Il Bacino d`Amantea è posto nella parte meridionale

Some remarks about the Mio-Pliocene tectonics in the Amantea Basin. Notaro Pasqualino Page 1 of 13
Some remarks about the Miocene-Pliocene tectonics in the
Amantea Basin, Calabria, S. Italy
Alcune osservazioni sulla tettonica mio-pliocenica del
Bacino di Amantea, Calabria, S Italia
ABSTRACT
The Amantea Basin is an extinct and uplifted sub-basin of the major Tyrrhenian rift.
The results of a structural geological survey of the central area of the Amantea Basin
are reported in the present paper. The map shows the presence of an alternation of
synformal and antiformal folds with a wave length of about 2 km and with axes
striking between EW and NWSE. These folds are not related to thrust tectonics. Two
different fault systems oriented NS and EW were evidenced; the first one is
constituted prevalently by normal faults, whereas, the faults of the second system
show often oblique slickenside lineations.
An extensional origin of some controversial structures is also suggested.
Key words:, Miocene-Pliocene tectonics, Tyrrhenian rift, Amantea basin, Calabria,
Italy.
RIASSUNTO
Il Bacino di Amantea è stato definito nella parte meridionale della Catena Costiera
calabra come una depressione strutturale collegata al rift Tirrenico (ORTOLANI et alii,
1979; TORTORICI, 1981; PATACCA et alii, 1990; COLELLA, 1995; MATTEI et alii, 1999;
2002). Esso si sarebbe individuato nel Serravalliano (MATTEI et alii, 2002) ed è
colmato da sedimenti silicoclastici del Tortoniano-Messiniano. Tuttavia, secondo
MUTO & PERRI (2002) il Bacino di Amantea non sarebbe una struttura estensionale
legata all’apertura del Tirreno, ma si sarebbe sviluppato in un regime compressivo
collegato con la progressiva migrazione di piani di sovrascorrimento verso
l’avampaese Ionico.
In questo lavoro si pone l’accento sulla presenza di una serie di strutture di
controversa interpretazione che potrebbero essere considerate evidenze di una
tettonica compressiva. Tuttavia, riportando la stratificazione ad una condizione di
pre-deformazione queste faglie, con apparente geometria inversa, sono interpretabili
come strutture estensionali basculate. Queste considerazioni non portano comunque
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ad escludere la presenza di una fase compressiva intra-messiniana, peraltro ben
caratterizzata in ORTOLANI et alii (1979).
Il rilevamento svolto ha evidenziato due sistemi di faglie ortogonali tra loro: 1) NS; 2)
EW; tipici di un sistema arco-avanfossa in migrazione verso est (DOGLIONI et alii,
1995).
Termini chiave: tettonica mio-pliocenica, rift tirrenico, bacino di Amantea, Calabria.
INTRODUCTION
The western Mediterranean is composed of several basins, progressively younger
from west to east, opened by back arc extension following westward subduction and
the progressive roll-back of the Ionian and Adria plate (MALINVERNO & RYAN, 1986;
FERRARI & MANETTI, 1993;TORTORICI et alii, 1995; GUEGUEN et alii, 1998). The
Tyrrhenian basin is the youngest of them (CATALANO et alii, 2001; DOGLIONI et alii,
1998, 1999; GUEGUEN et alii, 1998). The Amantea basin (Fig. 1) is a sub-basin related to the major Tyrrhenian rift (TORTORICI, 1981; PATACCA et alii, 1990) presently located on the western side of the southern part of the Calabrian Coastal
Chain. It is filled by upper Miocene to Pliocene sediments.
DI NOCERA et alii (1974) and ORTOLANI et alii (1979) analyze in detail the stratigraphy
and paleoenvironmental characteristics of the Amantea basin. These authors
consider it as an extensional basin formed since the Tortonian and distinguish two
sedimentary cycles, upper Tortonian – lower Messinian and Messinian - lower
Pliocene in age respectively. They are separated by an unconformity related to an
intra-Messinian compressive tectonic event responsible even for the thrusting of the
Paleozoic basement above the Miocene sediments.
According to COLELLA (1995) the first cycle of DI NOCERA et alii (1974) must be
subdivided into two depositional sequences (Fig. 2) separated by an angular
unconformity. The first depositional sequence (upper Tortonian) reaches a thickness
of over 120 m (Fig. 2) and lies on the alluvial fan or directly on the bedrock, it is made
up by a basal alluvial fan mainly emplaced by debris flow (COLELLA, 1995). Within the
higher part of this unit a volcanic ash horizon (7 m), rhyolitic in composition, is a keybed at the basin scale (GUERRERA & VENERI, 1989).
