Geological evolution of the Tacaná Volcanic Complex, -

by

1García-Palomo, A., 2Macìas J.L., 1Arce J.L, 2Mora, J.C., 3Hughes, S., 4Saucedo, R., 2Espindola, J.M., 5Escobar, R., and 6Layer, P.

1Departamento de Geología Regional, Instituto de Geología, UNAM Coyoacán 04510, México D.F., [email protected]

2Instituto de Geofísica, UNAM Coyoacán 04510, México D.F.,

3Departament of Geology, State University of New York, 876 Natural Science Complex, Buffalo, New York 14260, USA

4Instituto de Geología, UASLP San Luis Potosí

5CONRED, Guatemala

6University of Alaska Fairbanks

Manuscript to be submitted to a GSA Special Paper “Natural Hazards in ”

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 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 , in southern

Mexico, and in the 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 (Guzmán-

Speziale 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 , the main Mayan ceremonial center in the 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 (Guzmán-

Speziale 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 9-

12 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 -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

14 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, dark- gray, 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 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 DGAPA- UNAM (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 References Bergeat, A., 1894. Zur Kenntnis der jungen Eruptivgesteine der Republik Guatemala. Zeitschr. Geol. Ges., 131-157. Bravo, H.; Rebollar, C.J.; Uribe, A., and Jimenez, O., 2004. Geometry and state of stress of the Wadati-Benioff zone in the Gulf of Tehuantepec, Mexico. J. Geophys. Res., 109: B04307. Burkart, B. and Self, S., 1985. Extension and rotation of crustal blocks in northern Central America and effect on the volcanic arc. Geology, 13: 22-26. Campa, M.F., 1998. Una orogenía Miocénica en el sur de México. In: S. Alaniz-Alvaréz, Nieto-Samaniego, A., Ferrari, L (Editor), Primera Reunión Nacional de Ciencias de la Tierra, México, D.F. Carr, M.J., Rose, W.I. and Stoiber, R.E., 1982. Central America. In: R.S. Thorpe (Editor), Andesites. John Wiley & Sons, New York, pp. 149-166. Couch, R. and Woodcock, S., 1981. Gravity and structure of the continental margins of southwestern Mexico and northwestern Guatemala. Journal of Geophysical Research, 86: 1829-1840. Damon, P. and Montesinos, E., 1978. Late Cenozoic volcanism and metallogenesis over an active Benioff Zone in Chiapas, Mexico. Arizona Geological Society Digest, 11: 155-168. De Cserna, Z., Aranda-Gómez, J.J. and Mitre-Salazar, L.M., 1988. Mapa fotogeológico preliminar y secciones estructurales del Volcán Tacaná. Instituto de Geología, México. De la Cruz, V. and Hernández, R., 1985. Estudio geológico a semidetalle de la zona geotérmica del Volcán Tacaná, Chiapas. 41/85, Comisión Federal de Electricidad, México. De la Cruz-Reyna, S., Armienta, M.A., Zamora, V. and Juárez, F., 1989. Chemical changes in spring waters at Tacaná Volcano, Chiapas, México. Journal of Volcanology and Geothermal Research, 38: 345-353. De Mets, C., Gordon, R.G., Argus, D.F. and Stein, S., 1990. Current plate motions. Geophysical Journal International, 101: 425-478.

24 Donnelly, T.W., Horne, G.S., Finch, R.C. and López-Ramos, E., 1990. Northern Central America: The Maya and Chortis blocks. In: G. Dengo, and Case, J.E. (Editor), The Caribbean region. Geological Society of America, Boulder CO, pp. 37-76. Espíndola, J.M., Macías, J.L. and Sheridan, M.F., 1993. El Volcán Tacaná: Un ejemplo de los problemas en la evaluación del Riesgo Volcánico. In: I.I.-L.A. (IILA) (Editor), Simposio Internacional sobre Riesgos Naturales e Inducidos en los Grandes Centros Urbanos de América Latina. Serie Scienza No. 6, CENAPRED, México D.F., pp. 62- 71. Espíndola, J.M., Medina, F.M. and De los Ríos, M., 1989. A C-14 age determination in the Tacaná volcano (Chiapas, Mexico). Geofísica Internacional, 28: 123-128. García-Palomo, A., Macías, J.L. and Espíndola, J.M., 2004. Strike-slip faults and K- Alkaline volcanism at El Chichón volcano, southeastern Mexico. Journal of Volcanology and Geothermal Research, 136: 247-268. Guzmán-Speziale, M., Pennington, W.D. and Matumoto, T., 1989. The triple junction of the North America, Cocos, and Caribbean Plates: Seismicity and tectonics. Tectonics, 8: 981-999. Hey, R., 1977. Tectonic evolution of the Cocos-Nazca spreading center. Geological Society of America Bulletin, 88: 1404-1420. LeFevre, L. and McNally, K.C., 1985. Stress distribution and subduction of aseismic ridges in the Middle America subduction zone. Journal of Geophysical Research, 90: 4495-4510. Luhr, J.F., Carmichael, I.S.E. and Varekamp, J.C., 1984. The 1982 eruptions of El Chichón Volcano, Chiapas, Mexico: mineralogy and petrology of the anhydrite- bearing pumices. J. Volcanol. Geotherm. Res., 23: 69-108. Macías, J.L., Arce, J.L., Mora, J.C. and García-Palomo, A., 2004. The Agua Caliente Debris Avalanche deposit a northwestern sector collapse of Tacaná volcano, México- Guatemala. Geological Society of America Bulletin (Special Paper), Submitted. Macías, J.L., Espíndola, J.M., García-Palomo, A., Scott, K.M., Hughes, S., and Mora, J.C., 2000. Late Holocene Peléan style eruption at Tacaná Volcano, Mexico- Guatemala: Past, present, and future hazards. Bulletin of the Geological Society of America, 112 (8): 1234-1249.

