Geological evolution of the Tacaná Volcanic Complex, Mexico

Geological evolution of the Tacaná Volcanic
Complex, Mexico-Guatemala
by
1
García-Palomo, A., 2Macìas J.L., 1Arce J.L, 2Mora, J.C., 3Hughes, S.,
4
Saucedo, R., 2Espindola, J.M., 5Escobar, R., and 6Layer, P.
1
Departamento de Geología Regional,
Instituto de Geología, UNAM
Coyoacán 04510, México D.F.,
apalomo@geologia.unam.mx
2
Instituto de Geofísica, UNAM
Coyoacán 04510, México D.F.,
3
Departament of Geology, State
University of New York, 876 Natural
Science Complex, Buffalo,
New York 14260, USA
4
Instituto de Geología, UASLP
San Luis Potosí
5
CONRED, Guatemala
6
University of Alaska Fairbanks
Manuscript to be submitted to a
GSA Special Paper “Natural Hazards in Central America”
December 13, 2004
Abstract
The Tacaná Volcanic Complex represents the northernmost active volcano of the
Central American Volcanic Arc. The genesis of this volcanic chain is related to the
subduction of the Cocos plate beneath the Caribbean plate. The Tacaná Volcanic
Complex (TVC) is influenced by an important tectonic structure as it lies south of the
active left-lateral strike-slip Motozintla fault related to the Motagua-Polochic fault zone.
The geological evolution of the TVC and surrounding areas is grouped into six major
sequences dating from the Mesozoic to the Recent. The oldest basement rocks are
Mesozoic schists and gneisses of low grade metamorphism. These rocks are intruded
by Tertiary granites, granodiorites and tonalities ranging in age from 13 to 39 Ma
apparently separated by a gap of seven million years. The first intrusive phase occurred
during late Eocene to early Oligocene, and the second during early to middle Miocene.
These rocks are overlain by deposits from the Calderas San Rafael (2 Ma), Chanjale (1
Ma), and Sibinal grouped under the name Chanjale-San Rafael sequence of late
Pliocene-Pleistocene age. The activity of these calderas produced thick block-and-ash
flows, ignimbrites, lavas, and debris flows. The TVC began its formation during late
Pleistocene nested in the pre-existing San Rafael Caldera. The TVC formed through the
emplacement of the following four volcanic centers. Chichuj volcano, the first, was
formed by andesitic lava flows and later destroyed by the collapse of the edifice. Tacaná
volcano, the second, formed through the emission of basaltic-andesite lava flows, as
well as andesitic and dacitic domes that produced extensive block-and-ash flows about
38,000, 28,000, and 16,000 yr BP. An andesitic coulee called Plan de las Ardillas was
emplaced on the high slope of the Tacaná some 32,000 yr BP. Finally, San Antonio
2
volcanic center was built through the emission of lava flows, andesitic and dacitic
domes and a block-and-ash flow deposit 1,950 yr BP. The TVC was emplaced following
a NE-SW direction beginning with Chichuj, and followed by Tacaná, Las Ardillas, and
San Antonio. This direction is roughly the same of the Tacaná graben and the faults and
fractures exposed in the region. The TVC magmas have a calc-alkaline trend with
medium-K contents, negative anomalies of Nb, Ti, and P, and enrichment in LREE that
is typical of subduction zones.
Keywords: Southern Mexico, Guatemala, Tacaná, structural geology, volcanic
evolution.
Introduction
The Tacaná Volcanic Complex (TVC) is located in the State of Chiapas, in southern
Mexico, and in the San Marcos Department of Guatemala (Fig. 1). The TVC represents
the northwestern-most volcano of the Central American Volcanic Arc (CAVA), a WNW
oriented volcanic arc that extends for over 1,300 km, from the Mexico-Guatemala
border to Costa Rica. CAVA is parallel to the trench and consists of several
stratovolcanoes that have erupted calc-alkaline magmas from the Eocene to the Recent
(Carr et al., 1982; Donnelly et al, 1990). The origin of this volcanic chain is related to the
subduction of the Cocos plate beneath the Caribbean plate, which together with the
North-American plate create a complicated triple point junction in the region (GuzmánSpeziale et al., 1989). The TVC includes Tacaná volcano which has shown some recent
although minor activity as reported early by Bergeat (1894) and Saper (1927). Tacaná
3
reawakened in 1950 and 1986 with small phreatic explosions that reminded villagers
and authorities of its potential threat in case of future activity (Mullerried, 1951; De la
Cruz-Reyna et al., 1989). Prior to the 1986 eruption, the National Power Company
(Comisión Federal de Electricidad, CFE) conducted a geological survey to evaluate the
geothermic potential of Tacaná Volcano (De la Cruz and Hérnandez, 1986). These
authors presented the first geological map of the volcano, showing that Tacaná was
emplaced on top of Tertiary granodioritic rocks, and that it consisted of lava flows and at
least three Quaternary fans of pyroclastic flows that they labeled, from youngest to
oldest, Qt1, Qt2, and Qt3. De Cserna et al. (1988) presented a geologic map of the
volcano based mostly on photointerpretation. These authors also recognized the
existence of three volcanic episodes in the formation of Tacaná. The first radiometric
determinations at the volcano were obtained from charcoal samples extracted from Qt3
and revealed a late Pleistocene eruption dated at 38,000 yr BP (Espíndola et al., 1989)
that was confirmed afterwards (Espíndola et al., 1993). These latter authors assigned
an age of circa 28,000 yr BP to unit Qt2. Mercado and Rose (1992) published a
geologic map based mostly on photogeology and presented the first volcanic hazard
zonation of the volcano. This hazard zonation considered pyroclastic flows, debris
avalanches, pyroclastic fall and lahars. The first study that considered Tacaná as part of
a volcanic complex was carried out by Macías et al. (2000). They concluded that
Tacaná consisted of three edifices, from oldest to youngest, the Chichuj volcano (3,800
m a.s.l. [above sea level]), the main summit Tacaná (4060 m), and San Antonio Volcano
(3,700 m). They proposed that activity of the TVC has migrated from the northeast
(Chichuj) to the southwest (San Antonio), inside a 9-km wide caldera hereafter called
4
San Rafael. They studied in detail a Peléan type eruption originated at San Antonio
volcano some 1, 950 yr ago that emplaced the Mixcun flow deposits and associated
lahars. These events very probably caused the temporal abandonment of Izapa, the
main Mayan ceremonial center in the Soconusco region. Based on these results the
authors proposed a hazard zonation for pyroclastic flows produced by Peléan-type
eruptions at Tacaná, associated lahars, and debris avalanche deposits (Macías et al.,
2000). Mora et al. (2004) carried out a petrologic and chemical study of the TVC for the
last 40,000 yr BP concluded that the magmas feeding the complex are typical of
orogenic zones.
