Geological Evolution of the Tacaná Volcanic Complex, Mexico-Guatemala
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Geological evolution of the Tacaná Volcanic Complex, Mexico-Guatemala 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 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 (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 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 (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.