Paleomagnetism of the Todos Santos and La Silla Formations, Chiapas: Implications for the Opening of the Gulf of Mexico

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Paleomagnetism of the Todos Santos and La Silla Formations, Chiapas: Implications for the opening of the Gulf of Mexico

Antonio Godínez-Urban1, Roberto S. Molina Garza2, John W. Geissman3, and Tim Wawrzyniec3 1Posgrado en Ciencias de la Tierra, Universidad Nacional Autónoma de México, Campus Juriquilla, Querétaro, 76100, Mexico 2Centro de Geociencias, Universidad Nacional Autónoma de México, Campus Juriquilla, Querétaro, 76100, Mexico 3Department of Earth and Planetary Sciences, MSC 03 2040, 1 University of New Mexico, Albuquerque, New Mexico 87131-0001, USA

ABSTRACT an Euler rotation pole for the Maya Block 1992) as well as the tectonic grain in the deep for this time period in the eastern gulf. The Gulf of Mexico (Scott and Peel, 2001). The We report paleomagnetic data for the apparent polar wander path defi ned by granitoids of the Chiapas Massif record west- Lower to Middle Jurassic La Silla and Todos paleomagnetic poles for the Chiapas Massif directed characteristic magnetizations that yield Santos formations of southern Mexico, in and Jurassic rocks reported here suggests a paleomagnetic pole position in the central west-central Chiapas and the Tehuantepec that relative motion between North America equatorial Pacifi c. These data have been inter- Isthmus region. Volcanic rocks and red beds and the Maya Block occurred between Late preted to refl ect large magnitude counterclock- of these formations were deposited prior to Permian and Early Jurassic time, during a wise rotation of the Maya Block with respect to or during the early stages of Gulf of Mexico protracted rifting phase, and then in the Late North America (Molina-Garza et al., 1992). The opening. Dual-polarity characteristic mag- Jurassic in association with seafl oor forma- rotation indicated by paleomagnetic data for the netizations reside primarily in hematite and tion in the Gulf. Chiapas Massif, however, is larger (~70°) than pass intraformational conglomerate, regional suggested in plate reconstructions (35° –55°). tilt, and reversal tests; they are thus inter- INTRODUCTION AND The basement structure of the western Gulf preted as primary magnetizations. Our sam- PREVIOUS WORK of Mexico is thought to be dominated by attenu- pling sites are concentrated in three locali- ated crust whose primary features were defi ned ties; around La Angostura Lake, 17 accepted It is generally accepted that the Gulf of by the south-southeast drift of the Maya Block sites yield a tilt corrected mean of declination Mexico formed by counterclockwise rotation along the eastern margin of Mexico (Buffl er, (Dec) = 325°, inclination (Inc) = 4.6° (k = 11.9, of the Maya (or Yucatán) Block in the Late 1983; Buffl er and Sawyer, 1985; Ewing, 1991; α 95 = 10.8°); in the Matías Romero region, Jurassic following a protracted episode of con- Marton and Buffl er, 1994; Wawrzyniec et al., the mean is Dec = 312.9°, Inc = 3.2° (based tinental rifting that initiated in the Late Triassic 2003; Wawrzyniec et al., 2004; Ambrose et al., on only seven sites); and in the Custepec (Pindell and Dewey, 1982; Hall et al., 1982; 2003). This is supported by a steep north-south– area, Jurassic andesitic dikes intruding rocks Pindell, 1985; Buffl er and Sawyer, 1985; Pin- trending basement step and associated geo- of the Permian Chiapas Massif yield a cor- dell et al., 2006). Independent motion of the physical anomalies, including a coast-parallel rected mean of Dec = 335.0°, Inc = 5.0° (six Maya Block with respect to both North and magnetic maximum and a steep gravity gradient sites). The mean directions are discordant South America is supported by the development offshore of the Mexican states of Tamaulipas and with respect to expected North America ref- of rift basins as well as Upper Jurassic passive Veracruz. These anomalies appear to follow the erence directions, and indicate a counter- margin sequences surrounding it. The inferred trace of the Tamaulipas-Golden Lane-Chiapas clockwise rotation of 35° to 40°. Inclinations timing of counterclockwise rotation in the Late transform (Pindell, 1985), a structure considered indicate deposition or emplacement at near Jurassic is constrained by (1) plate kinematics to have accommodated