On early 2001 we began a systematic study of the TVC that comprises different aspects
of the complex, including geological mapping, determination of structural features,
volcanic stratigraphy, and petrology. The results of these studies have been integrated
in this work with the aim of determining the geological evolution of the TVC and
surrounding areas. In particular, in this paper we aim to establish the relationship of the
TVC to the tectonic framework, to present the geological units in the area supported by
radiometirc dating, and finally to synthesize the volcanic evolution of the area. An
additional outcome of these results will be the assessment of the geologic hazard in the
area
Tectonic setting of Southern Mexico
Southeastern Mexico and northwestern Guatemala are characterized by the
conspicuous Cocos-North America-Caribbean triple point junction delineated by the
Middle American Trench (MAT) and the Motagua-Polochic Fault System (GuzmánSpeziale et al., 1989). The precise location of this triple junction however is still a matter
5
of controversy (Guzmán-Speziale et al., 1989). In southern Mexico, the Cocos plate
subducts in the N45°E direction at an average rate of 76 mm/yr (DeMets et al., 1990).
This process is complicated by the subduction of the Tehuantepec Ridge (TR), an
aseismic ridge at 95° W (LeFevre and McNally, 1985), where no earthquakes larger
than 7.6 Mb have occurred during the last 190 years (Singh et al., 1981; McCann et al.,
1985). The TR is a narrow linear feature with a maximum vertical relief of 2,000 m that
separates shallower sea floor (3,900 m) to the NW from deeper sea floor (4,800 m)
towards the SE, that is the Guatemala basin (Truchant and Larson, 1973; Couch and
Woodcock, 1981; LeFevre and McNally, 1985). The TR also separates oceanic crusts
of different ages; to the west, the crust has an average age of 12 Ma (Nixon, 1982)
dipping 25° (Pardo and Suarez, 1995; Rebollar et al., 1999), whereas to the east the
crust has an age of 28 Ma (Nixon, 1982) and dips 40° (Rebollar et al., 1999). The
thickness of the crust at the TR is 28.5 ± 3.5 km (Bravo et el., 2004) dipping at circa 38°
according to Ponce et al. (1993) or 30-35° according to Rebollar et al. (1999). The
subduction of the TR together with the location of the Cocos-North America-Caribbean
triple point junction correlate with the apparent truncation of volcanism at CAVA in
southern Chiapas, the inland migration of scattered volcanism at the Chiapanecan
Volcanic Arc (Damon and Montesinos, 1978), and two sites of alkaline volcanism El
Chichón in northern Chiapas (Thorpe, 1977; Luhr et al., 1984; García-Palomo et al.,
2004) and Los Tuxtlas Volcanic Field in Veracruz (Nelson et al., 1995). The thickness of
the continental crust is 39 ± 4 km under Chiapas (Rebollar et al., 1999). Considering
these data, the TVC is located at about 240 km from MAT, and the projected slab
underneath would be at a depth of ca. 100 km.
6
Geomorphology
The four aligned volcanic structures mentioned before: Chichuj, Tacaná, Plan de Las
Ardillas and San Antonio are shown in Figs. 2 and 3. The difference in elevation of the
TVC with respect to the surrounding terrain is ~3,000 m in the SW part whereas in the
NE part is (Guatemala) ~1,300 m. These major differences in elevation as well as its
asymmetric shape are controlled by tilting of the basement rocks and the array of
calderic structures (Fig. 3).
Chichuj, the oldest center consists of a collapsed volcanic structure at a height of 3,800
m. Tacaná volcano (4,060 m), has a 600 m wide summit crater breached by a
horseshoe-shaped escarpment opened to the NW that contains an andesitic central
dome. The escarpment was produced by a flank collapse. The Plan de Las Ardillas
dome has an asymmetric shape, and the San Antonio dome has a horse shoe-shaped
crater opened to the south. The TVC was built within the remains of a 9 km semicircular
caldera structure called San Rafael Caldera that borders the northern flank of the
volcano, whereas the southern part is characterized by wedges or blocks composed of
granitic and old volcanic rocks. These wedges are named hereafter as Desenlace and
Agua Caliente-El Aguila. The first is formed by granitic rocks of Tertiary age covered by
Pliocene volcanic rocks. The second, located in the western portion of the volcano, is
made of Tertiary granitic and Pliocene volcanic rocks tilted toward the west. This wedge
is formed by the intersection of NW and NE fault systems and is cut by N-S faults.
The southern portion of the TVC is characterized by large and coalescing pyroclastic
and debris fans that reach the coast-line in the Pacific Ocean, whereas to the north the
pyroclastic fans pinch out against the walls of the rim of the San Rafael caldera.
7
Stratigraphy
Based on photogeology and satellite image analysis, field mapping, and radiometric
dating, six main stratigraphic units in the TVC area were recognized (Fig. 4). From
oldest to youngest they consist of metamorphic rocks intruded by two phases of igneous
rocks, the first one during late Eocene to early Oligocene and the second one during
early to middle Miocene. Unconformably, on top of these older units, rests the San
Rafael-Chanjale Sequence constituted by three Pliocene to Pleistocene calderas. On
top of these rocks sits the TVC which is divided in four sequences ranging in age from
Pleistocene to Recent. This stratigraphic sequence is described in detail below.
Mesozoic
Outcrops of this age are randomly exposed in the northwestern portion of the area near
the junction of the San Rafael and Coatán Rivers. The best exposures of these rocks
appear at sites TAC (9875, 0308a, 0358, 0359, 0367). These outcrops consist of
alternating schists and gneisses, which that at site TAC0358 have a minimum thickness
of 20 m. The schists are light to dark green forming centimetric thick layers. The
gneisses are composed of alternating green and white centimetric thick bands (Fig 5,
site TAC0367) with a schistosity and foliation trending of N60°W-70°NE. At site
TAC0308a, a slightly metamorphozed lava flow, deeply altered and faulted crops out. A
K-Ar determination carried out by Mugica (1987) in similar rocks of the area yielded an
age of 142 ± 5 Ma (Early Cretaceous) (Table 1).
Tertiary
These rocks were first described as part of the Coastal Batholith of Chiapas by Mugica
(1987). This batholith is 270 km long and 30 km wide and covers an area of 8,000 km2.
8
According to this author, at outcrop scale, the rocks are light-gray to pink; and
petrographically they are granodiorites composed of Na-Plg + qz + microcline + bi + hb
+ disseminated oxides, affected by dynamic metamorphism along major faults. It has a
late Oligocene-middle Miocene age 15-29 Ma obtained from biotite concentrates
(Mugica, 1987).
Despite their distribution, these rocks are difficult to map due to the thick forest cover
and to the deeply altered soil. Therefore the following description is mostly based on
radiometric dating and separated into two groups: the first is late Eocene to early
Oligocene and the second is early to middle Miocene.
