Miocene Plutons and the Tonalá Shear Zone: Syntectonic Emplacement and Rapid Exhumation During Sinistral Transpression

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 composition 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 dominantly 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 estimates (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.

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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

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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 ...... 29.6 Polochic F...... Tehuantepec ...... LO F. 29 29 ón ... ec North America a F. ... al PV M 4 Motagu ... am M plate id Ch dle ... .. 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 along the sinistral Polochic-Motagua fault sys- transpressional plate motion between North America and the Caribbean tem, and offshore in the Cayman Trough. The North America–Caribbean– plate in the late Miocene, which gradually was transferred to the Polo- Cocos triple junction migrated southeastward along the Middle America chic and Motagua fault systems. Trench (Pindell et al., 2005; Karig et al., 1978; Meschede and Frisch, 1998), but relative plate motions and plate interaction have resulted in deforma- SAMPLING AND METHODS FOR GEOCHRONOLOGY AND tion over a very broad region extending from the Tehuantepec isthmus, in GEOCHEMISTRY Mexico, to central Honduras (Ratschbacher et al., 2009; Guzmán-Speziale, 2010; Rogers and Mann, 2007). The Tonalá shear zone appears to link the In order to better establish the age of the Miocene Chiapas arc, we active Polochic-Motagua fault system with older faults along the continental collected a sample of fresh, undeformed monzonitic facies from a plu- margin of southern Mexico such as the Chacalapa and Tierra Colorada shear ton exposed northeast of Tepanatepec (sample 34-TEP; Fig. 2), and a zones (Tolson, 2005; Solari et al., 2007; Riller et al., 1992; Fig. 1). sample from a sheared granodiorite exposed west of Pijijiapan (VF48;

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10º

Zanatepec

34TEP 35 Tepan.

33 24º Chahuites 4 30 S I E R R A S O C O N U S C O 36 ARRIAGA 31 5º

46 45 TONALA 29 18º 16ºN 38 39 25 km La Polka 2 44 Los Patos 30º

48 P A C I F I C O C E A N PIJIJIAPAN Quaternary

Miocene Chiapas Arc 94ºW plutons Cretaceous carbonate rocks Upper Mesozoic (?) MAPASTEPEC meta-sedimentary rocks Lineament Jurassic volcanic and Fold axis sedimentary rocks Permian-Triassic Chiapas Massif Normal fault Upper Paleozoic Thrust fault sedimentary rocks 38 Sampling station Pre batholithic Strike slip fault metamorphic rocks ( ~ mylonitic) Road 15ºN 93ºW

Figure 2. Simplified geologic map of southern Chiapas, modified from Consejo de Recursos Minerales (2000), showing sampling for geochronology, petrography, and geochemistry along the Tonalá mylonite belt and the intrusions of the Miocene Chiapanecan arc. Sample localities (numbers) are used in the text with the prefix TON (e.g., TON29), except for stations numbered 46 or higher, for which we use the prefix VF (e.g., VF48).

Fig. 2). These samples were used for U-Pb zircon dating. Zircon crys- Sensitive high-resolution ion microprobe–reverse geometry tals were separated after rock crushing using conventional magnetic and (SHRIMP-RG) U-(Th)-Pb analyses were performed on individual zir- heavy liquid techniques at Centro de Geociencias (Universidad Nacional con grains using the ion microprobe housed at Stanford University, Cali- Autónoma de México [UNAM]). Zircon standard (R33) and unknowns fornia. The detailed procedures used in this study are similar to those were mounted in epoxy resin and polished down to expose the near- reported in Williams (1998) and Nourse et al. (2005). Briefly, the pri- equatorial section of the grains. The mounts were cleaned in distilled mary oxygen ion beam (O2-), operated at ~2–4 nA, excavated an area water and in 1 N HCl and gold-coated for maximum surface conduc- of ~20–40 mm in diameter (adjustable depending on grain size) to a tivity. Cathodoluminescence (CL) images were obtained using a JEOL depth of ~1–2 mm; sensitivity was within the range 5–30 cps per ppm 5600 instrument (SEM) at Stanford University. Pb. Data for each spot were collected in sets of five scans through the

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mass range. Nine peaks were measured sequentially for zircons with Aliquots of mineral separates were packaged in copper capsules and 196 204 a single collector: Zr2O, Pb, background (0.050 mass units above sealed under vacuum in quartz tubes to be then irradiated for 16 h in alu- 204Pb), 206Pb, 207Pb, 208Pb, 238U, 248ThO, and 254UO. The reduced ratios minum containers in the central thimble facility at the TRIGA reactor were normalized to the zircon standard R33 (Black et al., 2004). For the (GSTR) at the U.S. Geological Survey in Denver, Colorado. The monitor closest possible control of U/Pb ratios, one standard was analyzed after mineral used for the different irradiations was Fish Canyon Tuff (FCT- every 4–5 unknown samples. Uranium concentrations were monitored 3) sanidine with an age of 27.79 Ma (Kunk et al., 1985; Cebula et al. by analyzing a standard (MAD) with ~4200 ppm U composition. The 1986) relative to MMhb-1 with an age of 519.4 ± 2.5 Ma (Alexander et al., U and Th concentrations are accurate to ~10%–20%. Ion microprobe 1978; Dalrymple et al., 1981). The type of container and the geometry of isotopic raw data were reduced and projected to the Tera-Wasserburg samples and standards are similar to that described by Snee et al. (1988). concordia diagram along a model common Pb line based on Stacey and All of the samples were analyzed at the U.S. Geological Survey Thermo- Kramers (1975). All the U-(Th)-Pb age data presented in Table 1 were chronology laboratory in Denver, Colorado, using a VG Isotopes, Ltd., plotted using the software Isoplot/Ex 3.75 (Ludwig, 2012). Model 1200B mass spectrometer fitted with an electron multiplier. We In addition to U-Pb zircon dating, hornblende, biotite, and feldspar used the 40Ar/39Ar step-heating method to determine the age of the mineral mineral separates from three samples from the coastal plutons were pre- concentrates. All the argon step-heating data (Table 2) were reduced using pared at the U.S. Geological Survey, Denver, for 40Ar/39Ar thermochronol- an updated version of the computer program ArAr* (Haugerud and Kunk, ogy studies. One of them corresponds to sample 34-TEP also used for 1988). Inverse isotope correlation analysis of the step-heating data was U-Pb dating; TON36 was from a granodioritic intrusion exposed west of used to assess if nonatmospheric argon components were trapped in any Arriaga (Fig. 2), and TON38 was from a granitoid exposed east of Tres samples, and to calculate an inverse isochron age using the method of York Picos, which we have named La Polka (Fig. 2). The La Polka intrusion is (1969). For additional information on the analytical procedure for both bordered on the north by mylonite of the Tonalá shear zone. An additional techniques, see Kunk et al. (2001) and Iriondo et al. (2003, 2004). sample was collected from white-mica–bearing marble mylonite at Los In order to better understand the tectonic setting and magmatic origin Patos station (Fig. 2). We used for the analysis the largest size fraction of the Miocene coastal plutons, nine samples were selected for major- possible of the target mineral free of inclusions from other mineral phases. and trace-element analysis; samples TON29, TON4, TON31, TON33, Separates were produced using magnetic separation, heavy liquids, and TON35, TON36, TON38, TON44, and TON45 are representative of the handpicking techniques to a purity of >99%. The separates were later major intrusions studied. Major-element compositions were analyzed by washed in acetone, alcohol, and deionized water in an ultrasonic cleaner X-ray fluorescence at the Instituto de Geología (UNAM), and trace-ele- to remove dust and then resieved by hand using a 125 µm sieve. ments concentration was analyzed using an inductively coupled plasma–