rotation of the Maya equatorial paleolatitudes (2.1°N ± 3.4°). This (Pindell and Dewey, 1982; Pindell et al., 1988; Block. It also has been proposed that the Maya paleolatitude estimate is statistically indis- Bird et al., 2005; Pindell et al., 2006); (2) the Block was rotated about an Euler pole in the tinguishable from those previously observed Jurassic stratigraphy of the Gulf and northeast eastern Gulf region, located near the southern in the La Boca Formation of the Huizachal Mexico (e.g., Winker and Buffl er, 1988; Salva- Florida peninsula or western Cuba (e.g., Pindell Group in northeast Mexico. The localities we dor, 1987; Goldhammer, 1999), as it is gener- and Dewey, 1982; Hall and Najmuddin, 1984; sampled in southern Mexico are separated by ally assumed that rotation of Yucatán post-dates Marton and Buffl er, 1994; Pindell et al., 2006). ~150 km, suggesting that the paleomagnetic Callovian salt deposition; (3) isotopic (190.4 ± Because of uncertainty in the magnitude of signature of these rocks refl ects regional- 3.4 Ma) age determinations of rift-related dia- rotation, several details in the reconstruction scale rather than local deformation. These base dikes offshore of Yucatan (Schlager et al., of the Maya Block in the northern Gulf region Jurassic paleomagnetic directions support 1984); and (4) paleomagnetic data for the San remain unsolved, as illustrated in Figure 1. For a rotational origin for the Gulf of Mexico. Ricardo Formation along the northeast fl ank example, Ross and Scotese (1988) reconstruct The data are also consistent with an Early to of the Chiapas Massif (Guerrero-García et al., the Maya Block farther west than other mod- Middle Jurassic reconstruction that places 1990). The rotation of the Maya Block itself is els. Pindell and Kennan (2009), on the other the Chiapas Massif offshore the Tamauli- supported by paleomagnetic results for the Late hand, propose a tighter fi t, and the Maya Block pas state in the western Gulf of Mexico, and Permian Chiapas Massif (Molina-Garza et al., is placed farther north than other models, while

Geosphere; February 2011; v. 7; no. 1; p. 145–158; doi: 10.1130/GES00604.1; 8 ﬁ gures; 3 tables.

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Dickinson and Lawton (2001) locate the Maya 180 Ma Block farther east than other models. Similarly, the orientation of the rift structures in continental lithosphere that facilitated opening of the Gulf is uncertain, and somewhat controversial (Pindell et al., 2006; Exxon, 1985; Salvador, 1987). Some authors have suggested that relative motion between the Chiapas Massif and the Maya Block occurred during the rifting process or at a later time (e.g., Ross and Scotese, 1988; Dickinson and Lawton, 2001). Therefore paleomagnetic data of Permian age from the Permian Ross and Scotese, 1988 Massif may not refl ect with suffi cient accuracy the rotation of the Maya Block during opening Tr-J of the Gulf of Mexico. Furthermore, plutonic rocks lack reference to paleohorizontal, and, in the absence of robust fi eld relations involving overlying stratifi ed rocks, paleomagnetic data from them are inherently of lower reliability than those from stratifi ed rocks. The limited paleomagnetic data for the Todos Santos Formation ? reported by Molina-Garza et al. (1992) suggest that Jurassic rotation of the Maya Block is closer ? to the 35° –45° estimate that has been recently proposed in several paleogeographic reconstruc- Dickinson and Lawton, 2001 tions (e.g., Mickus et al., 2009; Pindell et al., 2006), but the overall quality of that Todos San- tos paleomagnetic data set from the Tehuantepec 190 Ma region is inadequate for a reliable reconstruction. The paleopole for the Chiapas Massif, and the larger rotation inferred by Molina-Garza et al. (1992), may thus refl ect internal deformation of - the Maya Block during the rifting phase of continental breakup, or other tectonic processes such as Neogene deformation. Other paleomagnetic data suggest a more complex scenario for Mesozoic deformation (Steiner, 2005). Paleomagnetic data from the Santa Rosa Group in the Maya Mountains have been interpreted by Steiner (2005) to indicate Pindell and Kennan, 2009 ~180° of rotation of the Maya Block since the mid-Permian. In essence, paleomagnetic data 165 Ma from the Maya Mountains are similar to results from the Chiapas Massif. In the Maya Moun- tains, early