Islands /Islas Orcadas archipelagus and the strong shaking caused some damages (no
casualties) to the structures of the permanent Argentinean base Orcadas, located about 70 km to the
South-West from the epicentre, also determining extensive cracks in the ice pack all along the shores
of the island (Laurie Island) hosting the base. During one year after the main event several thousands
aftershocks with magnitudes up to 5.8 and epicenters spanning a ~150x50 km2 East-West elongated
area followed the main shock. At that time the Antarctic Seismographic Argentinean Italian Network
ASAIN already consisted of four broad-band seismographic stations installed in Antarctica (Base
Orcadas, ORCD and Base Jubany, JUBA) and Argentinean Tierra del Fuego (Estancia Despedida,
DSPA and Ushuaia, USHU). The whole network recorded both the main shock and all the stronger
aftershocks while station ORCD, due to its favourable location, also recorded thousands of minor
shocks belonging to the “Centenary Earthquake” series. August 4th, 2003 has been a fundamental date
for the progress of seismometry and seismological studies in the Scotia Sea providing a practical
demonstration of the usefulness of a permanent regional seismographic network in the area.
During the nineties and the past ten years several seismological research groups have been working in
the Scotia sea and neighbouring areas but the realization of a permanent seismographic network is
mainly the result of the joint effort of the Italian Programma Nazionale di Ricerche in Antartide
(PNRA) / Ist. Naz di Oceanografia e di Geofisica Sperimentale–OGS and the Argentinean Dirección
Nacional del Antártico (DNA) / Instituto Antártico Argentino (IAA) groups which installed the first
ASAIN station at Base Esperanza at the beginning of 1992. Today the ASAIN consists of seven broadband stations sending their recordings using internet and satellite links provided by Argentina to the
OGS and the IAA. ASAIN real time data are also transmitted to the ORFEUS Data Centre as a
contribution to the Virtual European Broadband Seismographic network (VEBSN). A review of the
network activity during its 20 years life and some information about the on going work will be
presented. The most exciting planned activity is represented by the installation at Belgrano II station
during the 2010-2011 Antarctic campaign of a polar seismometer which can properly operate at
temperatures reaching -50° C.
OROGENESIS REFLECTED IN THE TRANSITION FROM EXTENSIONAL RIFT
BASIN TO COMPRESSIONAL FORELAND BASIN IN THE SOUTHERNMOST ANDES
(54.5°S): NEW PROVENANCE DATA FROM BAHÍA BROOKES AND SENO OTWAY
4-13
McAtamney, J.1*, Klepeis, K.1, Mehrtens, C.1, Thomson, S.2
(1) University of Vermont, Dept. of Geology, 180 Colchester Ave, Burlington VT 05401
(2) University of Arizona, Dept. of Geosciences, 1040 E. 4th St, Tucson AZ 85721
* Presenting author’s e-mail: mcatamney@gmail.com
South of 51ºS latitude, in the southernmost Andes, the Cretaceous inversion of the Late Jurassic Rocas
Verdes rift basin created the Cretaceous-Neogene Magallanes foreland basin between an active
volcanic arc and the South American craton. We present stratigraphic and sedimentary provenance
data from the Lower and Upper Cretaceous sedimentary units, known as the Zapata Formation and
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Punta Barrosa Formation, that record this tectonic transition to test previous models of rift basin
inversion and foreland sedimentation patterns. We present the results from sandstone petrography, an
analysis of sandstone detrital modes and mudstone Rare Earth Element geochemistry, and U/Pb
detrital zircon ages from pre- and post-inversion sediments within the Zapata and Punta Barrosa
Formations in two little studied parts of the Magallanes foreland basin: Bahía Brookes and Seno
Otway. The results constrain the timing of uplift and deundation of source terrane in the internal part
of the orogen and characterize the depositional setting and provenance during the transition from rift
to foreland basin sedimentation in the southernmost Andes.
Three kilometers of measured section record the stratigraphic transition from the Zapata Formation to
the Punta Barrosa Formation. Thinly bedded shallow marine mud and incomplete Bouma sequences
characterize the Zapata Formation. Fining-upward packages of thickly bedded coarse-grained sand
mark the onset of deposition of the Punta Barrosa Formation. Complex paleocurrent patterns from
both units support a sedimentation model of multiple back-arc submarine fans during the initiation of
the foreland basin. By Late Cretaceous time paleocurrent data indicate a north-south trending axial
channel transport system parallel to the orogen. Modal analysis and petrography of sandstone from
both units shows sediments are compositionally immature, highly feldspathic, and derived from a
volcanic arc. Detrital modes record the transition from dominantly volcanic lithic fragments in the
Zapata Formation to dominantly metamorphic lithic fragments in the Punta Barrosa Formation. Rare
Earth Element fractionation shows compositional overlap between the two formations in the southern
basin, and compositional distinction between formations in the northern basin. Detrital zircon age
spectra yielded maximum depositional ages between 89-88 Ma for the base of the Punta Barrosa
Formation and 81-80 Ma for the top of the Punta Barrosa Formation.
Andean orogenesis began as a narrow submarine thrust wedge behind a Late Cretaceous volcanic arc.
The Late Cretaceous sedimentary fill of the Magallanes foreland basin was sourced from uplifted
horst-and-graben style blocks proximal to an active volcanic arc. Preferential uplift and erosion of
pre-rift basement schists and Upper Jurassic volcanic rocks occurred after about ~82 Ma. The
depositional environment during initiation of the Magallanes fold-thrust belt included multiple apron
style submarine fans prograding from the rising Cordillera.
CRUSTAL DEFORMATIONS ASSOCIATED TO THE M 8.8 MAULE
EARTHQUAKE IN CENTRAL CHILE, 27 FEBRUARY 2010, DETECTED
BY PERMANENT GPS STATIONS IN ARGENTINA
4-14
Mendoza, L.1,2*, Vasquez, J.1,2, Del Cogliano, D.1,2
(1) Grupo de Geodesia Espacial, Facultad de Ciencias Astronómicas y Geofísicas, Universidad
Nacional de La Plata, Paseo del Bosque, B1900FWA La Plata, Argentina
(2) CONICET, Argentina
* Presenting author’s e-mail: lmendoza@fcaglp.unlp.edu.ar
The analysis of records from permanent GPS stations registered before, during and after an
earthquake provides direct measurements of the co-seismic offsets and the post-seismic deformation
associated to the seismic event. The magnitude and distribution of the observed deformation may help
to infer the nature of the fault at depths; on the long term the post-seismic relaxation observed by GPS
could be useful to other techniques (e.g. fit together InSAR scenes).
On the other hand, the co-seismic displacements and the post-seismic velocities of the GPS stations
are required in order to evaluate the impact of the earthquake on the regional geodetic infrastructure
itself and to assure, if necessary, a smooth redefinition of the affected geodetic networks; this situation
necessarily concerns to not scientific applications including the surveying, the engineering and the
land registry.
We present results from a consistent processing of GPS records, spanning 16 months and centred
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around the M 8.8 Maule earthquake in central Chile (27 February 2010), involving permanent GPS
stations from the RAMSAC network (Argentina) and GPS stations from the SIRGAS-CON and the
IGS tracking networks. The magnitude and spatial distribution of the observed deformation are
presented and discussed, with special attention to the implication for the geodetic infrastructure of the
country.
STRUCTURAL GEOLOGY OF THE EASTERN TIERRA DEL FUEGO ISLAND
Menichetti,
M.1*,
Tassone,
A.2,
Lippai,
4-15
H.2
(1) Dipartimento di Scienze menichetti@uniurb.it
Universidad de Buenos Aires. Ciudad Universitaria. Pabellón 2. CP - C1428EHA- Buenos Aires.
Argentina
* Presenting author’s e-mail: menichetti@uniurb.it
The eastern part of Tierra del Fuego, along the Atlantic coast, is characterized by continuous outcrops
of Cenozoic rocks pertaining to the Magallanes perisutural basins developed in front of the external
margin of the southernmost Andean Cordillera.
This basin is located on continental crust, extends to the Atlantic on-and-off-shore and is filled by a
5-km-thick siliciclastic sedimentary succession spanning in age from Cretaceous to Holocene. The
basin is located in front of the Cordillera, has a concave arcuate shape and a depocenter in southern
Patagonia. On the western side of the Tierra del Fuego Island the basin is oriented NW-SE with a
width of many hundreds km while toward the east, along the Atlantic coast, it strikes roughly E-W,
following the Fuegian Andes curvature, and is restraining for about one hundred km. Since the Late
Cretaceous its southern margin underwent significant compressional deformation that originated the
present-day Magallanes fold-and-thrust belt. The geometry of the basin is controlled by northward
thrust propagation involving the foreland in the deformational process. The stratigraphic succession
from the Late Cretaceous to the Miocene can be subdivided into four stratigraphic units separated by
syn-tectonic angular unconformities of the Late Cretaceous, Palaeocene, Eocene and the Lower
Miocene. Marine sandstones and mudstones of Andean provenance were deposited in a system of
clastic wedges progressively shifted toward the foreland. The syn-tectonic angular and progressive
unconformities constrain the timing of the thrusts propagation in the frontal part of the chain.
In the last years, several kinematic models have been proposed based mainly on stratigraphic and
sedimentary methodologies, in several cases not supported by field evidence. A simple fold-and-thrust
belt geometry with a basal detachment level in the Tertiary mudstone has been proposed for the area
without consideration of the structural complexity of the region affected by at least two tectonic
phases: compressional, from late Cretaceous to Oligocene, and transtensional, from Late Oligocene
on. The basins subsidence, at least until the Late-Middle Eocene unconformity, has been mainly
controlled by several transtensional faults running broadly parallel to the compressional front. Since
the late Oligocene, a left-lateral strike slip fault system has been overprinting the thrust structures and
partitioning the deformation along different fault arrays of the Atlantic Coast, from Cabo S. Diego to
Cabo S. Pablo. The main compressional structures mapped along the coast present a northward
verging fault-bend fold geometry, with an amplitude of a few km, a detachment level located in the
marls layers, low angle thrust planes and a shortening of about 30 %. The external compressional front
of the orogen can be located at Punta Gruesa, where the anticline thrusts over the slightly deformed
structures of Cabo S.Pablo. The main E-W left lateral strike-slip faults are located at Cabo Leticia,
Capo Campo del Medio and Cabo Irigoyen, and are associated with a set of N-S and NE-SE
extensional faults. These later fault systems with offsets of many hundreds of meters represent the
fault array associated with the Magallanes-Fagnano transform fault. Many seismically-triggered sand
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intrusions are present throughout the stratigraphic succession in the Late Cretaceous, the Late
Palaeocene and the Middle Miocene marking important tectonic events. Several of these sedimentary
and structural features can be easily correlated with the structures observable in the off-shore seismic
reflection profiles. The joint analysis of these data, document a multi-stage evolution of the basin with
the presence of five seismic units, separated by angular unconformities recording different
tectonosedimentary events. Several pull-apart basins are located along the off-shore alignment of the
principal deformation zone of the Magallanes-Fagnano fault and have their on-shore counterparts in
the Cabo Malenguena basin and western Lago Fagnano.
THE PRE-FARELLONES DEFORMATION (PEHUENCHE PHASE),
CORDILLERA PRINCIPAL AND FRONTAL (31°45’LS), SAN JUAN PROVINCE,
ARGENTINA
4-16
Pérez D.J.1*, Sanchez Magariños J.M.1
(1)Laboratorio de Tectónica Andina, Universidad de Buenos Aires, Ciudad Universitaria, Pabellón II
(1428), Buenos Aires, Argentina
* Presenting author’s e-mail: daniel@gl.fcen.uba.arl
Introduction
The objective of this study is to analyze the volcanic rocks and structure of the Mondaca river
region and their relationship within Mesozoic sedimentary deposits. New age data for Los
Pelambres Formation, and field structural data from these volcanic deposits, indicate an Oligocene
and Miocene deformation related to the Pehuenche phase. The study area is located in the Frontal
and Principal Cordillera at 31º45’S and 70°15’W, on the boundary with Chile, in the San Juan
province, Argentina. This region corresponds to the southern part of the non-volcanic “flat-slab”
region betwen 28°S and 33°S (Cahill e Isacks, 1992) under which the Nazca plate forms a broad
sub-horizontal bench between about 100 and 150 km. The first studies in the region were done by
Groeber (1951), Polanski (1964), Olivares Morales (1985), Rivano and Sepulveda (1991), and
more recently, by Alvarez (1996), Pérez (2001), Ramos et al. (1998).
Geology and structure
The stratigraphic sequences of the region begin with Permo-Triassic rhyolitic and rhyodacitic
rocks of the Choiyoi Group; then continue with Triassic rocks of the Rancho de Lata Formation
and Jurassic sequences of the Los Patillos, La Manga and Tordillo Formations. Then, follows the
Auquilco formation (gypsum and diapirs), but without stratigraphic relationships, and a sequence
of volcanic rocks follows, defined in the Chile region as Los Pelambres Formation (Rivano and
Sepúlveda, 1991) and Alitre and Mondaca Pass in Argentina. These same rocks in the La Ramada,
located immediately to the south of the study area, were related to the Juncal Formation (Ramos
et al., 1990); these same volcanic rocks immediately to the north of the study region, upper
Oligocene and lower Miocene in age, are assigned to the Pelambres and Pachón Formations
(Fernández et al., 1974; Mpodozis et al., 2009). These are correlated with the Abanico Formation.
Unconformable above these rocks, are the volcanic sequences of Farellones Formation, of
Miocene age. Quaternary deposits cover all the units.
The study region presents two structural styles: a thin skinned and a thick skinned styles, which
affected different rocks and in different periods of time. The first style can be recognized in the
Mondaca and Carnicerias River, where the low-angle thrust of Los Pelambres is uplifting the
volcanic sequences of upper Oligocene and lower Miocene over the Jurassic Los Patillos
Formation. Toward the west and in the Chilean territory, another thrust of low angle would be the
responsible of uplifting of the Cretaceous deposits above the Tertiary volcanic rocks. These events
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are attributed to different deformation phases (out of sequences thrust) of Miocene times. Similar
structure have been already describes to the south of the study region which affected OligoMiocene volcanic rocks of Los Pelambres Formation.
Conclusions
The volcanic deposits of the Los Pelambres Formation, previously assigned to the Cretaceous,
were redefined in age to the Oligocene, 33.4-25.2 Ma. The volcanic and volcaniclastic deposits of
the Pachón (Abanico) Formation were recognized; the age of this Formation, previously assigned
to the Cretaceous, was redefined to the late Oligocene - early Miocene, 25-21 Ma. The volcanic
deposits of Los Pelambres and Pachón Formations (Oligocene-Miocene) that constitute the
Abanico basin, are highly deformed, and are underlying the Farellones Formation of middle
Miocene (18,3 Ma). This unconformable relationship would be indicating a very important
deformation phases occurred during the upper Miocene (~21-18 Ma) and corresponding to the
Pehuenche phase.
REFERENCES
• Alvarez, P.P., 1996. Los depósitos triásicos y jurásicos de la Alta cordillera de San Juan. En V.A. Ramos (ed). Geología de la
región del Aconcagua, provincias de San Juan y Mendoza. Subsecretaría de Minería de la Nación. Dirección Nacional del
Servicio Geológico. Anales 24 (5): 59-137, Buenos Aires.
• Cahill, T. y Isacks, B.L., 1992. Seismicity and shape of the subducted Nazca plate. Journal of Geophysical Research . Nº97,
p. 17503-17529.
• Fernández, R.R., Brown, R.F., y Lencinas, A.N., 1974. Pachón un nuevo pórfido cuprífero argentino, Dto. de Calingasta, prov.
de San Juan, República Argentina. 5º Congreso Geológico Argentino, Actas II: 77-89, Buenos Aires.
• Groeber, P., 1951. La Alta Cordillera entre las latitudes 34° y 29°30’. Instituto Investigaciones de las Ciencias Naturales. Museo
Argentino de Ciencias Naturales B. Rivadavia, Revista (Ciencias Geológicas) I(5): 1-352, láminas I-XXI, Buenos Aires.
• Mpodozis, C., Brockway, H., Marquardt, C. y Perelló, J., 2009. Geocronología U/Pb y tectónica de la región de Los PelambresCerro Mercedario: Implicancias para la evolución cenozoica de los Andes del centro de Chile y Argentina. XII Congreso
Geológico Chileno, Actas S9_059, 22-26, Santiago
• Olivares Morales América Patricia, 1985. Geología de la Alta Cordillera de Illapel entre los 31°30 y 32° Latitud Sur. Tesis de
Grado, Universidad de Chile, Facultad de Ciencias Físicas y Matematicas Departamento de Geología y Geofísica.
• Pérez, D.J., 2001. El volcanismo neógeno de la cordillera de las Yaretas, Cordillera Frontal (34°S) Mendoza. Revista de la
Asociación Geológica Argentina, 56 (2):221-23, Buenos Aires.
• Polanski, J., 1964. Descripción geológica de la hoja 25a Volcán San José, provincia de Mendoza, Dirección Nacional de
Geología y Minería, Boletín 98: 1- 94, Buenos Aires.
• Ramos, V.A., Rivano, S., Aguirre-Urreta M.B., Godoy, E. y Lo Forte, G.L., 1990. El Mesozoico del Corcón del Límite entre
Portezuelo Navarro y Monos de Agua (Chile-Argentina). XI Congreso Geológico Argentino, Actas II: 43-46, San Juan.
• Rivano, G. y Sepulveda, H.,1991. Hoja Illapel Región de Coquimbo, Servicio Nacional de Geología y Minería, Carta Geológica
de Chile. Nº69. 132pp..
.
CORRELATIONS OF TECTONO-MAGMATIC EVENTS IN THE SOUTH
VERKHOYANSK OROGENIC BELT (EASTERN SIBERIA, NORTHEAST ASIA)
4-17
Prokopiev A.V.1*
39, Lenin Avenue, Yakutsk, 677980, Russia
* Presenting author’s e-mail: prokopiev@diamond.ysn.ru
The South Verkhoyansk orogenic belt extends submeridionally for 850 km along the southeastern
margin of the North Asian craton. Three tectonic zones are recognized there from west to east: Kyllakh
(frontal), Sette-Daban, and Allakh-Yun’; in the hinterland of the belt there is the Okhotsk cratonal
terrane (Fig. 1).
The history of the belt includes the following major geodynamic events (Fig. 2):
1. Within the Kyllakh and Sette-Daban tectonic zones, the Riphean rifting processes were followed by
pre-Vendian folding and thrusting. Here the folds and thrusts deforming the Riphean rocks are
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overlain,
with
an
angular
unconformity, by Vendian deposits.
Within the Okhotsk terrane, these
events are recorded by unconformities
at the base of Middle Riphean,
Vendian, and Cambrian sections.
2. The earliest Phanerozoic subduction
events are reported from the eastern
(hereafter in present-day coordinates)
margin of the Okhotsk terrane, which
occurred in the Middle-Late Ordovician.
At that time, the granitoid complex of a
continental-marginal arc was forming
there. Synchronous with the crustal
extension, within the Kyllakh and,
likely, the Sette-Daban tectonic zones,
dikes and sills of basic composition
were emplaced. Their age was first
determined as Late Ordovician
(451±24, 450±12 Ma) from U-Pb
dating of baddeleyite.
3. Next pulse of subduction events in
the eastern Okhotsk terrane (North
Okhotsk magmatic arc) is confidently
dated as late as the Late Devonian on
the basis of subduction-related
granitoids (U-Pb, 375.3±2.3 Ma,
zircon) and calc-alkaline volcanics
present there. Concurrent with this,
continental rifting processes occurred
within the Sette-Daban and AllakhYun’ tectonic zones, which manifested
themselves as normal faulting and
eruptions of alkali basalts of
Devonian-Early Carboniferous age.
Within the Kyllakh zone this time
interval was marked by the
Fig. 1 - Structural setting of the South Verkhoyansk orogenic belt.
emplacement of basic dikes and sills.
4.
The
Middle-Late
Triassic
subduction in the eastern Okhotsk terrane is evidenced by calc-alkaline volcanics and exhumation of
the crystalline basement as seen from (U-Th)/ He low-temperature thermochronometry data. No
evidence of this event is found within the tectonic zones in the west.
5. The Late Mesozoic geodynamic events were related to the subduction of the paleo-Pacific beneath
the eastern margin of the Okhotsk terrane and the formation of the Uda (Late Jurrasic-Neocomian)
and the Okhotsk-Chukotka (Albian-Late Cretaceous) active continental margins. The first pulse of
Late Mesozoic thrusting and dislocation metamorphism occurred within the Sette-Daban tectonic
zone in the latest Late Jurassic (40Ar/39Ar, 160±1 Ma). This metamorphic event marks the initiation of
folding and the onset of the formation of a metamorphic belt in South Verkhoyansk region. These
deformations are supposed to be the result of accretion processes that occurred along the subduction
zone of the Uda active continental margin. In the Allakh-Yun’ zone, the folding and metamorphic
processes occurred in the Late Neocomian and Aptian (40Ar/39Ar, 119±0.5 Ma). On the southern flank
and in the rear part of the Okhotsk terrane, deformation processes related to subduction along the Uda
magmatic arc continued. It is established that folding was initiated in the Sette-Daban zone much
earlier than in the neighboring Allakh-Yun’ zone. The timing of dislocation metamorphism in the
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Fig. 2 - Schematic correlation of tectonomagmatic events in the south Verkhoyansk orogenic belt.
latter zone coincides with the crystallization age of major granite plutons (U-Pb, 120-123 Ma, zircon).
In the Late Cretaceous, synchronous with the formation of volcanics of the Okhotsk-Chukotka
volcanogenic belt above the subduction zone, emplacement of granitoids aged at 95 Ma (U-Pb,
zircons) and left-lateral transpression reverse and strike-slip motions occurred within the Allakh-Yun’
zone.
THE LATE OLIGOCENE-MIOCENE ÑIRIHUAU FORMATION INTERPRETED
AS A FORELAND BASIN IN THE NORTHERN PATAGONIAN ANDES
4-18
Ramos, M.E.*, Orts, D., Calatayud, F., Folguera, A., Ramos, V.A.
Laboratorio de Tectónica Andina, Departamento de Ciencias Geológicas, Facultad de Ciencias
Exactas y Naturales, Universidad de Buenos Aires. Ciudad Universitaria, Pabellón II, Ciudad de
Buenos Aires, 1428, Argentina. CONICET
* Presenting author’s e-mail: miguelestebanramos@hotmail.com
A field study of the Ñirihuau basin southwest region, nearby the Maitén range, between the 42º00´S
and the 42º20´S, has revealed new stratigraphic relationships. The structural analysis of this Andean
segment, allowed a new interpretation of the tectonic setting of these Patagonian foothills.
The stratigraphic sequence consists of four units: The base is represented by the Oligocene Ventana
Formation, formed mainly by andesitic volcanic and pyroclastic rocks defining the Maitén eruptive
belt (Rapela et al., 1988). Based on chemical analyses, these authors recognized this sequence as a
typical arc succession. The shales, sandstones and coal beds of the Ñirihuau Formation of ~22-17 Ma
rest in general conformably on the previously described volcanic piles (González Bonorino, 1973;
Cazau, 1980). More recently, Paredes et al. (2009) made a detailed sedimentary analysis in the northern part of the Ñirihuao basin. Here, they differentiated a series of lithofacies associations in strati154
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graphical order: deep lacustrine, volcaniclastic flows, shallow lacustrine interbedded with Gilberttype deltas, volcaniclastic flows, fluvial channels, and volcanics at the top.
We identified a series of sections, circumscribed to the southern part of the basin, that can be compared to the ones described by Paredes et al. (2009) to the north. These start with a thin carbonaceous
section that could be associated with a relatively deep lacustrine association 10 meter thick, constituting an excellent stratigraphic marker through the basin. This is followed by a 50 meter thick volcaniclastic sandtstones. The upper section begins with liquefact levels, associated with Gilbert-type deltas,
and floodplain deposits 350 meter thick. Finally a fluvial system progrades on top of the deltaic section represented by a series of channels and associated flood plain deposits.
These sequences are exhumed at the eastern slope of the Maitén range which is interpreted as an eastverging thick-skinned structure dismembered by a series of synthetic-to the main thrust front structures (eg. El Pantano thrust), affected by a series of west-verging backthrusts developed in the back
limb (e.g. El Maitén thrusts). Basement shortening produced by these structures was transmitted to
the upper sedimentary cover, where a frontal monoclinal ramp was formed (Giacosa and Heredia,
2004a), absorbing discrete amounts of thin-skinned deformation related to fault bend folds associated with flexural slip (Fig. 1).
In the eastern sector, four sets of progressive unconformities from the base to the top of the Ñirihuau
Formation and the base of the Collón Cura Formation were recognized. These were explained applying the concept of growth strata bed-by-bed by kink-band migration (Suppe et al., 1997). These find-
Fig. 1 - Structural cross section and corresponding palinspastic restoration. Note a thick skinned domain in the
westernmost part and a thin-skinned deformation at the eastern section associated with flexural slip
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ings imply that this basement structure was created at the time of both Ñirihuao and Collón Curá sedimentation at the foothills and therefore that these constitute remnants of a proximal foreland basin.
These progressive unconformities are typically found at the fold and thrust belt top wedge (DeCelles
et al., 1996), implying that the early to late Miocene orogenic front was located in the Maitén range.
Based on these findings, four pulses of contractional deformation are proposed for the fold and thrust
belt at these latitudes coetaneous to the sedimentation of the Ñirihuao and the base of the Collón Cura
Formations (>22 to ~15 Ma), implying a mechanism of subsidence associated with orogenic loading
of the El Maiten range.
REFERENCES
• Cazau, L.B.; 1980. Cuenca de Ñirihuau-Ñorquinco-Cushamen Leanza, A. (ed.), Geología Regional Argentina 2, Academia
Nacional de Ciencias de Córdoba, Córdoba, Argentina, pp. 1149-1171.
• DeCelles, P.G., Giles K.A.; 1996. Foreland basin systems. Basin Research Vol.8, pp. 105-123.
• Giacosa, R.E., Heredia N.; 2004a. Structure of the North Patagonian thick-skinned fold-and-thrust belt, southern central Andes,
Argentina (41°–42°S) Journal of South American Earth Sciences, Vol.18, pp. 61–72
• Giacosa, R., Heredia, N.;2004b. Estructura de los Andes Nordpatagónicos en los cordones Piltriquitrón y Serrucho y en el valle de
El Bolsón (41 30‘–42 00‘ S), Río Negro. Revista Asociación Geológica Argentina, Nº59 Vol.1,pp. 91–102.
• González Bonorino, F.;1973. Geología del área entre San Carlos de Bariloche y Llao Llao. Fundación Bariloche, Dpto. Rec. Nat.
Energ. Buenos Aires, Publ. 16, pp. 1-53.
• Paredes, J.M., Giacosa, R.E., Heredia, N.; 2009. Sedimentary evolution of Neogene continental deposits (Ñirihuau Formation)
along the Ñirihuau River, North Patagonian Andes of Argentina. Journal of South American Earth Sciences, Vol. 28, pp. 74-88.
• Rapela, C., Spalletti, L., Merodio, J., and Aragón, E.; 1988. Temporal evolution and spatial variation of early Tertiary volcanism in
the Patagonian Andes (40° S - 42°30’ S): Journal of South American Earth Sciences, Vol. 1, pp. 75-88.•
• Suppe, J.; 1997. Bed-by-bed fold growth by kink-band migration: Sant Llorenq de Morunys, Eastern Pyrenees. Journal of Structural
Geology, Vol. 19 Nos. 3-4pp, 443-461.
A CASE OF PALEOHORIZONTAL RESTORATION OF PLUTONIC BODIES USING
PALEOMAGNETIC DATA: THE SIERRA DE VALLE FÉRTIL MAGMATIC COMPLEX,
WESTERN ARGENTINA
4-19
Rapalini, A1*, Pinotti, L.2, D’Eramo, F.2, Otamendi, J.2, Vegas, N.3,
Tubía, J.3, Singer, S.1, Vujovich, G.4
(1) Institutode Geofísica Daniel A. Valencio (INGEODAV)- Departamento de Ciencias Geológicas FCEN- Universidad de Buenos Aires. CONICET
(2) Departamento de Geología, FCEFQyN, Universidad Nacional de Río Cuarto.CONICET
(3) Departamento de Geodinámica- Universidad del País Vasco-España
(4) Departamento de Ciencias Geológicas - FCEN- Universidad de Buenos Aires. CONICET
* Presenting author’s e-mail: rapalini@gl.fcen.uba.ar
Introduction
Reliable paleohorizontal control is essential in any tectonic or paleogeographic application of
paleomagnetic studies. As such, the use of paleomagnetic data from plutonic bodies, for which
paleohorizontal control is frequently lacking, is severely restricted. However, if magnetization age can
be determined or constrained and the expected paleomagnetic direction is already known,
paleomagnetic information can be very useful in reconstructing the original position of plutonic
bodies, providing important information for the understanding of emplacement processes.
Recently Castro et al. (2008) proposed that the magmatic structures related to the mechanical
interaction between mafic magmas and granitoids in the Early Ordovician Sierra del Valle Fértil
magmatic complex are the result of top-to-down intrusions of a mafic magma into a granodioritetonalite mass. These sinking structures would be the result of a reverselly stratified magma chamber
with gabbros and diorites at the top and granodiorite-tonalite at the bottom. This interpretation would
not be valid if the plutons have been subsequently tilted so that sub-vertical to high inclination pipelike structures were originally subhorizontal, reflecting a different layering process.
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A paleomagnetic study was carried out
on fifty-one oriented cores from the
Sierra de Valle Fértil magmatic
complex in order to evaluate to which
extent these plutonic rocks have been
subsequently tilted, and in such way to
provide independent information to
improve the petrological model of
Castro et al. (2008).
Geological Setting
The magmatic complex of Sierra de
Valle Fértil (San Juan province,
Argentina) consists of igneous and
metamorphic rocks. This complex
formed at low- to middle-crustal paleodepths within the Early Ordovician
subduction-related magmatic belt from
central- and northwestern Argentina,
which is known as Famatinian arc. The
age of magmatism in this sector has
been well constrained between 490 and
465 Ma (Pontoriero and Castro de
Machuca, 1999; Pankhurst et al.,
2000). Recent ages of igneous plutonic
rocks yielded ages between 469 to 477
Ma (Ducea et al., 2010). Within the
western belt of the currently exposed
Fig. 1 - Main geological units of central western Argentina and
Famatinian magmatic arc, the Sierras
location of the study area.
de Valle Fértil contain fairly wellexposed sections showing the transition
between lower-crustal and upper-crustal levels (Otamendi et al., 2008). The reconstruction of the
Famatinian arc suggests that this deep seated crustal sequence formed in an outboard belt (e.g.
Quenardelle and Ramos, 1999). Tilting and uplifting of the studied crustal section during
emplacement in the upper crust might be primarily related to the collision between the Laurentiaderived terrane of Cuyania (or “Precordillea”) and the proto-Pacific margin of Gondwana (Thomas
and Astini, 1996; Ramos et al., 1996).
From west to east, the lithologic units display a progression towards more evolved igneous
compositions. In general, the stratigraphy may be described considering four units: (Mirré, 1976;
Vujovich et al., 1996) : (1) A layered mafic unit including Ultramafic, Ol-rich and Px-rich cumulates.
(2) A tonalite-dominated igneous unit comprising coarse grained biotite tonalites and extremely
heterogeneous rocks with mafic enclaves. (3) A felsic igneous unit making up a batholith-scale Bt ±
Amph granodiorite which hosts chilled mafic dikes, enclaves, and blocks of Amph-bearing gabbros.
(4) Migmatites (metatexite to diatexite) appearing as kilometric strips interlayered with igneous
mafic, intermediate and felsic rocks. The two silicic units, tonalites and granodiorites, contain
abundant mafic microgranular enclaves (Castro et al., 2009).
Paleomagnetic Study and Results
Sixty-two specimes were selected to perform stepwise AF (47) and thermal (15) demagnetization.
Both methods proved to be equally efficient to isolate the characteristic remanence. Most samples are
carrier of a stable magnetic remanence which may be determined with the standard techniques. Each
magnetic component was defined by principal component analysis (Kirschvink, 1980). Remanece
coercivity and unblocking temperatures suggest magnetite as the main carrier in most samples. The
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Fig. 2 - a) Paleomagnetic pole position for the Sierra de Valle Fértil complex from “in situ” mean remanence direction
(A) and after untilting into “paleohorizontal” (B) and “paleovertical” (C) positions for magmatic structures. b)
Comparsion of pole positions A and C with other poles from the Sierras Pampeanas, Famatina system, Eastern Puna
and Early Ordovician reference pole for Gondwana. More references in the text.
occasional occurrence of pyrrothite is suspected. After erasing a very soft and randomly directed
component, a consistent characteristic remanence was defined in 50 specimens. Mean remanence
direction is Dec: 14°, Inc: -73°, ·95: 5°. Only three samples showed a reverse polarity. This direction
is 23° apart from the geocentric axial dipole direction and even farther from the present-day Earth
magnetic field direction at the sampling locality, nor is it coincident with any expected direction for
the last 200 m.y.
A paleomagnetic pole was computed from this mean direction. This is presented in African
coordinates according to a Gondwana reconstruction (Reeves et al., 2004) in Fig. 2 (A). In order to
evaluate possible tilting of the intrusive bodies, two alternative paleomagnetic poles were computed.
The first (B) was obtained after untilting of 50° around a horizontal axis of Az 165° (right–hand rule).
This rotation turns most structures related to mechanical interaction between mafic magmas and
granitoids subhorizontal. The other (C) was the product of untilting of 40° around a subhorizontal axis
trending 345° (right - hand rule). This would turn the same structures subvertical. According to
geologial information in the area, tilting around a NNW trending axis is likely due to the Andean
orogeny in the Tertiary. Comparison of three alternative poles with reference poles clearly indicates
that rotation into subhorizontal (B) produces a pole position incompatible with any Phanerozoic pole
position. Alternative A produces a pole position roughly consistent with Devonian paleomagnetic
poles from the Devonian Achala Batholith (Geuna et al., 2009) and Baritú Fm. (Spagnuolo, 2009).
This option may be valid if the studied rocks were remagnetized in Devonian times and no significant
tilting due to Andean or other orogenesis occurred since then. In the third case (C) rotation into
paleovertical position for most of the already mentioned magmatic structures produces a pole position
consistent with previous paleomagnetic poles from Early Ordovician plutons and lavas from the Sierra
de Famatina and Eastern Puna (Conti et al., 1996, Spagnuolo et al., 2008), coeval with the studied
rocks. These results strongly support the third alternative suggesting that the described magmatic
structures were produced in a subvertical position which is consistent with the petrological model of
Castro et al. (2008). Neogene sediments in the area show minor to moderate dips towards the ENE,
consistent with tilting indicated by the paleomagnetic data.
REFERENCES
• Castro, A., Martino, R., Vujovich, G., Otamendi, J., Pinotti, L., D’Eramo, F., , Tibaldi, A., Viñao, A., 2008. Top-down structures of
mafic enclaves within the Valle Fértil magmatic complex (Early Ordovician, San Juan, Argentina). Geologica Acta: Vol. 6, 217-229.