Late Eocene -early Oligocene
Mugica (1987) described a Bi-Hb granodiorite exposed southwest of the Santa Rosa
village (35 ± 1 Ma), and a hb-bi gneissic diorite exposed in the vicinity of the 11 de Abril
village (39 ± 1 Ma) (Table 1). These dates, obtained withe K-Ar method from biotite
concentrates, correspond to a late Eocene-early Oligocene event.
Additional granitic rocks are exposed northwest of the El Aguila village at site
TAC0364c. Here, the rock is a white granite with equigranular texture (3 mm average
length) composed of quartz, plagioclase (up to 0.8 cm), and biotite (up to 1.7 cm). The
granite hosts light-gray enclaves up to 46 cm in diameter. These are subrounded to
elongated with sharp boundaries and reaction rims that are dark-gray with widths of 912 cm. Biotite separates of this rock were dated with the Ar-Ar method yielding an
average age of 29.4 ± 0.2 Ma. This age correlates with a K-Ar date of 29 ± 1 Ma
obtained in a gneissic tonalite cropping out along the Huixtla-Motozintla road (Mugica,
1987).
9
Middle-late Miocene
The rocks collected south of the TVC at Monte Perla (15°03’39”N, 92°05’35”W) a bi-hb
gneissic tonalite, and at Finca Zajul, Tapachula (15°13’31”N; 92°15’16”W) a bi-hb
granodiorite yielded ages of 20 ± 1 Ma in biotite (Table 1), respectively (Mugica, 1987).
In this work, we report a new granodioritic stock exposed in the northwestern part of the
area in the vicinity of the San Rafael River (site TAC0364). The granodiorite consists of
cm-sized K-feldspars, plagioclase, biotite and minor quartz. Ar-Ar analysis of biotite
grains yielded ages of 13.3 ± 0.2 and 12.2 ± 0.1 Ma (Table 2). Another small intrusive is
exposed south of San Antonio volcano at site TAC0359c. Here, a granitic rock is
intruded by veins rich in cm-size biotite, this mineral was dated with the Ar-Ar method at
13.9 ± 0.1 Ma (Table 2). These rocks can be correlated with the youngest part of the
Coastal Chiapas Batholith (Mugica, 1987).
Late Pliocene-Pleistocene
This volcanic sequence consists of three caldera structures named here San Rafael,
Chanjale and Sibinal, located in the northern portion of the studied area.
San Rafael Caldera (2 Ma)
This caldera has a discontinuous structure, with its northern and eastern walls well
exposed. By projecting these wall remnants in the geological map, a 9 km diameter is
estimated for this structure. The San Rafael Caldera walls are constituted mainly of a
green ignimbrite and capping lava flows, with a debris avalanche deposit inside.
The basal unit is represented by a 200 m thick ignimbrite exposed in the caldera rim on
top of the Tertiary granites, the ignimbrite starts to crops out at an elevation of 1,600 m
(site TAC0328a). It appears as a lithified green ignimbrite composed of angular to
10
subrounded lithics, mainly dark-gray andesites and rounded pumice fragments
embedded in a fine-grained matrix (medium sand). Towards the top, this unit becomes
enriched in pumice and scoria fragments embedded in a fine ash matrix. This unit crops
out at an elevation of 1,800 m (TAC0330), and has juvenile fragments with a
composition of 53.8 % wt. in silica (Table 3; Figure 6). A thick sequence of metric (>20
m) lava flows covers the green ignimbrite along the northern rim of the San Rafael
caldera. At site TAC0349c a dark-gray lava flow was dated with the Ar-Ar method at
1.87 ± 0.02 Ma (Table 2). This lava has a composition of 57.94% of silica being
therefore an andesite (Table 3).
Inside the Agua Caliente-El Aguila wedge, southwest of the town of Agua Caliente,
several dark-gray lava flows showing a porphyritic texture are bearing plagioclase,
pyroxene, and rare olivine crop out. Here the lava flow units are 1 m thick and have
fractures and rounded vesicles. An Ar-Ar date obtained at site TAC0323a yielded an
age of 1.99 ± 0.08 Ma (Table 2). This age is similar to those of the lava flows on the
northern rim of the caldera. According to this radiometric data, the San Rafael caldera
was active from late Pliocene to the Pleistocene.
Here the lava flows are covered by a massive, matrix supported block-and-ash flow
deposit, composed of dark-gray andesites and minor red altered andesites set in a
medium to coarse ash. The blocks of andesite are subangular to subrounded and
contain plagioclase phenocrysts. The dark-gray andesites have a silica content of 54.66
wt% (Table 3).
Inside the northern rim of the caldera, near La Vega del Volcan village (sites TAC035c,
TAC037c), a thick debris avalanche deposit is exposed. It consists of megablocks with
11
jigsaw-puzzle structures up to 4 m in diameter. The blocks are banded gray to red
porphyritic andesites with hornblende, pyroxene and white feldspar. The debris
avalanche deposit covers unconformably the granitic rocks (TAC0338c); however, its
stratigraphic position relative to others units suggest that it is younger than the green
ignimbrite and the lava flows.
Chanjale Caldera (1 Ma)
The Chanjale caldera dominates the western portion of the study area. It is a 6.5
km wide crater opened to the east and cut by the Coatán River. The caldera rim
consists of several units of lavas, pyroclastic and debris flows (Fig. 7). The flanks of the
structure near Malacate village, exposes a gray porphyritic lava flow rich in plagioclase
and piroxene (TAC0333) (Fig. 7). This lava flow has a silica composition of 58.39 wt %
and yielded an Ar-Ar date of 0.81 ± 0.16 Ma (Table 2). The total thickness of the unit is
approximately 200 m. A white ignimbrite altered to brown color is exposed around the
lava flows. It consists of plagioclase, quartz, and altered ferromagnesians, embedded in
a white fine matrix. On the southern flank of the caldera, in the vicinity of Chespal
(TAC0335), a white to light-yellow pyroclastic flow is exposed with clasts up to 1 m in
diameter embedded in a fine ash matrix with mm-size pumice fragments. This
pyroclastic flow deposit incorporated xenoliths of biotite-bearing granites. The clasts are
subangular to angular, dense and deeply altered.
On top of the sequence several indurated debris flow deposits up to 12 m thick
appear forming a fan toward the southern flank of the caldera. These deposits are
heterolithologic with boulders up to 2 m in diameter embedded in a sand-size matrix
(TAC0332).