Paleozoic intrusions that have been affected by a late Paleozoic thermotectonic event and yield Permian–Triassic K-Ar dates yield a paleomagnetic pole in the equatorial Pacifi c near the pole for the Chiapas Massif. Mid-Permian sedimentary rocks of the Santa Rosa Formation yield shallow, south-southwest magnetizations defi ning a pole at 62.5°N/22.6°E. The Santa Rosa Formation paleopole for the Maya Mountains is at an angular distance of 27° ± 17° from the pole reported for the upper Paleozoic sequence from Bird and Burke, 2006 the Chicomuselo uplift in Chiapas (the pole is located at 74.2°N–95.4°E; Gose and Sánchez- Figure 1. Selected reconstructions of western equatorial Pangea, with emphasis on the posi- Barreda, 1981). Both poles fall near the Juras- tion of the Yucatan block. Modifi ed after Ross and Scotese (1988), Dickinson and Lawton sic segment of the North American apparent (2001), and Pindell and Kennan (2009). polar wander path. Molina-Garza et al. (1992)

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reported similar, shallow and south-directed, deposited in alluvial fan, fl uvial, and lacustrine of the Todos Santos Formation and the under- magnetizations for Todos Santos strata in the environments (Blair, 1987). In the study area lying La Silla volcanic sequence. Most of the Motozintla area of the Chicomuselo uplift, near of the Sierra Homocline province of west- sites were collected from the volcanic-bearing the point where the Polochic fault system crosses central Chiapas, Godínez-Urban et al. (2011) basal strata assigned to La Silla Formation and the Mexico-Guatemala border. They thus inter- report a Pliensbachian U-Pb age for volcanic from a sequence of intercalated red sandstone preted the shallow south-directed magnetiza- rocks below the Todos Santos sequence (191 ± and mudstone of the lower Todos Santos El tions in Permian strata to be sec ondary, refl ecting 3.0 Ma) assigned to La Silla Formation, and Diamante Member (Fig. 2A). Four sites were a time period of remagnetization in the Jurassic. detrital zircons in the range from 196 to 161 Ma collected from Concordia facies of the Jericó Steiner (2005) argued, however, for a primary, in the lower member of the Todos Santos For- Member (high in the Todos Santos Formation), Permian age of the magnetization of the Santa mation. La Silla Formation is correlated with the and three sites were collected in conglomeratic Rosa Formation, and thus a larger rotation than Pueblo Viejo andesite (Meneses-Rocha, 1985); sandstones of the Jericó Member of the Todos inferred by Molina-Garza et al. (1992). There is, they have similar stratigraphic positions and iso- Santos Formation. In addition, we sampled 11 however, relatively good agreement between an topic age determinations. Godínez-Urban et al. undeformed dikes of intermediate composi- Early Triassic paleopole for the Maya Moun- (2011) proposed subdividing the Todos Santos tion. These intrude rocks of the Chiapas Massif tains and a Permian–Triassic paleopole for the Formation into a lower volcaniclastic unit and yield Early to Late Jurassic 40Ar-39Ar dates Chiapas Massif. named El Diamante Member and an overlying (Godínez-Urban et al., 2011). Most of the dikes In order to better understand the kinematics arkosic member named Jericó. were collected along the road from El Diamante of the Maya Block during the opening of the The term Sierra Homocline for the area east to Custepec (Fig. 2C), additional sites were col- Gulf of Mexico, we collected paleomagnetic of the Chiapas Massif (Fig. 2) is somewhat lected in the valley of the Tablón River, but all data from the Todos Santos Formation in central misleading, as the Todos Santos Formation but one failed to provide useful results. Finally, Chiapas and the Tehuantepec Isthmus region. around the village of Independencia (Fig. 2A) fi ve sites were collected in the Tehuantepec isth- Additional data were collected from the La Silla is exposed within NW trending folds, perhaps mus, near localities reported by Molina-Garza Formation, a recently recognized sequence of associated with the Chicomuselo uplift—an et al. (1992) along the Trans-Isthmus highway volcanic rocks that underlies Todos Santos strata oblique extension of the Chiapas Massif south- outside of Matías Romero (Fig. 2B), where (Godínez-Urban et al., 2011), and from associ- east of the study