• Conti, C.M., Rapalini, A.E., Coira, B. y Koukharsky, M. 1996. Paleomagnetic evidence of an early Paleozoic rotated terrane in
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Northwestern Argentina, a clue for Gondwana-Laurentia interaction?. Geology 24: 953-956.
• Ducea, M.N., Otamendi, J.E., Bergantz, G., Stair, K., Valencia, V., and Gehrels, G., 2010. Timing constraints on building an
intermediate plutonic arc crustal section: U-Pb zircon geochronology of the Sierra Valle Fértil, Famatinian Arc, Argentina.
Tectonics, in press.
• Geuna, S.E. , Escosteguy, L.D., Miró, R., 2008. Palaeomagnetism of the Late Devonian–Early Carboniferous Achala Batholith,
Córdoba, central Argentina: Implications for the apparent polar wander path of Gondwana. Gondwana Research, 13, 227-237.
• Kirschvink, J.L. 1980. The least - squares and plane and the analysis of paleomagnetic data. Geophysical Journal of the Royal
Astronomical Society, vol. 67, p. 699-718.
• Mirré, J.C., 1976. Descripción geológica de la Hoja 19e, Valle Fértil, Provincias de San Juan y La Rioja. Servicio Geológico
Nacional, Boletín N° 147. Ministerio de Economía, Buenos Aires, 70 pp.
• Otamendi, J.E., Tibaldi, A.M., Vujovich, G.I., Viñao, G.A., 2008. Metamorphic evolution of migmatites from the deep Famatinian
arc crust exposed in Sierras Valle Fértil-La Huerta, San Juan, Argentina. Journal of South American Earth Sciences 25, 313–335.
• Pankhurst, R.J., Rapela, C.W., Fanning, C.M., 2000. Age and origin of coeval TTG, I- and S-type granites in the Famatinian belt of
NW Argentina. Transaction Royal Society of Edinburgh: Earth Sciences, 91, 151-168.
• Pankhurst, R.J., Rapela, C.W., Saavedra, J., Baldo, E., Dahlquist, J., Pascua, I., Fanning, C.M., 1998. The Famatinian magmatic arc
in the central Sierras Pampeanas: an Early to Mid-Ordovician continental arc on the Gondwana margin. In: Pankhurst, R.J., Rapela,
C.W. (eds.). The Proto-Andean Margin of Gondwana. Special Publication. Geological Society, London, 343-368.
• Pontoriero, S., Castro de Machuca, B., 1999. Contribution to the age of the igneous-metamorphic basement of La Huerta range,
province of San Juan, Argentina, II South American Symposium of Isotopic Geology, Proceedings, 101-104.
• Quenardelle, S., Ramos, V., 1999. The ordovician western Sierras Pampeanas magmatic belt: record of Argentine Precordillera
accretion. In: Ramos, V.A., Keppie, D. (Eds.), Laurentia Gondwana Connections before Pangea, vol. 336. Geological Society of
America, Boulder, pp. 63–86. Special Paper.
• Ramos, V.A., Vujovich, G.I., Dallmeyer, R.D., 1996. Los klippes y ventanas tectónicas de la estructura preándica de la Sierra de Pie
de Palo (San Juan): edad e implicanciones tectónicas. In: Proceedings of the Actas XIII Congreso Geológico Argentino y III
Congreso de Exploración de Hidrocarburos, vol. 5. pp. 377–392.
• Reeves, C.V., de Wit, M.J., Sahu, B.K. 2004. Tight reassembly of Gondwana exposes Phanerozoic shears in Africa as global tectonic
players, Gondwana Res. 7 ; 7–19
• Spagnuolo CM (2009). Evolución paleogeográfica del Noroeste Argentino en el Paleozoico temprano en base a estudios
paleomagnéticos. PhD Thesys (unpublished). Buenos Aires Uiversity, Argentina. 325 pp.
• Spagnuolo C.M., Rapalini, A.E. y Astini R.A. 2008. Palaeomagnetic confirmation of Palaeozoic clockwise rotation of the Famatina
Ranges (NW Argentina): implications for the evolution of the SW margin of Gondwana. Geophysical Journal International 173 (1):
63.
• Thomas, W.A., Astini, R.A., 1996. The Argentine precordillera: a traveler from the ouachita embayment of North American
Laurentia. Science 273, 752–757.
• Vujovich, G.I., Godeas, M., Marín, G., Pezzutti, N., 1996. El complejo magmático de la Sierra de La Huerta, provincia de San Juan,
Actas XIII Congreso Geológico Argentino y III Congreso de Exploración de Hidrocarburos, Argentina, 465-475.
SEISMICITY AND EARTHQUAKE HAZARD IN TIERRA DEL FUEGO PROVINCE,
ARGENTINA
4-20
Sabbione, N.C.1*, Buffoni, C.1,2, Barbosa, N.1, Badi, G.1, Connon, G.3, Hormaechea, J.L.1,3
(1) Facultad de Ciencias Astronómicas y Geofísicas, Universidad Nacional de La Plata, Argentina
(2) CONICET, Argentina
(3) Estación Astronómica Río Grande, Tierra del Fuego. CONICET and Universidad Nacional de La
Plata, Argentina
* Presenting author’s e-mail: nora@fcaglp.unlp.edu.ar
Since the 1990s a local seismometric network installed in Tierra del Fuego, including permanent
broadband and short period stations, provides the seismological information required to afford
different kind of studies. This network also allows proper monitoring of Tierra del Fuego seismicity
for civil protection purposes considering the high risk of heavy damages in the towns of Ushuaia and
Rio Grande in the eventuality of a major earthquake such the event occurred in 1949.
Seismicity has been studied and records have been processed by Seisan Software, from Bergen
University (Norway); a significant level of low to medium magnitude earthquakes with epicentre in
Tierra del Fuego continental region and oceanic surrounding areas was found. The obtained
seismicity map shows that beyond rather dispersed seismicity related to the Magallanes-Fagnano fault
system (MFFS), a concentration of epicentres is found in the Darwin Cordillera and in the margins of
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the Scotia plate. It is well known that the region has a complex tectonic setting: the island is crossed
by the MFFS which divides Tierra del Fuego in two continental blocks. This MFFS constitutes the
major continental segment of the South America-Scotia plate’s border.
Results obtained are then used to calculate shear wave attenuation parameters with the spectral
technique. Adjustment of the Q spectral S wave is made in the frequency band between 1 and 9 Hz,
which results in quality factor values of about 100 for short distances and 300-400 for more distances,
according to a region with an important tectonic activity where earthquakes of magnitude 2-3.5 occur.
An earthquake hazard map for Tierra del Fuego Province, Argentina is here presented. Among the
important concerns are the completeness of the catalogues and whether sufficient studies have been
conducted to estimate the earthquake sizes and locations in order to assess the implications of a
modern recurrence. The most fundamental information for a hazard assessment is the record of past
earthquakes. The island has an important seismological history which includes an event of magnitude
7.8 occurred on December 1949. Reports of earthquakes occurred in 1929, 1930, 1944, 1949 and
1970 are known by a study of historical seismicity.
SYNOROGENIC SEQUENCES ASSOCIATED WITH THE ANDEAN
FRONT AT 37º S AS A CLUE FOR AGE EXHUMATION AND STRUCTURATION
OF THE FORELAND AREA
4-21
Sagripanti, L.*, Naipauer, M., Folguera, A., Ramos, V.A.
Laboratorio de Tectónica Andina, Departamento de Ciencias Geológicas,
Facultad de Ciencias Exactas y Naturales, Universidad de Buenos Aires
Ciudad Universitaria, Pabellón II, Ciudad de Buenos Aires, 1428, Argentina. CONICET
* Presenting author’s e-mail: usagripanti@gmail.com
The eastern Andean slope at 37º S was formed by the Malargüe fold and thrust belt (Kozlowski et al.,
1993), whose orogenic front exhumed a lower angular unconformity between Eocene terms of the
Pircala Formation and late Oligocene to early Miocene rocks of the Palauco Formation and an upper
uncorformity between Palauco Formation and a Neogene sedimentary cover (Tristeza Formation) and
Pliocene to Quaternary volcanic beds of the Payenia volcanic field. In order to constrain the relative
importance between these different contractional pulses that led to the formation of the orogenic front
at these latitudes, a sedimentological and petrographical study was carried out through profiles in the
Neogene sequences (Tristeza Formation) of the Pampa de Carrizalito syncline eastward the Sierra de
Reyes anticline (Fig. 1) plus a series of U/Pb datings in detrital zircons. The Sierra de Reyes anticline
is a basement structure that was produced by tectonic inversion of Late Triassic normal faults
(Kozlowski et al., 1993; Zamora and Zapata, 2005; Giambiagi et al., 2009).
Three contractional stages have been related to exhumation in the area (Late Cretaceous, late Eocene
and late Miocene), based on different studies (Cobbold and Rosello 2003; Orts and Ramos 2006;
Tunik et. al., 2010). The Upper Cretaceous contractional stage affected just the westernmost zone and
probably did not reach the present orogenic front where this study is hosted. The second contractional
event took place at the present orogenic front, although this event is just associated with a 50 – 100 m
thick shales and sandstones of the Pircala Formation (Kozlowski et al., 1987). The third mountain
building episode registered at the orogenic front developed in late Miocene times and is associated
with synorogenic sedimentation. We have focussed our attention in this last episode to compare this
sedimentary record and evaluate total denudation.
Historically the Tristeza Formation hosted in the Pampa de Carrizalito depocenter was assigned to the
late Miocene, based on their occurrence on top of the Palauco Formation whose upper part was dated
in 18,12 ± 0,24 Ma (Ar-Ar; Silvestro and Atencio, 2009). Two profiles located in both syncline flanks
(A – A’ and B – B’; see Fig. 1 for location) were done in order to constrain the geometry, thickness
variation next to the orogenic front, and detrital compositional changes, of main sequences. Based on
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Fig. 1 - Geological map of the Neogene synorogenic sequences developed on top of the orogenic wedge front, at the
Sierra de Reyes latitudes; a-f are the defined units in the sedimentary and petrologic analyses performed in the
Neogene synorogenic deposits (see Fig. 2).
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Fig. 2 - Progressive exhumation of the eastern flank of the Sierra de Reyes is recognized and correlated through the
Neogene Pampa de Carrizalito foreland basin. Pampa de Carrizalito depocenter represents a clear unroofing sequence.
Total exhumation of this system is recorded in Neogene times which show that older deformations have not produced
significant topography.
the sedimentological and petrographical analysis plus the results of U/Pb dating, 6 different units
(a,b,c,d,e and f in A – A’ profile) could be identified in a series of terms characterized by contrasting
sedimentary sources. In detail basal sections (a+b units) are mainly composed of ignimbrites and
volcaniclastic beds containing basalt clasts coming from the underlying Palauco Formation; then there
are sandstone clasts on top of the previous units (c unit), followed by beds with limestone detritus (d
unit), then gypsum clasts (e unit) and finally shale clasts (f unit) (Fig. 2). The uppermost section
contains clasts of polycrystalline quartz associated with a lithic metamorphic source. This same
compositional variations are also recognized in the eastern profile (B-B’; see Fig. 2), with the only
exception of the lowest volcaniclastic and the gypsum clast horizons. The lower sandstones are derived
from the erosion of the Neuquén Group, while the limestone fragments are related to the erosion of
the Mendoza Group, and the gypsum clasts to the Auquilco Formation. Finally, the shales are related
to the Bardas Blancas Formation. All these rocks are in a reverse order represented in the eastern
Sierra de Reyes flank. This implies that the succession hosted in the Pampa de Carrizalito syncline
constitutes a typical unroofing sequence: At the time when the Sierra de Reyes was uplifted its
exhumation led to the deposition of a sequence characterized by a variable clastic composition.
Finally, metamorphic clasts indicate that the foreland area was uplifted and exhumed at this time,
implying the cannibalization of the foreland basin (Fig. 2).
Since the geochronological dating of detrital zircons is a valuable tool in provenance analysis of
sedimentary basins, we have obtained several U-Pb (LA-ICP-MS) ages from six samples in the late
Miocene Tristeza Formation. Analyzed detrital zircons in unit b yielded prominent U/Pb age peaks for the
early Jurassic (ca. 190-180 Ma) and Permo-Triassic (ca. 270-260 Ma). In addition, minor peaks appear
with Paleozoic ages (Devonian and Carboniferous). Equivalently, the detrital grains from units d to e are
characterized for prominent peaks located in the early Jurassic and Permo-Triassic, but in addition they
have an important group of early Cretaceous ages (ca. 98 Ma). Subordinate peaks appear in the
Carboniferous, Devonian, and Neoproterozoic-Paleozoic. The U-Pb age spectra of the younger unit (f)
showed significant changes in the pattern of detrital zircon ages respect to the previously described units.
In the former, the early Cretaceous zircons are absent, and the Jurassic and Permo-Triassic zircons become
less important, although several populations of zircons of Mesoproterozoic, Neoproterozoic and early
Paleozoic ages begin to dominate, with subordinate Paleoproterozoic ages (ca. 2200 Ma).
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The obtained detrital zircon ages are clearly in line with the results of the microscopic and macroscopic
detrital analyses. The main characteristics can be summarized as follows: An igneous source (volcanic and
plutonic) in the basal half of the sequence that is probably coming from the Jurassic to Cretaceous Andean
arc roots, located to the west in the main Andes. Additionally a volcanic component is associated with the
Permian to Triassic Choiyoi Group. The origin of the Carboniferous and Devonian zircons without a clear
neighbor source remains more controversial. Finally, is worth noting that at the top of the sequence
microscopic evidence of a metamorphic source appears. Equivalently, the ages obtained are consistent
with a source area formed during the Grenville, Pampean, and Famatinian cycles. The San Rafael-Las
Matras block, located about 200 km east of this Neogene synorogenic depocenter is the best candidate as
a source area.
The fact that an entire unroofing sequence is registered in a Neogene depocenter next to the orogenic front
implies that complete exhumation of the Sierra de Reyes started about this time, and therefore that
previous phases are negligible in the study region. We can conclude that the Neogene contractional stage
was the responsible for the exhumation of the orogenic front at these latitudes and created the present
topography. These provenance studies show that the wedge top foreland basin was cannibalized during the
exhumation of the last stages of the sedimentary record when the San Rafael-Las Matras block was
exhumed to the east in the broken foreland final stage.
REFERENCES
• Cobbold, P. and Rossello, E.; 2003: Aptian to Recent compressional deformation in the foothills of the Neuquén basin Argentina. Marine
and Petroleum Geology, 20, 429-443.
• Giambiagi, L., Ghiglione, M., Cristallini, E. and Bottesi, G.; 2009: Caracteristicas estructurales del sector sur de la faja plegada y corrida
de Malargüe (35º - 36º): Distribución del acortamiento e influencia de estructuras previas. Revista de la Asociación Geológica Argentina,
65 (1), 140-153.
• Kozlowski, E., Cruz, C. and Rebay, G.; 1987: El Terciario volcaniclástico de la zona Puntilla del Huincan. Mendoza. X Congreso
Geológico Argentino, Actas 4, 229-232. Tucumán.
• Kozlowski, E., Manceda, R. and Ramos, V.A.; 1993: Estructura. In: Ramos V.A. (ed), Geología y Recursos Naturales de Mendoza. XII
Congreso Geológico Argentino and II Congreso de Exploración de Hidrocarburos. Relatorio 1, 18, 235-256.
• Orts, S. and Ramos, V.A.; 2006: Evidence of middle to late Cretaceous compressive deformation in the high Andes of Mendoza,
Argentina. Backbone of the Americas, abstract with Programs 5, 65, Mendoza.
• Silvestro, J. and Atencio, M.; 2009: La cuenca Cenozoica del Río Grande y Palauco: edad, evolución y control estructural. Faja plegada
de Malargüe (36°S). Revista de la Asociación Geológica Argentina. 65 (1), 154-169.
• Tunik M., Folguera A., Naipauer M., Pimente M. and Ramos V.A.; 2010: Early uplift and orogenic deformation in the neuquén basin:
constraints on the Andean uplift from U – Pb and Hf isotopic data of detrital zircons. Tectonophysics, 489, 258-273.
• Zamora Valcarce G. and Zapata T.R.; 2005: Estilo estructural del frente de la faja plegada neuquina a los 37º S. VI Congreso de
exploración y desarrollo de Hidrocarburos. Electronic files. Mar del Plata.
FURTHER EVIDENCE OF LOWER PERMIAN REMAGNETIZATION
IN THE NORTH PATAGONIAN MASSIF, ARGENTINA
4-22
Tomezzoli, R.1*, Rapalini, A.E.1, Lopez de Luchi, M.G.2
(1) Instituto de Geofísica “Daniel A. Valencio (INGEODAV)”, Departamento de Ciencias Geológicas,
Facultad de Ciencias Exactas y Naturales, Universidad de Buenos – CONICET
(2) Instituto Nacional de Geocronología y Geología Isotópica (INGEIS), CONICET-Universidad de
Buenos Aires
* Presenting author’s e-mail: renata@gl.fcen.uba.ar
The origin of Patagonia has long called the attention of South American earth scientists. In recent
years a dispute over whether it is an accreted crustal block that collided with Gondwana in Late
Paleozoic times or an autochthonous part of South America has taken place. Scarce paleomagnetic
data mostly younger than Devonian, preclude a definite paleomagnetic test on the origin of this
terrane. The presence of well-dated and undeformed Ordovician granitoids discordantly covered by
the Silurian-Devonian Sierra Grande Fm in the northeastern corner of the North Patagonian Massif
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(NPM) are a suitable target to undertake such test. The plutonic rocks belong to the Punta Sierra and
related granitoids which have recently yielded U-Pb crystallization ages between 472-476 Ma
(Pankhurst et al., 2006, Varela et al., 2009 and references therein). A paleomagnetic study of these
intrusives (41.5°S, 65.0°W) was carried out. As part of a multidisciplinary study, anisotropy of
magnetic susceptibility (AMS) measurements, systematic analyses of petrographic thin sections and
rock magnetic analyses, have also been performed. About one hundred specimens were processed,
comprising ten sites on the granite and two sites on the quartzites and sandstones of the Early
Devonian Sierra Grande Formation, with five cores each. Demagnetization at high temperatures
isolated a reverse characteristic remanent magnetization, suggesting of being acquired during the
Kiaman reverse superchron. Structural correction of paleomagnetic data from plutons was available
only at a few sites from bedding attitudes of the Devonian or Tertiary sedimentary rocks. Seven out
of twelve sites, all in the granites, provided consistent remanence directions. Structural correction
could be applied at a single site worsening the statistical parameters of the mean site direction and
suggesting a secondary magnetization. A paleomagnetic pole (PP) was computed from the mean of
the seven site directions. The position of this PP on the apparent polar wander path of South America
is at: 11.5°E, 65.0°S; A95=12°, K=24.5 suggesting that magnetization was acquired during the Early
Permian, being this pole consistent with previous poles of that age from South America (Tomezzoli,
2009). A remagnetization during the late Early Permian has been already reported on some outcrops
of the Devonian Sierra Grande Fm. in the same area (Rapalini and Vilas, 1991). Our data suggests that
the remagnetization was pervasive and affected the Ordovician granitoids as well. Whether this
remagnetization is due to the widespread Permian magmatism that affected the NPM or to the
deformational phase ascribed in some models to the collision of Patagonia against the Gondwana
margin is to be determined. However, it is significant that several South American Lower Permian
paleomagnetic poles have been computed from syntectonic magnetizations (e.g. Ponón Trehue; Tunas
I PP; Cochico PP; Río Curacó PP; and Sierra Chica PP, among others). All these PPs come from
localities along a 500 km long WNW-ESE orogenic belt that extends from the San Rafael block in the
province of Mendoza to the Ventana System in the province of Buenos Aires. Deformation along it
has been assigned to the San Rafaelic orogenic phase and dated at approximately 290 Ma. This phase
has been recognized mainly in the western areas of Argentina and has been linked to remagnetization
of a regional scale (Rapalini and Astini, 2005). Time coincidence of remagnetizations suggests that
causal links for all of them including those in the NPM are likely. Regional remagnetization associated
to this major orogenic phase and its geotectonic framework should be explored.
REFERENCES
• Pankhurst, R.J., Rapela, C.W., Fanning, C.M. and Márquez, M. 2006. Gondwanide continental collision and the origin of Patagonia.
Earth-Science Reviews 76(3-4): 235-257.
• Rapalini, A.E. and Vilas, J.F., 1991. Preliminary paleomagnetic data from the Sierra Grande Formation: Tectonic consequences of
the first mid-Paleozoic paleopoles from Patagonia. J. South Am. Earth Sci. 4(1-2): 25-41.
• Rapalini, A.E. and Astini, R.A.. La remagnetización Sanrafaélica de la Precordillera en el Pérmico: Nuevas evidencias. Revista de
la Asociación geológica Argentina: 60(2): 290-300.
• Tomezzoli, R.N., 2009. The Apparent Polar Wander Path for South America During the Permian-Triassic. Gondwana Research, 15:
209 – 215.
• Varela, R., Sato, K., González, G.P., Sato, A.M., and Basei, M.A.S, 2009. Revista de la Asociación geológica Argentina: 64(2): 275284.
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TECTONIC CONTROL ON THE EVOLUTION OF
MAASTRICHTIAN-PALEOGENE SYNOROGENIC SEQUENCES
OF THE FUEGIAN THRUST FOLD BELT, ARGENTINA
4-23
Torres Carbonell, P.J.1*, Olivero, E.B.1, Dimieri, L.V.2
(1) Centro Austral de Investigaciones Científicas (CADIC-CONICET). B. A. Houssay 200, 9410
Ushuaia, Tierra del Fuego, Argentina
(2) Instituto Geológico del Sur (INGEOSUR-CONICET), Departamento de Geología, Universidad
Nacional del Sur. San Juan 670, 8000 Bahía Blanca, Buenos Aires, Argentina
* Presenting author’s e-mail: polmacleod@hotmail.com
A geological study of the Atlantic coast of Tierra del Fuego between the punta Gruesa (54º 21’ S; 66°
38.5’ W) and the río Policarpo (54º 39’ S; 65º 30’ W) (Fig. 1), and other sectors of the Fuegian Andes,
Fig. 1 - Geologic map of the study area. Trace of the composite cross-section of Fig. 2 is shown.
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Fig. 2 - Kinematic evolution of the studied portion of the Fuegian thrust fold belt, with the contractional stages depicted.
PT: Policarpo thrust, CLT: Cabo Leticia thrust, PT: Punta Torcida, PAT: Punta Ancla thrust, CDMB: Campo del Medio
backthrust, MB: Malengüena backthrust, PGIS: Punta Gruesa imbricate system, LCT: La Chaira thrust, FTS: Fagnano
transform system. Stratigraphic units: 1, Upper Jurassic; 2, Lower Cretaceous; 3, Upper Cretaceous-Danian; 4, Paleocene;
5, Ypresian; 6, Paleocene-Ypresian; 7, Lutetian; 8, upper Lutetian-Priabonian; 9, Oligocene; 10, uppermost OligoceneMiocene; 11, upper Miocene-?Pliocene. The segments of this cross-section are located in Fig. 1.
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allowed us to define the stratigraphy and sedimentology of synorogenic successions from the Austral
basin, and their genetic relations with the geometry and kinematics of the Fuegian thrust fold belt.
We define seven sequences between the Maastrichtian and the Miocene, bounded by syntectonic
unconformities: Maastrichtian-Danian (180 to 800 m), Paleocene (50 to 370 m), Ypresian (450 to 650
m), Lutetian (80 m), upper Lutetian-Priabonian (1200 m), Oligocene (1600 to 200 m) and uppermost
Oligocene-Miocene (200 m). These successions are composed of marine sedimentites, mostly
deposited by gravity flows below the storm-wave base. The paleocurrent directions and the
petrography indicate sediment provenance areas in the volcanic arc along the Pacific margin of the
Andes, and in the core of the Fuegian Andes, the former dominant between the Maastrichtian and the
Lutetian, and the latter since the late Lutetian.
The detailed mapping of the Fuegian thrust fold belt structures allowed us to construct two balanced
cross-sections that depict their subsurface geometries. The southern cross-section shows main
décollements at the base of the Cretaceous and above the Maastrichtian-Danian, and thrust-related
folding of the Cretaceous-Miocene sedimentary cover. The total shortening in that cross-section is of
41.8 km. The northern cross-section has a main décollement at the base of the Paleocene-Ypresian
rocks, and minor ones in Bartonian-Priabonian levels. This section shows thrust-related folding of the
Paleocene-Miocene sedimentary cover, with a total shortening of 17.8 km.
By combining both cross-sections, estimating their location before the development of the Neogene
Fagnano transform system (Fig. 2), six contractional stages are defined in relation to the evolution of
the thrust-fold belt, age-constrained by the biostratigraphy of the synorogenic successions recognized:
Df1 (Danian) with low percentages of layer parallel shortening in the foreland; Df2 (Ypresian) with
development of thrust-related folding in a forward thrust-sequence and shortenings between 7 and
18.8 km (21%); Df3 (Lutetian) with development of out-of-sequence structures and a shortening of
6.6 km (7.3%); Df4 and Df5 (Oligocene) related to backthrusting with a shortening of 13.6 km
(15.2%); Df6LC and Df6PG (latest Oligocene-Miocene), the last contractional stages recorded in the
eastern Fuegian thrust-fold belt, comprising thrust-related folding within the belt with a shortening of
2.8 km (3.1%), and in the leading edge of deformation with a shortening of 10.5 km (11.6%).
The thrust-fold belt reveals an episodic evolution that can be analyzed in terms of the Coulomb wedge
theory, obtaining a model with three main stages: a critical wedge during the Danian to Ypresian
(stages Df1 and Df2) with forward directed thrusting and a progressively diminishing taper angle, a
subcritical wedge between the Lutetian and the ‘mid’ Oligocene (Df3 a Df5) with development of outof-sequence structures and backthrusts and a tendence to attain a critical taper, and a critical wedge
during the latest Oligocene to early Miocene (Df6PG) with foreland displacement of the thrust wedge
and propagation of the basal décollement to shallower levels.
The Austral foreland basin system evolved as a single depocenter (foredeep) during deposition of the
Maastrichtian-Danian, Paleocene and Ypresian sequences, the latter posibly also accumulated in
depocenters atop active structures (wedge-top). Between the Lutetian and the Miocene, sedimentation
occured in two depocentres: the wedge-top and the foredeep. During the Oligocene this segmentation
of the basin resulted in a thicker succession within the wedge-top, which distinguishes it from classic
tectonostratigraphic models. During the Miocene, cessation of contractional deformation in the thrustfold belt was simultaneous with the development of the last foredeep of the basin.
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STRATIGRAPHY
AND SEDIMENTOLOGY
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PRESERVATION OF TOTAL ORGANIC CARBON AND EVALUATION
OF CORG/NTOT ATOMIC RATIO IN A SEDIMENT OUTCROP LOCATED S-E
OF THE LAGO FAGNANO (TIERRA DEL FUEGO, ARGENTINA)
5-01
Caffau, M.1*, Comici, C.2, Zecchin, M.1, Presti, M.1, Lodolo, E.1, Tassone, A.3,
Lippai, H.3, Menichetti, M.4
(1) Istituto Nazionale di Oceanografia e Geofisica Sperimentale (OGS) – Trieste, Italy
(2) Dipartimento di Biologia e Oceanografia (OGS) – Trieste, Italy
(3) Dept. de Geofisica, Universidad de Buenos Aires, Argentina
(4) Dipartimento di Scienze Geologiche - Università di Urbino, Italy
* Presenting author’s e-mail: mcaffau@ogs.trieste.it
This study evaluates the content of the Total Organic Carbon (TOC), the Total Nitrogen (TN) and the
Corg/Ntot atomic ratio in the sediments outcropping at a cliff in the south-eastern corner of the Lago
Fig. 1 - Map of Tierra del Fuego,
showing the location of the
studied sequence.
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Fig. 2 - Sand-silt-clay distribution in the sequence. (a) Detail of wavy and flaser bedding commonly found in the upper
part of the sequence. (b) Detail of planar lamination that is dominant in the lower part of the sequence.
Fagnano. It consists of muddy to sandy deposits arranged to form a succession about 8 m thick.
The studied deposits are part of a larger depositional system referred here as the Lago Fagnano
Gilbert-type delta (Fig. 1), which has been interpreted as a proglacial delta (Bujalesky et al., 1997);
this sedimentary body is interbedded between levels of basal till (Bujalesky et al., 1997). The latter is
composed of heterogeneous conglomerate-size clasts immersed in a muddy to sandy matrix and
containing vegetable remnants (Bujalesky et al., 1997; Coronato et al., 2009).
The Lago Fagnano delta has prograded on the eastern margin of the lake, and is composed of finegrained bottomsets, gravelly foresets, and fine to coarse-grained topsets. Delta foresets are inclined
up to 30°, near the angle of repose, and rapidly wedge-out toward the north-east from about 20 to 0
m of thickness. They rest abruptly on the bottomset strata. The delta topset, consisting of peaty
lacustrine and fluvio-glacial deposits (Bujalesky et al., 1997), is overlain by younger till deposits.
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The examined sequence consists of fine- to medium-grained sands that are interlaminated with light
silt and clay, forming planar to ondulate lamination, wavy and flaser bedding. In detail, planar
lamination is dominant in the lower part (Figs. 2, 2b), whereas wavy to flaser bedding is common in
the upper part (Figs. 2, 2a). This feature allows to subdivide the bottomset sediments into an upper,
mostly rippled unit (Unit 1, from cm 325 to the core top), a middle, mostly planar laminated unit (Unit
2, from cm 532 to cm 325 depth) (Fig. 2) and a lower, muddy and planar thin laminated unit (Unit 3,
from cm 532 to the core bottom).
Sand and mud laminae of Unit 3 are 1 to 3.5 mm thick. Sand contents average 28,4%, ranging from
56,9% (sample at cm 688) to 9,7% (cm 560). Silt contents are inversely correlated to sand. Clay
contents are low, varying from 1.8 % (cm 580) to 0.4% (cm 684).
Sand and mud laminae of Unit 2 are 1,5 to 6 mm thick (Fig. 2b). Sand contents average 41.9%,
ranging from 70.9% (sample at cm 461) to 18.1% (cm 401). Silt contents are inversely correlated to
sand. Clay contents are very low, varying from 1.3 % (cm 483) to 0.1% (cm 524).
Ripples of Unit 1 are 0.5 to 2 cm thick, are typically symmetrical to weakly asymmetrical with
rounded crests, and locally show a triangular shape and sharp crests (Fig. 2a). Foreset laminae are
poorly visible yet they may be evidenced by mud drapes, showing local sigmoidal forms and variable
accretion patterns. Also ripple profiles are highlighted by mud drapes, which may be overlain by
interlaminated sand and mud forming climbing sets (Fig. 2a). Rare coarser-grained lenses 1 cm thick
are present in the upper part of the bottomset deposits. In Unit 1, Sand contents average 49.7%,
ranging from 87.5% (sample at cm 141) to 8.1% (cm 284). Silt content is inversely correlated to sand.
Clay contents are very low also within this unit, ranging from 1.3 % (cm 202) to almost 0 (cm 180).
Samples displaying sand contents above 75% (highlighted in Fig. 2) are those corresponding to the
lower and coarser parts of the ripples.
Preliminary analyses highlight that the sediments contain very poor Total Organic Carbon (TOC) and
Total Nitrogen (TN); similar contents of TOC were found by Harvard et al. (1999) in the southern
Taymir Peninsula (Central Siberia) and Waldmann et al. (2009) from Lago Fagnano bottom sediments.
The TOC content shows initial very low values with an increase in the central part and a subsequent
decrease at the top. The TN content shows constant low values throughout the sequence.
These low values are probably due to the coarse grain size of sediments, the age of the glaciolacustrine
sediments, and to processes that impact the organic matter in the relatively short time between its
synthesis and burial, such as the degradation during sinking, the bioturbation of bottom sediments that
causes oxidation of the organic matter and the alteration by anaerobic bacteria (Meyers and Ishiwatri,
1993; Meyers, 1994; Waldmann et al., 2009).
In agreement with the sedimentological description, the TOC and TN values identify three units. In
Unit 1 the content of TOC ranges from 0.13% dwt (dry weight) and 0.04% dwt with an average value
of 0.07% dwt. The average TOC in Unit 2 is 0.21% dwt an it ranges from 0.31% dwt to 0.10% dwt.
Unit 3 is characterized by an average TOC of 0.07% dwt and ranges from 0.08 to 0.05 % dwt. The
Corg/Ntot atomic ratio provides information on the origin of the organic matter of the lake sediments
(Meyers, 1994; Meyers and Lallier-Vergès, 1999). The Corg/Ntot atomic ratios in the studied
sequence show generally low values with a little increase in the central part. In Unit 1 the average
Corg/Ntot atomic ratio is 3.03 and it ranges from 16.16 to 4.19, in Unit 2 the average Corg/Ntot
atomic ratio is 8.93 and the maximum and minimum values are 16.16 and 4.19 respectively; in Unit
3 the average Corg/Ntot atomic ratio is 2.87 and it varies from 4.37 to 2.06. Probably these data
indicate an autochtonous origin of the organic matter in the sediments.
Process interpretation of bottomset deposits and TOC and Corg/Ntot contents
The sand-mud interlamination of bottomset sediments testifies phases of active hydrodinamics and
sediment availability that alternate with quieter periods characterized by the settling of mud by
suspension. The abundance of symmetrical ripple profiles in the upper part of the bottomset (Unit 1),
the presence of sharp and rounded crests, sigmoidal foresets and of local migration suggest the action
of waves combined with currents producing traction on the bed (Yokokawa et al., 1995; Myrow et al.,
2002). These bedforms, therefore, can be referred as wave and combined-flow ripples. Climbing
patterns are referred to high rates of sediment supply. The observed alternation between high- and
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low-energy phases may reflect seasonal flood events, probably associated to ice melting, that were
also responsible for the avalancing of the large-scale delta foresets. Floods triggered density flows
along the delta foresets transporting sandy sediment in the bottomset area. The low average values of
TOC (0.07%) and organic matter (3.03) are in accordance with this depositional context, mainly
characterized by transport of sediments due to active hydrodinamics, a high sedimentation rate and a
high average sand value (49.7%).
The vertical sequence from the planar laminated Unit 3 to the rippled Unit 1 suggests a shallowingupward trend, with the superposition of more energetic facies including traction structures on more
distal and deeper facies accumulated below wave base. The described facies show some resemblance
with those illustrated by Myrow et al. (2008) in prodeltaic systems. In Unit 2, the TOC average value
is 0.21% and the organic matter average value is 8.93. These values testify a quieter period with
settling of mud by suspension. The low values of TOC (0.07%) and organic matter (2.87) found in
Unit 3 are probably correlated with an alteration of the organic matter content in a post diagenetic
phase, as testified by the abundant oxidation found in the sand levels that are present in this unit.
REFERENCES
• Bujalesky G., Heusser C., Coronato A., Roig C., Rabassa J. (1997): Pleistocene glaciolacustrine sedimentation at Lago Fagnano,
Andes of Tierra del Fuego, Southernmost South America. Quaternary Science Review 16, 767-778
• VCoronato A., Seppälä M., Ponce J.F., Rabassa J. (2009): Glacial geomorphology of the Pleistocene Lake Fagnano ice lobe, Tierra
del Fuego, southern South America. Geomorphology 112, 67-81.