12
Sibinal Caldera
The Sibinal caldera is exposed in the northeastern portion of the area with the town of
Sibinal, Guatemala, located in its center. Its sequence consist of dark-gray andesitic
lava flows with aphanitic texture, and a total thickness of 60 m, composed of several
units up to 4 m thick. In hand specimens plagioclase and some ferromagnesians are
common (site TAC0340). A lava flow is located on top of the Sibinal caldera overlying
the Tertiary granite. On the northern slopes of the caldera (TAC0341) 2-m-thick lava
flows are exposed. These rocks consist of porphyritic gray andesites with plagioclase
and altered ferromagnesians encircling the Chamalecón couleé, which is composed of
pink to reddish andesites. These rocks contain tabular and subeuhedral phenocrysts of
plagioclase and pyroxene embedded in a fine red matrix composed of glass and
plagioclase microliths. They are deeply altered to spheroidal weathering, and are older
than the lavas of Sibinal caldera. The youngest unit of this caldera is a thick
volcaniclastic deposit that forms an apron of debris flows interbedded with the 32,000 yr
BP Sibinal Plinian fall deposit of Tacaná volcano. According to this stratigraphic
relationships the Sibinal caldera should be younger than the San Rafael and Chanjale
calderas.
Pleistocene-Holocene
The TVC is located in the central portion of the area and consists of the following units:
Chichuj Sequence
Chichuj volcano has a collapse structure facing west where it is sealed by Tacaná
volcano. Most of the Chichuj’s geology is exposed in its eastern flank, where six units
13
were recognized. Near to the Chocabj village (TAC0342) a unit of gray andesite lava
flows crops out, it is composed of plagioclase, dark-green hornblende, and pyroxene set
in a fine gray glassy matrix. These lavas host dark-gray to reddish xenoliths with
plagioclase, mafic minerals, and vesicles. The xenoliths are rounded and show reaction
rims. The lava flow sequence is 10 m thick with individual units of ca. 1 m. This lava
covers the granitic rocks at site TAC0336a.
At site TAC9876 a pink debris avalanche deposit at the base of the Muxbal gully is
exposed. This deposit is massive with yellow to orange hydrothermally altered zones
and meter-sized jigsaw blocks in a shattered matrix of coarse grained ash. The Muxbal
debris Avalanche is also exposed in the eastern flanks of Chocabj. Here the debris
avalanche is covered by a lacustrine sequence and a 28,540 ± 260 yr BP block-and-ash
flow deposit of Tacaná volcano (Table 4). At TAC0334c, a massive block-and-ash flow
deposit consists of dark-gray andesite and altered scoria clasts embedded in a medium
ash matrix. The andesite clasts contain plg + hn + px ± bi. The deposit is up to 25 m
thick and covers the lava flow unit exposed at site TAC0336a. A light-gray porphyritic
lava flow covering the above sequence (TAC0332a), consists of plagioclase,
hornblende, and pyroxene embedded in a fine, compact matrix. Atop of the Chichuj
sequence there is a ∼15 m thick unit constituted by pink to dark-gray breccias (∼ 2.5 m
thick) and laminate lavas with flow banding (50 cm thick) grading to massive (∼1m
thick). These rocks consist of plg > px > hn > set in a matrix with resorbed rims and
rounded light-gray enclaves up to 4 cm in diameter.
Tacaná Sequence
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Ten volcanic units can be recognized within the Tacaná Volcano Sequence, most of
them ranging in age from late Pleistocene to the Holocene. The sequence consists of
lavas, debris avalanches, block-and-ash flows, fallouts, and lahar deposits. The oldest
units of Tacaná are andesitic lava flows exposed only at the base of the gullies
surrounding the volcano (TAC9869, 9870, 0338a and 0324a). The lava shows flow
banding and has a chemical composition of 60.76% SiO2. These lava flows are overlain
by block-and-ash flow deposits (BAF) widely distributed around the volcano forming two
main fans.
The oldest BAF is exposed on the southern flank of Tacaná from the village of Talquian
up to Santo Domingo. The deposits are massive and consist of light-gray dense
andesitic blocks, minor red andesites embedded in a coarse ash matrix. The andesitic
blocks consist of plagioclase, pyroxene, and dark-brown glass. The deposits contain
charred logs that were dated at ca. 42,000 yr BP (Espindola et al, 1989) and 38, 630
+5100/-3100 yr BP (Espíndola et al., 1993) (Table 4). Another light-gray block-and-ash
flow deposit overlies this BAF unit in the vicinity of the Cordoban village and over the
Muxbal debris avalanche at the Muxbal coffee plantation. Here, the BAF consists of
glassy juvenile andesites and gray dense andesites embedded in a fine sand matrix.
Both types of clasts contain plagioclase, pyroxene, and amphibole. The deposit
contains disseminated charcoal that yielded an age of 28,540 +/- 260 yr BP (Table 4).
The northern flank of the volcano is dominated by a BAF deposit that pinches out
against the rim of San Rafael caldera. This BAF consists of at least four massive gray
units, each one composed of andesitic blocks supported by a fine ash matrix. At site
15
TAC9752, near the San Rafael village a charcoal sample yielded an age of 16, 350 yr
BP (Table 4) (Mora et al., 2004).
The E-NE slopes of the TVC exhibit a complex sequence of deposits. The deposits
consist of three fallouts rich in pumice interbedded with ash flow layers with minor
pumice and dense clasts set in an ash matrix, and several laminated, cross-stratified
surges. The juvenile fragments of this Plinian deposit are andesites similar to the most
recent products of the volcano, and the mineral assemblage is represented by
plagioclase, clinopyroxene, orthopyroxene, amphibole, and Fe-Ti oxides, set in a glassy
groundmass. A piece of charcoal found at the base of the deposit (TAC0335c) yielded
an age of 32,290 +2155/-1695 yr, BP (Table 4).
The Agua Caliente debris avalanche deposit crops out on the northwestern part of
Tacaná (Macías et al., 2004). The deposit is confined by the San Rafael and Tochab
rivers up to their junction with the Coatán River. It is 8 km long, covers an area of 6 km2
and has a volume of ca. 1 km3. It exhibits a block facies with a rare light-brown ash
matrix. The deposit has a maximum thickness of 200 m close to the Coatán River.
One of the youngest units is a clast-supported yellow pyroclastic flow (TAC0343c) rich
in pumice. The deposit is 6 m thick and is covered by another 6 m of reworked material.
The pumice clasts are white with pyroxene, amphibole, and plagioclase supported by a
glassy matrix. A charcoal sample collected in this unit yielded and age of 6,700 yr BP
(Table 4).
Plan de Las Ardillas Sequence
This sequence is exposed between the San Antonio and Tacaná volcanoes. It consists
of a central dome with two flows running along the NW and SE flanks of San Antonio
16
and Tacaná volcanoes. The dome consists of porphyritic gray andesitic lavas made up
of plagioclase and amphibole and abundant dark-gray enclaves set in a glassy matrix.