The second depositional sequence (upper Tortonian - lower Messinian), reaching a
thickness of about 180 m and occupying a wide area of the Amantea Basin, is made
predominantly by bioclastic sediments (Fig. 2). Locally this second sequence lies
Some remarks about the Mio-Pliocene tectonics in the Amantea Basin. Notaro Pasqualino Page 3 of 13
directly on the pre-Miocene bedrock. The third sequence is characterized by gypsum
beds related to the Messinian salinity crisis (Fig. 2).
COLELLA (1995) also supports the extensional origin of the Amantea Basin and
hypothesizes a transpressional event responsible for the unconformity between the
first two cycles.
Even MATTEI et alii (1999; 2002) agree with an extensional history of the Amantea
basin and highlight that the faults have a strike variable between NNE-SSW e NESW and dip generally toward the East. The authors point out that the whole
Calabrian block underwent a 15-20° clockwise rotation, probably during the
Pleistocene, i.e. after the extensional tectonic event.
According to MUTO & PERRI (2002) the stratigraphic succession can be subdivided
into five main units bounded by stratigraphic discontinuities. These authors point out
the occurrence in the basin of a thrust system, active from the Tortonian up to the
early-middle Pliocene, evidenced by fault propagation folds, questioning the
relationships between the Amantea basin and the Tyrrhenian rifting; according to
these authors the extensional tectonics began only in the late Pliocene (Fig. 3). Even
CESARANO & TURCO (2002) emphasize, in the surrounding area, the presence of
compressive features.
According to BONARDI et alii (1994, 2001, 2004), due to the presence of remnants of
the Stilo Unit, the Amantea Basin is located in a complex interaction area between
the two sectors (Fig. 1) that constitute the Calabria Peloritani Arc.
The results of a structural geological survey of the central part of the Amantea basin
are reported in the present paper, aimed at presenting evidence that some tectonic
structures, previously considered as indicative of thrust tectonics, can be interpreted
as tilted normal faults.
Neogene structures of the Amantea Basin
A geological-structural map (Fig. 4), at 1:10.000 scale, of about 30 kmq in the
Amantea basin, focusing on the analysis of the Neogene structures, has been
surveyed. The mapping has been carried out according to the stratigraphic
subdivision proposed by COLELLA (1995). The pre-Miocene basement is formed
prevalently by the Stilo Unit that is the uppermost nappe of the whole Calabria
Peloritani Arc (BONARDI et alii, 1984). In the studied area this tectonic unit is
represented both by Paleozoic metamorphites, intruded by Late-Hercynian
Some remarks about the Mio-Pliocene tectonics in the Amantea Basin. Notaro Pasqualino Page 4 of 13
granitoids, and by a Mesozoic-Cenozoic carbonate cover (BONARDI et alii, 1984;
2002).
The attitude of the Neogene deposits shows an alternation of synformal and
antiformal folds with a wave length of about 2 km and axes strike varying from E-W,
in the western part of the area, to NW-SE in its eastern part (Fig. 4). In the northern
part of the area the basement crops out and it is represented by an ophiolitic unit
dipping toward S-SSW. In the southern part of the surveyed area phyllites overlying
metabasalts belonging to the same ophiolitic unit dip toward N-NNE. Therefore it is
possible to recognize a synformal fold with the Stilo unit in the core.
The relationships between this synform and the minor folds affecting the siliciclastic
sediments are not clear. The synform could either pre-date the clastic deposits or to
be syn-sedimentary.
Two orthogonal fault systems have been recognized in the area (Fig. 5): the first one
striking approximately N-S and dipping to the east, with a dip angle variable from 45°
to 75°; the second system, striking approximately E-W with dips of 45-85°, is less
evident manifest because it is parallel to the extension of the area studied.
Faults of the first system are normal. They strongly control the outcrops of basement
units, e.g. the Stilo Unit outcrops only in a post-Messinian horst bounded by faults of
this group (Fig. 6).
Faults of the second system often exhibit oblique slickenside lineations, with a pitch
of 10-45°. They usually cut the faults of the first system and give rise to transpressive
effects (Fig. 7).
Apart from these transpressive (Fig. 7) faults, the occurrence of a true thrust
tectonics was not recognized during the survey.
Along the road from C.se Magioni to C.zzo Ciani, is possible to observe some faults
cutting a conglomerate bed (Fig. 8a). They apparently show a reverse geometry.
However, restoring the beds to the horizontal pre-deformation attitude (Fig. 8b) these
faults appear interpretable as tilted normal faults and are also associated with
antithetic faults.