• Harvart S., Hagedorn B., Melles M., Wand U. (1999): Lithological and biochemical properties in seiments of Laa Lake as indicators
for the Late Pleistocene and Holocene ecosystem development of the southern Taymir Peninsula, Central Siberia. Boreas 28, 167180.
• Meyers P.A., Ishiwatari R. (1993): Lacustrine organic geochemistry-an overview o indicators of organic matter sources and
diagenesis in lake sediments. Organic Geochemistry 20/7, 867-900.
• Meyers P.A. (1994): Preservation of elemental and isotopic source identification of sedimentary organc matter. Chemical Geology
114, 289-302.
• Meyers P.A., Lallier-Vergès E. (1999): Lacustrine sedimentary organic matter records of Late Quaternary paleoclimates. Journal of
Paleolimnology 21, 345-372.
• Waldmann N., Aritzegui D., Anselmetti F.S., Austin Jr. J. A., Moy C. M., Stern C., Recasens C., Dunbar R.B. (2009): Holocene
climatic fluctuations and positioning of the Southern Hemisphere westerlies in Tierra del Fuego (54° S), Patagonia. Journal of
Quaternary Science. DOI: 10.1002/jqs.1263.
• Myrow P.M., Fischer W., Goodge J.W. (2002): Wave-modifiedturbidites: Combined-flow shoreline and shelf deposits, Cambrian,
Antarctica. Journal of Sedimentary Research 72, 641-656.
• Myrow P.M., Lukens C., Lamb M.P., Houck K., Strauss, J. (2008): Dynamics of a transgressive prodeltaic system: Implications for
geography and climate within a Pennsylvanian intracratonic basin, Colorado, U.S.A. Journal of Sedimentary Research 78, 512-528.
• Yokokawa M., Masuda F., Endo N. (1995): Sand particle movement on migrating combined-flow ripples. Journal of Sedimentary
Research A65, 40-44.
STRATIGRAPHIC AND STRUCTURAL REVIEW OF CAÑADÓN ASFALTO BASIN,
CHUBUT PROVINCE, ARGENTINA
5-02
Figari, E.1*, Ramos, V.A.2
(1) Repsol Exploracion S.A., Dirección General Exploración Upstream, Madrid, Spain
(2) Laboratorio de Tectónica Andina, Universidad de Buenos Aires, Argentina
* Presenting author’s e-mail: efigari@repsol.com
The Cañadón Asfalto Formation is a succession of volcanic, biochemical, pyroclastic and epiclastic
deposits which are exposed in the central and northern regions of the province of Chubut close to the
limit with the Río Negro Province. These levels were originally described only in the Middle Valley
of the Chubut River and for several years the main uncertainty was the age, extension, and genesis of
the related basin, because classically the area was known as the North Patagonian Massif (Fig. 1).
Detailed field studies suggest that under the denomination of the Cañadón Asfalto Formation (sensu
lato) different litostratigraphic units have been grouped or misidentified, such as Cañadón Calcáreo
Formation, the Almada Beds, and inclusively the Chubut Group generating an extended confusion and
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Fig. 1 - Geologic Map of Cañadón Asfalto Basin in the area of the Middle Valley of Chubut River.
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Fig. 2 - Stratigrahic column of Cañadón Asfalto Basin (based on outcrop information).
Fig. 3 - Cross section showing the dramatic lateral thickness and facies changes of Sierra de La Manea Fm. and the
unconformable relationship over C. Asfalto Fm.
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severe problems in the stratigraphic correlation and tectonic interpretation over the whole region (Fig.
2).
This reconsideration allows recognizing, essentially with outcrop data, a north-western trending rift
system that occurred between the Middle Jurassic up to the Early Cretaceous, formed by a series of
halfgrabens and linked among them by accommodation zones. In the North-eastern part these
outcrops disappear below the Somuncura Volcanic Plateau.
We suggest the use of a more dynamic nomenclature such as Megasequences in order to describe these
sedimentary units related with rifting evolution. However, from the litostratigraphic point of view and
mapping we propose to keep the name Cañadón Asfalto Fm. exclusively for the older unit (OxfordianCallovian), and to introduce the name Sierra de la Manea Fm. for the younger (Titho-Neocomian?)
and more extended unit that covers the previous one with a regional unconformity (Fig. 3).
The main orientation of the extensional stress field that created the available space for generating this
basin could be related to the opening of the Weddell Sea during Middle Jurassic times and with the
Gondwana break-up in the Early Cretaceous.
Compressive east-west stresses (related with the Andean Orogeny in an internal intraplate setting)
were responsible for the partial tectonic inversion of the basin, mainly along its westernmost margin
(Agnia and Lonco Trapial Hills).
THE CONTROVERSY ABOUT MIOCENE MARINE SEDIMENTATION ALONG
THE FORELAND OF ANDES, SOUTH AMERICA: THE CASE
OF SANTA MARIA GROUP, ARGENTINA
5-03
Gavriloff, I.J.C.1*, Arce, M.N.1
(1) Facultad de Ciencias Naturales e Instituto Miguel Lillo, Universidad Nacional de Tucumán,
Miguel Lillo 205, 4000 San Miguel de Tucumán, Tucumán, Argentina
* Presenting author’s e-mail: igor@csnat.unt.edu.ar
The middle/upper Miocene was characterized by large changes in the geodynamic processes of the
planet. Some of these changes were: the accelerated uplift of large mountain ranges in almost all
continents, the drastic changes in global sea level and shifts in oceanic circulation, and the transition
from an optimum climatic during the middle Miocene to the development and growth of the ice sheet
in the East and West Antarctica during the upper Miocene.
In this global scenario, the uplift of the Andes in South America formed several foreland basins that
have been interpreted as marine during the Miocene, but with considerable controversy. For example,
in the Amazonia Basin, the Pebas, Solimôes and others correlated formations are assigned to either a
tidal environment by some authors as a fluvial environment by others. Recently, in the Chaco-Parana
Basin, the Ituzaingo Formation (upper Miocene-Pliocene), historically considered fluvial, has been
assigned to tidal environment. This formation is located immediatly above the marine Parana
Formation.
The “Paranaense Sea” (middle/upper Miocene) in Argentina, shows a clearly marine gradient from the
East to the West into the Chaco-Parana basin. To the West, near the Andes, several formations present
paranaense’s foraminifera together with groups of brackish or fresh-water macro and microfauna. In
this region, the Santa Maria Group (Sierras Pampeanas, Tucuman and Catamarca provinces), includes
two formations with thick lacustrine sequences, the San Jose Formation (middle Miocene) and the
Chiquimil Formation (upper Miocene). Since the foraminifera discovery in this group in the ’80
decade, the sedimentary rocks that hold them were in general assigned to the San Jose Formation,
even though without a precise stratigraphic control. In this first stage of our research, the goals are:
the sistematic identification of the foraminifera fauna, the stratigraphic identification into
lithostratigraphic units of the layers containing them and the proposition of hypotheses explaining the
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presence of foraminifera in lacustrine facies. Metodologically we have made facial and sistematic
field sampling of layers assigned as fertile in foraminifera; samples supplied by YPF petroleum
company have also been studied. From a total of 35 samples, 14 were positive containing foraminifera.
Four samples correspond to Chiquimil Formation with the following species: Ammonia parkinsoniana
(d’Orbigny) and Lippsina demens (Bik), forma santamariana Zabert. In this formation is noteworthy
the joint presence of Characids with Ammonia parkinsoniana (d’Orbigny) in the same stratigraphic
level. Ten samples of San Jose Formation gave the following species: Ammonia parkinsoniana
(d’Orbigny) and Nonion sp. We conclude that both lacustrine formations of Santa Maria Group are
carrier of a foraminiferal microfauna. The direct relationship of the lakes with the Paranense Sea,or
the possible sowing of foraminifera in the coastal lakes due to the migration of birds, are the two
hypothesesthat must be considered to explain the presence of foraminifera in the Santa Maria Group.
GEOLOGY OF THE LAGO FAGNANO AREA (FUEGIAN ANDES,
TIERRA DEL FUEGO ISLAND)
5-04
Menichetti, M.1*, Tassone, A.2, Lippai, H.2, Lodolo, E.3
(1) Dipartimento di Scienze Geologiche, Tecnologie Chimiche Ambientali - Università di Urbino, Italy
* Presenting author’s e-mail: menichetti@uniurb.it
Lago Fagnano is the southernmost ice-free lake on Earth. It is elongated in an E-W direction for 105
km and has an average width of 8 km and a maximum depth of 206 m. Its basin covers an area of
approximately 1900 km2 and has an irregular shape, especially in its eastern part, where is the
dividing line between the Atlantic and Pacific/Magallanes Strait. The morphostructural evolutionary
history of the region is complex, particularly in the Quaternary, which has strongly influenced the
regional water divide and drainage of the main tributaries that run along the N-S and E-W directions.
To the east, the border river runs close to the shore of the lake, while in other areas it is difficult to
identify because of flat topography with large ponds and bogs.
Along the southern and western lake shores, Upper Jurassic volcanoclastic metasediment of the Rocas
Verdes marginal basin with basalts of the Lemaire Fm. outcrop. Rhyolites of the M.te Buckland Fm.
in the Sierra de Alvear are present. Andesitic volcanoclastic turbidites of the Lower Cretaceous
Yahgán Fm. outcrop in the east side of the Lago Fagnano in the Sierra Lucas Bridge. Sediments
pertaining to the Cenozoic Magallanes foreland basin outcrop in the north side of the basin. In the
Sierra Inju Gooiyin (Sierra de Beauvoir), metasediments of dark slates, marls and tuffs of the
Beauvoir Fm. are present, while to the east, in the Sierra de las Pinturas, outcrops of marls, sandstones
and siltstone of Upper Cretaceous of the Cerro Matrero Fm. can be seen. Two Upper Cretaceous
intrusive plutons are located south of Tolhuin (Cerro Jeujepen), and in the southern slope of Sierra de
Inju Gooiyin (Cerro Krank). Glacio-fluvial and Quaternary sediments with moraine, glacio-fluvial
and glacio-lacustrine facies cover all the south-eastern sectors of the Lago Fagnano area. Tens of
meters-thick glacial moraines, recoding different ice advances linked to the Cordillera Darwin ice cap
that flowed eastward, are exposed along the river incisions and in the south-eastern shore of the lake.
The sedimentary cover of Lago Fagnano, surveyed by geophysical measurements and core samplings,
is composed of at least two glacial and glacio-lacustrine stratigraphic units, resting on a deformed
basement.
Lago Fagnano is located in the frontal part of the main stack of the Fuegian Cordillera basement sole
thrust front, at the boundary with the Magallanes foreland. A system of low angle, NNE-verging
thrusts constitute the main compressional structures related to the Upper Cretaceous Andean
collisional tectonic phase. A thick-skinned tectonic geometry with arrangements of imbricate fan
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thrust systems characterize the internal zones of the Sierra del Alvear and Sierra Valdivieso, where
basement sole thrusts can be observed. Thin-skinned tectonics with thrust duplex geometries prevail
in the foreland north of the lake. Several Oligocene E-W left-lateral strike-slip faults that define the
principal deformation zone of the Magallanes-Fagnano transform faults, superimpose and possibly
reactivate older compressive structures. The most prominent structure of the area is the Co. HopeCatamarca-Knokeke fault, running E-W through the lake area over a distance of more than 100 km.
Several outcrops allow the characterization of the geometries and the kinematics of this left-lateral,
transtensional, sub-vertical, south-dipping structure. Furthermore, the fault trace from the eastern
arms of the Magallanes Strait to the Atlantic offshore shows strong morphostructural evidence. Two
other faults, along both the north and the south lake edges, the Rio Turbio-Las Pinturas and the S.
Rafael, respectively, form a releasing step-over within the principal deformation zone. The kinematics
of these sub-vertical, E-W faults is mainly transcurrent with an important extensional component. The
fault arrays form complex structures that could derive from deformational partitioning involving the
distribution of strain orientation or intensity in various domains. Reactivations of oldest faults and preexisting structural weaknesses are common especially where the thrusts and wrench faults strikes
intersect at an angle of a few tenths of degrees.
The geodynamic evolution of the Lago Fagnano pull-apart basin can be related to the westward
migration of several step-overs along the Co. Hope-Catamarca-Knokeke strike-slip fault. The age of
the fault activity can be inferred by regional geological data since the Lower Miocene, while the
geomorphological evidence and the seismicity of the area indicate a still active tectonic activity. A set
of extensional faults NW-SE oriented, intercept and in same case cut the E-W strike-slip faults.
THE AGE OF DINOSAURS IN SOUTH AMERICA
5-05
Novas F.*
Museo Argentino de Ciencias Naturales, Buenos Aires, Argentina
* Presenting author’s e-mail: fernovas@yahoo.com.ar
Since dinosaurs were first documented in South America in 1890, the knowledge of their evolutionary
history remained fragmentary and restricted to a handful of poorly understood species. However, work
carried on since the end of the 1970´s in Mesozoic outcrops in Argentina, resulted in a remarkable
string of fossil findings, revealing that a rich and complex evolutionary history took place in this
southern continent.
The fossil evidence amassed in this continent (consisting in skeletons, footprints, eggs, nests and skin
impressions belonging to the main evolutionary streams of Saurischia and Ornithischia) sheds lights
on a variety of interesting paleobiological aspects, such as the rise of dinosaurs, the effects of
continental break-up on their evolutionary history, the emergence of birds and their flight, the
development of the largest land vertebrates, etc.
A brief review of the available information will be offered in this talk.
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SEDIMENTATION ENVIRONMENTS AND FACIES DEVELOPED
5-06
DURING THE MIOCENE TO EARLY PLIOCENE IN THE EASTERN BASIN OF FALCON,
WESTERN VENEZUELA
Romero, B.F1*, Bastos, P.1, Strakos, K.2, Baquero, M.1
(1) PDVSA EXPLORATION, PUERTO LA CRUZ, VENEZUELA
(2) BEICIP FRANLAB, PUERTO LA CRUZ, VENEZUELA
* Presenting author’s e-mail: romerofao@pdvsa.com;freddiromba@hotmail.com
A combined fieldwork and well data and biostratigraphic analysis on Cenozoic rocks from the eastern
Falcón Basin, Western Venezuela was conducted. The results are used to characterize the different
sediment types and to investigate the depositional processes and facies in the basin from Lower
Miocene to Lower Pliocene. These data allowed us to analyze the distribution of integrated paleofacies
in the basin. The deposits are organized into four sequences (SM1, SM2, SM3 and SM4) defined by
lithofacies and bounded by unconformities. Several maps were drawn up for each of the identified
sequences.
The deposition of the Sequence SM1 ranges from the Upper Aquitanian to Lower Burdigalian, it
begins with neritic deposits, with the calcareous facies of the Member Cauderalito (Agua Clara
Formation) sandy facies of the Pedregoso Formation, and conglomeratic facies of the Guarabal
Formation, in the west of the area. In the southeast part of the basin, the SM1 sequence is represented
by the sandy-clay facies of the San Lorenzo and Agua Linda formations. The clay facies of the Agua
Clara and San Lorenzo formations are bathyal.
The deposition of the sequence SM2 starts in the Burdigalian, in the south of the Cumarebo structure,
with sandy-clay facies of the Cerro Pelado Formation in fluvial environments, with marine influence.
During the Langhian, in the northern areas the sequence is represented by a succession of inland and
coastal conglomerates, sandstones, delta and lagoons clays and platform limestones of the Cantaure
Formation. The maximum of the transgression is represented by the bathyal clays of the Querales
Formation.
The Sequence SM3, preceded by a period of erosion, as shown by the absence of late Langhian in the
zone north, began with the Socorro Formation (Late Langhian-Serravallian) which contains two
facies; the lower clay facies corresponding to a transgressive event and the upper clay-sandy facies,
the retrograde part of the SM3. The depositional environment was middle-outer neritic in the structure
south of the La Vela Tierra. In the offshore, from north and east of the La Vela Tierra facies indicate
bathyal environments, as evidenced by the decrease in the sandy layers. Toward the southeast the
facies gradationally become more argillaceous, indicating the deposition in a bathyal environment
(Pozón Formation), and the Capadare Formation of internal neritic setting, gradually pass to coastal
deposits of the Agua Linda Formation The abrupt contact between the bathyal facies (Pozón
Formation) and inner neritic to continental facies (Upper Agua Linda Formation) is seen in this
southeast part.
The boundary of the Middle-Late Miocene (SM4) corresponds to a tectonic event that is manifested
by erosion at the top of Socorro Formation (incised valleys). The trend is regressive during this period.
Bathyal sediments are located in the southeast, at the base of the sequence with the Pozón Fomation,
grading into coastal to shallow marine facies of the Ojo de Agua Formation. In the north, the Caujarao
Formation is middle neritic with more marine influence towards the top. In the La Vela Tierra east nine
rhythmic sequences of limestone, sand and clay are recognized in outcrop representing the lower part
of the formation, whereas clay facies dominate the top of the formation coming to settle in at lower
Pliocene.
Sedimentation is slightly influenced by eustatic cycles in the area of study and according to the
conception of this study, the limits of the sequences are regional unconformities of tectonic origin:
extensional during the base of the Aquitanian and compressive since the Langhian to the Pleistocene.
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AN EXAMPLE OF COMPLEX FLUVIO-AEOLIAN SEDIMENTATION:
THE UPPER MEMBER OF THE MIOCENE-PLIOCENE RÍO NEGRO FORMATION,
NORTHERN PATAGONIA, ARGENTINA
5-07
Umazano, A.M.1*, Visconti, G.2, Pérez, M.2
(1) INCITAP (CONICET-UNLPam); Av. Uruguay 151, 6300 Santa Rosa, La Pampa, Argentina
(2) Fac. de Cs. Exactas y Naturales (UNLPam); Av. Uruguay 151, 6300 Santa Rosa, La Pampa,
Argentina
* Presenting author’s e-mail: amumazano@exactas.unlpam.edu.ar
The Miocene-Pliocene Río Negro Formation is a sandstone-dominated unit that outcrops in several parts of
northern Patagonia from the Andean foothill to Atlantic coast. The formation includes three members
denominated lower (aeolian), middle (marine) and upper (fluvio-aeolian). Two basal members crops out in
coastal cliffs between 41° 09’- 41° 07’latitude, whereas the upper member is mainly exposed in both margins
of the Negro river valley. Goal of this contribution is to analyze the complex fluvio-aeolian interaction
recorded in the upper member of the unit from three-dimensional sections exposed at Carmen de Patagones
(40°47’41’’ S, 63°0’4’’ W; Buenos Aires province).
The studied sedimentary succession has a maximum thickness of 12 m and a measured lateral extension of
121.5 m. Stratigraphically, the succession overlies to middle member, although the contact is not exposed,
and is covered by the Rodados Patagónicos. It is mostly composed by volcaniclastic, medium to fine-grained
sandstones with minor occurrence of mudstones, conglomerates and vitric, fine-grained tuffs. The
succession was studied in both sides of a NNE oriented route-cut and other two near orthogonal exposures.
Methodology included the measurement of nine detailed sedimentary logs, as well as facies and architectural
analysis, the later using four photomosaics.
Six facies associations (FA) were distinguished: sandstone dominated aeolian deposits, including dune and
dry interdune zones (FA1); sandstone-loessic aeolian deposits with water reworking and soil development,
deposited in relatively flat areas (FA2); intermittent mudstone fluvial channel-belt deposits (FA3); permanent
sandstone fluvial channel-belt deposits (FA4); pyroclastic deposits reworked by unconfined fluvial flows and
later subjected to pedogenesis (FA5) and shallow lacustrine deposits (FA6). Most of the paleosoils did not
display differentiation of horizons nor pedic structure. Complex spatial arrangement of FA can be
summarized as follows: 1) a lower sector dominated by FA1 deposits, which displays abundant simple and
complex fluvial channel-belt deposits of FA3 in the upper part; 2) a middle sector composed of FA2 deposits
with scarce amount of simple fluvial channel-belt deposits of FA4; 3) an upper sector constituted by deposits
of FA5 in the base and FA6 in the top. The boundary between middle and upper sectors is pointed by the best
developed paleosoil of the succession, which shows two horizons separated by a diffuse limit, as well as
microscopic evidences of clay lixiviation. Vertical distribution of the FA suggests an increment in water
availability probably linked to wetter climatic condition.
THE BRUNHES/MATUYAMA BOUNDARY AND ROCK MAGNETIC
PARAMETERS IN PLEISTOCENE LOESS DEPOSITS OF CAMET,
MAR DEL PLATA (ARGENTINA)
5-08
Bidegain, J.C.1*, Gomez Samus, M.1
(1) Laboratorio de Entrenamiento Multidisciplinario para la Investigación Tecnológica-CIC Calle 52
e/ 121 y 122, La Plata, Buenos Aires. Argentina
* P resenting author’s e-mail: Gomez_Samus@yahoo.com.ar
The Pleistocene-Holocene loess/paleosoils sequences exposed at the north of Mar del Plata have been
studied by several researchers from the beginning of the last century (Ameghino, 1908), more recently
by Schnack et al., (1982) and Fassano (1991). The present contribution refers to the sedimentary
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Fig. 1 - The Camet stratal sequence; main units (U1-U5) identified, lithostratigraphic characteristics, positions of
paleomagnetic samples and inferred magnetostratigraphy.
sequence exposed at Camet (37º 53´39,11´´S and 57º 31´17,8´´W) which contents records of normal
and reverse polarity that can be assigned to Brunhes and Matuyama Polarity Chrons. First
paleomagnetic studies were carried out in the sector Santa Clara del Mar and Arroyo La Tapera by
Bidegain et al. (2005). Paleomagnetic directions and magnetic parameters were measured on samples
collected from a section of five units separated by discontinuities. The present contribution confirm
previous ones in the concern of paleomagnetic zonation i.e. the unit labeled U1 at the base of the
profile (Fig. 1) presents reverse polarity levels while the units U2 to U5 present normal polarity. The
former is assigned to upper Matuyama (> 0.78 Ma) and the latter to Brunhes Normal Polarity
Chronozone (< 0.78 Ma). According to the records of magnetic parameters the pattern of behavior
follows that obtained in the north of the Buenos Aires province. The less pedogenized materials show
the higher LF susceptibility (330 m3/kg), B and BC horizons show values ranging between 80 and
200 m3/kg and the gley horizons 20 and 90 m3/kg. Frequency dependent susceptibility (¯fd) defined as
¯fd= (¯ 470 Hz - ¯4700 / ¯ 470 Hz ) x 100 was employed to estimate the superparamagnetic contribution. This
contribution is low along the profile, it increases in the pedogenized horizons (5%) and decreases
noteworthy in the loess layers (0.6 %).
REFERENCES
• Ameghino, F. 1908. Las formaciónes sedimentarias de la región litoral de Mar del Plata y Chapadmalal. Anales Museo Nacional de
Buenos Aires. Serie 3ª, X, 843-428.
• Bidegain, J.C., Osterrieth, M.L., Van Velzen, A.J., Rico, Y. 2005. Geología y registros magnéticos entre arroyo La Tapera y Santa
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Clara del Mar, Mar del Plata. Rev. Asoc. Geol. Arg., 60 (3): 599-604.
• Fasano, J.L. 1991. Geología y Geomorfología. Región III Faro Querandí-Mar de Cobo, Provincia de Buenos Aires. Informe Final.
Convenio de cooperación horizontal entre el Consejo Federal de Inversiones y la Universidad Nacional de Mar del Plata, 118p.
• Schnack, E.J., Fasano, J.L. Isla, F.I. 1982. The evolution of Mar Chiquita lagoon coast, Buenos Aires province, Argentina. En
Colquhom, D.J. (Ed.), Holocene Sea Level Fluctuations, Magnitude and Causes. IGCP-INQUA, Colombia S.C. U.S.A.: 143-155.
THE LATE CENOZOIC SEDIMENTARY SEQUENCES IN THE
CHAPADMALAL AREA (BUENOS AIRES). POLARITY CHANGES
AND MAGNETOCLIMATOLOGY
5-09
Bidegain J.C.1, Rico Y.1
(1) Laboratorio de Entrenamiento Multidisciplinario para la Investigación Tecnológica (LEMIT-CIC).
Calle 52 e/ 121 y 122, Provincia de Buenos Aires (Argentina)
* Presenting author’s e-mail: Yamrico@hotmail.com; jcbidega@yahoo.com.ar
The present contribution is focused on the polarity changes and magnetic parameters obtained in cliffs
of Chapadmalal area, Buenos Aires province. The sedimentary section comprises the Vorué, San
Andres, Miramar and Arroyo Seco Formations. The Vorohué Formation and lower part of San Andrés
Formation content records of normal polarity and both were assigned to the Gauss Chron (> 2.6 Ma).
The main section of San Andrés Formation appears to have been deposited during lower and middle
Matuyama including Olduvai (1.9 Ma). The boundary between Miramar and San Andrés Formations
coincides with a discordance and a new polarity change attributed to Middle Matuyama/Jaramillo
(0.99-1.05 Ma). The Miramar Formation shows normal polarity at the base (Jaramillo) and reverse
polarity at the top, those reverse levels (Upper Matuyama) were also recorded in the lower part of the
onlying Arroyo Seco Formation. Finally, the upper part of Arroyo Seco Formation was assigned to
Brunhes normal polarity chron (< 0.78 Ma).
Paleomagnetic data are in agreement – to some extend- with the previous ones carried out in the area
by Orgeira, 1988, 1990, and Ruocco,1990, however, it is arising some differences as regarding older
geological units which should encourage deeper investigations and further discussion.
The magnetic susceptibility as environmental proxy data in San Andrés shows a clear alternated
sequence of highs and lows that may be referred to changes in environmental conditions (Bidegain et
al., 2005, 2007). The major contrast in susceptibility values among layers is “enhanced” by the
presence of calcrete layers producing the almost total depletion of records. Both concentration
parameters (susceptibility and SIRM ) confirm the pattern of magnetic behavior of loess studied in
the north of the Buenos Aires province but there are also some differences in this concern. Highest
susceptibility values are > 300x 10-8 m3/kg in contrast with the similar ones of La Plata-Baradero area,
usualy between 100-200 x10-8 m3/kg (Bidegain et al., 2009). Paleosols and less pedogenized materials
do not show so sharp differences as regarding IRM, the SIRM values range between 22 A/m to 28 A/m
in both materials. Due to the low content of magnetic minerals the calcrete layers show very low
SIRM values (1.35 to 7.1 A/m). The saturation (SIRM) is reached at low field (< 0.5 T) in all samples
and the coercitivity of remanence (Bcr) varied between 29.5- 34 mT which corresponds to low
coercitivity magnetites.
The influence of pedogenesis in the enhancement of F factor seems not to have been relevant - as in
other localities studied until now- being the higest value around 7% .
REFERENCES
• Orgeira, M.J. Estudio Paleomagnético de los Sedimentos del Cenozoico Tardío en la Costa Atlántica Bonaerense. RAGA 1988. XLII
(3-4): 362-376.
• Orgeira, M.J. Paleomagnetism of late Cenozoic fossiliferous sediments from Barranca de Los Lobos (Buenos Aires,Argentina). The
magentic age of the South American land –mammal ages. Physics of the Earth and Planetary Interiors 1990. 64 :121-132.
• Ruocco, M. A 3 Ma paleomagnetic record of coastal continental deposits in Argentina.Palaeogeogr. Palaeoclimat. Palaeoecol 1989.
72: 105-113.
• Bidegain, J.C, M.E. Evans, A.J. van Velzen A magnetoclimatological investigation of Pampean loess, Argentina. Geophs., J.Int.
2005. 160: 55-62.
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• Bidegain, A.J. van Velzen, Y. Rico The Brunhes/Matuyama boundary and magnetics parameters related to climatic changes in
Quaternary sediment of Argentina, South Am. Earth Sciences 2007. 23: 17-29.
• Bidegain, J.C., Y. Rico, A. Bartel, M. Chaparro, S. Jurado. Magnetic Parameters Reflecting Pedogenesis in Pleistocene Loess
Deposits of Argentina. Quaternary International 2009. 209: 175-186.
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SURFACE PROCESSES
AND PALEOCLIMATE
GEOSUR2010
RECONSTRUCTION OF LATE-GLACIAL TO HOLOCENE CLIMATE AND
EARTHQUAKE HISTORIES ACROSS SOUTHERN CHILE BASED
ON THE SEDIMENTARY RECORD OF 21 LAKES
6-01
De Batist, M.1*, Moernaut, J.1, Heirman, K.1, Van Daele, M.1, Bertrand, S.1,
Abarzua-Vasquez,A.M.1, Pino, M.2, Brümmer, R.2, Urruti, R.3, Vila, R.4, Roberts, S.5,
Kilian, R.6, Verleyen, E.7, Vyverman, W.7, Keppens, E.8, Fagel, N.9, Gieles, R.10,
Sinninghe Damsté, J.10, Hebbeln, D.11, Gilli, A.12, Brauer, A.13,
the ENSO-CHILE, CHILT Project Members
(1) Renard Centre of Marine Geology (RCMG), Universiteit Gent, Gent, Belgium
(2) Instituto de Geociencias, Universidad Austral de Chile, Valdivia, Chile
(3) Centro EULA, Universidad de Concepción, Concepción, Chile.
(4) Laboratorio de Palinología, Facultad de Ciencias, Universidad de Chile, Santiago, Chile
(5) British Antarctic Survey (BAS), Cambridge, United Kingdom
(6) Department of Geology, University of Trier, Trier, Germany
(7) Protistology & Aquatic Ecology, Universiteit Gent, Gent, Belgium
(8) Department of Geology, Free University of Brussels, Brussels, Belgium
(9) Department of Geology, University of Liège, Liège, Belgium
(10) Royal NIOZ, Den Burg, Texel, The Netherlands
(11) MARUM, University of Bremen, Bremen, Germany
(12) ETH Zürich, Zürich, Switzerland
(13) GFZ, Potsdam, Germany
* Presenting author’s e-mail: Marc.DeBatist@UGent.be
During the past years and in the context of a succession of international research initiatives (o.a. the
ENSO-CHILE and CHILT Projects), the sedimentary infill of 21 lakes in southern Chile, extending
from 37°20’ S in the north to 53°35’ S in the south, was investigated by means of dense grids of highresolution reflection seismic profiles and of multi-proxy analyses of multiple short gravity cores and
long piston cores. This combined data set represents a vast sedimentary “library”, from which the
history of climate change, seismicity, volcanic activity and human impact in South-Central and South
Chile can be reconstructed, including its regional and latitudinal variability. As most of these lakes are
glacial in origin, the time window of investigation is in most cases effectively limited to the last ~1214 ka. However, some of the studied lakes contain unique, continuous sediment records that extend
much further in time, down to ~20 ka or even to ~40 ka. Moreover, many of these records are
characterized by high sedimentation rates and are annually laminated and they can thus potentially
produce very detailed paleo-reconstructions at annual resolution.
In this presentation, an overview will be given of recent research results obtained from the study of
these lake records. Emphasis will be given on the potential (advantages and disadvantages) of these
records for paleoclimate studies (at high resolution) and for paleoseismology.
RAPID CRUSTAL UPLIFT IN PATAGONIA AS A CONSEQUENCE
OF INCREASED ICE LOSS
6-02
Dietrich, R.1*, Ivins, E.R.2, Casassa, G.3, Lange, H.4, Wendt, J.†3, Fritsche, M.1
(1) Institut fuer Planetare Geoda¨sie, Technische Universitaet Dresden, 01069 Dresden, Germany
(2) Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA 91109, USA
(3) Centro de Estudios Cientificos, Arturo Prat 514, Valdivia, Chile
(4) TERRASAT S.A., Av. Eliodoro Yañez 2050, Santiago de Chile, Chile
* Presenting author’s e-mail: dietrich@ipg.geo.tu-dresden.de
GPS observations were carried out between 2003 and 2006 at the northeastern edge of the Southern
Patagonian Icefield. The data analysis was performed with the Bernese Software and revealed uplift
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rates of up to 39 mm/yr. For the region an accelerated glacier wasting has been observed since the termination of the Little Ice Age. This increasing ice loss continues up to present time. Advanced modeling shows that the rapid ice melting in combination with relatively low viscosity of the Earth’s mantle caused by the unique regional slab-window tectonics is central for the interpretation of the results.
The profile of GPS observations link ice loss to the soft viscoelastic isostatic flow response over the
time-scale of the Little Ice Age (LIA), including ice loss in the period of observation.
IMPLEMENTATION OF AQUIFER PROTECTION ZONING
6-03
Dustay, S.1*, Nel J.1, Xu, Y.1, Massone, H.2
(1) UNESCO Chair Centre, University of the Western Cape, Bellville, South Africa
(2) Centro de Geología de Costas y del Cuaternario, Facultad de Ciencias Exactas y Naturales,
Universidad Nacional de Mar del Plata, Funes 3350, 7600 Mar del Plata, Argentina
* Presenting author’s e-mail: 2541129@uwc.ac.za
Introduction
Two thirds of South Africa’s and Argentina’s population depends on groundwater for their domestic
water needs. Currently limited progress is made in South Africa (and Africa) and Argentina on the
protection of water. To achieve the objective of water for growth and development and to provide
socio-economic and environmental benefits of the communities, significant aquifers and well fields
must be adequately protected. Implementation of groundwater protection zones is seen as an
important step in this regard. From initial literature review the implementation of protection zoning in
developing countries like Africa is new or non-existent.
Methodology
Initial protection-zone delineation will be made using published reports and database data. This initial
delineation will probably be based on a simplified 2D model created with the US-Environmental
Protection Agency wellhead-delineation software, “WhEAM”. From these capture zones the planning
of the detailed study will be conducted. An inventory of the activities that can potentially impact water
quantity will be made and ranked according to their degree of risk for impacting water sources. This
information will be used to prioritize areas where more data is needed and where additional data
gathering is required. This data can be collected through a hydro census and through aquifer tests.
Aquifer tests that will be conducted include tracer tests, Flowing FEC and constant discharge tests.
This improved information can be used to build the conceptual model and implement a trustworthy
groundwater protection plan.
Expected Results
Development of protection zones and protection plans should lead to sustainable water use that
include environmental and ecosystem health as well as provision of water for human needs. The
expected results will have applicability to groundwater management in general. The methodology and
guidelines developed for the purpose of this project will be used to update and improve policy
implementation. The protection of these groundwater resources will ensure its sustainability from a
quality perspective for future generations.
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TERRESTRIAL AND LACUSTRINE EVIDENCE OF HOLOCENE
GLACIER ACTIVITY IN TIERRA DEL FUEGO (SOUTHERNMOST SOUTH AMERICA)
6-04
Maurer, M.1, Menounos, B.1, Clague, J.J.2, Osborn, G.3, Rabassa, J.(*)4,5, Ponce, J.F.4,
Bujalesky, G.4,5, Fernández, M.4, Coronato, A.4,5
(1) Geography and Natural Resources and Environmental Studies Institute, University of Northern
British Columbia, Prince George, BC, Canada, V2N 4Z9
(2) Department of Earth Sciences, Simon Fraser University, Burnaby, BC, Canada, V5A 1S6
(3) Department of Geosciences, University of Calgary, Calgary, AB, Canada, T2N 1N4
(4) CADIC-CONICET, Ushuaia, Tierra del Fuego, Argentina
(5) Universidad Nacional de la Patagonia-San Juan Bosco, Sede Ushuaia
* Presenting author's e-mail: jrabassa@gmail.com
The synchronicity of Holocene glacier fluctuations in the northern and southern hemispheres is a
subject of debate. Our research addresses this issue by examining the lacustrine and terrestrial
evidence of glacier activity in Tierra del Fuego. In April 2009, we performed a bathymetric survey of
glacier-fed Lago Roca (S 54º 48’, W 68º 38’) and recovered four percussion cores from the lake floor.