TABLE 1. U-Th-Pb ANALYTICAL DATA FOR SHRIMP-RG SPOT ANALYSES ON ZIRCON GRAINS FOR PLUTONIC ROCKS FROM CHIAPAS, MÉXICO Spot Comments Common 206Pb U Th Th/U 238U/206Pb* Error 207Pb/206Pb* Error 206Pb/238U† Error 206Pb/238U† Error name core/rim? (%) (ppm) (ppm) (%) (%) (absolute) age (Ma) (Ma)

VF-48-AB Pijijiapan tonalite, Chiapas (small paleomag sample), Mount Alex-20 VF48-3 Rim 0.626 107 64 0.62 685.49 ± 4.80.05117 ± 18.6 0.00145± 0.00007 9.34 ± 0.46 VF48-11 Rim 0.269 185 125 0.70 654.26 ± 3.70.04836± 12.8 0.00152± 0.00006 9.82 ± 0.37 VF48-6 Rim 1.150 167 131 0.81 629.99 ± 3.80.05532± 13.0 0.00157± 0.00006 10.11± 0.39 VF48-5 Rim 0.444 103 62 0.62 630.89 ± 4.60.04974± 17.7 0.00158± 0.00007 10.16± 0.48 VF48-12 Rim –0.491 69 31 0.46 636.74 ± 5.40.04236± 22.6 0.00158± 0.00009 10.17± 0.56 VF48-4 Rim 1.222 130 74 0.59 620.72 ± 4.20.05589± 14.8 0.00159± 0.00007 10.25± 0.44 VF48-2 Rim –0.477 57 26 0.47 622.00 ± 6.20.04248± 27.4 0.00162± 0.00010 10.41± 0.66 VF48-10 Rim 0.325 155 105 0.70 614.49 ± 3.80.04881± 13.9 0.00162± 0.00006 10.45± 0.41 VF48-9 Rim 0.387 187 186 1.03 606.44 ± 3.50.04931± 12.2 0.00164± 0.00006 10.58± 0.38 VF48-7 Rim 1.939 151 137 0.93 589.85 ± 4.10.06157± 13.7 0.00166± 0.00007 10.71± 0.45 VF48-1 Rim –1.005 109 70 0.66 579.51 ± 5.40.03832± 24.1 0.00174± 0.00010 11.23± 0.62 (MSWD = 1.00, n = 11) Weighted mean 206Pb/238U age = 10.24± 0.27 34-TEP Tepanatepec granodiorite, Chiapas (large sample), Mount Alex-21 TEP-2 Rim 0.152 182 70 0.40 633.67 ± 3.95 0.04744± 15.880.00158± 0.00006 10.15± 0.41 TEP-3 Rim 1.467 178 53 0.31 615.26 ± 4.16 0.05783± 14.820.00160± 0.00007 10.32± 0.44 TEP-4 Rim 1.311 166 56 0.35 609.73 ± 4.07 0.05660± 15.280.00162± 0.00007 10.43± 0.44 TEP-7 Rim –1.233 190 74 0.40 613.90 ± 3.83 0.03650± 17.300.00165± 0.00006 10.62± 0.41 TEP-6 Rim 1.221 300 85 0.29 592.64 ± 3.16 0.05590± 11.090.00167± 0.00005 10.74± 0.35 TEP-9 Rim 0.687 263 79 0.31 583.06 ± 3.25 0.05169± 11.930.00170± 0.00006 10.97± 0.37 TEP-10 Rim –0.422 223 115 0.54 586.61 ± 3.45 0.04292± 14.110.00171± 0.00006 11.03± 0.39 TEP-8 Rim 0.252 191 63 0.34 577.24 ± 3.71 0.04825± 14.850.00173± 0.00007 11.13± 0.42 TEP-11 Rim 0.763 313 141 0.46 568.09 ± 3.01 0.05229± 10.880.00175± 0.00005 11.25± 0.35 TEP-1 Rim –1.465 226 62 0.28 565.77 ± 3.58 0.03469± 16.570.00179± 0.00007 11.55± 0.42 TEP-5 Rim 0.266 189 36 0.19 493.96 ± 3.51 0.04840± 13.880.00202± 0.00007 13.00± 0.47 (MSWD = 1.17, n = 10) Weighted mean 206Pb/238U age = 10.84± 0.25 Note: Individual zircon ages in bold were used to calculate the weighted average 206Pb/238U age and its mean square of weighted deviates (MSWD). All errors given are at the 1 sigma level except for the weighted average 206Pb/238U age, which is reported at 2 sigma. *Uncorrected atomic ratios. †Atomic ratios and ages corrected for initial Pb using the amount of 207Pb.

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TABLE 2. 40Ar/ 39Ar STEP-HEATING DATA FOR IGNEOUS ROCKS FROM CHIAPAS, MÉXICO

39 39 40 39 Step Temp. % Ar of Radiogenic Ark Ar*/ Ark Apparent Apparent Apparent age Error (°C) total yield (moles × 10–12) (K/Ca) (K/Cl) (Ma) (Ma) (%)

TON-36 Granodiorite, Chiapas, biotite, total fusion J = 0.004655% ± 0.50%, wt = 4.3 mg, #94KD48 A 1450 100.0 71.9 0.531824 1.14126426 9.56 ± 0.1

TON-36 Granodiorite, Chiapas, K-feldspar, J = 0.004303% ± 0.50%, wt = 3.65 mg, #164KD48 A 850 0.2 17.2 0.00224 2.0936 81 16.18± 1.11 B 900 0.6 33.6 0.00540 1.48516221 11.49± 0.61 C 950 1.3 52.3 0.01183 1.24327366 9.63 ± 0.22 D 1000 2.4 71.9 0.02238 1.24719490 9.66 ± 0.12 E 1050 4.0 69.9 0.03718 1.1952010009.25± 0.08 F1100 5.7 72.3 0.05376 1.1922016679.23± 0.06 G1150 7.2 67.5 0.06721 1.1812320839.14± 0.07 H 1200 7.6 78.2 0.071001.176 35 5000 9.10 ± 0.05 I 1250 7.2 80.9 0.06731 1.1985758829.28± 0.06 J 1300 6.5 73.5 0.06105 1.2162920009.41± 0.05 K 1350 5.8 44.2 0.054481.223 52 1010 9.47 ± 0.07 L 1450 19.8 22.1 0.18567 1.31135256 10.15± 0.08 M 1550 29.2 15.8 0.27439 1.36382153810.55 ± 0.10 N 1650 1.8 3.7 0.016801.989 25 16215.38 ± 0.94 O 1650 0.7 3.9 0.00781 2.32286183 17.94± 1.31 Total gas 100.0 0.93850 10.04