Another example is located in the Cannavino area (Fig. 9), where it is possible to
observe some low angle faults cutting sub-vertical beds, apparently showing a
reverse displacement (Fig. 9a). Restoring the beds to a pre-deformation stage, even
these faults can be interpreted as normal listric (Fig. 9b).The tilting of this fault
system could be related to younger N-S listric normal faults.
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CONCLUSIONS
The survey of the central part of the Amantea Basin shows the presence of two fault
systems oriented about N-S and E-W. Actually, in a back-arc basin, related to a
subduction toward the west, the contemporaneous existence of these directions of
extension is a common feature (DOGLIONI et alii, 1995). Similar structures can be also
seen in the Catanzaro and Crati grabens (Fig. 1).
Field data do not confirm that the folds affecting the Tortonian - lower Messinian
sediments are related to thrust tectonics as suggested by MUTO & PERRI (2002).
Furthermore some structures, considered by the authors as evidence of thrust
tectonics, are interpretable as tilted normal faults or as transpressive faults (Fig.
7,8,9). Actually they are not unequivocally interpretable. Their interpretation as
extensional structures seems more in agreement with the knowledge about the
geodynamic setting of the Calabria Arc (AMATO et alii, 1993; CASSINIS et alii, 2003;
MELE, 1998).
Probably the folding is related to the Pliocene rotation of the Amantea Basin
(COLELLA, 1995; MATTEI et alii 2002). Actually the folds are cut by normal faults that
are still active along the Tyrrhenian side of the Calabrian Arc (TORTORICI et alii,
1995).
Some observations, in the surrounding areas, point out that the extensional tectonics
were active even before the rotation of the Amantea Basinn (MATTEI et alii, 2002), as
is evidenced by Stilo Unit outcrops preserved prevalently in graben and halfgraben
structures (NOTARO, 2004).
Acknowledgements
I’d like to thank Professor Bonardi for his valuable discussion and suggestion.
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Figura 1. Carta tettonica schematica della Calabria; sono distinti i due settori (meridionale e
settentrionale) ed il bacino di Amantea.
Tectonic sketch of Calabria, showing the southern and northern sectors and the Amantea
basin.
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Figura 2. Colonna stratigrafica dei depositi del bacino di Amantea, secondo COLELLA (1995),
leggermente modificata.
Stratigraphy of the Amantea basin deposits, slightly modified after COLELLA (1995).
Figura 3. Sezioni schematiche sulle differenti ipotesi sulla genesi del Bacino di Amantea: A) bacino di
rift; B) bacino thrust top.
Schematic sections illustrating different hypotheses about the Amantea basin genesis: A) rift
basin; B) thrust top basin.
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Figura 5. Diagramma stellare delle faglie nei primi due cicli sedimentari del Bacino di Amantea. Si
nota, nei due casi, la prevalenza delle faglie N/S e NNE/SSW immergenti verso E.
Rose Rose diagram of the faults affecting the first two sedimentary cycles of the Amantea
Basin. The prevalence of N/S and NNE/SSW faults dipping toward the E is clear in both situations.
Figura 6. Faglia diretta che mette a contatto i calcari giurassici dell’Unità di Stilo, a letto, con i depositi
silicoclastici del Tortoniano-Messiniano a tetto. Sono evidenti le strie sul piano di faglia orientate
secondo l’immersione.
A normal fault has brought the Jurassic limestones of the Stilo Unit (footwall) into contact with
Tortonian-Messinian siliciclastic deposits (hangingwall). Dip slickenside lineations are evident.
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Figura 7. Il piano di faglia principale taglia gli altri con un movimento transpressivo sinistro. Le strie
variano da orizzontali a leggermente oblique (pitch= 30°) (località Petrone, Amantea).
The master fault cuts the other fault planes with a left-lateral displacement. Slickenside
lineations are horizontal or slightly oblique (pitch 30°) (Petrone, Amantea).
Figura 8. A) La faglia in basso taglia, con un rigetto apparentemente inverso, la linea nera che segna
la base del livello conglomeratico. Riportando gli strati ad una condizione di pre-deformazione (B) il
sistema appare estensionale.
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A) The lower fault cuts, with an apparently reverse displacement, the black line that marks the
bottom of the conglomeratic bed. Restoring the layers to a pre-deformation condition (B) the system
changes to extensional.
Figura 9. Le faglie tagliano il livello pelitico, indicato con la linea nera, con un rigetto apparentemente
inverso; come nel caso precedente, riportando gli strati all’orizzontale, appare come un sistema
estensionale ad alto angolo.
The faults cut the pelitic bed, marked with a black line, showing an apparently reverse
displacement. Even in this case, restoring the layers to the horizontal, they change into an high angle
extensional system.
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Figura 4. Carta geologica dell’area di Amantea.
Geological map of the Amantea area.