The cores are 1-2 m in length and consist of inorganic, rhythmically laminated silt and clay. Laminae
are 1-2 mm thick, silt-clay couplets that appear to be clastic varves. Magnetic susceptibility generally
increases upward to a peak 1.5 m below the top of the longer cores. The uppermost 20 cm of the cored
sediments are the least clastic-rich and have the lowest magnetic susceptibility. Organic content is
inversely related to magnetic susceptibility, with low values near the top and bottom of the cores.
A field study at Stoppani Glacier, 25 km northeast of Lago Roca, in December 2009 provided
evidence of several advances of the glacier during the past 4000 years. The left lateral moraine of the
glacier is composed of multiple tills separated by glaciofluvial and glaciolacustrine sediments.
Radiocarbon ages of stumps in growth position, detrital wood in till units, and vegetation mats
indicate that the glacier repeatedly advanced between 3510 ± 15 and 184 ± 15 14C yr BP (3830–150
cal yr BP). The advances broadly coincide with documented intervals of glacier expansion in the
Northern Hemisphere.
Additional work on the Lago Roca sediment record is underway. Those data will supplement the
discontinuous terrestrial record preserved at Stoppani Glacier. The work includes analysis of pollen,
diatom, and phytoliths recovered from the sediment cores. The results will be compared with
palynologic records from Holocene peat bogs, which are abundant on Tierra del Fuego. These new
lake and terrestrial records will help constrain the age and magnitude of past glacier advances in
Patagonia, allowing an assessment of inter-hemispheric synchronicity of climate events during the
Holocene.
ROCK MAGNETISM STUDY ON SEDIMENTS FROM STREAMS
OF THE PARANÁ DELTA (ARGENTINA)
6-05
Mena, M.*, Dupuy, J.L.
CONICET-INGEODAV, Dpto. Ciencias Geológicas, Facultad de Ciencias Exactas y Naturales,
* Presenting author’s e-mail: mena@gl.fcen.uba.ar
We present a rock magnetism study performed on bed sediments from Paraná Delta, Buenos Aires,
Argentina. The sampling comprises eighty sites located along 30 km of streams. At each GPS-located
site, 300 g of bottom sediments were extracted using a dredge sampler. Four specimens were taken
from each sample, previously dried at room temperature. The magnetic susceptibility at three
frequencies was measured for all specimens. For each site, mean and standard deviations for mass
magnetic susceptibility (¯) and frequency-dependent susceptibility factor (FDF) were calculated.
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Measurements of temperature dependence and field dependence of magnetic susceptibility were
performed on one specimen per site using a MFK1-FA + CS4 Kappabridge susceptibilimeter.
Stepwise acquisition of isothermal remnant magnetization (IRM) was performed on another
previously consolidated specimen from each site. The ¯ presents an increasing tendency from rural to
urbanized areas. The FDF is relatively low (<10%) in all sites.
The statistical analysis of IRM acquisition curves allowed defining four magnetic components with
different coercivity ranges. The absolute contributions of the biggest coercivity component (400-600
mT) and the intermediate coercitivity component (100-300 mT) vary moderately among the samples.
This suggests that these magnetic phases should be representative for the detrital input into the river.
The two components with smaller coercivity (soft component: 5-20 mT) and (moderately soft
component: 20-70 mT) have low values of their coercivity dispersion parameters. The absolute
contributions of these components are widely variable among the samples. Their coercivities are
comparable to those expected for magnetic grains of authigenic chemical and bacterial origin.
A direct relationship among the magnetic enhancement and the location of the alleviation channel
mouths, access creeks to nautical neighbourhoods and more urbanized areas was found. Many of these
¯ peaks are associated to FDF minima, and all of them coincide with major contributions of the low
coercivity components, especially the moderately soft. The presence of mainly biogenetic
ferrimagnetic minerals may be related to decreased pH by the presence of industrial effluents and
sewage. A ¯ peak associated with a local maximum FDF and local increase of the contribution of both
low coercivity phases is located in front of a nautical fuel dispenser. These magnetic properties may
be due to both biogenic and chemical formation of ferrimagnetic minerals, probably by the combined
effects of small fuel spills which increase the acidity of the medium and generate reductive
environments, of internal combustion engines and of urbanization.
The drops in the moderately soft component contribution match the local ¯ minima located at the exit
of streams in less populated zones. These minima could be associated with more oxygenated local
environments. The presented results support the usefulness of employing magnetic properties of bed
load sediments to monitor the environmental evolution of riverbeds in the area.
CHARACTERIZATION OF COMPLEXITY OF FRACTURED ROCK AQUIFERS
6-06
Nel, J.1*, Xu, Y.1, Batelaan, O.2,3
(1) UNESCO Chair Centre, University of the Western Cape, Bellville, South Africa
(2) Department of Hydrology and Hydraulic Engineering, Vrije Universiteit Brussel, Pleinlaan 2, 1050
Brussels, Belgium
(3) Department of Earth and Environmental Sciences, Katholieke Universiteit Leuven, Celestijnenlaan
200e - bus 2410, 3001 Heverlee, Belgium
* Presenting author’s e-mail: jmnel@uwc.ac.za
Characterization of fractured rock aquifers using borehole logging tools like calliper, acoustic
televiewer, neutron and gamma are well established in the oil and water industry to determine
geological zones of suitably high oil or water yields. Fractures identified with these methods are
however not necessarily active and would therefore not all contribute to flow and transport. Methods
like packer testing and borehole flow meters have been used by some researchers to identify hydraulic
active fractures in boreholes and to identify fracture zones connecting different boreholes. These types
of equipment are very expensive and not commercially available in South Africa, requiring the import
of custom built setups for working around 200 m depths.
Alternative cheaper methods with simple field application were therefore investigated to identify
fracture positions and hydraulic properties. Borehole temperature data and electrical conductivity
logging data is used in this paper to identify fracture positions and the change in flow conditions under
different pumping conditions. Borehole temperature profiles and Fluid Electrical Conductivity (FEC)
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logs were measured using a YSI 6600 multi-parameter sonde with specific conductance, temperature
and depth logging capabilities.
The specific conductance in the borehole was slightly increased by dispersing table salt through the
borehole column using an injection sock. As soon as possible after the salt is added to the borehole
the specific conductance of the entire borehole column was logged. This logging of the borehole
column is then repeated over time. Any flow through the borehole will dilute the salt concentration
back to natural conditions. This data of salt dilution at specific points gives an indication of flow zones
in the borehole.
The data obtained from the FEC logs were be used to simulate flow rates of individual fractures. The
2-dimensional finite difference model BORE II software was used to simultaneously solve for flow
and mass transport on the borehole scale. Initial concentrations are provided as input, with fracture
flow rates and quality simulated as the borehole water is diluted. Calibration is done against the
measured data using a trial-and-error approach. Both temperature and FEC logging was successfully
employed to determine fracture positions and ambient flow conditions in the aquifer.
ENVIRONMENTAL MAGNETISM STUDY OF A HOLOCENE EOLIAN
SEDIMENTS AND PALEOSOLS SEQUENCE IN THE NORTH OF
TIERRA DEL FUEGO (ARGENTINA)
6-07
Orgeira, M.J.1,2*, Coronato, A.3,4, Vásquez, C.A.2,5, Ponce, A.3, Moretto, A.3,
Egli, R.6, Onorato, M.R.7
(1) Depto. Cs. Geológicas-FCEN, UBA. Cdad Universitaria Pab II, 1428 Buenos Aires, Argentina
(2) INGEODAV/UBA-CONICET, Buenos Aires, Argentina
(3) CADIC/CONICET, Ushuaia, Argentina
(4) FHCS-UNPSJB, Ushuaia, Argentina
(5) CBC-UBA, Buenos Aires, Argentina
(6) Dept. of Earth and Environmental Sciences, Ludwig-Maximilians University, Munich, Germany.
(7) FCEFN-UNSJ, San Juan, Argentina
* Presenting author’s e-mail: orgeira@gl.fcen.uba.ar
The studied sequence is located in the northern region of Isla de Tierra del Fuego, Argentina (53° 42’
48.6’’S, 68° 18’ 20.3’’W, Fig. 1), altitude 71 m asl (Coronato et al, 2009). The present mean annual
rainfall in the area is around 380 mm and the mean annual temperature is 5.2° C. The sequence is
located in a large area of low pressure under the effect of both the westerlies and the Polar Front. Wind
frequency is daily, with and average rate of 25 km/h, with frequent periods of higher wind speeds. The
influence of Antarctic air produces short periods of colder and drier climate. The studied sequence is
located in the semiarid Fuegian steppe where Festuca gracillima (coirón) is the dominating specie.
The sequence comprises a succession of 20 m of eolian silty-fine sand-clay sediments with 8 paleosols layers interbedded.
Paleosols can be clearly identified on the basis of typical pedofeatures such as, coal grains, clay coatings, organic content and dark colour (10Y/R to 5YR). The paleosols layers contain abundant fossil
record represented by Lama guanicoe and Ctenomys sp, and three or four different horizons have been
identified in each paleosol.
Radiometric data of the intermediate paleosoil 4 (5800 yr) and its overlying tephra layer (ca 3200 yr)
allow to infer that the eolian deposition and the edaphic processes started during the Early/middle
Holocene or earlier.
The origin of the deposits is not clear yet. Two hypothesis are being analyzing: a) they could be the
result of deflation during dissication periods of a shallow lake located near the eolic deposit, or b) they
could be deposited forming perched dunes, ciclically eroded, edaphized and buried by new deposits.
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Fig. 1 - Overview of the studied Holocene eolian sequence.
Fig. 2 - Details of sampling. View of the paleosol 5 located in the middle of the profile
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Fig. 3 - W values (moisture ratio) calculated for the studied area in Tierra del Fuego, Argentina (a)
and Alaska, USA (b).
In case a) the source of the sediments must be the lake bottom formed by weathered marine Tertiary rocks
and probably till; in case b) the source must be the weathered silty-sand bedrock which form the cliff over
which de deposits lay. On going geomorphological studies will help to explain this depositional sequence.
Additionally, this sequence is interesting due to archeological record. Abundant lithic and faunal archeological materials, formely buried are exposed by present eolian erosion.
This contribution includes preliminary results, mainly focused in environmental magnetism measurements of 93 samples collected along the sequence and present soils of the same area (Fig. 2).
The magnetic results include magnetic susceptibility measurements at room temperature and two frequencies (470 and 4700 Hz) (Bartington MS2), Vibrating samples magnetometer (Molspin VSM) measurements at room temperature and some measurements of susceptibility at high and low temperatures (Kappa
bridge) in selected samples.
The concentration of ferrimagnetic mineral recorded along the sequence (mass specific magnetic susceptibility, c between 17 and 7 E-7 m3/Kg) is similar to those of Pampean loess sequences. This fact allows to
compare magnetic results from both areas in order to evaluate wind impact in the magnetic signal.
The obtained magnetic results are used to prove, for high-mid latitudes (Tierra del Fuego and Alaska), the
quantitative model for magnetic signal proposed by Orgeira et al (2010). Fundamentally, the model is
based on the hypothesis of ultrafine magnetite precipitation during altenating wetting and drying cycles
in the soil micropores. The rate at which this occurs depends on the frequency of drying/wetting cycles,
and on the average moisture of the soil.
In order to do the comparison cited above, the W values (moisture ratio = precipitation/evapotranspiration, Orgeira et al., 2010) were calculate for both areas (Figs. 3a and 3b). Therefore, the magnetic signal
of the area and its W value were compared with those obtained in a loess deposit from Alaska (Lagroix
and Banerjee, 2002).
After the comparison above mentioned, the magnetic results obtained in the paleosols can be transformed
in paleo-average moisture.
Finally, these magnetic proxi plus the other multiproxi data obtained in the studied sequence define the
Holocene climatic pattern for the area, wich is characterized by an important climate variability occurred
in the southern extremity of the Americas. Based on this study we hope to contribute to the knowledge of
the variability of the southern atmospheric circulation for the most recent geological times.
REFERENCES
• Lagroix F. and Banerjee S.K.; 2002: Palaeowind directions from the magnetic fabric of loess in central Alaska, Earth Planet. Sci.
Lett., 195, 99-112.
• Orgeira M.J., Egli R. and Compagnucci R.H.; 2010: A quantitative model of magnetic enhancement in loessic soils. Chapter in Earth
magnetic Interior (IAGA Special Sopron Book series), Springer; in press.
• Coronato A., Fanning P., Saleme M., Oría J. and Pickard J., 2009: Aeolian paleosoils and the archeological record at Lake Arturo,
Nothern Tierra del Fuego, Argentina. IV Cong. De Cuaternario y Geomorfología. XII Congresso da Associacaode Studos Do
Cuaternario. Abstracts: 234.
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MANAGEMENT AND CONTROL OF THE WATER RESOURCES
OF SAN LUIS PROVINCE (ARGENTINA)
6-08
Pedersen*, O.A.
San Luis Agua S.E. Justo Daract 1695 San Luis Capital
* Presenting author’s e-mail: Pedersen.oscar@gmail.com
To effectively manage the water resources of the Province of San Luis, at the end of 2009 the San
Luis Agua SE company was created, based on high standards of a private management company.
Plan, regulate, control and manage resources in a given territory, constitute a complex task. Part
of the problem is to identify the variables involved in the process of management of water
resources. It is also important to understand and analyze the interactions among these variables.
From those premises, it is possible to build a digital model to simulate possible behaviors in order
to drive efficient use of natural resources of the province, and to be able to react quickly to unforeseen situations. The GIS (Geographic Information System) uses a set of graphical information in
the form of geo-referenced maps and methods of management of databases. On this basis, the
company uses the GIS as a tool for information management and geospatial data for an ideal use
of its capital. The company has also created, within the GIS, a system where users of the province
have access to information of public interest like water wells, waterworks, etc. The goal of San
Luis Agua SE is to maximize its resources, both human and infrastructure, result in a product economically viable over time, reducing operating costs and management. The GIS provides the technical tools for these purposes and provides an environment for work and consultation that best
suits the present needs.
SEDIMENTARY IMPRINT OF THE 2007 AYSÉN EARTHQUAKE AND
TSUNAMI IN AYSÉN FJORD (CHILEAN PATAGONIA)
6-09
Van Daele, M.1, De Batist, M.1*, Versteeg, W.1, De Rycker, K.1, Cnudde, V.2,
Gieles, R.3, Duyck, P.4, Pino, M.5, Urrutia, R.6
(1) Renard Centre of Marine Geology (RCMG), Ghent University, Krijgslaan 281/S8, B-9000 Gent,
Belgium
(2) Department of Geology and Soil Science, Ghent University, Krijgslaan 281/S8, B-9000 Gent,
Belgium
(3) Royal Netherlands Institute for Sea Research (NIOZ), Landsdiep 4, 1797 SZ’t Horntje (Texel),
Netherlands
(4) Department of Radiology and Medical Imaging, Ghent University Hospital, De Pintelaan 185, B9000 Gent, Belgium
(5) Instituto de Geociencias, Universidad Austral de Chile, Casilla 567, Valdivia, Chile
(6) Centro EULA, Universidad de Concepción, Casilla 160-C, Concepción, Chile
* Presenting Author e-mail: Marc.DeBatist@UGent.be
On 21 April 2007, the Mw 6.2 Aysén earthquake caused several subaerial mass movements
(landslides, rockfalls) along the slopes of Aysén fjord (Fig .1). The three most voluminous of these
triggered several tsunamis. These landslide-induced tsunamis not only destroyed small villages,
houses and salmon farms along the shores of the fjord, but also resulted in 3 casualties and 7 missing
persons. The earthquake had its epicentre at a depth of < 9 km below the fjord. It was the most
important earthquake of a seismic swarm, which had lasted 3 months (starting on January 22) and in
which more than 7000 earthquakes were registered. Intensities as high as VIII to IX were recorded
around the epicentral zone, i.e. where the landslides occurred (Fig. 1). In Puerto Chacabuco and
Puerto Aysén (at the end of the fjord) intensities of VII were recorded (Naranjo et al., 2009; Sepulveda
and Serey, 2009).
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Fig. 1 - Location map of the study area, with indication of the epicentral zone of the earthquake swarm, location of
landslides and cores studied.
This type of mass movements and their related tsunamis can leave an eventdeposit in the fjord’s
sedimentary record. Such deposits and the processes forming them have been described by several
authors. However, coupling the nature of these deposits to ‘what occurred on which slope’ and to
earthquake intensity is a complicated assignment. Historical records are often incomplete and only
range back in time a few hundred years, depending on the region. As such, examples where the
original processes are well documented are scarce.
In order to gain a better insight in the sedimentological characteristics of event deposits caused by
landslide-induced tsunamis, we conducted a multidisciplinary study of the recent sedimentary
infill of Aysén fjord. By studying the 2007 event-deposit with multibeam bathymetry, highresolution reflection seismics and a multiproxy analysis on 20 short gravity cores (Fig. 1), we aim
to fingerprint this deposit in the highest detail. Multibeam bathymetry and reflection seismics are
used to map out the occurrence, morphology and thickness of the deposit throughout the fjord, and
gravity cores are taken to ground-truth the geophysical data. Sedimentological characterisation of
the event-deposit is achieved by combining CT-scans, high-resolution (1 mm) grain-size analyses,
XRF-scanning (1 mm) and magnetic-susceptibility measurements (2.5 mm) of the sediment cores.
The deposit is very heterogeneous in space and has a varying thickness (centimetre- to metrescale). The internal structure varies between parallel laminations, fine cross-bedding, ripples and
homogeneous, with grain sizes ranging from fine clay to gravel. Different phases in the deposition
can be correlated between most of the cores and thereby allow us to gain an insight into the
evolution of this deposit. Comparing this complex sedimentary imprint with eye-witness reports,
field observations, records of seismic-shaking and macro-intensities allows us to better understand
the processes forming these deposits.
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REFERENCES
• Naranjo, J.A., Arenas, M., Clavero, J. and Munoz, O., 2009. Mass movement-induced tsunamis: main effects during the Patagonian
Fjordland seismic crisis in Aisen (45 degrees 25’S), Chile. Andean Geology, 36(1): 137-145.
• Sepulveda, S.A. and Serey, A., 2009. Tsunamigenic, earthquake-triggered rock slope failures during the April 21, 2007 Aisen
earthquake, southern Chile (45.5 degrees S). Andean Geology, 36(1): 131-136.
RECONSTRUCTION OF THE EVOLUTIVE STAGES OF LLANCANELO LAKE
6-10
AND SURROUNDINGS (SOUTHERN MENDOZA PROVINCE, WESTERN ARGENTINA)
Rovere, E.I.1, Violante, R.A.2*, Osella, A.3, De la Vega, M.3, López, E.3
(1) Dirección de Geología Regional, Servicio Geológico Minero Argentino SEGEMAR, Av. Julio A
Roca 651. Buenos Aires C1067ABB, Argentina
(2) Servicio de Hidrografía Naval, Departamento Oceanografía, División Geología y Geofísica Marina.
Av. Montes de Oca 2124, Buenos Aires C1271ABV, Argentina
(3) Departamento de Física, Facultad de Ciencias Exactas y Naturales, Universidad de Buenos
Aires, Ciudad Universitaria, Buenos Aires C1428EGA, Argentina
* Presenting author’s e-mail: Roberto A. Violante, violante@hidro.gov.ar
Introduction
Llancanelo Lake (southern Mendoza province, western Argentina, 35º35´S-69º09´W) is located at the
foot of the Andes cordillera in a transitional region between northwestern Patagonia and western dry
Pampas. It occupies a tectonic depression surrounded by extensive volcanic fields largely composed
of volcanic cones and basaltic flows. As a result it behaved as the main regional depocenter through
its evolutionary history, which has been affected by diverse intra and extra basinal sedimentation
through different sedimentary processes. Consequently, thick sedimentary and volcaniclastic
sequences were deposited in the basin, controlled by tectonic and volcanic factors under fluctuating
climatic changes. As the last stages of evolution were dominated by arid conditions, the lake
progressively changed from an ancient, largely extended water body, to a presently smaller, endorreic,
shallow and highly saline lacustrine environment. Therefore, in the lake and its basin the records of
the regional evolution are preserved, what makes the area a key region for paleoenvironmental,
paleoclimatic and paleovolcanic reconstructions.
Geophysical and geological surveys have been performed in order to: 1) define the depocenter
geometry and its substratum; 2) evaluate the extension and characteristics of the sedimentary
sequences; 3) recognize the volcanic events that occurred during its evolution; 4) reconstruct the
evolutionary history of the lake. This contribution synthesizes the present knowledge on aspects
related to the conditioning factors that influenced the evolution of the region.
Geological setting
Llancanelo Lake occupies the southern extreme of the so-call Huarpes Depression, a tectonic basin
extended from N to S between the Andes cordillera to the W and the San Rafael Block to the E. The
southern boundary of the basin is the Payenia Volcanic Field. Thickness of the alluvial sedimentary
sequences filling the basin where the lake is contained ranges between 500 and 1000 m (Ostera and
Dapeña, 2003). Although the present lake covers an area of around 370 km2 with an average depth not
exceeding 1 m, evidence arising from regional geological, morphological, faunistic and archeological
records (Groeber, 1939; Delpino, 1993; Dieguez et al., 2004; Gil et al., 2005; Guerci et al., 2006;
among others) suggest that an ancient “big” lake containing not only the present Llancanelo Lake but
also the present El Nihuil Lake, covered in the past an area of around 5000 km2 reaching a level of
+50 m above the present lake, although no evidence exist about the depth of that ancient lake.
The Payenia Volcanic Field located immediately south of (and partially surrounding) Llancanelo
Lake, is a Tertiary-Quaternary back-arc basaltic province (Delpino, 1993; Bermúdez et al., 1993) in
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which volcanism has been very intense during the Cenozoic, when some 800 volcanoes have been
active at different times. Basaltic lava flows were the main volcanic products, mostly originated in
complex volcanoes and monogenic cones, some of them hydroclastic and maars (Delpino, 1993;
Bermúdez et al., 1993; Risso et al., 2008; among others). On the other hand, the neighboring Andean
arc contains -in the 300 km long stretch of mountains that extend west of Llancanelo Lake and
Payenia- around 12 volcanoes, some of them active up to very recent times (eg.: Quizapu, Peteroa);
dominantly acid explosive eruptions occurred there during most of the Cenozoic under the effect of
strong westerly winds, therefore they contributed with huge volumes of tephras to the regions located
to the east, which were partially accumulated in the Llancanelo Lake depression.
Results
The lake substratum and surroundings are mainly formed by basaltic lava flows. Geophysical surveys
(geoelectric and electromagnetic induction) performed on the lacustrine plains and beyond in a W-E
transect in the middle part of the lake, detected both very high and very low resistivity layers, which
were respectively associated to unconsolidated sedimentary sequences and basaltic rocks. In the
western lacustrine plain, the basaltic substratum crops out several km to the W of the shoreline and
progressively dips to the E reaching depths of at least 30 m at the lake´s shore. In the eastern lacustrine
plain the basaltic substratum (found with geoelectric soundings at depths deeper than 100 m) rapidly
rises to the E in a stepping geometry, and crops out close to the eastern lake shoreline. This stepping
and rapid decreasing in depth of the substratum is considered as an evidence of the Llancanelo fault,
which is a major N-S-trending normal fault that marks the eastern tectonic boundary of the basin
(Ramos and Folguera, 2005; among others). The geoelectric surveys also allowed to recognize layers
of low resistivity sequences indicative of soft, probably lacustrine sediments, interbedded between
high resistivity layers associated to basaltic flows, what preliminary indicates that the lake evolution
could have been sometimes interrupted by volcanic episodes.
The uppermost subsoil sedimentary sequences were recognized along the W-E transect by both
shallow geophysical surveys and drillings. Mainly lacustrine, paludal and eolian (most of the times
volcaniclastic) sediments including levels of tephras, paleosoils and evaporites (gypsum) constitute
the sequences. Sedimentological and microfaunistical analysis reveal the larger extension of the lake
in the past as well as fluctuations in its size and depth. Volcanism was defined as a major conditioning
factor in the regional evolution. Besides the evident surface records of volcanic activity, main
subsurface volcanic evidence are represented by geoelectrical/sedimentological anomalies
preliminary hypothesized as representing buried volcanic edifices (possible maars and diatremes). On
the other hand, at least one discrete although highly significant ash layer was identified at the top of
the sedimentary sequences at a regional level, which is the product of the Quizapú volcano eruption
occurred in 1932.
Conclusions
The evolution of the region was conditioned by tectonic, volcanic and climatic factors that acted
together, although three main stages dominated by each of them can be synthesized.
Stage 1: Tectonic (Pre-Pliocene): Prior to the Pliocene, a complex tectonic history dominated, which
varied from extensional phases to contractional deformation, development of the Malargüe fold and
thrust belt and subsidence of the Rio Grande foreland basin (Ramos and Folguera, 2005). Faulting is
evidenced by the Llancanelo fault in the eastern margin of the lake.
Stage 2: Volcanic (Pliocene / Early Quaternary): Payenia volcanism has been active since the middle
Pliocene, around 3.4 Ma ago, with significant reactivations at 1.7 and 1.2 Ma. In the late Pleistocene
(after 450 Ka) several large volcanic events that gave origin to some of the more extensive lava flows
took place (Germa et al., 2007; Quidellieur et al., 2008; Pasquaré et al., 2008).
Stage 3: Climatic (late Pleistocene / Holocene): During these times, climatic changes were the most
important factor involved in the regional evolution, associated to alternating cold (glacial) and warm
(interglacial) periods related to the large Quaternary glaciations. However, no evidences of glacial
activity in the study area have been found up to the present. Despite the relative predominance of
climate variability, volcanism was still somehow relevant as evidenced by some records of volcanic
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events, like that occurred around 7 Ka (Germa et al., 2007; Quidellieur et al., 2008).
REFERENCES
• Bermúdez, A., Delpino, D., Frey, F. and Saal, A. (1993). Los basaltos de retroarco extraandinos. 12º Congreso Geológico Argentino
and 21º Congreso de Exploración de Hidrocarburos, Mendoza. Relatorio: 161–172.
• Delpino, D.H. (1993). Fue el sur Mendocino similar a Hawaii? Evidencias del Pasado para entender el Presente. Primeras Jornadas
Nacionales de Vulcanismo, Medio Ambiente y Defensa Civil, Asociación Geológica de Mendoza-Subsecretaría de Ciencia y
Técnica, Malargüe (1992): 67–77.
• Dieguez, S., De Francesco, C., Páez, M., Navarro, D., Quintana, F., Guerci, A., Zárate, M., Giardina, M., Neme, G. and Gil, A.
(2004). Paleoambiente y ocupación humana en el valle del río Atuel: trabajos recientes. Resúmenes del 15º Congreso Nacional de
Arqueología Argentina (Río Cuarto, Córdoba): 39–49.
• Germa, A., Quidellieur, X, Gillot, P.Y. and Tchilinguirian, P. (2007). Volcanic evolution of the back-arc complex of Payun Matru
(Argentina) and its geodynamic implications for caldera forming eruption in a complex-slab geometry setting. IUGG, Perugia.
Abstract: 10028.
• Gil, A., Zarate, M. and Neme, G. (2005). Mid-Holocene paleoenvironments and the archeological record of southern Mendoza,
Argentina. Quaternary Internacional, 132: 81-94.
• Groeber, P. (1939). Informe Geológico sobre la Zona de Embalse del Proyectado Dique en Nihuil (Provincia de Mendoza).
Dirección de Minas y Geología, 53 p.
• Guerci, A., Paez, M., Dieguez, S., De Francesco, C., Gil, A., Neme, G. and Polimeni, M. (2006). Estudios paleoambientales al sur
del río Atuel durante el Holoceno. Resúmenes del 13er. Simposio Argentino de Paleobotánica y Palinología (Bahía Blanca): 126135.
• Ostera, H. and Dapeña, C. (2003). Environmental isotopes and geochemistry of Bañado Carilauquen, Mendoza, Argentina. In: IV
South American Symposium on Isotope, Geology, Short Papers: 461-464.
• Pasquaré, G., Bistacchi, A., Francalanci, L., Bertotto, G.W., Boari, E., Massironi, M. and Rossotti, A. (2008). Very long Pahoehoe
inflated basaltic lava flows in the Payenia volcanic province (Mendoza and La Pampa, Argentina). Asociación Geológica Argentina,
63: 131-149.
• Quidellieur X., Carlut, J., Tchilinguirian, P., Germa A. and Gillot, P.Y. (2008). Paleomagnetic directions from mid-latitudes sites in
the southern hemisphere (Argentina): Contributions to Time Averaged Field models. Physics of the Earth and Planetary Interiors,
172 (3-4): 199-209.
• Ramos, V.A. and Folguera, A. (2005). Los Andes Australes: una evolución tectónica excepcional entre el sur de Mendoza y el Norte
de Neuquén. In: VI Congreso de Exploración y Desarrollo de Hidrocarburos, Mar del Plata, Actas: 75-83.
• Risso, C., Nemeth, K., Combina, A.M., Nullo, F. and Drosina, M. (2008). The role of phreatomagmatism in a Plio-Pleistocene highdensity scoria cone field: Llancanelo Volcanic Field (Mendoza) Argentina. Journal of Volcanology and Geothermal Research, 169:
61–86.
ROCK-MAGNETISM CHARACTERIZATION OF A LATE QUATERNARY
SOIL HORIZON (SAN SEBASTIÁN BAY, ISLA GRANDE OF TIERRA DEL FUEGO)
6-11
Walther, A.M.1-2*, Raposo, M.I.B.3, Vilas, J.F.1-2
(1) Departamento de Ciencias Geológicas Fac. Cs. Exactas y Naturales, Universidad de Buenos
Aires. Ciudad Universitaria Pab II, 1428 Buenos Aires
(2) Consejo Nacional de Investigaciones Científicas y Técnicas, Instituto de Geofísica Daniel
Valencio (INGEODAV) Depto. de Ciencias Geológicas FCEN UBA
(3) Laboratorio de Anisotropía Magnéticas y Magnetismo de Roca del Instituto de Geociencias de la
Universidad de San Pablo
* Presenting author’s e-mail: walther@gl.fcen.uba.ar
Introduction
An environmental magnetism study was performed on Quaternary sediments from a pedogenetic
horizon that lies on the lateral moraines of the Río Cullen glaciation (Cabo Vírgenes drift) and of the
San Sebastián glaciation (Punta Delgada drift), in San Sebastián Bay, Tierra del Fuego, Argentina
Rabassa et al. (2005).
The main objectives of the study were two-fold: (1) to perform a detailed determination of the nondirectional magnetic parameters in the analyzed profile sediments; (2) to perform a comparison of the
recorded magnetic signal against previous studies carried on paleosoils from the Pampean plain
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developed on sediments of different origins and
ages.
Fig. 1- Location Map in San Sebastián Bay, Tierra del
Fuego, Argentina.
Stratigraphy
In the San Sebastián Bay area Fig 1, located north
of Río Grande, two very well-defined alignments
of moraines outcrop on both margins of the
depression formed by Inútil and San Sebastian
bays, corresponding to the Punta Delgada and
Cabo Virgenes drifts Coronato et al. (2004). A
pedogenic horizon aged post-main Pleistocene
develops above both drifts. This horizon is a
sedimentary sequence of variable thickness of
1.15 m in the studied area. Four informal
sedimentary units were differentiated through a
macroscopic description of the profile and the
corresponding sampling from base to top.
Unit A (Fig. 2) lies unconformably on a gravel
and coarse sands deposit, presenting a thickness
of 0.52m. It is a poorly sorted silt-sandy
sediment, friable, which presents some poorlydefined prismatic structures (samples 1-4). The
upper limit is transitional.
Unit B (Fig. 2) has a thickness of 0.82 m, is friable and is constituted by light brown-colored argillaceous
silts. It presents prismatic block structure with abundant organic matter, and root molds and cutans are
common. Root molds appear coated with argillo and ferrocutans. Edaphic processes intensify towards the
unit top (samples 5-8). The upper limit is transitional.
Unit C (Fig. 2), with a thickness of 0.36 m, is composed of light brown-gray silty argillous sandy sediments,
unconsolidated, with abundant root molds. The upper limit is clearer and smooth (samples 9-14).
This unit is overlaid with dark grey
silt-sandy sediments where the
present soil is developing.
Fig. 2 - Geological profile (left side) and susceptibility, Ms and Mr
values as a function of depth
Rock magnetism study
Weather influence is a first order
factor in the formation of a soil.
Although the magnetic minerals
constitute a minor fraction within
the rocks and soils, their
sensibility to chemical changes
make them excellent detectors of
environmental changes (Verusob,
1995).
Fourteen successive levels were
sampled from the base of
sedimentary unit A to the top of
sedimentary unit C. The distance
between levels is 10 to 15 cm.
Bulk magnetic susceptibility
measurements at two frequencies,
hysteresis loops and isothermal
remanent magnetization (IRM)
acquisition experiments were
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performed.
Results
The interpretation of rock magnetism
parameters provides information
about the concentration, grain size
and magnetic mineralogy of the
analyzed sedimentary material.
Initial specific magnetic susceptibility
(X) was measured in low and high
frequencies (470 Hz and 4700 Hz),
recording values of XfD% (susc 470 Hz susc 4.700 Hz / susc 470 Hz) lower than
5% in all samples. These suggest the
existence of a non significant
superparamagnetic (SP) fraction in
the sequence.
Fig. 2 represents the fluctuations of
total magnetic X in two frequencies,
as function of the stratigraphic
position of the analyzed samples
throughout the profile. The values
for this parameter oscillate between
0.4 and 1.2 x 10-6 m3/kg.
A significant feature observed in
Fig. 3 - Magnetic grain size characteristics from hysteresis properties
of the samples shown in a modified bivariate Day plot. (Day et al.,
the X’s profile is the presence of a
1977; Dunlop, 2002).
decreasing tendency from base to
top in unit B, which intensifies in
unit C. The observed minimum X corresponds with the more edaphic sectors of the studied units.
Saturation magnetization (Ms) and remanent saturation magnetization (Mrs) values from hysteresis
loop (Fig. 2) show that both parameters present a decrease in the samples from unit C. This behavior,
similar in the three parameters, can be attributed to a lower amount of ferrimagnetic minerals.
Through the analysis of variations in the parameters of coercivity (Hc) and remanence coercivity
(Hcr) it was found that Hc values increases in unit C, while Hcr values increase in units B and C.
The fluctuations of Hc and Hcr in the studied material are those expected for magnetite and/or
titanomagnetite Dekkers (1988); Roberts et al. (1995). Hysteresis loops and IRM acquisition curves,
performed up to 1 T fields, display different characteristics, which can be divided in two groups:
Those that form very narrow loop, typical of minerals with low coercivity and multidomain (MD) or
pseudo simple domain (PSD) magnetic grain size. This behavior appears in levels 1 to 11.
Loop corresponding to minerals with low coercivity and elevated content of paramagnetic minerals,
belonging to samples 12, 13 and 14.