TON-38 La Polka monzodiorite, Chiapas, hornblende, J = 0.004243% ± 0.50%, wt = 236.3 mg, #155KD48 A 900 0.4 25.8 0.012412 10.655 0.35 12 79.76± 1.15 B 1000 0.7 27.0 0.022092 1.5830.143212.08 ± 0.32 C1100 14.0 60.8 0.456162 1.5540.122111.85 ± 0.03 D1125 21.7 70.7 0.707098 1.6170.112012.33 ± 0.01 E1150 39.4 84.8 1.283002 1.5580.122011.88 ± 0.02 F1175 5.0 72.2 0.164224 1.5740.122012.01 ± 0.05 G 1200 4.6 72.4 0.148432 1.5780.112012.04 ± 0.05 H 1225 5.7 76.4 0.187309 1.5930.112112.15 ± 0.04 I 1250 3.5 72.8 0.113038 1.6320.122012.45 ± 0.07 J 1300 4.2 71.7 0.137973 1.7020.122012.98 ± 0.06 K 1450 0.8 58.0 0.026246 1.5820.122012.07 ± 0.24 Total gas 100.0 74.9 3.258000 1.6180.122012.34

TON-38 La Polka monzodiorite, Chiapas, biotite, total fusion J = 0.004659% ± 0.50%, wt = 4.6 mg, #92KD48 A 1450 100.0 55.4 0.553054 1.07313182 9.00 ± 0.1

34-TEP Tepanatepec granodiorite, Chiapas, K-feldspar, J = 0.004895% ± 0.50%, wt = 19.7 mg, #90KD45 A 750 13.1 55.3 0.52997 1.19820.3532 10.55± 0.06 B 850 21.4 94.7 0.86544 1.12226.713284 9.88 ± 0.03 C 950 19.7 96.4 0.79831 1.12939.328211 9.94 ± 0.03 D1100 22.5 92.0 0.90772 1.13052.842819.96± 0.03 E 1200 23.3 83.4 0.94169 1.15035.8193 10.13± 0.04 Total gas 100.0 86.7 4.04314 1.14236.3949010.05 63.6% of gas on plateau in 850 through 1100 steps Plateau age =9.92± 0.1

PATOS-CHI, Marble mylonite, Chiapas, white mica, J = 0.004897% ± 0.50%, wt = 29.6 mg, #92KD45 A 900 8.3 40.9 0.57580 1.13620412 10.01± 0.10 B 950 6.0 75.9 0.42102 1.17977193710.39 ± 0.07 C 1000 7.1 86.1 0.49160 1.190107 2230 10.48± 0.07 D 1050 9.9 89.6 0.69162 1.178162 3182 10.38± 0.04 E1100 13.1 92.2 0.91545 1.180172 3413 10.39± 0.03 F1150 19.1 92.5 1.33336 1.178286 28510.38 ± 0.02 G 1200 23.3 93.4 1.62149 1.179305 3473 10.39± 0.02 H 1220 9.6 93.8 0.670221.186 166296110.45 ± 0.03 I 1240 3.5 92.0 0.24589 1.17758159210.37 ± 0.10 Total gas 100.0 86.8 6.96645 1.177196 2277 10.37 78.6% of gas on plateau in 950 through 1200 steps Plateau age =10.39 ± 0.1 Note: Ages were calculated assuming an initial 40Ar/36Ar = 295.5 ± 0. All precision estimates are at the one sigma level of precision. Ages of individual steps do not include error in the irradiation parameter J. No error is calculated for the total gas age.

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mass spectrometer at the Centro de Geociencias (UNAM; Table DR11). solutions. The only signs of hydrothermal (or retrograde?) alteration are We followed the methods described by Mori et al. (2007). This study small amounts of calcite and epidote in sample TON38G and incipient was further complemented by petrographic inspection of 12 samples, and chloritization of biotite in other samples. structural observations along the shear zone. Fabrics of the Miocene granitoids range from magmatic to solid state at the mesoscale. Textures indicate that most samples in the collection were CHIAPAS COASTAL PLUTONIC BELT affected by dynamic metamorphism. Crystal-plastic deformation textures in plagioclase (Fig. 3) indicate temperatures near 500 °C, but textures vary Rocks of the Miocene Chiapanecan arc are well exposed along the Chi- along the shear zone. The igneous textures observed are holocrystalline, apas coastal plain from the Tehuantepec isthmus to near the Guatemala inequigranular, and fine to medium (1–5 mm) grained. Typical textures are border (Fig. 2). Several exposures of the plutons form isolated topographic either porphyritic or seriate, where plagioclase tends to form the largest features resulting in an “island” aspect on the coastal plain. The plutons of crystals, which usually are euhedral or subhedral. Quartz and K-feldspar the Miocene belt comprise a complete magmatic series of quartz monzo- crystals (orthoclase or less frequently microcline) tend to be anhedral and nite, tonalite, granodiorite, and gabbro. Locally, some of the plutons have fill irregular spaces between the plagioclase crystals. Large crystals of well-developed magmatic foliations, but more often the plutonic rocks are K-feldspar sometimes contain poikilitic inclusions of plagioclase and/or strongly deformed and exhibit tectonic fabrics. The outcrop patterns of indi- biotite. Quartz commonly displays undulatory extinction, and, in those vidual intrusions are collectively elongated parallel to the strike of the mas- samples that are slightly deformed, quartz forms fine-grained polycrystal- sif (~310°). In plan view, the plutons have aspect ratios of ~6:1 or greater. line aggregates with sutured intracrystalline contacts. Biotite and horn- Typically, Permian granitoids are the host rock to the Miocene plutons, but a blende vary from euhedral (biotite) to anhedral (biotite and hornblende) little studied and conspicuous suite of metamorphic rocks, the protolith ages and tend to occur together associated with accessory minerals. In samples of which are unknown, is also intruded by the Miocene granitoids. TON38G and TON39H (both from the La Polka pluton), the ferromag- Outcrop patterns in the coastal Chiapas pluton belt are conspicuous nesian minerals define a weak foliation, which appears to be unrelated when compared with pluton exposures west of Tehuantepec, which are to dynamic metamorphism, as they are comparatively coarse grained and generally dispersed over a wider area (Fig. 1). In Chiapas, the plutons are independent of other signs of dynamic recrystallization. Therefore, these concentrated along the Tonalá shear zone, and they are elongated parallel to two rocks are classified as foliated tonalites. the shear zone on the basis of surface exposures. The largest volume of plutons in Chiapas appears to crop out near the western end of the Tonalá shear Geochronology zone near Zanatepec, where the fault curves and branches to the northwest. The Tehuantepec area is characterized by Jurassic red beds, mid-Cre- The 40Ar/ 39Ar cooling age estimates obtained in this study are within a taceous platform and basinal carbonate rocks, and fine-grained siliciclas- narrow range between ca. 10 and 12 Ma. Table 2 provides all step-heating tic rock of Upper Cretaceous age exhibiting low-grade metamorphism data and includes the identification of individual step, plateau, average, (Pérez-Gutiérrez et al., 2009). This area also contains widespread outcrops and total gas ages for each sample. An individual step age represents the of Oligocene and Miocene intrusions, which yield dates between ca. 29 apparent age obtained for a single temperature step analysis. Plateau ages and 13 Ma (Morán-Zenteno et al., 2000), and scattered outcrops of felsic are identified when three or more contiguous steps in the age spectrum volcanic rocks of similar age in the west. In some areas of the Tehuantepec agree in age, within the limits of analytical precision, and contain more 39 isthmus, such as near Ixtepec, Miocene conglomerate deposits hundreds than 50% of the ArK released from the sample. The average ages (shown of meters in thickness are preserved. These conglomerates are inferred to in Fig. 4) are calculated from contiguous steps forming no plateau but be also present offshore in the Tehuantepec Gulf; Sánchez-Barreda (1981) containing more than 50% of the gas with the intention of obtaining the described cuttings from an offshore well as composed mainly of volcanic best age approximation for the sample. and plutonic clast conglomerate, which we reinterpret here. TON36, a granodiorite deformed by dynamic metamorphism collected from the western Arriaga pluton, yields a biotite total fusion age of 9.56 Petrography ± 0.05 Ma. The Ar release spectrum of the K-feldspar separate is saddle shaped (Fig. 4) and does not yield a plateau age. The western Arriaga Samples of the Miocene intrusions along the Tonalá shear zone were body (Fig. 2) has an exposed width of ~10 km on its eastern side and an studied petrographically (Fig. 3). Modal compositions were determined elongated tail, ~30 km long, extending to the west-northwest parallel to by point counting, and these indicate that the rocks examined are granodi- the Tonalá shear zone. The pluton is delineated on its northern margin by a orite, quartz monzonite, and tonalite following the Le Bas and Streckeisen band of mylonitic granitoid. TON38 is from tonalitic facies of a somewhat (1991) classification. Mineral paragenesis in all the samples examined is smaller pluton of similar shape (with a long stretched tail extending to plagioclase > quartz > K-feldspar > biotite >> oxides. Samples TON34G the west-northwest; Fig. 2). A hornblende separate yields a plateau age of (Tepanatepec body), TON38G (La Polka), and TON29H (shear zone) 12.0 ± 0.05 Ma, and a total fusion biotite age of 9.0 ± 0.05 Ma (Table 2). also contain hornblende. Accessory minerals (<<1 vol%) in all the rocks 34-TEP was collected from an elongated granodiorite body; K-feldspar include titanite, apatite, and zircon. Primary mineralogy is well preserved from this sample yields a plateau age 9.92 ± 0.06 Ma. Sample 34-TEP in all but one sample (TON31D, Arriaga), where plagioclase is partially yields a U-Pb zircon weighted mean age of 10.8 ± 0.3 Ma (Table 1; Fig. 5). replaced by biotite, and the brittle fractures are filled with microcrystalline Finally, sample VF48 is from a protomylonite exposed within an elon- quartz. Sample TON31D also contains small clusters of opaque minerals gated tonalitic pluton northwest of Pijijiapan (Fig. 2). This sample yields along grain boundaries that likely reflect interaction with hydrothermal a weighted mean U-Pb zircon age of 10.2 ± 0.3 Ma. This intrusion is ~30 km long, with an aspect ratio of ~8:1, not untypical of the Miocene pluton 1 GSA Data Repository Item 2015107, Table DR1: ICP-MS trace element and series. In summary, the plutons of the Miocene Chiapas arc between Tep- FRX major element analysis of 9 samples of the Miocene magmatic suite of the anatepec and Pijijiapan appear to have been intruded between ca. 12.5 and Chiapas Miocene Arc, is available at www.geosociety.org/pubs/ft2015.htm, or on request from [email protected], Documents Secretary, GSA, P.O. Box 9140, 10 Ma, with rapid cooling to temperatures below 250 °C by ca. 10 Ma. Boulder, CO 80301-9140, USA.