The two lava flows are dark-gray andesites with porphyritic textures of plagioclase and
pyroxene phenocrysts in an aphanitic matrix. These flows developed steep flow fronts
and levees with breccias at their base made of meter-size blocks. Ar-Ar analysis of two
samples yielded ages of .032 ± .012 Ma (TAC0324a), and .013 ± .023 Ma (TAC0361c),
(Table 2). From field relations it is clear that the Plan de Las Ardillas domes are younger
than the Agua Caliente debris avalanche (< 26,000 yr BP; Table 4).
San Antonio Sequence
It is located southwest of the Plan de Las Ardillas dome. Near Santa María La Vega
village (site TAC0358c) it is composed by an alternating sequence of gray basaltic
andesitic lava flows. These rocks have an aphanitic texture with amphibole and
plagioclase phenocrysts. Two sites near Talquian village (TAC9802 and 9803) were
studied. At the first locality, the lava flow consists of several units of gray porphyritic
basaltic andesites. The hand specimens contain plagioclase, hornblende and olivine,
set in a medium grain groundmass. Individual flow units are 2 m thick with a total
thickness of 5 m. In the second locality it crops out as a porphyritic andesite with similar
features.
The youngest product of the San Antonio dome is the Mixcun pyroclastic flow deposit
exposed in its southern flank. This is a BAF deposit that consists of light-gray, darkgray, glassy, and banded andesites, minor altered red andesites, embedded in a coarse
ash matrix. The deposit has fumarolic pipes and juvenile lithics with cooling joints.
Disseminated charcoal found in this deposit yielded an age of 1,950 yr BP (Macías et
17
al., 2000). A thick sequence of debris flows, hiperconcentrated flows and fluviatile layers
is exposed at TAC98072 and TAC98073, near San Salvador Urbina and Union Rioja
towns. Each layer is up to 2 m thick and is constituted by accidental cobbles up to 20
cm in diameter, and pumice embedded in a coarse sand matrix.
Structural Geology
Burkart and Self (1985) proposed a structural geometry model of Guatemala and
eastern Mexico (Fig 8A). They constructed a regional cross section and recognized
three volcanotectonic zones. The eastern zone (III) was characterized by horst and
graben structures including the Guatemala and Ipala grabens with widespread
associated monogenetic volcanism; the central zone (II) characterized by an extreme
thinning of the crust, with the Atitlán Caldera bounded by two structural highs showing
polygenetic volcanism, and the western zone (I) dominated by volcanoes built upon
basement complexes. The TVC is precisely built upon the western zone, a structural
high characterized by a negative gravimetric anomaly (Fig. 9). Lithologically, the high is
made of metamorphic, granitic and volcanic rocks deeply altered, fractured, and faulted.
In the TVC region the structural high is affected by three important fault systems; the
oldest is located to the west of the complex and consists of fractures and NW-SE faults
affecting granitic rocks of Mesozoic and Tertiary age. The second fault system is
aligned to the TVC and has a NE-SW trend. The youngest fault system has a N-S trend
and crosscuts the other two fault systems. The NE-SW fault system is the most
conspicuous structure because it delineates the graben hereafter called the Tacaná
graben, inside of which the TVC has been built (Fig. 8B).
18
This graben is 30 km long and 18 km wide with vertical displacements of about 600 m. It
is bounded to the NW by a horst on which the Chanjale Caldera is located, and to the E
by a horst holding the Sibinal caldera. The Coatán and Suchiate rivers follow these
major faults, respectively, and act as the main boundaries of the Tacaná graben. The
structural analysis on the Coatán and Suchiate rivers (Figs. 10 and 11), indicates that
the main fracture systems have a NE-SW trend, which is supported by the analyses of
satellite images, photogeology, and normal faults (Fig. 12).
The graben became active after the emplacement of the San Rafael (2 Ma) and
Chanjale (1 Ma) calderas, and before the emplacement of the Chichuj volcano, as the
rocks forming this structure are not affected by the NE-SW faults. This fact indicates
that the graben has controlled the birth and evolution of the TVC. This is supported by
the alignment of the Chichuj, Tacaná, Plan de Las Ardillas, and San Antonio landforms,
which has a general trend of N65ºE.
According to our analysis, the region was affected by a NNW stress field with a
minimum principal stress σ3 during late Pleistocene that correlates with the focal
mechanism determined in the region by Guzman-Speziale et al., 1998) (Fig 13). The
origin of the stress is related to the direction of movement of the Cocos plate beneath
the Caribbean plate (De Cserna et al., 1998)
Discussion
Tectonic and volcanic evolution
Based upon the stratigraphic relations and the radiometric and radiocarbon data, we
propose the following tectonic and volcanic evolution of the TVC and surrounding areas.
19
The oldest rocks in the region are gneisses, schists, metavolcanics, slates, and granites
of Cretaceous age. The origin of these rocks remains uncertain due to the scarcity of
absolute dates and other detailed studies. However, the sequence can be correlated
with rocks of the same age of Central America (Meschede and Frisch, 1988)
During the Tertiary, the metamorphic rocks were intruded by two main magmatic pulses,
including granites and tonalites. The first pulse occurred during late Eocene to early
Oligocene probably related to the subduction of the Farallon plate underneath the North
America plate (Meschede and Frisch, 1988). The second pulse occurred during early to
middle Miocene likely associated to the subduction of the Cocos plate under the
Caribbean plate. The seven million years gap between these two intrusive pulses could
have been related to the reorganization of the Pacific region during the fragmentation of
the Farallon plate into smaller microplates (i.e. Cocos plate) (Hey, 1977; Stock and Lee,
1994).
The region was later affected by a tectonic Miocene compressive event accommodated
through strike-slip and reverse faults. This compressive event could be related to motion
and kinematics of the Motagua-Polochic fault and to the position of the Tacaná Volcanic
Complex at a compressive stepover placed between the Chamalecón and Motagua
faults. This tectonic phase has been recorded in other parts of southern Mexico
(Campa, 1998), at El Chichón volcano (García-Palomo et al., 2004), and at the Ixtapa
Graben in central Chiapas (Meneses-Rocha, 2001). The analysis of these sites
indicates a NE-SW trend for the main principal stress σ1 as attested by slickensides,
sigmoidal gouge faults, non-cohesive breccias, and other sigmoidal structures.
20
The basement rocks were uplifted and tilted after the middle Miocene and before the
Pliocene as a consequence of the subduction process. The basement rocks suffered
deep erosion and weathering that created coalescent debris fans widespread in the SE
portion of the area.
During the Pliocene three caldera structures San Rafael (2 Ma) , Chanjale (1 Ma), and
Sibinal were emplaced unconformably on the tilted basement rocks that canalized
volcanic products toward the southern portion of the area. This process produced
widespread fans of pyroclastic and debris flow deposits. The caldera structures were
affected by normal faulting in the late Pliocene-early Pleistocene originating the NE-SW
trending Tacaná graben. The TVC was emplaced later inside the graben.