The IRM curves done up to 1 T present a typical behavior of low coercivity minerals such as
magnetite and titanomagnetite, which saturate at less than 300 mT. Neither the hysteresis loops nor
the IRM curves detect the presence of antiferrimagnetic minerals in the edaphic levels of the
sequence.
Since the ferrimagnetic minerals are dominant in the sequence, it would be valid to use the Mrs/Ms
vs. Hcr/Hc ratios in the graph by Day et al. (1977) modified by Dunlop (2002) for a magnetite
unimodal aggregate. Fig. 3 displays the Mrs/Ms vs. Hcr/Hc relations. It can be observed that the
corresponding values fall predominantly in the pseudo single domain (PSD) field.
Discussion
The presence of organic matter, molds coated by cutans and prismatic structures observed throughout
the profile and more intensely in unit C, indicate the action of edaphic processes which would be
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linked to the existence of a benign (temperate???) climate which would have allowed the development
of vegetation.
The decrease of magnetic parameters (X, Ms and Mrs) in the profile can be attributed in the first
instance to a process of gradual decrease of the amount of detritic ferrimagnetic particles, and in the
second instance to a mineralogical change.
The decrease of these parameters in the profile matches the intensification of pedogenetic features
observed in the field. The rise in edaphic activity at these levels would be associated to the fall in the
amount of ferrimagnetic particles and mineralogical changes on the surface of the magnetic grain.
Bidegain et al. (2001) arrive to similar conclusions regarding the magnetic behavior of loessical
sediments studied near the La Plata city (Buenos Aires, Argentina), suggesting “dissolution” of
magnetite as a consequence of the weathering phenomena. On the other hand, the presence of high
coercivity magnetic minerals is not detected by the analysis of hysteresis cycles. Then, the pedogenetic
processes that affected the sequence studied here from the point of view of magnetic behavior could
be reflecting a decline of the detritic ferrimagnetic fraction (magnetite and/or titanomagnetite) in the
paleosol.
The registered behavior in the analyzed paleosol is similar to the one occurring in the Pampean plain.
Moreover, there is no detection of neoformation of high coercivity minerals (hematite and/or goethite)
or of low coercivity ones (ultra fine magnetite), a very conspicuous process in the Pampean plain
paleosols developed under weathers with a pronounced dry season (Walther et al., 2004). Although
the average temperatures during the development of this paleosol and the Pampean plain ones are
different, the benign climatic events that generated them should have affected the magnetic minerals
in the same way.
The glacial advance of Cabo Virgenes drift, occurred during Stage 12 and the Punta Delgada drift
advance, happened during Stage 10, are prior to this unit’s deposition. In the study area, the next
glacial advance is the Primera Angostura drift, which was estimated to have occurred during Stage 6.
Therefore it is probable that these sediments were deposited during this period or later, and then were
edaphized during the warmer Isotope Stage 5 (125 ky-75 ky B.P.), during the substage 5e (the warmest
and longest), or after it.
Conclusions
The magnetic parameters in the studied paleosol indicate a depletion of ferromagnetic minerals in the
edaphic levels compared with the inferior levels. This loss would be due to the edaphic processes that
acted during the soil formation. Similar behavior was observed both in eolic sediments as well as in
fluvial sediments of different ages in the Pampean plain.
Neoformation of magnetite and/or maghemite (SP magnetic grain size) was not detected. There was
no manifestation of neoformation of antiferromagnetic minerals by dehydration of amorphous gels
created during edaphic-synthesis processes. The absence of these minerals is associated with periods
of warm weather, with no dry season.
This soil is probably reflecting the climate change occurred in the area during Isotope substage 5e.
REFERENCES
• Bidegain, J. C.and van Velzen, A. J.,Rico, Y.; 2001: Parámetros magnéticos en una secuencia de loess y paleosuelos del Cenozoico
tardío en la cantera de Gorina, La Plata: su relevancia en el estudio de los cambios paleoclimáticos y paleoambientales. Revista de
la Asociación geológica Argentina. 56(4): 503-516.
• Day, R., Fuller, M. and Schmidt, V. A., 1977: Hysteresis properties of titanomagnetites: grain-size and compositional dependence.
Physics of the Earth and Planetary Interiors, 13: 260-267.
• Dekkers, M. J., 1988: Some rock magnetic parameters for natural goethite, pyrrhotite and fine grained hematite. Geologica
Ultraiectina, N 51 Ph. D. Thesis, University of Utrecht, 231, p. Utrecht
• Coronato, A., Meglioli, A., and Rabassa, J. 2004: Glaciations in the Magellan Straits and Tierra del Fuego, southernmost South
America. Quaternary Glaciations- Extent and Chronology, Part III Editors Ehlers and P L Gibbard. Elsevier.
• Dunlop, D. J., 2002. Theory and application of the Day plot (Mrs/Ms versus Hcr/Hc) 2.Aplication to data for rocks, sediments, and
soils. Journal of Geophysical Research vol 107 (B3) 10.1029-2001 JB000487
• Rabassa, J., Coronato, A.M., and Salemme, M. 2005: Chronology of the late Cenozoic Patagonian glaciations and their correlation
with biostratigraphic units of the Pampean region (Argentina).Journal of South American Earth Sciences 20 81-103 Elsevier
• Roberts, A. P, Yulong Cui Y., and Verosub, K.L., 1995 : Wasp-waisted hysteresis loops: Mineral magnetic characteristics and
discrimination of components in mixed magnetic systems. Journal of Geophysical Research, vol. 1000. N0 B9 17909-17924.
Washington.
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• Verosub, K. L. and Roberts, A. P. 1995: Environmental magnetism: Past, present, and future. Journals Geophysics Research 100:
2175-2192.
• Walther, A. M., Orgeira, M. J., and Lippai H. F., 2004: Magnetismo de rocas en sedimentos cenozoicos tardíos en San Antonio de
Areco provincia de Buenos Aires. Revista de la Asociación Geológica Argentina Vol. 59 (3): 433-442.
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Session 7
MARINE GEOLOGY
AND GEOPHYSICS
GEOSUR2010
CHARACTERIZATION OF THE MAGNETIC RESPONSE OF
7-01
THE NORTHERN ARGENTINE CONTINENTAL MARGIN (SOUTH ATLANTIC OCEAN)
Abraham, D.A.1,2*, Ghidella, M.3, Paterlini, M.1,4, Schreckenberger, B.5
(1) COPLA. Comisión Nacional para la Determinación del Límite Exterior Argentino
(2) Escuela de Ciencias del Mar, Instituto Universitario Naval, Argentina
(3) Instituto Antártico Argentino, Argentina
(4) Servicio de Hidrografía Naval, Argentina
(5) Federal Institute for Geosciences and Natural Resources (BGR), Germany
* Presenting author's e-mail: abrahdaniel@gmail.com
Introduction
We have compiled a magnetic anomaly grid by using several data sets collected along the Argentine
continental margin in the sector comprised between 35º S and 48º S. We have calculated the anomaly
field’s numerical curvature along specific directions. The results were then overlaid on the original
anomaly grid with a transparency factor, and this resulted in the enhancement of the magnetic
alignments and the improvement of their outline, thus allowing a better mapping of these features. We
have identified both numerous anomaly alignments and their fragmented character. We have also
improved the definition of magnetic provinces as a result of a further qualitative analysis. These
results were correlated with the proposed segmentation of this sector of the Argentine continental
margin at the time of the ocean opening, as asserted by several authors.
Margin structure
The sector under study is a typical extensional volcanic margin. One of the main characteristics of this
type of margin is the presence of volcanic wedges, recognized from multichannel seismic reflection
as seaward dipping reflector sequences (SDRS). They are mainly represented by buried basaltic flows.
Based on seismic data analysis, Franke et al. (2007) have identified four segments in the Argentine
margin, being their boundaries interpreted as transfer zones. These are: the Malvinas, Colorado,
Ventana, and Salado transfers. The main criterion to map the abovementioned segments was the
presence of great lateral displacements in the seismic reflector wedges as well as abrupt changes in
their architecture. The transfer zones may represent old weakness areas which date from the beginning
of the oceanic opening in the Late Cretaceous.
The first margin segment (segment I), located between the Malvinas fracture zone and the Colorado
transfer, comprises the transition between the sheared margin and the typical volcanic margin.
Segment II is between the Colorado transfer and the Ventana transfer. The Colorado transfer area is
characterized by a first-order magnetic anomaly (Ghidella et al., 1995). This prominent anomaly was
interpreted as the limit between the Rio de la Plata and the Patagonian cratons (Max et al., 1999). The
well developed SDRS disappear south of the magnetic discontinuity. Segment III corresponds to a
volcanic margin with multiple seismic reflector wedges, and Segment IV is the area to the north of
the Salado transfer (Franke et al., 2007). We can see those segments in Figs. 3 and 4.
Magnetic anomalies
The Colorado discontinuity divides two sectors, north and south, with different magnetic responses.
The northern sector belongs to the extensional margin of the Río de la Plata craton. The southern one
extends until the Malvinas fracture zone. This is the extensional Patagonian continental margin.
From the coastline to the east, the predominant trend of the magnetic anomalies becomes parallel to
the continental slope. Here there are significant SW-NE alignments attributed to the volcanic activity
when the margin was formed.
South of the Colorado Basin there is a highly intense magnetic anomaly (Tona anomaly) that
disappears abruptly at the Colorado discontinuity. The magnetized bodies that are sources for this
anomaly do not show important density contrasts in the free air anomaly map. According to Max et
al. (1999), the Tona sources seem to be too deep to correspond to volcanic intrusions and too shallow
to be represented by basaltic underplating.
Along the Atlantic volcanic margins in general and in the Argentine margin in particular there are large
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Fig. 1 - Data coverage. Survey track lines over a
bathymetric grid.
Fig. 2 - Magnetic provinces over the anomaly magnetic
model. G: G anomaly area. G1: magnetic province shifted
eastward with respect to anomaly G. CO_M: Oceanic region
where the M series magnetic alignments can be identified.
CO_M1: here there are oceanic magnetic alignments, but not
as sharp as in CO_M. LPD: Río de la Plata craton domain.
ZQ: Cretaceous quiet magnetic zone. PMPP: magnetic
province of the Patagonian shelf. ZP: porphyry zone.
deep igneous bodies located in the lower crust just above the Moho discontinuity that have been detected
and recognized by seismic refraction experiments (“high velocity bodies”). In the Argentine margin those
bodies appear below the volcanic wedges (Neben et al., 2002); they are also present in the Namibia margin.
The magnetic response of those lower crustal bodies is not clear. They are very deep in transitional or
oceanic crust and their temperatures may be above the Curie point. Furthermore two dimensional magnetic
modeling (Ghidella, et al, 2005, http://dna.gov.ar/mararg/pictr2002/Interpretacion/tona/index.html) can
reproduce the anomalies without including such a body.
The prominent anomaly G is an isostatic gravity anomaly with associated magnetic expression which was
determined using magnetic and gravity data and interpreted as marking the transition between oceanic and
continental crust (Rabinowitz and LaBrecque, 1979). The G anomaly is related to the presence of SDRS
(Hinz et al., 1999).
With the available magnetic data (Fig. 1) we developed a model for total field anomalies where magnetic
provinces are distinguished (Fig. 2). Areas not surveyed were completed with the WDMAM (World Digital
Magnetic Anomaly Map) compilation (http://ftp.gtk.fi/WDMAM2007/WDMAM_1.0_DVD_2007_Edition/).
We distinguish four areas north of the Colorado discontinuity:
G: G anomaly area. This zone reflects the SDRS magnetic expression. The zone exhibits
fragmentations, some of which correspond to the transfer zones determined by Franke et al. (2002,
2007). The variable width of the segments becoming narrow northward supports the inference of a
diachronic opening of the margin from south to north, posed by several authors. The western edge of
this zone largely corresponds to the 2000 m isobath.
CO_M: This is the oceanic region where the M series magnetic alignments can be identified. In order
to improve their definition, we resorted to the numerical curvature of the total anomaly field with
which we performed a semi-transparent mask and overlaid it on the anomaly map. The anomaly field
curvature was calculated according to its definition:
K=
d 2 ÄT
dl 2
3
⎛ ⎛ dÄT ⎞ 2 ⎞ 2
⎜ 1+ ⎜
⎟ ⎟
⎝ ⎝ dl ⎠ ⎠
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Fig. 3 - Magnetic anomaly map with the curvature
image superimposed with transparency. Magnetic
alignments are enhanced.
Fig. 4 - Magnetic provinces. Sea-floor magnetic
lineaments in the CO_M and CO_M1 zones. Magnetic
discontinuities. Transfer zones from Franke et al. (2007).
Where:
K: curvature of the magnetic anomaly field calculated in a specific direction.
¢T: magnetic anomaly field.
The first and second derivatives are directional. This direction can be characterized by an angle ·,
which is the angle between the E-W direction and the derivative direction measured counterclockwise. The expression of the parameterized curvature is as follows:
K=
∂2 ÄT
∂2 ÄT
∂2 ÄT 2
2
sin á
cos
sin
+
cos
+2
á
á
á
∂x 2
∂x ∂y
∂y 2
3
2
⎛ ⎛ ∂ÄT
⎞ ⎞2
∂ÄT
sin á ⎟ ⎟
cos á +
⎜ 1+ ⎜
⎠ ⎠
∂y
⎝ ⎝ ∂x
We found that an angular parameter of 40° highlighted the seafloor magnetic lineaments, identified
from M0r to M5n (Fig. 3), according to the Gradstein et al. (2004) Reversal Polarity Time Scale.
LPD: Río de la Plata craton domain which extends from anomaly G to the coastline. The Tona
anomaly stands out with its prominent amplitude and three-dimensional morphology.
ZQ: Cretaceous quiet magnetic zone.
From the Colorado discontinuity to the Malvinas transfer zone, extends the volcanic passive
Patagonian margin. A flat low-amplitude anomaly field characterizes the magnetic province of the
Patagonian shelf (PMPP). To the west of this area next to the coastline we identified a porphyry zone
(ZP), with a high-frequency and medium-amplitude magnetic response. This region has no alignments
in any predominant direction. Area G1, centered at about 46º S and 58° W, is a magnetic province
shifted eastward with respect to anomaly G. This offset discontinuity occurs in the Colorado transfer
and may be interpreted as an early continuation of the anomaly G zone southward. Adjacent to this
area, we identified the CO_M1, centered on 47º S and 56º W, where there are oceanic magnetic
alignments, but not as sharp as in CO_M. The lineaments observed here have a predominant N-S
direction. We have identified magnetic alignment fragmentations in the G area and between CO_M1
and CO_M zones partially corresponding to the transfer zones identified by Franke et al. (2007).
We note that the Colorado discontinuity plotted on the magnetic anomaly map is similar to the
Colorado transfer as determined from seismic work, although there are some differences. The abrupt
vanishing of the anomalies on this line probably marks the end of their causative bodies, thus defining
a first order discontinuity, although the geological history of their emplacement cannot be explained
from the magnetic point of view only. Further magneto-gravimetric modeling, based on the geometry
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imaged by seismic data, may improve significantly the geological knowledge of this margin.
Conclusions
This paper attempts to contribute to the knowledge of the Argentine continental margin, based on
magnetic data interpretation. We have developed a method that improves the definition of structural
alignments in the oceanic crust. We have also characterized extensional margin volcanic provinces and
identified magnetic alignments although the classification of those areas does not allow a univocal
determination of the intra-basement bodies that produce the magnetic signal.
REFERENCES
• Franke, D., Neben, S., Hinz, K., Meyer, H., and Schreckenberger, B., 2002. Deep crustal structure of the Argentine continental
margin from seismic wide-angle and multichannel reflection seismic data. AAPG Hedberg Conference, Hydrocarbon Habitat of
Volcanic Rifted Passive Margins, September 8-11, 2002, Stavanger, Norway.
• Franke, D., Neben, S., Ladage, S., Schreckenberger, B. and Hinz, K., 2007. Margin segmentation and volcano-tectonic architecture
along the volcanic margin off Argentina/Uruguay, South Atlantic. Marine Geology 244, 46-67.
• Ghidella, M.E., Paterlini, M., Kovacs, L.C. and Rodríguez, G., 1995. Magnetic anomalies on the Argentina Continental Shelf. 4th
International Congress of the Brazilian Geophysical Society and 1st Latin American Geophysical Conference. Expanded abstracts,
269–272, Río de Janeiro.
• Gradstein, F.M., Ogg, J.G., Smith, A.G., Agterberg, F.P., Bleeker, W., Cooper, R.A., Davydov, V., Gibbard, P., Hinnov, L.A., House,
M.R., Lourens, L., Luterbacher, H.P., McArthur, J., Melchin, M.J., Robb, L.J., Shergold, J. and Villeneuve, 2004. A Geologic Time
Scale 2004. Cambridge University Press, 589.
• Max, M.D., Ghidella, M., Kovacs, L., Paterlini, M. and Valladares, J.A., 1999. Geology of the Argentine continental shelf and margin
from aeromagnetic survey. Marine and Petroleum Geology 16, 41–64
• Neben, S., Franke, D., Hinz, K., Schreckenberger, B., Meyer, H. and Roeser, H.A., 2002. Early Opening of the South Atlantic: PreRift Extension and Episodicity of Seaward Dipping Reflector Sequence (SDRS) Emplacement on the Conjugate Argentine and
Namibia Continental Margins. AAPG Hedberg Conference, Hydrocarbon Habitat of Volcanic Rifted Passive Margins, September
8-11, 2002, Stavanger, Norway. American Association of Petroleum Geologists.
LINKING SEAFLOOR MORPHOLOGY, HYDROSEDIMENTARY PROCESSES
AND LIVING RESOURCES IN SUBMARINE CANYONS OF
THE NW MEDITERRANEAN SEA: A UNIQUE STUDY CASE
7-02
Canals, M.*
GRC Geociències Marines - Universitat de Barcelona
* Presenting author's e-mail: miquelcanals@ub.edu
Several submarine canyons deeply dissect the continental shelf and slope of the Gulf of Lion and the
northern Catalan margin in the NW Mediterranean Sea. These canyons are efficient conduits from the
continental shelf to the deep basin, as they are able to rapidly transport large amounts of dense water,
sedimentary particles, organic matter and pollutants when specific environmental conditions occur.
Two main modes of massive transport events from shallow to deep have been identified in the last few
years: i) Dense Shelf Water Cascading (DSWC) that is triggered by persistent, cold and dry northerly
winds blowing mainly in winter months, and ii) highly energetic coastal storms that may occur at any
time during the year. The occurrence of DSWC is further favoured when low river discharge has been
low during the previous months, as this diminishes the buoyancy of upper waters that, therefore, may
easily reach the critical densities required to initiate cascading flows. Near-bottom speeds of 1 m s-1
and sand contents as high as 80% have been measured in situ during cascading episodes. Furthermore,
canyon floors and flanks display large-scale furrows that are attributed to the abrading effect of the
sand load involved in cascading events. These canyons are preferred habitats for cold-water corals in
the warm Mediterranean Sea. It has been also demonstrated that DSWC causes the temporary collapse
of the most valuable fishery in the area, the rose shrimp Aristeus antennatus one, which was
previously attributed to overfishing and pollution.
Highly energetic coastal storms erode the beaches and sand bodies on the inner shelf therefore easing
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the trapping of coarse sediment into canyon heads. Furthermore, such storms create a generalised
resuspension of fine sediment over most of the continental shelf, which is subsequently exported to
other areas by currents and down canyon flows. A succession of large coastal storms occurred from
December 2008 to March 2009 for which the most outstanding in situ observations will be presented.
High concentrations of organic pollutants have been found in both deep slope and rise sediments and
in organisms living there, which is likely related to transfers from shallow to deep due to DSWC and
large coastal storms. The occurrence and impact of similar processes in other margin areas, such as
the Patagonian margin, is certainly worth investigating.
MORPHOSTRUCTURE OF THE WESTERN SECTOR OF
THE NORTH SCOTIA RIDGE
7-03
Esteban, F.D.1*, Tassone, A.1, Lodolo, E.2, Menichetti, M.3
Universidad de Buenos Aires. Argentina
(2) Istituto Nazionale di Oceanografia e di Geofisica Sperimentale (OGS) – Trieste. IItaly
(3) Istituto di Scienze della Terra, Università di Urbino – Campus Universitario. Urbino. Italy
* Presenting author’s email: esteban@gl.fcen.uba.ar
Introduction
The North Scotia Ridge (NSR) is the submerged morpho-structural expression of the Scotia plate
northern edge. It is constituted by the Tierra del Fuego continental margin, isla de Los Estados,
Burdwood (BB), Davis and Aurora banks, and the Georgias islands shelf (Parker et al., 1996; Barker,
2001; Giner-Robles et al., 2003; Pandey et al., 2010). About 40 Ma these blocks were grouped
forming a continental link between Tierra del Fuego and Antarctica. Afterwards, with the development
of the Scotia plate, the blocks drifted towards the east to their actual position (Barker, 2001; Pandey
et al., 2010). Several authors have established that the actual movement of the South America - Scotia
plate boundary is left-lateral (Forsyth, 1975; Pelayo and Wiens, 1989; Giner-Robles et al., 2003;
Thomas et al., 2003; Smalley et al., 2007). In the Tierra del Fuego region, the plate boundary is
represented by a mostly transtensional fault system known as Magallanes-Fagnano (Lodolo et al.,
2002, 2003, 2006, Tassone et al., 2008; Menichetti et al., 2008). Towards east, the boundary is located
in the Malvinas trough, at the north of the BB and it would be transpressive (Cunningham et al., 1998;
Giner-Robles et al., 2003; Bry et al., 2004). The change of the tectonic regime (transtensional to the
W to transpressional to the E) would occur at 63.5 ºW (Lodolo et al., 2003; Yagupsky et al., 2003).
As part of a study of the evolution of the SW Atlantic continental margin, we analyze and describe the
morpho-structure of the western sector of the North Scotia Ridge (Figs. 1 and 2).
Sources and methods
One hundred sixty eight unpublished multi-channel seismic lines were integrated with seismic
sections taken from literature (Platt and Philip, 1995; Yagupsky et al., 2003; Bry et al, 2004; Lodolo
et al., 2006; Tassone et al., 2008). In addition, single-channel seismic lines available on the web
(GeoMapApp, GeoDas), bathymetric (GEBCO) and gravimetric data (Sandwell and Smith, 2009)
were also used.
The metodology consisted in the recognition of the acoustic basement in the seismic lines. Then, the
points of the top of the acoustic basement between 0.5 and 3.5 seconds two-way travel time (twtt)
every 0.5 seconds were drawn in the map. The points with the same twtt were connected in lines
(isobaths). The bathymetric data was used to assist the interpolation between lines, specially when the
distance between two nearest lines was considerable.The lines were interpreted by using the Kingdom
software suite (version 8.2). The lines taken from the published data and from the Web (in image
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Fig. 1 - A) Map with the location of the main tectonic plates. B) Bathymetric map of the studied region with the
contours lines of the gravimetric anomalies every 50 mGal (dotted lines), and the twt isobaths of the acoustic basement
every 0.5 seconds (solid lines). Plate boundary (segmented lines) from PLATES Project
(www.ig.utexas.edu/research/projects/plates/). A-B segment indicates the location of the seismic section of Figure 2.
Used abbreviations: AB, Austral (Magallanes) Basin; BB, Burdwood Bank; EFZ, Endurance Fracture Zone; IE, isla de
Los Estados; MB, Malvinas Basin; MT, Malvinas Trough; QFZ, Quest Fracture Zone; SAM, South American plate;
SCO, Scotia plate; SMB, South Malvinas Basin; TdF, Isla Grande de Tierra del Fuego.
format) were converted into Segy format with Image2segy software (Farran, 2006), which runs under
MatLab. The seismostratigraphic units recognized in the foredeep (Fig. 2) were obtained by
correlating seismic lines and published results (Yagupsky et al., 2003; Tassone et al., 2008).
Western edge of the North Scotia Ridge. Geophysical interpretation.
In Fig. 1, the distribution of the isobaths of the top of the basement clearly defines the structural highs
that constitute the NSR. These highs are oriented WSW-ENE to W-E (with a break at ~62 °W), and
have sharp limits as indicated by the proximity of the isobaths. This is particularly evident between 60
°W and 66 °W, where these highs border the Austral and Malvinas basins. More gentle slopes are
located in the extreme eastern of the BB. The 0.5 seconds isobaths, unlike the others, allow
recognizing two units. The first, located to the east, extends from the isla Grande de Tierra del Fuego
to the isla de Los Estados. The other unit, located to the west, corresponds to the BB.
In the seismic lines over the structural highs of the NSR a notable contrast in the sedimentation infill
has been recongnized (Fig. 2, see also Fig. 2 of Kimbell and Richards, 2008). Over the southern flank
of these structural highs, the sedimentation is very reduced. This can be visualized by the good
correlation between the isobaths and the bathymetry (Fig. 1). On the other hand, the important
sedimentary infill of the Austral and Malvinas basins (Galeazzi, 1996; Tassone et al., 2008) has
partially covered the northern flank of these structural highs (Fig. 2). This can be seen specially near
the northern shore of isla de Los Estados where there is no correlation between the isobaths and the
bathymetry (Fig. 1).
The gravimetric anomalies (Fig. 1; Smith and Sandwell, 2009) are distributed along E-W-trending
belts and range from -150 mGal to 150 mGal. The distribution is characterized by a positive belt over
the southern edge of the NSR, and a parallel negative belt which coincides with the Malvinas Trough
and the northern edge of the NSR. In the eastern sector, over the BB, Bry et al. (2004) and Kimbell
and Richards (2008) correlated this high-low pattern with the overthrust of the BB over the South
America plate southern edge. The relative high level of the BB generates a positive anomaly, while
the downwards flexure of the South America plate explain the negative anomaly. Instead, offshore of
Tierra del Fuego, Lodolo et al. (2003) correlated the gravimetric minima with the sedimentary fill of
pull-apart basins (Tassone et al., 2008; Esteban et al., 2009) developed along the Magallanes-Fagnano
system fault.
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Fig. 2 - S-N multichannel seismic section. Location in Fig. 1. NSR: North Scotia Ridge. FD: Malvinas Foredeep.
Nomenclature of the seismic units recognized in the foredeep follows Yagupsky et al. (2003) and Tassone et al. (2008).
The N-S seismic section in Fig. 2 is located in the central part (~62 ºW) of the studied area (Fig. 1).
In general, the bathymetry is flat (less than 600 m), except for the southern edge, where there is a steep
slope (up to 2000 m). In the seismic section, two sectors can be distinguished. The first, from the SPs
16800 to the north, is characterized by a shallow acoustic basement (less than 2 s twt), a minimal
sedimentary fill, strong multiples and numerous vertical faults (some exceed 2 s twt). The basement
is seismically homogeneous, with poor lateral continuity and moderate amplitude, resulting in a
chaotic arrangement. Between SPs 18000 and 18500, a small basin limited by faults can be
recognized. To the north, levels of folded reflector can be seen and would correspond to the fold-andthrust belt (SPs 17500 to 17000; Lodolo et al., 2003; Bry et al., 2004; Fish, 2005; Tassone et al., 2008).
In contrast, the northern sector (SP 16800 to the south) corresponds to the Malvinas basin foredeep
and the important sedimentary fill observed was correlated with published seismic lines (Yagupsky et
al., 2003; Tassone et al, 2008). The five seismic units (units 2, 3, 4, 5a and 5b of Tassone et al., 2008)
span from Middle-Late Jurassic to Holocene, and include syn rift (unit 2), sag and foredeep (units 3
and 4), and foreland (5a and 5b) tectonic phases.
REFERENCES
• Barker, P.F., 2001. Scotia Sea regional tectonics evolution: Implications for mantle flow and paleocirculation. Earth Science
Reviews, 55, 1-39.
• Bry, M., White, N., Singh, S., England, R., Trowell, C., 2004. Anatomy and formation of oblique continental collision: South
Falkland basin. Tectonics, 23, TC4011, doi: 10.1029/2002TC001482.
• Cunningham, A.P., Barker, P.F., Tomlinson, J.S., 1998. Tectonics and sedimentary environment of the North Scotia Ridge region
revealed by side-scan sonar. Journal of Geological Society, 154, 849-862.
Esteban, F., Tassone, A., Menichetti, M., Lodolo, E., 2009. Morfoestructuras de las cuencas sedimentarias asociadas al límite de placa
Sudamérica-Scotia Atlántico SW. VII Jornadas Nacionales de Ciencias del Mar, Bahía Blanca 30 Nov. – 4 Dec. 2009, ISBN 978987-25479-0-5.
• Farran, M., 2006. IMAGE2SEGY: Una aplicación informática para la conversión de imágenes de perfiles sísmicos a ficheros en
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formato SEGY. Geo-Temas 10, pp. 1215-1218. ISSN: 1567-5172.
• Fish, P., 2005. Frontier South, East Falkland basins reveal important exploration potential. Oil and Gas Journal, 103, 34–40.
• Forsyth, D.W., 1975. Fault plane solutions and tectonics of the South Atlantic and Scotia Sea. Journal Geophysical Research. 80,
1429–1443.
• Galeazzi, J.S., 1996. Cuenca de Malvinas. Geología y Recursos Naturales de la Plataforma Continental Argentina, Ramos, V. A.,
Turic, M. A. (Eds.). XIII Congreso Geológico Argentino y III Congreso de Exploración de Hidrocarburos vol. 15: 273-309.
• GEBCO_08 Grid, version 20091120, http://www.gebco.net
• Giner-Robles, J.L., González-Casado, J.M., Gumiel, P., Martín-Velázquez, García-Cuevas, C. 2003. A kinematic model of the Scotia
plate (SW Atlantic Ocean). Journal of South America Earth Science, 16, 179-191.
• Lodolo, E., Menichetti, M., Tassone, A., Sterzai, P., Lippai, H., Hormaechea, J.L., 2002. Researchers Target a continental transform
Fault in Tierra del Fuego. EOS Transactions AGU, 83, 1, 1-6.
• Lodolo, E., Menichetti, M., Bartole, R., Ben-Avraham, Z. Tassone, A. y Lippai, H., 2003. Magallanes-Fagnano continental
transform fault, (Tierra del Fuego, southermost South America). Tectonics, 22(6): 1076, doi: 10.1029/2003TC001500.
• Lodolo, E., Donda, F., Tassone, A. 2006. Western Scotia Sea margins: improved constraints on the opening of the Drake Passage.
Journal Geophysical Research, 111, B06101, doi:10.1029/2006JB004361.
• Kimbell, G.S., Richards, P.C., 2008. The three-dimensional lithospheric structure of the Falkland Plateau region based on gravity
modelling. Journal of Geological Society, 165, 4, 795-806.
• Menichetti, M., Lodolo, E., Tassone, A., 2008. Structural geology of the Fuegian Andes and Magallanes fold-and-thrust belt – Tierra
del Fuego Island. Geologica Acta, 6, 1, 19-42.
• Pandey, A., Parson, L., Milton, A., 2010. Geochemistry of the Davis and Aurora Banks: Possible implications on evolution of the
North Scotia Ridge. Magine Geology, 26, 1-4, 106-114.
• Parker, G., Violante, R.A., Paterlini, M.C., 1996. Fisiografía de la plataforma continental. Geología y Recursos Naturales de la
Plataforma Continental Argentina, Ramos, V. A., Turic, M. A. (Eds.). XIII Congreso Geológico Argentino y III Congreso de
Exploración de Hidrocarburos vol. 15: 1-16..
• Pelayo, A. M., Wiens, D.A., 1989. Seismotectonics and relative plate motions in the Scotia Arc region, Journal Geophysical
Research, 94, 7293 – 7320.
• Platt, N.H., Phillip, P.R., 1995.Structure of the southern Falkland Islands continental shelf: Initial results from new seismic data.
Marine and Petroleum Geology. 12, 759-771.
• Sandwell, D.T., Smith, W.H.F, 2009. Global marine gravity from retracked Geosat and ERS-1 altimetry: Ridge segmentation versus
spreading rate. Journal of Geophysical Research, 114, B01411, doi:10.1029/2008JB006008.
• Smalley, R., Dalziel, I.W.D., Bevis, M.G., Kendrick E., Stamps, D.S., King, E.C., Taylor, F.W., Lauría, E., Zakrajsek, A., Parra, H.,
2007. Scotia arc kinematics from GPS geodesy, Geophysical Research Letters, 34, L21308, doi:10.1029/2007GL031699.
• Tassone, A., Lodolo, E., Menichetti, M., Yagupsky, D., Caffau, M., Vilas, J.F., 2008. Seismostratigraphic and structural setting of
the Malvinas Basin and its southern margin (Tierra del Fuego Atlantic offshore). Geologica Acta, 6, 1, 55-67.
• Thomas, C., Livermore, R., Pollitz, F., 2003. Motion of the Scotia Sea plates. Geophyical Journal International. 155, 789–804.
• Yagupsky, D., Tassone, A., Lodolo, E., Vilas, J.F., Lippai, H. 2003. Estudio sismoestratigráfico del sector sudoccidental de la cuenca
de antepaís de Malvinas. Margen continental atlántico. Argentina. 10° Congreso Geológico Chileno. 6-10 de octubre. Concepción.
Chile: 10 p. En CD de Actas del Congreso.
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GIANT MOUNDED DRIFTS IN THE ARGENTINE CONTINENTAL MARGIN
7-04
Hernández-Molina, F.J.1*, Paterlini, M.2, Somoza, L.3, Violante, R.2, Arecco, M.A.4,
De Isasi, M.4, Rebesco, M.5, Uenzelmann-Neben, G.6, Marshall, P.4
(1) Facultad de Ciencias del Mar, Universidad de Vigo, 36200 Vigo, Spain
(2) División Geología y Geofísica Marina, Servicio de Hidrografía Naval (SHN), Montes de Oca 2124,
Buenos Aires, C1270ABV, Argentina
(3) Instituto Geológico y Minero de España (IGME), c/ Ríos Rosas, 23, 28003 Madrid, Spain
(4) Argentine National Commission of the Outer Limit of the Continental Shelf (COPLA), Montes de
Oca 2124, Buenos Aires, C1270ABV, Argentina
(5) Istituto Nazionale di Oceanografia e di Geofisica Sperimentale (OGS), Borgo Grotta Gigante
42/C, 34010 Sgonico, Italy
(6) Alfred Wegener Institute (AWI), Foundation for Polar and Marine Research Geophysics, P.O. Box
12 01 61, Am Alten Hafen 26, 27515 Bremerhaven, Germany
* Presenting author’s e-mail: fjhernan@uvigo.es
Introduction
Whilst interaction between down-slope and along-slope sedimentary processes is common over
continental margins when along-slope processes dominate, a Contourite Depositional System (CDS)
may develop as an association of
various depositional features
(drifts) and erosive features
(Rebesco and Camerlenghi, 2008).
The interaction of one or more water
masses with a smooth-morphology
margin may cause large drifts, but a
complex physiography can create
multiple vortices associated with
each water mass, and both the
erosive and depositional features can
become difficult to decipher. Some
contourite deposits represent giant,
mounded elongated contourite drifts
(hereafter, giant drifts) extending
along large distances, such as those
described in the Weddell and Scotia
Basins, south-westernmost Indian
Ocean and Greenland margin
(Rebesco and Camerlenghi, 2008).