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Figure 3. (A–B) Undeformed granodiorite with inequigranular-seriate texture (sample TON34-G). K-feldspar was stained with Na cobalti- nitrite and has a cloudy appearance in plane polarized light (ppl.). Bio- tite is slightly altered to chlorite. (C) Quartz commonly displays marked undulatory extinction in undeformed or mildly deformed rocks; plagioclase is slightly altered to fine-grained sericite. K-feldspar occasionally has gridiron twinning (sample TON30F). (D–E) Mildly deformed granodiorite: Some biotite crystals appear “smeared,” defining a weak foliation, quartz has experienced polygonitization and grain-size reduction, and some plagioclase crystals display slightly bent twins due to internal deformation (sample TON36E, crossed nichols). Biotite located along the foliation is altered to chlorite and secondary spinel (E: ppl). (F) Igne- ous texture in intensely deformed bands within the protomylonites has been obliterated. A marked foliation, defined by secondary biotite and finely recrystallized quartz, wraps around rounded plagioclase crystals. Primary biotite has pres- sure tails formed by secondary, relatively coarse-grained biotite. The fine-grained aggregate was counted as matrix (M) during the modal analysis (sample TON2I, crossed nichols). (G–H) Sample TON46E is unique in the studied set as it displays a marked foliation defined by very fine-grained secondary biotite and recrystallized quartz. Porphyroclasts are elongated parallel to the foliation and show intense internal microfractur- ing and undulatory extinction. The foliation is cut by brittle microfaults (arrows). Key for abbreviations: Ks—K-feldspar, Pl—plagioclase, Q—quartz, Bi—biotite, Ch—chlorite, Hb—hornblende, Sp—sphene, My—myrmekite, M—matrix. The scale bars are 200 μm.

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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. e e E C B D m 0. 8 0. 5 H m r s E F G I D s J N-38 C 0. 4 .6 K Chiapa TO K K Ar 34-TE P Chiapa Ar Ar 39 Ar 39 hornblende 40 40 A / 0. 3 / K-feldspa La Polka monzodiorit Ar Ar panatepec granodiorit 0. 40 39 39 Te 0. 2 1.56 ± 0.25 Ma 1 B = 335 ± 18 Inverse-isotope correlation diagra i = 319 ± 3 i Age =

Inverse-isotope correlation diagra Age = 9.88 ± 0.06 Ma

0. 2 Ar ] on Ar ] D through I with 79.9% of on 36

0. 1 A through E with 100% of 36

Ar / Ar / 40 Isoc hr MSWD = 0.961 Step s [ 40 Isoc hr [ MSWD = 0.71 Step s A

0 0

/ 0.00 3 0.00 2 0.00 1 Ar

0.00 3 0.00 2 Ar 0.00 1

/ Ar Ar

40 36

40 36 0. 1 10 0 10 1 10 0 10 0 e 80 80 e r Released Released K K 60 60 s s Ar Ar 1.56 ± 0.25 Ma 39 39 1 Ag e

N-38 K/C a K/Ca 34-TE P Chiapa Chiapa TO Platea u 40 40 Age = 9.92 ± 0.06 Ma Age = K-feldspa n hornblende panatepec granodiorit ro La Polka monzodiorit Te Platea u 20 20 Isoc h Cumulative % Cumulative % 0 0

8 6 4 2 0 0 5

20 18 16 14 12 10 15 10 20

) (Ma Age Apparent ) (Ma Age Apparent I H H F G E I 0.8 F J D E 0.6 D C G S B TO N-36 0.6 PA C K TO K K-spar Ar Ar Ar 0.4 Ar 39 40 39 K LOS / 40 / white mica Ar Granodiorite, Chiapas Ar 0.4 39 A 39 1 Marble mylonite, Chiapas with 68.5% of B 0.2 L = 302 ± Inverse-isotope correlation diagram i = 296 ± 12 Inverse-isotope correlation diagram i Age = 9.27 ± 0.12 Ma

0.2 Age = 10.40 ± 0.09 Ma

Ar] M on Ar] on 36 36 A Ar/ Ar/ 40 40 MSWD = 1. 0 Steps I through M [ Isoc hr MSWD = 0.17 Steps B through I with 91.7% of [ Isoc hr O N