The initial episodes of formation of the TVC began in late Pleistocene with the
emplacement of the Chichuj volcano. It started with the emission of andesitic lava flows
followed by explosive activity that destroyed the volcanic edifice producing the Muxbal
debris avalanche. The debris avalanche was controlled by the morphology of the region,
it was emplaced toward the east pinching out against the San Rafael caldera walls and
then toward the south. The volcano grew though the emission of lavas and minor
pyroclastic activity. Chichuj volcano finally collapsed towards the W-SW leaving the
remains of the volcanic edifice that we see today.
Tacaná volcano was built west of the remains of Chichuj initially through emissions of
andesitic lava flows followed by Peléan, Plinian, and effusive eruptions. The original
edifice of Tacaná was later destroyed in part by a sector collapse directed to the NW of
the crater and perpendicular to the NNE-SSW normal faults. The debris avalanche
pinched out against the caldera wall and was stopped by the Coatán River. The event
21
was followed by a series of block-and-ash flows without the generation of juvenile
material (Macías et al., 2004). These facts suggest that the collapse was triggered by
motion along the NNE-SSW normal faults.
The activity of the volcano continued with the emplacement of the Plan de Las Ardillas
Dome (≥ 32,000 yr), likely coeval with the collapse of Tacaná. This dome was intruded
SW of Tacaná following a NW-SE trend that correlates with the NE-SE normal faults.
Finally, the San Antonio Volcano was constructed in the SW tip of the NE-SW volcanic
alignment with steep slopes of andesitic lava flows. A magma mixing event produced a
Peléan type eruption with the generation of the Mixcun pyroclastic flow 1,950 yr B.P.
(Macías et al., 2000).
Conclusions
The TVC is built upon Mesozoic gneisses, schists, metavolcanics, slates, and granites
rocks. These rocks were intruded by two episodes of magmatism during late Eocene to
early Oligocene probably related to the subduction of the Farallon plate underneath the
North America plate, and during early to middle Miocene likely associated to the
subduction of the Cocos plate under the Caribbean plate. These rocks are affected by a
tectonic Miocene compressive event that was accommodated through strike-slip and
reverse faults. The main principal stress σ1 of this event had a NE-SW trend as
supported by slickensides, sigmoidal gouge faults, no cohesive breccias, and sigmoidal
structures. These basement rocks were uplifted and tilted after the middle Miocene and
before the Pliocene as a consequence of the subduction process. During the Pliocene
three caldera structures (San Rafael, Chanjale, and Sibinal) formed on the basement
rocks and during the Pliocene-early Pleistocene these calderas were affected by NE-
22
SW normal faults originated the Tacaná graben inside of which was emplaced the
Tacaná Volcanic Complex during late Pleistocene. The TVC formed through the
subsequent formation of four NE-SW aligned structures named Chichuj, Tacaná, Plan
de las Ardillas and San Antonio.
Acknowledgments
This project was supported by grants from CONACYT (38586-T to J.L.M.) and DGAPAUNAM (IX101404 to J.L.M.). We are indebted to F. Ortega and M. Alcayde for their
review of the first version of this manuscript.
23
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27
Figure captions
Figure 1. Tectonic setting of southern Mexico showing the boundary between plates,
the isodepth contour lines (Pardo and Suarez, 1995), convergence rates (DeMets et
al., 1990), major fault system (Burkart and Self, 1985), and location of volcanoes after
García-Palomo et al., 2004). Location of the Tacaná Volcanic Complex (TCV) in
southern Mexico.
Figure 2. View from the south of Chichuj (Ch), Tacaná (T), Plan de Las Ardillas (PA),
and San Antonio (SA) edifices forming the Tacaná Volcanic Complex. In the
foreground appears the town of Santo Domingo located on the southeastern slopes of
the volcanic complex.
Figure 3. Cross section C-C´ shows the geomorphologic features of the TVC, such as
major difference in elevation, asymmetric shape and its control by the tilting of the
basement rocks and the calderic structures. See location of cross section in figure 4.
Figure 4. Simplified geological map of the TVC displayed upon a digital elevation model
of the area. The map shows main units separated on the basis of stratigraphic
correlation and radiometric dating as shown in the stratigraphic column. The B-B’ and
C-C’ cross sections, the latter is shown in figure 3. Asterisks indicate the location of
the K-Ar or Ar-Ar dates mentioned in the text. Dots and numbers are the sites of
stratigraphic sections.
Figure 5. Photograph of the metamorphic basement along the San Rafael river at site
TAC0367. Here the rocks exhibit crenulation cleavage (A), and banding of minerals
(B).
Figure 6. Alkali vs. Silica classification diagram modified after Le Bas et al. (1986).
TVC, Tacaná Volcanic Complex composed by Chichuj, Tacaná and San Antonio
Volcanoes (Mercado and Rose, 1992; Mora et al., 2004). The TVC rocks fall in the
medium-k content field. The caldera rocks and Las Ardillas dome follow the same trend.
Figure 7. View of vertical walls of the Chanjale Caldera lava flow at site TAC0333 along
the Chespal-Pavincul road. Notice columnar jointing of the rocks.
Figure 8. Structural geometry of the Central America region according with Burkart and
Self (1985). A) Regional cross section that divides Guatemala in three main
volcanotectonic zones: The western zone dominated by volcanoes built upon
28
basement complexes, the central zone characterized by extreme crustal thinning, and
the eastern zone characterized by horst and graben structures including the
Guatemala and Ipala grabens. B) Section of the western zone at the Tacaná Volcanic
Complex.
Figure 9. Regional gravimetric map of western Central America (Burkart and Self,
1985). The Tacaná Volcanic Complex (TVC) appears on the southwestern edge of the
area. The TVC is built upon a basement high that is represented by a gravimetric
anomaly.
Figure 10. Detail of the Coatán River normal fault that bounds two different lithologies,
to the left block-and-ash flow deposits of the Sibinal Caldera and to the right Tertiary
granites.
Figure 11. Characteristics of Suchiate River fault, affected to granitic rocks, a fault
gouge, and riedel structures that indicated a normal fault. Hammer as scale
Figure 12. Analysis of faults and fractures in the TVC area. Rose fracture diagrams of
the Suchiate (A) and Coatán (B) river faults, showing N-S and NE-SE trends,
respectively. C) Rose diagram that displays the lineaments obtained from
photogeology and D) stereographic projection of normal faults (lower hemisphere in
the Wulf diagram).
Figure 13. Sketch map showing focal mechanisms (circles) and maximum horizontal
stress (bars) from borehole elongations in the region (from Guzmán-Speziale and
Meneses-Rocha, 2000).