These giant drifts are generated
during a long period of relatively
stable hydrological conditions that
lead to long-term bottom water
flows. Onset of many of these drifts
is related to a gateway opening or
deepening, owing to long-term
plate-tectonic evolution, and/or
Fig. 1 - Location of the Argentine
Margin, with the regional bathymetric
map, study area and the general
circulation of surface- and deep-water
masses
indicated.
Simplified
hydrographic sections are shown below
(modified from Hernández-Molina et
al., 2010).
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Fig. 2 - Multichannel seismic reflection profile across the buried giant-drifts, where morphosedimentary features,
seismic units (LU, IU & UU) and seismic facies are shown.
large-scale palaeoceanographic changes associated with climatic changes. Therefore, giant drifts are
essential to understanding major tectonic or palaeoceanographic changes, and consequently, to
understand how the bottom circulation and climate were in the past. Moreover, they have great
potential for mineral and energy resource exploration, since they are often amenable to generation of
poly-metallic nodules and to accumulation of hydrates and free gas (Rebesco and Camerlenghi, 2008).
This abstract is base on the recent contribution by Hernández-Molina et al. (2010) and describes the
partially buried giant drifts located on the extensional Argentine Continental Margin (Fig. 1), in the
southern portion of the South Atlantic Ocean, in an area crucial for geologic and palaeoceanographic
reconstruction between the Atlantic and Antarctica. Its genesis and evolution in the Argentine Basin
are explained, and its global implications are discussed, primarily based on the bathymetric,
multichannel seismic reflections profiles (MCS) and gravimetric broad database (Fig. 1). This margin
encompasses the Brazil/Malvinas Confluence (BMC), as well as the interaction of Antarctic water
masses (Antarctic Intermediate Water [AAIW], Circumpolar Deep Water [CDW] and Antarctic
Bottom Water [AABW]), with the Brazil Current, re-circulated AAIW and North Atlantic Deep Water
(NADW) (Piola and Matano, 2001; Carter et al., 2009) at different depths (Fig. 1). The surface
circulation around the Argentine margin results from interaction of the Malvinas Current toward the
north-northeast with the Brazil Current toward the south-southwest, both of which determine the
BMC. The intermediate water masses circulation south of this confluence is conditioned by the
circulation toward the north of the Antarctic Intermediate Water (AAIW), and of the two CDW
fractions: Upper Circumpolar Deep Water (UCDW) and Lower Circumpolar Deep Water (LCDW).
Northward of the confluence, apart from the aforementioned water masses, the NADW develops,
circulating toward the south (Fig. 1). The deep circulation is caused by the displacement of AABW
(Fig. 1), which is partially trapped in the basin, generating a large cyclonic gyre, the influence of
which is felt at depths greater than 3500 to 4000 m. This oceanographic regime is clearly significant
in controlling sedimentary processes across the entire ocean basin, and particularly on the Argentine
margin (Hernández-Molina et al., 2010; Violante et al., 2010).
The giant drifts
There are two buried giant-drifts within the Argentine CDS, in the transition between the base of the
slope and the abyssal plain, at 5300 to 5400 m water depth, south and north of a large seamount (The
Austral Seamount). The southern drift is the biggest (ca. 40 to 50 km wide and 250 to 300 km long)
and has a sedimentary thickness of 830 to 950 m. The asymmetrical external shape is characterised
by a steep west side and a gently-dipping smooth east side, and its local internal reflections prograde
eastward (Fig. 2). In contrast, the northern drift is ca. 35 km wide and has a sedimentary thickness of
767 m. Its geometry is opposite to that of the southern zone: its west side is smooth and its east side
is steep. Likewise, its internal reflections prograde westward.
Three seismic units have been defined and correlated regionally: the Lower Unit (LU, EoceneOligocene boundary / early middle Miocene), Intermediate Unit (IU, middle Miocene) and Upper
Unit (UU, late middle Miocene / present day). The giant-drifts developed essentially during deposition
of the LU, which exhibits the major progradation stage and local maximum of thickness, and are
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buried by the UU (Fig. 2). The
seismic facies of the giant-drifts
shows very weak to transparent
acoustic
response,
with
discontinuous reflections and
abrupt changes in the acoustic
facies, indicating a fluidified
seismic signature. Some high
amplitude reflections with greater
lateral continuation reveal that
long-term cycles of drape
deposition and erosion have
combined to form the giant-drifts
(Fig. 2).
Final considerations
Buried, asymmetrical, mounded,
elongated contourite drifts are
located in the southernmost sector
of the extensional Argentine
margin, below the presently active
Contourite Depositional System
(CDS). These giant-drifts have a
northerly trend at the base of the
slope, at water depths of 5300 to
5400 m. Their summit outcrops at
present seafloor, generating a
bathymetric jump that represents
an important change in the slope
gradient trend at the base of the
slope. Based on its position,
morphology
and
internal
Fig. 3 - Evolutionary sketches for: A) development of the giant drifts
characteristics, it has been deduced
(LU) and B) the present scenario. Major tectonic and
that these giant-drifts were
morphosedimentary features as well as the seismic units and
generated, in an open deep marine
discontinuities are shown. Water masses distribution along the Segment
environment, by the AABW, from
I and southernmost part of Segment II are indicated in every sketch. UU
= upper unit; IU = intermediate unit; and LU = lower unit (from
the Eocene-Oligocene boundary
Hernández-Molina et al., 2010).
until the middle Miocene. The drifts
were inferred to record a major
palaeoceanographic change between the middle to very late Miocene (Hernández-Molina et al., 2010),
when a new oceanographic scenario was established, starting to develop the present day
morphosedimentary features of the CDS as a result of the northward flow of the Antarctic water masses.
The results presented in this contribution are testament to how large contourite drifts in deep marine
environments can yield evidence for reconstructing palaeoceanographic changes, and help to explain
Thermohaline Circulation and climate in the past.
Acknowledgements
This work has been partially funded by the Mobility Award of the Spanish Ministry of Education and
Science (PR2007-0138). This work was supported through projects CTM 2008-06399-C04/MAR,
CTM2008-06386-C02/ANT, and ANPCyT – PICT 2003 Nº 07-14417. The authors thank COPLA
(Argentina) and the BGR (Germany) for allowing us to use their bathymetric and multichannel
seismic (MCS) reflections profiles database.
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REFERENCES
• Hernández-Molina, M. Paterlini, M., Somoza, L., Violante, R., Arecco, M.A., de Isasi, M., Rebesco, M., Uenzelmann-Neben, G.,
Neben, S., Marshall, P., 2010. Giant mounded drifts in the Argentine Continental Margin: Origins, and global implications for the
history of thermohaline circulation. Marine and Petroulem Geology. , 27: 1508-1530.
• Violante, R., Paterlini, C.M., Costa, I.P., Hernández-Molina, F.J., Segovia, L.M., Cavallotto, J.L., Marcolini, S., Bozzano, G.,
Laprida, C., García Chapori, N., Bickert, T. and Spieß, V. (2010). Sismoestratigrafía y evolución geomorfológica del talud
continental adyacente al litoral del este bonaerense. Latin American Journal of Sedimentology and Basin Analysis. In press.
• Piola A.R., Matano, R.P., 2001. Brazil and Falklands (Malvinas) Currents. In: Steele, J.H., Thorpe, S.A., and Turekian, K.K. (Eds),
Encyclopedia of Ocean Sciences, London, Academic Press, 1: 340 - 349.
• Carter, L., McCave, I.N., Williams, M.J.M., 2009. Circulation and Water Masses of the Southern Ocean: A Review. In: Fabio
Florindo and Martin Siegert (Eds.), Developments in Earth and Environmental Sciences. Antarctic Climate Evolution, The
Netherlands: Elsevier, vol 8: 85–114.
PLIOCENE SUBMERGED CRATERS AT THE UPPER CONTINENTAL
SLOPE OF MAR DEL PLATA (ARGENTINA)
7-05
Isla, F.1*, Madirolas A.2
(1) Instituto de Geología de Costas y del Cuaternario, CONICET-UNMD, Funes 3350, Mar del Plata,
Argentina
(2) INIDEP, Escollera Norte s/n, Mar del Plata, Argentina
* Presenting author’s e-mail: fisla@mdp.edu.a
Introduction
Bolides, including meteoroids, asteroids or comets (Greeley, 1994; Poag, 1997), used to strike on
Earth episodically, although some recurrence has been suggested (Torbett, 989). Considering the
present distribution of continents and oceans, there is only 38% of probability to leave a record on a
continent. Notwithstanding this probability, many of the craters that stroke on Earth may have
disappeared due to erosion, plate consumption at convergent margins, or they can be buried by
sediments. In this regard, comparative planetology proposed to search former bolide rains in the closer
planets or moons without plate tectonics, or without a liquid cover. On the Earth, crater erosion is
more expected at the continents, and sediment burial at the ocean.
The continental slope is part of the plate margin dominated by gravity-transport phenomena. Erosion
is restricted to the location of submarine canyons where turbidity currents may occur frequently.
Sedimentation is more frequent at the bottom of these canyons, and dominantly at the foot of the
continental slope. The upper slope (between 100 and 500 m depth) is therefore dominated by sediment
transport; and can be considered with less effects of erosion or deposition.
a
b
Fig. 1 - a) MBES record of the major crater found at about 350 m water depth. b) two profiles from the same feature.
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In this paper, we present the
morphological evidence supported
by swath bathymetry surveys from
the upper slope of the continental
margin of Mar del Plata, Argentina.
These evidences are related to
previous
geological
studies
suggesting an impact that have
occurred about 3.3 million years ago
(Ma).
Geological setting
The continental shelf of Buenos
Aires has been surveyed using highresolution seismic methods and
sampling piston cores. The more
Fig. 2 - Minor craters detected 10 km to the south of the major crater.
recent siliciclastic deposits have
been
discriminated
into
5
depositional sequences, 4 of them
were assigned to the Plio-Pleistocene “pampean” series, and the uppermost to the Holocene
transgression-regression cycle (Parker et al., 2008).
Methods
Several legs were performed using a SIMRAD EM1002 multibeam echosounder (MBES) mounted
on the R/V Cap. Oca Balda (INIDEP). These legs were composed into detailed bathymetric charts
with different colours either for different depths and sound dispersion. One major crater was found
with two small craters close to it. From these data, bathymetry allows to analyze the slopes of the walls
of the major crater.
Results
The larger crater is located on the continental slope at 38º 14.7´S and 55º 19.1’W, between 347 and
377 m water depth (Fig. 1a). The wall located at a shallower depth (N-W side) has a maximum slope
of 30%, the wall towards the slope (S-E side) is also steep, about 20% (Fig. 2b). The hollow is 30 m
depth. The 500 m diameter rim is more evident to the down-slope border. At the bottom of the crater,
the diameter is about 50 m.
Some km to the south (38º 20´S and
55º 18’ 30”W), other two small
craters were surveyed at a greater
depth (at about 400 m water depth;
Fig. 2).
During many years the PlioPleistocene sequence outcropping at
the coastal cliffs south of Mar del
Plata was known by their escorias
content and reddish bricklike
remains (Fig. 3). Today, these
remains were known as tektites
related to an impact that have
occurred about 3.3 Ma (Schultz et
al., 1998). As the crater has never
been found, it was assumed that it
might have fallen on the present
Fig. 3 - Dispersed escorias, bulk of 0.70 x 0.40 m, with pieces of
continental shelf and eroded during
reddish bricklike interfingered remains (courtesy L. Cortizo)
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the Quaternary marine transgressions.
Discussion and conclusions
Although the sizes of these craters could not explain the extension (hundreds of km) of tektite remains
(escorias), they are probing a rain of bolides in the region. However, these are the only features of
impacts on the region. In this sense, the dating of the tektite layer allows to assign an Upper Pliocene
age to these hollows on the upper portion of the Argentine continental slope.
We can conclude that:
A hollow of 500 m diameter and 30 m depth is found at the upper slope offshore Mar del Plata.
The presence of other smaller hollows at the surroundings forced to consider a bolide rains.
The evidence of Upper Pliocene tektites in the region led to consider this age for these meteoric
impacts.
REFERENCES
• Greele, R., 1994. Planetary landscapes. Chapman and Hall, New York, 286 pp.
• Parker, G., Violante, R. A., Paterlini, C. M., Marcolini, S., Costa, I. P. And Cavallotto, J. L., 2008. Las secuencias
sismoestratigráficas del Plioceno-Cuaternario en la plataforma submarina adyacente al litoral del Este Bonaerense. Latin American
Journal of Sedimentology and Basin Analysis 15, 2, 105-124.
• Poag, C. W., 1997. The Chesapeake Bay bolide impact: a convulsive event in Atlantic Coastal Plain evolution. Sedimentary Geology
108, 45-90.
• Schultz, P. H., Zárate, M., Hames, W., Camilión, C. and King, J., 1998. A 3.3-Ma impact in Argentina and possible consequences.
Science 282, 2061-2063.
• Torbett, M. V., 1989. Solar system and galactic influences on the stability of the Earth. Global and Planetary Change 75, 3-33.
THE 2005-2006 EXPERIMENT IN ANTARCTICA WITH
MABEL SEAFLOOR MULTIDISCIPLINARY OBSERVATORY
7-06
Marinaro, G.*, Falcone G., Frugoni, F., Favali, P.
Istituto Nazionale di Geofisica e Vulcanologia, Rome, Italy
* Presenting author’s e-mail: giuditta.marinaro@ingv.it
MABEL (Multidisciplinary Antarctic BEnthic Laboratory) is an Italian PNRA (Programma
Nazionale di Ricerche in Antartide) project performed by INGV (Istituto Nazionale di Geofisica e
Vulcanologia) in collaboration with AWI (Alfred Wegener Institut, Bremerhaven). MABEL is a deepsea multidisciplinary observatory for long-term autonomous observations. It was deployed by R/V
Polarstern on December 5th, 2005 on the seafloor of Weddell Sea (69° 24, 29 S and 5° 32,2 W) at 1884
m w.d.
MABEL is able to measure and record autonomously and automatically data for one year with the following instruments:
three component broad band seismometer (100 Hz per 3 channel);
conductivity, pressure and temperature (CDT, 1 sample hour);
light transmissometer (1 data/hour);
All these instruments are time-referenced with a high precision rubidium clock. Data acquisition started on December 6th, 2005 and lasted until December 31st, 2006, when the observatory automatically
ended acquisition and all instruments were switched off. On December 16th, 2008 MABEL was recovered always using R/V Polarstern and using MODUS (MObile Docker for Underwater Sciences) of
the BEUTH (former TFH) and TUB.
The status of MABEL after about three years in deep-sea environment was very satisfactory with no
significant corrosion. There was no water intrusion inside these vessels and the most important part
of MABEL, which is the DACS (Data Acquisition and Control Unit) was found in very good conditions. The data acquisition covered the whole period of the mission, from December 6th 2005 to
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Fig. 1 - Seismograms recorded by Mabel OBS station. Teleseismic event (May 3, 2006; Tonga, Mw=8.0): a) unfiltered
raw data; b) bandpass filtered data (0.5-2 Hz) and c) corresponding spectrogram.
Ice quake (April 7, 2006) d) unfiltered raw data; e) bandpass filtered data (2-8 Hz) and f) corresponding spectrogram
December 31st 2006, as we expected, with a total amount of data of more than 13 GBytes.
The continuous recording and acquisition of data from seismometer allows to focus on local activity,
also with the integration of the data collected on Neumayer “on-land” seismic network.
The teleseismic waveform monitoring will be integrated with the records collected by ASAIN
(Antarctic Seismographic Argentinean Italian Network) of the OGS (Istituto Nazionale di
Oceanografia e di Geofisica Sperimentale).
Acknowledgments
We thank PNRA (Programma Nazionale di Ricerche in Antartide) for the support to the project.
A particular thank to the other Institutes and Companies participated to the activities:
Tecnomare S.p.A. (Francesco Gasparoni, Flavio Furlan) for the engineering of the system;
Beuth Hochschule für Technik (BEUTH), Berlin (Hans W.Gerber) for deployment/recovery activities;
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Technische Universität Berlin (TUB) (Haiko de Vries) for deployment/recovery activities;
Alfred Wegener Institute for Polar Sciences, Bremerhaven (Wilfried Jokat);
Istituto Nazionale di Oceanografia e Geofisica Sperimentale (OGS) (Marino Russi);
The Captain and the crew of R/V Polarstern for their precious and valuable support;
Massimo Calcara, Nadia Lo Bue, Capt. Emanuele Gentile and Marco Lagalante for participation to the
cruises.
COASTAL EROSION IN MAR DE COBO (BUENOS AIRES PROVINCE)
7-07
San Martín, L.*, Marcomini, S.C., López, R.A.
Dpto. de Geología, Facultad de Ciencias Exactas y Naturales, Universidad de Buenos Aires, Argentina
* Presenting author’s e-mail: sanmartin.laura@gmail.com
Introduction
Mar de Cobo, located in Buenos Aires Province (Fig. 1), is considered to have a high rate of coastal
recession, ranging from 3.5 to 6 m/yr according to different authors (Schnack et al., 1983; López and
Marcomini, 2002). Values calculated for Mar Chiquita, located 4 km northwards, include 6 m/yr (Isla and
Villar, 1992), 7 m/yr (Isla, 1997) and 5.16 m/yr (Merlotto and Bértola, 2007 and 2008). This area is
naturally erosive, fact that is strongly reinforced by human activities (Isla and Bértola, 2005; Merlotto and
Bértola, 2008).
The aim of this study is to characterize the erosion acting in this area trough beach parameters (width and
slope) and statistical parameters (average, selection and asymmetry), together with geomorphologic
features from beach profiles (abrasion platform, marine abrasion terrace, conservation degree of the
coastal ridge, presence of berms or submerged bars).
Regarding beach parameters, a wider beach indicates lower vulnerability to erosion, the same as active
coastal dunes (Marcomini et al., 2007). What is more, an increase in beach slope also points to beach
erosive conditions (Marcomini and López, 1995).
Relationships between grain size and erosive or aggrading beaches were considered by Mazzoni and
Spalletti (1980), who related a bigger grain size average and a lower selection value to coastal areas in
erosive stages. At the same time, they consider this study area to be within a zone with backward
movement of the coast line, high energy and strong mechanical activity.
In addition to the natural erosive characteristics of this area, already determined by Teruggi et al. (1959)
and Spalletti and Mazzoni (1979), protection structures for coastal moderation increases rates of erosion.
This structures, although causing the recovery in volume of beach sediments, also generate a loss of
saturation downwater in the litoral drift which in turn increases rates of erosion (López and Marcomini,
2002; Isla and Bértola, 2005). At the same time, protection structures located in Mar del Plata have played
a unique role in causing induced erosion in this study area (Isla and Villar, 1992; Schnack et al., 1983).
Methodology
Six beach profiles were analyzed along Mar de Cobo´s shore, and numbered from south to north. The
distance between them is about 400 meters. The equipment used for this task was Total Station, which
assembles transverse beach profiles from a fixed point, located in the dune, up to the low tide. This
profiles provided the information to calculate beach parameters. Parameters used were total beach width
and slope and backshore width.
Furthermore, we collected samples of sand from the different beach environments in three of those
profiles (MC1, MC3 and MC5) in order to perform grain size analysis. The samples were sifted with a
Ro-Tap, using half phi divisions. Granus software (Perillo et al., 1985) was used to calculate statistical
values (average, selection and asymmetry).
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Fig. 1 - Location
map showing the
six beach profiles.
Results
The study of Mar de Cobo site presents both natural and man-made unique characteristics, which made
the comparison between profiles difficult. We selected total beach width and back shore width, since they
are affected mostly by long term dynamics.
Results are shown in Fig. 2, with an important decrease in total beach width from south to north and a
local increase in MC4, which is located between two jetties. There is also a slight increase in beach width
in MC6 that could be related to the end of the urban section and stabilized dunes. Lower values point
towards profiles MC3 and MC5. Backshore width displays much more variability, with null values for
MC3 and MC5, therefore represent the most erosive ones. Globally, data follows the same pattern as total
beach width.
Total beach slope was also taken into account, and it shows the same pattern as total beach width (Fig. 2).
That would be an increase in slope values from south to north, showing an increase in erosion, except for
MC4 and MC6. Beach slope data also indicates MC3 and MC5 as the most erosive profiles.
Regarding grain size, samples from backshore and foreshore are strongly affected by jetties and
geomorphologic features in this study area, so they were discarded. Instead, we selected the fore dune
ridge since it nourishes from the beach and it would be the less affected.
Concerning coastal dune sediment average, there is an important increase in grain size from south to
north, from fine to medium sand. Selection is similar in the first two profiles, being moderately good to
good, but it turns into poor on the northern profile. In addition, the first two profiles present almost
symmetric distributions, but the northern one shows negative asymmetry (Fig. 3). In conclusion, analyzed
parameters showed an increase in erosion towards the north.
As for geomorphologic features, we studied the presence or absence of different features in the beach. We
observed there are no berms in any profile and only a submerged bar in one of them. These characteristics
are an erosion-sensitive feature, and their absence indicates erosive conditions in many beach models
(López and Marcomini, 2002).
Study area has natural erosive features, including a marine abrasion terrace and an abrasion platform. The
first one is approximately 400 meters long, has an irregular edge and locates in the foreshore of MC3, and
the second one is situated in MC6, is 50 to 60 meters wide and generates a “cape” type irregularity
situated in the north extreme of the city.
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Fig. 2 - Variation in beach
parameters through the coast
line, from south to north, along
the six profiles registered in
Mar de Cobo. There is no back
shore development in MC3 and
MC5.
Fig. 3 - Variation of
statistical parameters along
the beach line: average,
selection and asymmetry.
Coastal dune is registered in all profiles, even if it is only preserved as a relict nucleus or incipient dune.
Every studied profile presents some kind of alteration, which differs from each other. Mostly, the dune is
fixated by Carpobrotus edulis and is scarped. In every dune there is also some kind of autochthonous
flora, mainly Panicum racemosum, which allows incipient dunes to form (Table 1). MC1 shows a
particular feature, with blow out dunes over the coastal dune caused by vehicles.
Since fixed dunes increase coastal recession speed compared to active dunes (López and Marcomini,
2002), MC5 would be the most erosive profile because the coastal dune is only composed by isolated
Table 1 - Coastal dune characteristics in Mar de Cobo.
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relict nucleus with Carpobrotus edulis and scattered incipient dunes with Panicum racemosum.
Conclusions
Coastal parameters showed a strong erosive trend that increases to the north in the study area. Geoindicators best reflecting erosion for this area were total beach and backshore width, average and
selection. Protective structures for coastal moderation along the shore caused local modifications in
erosion and accumulation rates for this village. Higher erosion risk zones are near De Las Torres St. and
Del Trabajo St.
REFERENCES
• Isla, F., 1990. Tendencias litorales y transversales de transporte en playas y boca de marea: Mar Chiquita, Buenos Aires. Revista de la
Asociación Argentina de Mineralogía, Petrología y Sedimentología, 21(1): 75-87.
• Isla, F., 1997. Seasonal behaviour of Mar Chiquita tidal inlet in relation to adjacent beaches, Argentina. Journal of Coastal Research,
13(4): 1221-1232.
• Isla, F. y Bértola, M., 2005. Litoral bonaerense. Relatorio del XVI Congreso Geológico Argentino: Geología y Recursos Minerales de la
Provincia de Buenos Aires. pp. 265 – 276.
• Isla, F. y Villar, M., 1992. Ambiente Costero. Pacto ecológico. Universidad Nacional de Mar del Plata – Senado de la Provincia de Buenos
Aires, La Plata.
• López, R. y Marcomini, S., 2002. Pautas para el manejo en costas acantiladas y de dunas, Provincia de Buenos Aires. Revista de Geología
Aplicada a la Ingeniería y al Ambiente,18: 59–68.
• Marcomini, S. y López, R., 1995. Strategies for the coastal management of Villa Gesell, Argentina. Proc. Int. conf. “Coastal Change 95´
Bordomer-IOC, Bordeaux, p. 819–831.
• Marcomini, S., López, R. y Spinoglio, A., 2007. Uso de la morfología costera como geoindicador de susceptibilidad a la erosión en costas
cohesivas, Necochea, Buenos Aires. Revista de la Asociación Geológica Argentina, 62(3): 396–404.
• Mazzoni, M. y Spalletti, L., 1980. Características sedimentológicas de playas en erosión y en agradación. Revista de la Asociación
Geológica Argentina, 23(3): 355–363.
• Merlotto, A. Y Bértola, G., 2007. Consecuencias socio-económicas asociadas a la erosión costera en el Balneario Parque Mar Chiquita,
Argentina. Investigaciones Geográficas (Esp), Núm. 43, sin mes, 2007, pp. 143-160. Universidad de Alicante, España.
• Merlotto, A., Bértola, G., 2008. Evolución urbana y su influencia en la erosión costera en el balneario parque Mar Chiquita, Argentina.
Papeles de Geografía, Núm. 47-48, enero-diciembre, 2008, pp. 143-158 Universidad de Murcia España.
• Merlotto, A., Verón., E., Sabula, F., 2008. Riesgo de erosión costera en el Balneario Parque de Mar Chiquita. Párrafos Geográficos (7)1:
103-121.
• Perillo, G., Gómez, E., Aliotta, S. y Galíndez, D. 1985. Granus: un programa FORTRAN para el análisis estadístico y gráfico de muestras
de sedimentos. Revista Asociación Argentina Mineralogía, Petrología y Sedimentología, 16(1-4): 1-5.
• Schnack, E., Álvarez, J. y Cionchi, J., 1983. El carácter erosivo de la línea de costa entre Mar Chiquita y Miramar, Provincia de Buenos
Aires. Simposio Oscilaciones del nivel del mar durante el último hemiciclo deglaciar en la Argentina, INQUA, Mar del Plata. Actas pp.
118–130.
• Spalletti, L. y Mazzoni, M., 1979. Caracteres granulométricos de arenas de playa frontal, playa distal y médano del litoral bonaerense.
Revista de la Asociación Geológica Argentina, 34(1): 12 – 30
• Teruggi, M., Chaar, E., Remiro, J. y Limousin, T., 1959. Las arenas de la costa de la provincia de Buenos Aires entre Cabo San Antonio
y Bahía Blanca. LEMIT serie II, 77, 1-54.
CONDITIONING FACTORS AND RESULTING MORPHOSEDIMENTARY
FEATURES IN THE UPPER-MIDDLE CONTINENTAL SLOPE OFFSHORE EASTERN
BUENOS AIRES PROVINCE, ARGENTINA
7-08
Violante, R.A.1*, Paterlini, C.M.1, Hernández Molina, F.J.2, Bozzano, G.1,
Pastor Costa, I.1, Marcolini, S.1
(1) División Geología y Geofísica Marina, Departamento Oceanografía, Servicio de Hidrografía Naval,
Av. Montes de Oca 2124, Buenos Aires C1271ABV, Argentina
(2) Facultad de Ciencias del Mar, Universidad de Vigo. 36200, Vigo, Spain
* Presenting author’s e-mail: Roberto A. Violante, violante@hidro.gov.ar.
Introduction
The morphosedimentary characteristics of the Argentine Continental Margin are dominated by
interaction between alongslope and downslope processes, which gave rise to a complex sedimentary
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system composed of contourites, turbidites and debris flows deposits. The main sedimentary body was
defined as a Contourite Depositional System (CDS, Hernández Molina et al., 2009), genetically
related to termohaline circulation, which affects sedimentation and induces both erosive and
depositional action on the slope surface associated to the interfaces of the Antarctic water masses,
particularly the Antarctic Intermediate Water (AAIW) and Antarctic Bottom Water (AABW). The
CDS is better developed offshore the Patagonian region where the unidirectional south-to-north
oceanic circulation is clearly manifested. In the northern part of the slope adjacent to eastern Buenos
Aires province, the progressive northward decreasing transport capacity of the Antarctic-sourced
waters, together with their complex interaction with North Atlantic water masses and the oceanic
dynamic imposed by the Confluence Zone, influences the regional circulation. In addition, local
downslope processes associated to shelf-originated sediment supply and transport across the shelfslope transition, as well as slope-originated gravitational processes, are masking the regional
alongslope processes. As a result, the CDS and the related features loose there their typical
configuration.
Recent research was focused on the eastern Buenos Aires continental slope between 37-39ºS, where
detailed bathymetric as well as mono and multichannel high resolution seismic surveys were
performed, and sediment cores were recovered, during two campaigns carried out in 2009: M78/3
(R/V Meteor, Universities of Bremen and Kiel, Germany) and Litoral Bonaerense IV (LBIV, R/V
Puerto Deseado, Argentina Hydrographic Survey). Complementary data were gathered from former
cruises (R/V Meteor: M29/2 1994, M46/3 2000 and M49/2 2001; and R/V Puerto Deseado:
Fisiografía 1996, Litoral Bonaerense III 2000, Prueba de Coring 2001, Coring 2002 and Pistón II
2004). Additionally, seismic data from Lamont-Doherty Earth Observatory (Columbia, USA) and
Federal Institute of Geosciences and Natural Research (Hannover, Germany) were used. The analyzed
information, which reaches a total of around 6600 km of seismic and bathymetric lines, allowed to
define the margin architecture, major sedimentary structures and sedimentary bodies’ configuration.
Around 90 piston and gravity cores were available for studing the sedimentary characteristics.
This contribution deals with the description of the eastern Buenos Aires province continental slope as
well as the definition of the conditioning factors involved in the evolution of the main features and
depositional processes.
Major morphosedimentary features
The major morphosedimentary features are (Fig. 1): continental shelf, continental slope, continental
rise and Mar del Plata submarine canyon (Hernández Molina et al., 2009; Violante et al., 2010). The
Continental slope extends between 120 m and 3500 m water depth (shelf break and slope-rise
transition respectively). It is constituted by three lower-order features: upper, middle and lower slope.
The upper slope extends between the shelf break and 700-800 m depth and is characterized by a steep
gradient. Upslope the Mar del Plata canyon´s head, the upper slope reaches depths not deeper than 500
m as its foot is locally affected by gravitational erosive processes and consequent erosional retreat.
The middle slope extends downslope reaching around 1300 m depth and is regionally formed by a
low-gradient terraced surface named Ewing Terrace (ET), which is the morphological expression of
the CDS and associated erosive processes resulting from the dominating oceanographic conditions; it
is characterized by a sub-horizontal, slightly concave configuration in a transverse (WNW-SSE)
section with a longitudinal, alongslope mounded fringe at its outermost (seaward) side. A particular
feature in the region is another terrace, or wedge-shaped fan-like body (WF), which reaches depths
of 1100-1200 m and partially covers and distorts the western side of the Ewing Terrace. WF is
preliminary associated to deposition from the shelf as it is closely related to an incision that cuts the
shelf-upper slope at 38º50´S, although later redistribution by alongshore currents is also evident;
however, this feature is not well known yet and needs more detailed studies in order to define its origin
and evolution. Along the boundary between ET and WF, an alongslope, northwards flowing
longitudinal channel extends, which begins at around 900 m depth, flows towards the Mar del Plata
canyon and meets it at around 1300 m depth. At depths below 1300 m, the lower slope extends with
a steep slope reaching around 3500-3700 m depth from where it grades offshore to the continental
rise. The Mar del Plata submarine canyon dissects the continental slope at around 38ºS and
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Fig. 1 - Location map, detailed bathymetry and seismic example of part of the offshore area of the Buenos Aires province.
represents the major sediment pathway towards the base of the slope.
Sedimentary processes and deposits
The dynamic and oceanographic complexity of the region produces diverse sedimentary processes
which interact each other, giving rise to a variety of morphological features and sedimentary deposits.
The dominant regional sedimentary body in the middle slope is the CDS, which constitutes most of
the Ewing Terrace and is evidenced in the seismic records by agradational and progradational
configurations, usually mounded-shaped, up to 300-400 m thick, with high amplitude internal
reflections. Several minor internal unconformities are present, what allows to differentiate several
evolutive stages. These contouritic bodies are being formed since middle Miocene (Hernández Molina
et al., 2009, Violante et al., 2010), according to the age of the basal seismic horizon AR5 (Hinz et al.,
1999). Contouritic sediments recognized in cores have thicknesses up to 7 m and are mainly composed
of dark olive gray very fine sands with a “wet” appearance (Bozzano et al., 2010).
Turbidites and debris flows deposits constitute very important components of the sedimentary
sequences. Seismic records show large bodies of these deposits up to several tens thick, constituted
by stratified configurations for the turbidites and chaotic for the debris flows. Sedimentary records
observed in cores usually do not exceed a thickness of 10 cm for each single turbidite unit, which are
represented by well defined layers of black fine sands showing a marked stratified and upward
decreasing grain-size distribution, most of the times with sharp basal contacts with underlying
sediments and gradual transition to the overlying sediments. On the other hand, debris flows are
represented by chaotic accumulations of muddy pebbles included in a sandy-muddy matrix. It is
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common to find a sequence of basal debris flows grading to turbidites in an upward transition. The
presence of large blocks of rocks on the seabed at the canyon´s head and main channel reveals the
occurrence of active high-energy processes, sometimes related to collapse of the canyon´s walls. In
this sense, the structural, dynamic, sedimentary and oceanographic scenario around the canyon is
compatible with the occurrence of significant bottom instabilities manifested by submarine landslides
and other gravitational processes (Krastel et al., 2010).
A significant, locally extended terraced feature is the wedge-shaped fan-like body, which is
preliminary considered as formed by sediment transport from the shelf via a channel incised at the
shelf-slope transition and consequent deposition at the foot of the upper slope. It develops above the
Ewing Terrace and partially covers its western part, showing a marked asymmetrical, northwardelongated shape (Fig. 1). This geometry indicates a net alongshore drifting to the north, suggesting
that sediments are being largely incorporated to the contourite system as shown by the contouritic
character found in sediments contained in cores obtained on the fan. Although further detailed studies
are necessary on the WF, preliminary interpretation of seismic sections indicates that it could have
begun to be formed since Pliocene-Quaternary times, probably associated to higher sediment supply
from the shelf during lowstands periods.
Final remarks
Three main sedimentary processes are significant in the shaping of the region: 1) those related to
contouritic (alongslope) deposition/erosion on the middle slope driven by contourite currents
originated by the activity of the AAIW, which is the major morphosedimentary factor involved in the
formation and evolution of the Ewing Terrace; 2) those related to turbiditic-debris flows-submarine
landslides (downslope) deposition which are important in the steeper slopes, like the upper and lower
slope as well as the surroundings of the Mar del Plata submarine canyon; 3) those related to the
formation of a wedged fan body originated at the shelf break that was accumulated above the western
part of the Ewing Terrace primarily by gravitational sediment deposition and later reworking by
contour currents. The interaction among the regional contouritic processes and the local gravitational
processes occurring in the shelf, slope and submarine canyon, allowed the region to acquire very
complex morphosedimentary configurations.
REFERENCES
• Bozzano, G., Violante, R.A., Paterlini, C.M., Hernández-Molina, F.J, Hanebuth, T., Huppertz, T., Orgeira, M.J. and Krastel, S.
(2010). Depositional pattern of contourite facies on the terraced slope off Buenos Aires province (NE Argentina, SW Atlantic): a
sedimentological approach. International Congress “Deep-Water Circulation: Processes and Products”, Baiona, Pontevedra, Spain,
Abstracts: 41-42.
• Hernández-Molina, F.J., Paterlini, C.M., Violante, R.A., Marshall, P., de Isasi, M., Somoza, L. and Rebesco, M. (2009). Contourite
Depositional System in the Argentine Margin: an Exceptional Record of the Influence and Global Implications of Antarctic Water
Masses. Geology, 37 (6): 507-510.