0 0

0.003 /

0.002 0.001 Ar

Ar 0.0032 0.0024 0.0016 0.0008

/ 40 36 Ar Ar

40 36 20 0 80 60 40 100 100 10 1000 100 100 d d 80 80 Release Release S K K 60 60 Ar Ar TO 9.27 ± 0.12 Ma Age 39

39 PA K/Ca ON-36 K/Ca Plateau T Age = 40 K-spar 40 Age = 10.39 ± 0.05 Ma

LOS n 4. Argon release spectra and inverse correlation diagrams for samples from the Tonalá mylonite belt. MSWD—mean square of weighted deviates. of weighted MSWD—mean square belt. mylonite Tonalá the samples from for diagrams correlation and inverse spectra release Argon 4. Figure white mica ro Granodiorite, Chiapas Isoch Marble mylonite, Chiapas 20 Plateau 20 Cumulative % Cumulative % 0 0

8 6 4 2 0 8 6 4 2 0

20 20 18 16 14 12 10 18 16 14 12 10

) (Ma Age Apparent ) (Ma Age Apparent

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11 U U data-point error boxes are 2-sigma data-point error boxes are 2-sigma

) 11 11 238 238 5 4 U age = 9 U age = 10 9 12 23 8 23 8 10 7 Pb/ Pb/ 12 2 8 11 Pb/ Pb/ 12 206 206 206 206 1 1 MSWD = 1.0 (n Mean Mean MSWD = 1.17 (n 10) 10.24 ± 0.27 Ma 10.84 ± 0.25 Ma 13 13 A D a a 9 ) 750 700 U age = U age = 23 8 23 8 9 Pb/ Pb/ 20 6 20 6 10 MSWD = 1.0 (n 11 data-point error ellipses are 2-sigm data-point error ellipses are 2-sigm MSWD = 1.17 (n 10) Mean Mean 10.84 ± 0.25 Ma 10.24 ± 0.27 Ma 650

600 10 Pb Pb 11 206 206 U/ U/

12 11 238 238 e 550 500 13 12 e 14 13 onalit 15 epanatepec, Chiapas 34-TEP T Pijijiapan, Chiapas Granodiorit T VF-48-AB 14 16 450 400

. (B, E) Zircons used in mean in used Zircons E) (B, 2 s . are spots individual of ellipses Error arc. Miocene Chiapas the from samples for zircons of ratios isotope U-Pb for diagram Tera-Wasserburg D) (A, 5.

0.09 0.07 0.05 0.03 0.01 0.10 0.08 0.06 0.04 0.02

Pb / Pb Pb / Pb

206 207 206 207 Figure deviates. of weighted MSWD—mean square respectively. VF48 and 34-TEP, samples from of zircons F) Cathodoluminescence images (C, calculation. age

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The available geochronologic data indicate that the emplacement ages of cut by dikes and elongated porphyric intrusions associated with the shear deformed and undeformed plutons are indistinguishable. zone. Kinematic analysis of features in ultramylonite zones indicates a left-lateral regime (Fig. 9C). The contacts between the dikes and the Geochemistry porphyritic granitoids show evidence of magma mingling, which is interpreted to indicate magmatic fracturing. Near the contact between the ultr- The plutonic assemblage we sampled includes quartz monzonite, amylonite and the dikes, magmatic fabrics are overprinted or otherwise granodiorite, tonalites, and diorite, all of which are slightly peraluminous. obscured by solid-state fabrics. The assemblage has characteristics of typical arc melts. Multi-element At two sites east of Arriaga, there are pervasive mesoscale folds with diagrams (Fig. 6A) show enrichment in high-ionic-radius elements (Rb, northwest-striking axial planes in granites. Axial planes in host rocks Cs, Sr, Ba) with respect to high field strength elements. There is enrich- north of the shear zone between Tonalá and Arriaga are similar to those ment in light rare earth elements (La, Ce, Eu; Fig. 6B), relative to heavy within the shear zone. From a regional perspective, this observation, rare earth elements (Gd, Tb, Lu). Also, there are positive anomalies of Ba, together with the northeast-dipping foliation planes, is consistent with Pb, and Sr, and relative depletion of Nb and Ta. The latter is considered a northeast-directed shortening during pluton emplacement and strike-slip geochemical characteristic of magmatic arcs (Hawkesworth et al., 1993). motion along the shear zone, thus resulting in an overall transpressional Two samples (TON45 and TON29), from the Tonalá area, however, devi- setting during shear zone activity. A continuous >200-m-wide zone of iso- ate from this general pattern. These samples appear to belong to a different clinal folding evident at the mesoscale is exposed at a series of localities magmatic suite. Their rare earth element (REE) patterns show Eu anoma- east of Arriaga; this zone is located at the contact between the Tonalá shear lies, and less enriched light REEs relative to heavy REEs. zone and the Chiapas Massif (Fig. 9A). We refer to this zone as the “shear- zone contact,” for which axial planes strike northwest (mean trend of 335° TONALÁ SHEAR ZONE and mean dip 68° to the northeast; Fig. 9B). The folds are classified as Ramsay’s class 2 folds. Contact zone folds include lower-scale folds and We named the Tonalá shear zone after exposures at the outskirts of asymmetric folds, which show a SW-side-up shear sense. Tonalá city along the Río Zanatenco (Fig. 2). The shear zone is character- Between Tepanatepec and Zanatepec (Fig. 2), melanocratic phyllites ized by protomylonitic to ultramylonitic textures in Permian intrusions of are exposed as screens within mylonitized granitoids. They are interpreted the Chiapas Massif, the Miocene plutons of the coastal series, and sedi- to be part of the Upper Cretaceous marine-dominated sequence of Tehu- mentary carbonate and metasedimentary rocks for which protolith ages antepec described by Pérez-Gutiérrez et al. (2009). West of Zanatepec, the are uncertain. We estimate the mylonite belt to have a thickness of ~3–4 shear zone juxtaposes high-grade metasedimentary rocks to the north of km, based on the distribution of nearly continuous exposures north of the the mylonites with lithic graywacke and volcaniclastic conglomerates to La Polka pluton, and the estimate was verified by triangulation of the posi- the south. These field relationships also suggest north-side-up thrusting, tions of other sampling stations along the shear zone. The Tonalá shear produced by transpression in the shear zone. zone is a strikingly linear feature, extending for at least 120 km from expo- Coarse-grained granitoids with or without ductile deformation in and sures near Arriaga to east of Pijijiapan (Fig. 2). The Tonalá shear zone is around the shear zone occasionally contain pseudotachylyte vein net- associated with a high-amplitude and short-wavelength linear magnetic works. These are generally parallel arrays, but they occasionally form anomaly with a strike of ~310° that is superimposed on a longer-wave- conjugate arrays of veins with parasitic branching. Pseudotachylyte veins length and somewhat irregular magnetic low extending from Tehuantepec vary in thickness between ~1 and 20 mm, separated at distances between to Pijijiapan in coastal Chiapas (Fig. 7). The linear pattern of this anomaly ~1 and 10 m, and with variable offsets of up to 60 cm. They have an may be recognized between Mapastepec and Arriaga. A linear anomaly average east-northeast–west-northwest orientation and dip steeply to the extending northwest of Arriaga with a strike of 330° is a possible north- south (Fig. 9D). Our observations indicate that the pseudotachylytes are west extension of the Tonalá shear zone, possibly linking the Tonalá shear associated with extension along nearly E-W planes. They were formed at a zone with structures in the western Tehuantepec Gulf. shallower depth than the mylonite of the Tonalá shear zone, and they may A subvertical, west-northwest–striking (295° to 320°), northeast-dip- be associated with exhumation in a younger transtensional regime. ping foliation (mean trend of 319° and mean dip of 74° northeast) and subhorizontal lineation characterize the Tonalá shear zone (Figs. 8A and 8B). DISCUSSION In the mesoscale, some delta clasts structures indicate a left-lateral sense of motion, but generally intense shear produced planar fabrics from which A suite of calc-alkaline intrusions of predominantly intermediate com- kinematic indicators are ambiguous. In the microscale, we observed some position defines the Miocene Chiapanecan arc. The intrusions range in S-C′ structures indicating right-lateral shear. Seismic motion tensors for composition from mafic to felsic. The plutons have geochemical signatures earthquakes along the shear zone are left-lateral (Guzmán-Speziale, 2014). typical of an arc setting (Fig. 6). The plutons exposed in the region between At an isolated locality at Los Patos railroad station (Fig. 2), the mylonite is Zanatepec and Pijijiapan yield interpreted emplacement ages between developed in marbles of inferred Cretaceous age. White micas separated ca. 12 and 10 Ma, and they serve to better define a regional eastward- from these rocks yield a 40Ar/39Ar plateau age of 10.4 ± 0.1 Ma (Table 2). younging trend of plutons along the southwest Mexico Pacific margin. The The age estimate is slightly older than hornblende and biotite 40Ar/ 39Ar microscopic textures suggest that shear-zone–related deformation of the cooling age estimates of ca. 8.2 Ma from three mylonitic gneisses east plutons was at temperatures in excess of 500 °C, which, for relatively high of Los Patos in the shear zone, reported by Ratschbacher et al. (2009) geothermal gradients, suggest emplacement in the middle crust. Exposures and interpreted by those workers to represent the age of sinistral shear of mid-crustal-level rocks, rather than volcanic rocks as is the case in Oax- deformation. Rocks along the shear zone west of Arriaga have stretching aca and Guatemala (Fig. 1), are most likely related to recent exhumation of lineations to the northwest, but rocks east of Arriaga have shallow linea- plutons of the Pacific Chiapas margin and regional uplift. Ascent of magma tions varying from northwest to southeast. was most likely facilitated and controlled by the active Tonalá shear zone, Permian granitoids north of Arriaga (Fig. 2) contain numerous, and a crustal discontinuity along the southern margin of the Chiapas Massif. discrete, ultramylonite bands tens of centimeters in thickness, which are This is suggested by the distribution of plutons, their elongated shape, and