25 Meneses-Rocha, J.J., 2001. Tectonic evolution of the Ixtapa Graben, an example of a strike-slip basin of southeastern Mexico: Implications for regional petroleum systems. In: C. Bartolini, Buffler, R.T., and Cantú-Chapa, A. (Editor), The western Gulf of Mexico Basin: Tectonics, Sedimentary Basins, and Petroleum Systems. AAPG Memoir, pp. 183-216. Mercado, R. and Rose, W.I., 1992. Reconocimiento geológico y evaluación preliminar de peligrosidad del Volcán Tacaná, Guatemala/México. Geofísica Internacional, 31: 205-237. Meschede, M. and Frisch, W., 1998. A plate-tectonic model for the Mesozoic and early Cenozoic history of the Caribbean plate. Tectonophysics, 296: 269-291. Mora, J.C., Macías J.L., García-Palomo, A., Espíndola, J.M., Manetti, P., and Vaselli, O., 2004. Petrology and geochemistry of the Tacaná Volcanic Complex, Mexico- Guatemala: Evidence for the last 40 000 yr of activity. Geofísica Internacional, 43: 331-359. Mugica, R., 1987. Estudio petrogenético de las rocas ígneas y metamórficas en el Macizo de Chiapas. C-2009, Instituto Mexicano del Petróleo, México. Müllerried, F.K.G., 1951. La reciente actividad del Volcán de Tacaná, Estado de Chiapas, a fines de 1949 y principios de 1950. Informe del Instituto de Geología de la UNAM: 28. Nixon, G.T., 1982. The relationship between Quaternary volcanism in central Mexico and the seismicity and structure of the subducted ocean lithosphere. Geological Society of America Bulletin, 93: 514-523. Nelson, S.A., Gonzalez-Caver, E., and Kyser, T.K., 1995. Constrains on the origin of alkaline and calc-alkaline magmas from the Tuxtla Volcanic Field, Veracruz, Mexico. Contributions to Mineralogy and Petrology, 122: 191-211. Pardo, M. and Suárez, G., 1995. Shape of the subducted Rivera and Cocos plates in southern Mexico: Seismic and tectonic implications. Journal of Geophysical Research, 100: 12,357-12,373. Ponce, L., Gaulon, R., Suárez, G. and Lomas, E., 1992. Geometry and state of stress of the downgoing Cocos plate in the Isthmus of Tehuantepec, Mexico. Geophysical Research Letters, 19: 773-776.

26 Rebollar, C.J., Espíndola, V.H., Uribe, A., Mendoza, A. and Pérez-Vertti, A., 1999. Distribution of stress and geometry of the Wadati-Benioff zone under Chiapas, Mexico. Geofísica Internacional, 38: 95-106. Sapper, C., 1927. Vulkankunde. J. Engelhorns Nachf, Stuttgart,: 1-80. Singh, S.K., Astiz, L. and Haskov, J., 1981. Seismic gaps and recurrence periods of large earthquakes along the Mexican subduction zone: a reexamination. Bull. Seism. Soc. Am, 71: 827-843. Stock, J.M. and Lee, J., 1994. Do microplates in subduction zones leave a geological record? Tectonics, 13: 1472-1487. Thorpe, R.S., 1977. Tectonic significance of alkaline volcanism in eastern Mexico. Tectonophysics, 40: 1926. Truchan, M. and Larson, R.L., 1973. Tectonic lineaments on the Cocos plate. Earth and Planetary Science Letters, 17: 426-432.