• Hinz, K., Neben, S., Schreckenberger, B., Roeser, H.A., Block, M., Goncalves de Souza, K. and Meyer, H. (1999) The Argentine
continental margin north of 48º S: sedimentary successions, volcanic activity during breakup: Marine and Petroleum Geology, 16:
1-25.
• Krastel, S., Freudenthal., T., Hanebuth, T., Preu, B., Schwenk, T., Strasser, M., Violante, R.A., Wefer, G. and M78-3 shipboard
scientific party (2010). Sediment Dynamics and Geohazards offshore Uruguay and Northern Argentina: first results from the multidisciplinary Meteor-Cruise M78-3. 18th Meeting of Swiss Sedimentologists, Fribourg, Swiss, Abstract volume.
• Violante, R.A, Paterlini, C.M., Costa, I.P., Hernández-Molina, F.J., Segovia, L.M., Cavallotto, J.L., Marcolini, S., Bozzano, G.,
Laprida, C., García Chapori, N., Bickert, T. and Spieß, V. (2010). Sismoestratigrafía y evolución geomorfológica del talud
continental adyacente al litoral del este bonaerense. Latin American Journal of Sedimentology and Basin Analysis, 17 (1), MS188.
.
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CENOZOIC GROWTH PATTERNS AND PALEOCEANOGRAPHY
OF THE OCEAN BASINS NEAR THE SCOTIA-ANTARCTIC PLATE BOUNDARY
7-09
Maldonado, A.1*, Bohoyo, F.2, Galindo-Zaldívar, J.1,3, Hernández-Molina, F.J.4, Lobo, F.J.1,
Martos-Martin, Y.1, Schreider, A.A.5
(1) Instituto Andaluz Ciencias de la Tierra (IACT). CSIC/Universidad Granada. 18002 Granada, Spain
(2) Instituto Geológico y Minero de España (IGME), Ríos Rosas, 23, 28003 Madrid, Spain
(3) Departamento de Geodinámica, Universidad de Granada. 18071 Granada, Spain
(4) Facultad de Ciencias del Mar, Universidad de Vigo, 36200 Vigo, Spain
(5) P.P. Shirshov Institute of Oceanology, Russian Academy of Sciences. 23 Krasikova 117218
Moscow, Russia
*Presenting author’s email: amaldona@ugr.es
The tectonics and distribution of seismic units of the southwestern Scotia Sea are described based on
multichannel seismic profiles, swath bathymetry and magnetic anomalies. Recently acquired profiles
suggest that spreading of the Drake Passage was active prior to the Eocene/Oligocene boundary and
that gateways may have connected the Pacific and Atlantic oceans allowing a restricted circumpolar
current. After the initial breakup the Scotia Sea resulted from several spreading centers that developed
deep oceanic basins. The three youngest units identified in the Cenozoic deposits exhibit similar
seismic facies and are correlated at regional scale. The deposits show a variety of contourite drifts that
resulted from the interplay between the northeastward flows of Weddell Sea Deep Water (WSDW),
the Antarctic Circumpolar Current (ACC) and the complex bathymetry.
Introduction
A host of ocean basins and migrating spreading centers, with intervening banks of continental slivers,
were active along the Antarctic and the South American plates between the Oligocene and the present
(Fig. 1). The result of this tectonic evolution was the opening of the Drake Passage and the creation
Fig. 1 - Regional map showing the location o the BIO HESPERIDES profiles collected in the Scotia Sea and Weddell
Sea during the SCAN cruises.
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Fig. 2 - Map showing the tectonics and the location o the BIO HESPERIDES profiles of the southern Drake Passage
(modified from Aldaya and Maldonado, 1996).
of the Scotia Sea, one of the most important Cenozoic features of the Southern Ocean (Bohoyo et al.,
2007).The opening of Drake Passage during the Cenozoic created the final gateway for a continuous
deep-water circulation around Antarctica (Livermore et al., 2004; Maldonado et al., 2006). The
gateway allowed the instauration of the Antarctic Circumpolar Current (ACC), which is proposed to
have profound effects on paleoceanography, the evolution of the Antarctic climate and the beginning
of the north-south ocean circulation patterns. We describe the correlation of the basin depositional
units and show how their evolution was influenced by tectonics, which controlled the opening and
closing of gateways modifying global paleoceanographic events with remark impact on erosion and
depositional processes.
Methods
Multichannel seismic reflection profiles and magnetic anomalies, complemented with high resolution
TOPAS profiles, swath bathymetry and gravity data that we collected in the area during seven
oceanographic cruises with the BIO HESPERIDES allow us to establish the main tectonic events and
the geodynamics of the area (Figs. 1, 2). High resolution subbottom profiles were obtained with a
Topographic Parametric Sonar (TOPAS) Konsberg Simrad PS018. The swath bathymetric data were
obtained with a SIMRAD EM 12 system and post-processed with NEPTUNE software and
FLEDERMAUS for visualization. Total intensity magnetic field data were recorded every 5 s with a
Geometrics G-876 proton precession magnetometer along the ship track lines. Gravity data were
acquired with a Bell Aerospace TEXTRON BGM-3 marine gravimeter.
The ship tracks cover the study area reasonably well, although the profiles are widely spaced. The
stratigraphic analysis and the regional distribution of depositional units and discontinuities in the area
were complemented, however, with additional MCS profiles acquired in previous cruises by Italian,
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Fig. 3 - Representative multichannel seismic profile, and line drawing interpretation with the magnetic anomalies
across the southwestern Scotia Sea
Russian and Spanish institutions. The age of the main seismic units was tentatively calculated on the
basis of: (a) the age of the igneous basement provided by the magnetic anomalies, (b) the total
thickness of the depositional sequence for selected stratigraphic sections, (c) the sedimentation rate of
surface sediment cores, and (d) the results of ODP borehole sites in the area.
Basin age and seismic stratigraphy
The oldest magnetic anomaly previously reported in the southwestern Scotia Sea is chron C10,
(Lodolo et al., 1997), whereas our recently acquired magnetic profiles in the area show up to chron
C12n (30.5-31 Ma). Spreading of the Drake Passage was active prior to 31 Ma, although older
magnetic anomalies were, moreover, identified during the SCAN 2008 cruise southwestward of Terror
Rise. The distribution of deep basins in the southwestern Scotia Sea suggests an initial phase of
diffuse spreading and that rifting of the margins and shallow seaways between the Antarctic Peninsula
and South America existed prior to the Eocene/Oligocene boundary (Fig. 2).
The development of Protector Basin is well constrained by the seafloor magnetic anomalies (14.0-14.4
to 17.6 Ma), whereas the magnetic anomalies of the central Scotia Sea indicate an age of spreading
between 20.7 and 14.2 Ma (Maldonado et al., 2003, 2006; Bohoyo et al., 2007). Six main seismic units
are identified regionally, although locally older units may exist. The distribution and seismic features
of these deposits vary in relation to the bottom topography, which significantly influenced the
distribution of bottom flows (Maldonado et al., 2003, 2006). In the unconfined setting of the abyssal
plain, the types of contourite drifts are determined by the interplay of strong currents shearing along
the margins of submarine banks and the basement disruptions of the sea floor. The sheeted drifts
dominate in the abyssal plain, but along the margins of the banks slope plastered and giant elongatedmounded drifts are developed by the currents. The units are bounded by high-amplitude continuous
reflectors, named a to d from top to bottom. The three older units are of different age and seismic
facies in each basin and they generally correspond to the syn-drift deposits. The three youngest units
(3 to 1) exhibit, in contrast, rather similar seismic facies and can be correlated at a regional scale. The
contourite drifts that resulted from the interplay between the northeastward flows of the Weddell Sea
Deep Water (WSDW), the Antarctic Circumpolar Current (ACC) and the complex bathymetry.
Reflector c (~12.1-12.6 Ma) can be correlated basin.wide and it is coeaval with the timing of
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connection between the Scotia Sea and the Weddell Sea (Maldonado et al., 2006). Unit 3 (~Middle to
Late Miocene) shows the initial incursions of the WSDW into the Scotia Sea, which influenced a
northward progradational pattern, contrary to the underlying deposits. The tentative age calculated for
Reflector b is coincident with the end of spreading in the West Scotia Ridge (~6.4 Ma). The deposits
of Unit 2 (~Late Miocene to Early Pliocene) have abundant high-energy, sheeted deposits in the
northern Weddell Sea, which may reflect a higher production of WSDW as result of the advance of
the West Antarctic ice-sheet onto the continental shelf. Reflector a represents the last major regional
paleoceanographic change. The timing of this event (~3.5-3.8 Ma) coincides with the end of spreading
in the Phoenix-Antarctic ridge, but it may be also correlated with global events such as the initiation
of the permanent northern Hemisphere ice-sheet and a major sea level drop. Unit 1 (~Late Pliocene
to Recent) is characterized by high-energy contourite deposits, which suggest intensified deep water
production. Units 1 and 2 show, in addition, a cyclic pattern, more abundant wavy deposits and the
development of internal unconformities, all of which attest to alternative periods of increased bottom
current energy.
The older units of the isolated basins are generally affected by a northward thrusting below the margin
of the South Scotia Ridge, and a portion of the oldest crust was probably consumed by the subduction
processes (Fig. 3). These data bear evidences for an earlier than previously postulated opening of a
full circum-Antarctic gateway through Drake Passage, which at least allowed an eastward circulation
throughout the gateway of the superficial and intermediate water masses.
Discussion and conclusions
The Earth’s climate experienced a major change near the Eocene-Oligocene boundary, but whether it
can be explained strictly as a result of the opening of southern latitude oceanic gateways, or attributed
to changes in atmospheric CO2 concentrations, or is in fact the result of multiple causes, is a subject
of debate (Lawver and Gahagan, 2003; DeConto and Pollard, 2003; Livermore et al., 2004). The new
magnetic anomalies indicate the development of oceanic crust in Drake Passage and that an oceanic
gateway existed between South America and the Antarctic Peninsula prior to 30.9 Ma. Taking into
consideration the timing for breakup and the tectonics of the area, a gateway may have developed in
Drake Passage before the Eocene/Oligocene boundary. The ridges and basins that were active during
the early stages in the evolution of the Scotia Sea controlled the development of the Antarctic
Circumpolar Current and the deep water flows (Maldonado et al., 2003). The major regional
unconformity represented by Reflector c seems coeval with a major Miocene glaciation (Mi4), a
lowering of sea level (Ser3) and with the initiation of the permanent East Antarctic ice-sheet. This
reflector suggests a major event in the dynamics of bottom water circulation, which would represent
the connection between the Scotia Sea and the Weddell Sea across the South Scotia Ridge.
The Oligocene glaciers of Antarctica were isolated and a West Antarctic ice-sheet that advanced onto
the continental shelf did not develop until the Late Miocene (Miller et al., 2009), which seems to be
recorded by Reflector b and may also be coincident with the end of spreading at the West Scotia
Ridge. It has been proposed that a major factor for the present global ocean circulation and the Late
Pliocene formation of the West Antarctic and Northern Hemisphere ice-sheets was the closure of the
Isthmus of Panama, at about 3.7-3.0 Ma ago (Lawver and Gahagan, 2003). The coincidence between
the ages calculated for Reflector a and the timing of closure of the Panamanian seaway is remarkable.
Spreading ended almost coetaneous at the Phoenix Ridge, however, which may also had a more
significant influence on the paleoceanography of the area by modifying the deep water flows through
significant changes in the sea bottom topography of Drake Passage.
The ridges and basins that were active during the early stages in the evolution of the Scotia Sea
controlled the development of the Antarctic Circumpolar Current and the deep water flows and they
have, hence, a profound influence on paleoceanography and climate.
Acknowledgements
Spanish Comisión Interministerial de Ciencia y Tecnología (CICYT) supported this research through
Projects CGL2004-05646; POL2006 13836/CGL and CTM2008-06386-C02/ANT..
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REFERENCES
• Aldaya, F. and Maldonado, A.(1996): Tectonics of the triple junction at the southern end of the Shackleton Fracture Zone (Antarctic
Peninsula). Geo-Marine Letters, 16: 279-286
• Bohoyo, F., Galindo-Zaldívar, J., Jabaloy, A., Maldonado, A., Rodríguez-Fernández, J., Schreider, A. and Suriñach, E. (2007):
Extensional deformation and development of deep basins associated with the sinistral transcurrent fault zone of the Scotia-Antarctic
plate boundary.. Geological Society, London, Sp. Pub, 290: 203-217.
• DeConto, R. M., and D. Pollard (2003), Rapid Cenozoic glaciation of Antarctica induced by declining atmospheric CO2, Nature,
421, 245–249.
• Lawver, L. A., and L. M. Gahagan (2003), Evolution of Cenozoic seaways in the circum-Antarctic region, Palaeogeogr., Palaeoclim.,
Palaeoecol., 198, 11–37.
• Livermore, R., Eagles, G., Morris, P. and Maldonado, A. (2004): Shackleton Fracture Zone: No barrier to early circumpolar ocean
circulation. Geology, 32: 797-800.
• Lodolo, E., Coren, F., Schreider, A.A. and Ceccone, G., (1997): Geophysical evidence of a relict oceanic crust in the South-western
Scotia Sea. Mar. Geophys. Res. 19, 439-450.
• Maldonado, A., A. Barnolas, F. Bohoyo, J. Galindo-Zaldívar, J. Hernández-Molina, F. Lobo, J. Rodríguez-Fernández, L. Somoza,
and J. T. Vázquez (2003): Contourite deposits in the central Scotia Sea: the importance of the Antarctic Circumpolar Current and
the Weddell Gyre flows, Palaeogeogr., Palaeoclim., Palaeoecol., 198, 187–221.
• Maldonado A., Bohoyo F., Galindo-Zaldívar J., Hernández-Molina F.J., Javaloy A., Lobo F.J., Rodríguez-Fernández J., Suriñach E.
and Vázquez J.T. (2006): Ocean basins near the Scotia–Antarctic plate boundary: Influence of tectonics and paleoceanography on
the Cenozoic deposits. Marine Geophysical Researches, 27 (2): 83-107.
• Miller, K.G., Wright, J.D., Katz, M.E., Browning, J.V., Cramer, B.S., Wade, B.S. and Mizintseva, S.F. (2008): A view of Antarctic
ice-sheet evolution from sea-level and deep-sea isotope changes during the Late Cretaceous-Cenozoic. In: Cooper, A. K., P. J.
Barrett, H. Stagg, B. Storey, E. Stump, W. Wise, and the 10th ISAES editorial team, eds. (2008). Antarctica: A Keystone in a
Changing World. Proceedings of the 10th International Symposium on Antarctic Earth Sciences. Washington, DC: The National
Academies Press, pp: 55-70.
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Session 8
OIL
AND MINERAL RESOURCES
GEOSUR2010
OCCURRENCE OF SHALLOW GAS IN THE EASTERNMOST LAGO FAGNANO
(TIERRA DEL FUEGO)
8-01
Darbo, A.1, Baradello, L.1, Lodolo, E.1, Grossi, M.1, Tassone, A.2, Lippai, H.2
(2) Instituto de Geofísica “D. Valencio”, Dpto. de Geologia, Universidad de Buenos Aires, Argentina
High-resolution seismic profiles acquired on November 2009 in the Lago Fagnano (Tierra del Fuego)
have shown the presence of shallow gas layers in the south-easternmost sector of this basin. Data have
been acquired in the frame of an Italian-Argentinean scientific project funded by the Italian Foreign
Ministry. The survey consisted of single-channel seismic reflection profiles acquired using a Boomer
source and a single-channel streamer (10-hydrophones selectable array). These data complement and
complete the bathymetric and seismic surveys carried out previously in the Lago Fagnano (Lodolo et
al., 2007; Waldmann et al., 2010). Moreover, during this Campaign, some gravity piston cores have
been collected to analyze the stratigraphy of the most recent (Middle to Late Holocene) sedimentary
cover in the eastern sector of the basin.
The high-resolution seismic investigation has revealed an extensive area marked by poor seismic
penetration that is caused by the presence of shallow gas (Fig. 1). The gas-related features observed
on the seismic profiles include typical acoustic turbidity with a strong phase reversal reflector on top
that creates multiple reflections (Best et al, 2004) . The gassy sediments exhibit high attenuation
(blanking) that hide geological sub-surface structures. The lake-floor morphology does not reveal any
evidence of clear gas escape from the floor. The top of the acoustically turbid layer is located between
0-1 and 7-10 m below the lake-floor surface. It generally forms a sharp boundary, often marked by a
varying offset probably due to different levels of gas penetration which could be related to the
lithology (poorly consolidated muddy layers) of the overlying sediments. Shallow gas horizons are all
located in the south-eastern sector of Lago Fagnano where water depths vary from 20 to 50 m and
where a presence of a ground moraine in the vicinity of the lake shore is reported (Coronato et al.,
2009). This geographical distribution may be in some ways conditioned by shallow structural
lineaments associated to the left-lateral transform system which separates the continental South
American plate from the Scotia plate (Lodolo et al., 2003; Menichetti et al., 2008). Lago Fagnano
itself occupies a segment of the transform system, and is considered an example of pull-apart basin
developed in a series of graben-shaped, asymmetrical tectonic sinks disposed in an en-echelon
arrangement along the transform boundary (Lodolo et al., 2002; Lodolo et al., 2003). The tectonic
structure of the Lago Fagnano formed presumably during the Paleogene and was subsequently
modified by glacial erosion, especially during Late Quaternary (Menichetti et al., 2007).
Seismic characteristics of the profiles where shallow gas layers have been individuated seem to
suggest a low concentration of gas, most likely less than 1%. To confirm the actual presence of gas,
some gravity coring have been performed in correspondence of the high-resolution seismic profiles
Fig. 1 - Lago Fagnano location map (left); map of the shallow gas occurrence with the grid of the high-resolution
seismic profiles acquired in the easternmost sector of the basin (middle);example of a high-resolution line showing the
presence of a shallow gas layer characterized by a blanking effect (right).
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in both places where the blanking effect was most relevant, and in areas where shallow gas was not
evident. These cores were then analyzed in laboratory in order to quantify and define the type of gas.
Gas was not sampled from the cores, mostly because it was almost completely released in the lake
water during the core recovery. It can be thought of as the reduction of hydrostatic pressure from 40
m depth at which the samples have been retrieved to atmospheric pressure, has favoured the
immediate volatilization of gases in the water column. Further laboratory analyses will be carried on
the recovered sediments to eventually detect the presence of heavier gases. As a preliminary
interpretation, we may assume that the main origin of the gas could to some extent be linked to the
presence of a shallow, thin peat-rich layer of Middle-Late Holocene age.
To date, this is the first evidence of shallow gas layers in Tierra del Fuego lakes.
REFERENCES
• Best, A.I., Tuffin, M.D.J., Dix, J.K. and Bull, J.M. (2004). Tidal height and frequency dependence of acoustic velocity and
attenuation in shallow gassy marine sediments. Journal of Geophysical Research, 109, (B8), B08101.
• Coronato, A., Seppala, M., Ponce, J.F., Rabassa, J. (2009). Glacial geomorphology of the Pleistocene Lake Fagnano ice lobe, Tierra
del Fuego, southern South America. Geomorphology, 112, 67-81.
• Lodolo, E., Menichetti, M., Tassone, A., Geletti, R., Sterzai, P., Lippai, H. and Hormaechea, H-L. (2002). Researchers target a
continental transform fault in Tierra del Fuego. EOS, Trans., AGU, 83, 1-5.
• Lodolo, E., Lippai, H., Tassone, A., Zanolla, C., Menichetti, M., Hormaechea, J. L. (2007). Gravity map of the Isla Grande de Tierra
del Fuego, and morphology of Lago Fagnano. Geologica Acta, 4, 307-314.
• Lodolo, E., Menichetti, M., Bartole, R., Ben-Avraham, Z., Tassone, A., Lippai, H. (2003). Magallanes-Fagnano continental transfom
fault ( Tierra del Fuego, southernmost South America). Tectonics, 6, 1076.
• Menichetti, M., Lodolo, E., Tassone, A., Hormaechea, J. L., Lippai, H. (2007). Geologia dell’area del Lago Fagnano in Terra del
Fuoco (Sud America). Rend. Soc. Geol. It., 4, 251-254.
• Menichetti, M., Lodolo, E., Tassone, A. (2008). Structural geology of the Fuegian Andes and Magellanes fold-and-thrust beld Tierra del Fuego Island. Geologica Acta, 1, 19-42.
• Waldmann, N., Ariztegui, D., Anselmetti, F.S., Austin, J.A., Moy, C.M., Stern, C., Recasens, C., Dunbar, R.B. (2009). Holocene
climatic fluctuations and positioning of the Southern Hemisphere westerlies in Tierra del Fuego (54°S), Patagonia. Journal of
Quaternary Science, doi: 10.1002/jqs.1263.
GEOLOGY OF THE SAN PEDRO MINING DISTRICT,
SAN RAFAEL MASSIF (ARGENTINA)
8-02
Gómez, A.1*, Rubinstein, N.2
(1) CONICET. Departamento de Geología, Ciudad Universitaria, Buenos Aires
(2) CONICET-Universidad de Buenos Aires.
* Presenting author’s email: anabel@gl.fcen.uba.ar
Introduction
The San Pedro mining district is located in the central part of the San Rafael Massif (35º 21’ 58.6” S;
68º 23’ 22” W), province of Mendoza, Argentina. The San Rafael Massif is characterized by
widespread volcanic and pyroclastic rocks of Gondwanian age, known as Choiyoi Magmatic Cycle.
Two different suites can be distinguished within this volcanic sequence (Llambías et al., 1993). The
Lower Permian suite (lower section) has geochemical characteristics that indicate a subduction setting
and a transpressional deformation style. The Upper Permian suite (upper section) has a geochemical
signature which can be interpreted as transitional between subduction and continental intraplate
settings and a structural style typical of an extensional regime (Kleiman and Japas, 2009).
New information provided by fieldwork and petrographic studies allow redefining the hydrothermal
alteration assemblages and mapping the alteration zones.
Geological setting
In the studied area the Choiyoi Magmatic Cycle is represented by the lower section which includes
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Fig. 1 - Geology of the San Pedro mining district, showing the location of alteration zones and the polymetallic veins
outcropping in the area.
pyroclastic rocks intruded by a sub-volcanic intrusive (San Pedro Hill) cut by andesitic dykes. The
whole Permian sequence is overlain by Quaternary alkaline basalts (Fig. 1).
The pyroclastic rocks correspond mainly to a moderately welded massive ignimbrite of dacitic
composition that in the upper part of the deposit shows fine lamination and strong oxidation. It is
composed of quartz, feldspars and micas crystaloclasts and accessory and cognates lithic fragments
in a felsitic matrix with recrystallized shards and fiamme and scarce disseminated pyrite. In the northwestern part of the studied zone a pyroclastic breccia (probably a “block and ash” deposit) crops out,
and is composed of andesitic clasts up to 10 cm in length bounded by curviplanar surfaces immersed
in a very fine andesitic matrix.
The sub-volcanic intrusive has a porphyritic to granular texture and quartz-dioritic composition. It is
composed of plagioclase and minor clinopyroxene with scarce reddish brown biotite, pale green
amphibole and interstitial K-feldspar and quartz (occasionally conforming graphic intergrowth) and
abundant disseminated magnetite and pyrite crystals. In the western contact with the dacitic
ignimbrite it produced a poorly sorted volcanic breccia composed of ignimbritic clasts with ameboid
shape.
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Fig. 2 - Potassic alteration veins: a) Quartz veins with K-feldspar alteration halos. b) Quartz -magnetite veins.
The San Pedro Mining district
This mining district is hosted by the Permian pyroclastic and volcanic rocks of the area and consists
of an alteration zone with disseminated copper mineralization and a group of polymetallic veins that
were partially mined during the end of the XIX century and the beginning of the XX century (Salazar,
1974). It was preliminarily classified as a porphyry copper deposit (Delpino et al., 1993; Rubinstein
et al., 2002). Later fluid inclusions studies (Korseniewsky and Rubinstein, 2005) reveal the presence
of high salinity and high temperature inclusions with daughter crystal (including hematite and opaque
minerals) supporting the proposed genetic model.
Based on stratigraphic constraints, Rubinstein et al., (2002) suggested that the polymetallic veins are
genetically related to the porphyry copper system. Lead isotopes analyses confirmed the genetic link
between this mining district and the lower section of the Choiyoi Magmatic Cycle (Rubinstein et al.,
Table 1 - Ore paragenesis and hydrothermal alteration assemblage for the principal veins of Cerro San Pedro (modified
from Rubinstein et al., 2002). ag: silver; ap: apatite; az: azurite; bn: bornite; cb: carbonate mineral; cerussite: cer; cct:
chalcocite; ccp: chalcopyrite; chal: chalcanthite; chl: chlorite; ccl: chrysocolla; cv: covellite; gn: galena; gp: gypsum;
hem: hematite; ilt: illite; lm: limonites; mlc: malaquita; mol: molibdenita; py: pyrite; qtz: quartz; ser: sericite; rt: rutilo;
sp: sphalerite; str: stromeyerite tnt: tennantite.
Veins
Trend and dip
Ore paragenesis
Gangue
Supergene
paragenesis
Hydrothermal
alteration
La Julia
N25ºE/72º SE N35ºW/
vertical N70ºW/
vertical N42ºW/
72ºSW
py- ccp - gn- molbn
qtz
cv - cc- lm -mal
ser (ilt)-qtz(rt- ap - chl)
La Margarita
N74º W/ vertical
py – sp - ccp -(gn)
qtz
lm - gp - mlc -az –
ccl – chal
ser (ilt) – qtz
(rt -ap) cb - qtz
San Pedro
N70ºE/ 80ºSE
qtz
lm - az – mlc
ser - qtz
Santo Tomás
N80º E/ vertical; 75º
SE/ vertical N51ºE/
vertical N75ºW/
vertical
py
qtz
lm – mlc
ser (ilt) - qtz
Sin Nombre
65º/subvertical
hem- py - (ccp)
qtz
qtz - ser - (chl)
San Eduardo
N75ºE
gn - (sp - ccp - py)
qtz
ser –qtz cb– (qtz)
Juanita
290/ 68º S
gn - py
qtz
La Salvadora
N13ºE/ N25ºW 47ºSW
gn - cct - (ag - cp qtz - cb
bn-str-tnt-sp- hem)
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lm
ser – qtz cb - qtz
lm - mlc- cer
ser – qtz cb - qtz
GEOSUR2010
2004).
Outwards, the alteration zone consists of pervasive propylitization homogeneously distributed in the
dioritic intrusive; locally chlorite-epidote veinlets are observed (Fig. 1). The alteration assemblage
consists of chlorite, epidote, carbonate, tremolite, sericite and minor albite. The potassic alteration is
irregularly distributed within the intrusive (Fig. 1) and has an assemblage of K-feldspar, biotite,
magnetite and quartz. It occurs pervasively and in veinlets with parallel walls suggesting that they
were formed under a brittle regime. Three types of veins have been preliminary recognized based on
their morphology, mineralogy and character of the halos: barren K-feldspar veins without alteration
halo; quartz veins with no or very scarce pyrite and chalcopyrite with or without K-feldspar alteration
halos (Fig. 2a) and quartz - magnetite veinlets (Fig. 2b).
About 3 km south-east to San Pedro Hill, in La Totora River, there is a small outcrop (about 250 x 15
m) similar in composition to the quartz-dioritic intrusive (Fig. 1). It shows intense pervasive potassic
alteration with an assemblage of K-feldspar and a quartz stockwork structure with chalcopyrite and
pyrite. Veinlets with oxidation minerals (gypsum, malachite and azurite) are frequently observed.
Geochemical analyses carried out by Portal Resources (Davicino, 2008) returned values that reach
2.5% Cu and 542 ppm Mo. Geophysical surveys (Johanis, 2003) show a Th/K and U/K low and a
positive magnetometric anomaly in centre of San Pedro Hill coinciding with the central potassic zone.
Weak phyllic alteration is irregularly distributed in the quartz-dioritic intrusive and the dacitic
ignimbrite (Fig. 1). It bears pervasive silicification and sericitization (illite determined by short wave
infrared reflectance spectrometry, SWIR) with scarce disseminated pyrite and quartz-pyrite stockwork
structure with minor chalcopyrite, sphalerite and galena.
A late carbonatization process, pervasive and also present in veins with minor quartz, overprints both
the potassic and phyllic alteration.
The main characteristics of the polymetallic veins of the area (Fig. 1) are summarized in Table 1. They
crop out within or close to the alteration zone and are controlled by N-S, NW-SW and NW-SE
regional structures. Close to the contact with the veins the host rocks show pervasive and vein-type
silicification and pervasive sericitization (Rubinstein et al., 2002).
Conclusion
Studies carried out in the San Pedro mining district allow mapping the geological units and also
discriminating the alteration assemblages and their field distribution. In this way the new information
will contribute to characterize the ore deposits of the area and hence to establish its genetic model.
Acknowledgments
This study was financially supported by UBACyT X485 project (Universidad de Buenos Aires). We
thank the Servicio Geológico Minero Argentino (SEGEMAR) for supporting the field work.
REFERENCES
• Davicino, R.; 2008. A review of the Anchoris proyect, Argentina. Unpublished report, 39 p.
• Delpino, D., Pezzutti, N., Godeas, M., Donnari, E., Carullo, M., Núñez, E., 1993. Un cobre porfírico paleozoico superior en el centro
volcánico San Pedro, distrito minero El Nevado, Provincia de Mendoza. Comptes Rendus XII ICC-P, 1: 477-490. Buenos Aires.
• Johanis, P., 2003. Informe geofísico San Pedro – Las Chilcas. Unpublished report, 2p. Servicio Geológico Minero Argentino,
Buenos Aires.
• Kleiman, L.E., Japas, M.S.; 2009. The Choiyoi volcanic province at 34°S – 36°S (San Rafael, Mendoza, Argentina): Implications
for the Late Palaeozoic evolution of the southwestern margin of Gondwana. Tectonophysics, 473, (3-4):283 – 299.
• Korseniewsky L.I., Rubinstein, N., 2005. Estudio de inclusiones fluidas en la veta La Julia, Cerro San Pedro, provincia de Mendoza.
Congreso de Geología Económica, 1:171-174. Buenos Aires.
• Llambías, E .J., Kleiman, L. E. and Salvarredi, J. A., 1993. El magmatismo gondwánico. En: Geología y Recursos Naturales de
Mendoza, (Ed: Ramos, V.A.), Relatorio 12° Congreso Geológico Argentino: 53-64. (Mendoza).
• Rubinstein, N., Carpio, F., Mallimacci, H., 2002. Las vetas polimetálicas del área del Cerro San Pedro, provincia de Mendoza,
Argentina. 15° Congreso Geológico Argentino, 2: 263 – 266. Calafate.
• Rubinstein, N., Ostera, H., Mallimacci, H., Carpio, F., 2004. Lead isotopes from gondwanic ore polymetallic vein deposits, San
Rafael Massif, Argentina. Journal of South American Earth Science 16 (7): 595 – 602.
• Salazar, L., 1974. Distrito mineralizado “Costa del Nevado”, Cu, Pb, Zn, Ag. Unpublished report, Los Chalanes S.A., 20p. Servicio
Geológico Minero Argentino, Buenos Aires.
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INDUCED POLARIZATION–RESISTIVITY EXPLORATION IN THE POLYMETALLIC
PURÍSIMA-RUMICRUZ DISTRICT, JUJUY PROVINCE (ARGENTINA)
8-03
López, L.1,2*, Echeveste, H.1, Tessone, M.1
(1) INREMI. Facultad de Ciencias naturales y Museo
(2) CONICET. Consejo Nacional de Investigaciones Científicas y Técnicas
* Presenting author’s e-mail: lopezluciano@hotmail.com
Introduction
The studied area is located at the edge between the Puna and Cordillera Oriental geological provinces,
in the north-western Argentina. The district is 25 km east of Abra Pampa town, 1800 km north of
Buenos Aires. North-western Argentina has a long mining history. Aguilar is the most important mine
in the area and produces a Pb-Ag-Zn concentrate from an Ordovician SEDEX deposit type, with a
later remobilization event produced by intrusion-related metamorphism of Mesozoic granitic plutons
(Sureda, 1999). Purísima-Rumicruz has anomalous contents of Ag, As, Ni, Co, Cu and Pb, and
together with La Esperanza and La Niquelina, represents a small but uncommon metallogenic
province in Argentina. This province gathers together deposits of Puna and Cordillera Oriental
classified as five element deposit by Lurgo Mayón (1999). Induced Polarization (IP) - Resistivity
study is a well known geophysical method to determine the presence of sulfide ores, particularly
hosted in vein system. IP anomalies in sulfide-rich vein deposits are considered excellent exploration
guides to determining future targets for exploration.
Regional and structural geology
Purísima-Rumicruz veins are hosted in the Acoite Formation, a low-grade metamorphosed
sedimentary rocks composed by sandstones and dark shales that were deposited in a shallow wavedomain platform. A progradation sequence can be distinguished from base to top. Ortho-quartzite and
grainstones beds are interbedded in the sandstone/siltstone sequence. Profuse biostratigraphic studies
have been carried out on the Acoite Formation defining a late Tremadocian to Arenigian age for these
rocks. The deformation history of the region can be summarized into three main episodes: (1) a late
Ordovician deformation (Ocloyic orogeny); (2) an early Cretaceous to low Tertiary rifting process;
and (3) the Eocene to Present Andean orogeny, characterized by deformation of variable intensity
produced by subduction and generation of a magmatic arc.
Mineralization
Purísima-Rumicruz deposit has been mined on the sixties and seventies decades. The ore was mined
with 450 meters of underground galleries manually worked. Those galleries are actually inaccessible
because flooded and/or collapsed. Veins at surface are narrow, from 0.2 to 1 m wide, and have no
topographic contrast with the host rocks. They are mostly covered by Quaternary sediments and their
length is difficult to determine. Veins are characterized by brecciated textures, with sub-angular to
round-shaped clasts. At least five events of brecciation and stockwork can be defined in the veins.
First, a chalcocite, galena and chalcopyrite-bearing pulse, then a second pulse with coarse quartz, an
extensive third infill of calcite, barite-rich pulse, and finally a limonite, malaquite and azurite
stockwork. Alteration is subtle to weak, and it is mainly represented by oxidation of the vein and a few
centimeters on the sides of it. There is also some restricted argillic alteration in the margins of the
veins. Spatially related to this mineralization, another paragenetic association was described,
consisting in narrow veinlets of nickeline, amorphous uraninite, rammelsbergite, gersdorffite,
covellite, and sphalerite (Brodtkorb, 1973). Considering textures, structures, clast shape and size
distribution, Purísima-Rumicruz veins were interpreted as infill of dilatational fault zones with
mechanical brecciation produced by shear stress and minor chemical corrosion. A strong structural
control was defined in the area (López et al., 2008). Sulfide veins have an E-W trend (PurísimaRumicruz, La Nueva) barite-bearing veins a WNW-ESE trend (El Brechón) and quartz veins are
characterized by random orientations. Exploration of the area consisted in a lithological and structural
mapping at 1:20,000 scale, followed by a detailed mapping of the host rock, veins, structure and
alteration performed at 1:5,000 scale. In addition, geochemical data of the dump piles was performed
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Fig. 1 - A. IP (left) and Resistivity (right) plan maps (-57 m) of Purísima-Rumicruz, La Nueva and Brechón veins. B.