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A TON44Bb TON4FC TON33D TON31DB TON35G 100 TON36EC TON38GC 100

10 10 Rock/Chondrites

1 1 Ce Nd Sm Gd Dy Er Yb Ce Nd Sm Gd Dy Er Yb La Pr Pm Eu Tb Ho Tm Lu La Pr Pm Eu Tb Ho Tm Lu

TON44Bb TON29F TON33D

TON45G TON35G 100 100 TON38GC TON4FC

TON31DB TON36EC

10 10 Rock/Chondrites

1 1

Ce Nd Sm Gd Dy Er Yb Ce Nd Sm Gd Dy Er Yb La Pr Pm Eu Tb Ho Tm Lu La Pr Pm Eu Tb Ho Tm Lu

1000 1000 B TON38GC TON4FC TON44Bb TON31DB 100 TON35G 100 TON36EC TON33D

10 10

1 1 Rock/NMORB

.1 .1

.01 .01 Rb Th Nb La Pb Sr Nd Sm Ti Y Lu Rb Th Nb La Pb Sr Nd Sm Ti Y Lu Cs Ba U K Ce Pr P Zr Eu Dy Yb Cs Ba U K Ce Pr P Zr Eu Dy Yb

1000 1000 TON38GC TON45G TON44Bb 100 TON29F 100 TON35G TON33D TON4FC 10 10 TON31DB

TON36EC 1 1 Rock/NMORB .1 .1

.01 .01 Rb Th Nb La Pb Sr Nd Sm Ti Y Lu Rb Th Nb La Pb Sr Nd Sm Ti Y Lu Cs Ba U K Ce Pr P Zr Eu Dy Yb Cs Ba U K Ce Pr P Zr Eu Dy Yb

Figure 6. Geochemical data for Miocene rocks of coastal Chiapas. (A) Trace-element multi-element spider diagram, with mid- ocean-ridge-basalt (MORB)–normalized values after Sun and McDonough (1989). (B) MORB-normalized rare earth element data.

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Figure 7. Total magnetic anomaly map of southern Chiapas and eastern Oaxaca, modified from Servicio Geológico Mexicano (2008).

the syngenetic relationship between dikes and ultramylonite. Based on the and the Chiapas Massif. Other indicators of northeast-southwest regional geochronologic data reported here, as well as previously published data, we contraction are the intersection of shear-zone fabrics and the orientations infer that emplacement of magma was synchronous with strike-slip motion of dikes, the southeast-plunging stretching lineations of the Tonalá shear in the Tonalá shear zone. Interpreted crystallization ages of plutons of the zone, as well as the steep northeast dips of the axial planes of regional folds. Miocene Chiapas arc and cooling ages of neoformed micas in the shear Because of this, we suggest that the Chiapas Massif was displaced to the zone overlap with crystallization age estimates on plutons of 10.2 Ma near southwest above rocks now lying below the coastal plain, and that ascent Pijijiapan and ca. 10.8 Ma near Tepanatepec, bracketing the inferred cool- of the Miocene plutons was associated with an attending component of ing age of micas at Los Patos station (10.4 Ma). A 40Ar/ 39Ar cooling age for northeast-directed shortening. A similar structural relationship was inferred hornblende of ca. 12 Ma for the La Polka intrusion and a U-Pb zircon crys- by Authemayou et al. (2011) based on analysis of geomorphic features, and tallization age estimate reported by Witt et al. (2012) of ca. 12.3 Ma (their was implied earlier by Guzmán-Speziale and Meneses-Rocha (2000) in sample C13) from the coastal plutonic suite are interpreted to indicate that their investigation of the North America–Caribbean plate boundary. arc magmatism was active since ca. 12 Ma in the late middle Miocene in The Pacific margin of Mexico from ~106°W to the Tehuantepec isth- the Tonalá area, and it likely continued until ca. 9 Ma. mus at ~94°W is characterized by wide exposures of Mesozoic metavolca- The Tonalá shear zone experienced a complex history, and although spe- nic arc sequences and metasedimentary rocks of Jurassic and Cretaceous cific field relations provide ambiguous kinematic information, we interpret protolith ages, and these rocks are intruded by Late Cretaceous to Mio- the collective body of information to indicate that the Tonalá shear zone cene plutons (Talavera-Mendoza et al., 2013; Ducea et al., 2004; Morán- was a predominantly left-lateral transpressional structure with a component Zenteno et al., 2000; Herrmann et al., 1994). The Cretaceous to Miocene of northeast-directed shortening. Pervasive folding of rocks at the shear- intrusions, as well as regional metamorphism, overprint a previously zone contact indicates regional shortening between the Miocene granitoids assembled mosaic of older tectonic elements defined by Campa and Coney

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51 10º

29 Los Patos Zanatepec

17º Tepan.