29
Table 1. Summary of K-Ar dates of rocks of the Tacaná area according to previous works.
Sample
Material dated
2M-26-79
Bi
2M-23-79
Bi
GAP-589
Bi
GAP-586
Bi
GAP-582
Hb
40*Ar = Radiogenic 40Ar
n.a. = not available
40
*Ar (ppm)
0.00106
0.001603
0.001994
0.002208
0.0029
40
K (ppm)
7.85776
6.6859
8.0608
5.6196
0.3567
Age (Ma)
Location
20 ± 1
29 ± 1
35 ± 1
39 ± 1
142 ± 5
N15°03'39'' W91°05'35''
N15°17'33'' W92°21'59''
n.a.
n.a.
n.a.
Refs
Mugica, 1987
Mugica, 1987
Mugica, 1980
Mugica, 1980
Mugica, 1980
Table 2. 40Ar/39Ar analyses of rocks from Tacaná area.
Sample
Min.
Integrated Age (ka)
% 40Ar*
Tac0361c
WR#1
80 ± 64
3.7
138 ± 42
1.8
86 ± 51
WR#3
-65 ± 39
-0.8
-22 ± 36
-2.1
WR#5
-26 ± 29
-0.5
WR#1
-96 ± 85
-14.5
WR#2
Average
Tac0333
-142 ± 34
5 ± 19
1.1
WR#3
22 ± 16
3.9
WR#1
WR#2
946 ± 47
734 ± 38
49.8
46.6
Plateau information
Single shot
WR#2
WR#4
Average
Tac0324a
Plateau Age
2 fractions 74%
39
3 fractions 58%
39
2 fractions 72%
39
Ar released MSWD
= 0.2
3 fractions 93% 39Ar released MSWD
= 0.7
Average
Tac0349c
Average
Tac0323a
769 ± 23
50.5
WR#1
WR#2
WR#3
1859 ± 43
1744 ± 29
1729 ± 17
68.5
61.7
66.4
WR#1
WR#2
11.7 ± 5.1 Ma
1.2
5
WR#3
5.8 ± 1.4 Ma
3.3
Average
Tac0364
Tac0359C
Single shot
Ar released MSWD
=1.6
5 fractions, 90%
39
Ar released MSWD
=1.8
39 ± 15
32 ± 12 ka
811 ± 30
Single shot
1 fraction, 56% 39Ar released
816 ± 19
815 ± 16 ka
1834 ± 26
1887 ± 16
1872 ± 24 ka
1968 ± 147
2007 ± 98
1995 ± 82 ka
13.3 ± 0.2 Ma
89.1
13.3 ± 0.2 Ma
Bi/Pl
11.8 ± 0.1 Ma
63.4
12.2 ± 0.1 Ma
13.7 ± 0.1 Ma
39
3 fractions, 79%
21 ± 19
Bi
Bi
Ar released MSWD
=0.0
29 ± 28
13 ± 23 ka
5 fractions, 94%
WR#3
Ar released MSWD
=0.1
-37 ± 48
Ar released MSWD
= 0.1
Single shot
1 fraction 71% 39Ar released
1 fraction 57% 39Ar released
Lost 39Ar data
1 precise fraction, 20% 39Ar released
8 fractions, 75% 39Ar released MSWD
=2.4
4 fractions 88%
39
9 fractions 86%
39
Ar released MSWD
= 0.4
5 fractions 74% 39Ar released MSWD
= 0.9
13.9 ± 0.1 Ma
10 fractions 87%
Tac0364cgr
Bi
29.1 ± 0.2 Ma
29.4 ± 0.2 Ma
39
Ar released MSWD
= 1.4
39
Ar released MSWD
= 1.4
Bold: Preferred age for each sample (ages reported at ± 1 sigma). Plateau: 3+ consecutive fractions, MSWD <
~2.5, more than 50% 39Ar release. Abbreviations: WR = whole-rock; Bi =biotite, Pl = Plagioclase. All analyses
were performed at University of Alaska, Fairbanks.
Table 3. Whole-rock chemical composition of some rocks of the Tacana Volcanic Complex.
Volcano
Sample
San Rafael
San Rafael
Las Ardillas
Changajale
0323A
0349C
0333a
0324a
0361C
SiO2
54.66
57.94
58.39
59.58
59.91
TiO2
0.80
0.82
0.68
0.67
0.63
Al2O3
17.53
17.70
17.88
17.36
17.04
Fe2O3*
7.86
7.22
7.03
6.33
6.39
MnO
0.17
0.11
0.15
0.11
0.12
MgO
3.00
2.64
2.54
2.55
2.54
CaO
7.30
6.50
7.44
5.95
5.94
Na2O
3.74
3.74
3.89
3.67
3.43
K2O
1.75
2.16
1.56
2.02
2.12
P2O5
0.27
0.25
0.24
0.16
0.18
LOI
1.98
0.65
0.38
0.44
1.00
Total
99.06
99.73
100.18
98.84
99.30
Ba
788
788
821
714
732
Rb
39
70
32
47
52
Sr
840
726
714
507
493
Cs
1.6
1.6
1.5
1.7
2.0
Hf
3.7
4.2
3.4
3.4
3.7
Zr
155
152
135
136
126
wt. %
FeO
ppm
Y
17
15
20
17
16
Th
3.9
5.6
3.2
2.7
3.9
U
1.4
2.0
1.4
1.3
1.5
Cr
22.8
13.1
9.7
9.5
5.3
Co
18.5
15.8
13.9
14.3
14.2
Sc
11.7
10.6
11.8
11.8
12.6
V
141
139
116
156
105
Tb
0.6
0.6
0.6
0.6
0.5
Cu
14
14
9
17
10
Ni
10
Pb
7
5
4
Zn
90
89
89
77
86
La
20.6
22.1
17.8
15.9
17.7
Ce
35
40
32
27
35
Nd
18
22
17
13
17
Sm
4.09
4.22
4.08
3.12
3.52
Eu
1.39
1.28
1.40
1.08
0.93
Yb
1.47
1.35
1.97
1.47
1.56
Lu
22.00
0.20
0.29
0.23
0.24
Major and trace elements analyzed by Inductively Coupled Plasma Mass Spectrometry (ICP-MS) and Instrumental
Neutron Activation Analysis (INAA) (<0.01% major elements; Ba, 50 ppm; Cr, Pb, Nb, V, and Rb, 2 ppm; Ni, Sc, Sr, Y,
and Zr, 1 ppm; Cu, Zn and Ta, 0.5 ppm; Hf and Th, 0.2 ppm; U, 0.1 ppm; La, Ce, Nd, Sm, Tb and Yb, 0.1 ppm; Eu, 0.05
ppm, detection limits) at Activation Laboratories, Ancaster, Canada. B1, Fe2O3* is reported as total iron.
