RESEARCH Geology of the coastal Chiapas (Mexico) Miocene plutons and the Tonalá shear zone: Syntectonic emplacement and rapid exhumation during sinistral transpression Roberto S. Molina-Garza1, John W. Geissman2, Tim F. Wawrzyniec3,*, Tomás A. Peña Alonso1, Alexander Iriondo1, Bodo Weber4, and Jorge Aranda-Gómez1 1CENTRO DE GEOCIENCIAS, UNIVERSIDAD NACIONAL AUTÓNOMA DE MÉXICO, CAMPUS JURIQUILLA, 76230 QUERÉTARO, MÉXICO 2DEPARTMENT OF GEOSCIENCES, UNIVERSITY OF TEXAS AT DALLAS, 800 WEST CAMPBELL ROAD, RICHARDSON, TEXAS 75080-3021, USA 3DEPARTMENT OF NATURAL AND ENVIRONMENTAL SCIENCE, WESTERN STATE COLLEGE OF COLORADO, GUNNISON, COLORADO 81231, USA 4DEPARTAMENTO DE GEOLOGÍA, CENTRO DE INVESTIGACIÓN CIENTÍFICA Y DE EDUCACIÓN SUPERIOR DE ENSENADA, CARR. ENSENADA-TIJUANA 3918, 22860 ENSENADA B.C., MÉXICO ABSTRACT Late Miocene plutons in coastal Chiapas, Mexico, represent the roots of an extinct magmatic arc. Miocene granitoids of calc-alkaline com- position and arc chemistry intruded into and were deformed within the Tonalá mylonite belt in the middle to upper crust. The mylonite belt is a crustal-scale shear zone extending along the western margin of the Chiapas Massif for ~150 km. Deformation is characterized by a domi- nantly subhorizontal lineation and subvertical foliation along a strikingly linear zone that trends ~310°. Mylonitic fabrics contain ambiguous but dominantly sinistral shear indicators. Intrusions are interpreted as syntectonic on the basis of similar U-Pb zircon crystallization age esti- mates (ca. 10 Ma) and the cooling age estimates obtained on neoformed micas in the mylonite. The plutons are elongated, their long axis is parallel to shear zone, and some plutons show markedly asymmetric outcrop patterns, with sheared tails that trail behind the intrusions and that are consistent with sinistral displacement. Parts of plutons were mylonitized by continuous deformation in the Tonalá shear zone, locally developing intricate pseudotachylyte and cataclasite veins slightly oblique to the mylonite foliation. Outside of the shear zone, plutons pre- serve magmatic fabrics. These observations are consistent with features common to syntectonic granites interpreted to have been emplaced along strike-slip shear zones in a transpressional setting. We interpret the Tonalá mylonites as representing a relict transform boundary that was slightly oblique to the Polochic-Motagua fault system, which accommodated over 100 km of sinistral displacement between the Chortis block (on the Caribbean plate) and Chiapas (on the North America plate) in late Miocene time. LITHOSPHERE; v. 7; no. 3; p. 257–274; GSA Data Repository Item 2015107 | Published online 12 March 2015 doi:10.1130/L409.1 INTRODUCTION and Self, 1985; Guzmán-Speziale et al., 1989; Lyon-Caen et al., 2006; DeMets, 2001). Chiapas is included in the Maya block (Dengo, 1969), Products of middle to late Miocene arc volcanism are widespread in which is within the North America plate. The Maya block was defined to southern Mexico (Fig. 1; Morán Zenteno et al., 2000), as well as in the also include the Yucatán Peninsula and northern Guatemala. Its western- Central American highlands from Guatemala to Nicaragua (Burkart et al., most geology, along Sierra Soconusco (Fig. 2), is dominated by the Chi- 1987). However, volcanic rocks of this age range are conspicuously absent apas Massif. The massif is a plutonic and metamorphic complex of mostly in southernmost Mexico, in the state of Chiapas. Igneous activity in the Permian age (Weber et al., 2005), overlain along its northern margin by region between the Tehuantepec isthmus and Guatemala is instead repre- Jurassic to Cretaceous sedimentary rocks and limited volcanic rocks sented by a suite of plutonic rocks exposed along the Pacific coastal plain, (Meneses-Rocha, 2001). The sedimentary sequence is exposed in a fold- where it is often referred to as the extinct Miocene Chiapas arc (Damon and-thrust belt that was formed during the Chiapanecan orogeny between and Montesinos, 1978). Rather than a gap in magmatism, recent uplift and ca. 12 and 10 Ma (Witt et al., 2012; Mandujano-Velazquez and Keppie, deformation resulted in the exhumation of deeper crustal levels in Chiapas 2009; among others). Along its southern margin, the Chiapas Massif is than in Oaxaca to the west, or Guatemala to the east. The Miocene arc in delineated by a pronounced escarpment (~1500 m) that borders a narrow, Chiapas is poorly understood, which is problematic because it may be a very low-relief coastal plain ~25 km wide. The sediment cover along the key to understanding the evolution of the North America–Caribbean plate coastal plain is insignificant, and granitoids of the Miocene Chiapanecan boundary (Ratschbacher et al., 2009), and the causes of the