24º Chahuites 48

5º

TONALA Pijijiapan 18º 16ºN

25 km Los Patos 49 B 30º

Mean orientation PIJIJIAPAN 319/74º 94ºW

MAPASTEPEC 50b

Horcones

N=197 n=164 15ºN 93ºW Figure 8. (A) Structural data along the Tonalá mylonite belt. Small stereoplots show foliation planes at selected stations along the shear zone, with large hexagon symbol showing mean lineation where clearly present. Foliations show a relatively constant northwest strike along the shear zone. (B) Stereoplot of poles of foliation (open circles) and stretching lineation (closed circles) for all stations where these data were collected. They show the full variability of structural data along the shear zone.

(1983) as the Guerrero, Mixteco, Oaxaca, and Maya terranes (Fig. 1). The when plotted against distance along the continental margin measured from plutons along the coast become progressively younger toward the south- an arbitrary point in Puerto Vallarta, as selected in previous work by Schaaf east (Herrmann et al., 1994; Morán-Zenteno et al., 2000; Fig. 1), with ages et al. (1995) among others (Fig. 10), show a systematic decrease from west of ca. 90 Ma near Puerto Vallarta at 106°W (Fig. 1, inset; Köhler et al., to east, with magmatism migrating eastward at a rate of ~30 km/m.y. 1988), ages of ca. 60–70 Ma near Colima at ~104°W, ages of ca. 45 Ma The east-directed decrease in the age of plutons in the continental mar- near Zihuatanejo at ~101.5°W (Martini et al., 2010), and ages of ca. 13 Ma gin (shown in Fig. 1; quantified in Fig. 10) has been interpreted to reflect in the Tehuantepec region (Damon and Montesinos, 1978). Miocene plu- eastward migration of the trench-transform-trench triple junction formed tons in coastal Chiapas and Oaxaca, as described here and in other reports, by the Middle America Trench and a fault system that accommodated clearly extend the eastward-younging trend of calc-alkaline magmatism eastward motion of the Caribbean plate. This left-lateral strike-slip fault observed along the Pacific coast of Mexico, with crystallization ages of system parallel to the modern continental margin accommodated Carib- ca. 10 Ma in the Tonalá area. The U-Pb crystallization ages of plutons, bean–North America plate motion from Eocene to Miocene time (Karig et

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Mean orientation 335/68º

N=14 n=5

N=27 N=17 n=10 n=5

Figure 9. Structural observations: (A) Isoclinal fold at the contact zone between the Tonalá shear zone and the Chiapas Massif. (B) Stereoplot of northwest-trending mean axial fold plane corresponding to poles of the folds delineating the contact zone (open circles) and southeast-plunging hinge lines (closed circles). (C) Stereoplot of poles (open circles) and great circles of ultramylonite planes, including slip directions (black circles with arrows) and fault plane solution. (D) Stereoplot of poles (open circles) and great circles of pseudotachylyte planes, including slip directions (black circles with arrows) and fault plane solution.

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50 this section by Sánchez-Barreda (1981), we hypothesize that it is entirely Miocene in age. Our interpretation is based on the absence of a correlative Upper Cretaceous–Paleogene stratigraphy onshore, the presence of thick unconsolidated Miocene conglomeratic units onshore, and the likelihood 40 that fossils in cuttings from a single well may represent reworked material. A key factor in this interpretation is the presence of plutonic clasts, because there is evidence suggesting the Chiapas Massif was not exposed in the 30 Paleogene (Witt et al., 2012), and no other source of exposed plutonic rocks can be found in the region for the Late Cretaceous and the Paleogene. The distribution of metamorphic rocks in the northern Chortis block Age (Ma) suggests that in the process of continental truncation, continental frag- 20 ments may have been sheared along the transform systems between Chor- l tis and Mexico, they may have been left stranded along the margin, or they

c may have been captured by Chortis and transported eastward, as has been 10 suggested for the Las Ovejas complex (Torres-de León et al., 2012). The

huantepe eastern area of the Tehuantepec Gulf differs from the continental margin Zihuatanejo Arriag a Pinotepa Naciona Te to the west, thus south of the Xolapa terrane (Fig. 1), in that a forearc is preserved, and it is similar in size to that observed in the Central American 0 arc (Authemayou et al., 2011). The forearc in Chiapas may be a stranded 400 600 800 1000 1200 1400 1600 fragment of Chortis continental crust. If a forearc is still preserved south Distance (km) of the Miocene arc in Chiapas, this suggests that subduction erosion has Figure 10. Plot of age (U-Pb, various methods) vs. distance (measured not affected the Chiapas region, perhaps because of the young age of the from the city of Puerto Vallarta along the coast) for plutons in western subduction process in this area. Mexico from Zihuatanejo, Guerrero, to Tonalá, Chiapas. The plot includes Paleogeographic reconstructions place the Chortis block south of the a simple linear regression fit for which the slope is- 0.0335 and R2 = Mexican continental margin during Paleogene time (Pindell et al., 1988; 0.887. Figure was constructed with data reported by Morán-Zenteno et Schaaf et al., 1995; Rogers et al., 2007; Silva-Romo, 2008), as well as al. (2007), supplemented with data from Solari et al. (2007), Martini et al. (2010), and from this study. south of the Xolapa terrane (Fig. 1). Most authors suggest that the process of continental truncation involved the separation of the Chortis block from southern Mexico, as Chortis moved with the Caribbean plate. If the migration of the age of magmatism in the continental margin was al., 1978; Meschede and Frisch, 1998; Schaaf et al., 1995). The presence directly related to the rate of displacement of the Chortis block, then the of plutonic arc rocks less than 100 km from the present Mid-America rate of motion of Chortis along the margin was ~30 km/m.y. (Fig. 10). Trench, and thus the absence of a forearc region, has been interpreted Schaaf et al. (1995) and Morán-Zenteno et al. (2009) provided different as evidence of truncation of the continental margin in the Cenozoic. A rates of migration of magmatism, but in their estimate, they combined component of subduction erosion has also been invoked to explain the age estimates obtained using different isotopic methods, and this process absence of a forearc (Morán-Zenteno et al., 1996), and most likely played may have introduced some uncertainty. The rate of 30 km/m.y. is compa- an important role in the removal of continental crust from the forearc rable to the average spreading rate in the Cayman Trough. MacDonald region (Keppie et al., 2012). and Holcombe (1978) correlated magnetic anomalies east and west of the A series of margin-parallel regional shear zones such as the Tierra Colo- spreading center at the trough and suggested a total opening rate of 20 rada (Solari et al., 2007; Riller et al., 1992), Chacalapa (Tolson, 2005), and km/m.y. between 2.4 Ma and the present, with a much higher 40 km/m.y. Tonalá (described in this study) has been interpreted to support the model rate between 2.4 and 8.3 Ma. Rosencrantz et al. (1988) estimated an aver- of truncation of the continental margin resulting from eastward displace- age opening rate of 15 mm/yr between 15 and 30 Ma and the present, and ment of the Caribbean plate (Pindell et al., 1988). The Tonalá shear zone a faster rate of 27 mm/yr between ca. 30 and 45 Ma. Leroy et al. (2000) is the youngest mylonite belt along the margin, being active until motion estimated slower rates of total opening of 17 mm/yr between 20 Ma and was finally transferred to the Polochic-Motagua system and faults north of the present, 20 mm/yr between 26 and 40 Ma, and an average of 15 mm/ the Chiapas Massif after ca. 8 Ma (Guzmán-Speziale and Meneses-Rocha, yr between 42 and 49 Ma. We recognize, however, that magmatism along 2000; Authemayou et al., 2011). We propose that the transfer of motion the continental margin is not only dependent on the rate of displacement from the Chacalapa fault, active in the Oligocene between 29 and 24 Ma, of the Chortis block along southern Mexico. to the Tonalá shear zone, active in the late Miocene from ca. 12 to 8 Ma, The modern Chiapanecan arc (Pliocene to Holocene) is located in the resulted in important, regionally interconnected events in the Tehuantepec central Chiapas Highlands (Mora et al., 2012; Manea and Manea, 2006), area. These include left-lateral strike slip on the Chipehua fault (Fig. 1) some 150–200 km north of the Miocene arc and ~150 km farther inboard in the western Tehuantepec Gulf (Sánchez-Barreda, 1981); widespread from the Mid-America Trench (Fig. 1). We propose that recent migration normal faulting in the western Tehuantepec isthmus associated with a of arc magmatism to the north is due to interaction among the Cocos, pulse of Miocene (24–13 Ma) magmatism in this region (Morán-Zenteno North America, and Caribbean plates, with the latter represented by the et al., 2000); uplift and denudation of the western and eastern Tehuante- Chortis block (the continental component of the Caribbean plate). This pec region; and transtension in the Tehuantepec Gulf, forming basins into northward migration of the locus of magmatism may be explained by a which thick sequences of sediments derived from the uplifted region accu- decrease in the slab dip angle as the Chortis block is removed from the mulated. Sánchez Barreda (1981) reported up to 4 km of sandstone, shale, forearc region. The same process was proposed by Morán-Zenteno et and conglomerate, containing plutonic and volcanic clasts in the Tehuan- al. (1996) to explain uplift and exhumation of midcrustal plutons of the tepec Gulf. Although a Late Cretaceous to Holocene age was assigned to Xolapa terrane along the Pacific coast. The displacement of the Chortis