Geology, IP and Resistivity cross sections at line 600 in Purísima-Rumicruz veins.
on 19 rock chip samples, and a 7,300 m of IP-Resistivity survey was completed.
Methodology
The IP techniques are based on the study of secondary electric fields generated in the ground by
electric currents and is one of the most widely used techniques in ore deposit prospecting.
Geoelectrical works were focused on the central portion of the District, considering the location of the
veins. Three areas (Fig. 1) were defined “Purísima” (P), “La Nueva” (N) and “El Brechón” (B). A total
of 17 lines were realized to perform this study. The orientation of these lines was normal to the average
orientation of the veins. The length was variable; Purísima has all 500 m long lines, La Nueva 300 m
lines and in Brechón the length varies between 500 m and 200 m. Additionally, a topographic survey
along each line was realized with a hand clinometer (Abney Level) every 25 m, with GPS (Garmin
eTrex) control points at the beginning and the end of each line. The methodology of the survey was
carried out in a linear “multielectrodic” design, with a dipole-dipole configuration. The distance
between the electrodes “a” was 25 m and on each station depth level (n) vary from n=1 to n=8. The
duration of each cycle was two seconds. The receptor was a IPR-12 Time Domain IP/Resistivity
Receiver and the energization was performed with a IPC-9/200W, both made by Scintrex Company.
Apparent IP and resistivity data were processed with the RES3DINV 2.14 version software. Data
inversion was made with the Gauss-Newton (Loke and Dahlin, 2002) method, because the amplitude
of the resistivity data. The final result of the inversion shows a model of true (corrected) IP and
resistivity. The Root Mean Square (RMS) was always less than 10%. True IP and resistivity of each
line was presented in a geological section. Maps showing values of IP and resistivity results at 57m
underneath surface were plotted.
Results
The IP anomalies are interpreted as due to the presence of polarizable minerals, and according to
outcropping vein composition and structure, we assume the presence of sulfide-rich veins beneath the
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surface. According to IP data, areas with values higher than 50 mv/v are considered anomalies. Range
between intervals is 10mv/v. Three categories of IP anomalies were defined in decreasing order of
importance, according to intensity, vertical and lateral continuity, and spatial relationship with
outcropping mineralization. Despite this, caution must be taken in the interpretation because of
presence of disseminated pyrite in the Acoite Formation siltstone. Nevertheless morphology of
anomalies did not match with the strike of stratification, which could be interpreted as a pyrite rich
bed, and correspond with orientation of veins on surface. Similar categories were established in the
polymetallic vein of the Pingüino area, in the Santa Cruz Province. There, the most important anomaly
was drilled and one of the holes becomes the most important base metal intersection of the entire
project (Guido et al., 2009). Resistivity interpretation is more difficult than IP data. Usually high
values of resistivity could be related to the presence of low porosity of host rock, quartz veins or
silicified levels. Low values of resistivity could be associated to fault zones, sulfide-rich veins and/or
the presence of water in pores or fractures. By the above, sulfide rich veins could be represented by
high IP and low resistivity. In the area, some low values of resistivity fit with high values of IP,
showing a negative correlation between these two parameters. This correlation is evident in the
Brechón and in the La Nueva areas, but not very clear in Purísima-Rumicruz area.
Conclusions
A negative correlation between IP and Resistivity values was observed in the geophysical survey at
Purísima Rumicruz veins. This can be related to minor presence of quartz or the brecciated texture
of the veins, together with the presence of patches of massive sulfides.
Purísima Rumicruz is a small but rare deposit with an unusual paragenetic association, and in spite of
been mined in the past, there is still a potential ore deposit beneath surface that has the potential to be
explored. Geoelectric (IP - Resistivity) shows to be an excellent methodology in areas covered with
Quaternary sedimentation like this.
Although the northern Argentina have the longest history of mining, an updating of the exploration
procedures is necessary to achieve a more predictive method for discovering hidden ore bodies.
REFERENCES
• Brodtkorb de, M K.; 1973: Estudio de la mineralización del yacimiento “La Niquelina”, provincia de Salta y un análisis
.comparativo de sus posibles relaciones con los depósitos “Rumicruz” y “Esperanza”. Revista de la Asociación Geológica
Argentina, 27, 4: 364-368.
• Guido D. M., Jovic S. M., Echeveste H., Tessone M. O., Ramayo Cortes L., Schalamuk I. B.; 2009: Descubrimiento y modelización
de clavos mineralizados en vetas polimetálicas a partir de exploración geoeléctrica, proyecto Pingüino, Macizo del Deseado. Revista
de la Asociación Geológica Argentina 64 (3): 203 - 210 (2009).
• Loke, M.H. y Dahlin, T.;2002: A comparison of the Gauss-Newton and quasi-Newton methods in resistivity imaging inversion.
Journal of Applied Geophysics 49: 149-162.
• López L., Echeveste H., Schalamuk I. B.; 2008: Nuevos aportes en el distrito minero Purísima Rumicruz, provincia de Jujuy. XVII
Congreso Geológico Argentino. San Salvador de Jujuy. Actas (II): 607-608. Jujuy, Argentina. ISBN 978-987-22403-1-8.
• Lurgo Mayón, C. S.; 1999: Depósitos polimetálicos ricos en níquel, cobalto y arsénico de la Cordillera Oriental, Jujuy y Salta. In:
Recursos Minerales de la República Argentina (Ed. E. O. Zappettini), Instituto de Geología y Recursos Minerales SEGEMAR,
Anales 35: 999-1004, Buenos Aires.
• Sureda, J. R.; 1999: Los yacimientos sedex de plomo y zinc en la Sierra de Aguilar, Jujuy. En: Recursos Minerales de la República
Argentina (Ed. E. O. Zappettini), Instituto de Geología y Recursos Minerales SEGEMAR, Anales 35: 459-485, Buenos Aires.
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ROCK-MAGNETISM PROPERTIES FROM DRILL CUTTING AND
THEIR RELATION WITH HYDROCARBON PRESENCE AND
PETROPHYSICAL PARAMETERS
8-04
Mena, M.*, Walther, A.M.
CONICET-INGEODAV, Dpto. Ciencias Geológicas, Facultad de Ciencias Exactas y Naturales,
* Presenting author’s e-mail: mena@gl.fcen.uba.ar
We present a study of the vertical variations of magnetic properties, performed on cutting samples
from an exploratory hydrocarbon well drilled in the Golfo San Jorge basin, Argentina. The studied
stratigraphic section is 248 m long. The sampled levels correspond to tuffaceous sandstones and
sillstones, and silty to sandy tuffs. A total of 108 samples of drill cutting were used, each one
representative of a level 2 m to 3 m thick. The mass magnetic susceptibility was measured at three
different frequencies for five specimens per sample. The related frequency-dependent susceptibility
factor (FDF), the mean mass susceptibility (X) and the standard deviation for each level were
calculated. Different magnetic phases were identified on the basis of detailed measurement of
isothermal remanent magnetization (IRM) performed on previously consolidated specimen per each
sample. The different coercivity ranges defined suggest the important presence of titanomagnetite,
magnetite and magnetic pyrrhotite, and oxidized magnetite in lower proportion. Concentration
indexes (CI) for the identified magnetic minerals were calculated. Curves of X, FDF, saturation of
IRM (SIRM) and CI were drawn and analyzed together with lithological information and geophysical
logs.
The three parts of the profile where the cutting descriptions were performed indicate the presence of
hydrocarbon-impregnated tuffaceous sandstone levels, which coincide with higher X and SIRM
values. However, the correlation coefficients between the percent contribution of those lithologies and
the susceptibility and SIRM values are not statistically significant. The profile depths with
hydrocarbon-impregnated rocks that present certain productive interest are correlated with levels in
which the magnetite CI shows relative increases. But in general, the areas with higher porosity,
defined from density logs (DPHI) but especially those defined from sonic (SPHI) and neutronic logs
(NPHI) match with areas with higher magnetite CI. On the contrary, the peaks of titanomagnetite CI
coincide with areas of relative decrease of NPHI, DPHI and SPHI. The levels where gas was detected
coincide with the biggest magnetite CI values. In those sectors, the levels of higher magnetite and
pyrrhotite concentrations agree with the depths where higher porosity values (NPHI, DPHI and SPHI)
were registered. Although the gas peaks are located in levels where the relative content of magnetite
is higher, the immediately superior levels, about 3m above them, are the ones that present a larger
growth of magnetite and also of pyrrhotite content. This could indicate the authigenic formation of
magnetic minerals influenced by hydrocarbon microseepage from lower strata.
Magnetic susceptibility values and quantitative mineralogical information derived from rock
magnetism studies can provide accurate data concerning to lithological variations. The qualitative
correlations between magnetic data and key petrophysical parameters such as porosity, joined to the
association of magnetic mineralogy with hydrocarbon presence or migration, show the potential utility
of these techniques for subsurface exploration.
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USE OF ASTER IMAGERY TO IDENTIFY MINERALIZATION IN THE ANDEAN
CORDILLERA FRONTAL (31º45´S), SAN JUAN PROVINCE (ARGENTINA)
8-05
Pérez, D.J.1*, D’Odorico, P.2
(1) Laboratorio de Tectónica Andina, Universidad de Buenos Aires, Ciudad Universitaria, Pabellón II
(1428), Buenos Aires, Argentina
(2) ArPetrol Argentina S.A.
* Presenting author’s e-mail: daniel@gl.fcen.uba.ar
Introduction
The main aim of this study was to verify the potential uses of multispectral imagery for
geologic/mineral mapping, and show a methodology for search of mineral resources. We used ASTER
(Advanced Space borne Thermal Emission and Reflection Radiometer) data to detect the presence of
altered rocks associated with hydrothermal alteration of porphyry system. In arid environments, the
spectral signatures of diagnostic minerals are often not masked by water, vegetation, or superficial
materials. The ASTER data characteristics were originally selected with the purpose of achieving
remote mineralogical identifications and thus it became a power tool to apply in geology. Digital
image processing techniques were used with ASTER data to enhance lithologies and to detect
alteration associated with possible mineral deposits. ASTER imagery was combined with field
mapping and PIMA (Portable Mineral Infrared Analyzer) field data, into a geographic information
system; and was integrated in order to establish the relationship with a structural model of the
mineralized bodies.
The study region is located at 31°45’ S and 70°00’ W, in the Frontal Cordillera of San Juan Province,
in the south end of flat slab subduction segment. Climate is generally arid, and vegetation and soil
cover are poorly developed (Fig. 1).
Geology and structure
The stratigraphic sequence of the region is as follows (from bottom to top): A Carboniferous and
Permo-Triassic basement (Choiyoi Group), composed of rhyolites and granites. Mesozoic deposits
represented by the volcaniclastic, pyroclastic and sedimentary rocks of the Rancho de Lata Formation
(Triassic-Jurassic), interpreted as syn-rift deposits. Jurassic marine deposits of the Los Patillos and La
Manga Formations (Lias-Dogger), interpreted as a sag phase deposits, and continental sequences of
the Tordillo Formation (Malm). Without stratigraphic relationship is the Auquilco Formation, formed
by gypsum. Overlaying the latter sequence, continental sedimentary and volcaniclastic Cretaceous
sequences of the Diamante and Cristo Redentor Formations are developed. These deposits are overlaid
by the volcanic rocks of the Farellones formation, being this volcanism responsible for the
development of the hydrothermal alteration areas like Los Pelambres, Pachón, Carnicerías, La Coipa
and Altar. The structure of the region has two distinct styles, one of thick skinned, with tectonic
inversion evidence that involves the Permo-Triassic basement rock; and a thin skinned style, that
affects the Cenozoic and Mesozoic sediments.
Remote Sensing
Aerial photography has been the most commonly used type of remote sensing data, but satellite data
have advanced applications because of the larger number of image wavelengths. There are several
satellite platforms, many of them containing more than one sensor on board. ASTER is one of such
instruments currently available. The ASTER is a multispectral instrument mounted on the Earth
Observing System (EOS), TERRA. ASTER has 14 bands in 3 regions subsystems; three bands on
visible and near infrared (VNIR, 0.52-0.86 Ìm), six bands on short-wave infrared (SWIR, 1.60-2.43
Ìm), and five bands on thermal infrared (TIR, 8.125-11.65 Ìm); which have 15, 30, and 90-m spatial
resolution, respectively. It has also a stereo mode by the nadir looking band 3N and backward-looking
band 3B of VNIR.
Digital image processing techniques were used with ASTER data to enhance lithologies and to detect
alteration associated with mineral deposits. ASTER imagery was combined with field mapping and
PIMA data field, into a geographic information system. This data-set was integrated in order to
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Fig. 1 - Location map for the presented figures.
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Fig. 2 - Color composite, RGB 641.
Fig. 3 - Band ratios 4/5, 4/6, 4/7 (RGB)
establish the relationship with a geological model of the mineralized bodies.
Processing, analysis and methodology
Analysis of ASTER spectral reflectance data provides a basis for geological mapping and
distinguishes hydrothermal alteration zones. Different image processing techniques were applied in
order to extract the information. Samples were collected to be analyzed with a PIMA spectrometer in
order to determine what alteration could be distinguished by spectral reflectance alone. Individual and
combination minerals could be separated in terms of their reflectance. The integrated approach to this
study aided geologists to identify several mine prospects such as Carnicería, La Coipa, Yunque and
Yeguas Heladas. These deposits were found by using the ASTER imagery to identify areas with
Fig. 4 - Index OHI (a)
246
Fig. 5 - Index OHI (b).
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Fig. 6 - SAM classification
Fig. 7 - SAM classification
characteristics similar to known prospects such as Pachon, Los Pelambre and Altares.
An ASTER L1B image of January 21 2001 was used, and the following pre-processing were applied:
(1) correction of the effect Crosstalk; (2) correction for radiance; (3) atmospheric correction to obtain
data in apparent reflectance. Then, the following processing were carried out: color composites, color
ratios composites, Spectral Angle Mapper classification, and Ninomiya index.
Band combinations
The band combinations allow getting a first geological interpretation of the region. The band
combination 321 (RGB) allowed to identify and correlate different lithologies with olds maps. The
band combination 654 (RGB), allows a first regional identification of possible areas of hydrothermal
alteration (Fig. 2).
Band ratios
Because in many cases the spectral characteristic of the rocks are similar, is not possible to
discriminate different lithologies with imagery interpretation, from color bands combinations. For
these reason band ratios combinations in the SWIR were used for discriminating areas of
hydrothermal alteration, since these present picks of absorption and of reflectance characteristic in
this region of the electromagnetic spectrum. The band ratios combination of 4/5, 4/6, 4/7 (RGB),
allowed identify areas of hydrothermal alteration (Fig. 3).
Ninomiya – SWIR index
Minerals as montmorillonite and sericite present picks of absorption in band 6 of ASTER, while the
pirofilite presents a characteristic pick of absorption in band 5. Beside that, the caolinite and alunite
present characteristic picks of absorption in bands 5 and 6 respectively. Considering these parameters,
the qualitative estimate of the presence of these minerals was achieved, using the indexes.
• OHI
• OHI
• ALI:
• CI:
(a):
(b):
(band 4 * band 7) / (band 6 * band 6);
(band 4 * band 7) / (band 5 * band 5);
(band 7 * band 7) / (band 5 * band 8).
(band 6 * band 9) / (band 8 * band 8).
The OHI(a) index identifies minerals that present picks of absorption in the band 6 while the OHI(b)
index allows the identification of minerals that present picks of absorption in the band 5; the ALI
index allows to distinguish alunite for its pick of absorption in the band 8. Applying these indexes it
is possible to distinguis minerals or mineral groups with alteration, based on their respective spectral
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characteristics (Fig. 4 and Fig. 5).
Spectral Angle Mapper Classification – SAM
The supervised classification is an automated method for comparing image spectra to individual
spectra or a spectral library. SAM assumes that the data have been reduced to apparent reflectance
(true reflectance multiplied by some unknown gained factor controlled by topography and shadows).
The algorithm determines the similarity between two spectra by calculating the “spectral angle”
between them, treating them as vectors in a space with dimensionality equal to the number of bands
(nb). A simplified explanation of this can be given by considering a reference spectrum and an
unknown spectrum from two-band data. The two different materials will be represented in the 2-D
scatter plot by a point for each given illumination, or as a line (vector) for all possible illuminations.
For the present SAM classification the end-members from the ASTER image were collected (see Fig.
4). In the classification, various types of data were used, as the spectra database of the USGS
(speclib4). Another used database was those derived from samples collected in the field, and analyzed
with a spectrometer PIMA. These samples allowed to determine the present alteration minerals in the
region. With these spectra, and taking into account the spectra base of the USGS (Speclib4), the
following mixture of minerals was determined: illite, caolinite, jarosite, quartz and clorite. This
process allowed to identify those minerals in the regions of Altar, The Coipa, Pachón and Los
Pelambres (Figs. 6 and 7).
Summary
Analysis of ASTER spectral reflectance data provides a basis for geological mapping and allowed to
identify and classify several areas that present processes of hydrothermal alteration. Two of these areas
correspond to Pachón and Los Pelambres locations; a third area of hydrothermal alteration is the Altar
prospect; and a fourth area is La Coipa, which would be temporally linked before to the mentioned
deposits.
These results show the potential uses of multispectral ASTER for geologic mapping in regions where
superficial exposure of rocks are limited and for mapping areas of potential mining.
SEISMIC EVIDENCE OF A GAS HYDRATE SYSTEM IN
8-06
THE WESTERN ROSS SEA (ANTARCTICA) BY TOMOGRAPHY, AVO ANALYSIS AND
PRESTACK DEPTH MIGRATION
Picotti, S.1, Geletti, R.1*, Gei, D.1, Mocnik, A.2, Carcione, J.M.1
(1) Istituto Nazionale di Oceanografia e di Geofisica Sperimentale - OGS, Trieste, Italy
(2) Dept. of Geoscience, University of Trieste, Italy
* Presenting author’s e-mail: rgeletti@inogs.it
We present a seismic evidence of the presence of gas hydrates on the Lee Arch in the Terror Rift
(western Ross Sea - Antarctica) (Fig. 1). The presence of gas hydrates is inferred from a bottom
simulating reflection (BSR), the first identified in the Ross Sea (Geletti et al., 2008; Geletti and
Busetti, 2009 and 2010). The BSR (Fig. 2) was identified and analysed through targeted reprocessing
of the multichannel seismic reflection data (3000 m streamer, 120 channels, 60 fold) acquired in 1990
by the Italian research vessel OGS Explora. The BSR is characterised by high amplitudes of reverse
polarity, above interval velocities as low as 1.4 km/s, consistent with the presence of free gas; a second
reflection of normal polarity below (about 100 ms below the BSR) and parallel to the BSR is
interpreted to mark the base of the free gas zone (Bottom of free Gas Reflector - BGR). The BSR
cross-cuts stratal reflections of the Terror Rift and it is locally offset across faults of a positive flower
structure along its eastern flank (Lee Arch). The multichannel seismic data also reveal the presence
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GEOSUR2010
Fig. 1 - a) the map of
Antarctica shows the
location of the study area
in the western Ross Sea; b)
tectonic sketch-map and
bathymetry of the western
Ross Sea showing the
position of the studied
seismic profile (Figures 2
and 3).
a
b
Fig. 2 - a) seismic profile where a gas hydrate related BSR (Base of Gas Hydrate Stability Zone - BGHSZ) and the
Bottom of free Gas Reflector (BGR) are evident; b) the highlights from section in a) show the seismic character of the
BSR and BGR compared to the seafloor reflection (modified after Geletti and Busetti, 2010).
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Fig. 3 - P- (a) and S-wave (b) velocity reflectivity sections.
along the eastern flank of the rift of seabed elevations and depressions, some of which coincide with
irregularities in the BSR that are interpreted to indicate an upward migration of gas/fluid toward
seabed. Multibeam data acquired in 2006 by the R/V OGS Explora in the frame of the Italian
Programma Nazionale di Ricerche in Antartide (PNRA) confirm the seabed elevations to be mud
volcanoes (up to 80 m high and 1-2 km wide) in water depths of 450-750 m. The faults, which control
the migration of gas toward the sea-floor, could connect the free gas zone below the BSR and the mud
volcanoes on the sea-floor (Geletti and Busetti, 2010).
The seismic data were reprocessed in order to increase the signal/noise ratio by adopting a ‘trueamplitude’ approach which preserve the real amplitudes of the reflection signals, allowing a
successive AVO (Amplitude Variations with Offset) and tomographic analysis.
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Seismic tomography and AVO gave a fundamental contribution to characterise the geometry of the
subsurface structures and the seismic properties of the sediments. Besides obtaining more detailed
velocity fields, both for the P and S waves (Fig. 3), the travel-time tomographic procedure can
determine the depths of horizons associated with the reflected and refracted arrivals. The adopted
tomographic software CAT3D, based on the SIRT method and the minimum time ray tracing (Böhm
et al. 1999), estimates the velocity field and the reflector structure in sequence, from the upper to the
deeper horizon. Thus, using the tomographic velocity model as input for the pre-stack depth
migration, we obtained a detailed imaging of the gas seeping features, the BSR and the overlying and
underlying structures.
The AVO analysis allows us to extract information on both P- and S-wave velocity reflectivity. The
ratios between the P- and S-waves reflectivity are related to the Poisson’s ratio that supplies important
information on the fluid content within a porous medium and on the rigidity of the solid matrix
(Carcione and Tinivella, 2000). Figure 3 shows that the BSR is identified both in the P- and S-wave
velocity reflectivity sections, which denotes that there is a change both in the properties of the solid
matrix and in the pore fluids across the interface.
In addition, the combined use of travel-times and attenuation in the tomographic inversion (Rossi et
al., 2007; Picotti and Carcione, 2006) provides a multi-parameter velocity-Q (quality factor) model
which, using rock-physics theories and AVO analysis, allowed to map the spatial distribution of gashydrate and free gas bearing sediments.
Finally, using the obtained petrophysical model and adopting a numerical wave-modelling algorithm,
we reproduced the real seismic section of the study area.
REFERENCES
• Böhm G., Rossi G. and Vesnaver A.; 1999: Minimum time ray-tracing for 3-D irregular grids. J. of Seism. Expl., 8, 117-131.
• Carcione J. M. and Tinivella U.; 2000: Bottom-simulating reflectors: seismic velocities and AVO effects. Geophysics, 65, 54-67.
• Geletti R. and Busetti M.; 2009: Evidenze di bottom simulating reflector (BSR) e gas seep nel Mare di Ross occidentale (Antartide).
In: 28° National Meeting of GNGTS, Geofisica di esplorazione e produzione. Trieste, Italy, 16-19 novembre. Expanded Abstract,
644 – 648.
• Geletti R. and Busetti M.; 2010: Bottom Simulating Reflector (BSR) and Base of the free-Gas Reflector (BGR) linked to gas hydrate
and gas seepage in the western Ross Sea (Antarctica). J. Geophys. Res., submitted.
• Geletti R., Praeg D. and Busetti M.; 2008: Evidence of gas hydrates and mud volcanoes in the western Ross Sea, Antarctica. In: J.
Mienert, G. Westbrook, C. Paull, H. Haflidason (convenors), Gas hydrates in oceanic and permafrost environments - importance for
energy, climate and geohazards - Session GAH-01, 33rd International Geological Congress, Oslo, 6-14 August, abstract 1352489.
• Picotti S. and Carcione J. M.; 2006: Estimating seismic attenuation (Q) in the presence of random noise. Journal of Seismic
Exploration, 15, 165-181.
• Rossi G., Gei D., Böhm G., Madrussani G. and Carcione J. M.; 2007: Attenuation tomography: An application to gas-hydrate and
free-gas detection. Geophysical Prospecting, 55, 655-669.
251
AUTHOR INDEX
The code following the name indicates the session and the location in the book.
Abarzua-Vasquez A.M.
Abraham D.A.
Acevedo R.
Adams C.J.
Ader M.
Alvarez O.
Amarasinghe U.
Arecco M.A.
Ashchepkov I.V.
Avdeev D.V.
Badi G.
Bagú D.
Baldo, E.G
Baquero M.
Baradello L.
Barbosa N.
Basei M.A.S.
Bastos P.
Batelaan O.
Bellatreccia F.
Bergantz G.
Bermudez A.
Bertrand S.
Betka P.
Bidegain J.C.
Bingen B.
Boedo F.L.
Bohoyo F.
Boiocchi M.
Bozzano G.
Brauer A.
Brümmer R.
Buffoni C.
Bujalesky G.
Caffau M.
Calatayud F.
Calderon M.
Canals M.
6-01
7-01
3-11
1-01
1-22
3-02
1-05
7-04
2-14
1-08
4-20
3-01
1-20, 1-25
5-06
3-13, 8-01
4-20
1-17, 1-21
5-06
6-06
2-10
1-16
2-01, 4-04
6-01
4-01
5-08, 5-09
1-11
1-02, 1-27
7-09
2-10
7-08
6-01
6-01
4-20
6-04
5-01
4-18
4-06
7-02
Carcione J.M.
8-06
Cardona A.
1-10
Carugati G.
4-02
Casquet C.
1-20
Casassa G.
6-02
Castro J.
2-02
Cerredo M.E.
2-03, 2-07, 2-10, 2-12
Chernicoff C.J.
1-03
CHILT Project Members
6-01
Cingolani C.
1-04
Civile D.
4-10
Clague J.J.
6-04
Clark C.
1-05
Cnudde V.
6-09
Collins A.S.
1-05
Comici C.
5-01
Connon G.
4-20
Corbella H.
3-11
Coronato A.
6-04, 6-07
Cosentino N.J.
4-03
Costa P.
7-08
Cravos C.
4-12
Currie K.
1-24
Dalziel I.W.D.
1-06
Darbo A.
8-01
De Batist M.
6-01, 6-09
DeCelles P.
1-10
D’Eramo F.
4-05, 4-19
De Isasi M.
7-04
Del Cogliano D.
3-01, 3-06, 3-12, 4-14
De la Vega M.
6-10
Della Ventura, G.
2-10
Delpino G.D.
2-01, 4-04
De Saint Blaquat M.
2-02
De Rycker K.
6-09
Dickerson P.W.
1-07
Didenko A.N.
1-08
Dietrich R.
3-01, 3-12, 6-02
253
AUTHOR INDEX
Dimieri L.V.
Di Marco A.
DÓdorico P.
Dristas J.A.
Drobe M.
Ducea M.N.
Dupuy J.L.
Dustay S.
Duyck P.
Echeveste H.
Egli R.
Emmel B.
Engvik A.
Escayola M.
Escosteguy L.
Esteban F.D.
Fagel N.
Falcone G.
Fanning C.M.
Favali P.
Fazzito S.
Fernández M.
Figari E.
Folguera A.
Fritsche M.
Frugoni F.
Galindo-Zaldivar
García Morabito E.
García M.
García R.E.
Gavriloff I.J.C.
Gei D.
Geletti R.
Gehrels G.
Geuna S.
Ghidella M.
Ghidella M.E.
Gianibelli J.C.
Gieles R.
Gilli A.
Gimenez M.
Gimenez M.E.
Godoy E.
Goetze H.J.
254
4-23
4-05
8-05
2-04, 2-07
1-12
1-16
6-05
6-03
6-09
8-03
6-07
1-11
1-11
1-09
4-05
7-03
6-01
7-06
1-07, 1-18 1-20, 4-06
7-06
3-10
6-04
5-02
4-18, 4-21
3-12, 6-02
7-06
7-09
1-18
3-02
3-04
5-03
8-06
8-06
1-10
4-05
7-01
4-08
3-03, 3-04. 3-07
6-01, 6-09
6-01
3-02
4-02
4-06
3-05
Gómez A.
8-02
Gomez M.E.
3-06
Gomez S.M.
5-08
González M.
3-11
Götze H.J.
4-11
Griffin W.
1-04
Grossi M.
3-13, 8-01
Guevara N.O.
3-08
Guryano V.A.
1-08
Guseva G.L.
4-09
Hanson R.E.
1-07
Hebbeln D.
6-01
Helen J.
2-05
Hernández-Molina F.J.
7-04, 7-08, 7-09
Heirman K.
6-01
Hervé F.
1-18, 2-02, 3-09, 4-06
Hormaechea J. L.
3-06, 3-12, 4-20
Ibanez-Mejia M.
1-10
Inbar M.
2-11
Introcaso A.
4-02
Isla F.
7-05
Ivins E.R.
6-02
Jacobs J.
1-11
Kading T.
2-01
Kleinhanns I.
1-11
Klepeis K.
4-01, 4-13
Keppens E.
6-01
Kilian R.
6-01
Kinny P.D.
1-05
Kumar R.
1-11
Lagorio S.L.
2-06, 2-16
Lange H.
6-02
Lawver L.A.
4-08
Leychenkov G.L.
4-09
Linares E.
1-14
Lippai H.
2-03, 3-09, 3-13, 4-03,
4-15, 5-01, 5-04, 8-01
Llanos M.P.I.
4-07
Lobo F.J.
7-09
Lodolo E.
3-09, 3-13, 4-10, 5-01,
5-04, 7-03, 8-01
López E.
6-10
López L.
8-03
AUTHOR INDEX
López R.
López de Luchi M.G.
Lossada A.L.
Lücke O.H.
Luna E.
Madirolas A.
Mahlburg K.S.
Maldonado A.
Marcolini S.
Marcomini S.C.
Marinaro G.
Martínez J.C.
Martínez O.
Martínez P.
Martínez Dopico C.I.
Martos-Martin Y.
Matthew G.L.
Marshall P.
Marvin B.
McAtamney J.
Massonne H.J.
Maurer M.
Mehrtens C.
Mena M.
Mendoza L.
Menounos B.
Menichetti M.
Mocnik A.
Moernaut J.
Mora A.
Moretto A.
Mosher S.
Mutti D.
Nacif S.
Naipauer M.
Nel J.
Nemeth K.
Nogueira A.C.R.
Noelia A.M.
Novara I.L.
Novas F.
Novo R.
Nullo F.
3-10, 7-07
1-12, 1-14, 2-07, 4-22
1-13
4-11
3-02
7-05
2-05
7-09
7-08
3-10, 7-07
7-06
2-04, 2-07
3-11
3-02
1-12, 1-14, 2-07
7-09
2-05
7-04
5-06
4-13
2-04, 2-07, 6-03
6-04
4-13
6-05, 8-04
3-01, 3-12, 4-14
6-04
2-03, 2-10, 3-09
4-15, 5-01, 5-04, 7-03
8-06
6-01
1-10
6-07
4-01
4-05
3-02
1-24, 4-21
6-03, 6-06
2-11
1-22, 1-23
5-03
4-02
5-05
3-04, 3-07
2-11
O’Brien B.H.
1-15
Oberti R.
2-10
Olivero E.B.
4-23
Onorato M.R.
6-07
Orgeira M.J.
3-11, 6-07
Orts D.
4-18
Osella A.
6-10
Osborn G.
6-04
Otamendi J.E.
1-16, 4-19
Pankhurst R.J.
1-20
Paterlini M.
7-01, 7-04, 7-08
Pedersen O.A.
6-08
Peel E.
1-17, 1-21
Perdomo R.
3-06, 3-12
Perestoronin A.N.
1-08
Pérez D.J.
4-16, 8-05
Pérez M.
5-07
Peroni J.I.
2-03, 3-09
Peskov A.Y.
1-08
Pino M.
6-01, 6-09
PicottI S.
8-06
Pinotti L.
4-05, 4-19
Plasencia Linares M. P.
4-12
Plavsa D.
1-05
Poiré D.
1-20
Polvé M.
2-02
Ponce A.
6-07
Ponce J.F.
3-11, 6-04
Presti M.
5-01
Prezzi C.
3-10, 3-11, 4-07
Prokopiev A.V.
4-17
Quaglino N.
3-04
Rabassa J.
3-11, 6-04
Ramos M.E.
4-18
Ramos V.A.
1-18, 4-18, 4-21, 5-02
Rapalini A.E.
1-13, 1-14
1-19, 4-19, 4-22
Rapela C.W.
1-20
Raposo M.I.B.
6-11
Rebesco M.
7-04
Re G.H.
2-09
Remesal M.B.
2-03, 2-12
Renzulli N.A.
2-10
Reyes A.G.
3-08
255
AUTHOR INDEX
Riccomini C.
Richter A.
Rico Y.
Ridolfi F.
Risso C.
Roberts J.M.
Roberts S.
Rodriguez G.D.
Romero B.F.
Rovere E.I.
Rubinstein N.
Russi M.
Ruiz J.
Sabbione N.C.
Sagripanti L.
Salani F.M.
Sánchez A.
Sanchez Bettucci L.
Sanchez Magariños J.M.
San Martín L.
Sansjofre P.
Santos J.O.S.
Scheinert M.
Schmidt S.
Schreckenberger B.
Schreider A.A.
Schwabe J.
Seluchi N.
Selway K.
Siegesmund S.
Singer S.E.
Sinninghe Damsté J.
Smelov A.P.
Soares J.L.
Somoza L.
Somoza R.
Spagnotto S.
Spalletti L.A
Steenken A.
Strakos K.
Suarez I.
Tabare T.
Tancredi G.
256
1-23
3-12
5-09
2-10
2-11
1-07
6-01
3-04
5-06
6-10
8-02
4-12
1-10
4-20
4-21
2-12
2-02
1-13, 1-17, 1-21
3-04, 3-07
4-16
7-07
1-22
1-03, 1-04
3-01, 3-12
3-05
7-01
7-09
3-01
3-07
1-05
1-12
2-13, 4-19
6-01
2-14
1-22
7-04
2-17
3-02
1-20
1-12
5-06
3-07
3-08
3-04
Tassone A.A.
2-03, 2-10, 3-09, 3-13
4-03, 4-10, 4-15, 5-01
5-04, 7-03, 8-01
Tessone M.
8-03
Thomas R.T.
1-11
Thomson S.
4-13
Tohver E.
1-23
Tomezzoli R.
4-22
Torres Carbonell P.J.
4-23
Trindade R.I.F.
1-22, 1-23
Tubía J.
4-19
Ueda K.
1-11
Uenzelmann-Neben G.
7-04
Umazano A.M.
2-15, 5-07
Urruti R.
6-01, 6-09
Valencia V.
1-11
Van Daele M.
6-01, 6-09
Van Staal C.
1-09, 1-23
Varekamp J. C.
2-01
Vásquez C.A.
3-10, 3-11, 6-07
Vasquez J.
4-14
Vegas N.
4-19
Verdecchia S.O.
1-25
Verleyen E.
6-01
Vernikovskaya A.E.
1-26
Vernikovsky V.A.
1-26
Versteeg W.
6-09
Vila R.
6-01
Vilas J.F.
2-09, 4-03, 6-11
Violante R.A.
6-10, 7-04, 7-08
Visconti G.
5-07
Vizán H.
2-06, 2-16
Vujovich G.I.
1-02, 1-23, 1-27, 4-19
Vyverman W.
6-01
Walther A.M.
6-11, 8-04
Wemmer K.
1-12, 1-14, 2-04
Wendt J.
6-02
Willner A.P.
1-27
Xu Y.
6-03, 6-06
Zaffarana C.B.
2-17
Zaitsev A.I.
2-14
Zecchin M.
5-01
Zappettini E.O.
1-03

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