Chiapanecan arc are well exposed in riverbeds along the coastal plain, in isolated hills orogeny to the north (Mandujano-Velazquez and Keppie, 2009). near the coast, and in the foothills of Sierra Soconusco along the southern The Chiapas region is framed within a relatively complex plate-tec- escarpment (Fig. 2; Damon and Montesinos, 1978). tonic setting, affected by the interaction of the North America, Caribbean, The Pacific coastal plain also includes exposures of a laterally continu- and Cocos plates in an unstable and evolving triple junction (Burkart ous mylonite belt, striking parallel to the Chiapas Massif, that is exception- ally well exposed near the city of Tonalá. We present evidence here that *Deceased. this major shear zone, in the western Maya block, is a relict late Miocene LITHOSPHERE© 2015 Geological | Volume Society 7 of| AmericaNumber 3| |For www.gsapubs.org permission to copy, contact [email protected] 257 Downloaded from http://pubs.geoscienceworld.org/gsa/lithosphere/article-pdf/7/3/257/3050979/257.pdf by guest on 01 October 2021 MOLINA-GARZA ET AL. 99ºW 96ºW 93ºW 90ºW 18ºN MEXICO Maya Block Fig. 2 Chiapas 34.5 34.3 MT foldbelt ModC TCSZ CT A Guatemala 30 OT XT 16ºN 32.1 29 32 TSZ 32 CSZ 21 13 ... CSZ ....... 28 Gulf of 10 P ...... 29.6 olochic F. ... .. 29 Tehuantepec ..........LO F 29 ... ón ... lec North America ua F. a PV plate M 4 Motag ... am M idd -Ch le ... .. tán Am J Joco 90 e al AC ric tap a 19 ag Tre ua Chortis Block Caribbean nch F. 14ºN plate El Salvador Cocos Pacific Ocean plate 100 km Figure 1. Regional tectonic map of the distribution of Cenozoic plutonic rocks (red, with numbers indicating the age of emplacement) in southern Mexico and Guatemala and major tectonic features. The Chiapas Massif is shown in dark gray (Weber et al., 2005), and Cenozoic volcanic rocks are in light gray. Modern volcanic edifices are indicated by closed triangles. See inset for location. MT—Mixteca terrane, OT—Oaxaca terrane, XT—Xolapa terrane, CT— Cuicateco terrane, TCSZ—Tierra Colorada shear zone, TSZ—Tonalá Shear Zone, CSZ—Chacalapa shear zone, ModCA—Modern Chiapanecan arc, LO—Las Ovejas complex, CU—Chicomuselo uplift, CF—Chipehua fault, OF—Ocosingo fault, GF—Grijalva fault, CF—Concordia fault, MF—Malpaso fault, ModCA— modern Chiapanecan arc. In the inset PV—Puerto Vallarta; AC—Acapulco; M—Mexico City. crustal plate structural margin that accommodated considerable eastward Generally accepted models for the evolution of the North Amer- motion of the Caribbean plate and facilitated ascent of magma to the Mio- ica–Caribbean–Cocos plate circuit suggest that as the triple junction cene Chiapas arc. This mylonite belt was first recognized by Carfantán migrated to the southeast, the continental margin of Mexico was trun- (1976), and it was also mentioned by Meneses-Rocha (2001); it has been cated, and a continental fragment (the Chortis block) moved eastward referred to as the Tonalá-Motozintla fault. Authemayou et al. (2011) inter- with the Caribbean plate (Karig et al., 1978; Meschede and Frisch, preted the Tonalá fault as a trench-parallel fault linked to the Jaltapagua 1998; Schaaf et al., 1995). The Chortis block, which includes parts of fault in Guatemala. Jaltapagua is a fault bounding a forearc sliver sheared Guatemala, Honduras, El Salvador, and the Nicaragua Rise, shares with between the Cocos and Caribbean plates. Authemayou et al. (2011) fur- southern Mexico similar pre-Mesozoic and Mesozoic histories (Rogers ther proposed that the Tonalá-Motozintla fault marks a suture that allowed et al., 2007; Silva-Romo, 2008). Recent contributions have discarded the North America–Cocos–Caribbean triple junction to migrate south, and this model, but the alternative models proposed have not received much they assumed a component of southward thrusting near its intersection attention. Keppie and Morán-Zenteno (2005) proposed a Pacific origin with the Polochic fault system. We present the first detailed studies of the for the Chortis block, with no interaction with southern Mexico in the shear zone, including geochronologic information documenting the timing Paleogene. More recently, Keppie and Keppie (2012) proposed an origin of displacement along the shear zone, and the associated Miocene plutonic for Chortis in the Gulf of Mexico. A better understanding of the Mio- belt. The fault is here formally named the Tonalá shear zone. cene arc in Chiapas and the Tonalá shear zone is thus critical for the The Cocos plate is presently subducting beneath the North America and evaluation of competing models for the origin of the Chortis block. We Caribbean plates. Neogene to recent motion between the Caribbean plate suggest in this paper that the Tonalá shear zone accommodated relative and North America occurred