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block explains the eastward migration of magmatism observed along the chic, Motagua, and Jamalecón, as well as other faults (Guzmán-Speziale, southern Mexican Pacific coast, and also the age of mylonites parallel to 2010). Burkart et al. (1987) showed that the Polochic fault accommodated the margin. This includes the Tierra Colorada shear zone (also known as ~130 km of the total displacement along the plate boundary, but Brocard La Venta fault, active before 35 Ma), the Chacalapa fault (active between et al. (2011) suggested that most of the displacement along the Polochic 29 and 24 Ma), and the ca. 13–8 Ma Tonalá fault (Fig. 1). fault occurred prior to 7–10 Ma. We interpret the mylonite system of the In this contribution, we have described a belt of middle to late Mio- Tonalá shear zone to be a relict transform boundary slightly oblique to the cene plutons, and their host rocks, which are pervasively sheared along a Polochic-Motagua fault system, which accommodated a minimum of 100 well-defined west-northwest–trending mylonitic shear zone extending for km of displacement between the Chortis block (on the Caribbean plate) at least 120 km (but possibly up to 150 km) along the western margin of and Chiapas (on the North America plate) in late Miocene time. the Chiapas Massif (Fig. 2). The position of the middle to late Miocene The structural data from the Tonalá shear zone provide strong evidence arc along the Pacific coast of Chiapas, and the inland migration of mag- for northeast-directed shortening. This strain was contemporaneous with matism during the Pleistocene are features similar to those proposed for emplacement of the Chiapas Massif and suggests that the late Miocene the Xolapa area, but there are some differences. The dip of the subducted Chiapas fold belt north of the Chiapas Massif (Fig. 1), and similar-age slab must increase gradually to the east in Chiapas as “normal” subduc- structures in the Tabasco coastal plain buried by younger sediments are tion is ongoing under the Caribbean plate in Guatemala. As in the Xolapa kinematically linked to the Miocene interaction between the Chortis and terrane, slab flattening results in isostatically driven uplift and exhuma- Maya blocks along the Tonalá shear zone. tion of previously formed midcrustal plutons. At the trailing edge of the eastward-moving Chortis block, however, transtension has led to basin CONCLUSIONS development, as observed in the Tehuantepec area. The 40Ar/39Ar age spectra obtained from K-feldspar in sample 34-TEP The Miocene Chiapanecan magmatic arc was active in the region (a mylonitic monzonite north of Tepanatepec) yield a cooling age estimate of coastal Chiapas between ca. 13 and 9 Ma. Magma ascent and pluton that is ~1 m.y. younger than the U-Pb zircon crystallization age estimate. emplacement were most likely controlled by a crustal-scale shear zone, Using a closure temperature of the U-Pb system of ~750 °C and a maxi- herein formally named the Tonalá shear zone, which accommodated rela- mum closure temperature of 300 °C for the 40Ar/39Ar system in K-feldspar tive plate motion between the Caribbean and North America plates in the (e.g., Heizler and Harrison, 1988), this set of data implies a rapid, mini- late Miocene. The shear zone is a strikingly linear feature, extending for mum, cooling rate of ~450 °C/m.y. Witt et al. (2012) reported an apatite at least 120 km, and it is characterized by the Tonalá mylonitic belt. The fission-track and (U-Th)/He apatite age of ca. 8.9 Ma for a locality within plutons of the Miocene arc are syntectonic, as pluton crystallization ages 0.5 km of our 34-TEP sample, suggesting that plutons of the Miocene arc and ages of neoformed micas in the shear zone are indistinguishable. The were at shallow crustal levels soon after emplacement. Regional uplift in Tonalá shear zone and associated rocks show evidence of very rapid uplift, southern Chiapas probably continues today, as all rivers in the Chiapas which may be explained by flattening of the subducted slab as the Chortis Massif incise on bedrock. Locally, higher uplift rates may be linked to block was displaced eastward from the forearc region. Isostatic rebound fault relays such as the link between Tonalá and Polochic driving the Chi- related to erosion further contributed to contemporaneous uplift. comuselo uplift in Chiapas (Fig. 1), or at restraining bends (Authemayou et al., 2011). Uplift and erosion of the Chiapas Massif have also been ACKNOWLEDGMENTS responsible for sediment transport to the Gulf of Mexico since the early This research was possible with the support of Conacyt grant CB129862 to R. Molina, and an American Chemical Society–Petroleum Research Fund award to T. Wayrzyniec. Antonio Godínez Miocene, but more significantly since the late Miocene. Sediment removal and Linda Donohoo assisted with sample collection. We appreciate the technical support of in the humid Chiapas climate and concomitant isostatic rebound further Ofelia Pérez and Juan Tomás Vázquez. Iriondo would like to thank Mick Kunk for supervision contribute to modern uplift. undertaking the 40Ar/39Ar experiments. In addition, Joe Wooden is thanked for his supervision of the U-Pb sensitive high-resolution ion microprobe–reverse geometry analyses. We also thank The orientation of the Tonalá shear zone is somewhat oblique to the reviewers J. Goodge, M. Guzmán Speziale, T. 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