Table 4 . Summary of radiocarbon determinations carried out at analyses of charcoal samples. * AMS
Sample
TAC 0343C
Lab. No. 14C age yr BP
A-10442
9752
TAC0330a
A-12890
TAC9714*
TAC9332
TAC0335-C2C
Trinidad
La Trinidad
d13PDB(%o) Mat. dated Location
Refs
6910 +/- 95
-25.9
Charcoal
N 15°06'10'' W92°04'50''
This work
16, 350 +/- 50
-25.0
Charcoal
N15°09'29'' W92°06'97''
Mora et al., 2004
26340 +910/-820
-26
Macías et al., 2004
Wood
N15°09'35'' W92°10'38''
Charcoal
N15°05'03'' W92°04'35''
Mora et al., 2004
-25
Charcoal
N15°02'34'' W92°05'11''
Espíndola et al. 1993
28,540 +/- 260
6923
>30,845
A-13365
32,290 +2155/-1695
-20.8
Charcoal
N15°09'43'' W92°05'68''
This work
n.a.
42,000
not available
Charcoal
N15°02'12'' W92°06'51''
Espíndola et al. 1989
A-6924
38,630 +5100/-3100
-25.0
Charcoal
N15°02'12'' W92°06'51''
Espíndola et al. 1993
22°
N
NAP
30
20°
100 km
TVF
18°
Ch
an
C a y mu g h
Tro
Oaxaca
Salina Cruz
0
CVA
C VA
0
10
16°
30
20
0
CaP
60
40
20
68
MA
Guatemala Ca ribbean
C AV
T
Plate
A
CAV
A
66
TR
CoP
96°
76
14°N
Tacaná
94°W
92°W
19
90°W
88°W
Figure 1. García-Palomo et al
T
SA
PA
Ch
Figura 2 Garcia-Palomo et al
N76=E
S76=W
C
C`
Tacaná
San Rafael Caldera
Chichuj
Plan de las Ardillas
San Antonio
4000
4000 m
3000
3000
2000
Carrillo Puerto
village
1000
0
2000
1000
23.5 km
Figure 3. García-Palomo et al
92°15’ W
92°10 ’W
92°05’ W
Tacana
B
*
9875
0.8+.01
C
0333
*
1680000
20 +1
0364
Chanjale
San Rafael
* 12.2+.1
15°10’ N
1.9+.02
0367
0338a
0359
0330
*
*
35+1
0323
*
0335c2
035C
0324a
0349c
0334c
9752
0328a
037c
TV
0308a
0332a
0332
0333b
0335
0333
SA LA
*
Sibinal
Ch
142+5
0364
9870
*
1670000
9869
0340
0343c
29+0.2
15°05’ N
9803
9802
0358
*
El Edén
39+1
*
13.4+1
B
*
20+1
Santo Domingo
Carrillo Puerto
C
N
Legend
Tacana Sequence
Modern aprons
Late Pliocene-Pleistocene Sequence
Chichuj Sequence
Tertiary granitic rocks
Mesozoic metamorphic rocks
San Antonio Sequence
Las Ardillas Sequence
W
0
5 km
E
S
Figure 4. García-Palomo et al
Sequence
SM
San Antonio
Sequence
Age
Tectonic/volcanic
event
1902 Ash fall
Debris flow
Mixcum block-and-ash flow (1, 950 yr BP)
Third collapse
Dome
Plan de
las Ardillas
Sequence
Agua Caliente lava flow (32,000 yr BP)
Lava Dome
Lava dome (8,000 yr BP)
Block-and-ash flow (16,000 yr BP)
Lava flow (17,000 yr BP)
Tacana
Sequence
Block-and-ash flow (28,000 yr BP)
TVC
Debris avalanche deposit (» 26340 +910/-820 yr BP)
Plinian deposit (fall, flow and surges)
(32290 +2155/-1695 yr BP)
Second collapse
Lava flow (35,000 yr BP)
Block-and-ash flow (40,000 yr BP)
Andesitic lava flow
Pleistocene
Chichuj
Sequence
Muxbal debris avalanche
First collapse
Andesitic lava flows
Emplacement of the TVC
Pre-TVC
Tectonic phase with normal faulting
San Rafael
Chanjale
Sequence
Late Pliocene-Pleistocene
(0.2 +/- .08 to 0.81 +/- .016 Ma)
Disconformity
Second phase of igneous activity
Late Eocene early Oligocene
(29.1 to 39 5 Ma)
First phase of igneous activity
7-Ma gap
Uplift
Basement
Tectonic phase with reverse faulting
Early Miocene to middle Miocene
(13.7 to 20 Ma)
Early Cretaceous
(142 +/- 5 Ma)
Figure 4a. García-Palomo et al
A
B
Figure 5. García-Palomo et al
12
Tephriphonolite
Na2O+K2O wt.%
10
Trachyte
PhonoTephrite
San Antonio
San Rafael
8
Rhyolite
Chichuj
Tacana
6
Dacite
4
Basaltic
andesite
Basalt
Las Ardillas
Andesite
2
Chanjale
0
45
50
55
60
SiO2 wt.%
65
70
75
Figure 6. Garcia-Palomo et al
Figure 7. García-Palomo et al
S60E
N76W
S76E
N47W
Tacana Graben
N60W
B
Chanjale caldera
S47E
Suchiate River Fault
Coatán River Fault
B
3600
3600
2600
2600
1600
1600
600
600
0
31.3
TVC
Atitlán caldera
A
Volcan de Fuego
Guatemala
Graben
Western Zone
Central Zone
Ipala
Graben
A
Eastern Zone
Figure 8.
Garcia-Palomo et al
90=W
0
50
100 km
-4
16=N
Me
-4
0
-8
-8
0
-6
0
0
-11
xic
o
Gu
ate
ma
la
-100
N
0
-2
0
0
-10
0
Polochic Fault
0
-120
TVC
Motagua Fault
-4
0
-60
-10
-60
-80
Pacific Ocean
-40
Honduras
Jocotan Fault
0
-60
-40
-2
0
El Salvador
Anomaly isopleths in milligals
Figure 9 Garcia-Palomo et al
Block-and-ash flow
deposit
Granite
Figure 9 Garcia-Palomo et al
Figure 11 Garcia-Palomo et al
A
B
N
20
20
20
N
20
20
20
20
C
N
D
20
20
N
20
3
20
Figure 12 Garcia%palomo et al
Gulf of Mexico
5
4
7
6
8
Mexico
Guatemala
<
9
<
Caribbean Sea
Motagua fault
Focal Mechanims
<
TVC
<
<
< <
Polochic Fault
<
Main sinistral faults
Pacific Ocean
Figure 13. García-Palomo et al