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 40*Ar (ppm) 40K (ppm) Age (Ma) Location Refs 2M-26-79 Bi 0.00106 7.85776 20 ± 1 N15°03'39'' W91°05'35'' Mugica, 1987 2M-23-79 Bi 0.001603 6.6859 29 ± 1 N15°17'33'' W92°21'59'' Mugica, 1987 GAP-589 Bi 0.001994 8.0608 35 ± 1 n.a. Mugica, 1980 GAP-586 Bi 0.002208 5.6196 39 ± 1 n.a. Mugica, 1980 GAP-582 Hb 0.0029 0.3567 142 ± 5 n.a. Mugica, 1980 40*Ar = Radiogenic 40Ar n.a. = not available Table 2. 40Ar/39Ar analyses of rocks from Tacaná area. 40 Sample Min. Integrated Age (ka) % Ar* Plateau Age Plateau information Tac0361c WR#1 80 ± 64 3.7 Single shot 2 fractions 74% 39Ar released MSWD WR#2 138 ± 42 1.8 86 ± 51 = 0.2 3 fractions 93% 39Ar released MSWD WR#3 -65 ± 39 -0.8 -22 ± 36 = 0.7 3 fractions 58% 39Ar released MSWD WR#4 -142 ± 34 -2.1 -37 ± 48 =0.1 2 fractions 72% 39Ar released MSWD WR#5 -26 ± 29 -0.5 29 ± 28 =0.0 Average 13 ± 23 ka Tac0324a WR#1 -96 ± 85 -14.5 Single shot 3 fractions, 79% 39Ar released MSWD WR#2 5 ± 19 1.1 21 ± 19 =1.6 5 fractions, 90% 39Ar released MSWD WR#3 22 ± 16 3.9 39 ± 15 =1.8 Average 32 ± 12 ka Tac0333 WR#1 946 ± 47 49.8 Single shot WR#2 734 ± 38 46.6 811 ± 30 1 fraction, 56% 39Ar released 5 fractions, 94% 39Ar released MSWD WR#3 769 ± 23 50.5 816 ± 19 = 0.1 Average 815 ± 16 ka Tac0349c WR#1 1859 ± 43 68.5 Single shot WR#2 1744 ± 29 61.7 1834 ± 26 1 fraction 71% 39Ar released WR#3 1729 ± 17 66.4 1887 ± 16 1 fraction 57% 39Ar released Average 1872 ± 24 ka Tac0323a WR#1 - 1.2 - Lost 39Ar data WR#2 11.7 ± 5.1 Ma 5 1968 ± 147 1 precise fraction, 20% 39Ar released 8 fractions, 75% 39Ar released MSWD WR#3 5.8 ± 1.4 Ma 3.3 2007 ± 98 =2.4 Average 1995 ± 82 ka 4 fractions 88% 39Ar released MSWD Tac0364 Bi 13.3 ± 0.2 Ma 89.1 13.3 ± 0.2 Ma = 0.4 5 fractions 74% 39Ar released MSWD Bi/Pl 11.8 ± 0.1 Ma 63.4 12.2 ± 0.1 Ma = 0.9 9 fractions 86% 39Ar released MSWD Tac0359C Bi 13.7 ± 0.1 Ma 13.9 ± 0.1 Ma = 1.4 10 fractions 87% 39Ar released MSWD Tac0364cgr Bi 29.1 ± 0.2 Ma 29.4 ± 0.2 Ma = 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 San Rafael San Rafael Changajale Las Ardillas Sample 0323A 0349C 0333a 0324a 0361C

wt. %

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 FeO 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 ppm 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 Y 1715201716 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 Ni 10 7 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 Pb 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 Lab. No. 14C age yr BP d13PDB(%o) Mat. dated Location Refs TAC 0343C A-10442 6910 +/- 95 -25.9 Charcoal N 15°06'10'' W92°04'50'' This work 9752 16, 350 +/- 50 -25.0 Charcoal N15°09'29'' W92°06'97'' Mora et al., 2004 TAC0330a A-12890 26340 +910/-820 -26 Wood N15°09'35'' W92°10'38'' Macías et al., 2004 TAC9714* 28,540 +/- 260 Charcoal N15°05'03'' W92°04'35'' Mora et al., 2004 TAC9332 6923 >30,845 -25 Charcoal N15°02'34'' W92°05'11'' Espíndola et al. 1993 TAC0335-C2C A-13365 32,290 +2155/-1695 -20.8 Charcoal N15°09'43'' W92°05'68'' This work Trinidad n.a. 42,000 not available Charcoal N15°02'12'' W92°06'51'' Espíndola et al. 1989 La Trinidad A-6924 38,630 +5100/-3100 -25.0 Charcoal N15°02'12'' W92°06'51'' Espíndola et al. 1993 22° N

NAP

0 20° 3 100 km

TVF

18° Ch man Cay h roug 3 T Salina Cruz 00 2 CVA CVA0 0 1 9 16° 0 1 0 60 CaP 40 Guatemala Ca ribbean 20 Tacaná C Plate 8 A 6 M VA 14°N A T R 6 T 6 C AV A CoP 6 7

