Tectonic Setting

Southeastern Mexico is characterized by a complex triple junction of the North American, Caribbean and Cocos plates (Fig. 2.1). The tectonic setting is characterized by a regional zone of left-lateral strike-slip faults known as the Motagua-Polochic sinistral fault which separates the North American and Caribbean plates. The sinistral fault was created by the eastward migration of the Caribbean plate relative to the North American plate. A zone of convergence is created by the subduction of the Cocos plate under the North American and Caribbean plates along the Middle American trench (Nixon, 1982; Burbach et al., 1984). The Cocos plate is marked by a prominent bathymetric feature called the Tehuantepec ridge (Manea et al., 2005).

Structure is characterized by a Neogene fold and thrust belt and strike-slip fault zone in Mesozoic and Tertiary sedimentary rocks located in the central and northeast portions of Chiapas State (Fig. 2.2; Guzman-Speziale and Meneses-Rocha, 2000; Meneses-Rocha, 2001). The Neogene fold and thrust belt and strike-slip fault zone formed at the beginning of the middle Miocene as a result of a northeast-oriented maximum horizontal stress of the subducted Cocos plate (Meneses- Rocha, 2001). The structural zone is bounded to the south by the Chiapas massif and to the north by the Yucatan platform (Fig. 2.2). The fold and thrust fault belt is an elongated structural trend, convex to the northeast, formed by long, narrow anticlines offset along their flanks by reverse faults that generally overthrust the intervening synclines (Fig. 2.2; Mandujano-Velazquez and Keppie, 2009).

Chapter 2: Regional Geology CV_A CAVA TVF EAP CP TMVB North American Plate Cocos Plate Pacific Plate Caribbean Plate TR 500 km MPSF MAT Mexico City Cerro la Mina and El Chichón N

Figure 2.1 Tectonic map and distribution of volcanic arcs of Mexico and Central America from Cenozoic till recent. Abbreviations: CAVA - Central American volcanic arc, CVA–Chiapanecan volcanic arc, CP - Oligocene to Miocene

Cordilleran Province, EAP - Eastern Alkaline Province, MAT - Middle American Trench, MPSF - Motagua-Polochic Sinistral Fault, TMVB –Trans Mexican Volcanic Belt, TVF –Tuxtlas volcanic field, TR - Tehuantepec ridge. Modified from Robin (1982) and García-Palomo et al. (2004).

Yucatan Platform Chiapas M assif Sinistral fault Anticline Recumbant Anticline Syncline Thrust fault Ixtapa Graben Cerro la Mina 50 km N Motagua-Polochic Fault Mexico Guatemala

Neogene Fold and Thrust Belt Sinistral

Fault Zone

Figure 2.2 Generalised structural map of southeast Mexico. The Motagua-Polochic sinistral fault is a diffuse boundary

of faults that starts in northern Guatemala and continues into southeastern Mexico where the Neogene fold and fault belt and strike-slip zone occurs. This shear belt distributes the interplate strain along reverse and strike-slip faults and is bounded by the Yucatan platform and the Chiapas Massif. Modified from Guzman-Speziale et al. (2000) and Meneses-

Chapter 2: Regional Geology The fold axes trend northwest and generally plunge beneath the Pliocene and younger sedimentary rocks of the coastal Tabasco plain of the Gulf of Mexico (Duffield et al., 1984; Meneses-Rocha, 2001; García-Palomo et al., 2004). The strike-slip fault zone is 130 km wide and 360 km long formed by a set of up thrown and down thrown blocks, bounded by left-lateral strike-slip faults (Fig. 2.2; Meneses-Rocha, 2001).

The Ixtapa graben is located in the centre of the strike-slip fault zone and exposes rocks of Middle Cretaceous (Albian-Cenomanian) to Pleistocene age (Fig. 2.2; Meneses-Rocha, 2001). Since the initiation of the subducting trench, the Ixtapa graben has been mainly under transtension until the early Pliocene and shifted to transpression by the late Pliocene (Meneses-Rocha, 2001). The Quaternary is marked by an extension in the Ixtapa graben that was contemporaneous with the ini- tiation of the Chiapanecan Volcanic arc (CVA), during which time volcanic derived sediments were deposited (Meneses-Rocha, 2001).

The nature of the triple junction between the North American, Caribbean and the Cocos plates is controversial, as some (e.g., Burkart, 1983) interpreted it as a simple trench-trench-transform, whereas others (e.g., Guzman-Speziale and Meneses-Rocha, 2000; Meneses-Rocha, 2001) do not consider the triple junction of the three plates to be a classic triple point, but rather propose that the continuation of the western-most part of the Motagua-Polochic sinistral fault is a diffuse boundary of faults that starts in northern Guatemala and continues into southeastern Mexico (Figs. 2.1 and 2.2). According to this model, the Neogene fold and fault belt and strike-slip zone is a shear belt that dis- tributes the interplate strain along reverse and strike-slip faults in northeast and central Chiapas (Fig. 2.2).

The subduction architecture in southern Mexico consists of the Cocos plate subducting beneath the North American and Caribbean plates toward N45°E at an average rate of 76 mm/year (Fig. 2.1; De Mets et al., 1990). The plate is marked by a fracture zone called the Tehuantepec ridge, which began subducting around 8 Ma (Manea et al., 2005) and onshore propagation intercepts El Chichón volcano and the Cerro la Mina prospect (Fig. 2.1). The ridge is a narrow linear feature with a maximum vertical relief of 2000 m from the sea bottom that separates shallower sea floor (−3900 m) to the northwest from the deeper Guatemala basin sea floor (−4800 m) toward the southeast (Manea and Manea, 2008). The geometry of Wadati-Benoiff zone shows that the dip of subduction increases from 25º northwest of the Tehuantepec ridge to 40º southeast and the ridge itself dips at 30 to 35° according to Rebollar et al. (1999). Despite the different subduction angles of the down going Cocos plate there is no evidence for any major tear in the subducted plate associated directly with the Tehuantepec ridge (Burbach et al., 1984; Pardo and Suárez, 1995). The age of the Cocos plate increases from northwest to southeast of the Tehuantepec ridge by approximately 7 Ma (Manea et al., 2005). The depth of the slab below the Cerro la Mina prospect region is calculated to be 225 km (Havskov et al., 1982).

The Cocos plate subducts below the Maya terrane in southern Mexico, a crustal block that comprises mainly the Yucatan Peninsula, the Mexican states of Chiapas, Veracruz and parts of Oaxaca. Basement rocks of northern Chiapas are thought to be granitic and metamorphic, similar

Chapter 2: Regional Geology in age and composition to rocks exposed in the Chiapas Massif (described below; Meneses-Rocha, 2001). The Chiapas crust ranges in thickness from 17 to 28 km near the Middle American trench and 49 to 53 km further inland and in the proximity of the Cerro la Mina prospect (Narcía-López et al., 2004).