96° 94°W 92°W 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 Plan de las Ardillas San Rafael Caldera Chichuj San Antonio 4000 4000 m 3000 3000 2000 2000 Carrillo Puerto 1000 village 1000

0 23.5 km

Figure 3. García-Palomo et al 92°15’ W 92°10 ’W 92°05’ W

B Tacana

* 20 +1

9875 0.8+.01 0 C 0 0

0333 0 * 8

0364 6 San Rafael 1 Chanjale * 12.2+.1

N 1.9+.02

’ 0367

0

1 0359 0338a 0335c2 * ° 0349c 5 0330 035C 1 0324a * 9752 0334c Sibinal 0328a 037c * 0323 35+1 0308a TV 0332a 0333b 0332 0333 0335 * SA LA Ch 142+5 9869 0

9870 0340 0 0

* 0 0364 29+0.2 0343c 7 6 9803 1

N 9802

’

5

0

°

5 *

1 0358 13.4+1 B 20+*1 * 39+1 El Edén

Santo Domingo

Carrillo Puerto C

Legend N Tacana Sequence Modern aprons W E 0 5 km S Late Pliocene-Pleistocene Sequence Chichuj Sequence Tertiary granitic rocks San Antonio Sequence Mesozoic metamorphic rocks Las Ardillas Sequence Figure 4. García-Palomo et al Sequence Age Tectonic/volcanic event SM 1902 Ash fall Debris flow San Antonio Sequence Mixcum block-and-ash flow (1, 950 yr BP) Third collapse Dome Plan de Agua Caliente lava flow (32,000 yr BP) las Ardillas Lava Dome Sequence Lava dome (8,000 yr BP)

Block-and-ash flow (16,000 yr BP) Lava flow (17,000 yr BP) Tacana Block-and-ash flow (28,000 yr BP) Sequence Debris avalanche deposit (» 26340 +910/-820 yr BP) Second collapse Plinian deposit (fall, flow and surges)

C (32290 +2155/-1695 yr BP) V T Lava flow (35,000 yr BP) Block-and-ash flow (40,000 yr BP) Andesitic lava flow Pleistocene

Chichuj Muxbal debris avalanche First collapse Sequence

Andesitic lava flows Emplacement of the TVC

Tectonic phase with normal faulting

C San Rafael V Late Pliocene-Pleistocene T

- Chanjale (0.2 +/- .08 to 0.81 +/- .016 Ma) e r Sequence P

Disconformity Tectonic phase with reverse faulting

Early Miocene to middle Miocene

(13.7 to 20 Ma) Second phase of igneous activity t

f

i l t n 7-Ma gap p e Late Eocene early Oligocene U m e (29.1 to 39 5 Ma) First phase of igneous activity s a B

Early Cretaceous (142 +/- 5 Ma)

Figure 4a. García-Palomo et al A

B

Figure 5. García-Palomo et al 12 Tephri- Trachyte phonolite

10 Phono- Tephrite

San Antonio %

. San Rafael Rhyolite

t 8 w

Chichuj O 2 Tacana

K 6 Dacite

+

O

2

a N 4 Basaltic Las Ardillas Basalt andesite Andesite

2 Chanjale 0 45 50 55 60 65 70 75

SiO2 wt.%

Figure 6. Garcia-Palomo et al Figure 7. García-Palomo et al S60E N76W S76E N47W

N60W Tacana Graben S47E B B Suchiate River Fault Chanjale caldera Coatán River Fault 3600 3600 2600 2600 1600 1600 600 600

0 31.3

TVC Atitlán caldera Volcan de Fuego A Guatemala Ipala Graben Graben A

Western Zone Central Zone Eastern Zone

Figure 8. Garcia-Palomo et al 90=W 0 50 100 km - 0 N 4 -2 0 0 - -8 6 16=N -100 0 0 o la ic a -1 1 x m 0 e e t - 4 M a -1 0 u 00 G Polochic Fault -8 0 -120 TVC Motagua Fault -4 0 -60 Honduras

-1 Jocotan Fault 00 -6 0 -80 Pacific Ocean -60 -40 - 4 0 El Salvador -2 0 Anomaly isopleths in milligals

Figure 9 Garcia-Palomo et al Block-and-ash flow Granite deposit

Figure 9 Garcia-Palomo et al Figure 11 Garcia-Palomo et al N N 20 A 20 B

20 20 20 20

20

N N C 20 D

20 20

3

20

Figure 12 Garcia%palomo et al Gulf of Mexico

4 5 6 7 8

Mexico Guatemala

9

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Caribbean Sea

Polochic Fault

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< TVC < Focal Mechanims Motagua fault

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< Main sinistral faults Pacific Ocean

Figure 13. García-Palomo et al