area. Also, two major strike-slip Todos Santos strata are gently tilted to the north. ated Jurassic dikes in the Chiapas Massif. These faults are recognized in this sector of the Sierra Most samples were obtained with a gas- data, together with a reappraisal of the relevant Homocline. One, the trace of which is north of powered drill, but a few site collections con- circum-Gulf Jurassic geology, allow us to better La Angostura Lake, trends WNW; the other is sisted of oriented hand samples, from which defi ne the rotation parameters (the Euler pole a conjugate structure that trends north-south, standard cores 2.5 cm in diameter were prepared and amount of associated rotation, with error and crosses the study area near El Diamante in the laboratory. Samples were oriented with a estimate) of the Maya Block for the Jurassic. (Fig. 2A). The homoclinal structure character- magnetic compass and clinometer, and where izes outcrops of Cretaceous carbonate rocks at possible with a sun compass. Sites were selected REGIONAL GEOLOGIC SETTING La Angostura Lake, and east of it deformation from localities where bedding attitudes could be of Neogene age formed the Chiapas foldbelt clearly discerned, but for a handful of sites in Todos Santos strata crop out along a nearly (Meneses-Rocha, 2001). andesite fl ows of the La Silla Formation bed- continuous belt extending from western Guate- The Todos Santos Formation in the Tehuan- ding attitudes are diffi cult to determine. In those mala (Clemons et al., 1974), along the northern tepec Isthmus, in the Mixtequita region, overlies cases, we combined information from nearby margin of the Chiapas Massif, westward into basement rocks of the Grenville age Guichicovi sites with good bedding control with informa- the western margin of the Veracruz basin, and Complex (Weber and Kohler, 1999; Fig. 2B). It tion from anisotropy of magnetic susceptibility. into the Zongolica foldbelt (Fig. 2). The non- consists of polymictic conglomerate, sandstone, In most cases, we observed that the bedding marine Todos Santos Formation was deposited and mudstone. Volcanic rocks have been rec- inferred from nearby sites in sedimentary rocks in extensional basins during the early stages of ognized in the Mesozoic sequence east of the coincided with the orientation of the magnetic continental rifting in the Gulf region. Elsewhere isthmus in the Uzpanapa river area (Herrera-Soto susceptibility foliation plane. in Mexico, comparable sequences of nonmarine and Estavillo-González, 1991). South of Matías For measurements of the natural remanent detrital strata have been assigned to La Boca and Romero, the Todos Santos Formation is in thrust magnetization (NRM) we used a 2G Enterprises La Joya formations of the Huizachal Group in contact with an allochthonous Upper Cretaceous DC-SQUID–based superconducting magnetome- the Tamaulipas area (e.g., Mixon et al., 1959; sequence of metasedimentary and metavolcanic ter or a JR5 spinner magnetometer. Both instru- Michalzik, 1991), or the Cahuasas Formation rocks (Fig. 1B; Pérez-Gutiérrez et al., 2009). ments are hosted in shielded rooms, at the in the Huayacocotla area (Ochoa-Camarillo These authors report detrital zircon data from University of New Mexico and the Centro de et al., 1998). Todos Santos strata were directly sandstones in the Todos Santos sequence indi- Geociencias, respectively. The samples were deposited on the basement of the Maya Block, cating a Late Triassic maximum deposition age. subjected to progressive alternating fi eld (AF) as represented by the Permian metamorphic and In the isthmus, Todos Santos is overlain by the and thermal demagnetization. The vector compo- plutonic complex of the Chiapas Massif and Upper Jurassic Mogoñé Formation (Herrera- sition of the NRM was determined from visual Paleozoic rocks of the Altos Cuchumatanes. The Soto and Estavillo-González, 1991). inspection of orthogonal demagnetization dia- Todos Santos Formation in Chiapas is overlain grams (Zijderveld, 1967), and the directions of by the Upper Jurassic–Lower Cretaceous San METHODS AND SAMPLING components present were determined using stan- Ricardo Formation (Meneses-Rocha, 2001). dard principal component analysis (Kirschvink, Here, Todos Santos consists of polymictic con- In the La Angostura Lake area of the Sierra 1980). Site means and formation means were glomerate, sandstone, mudstone, volcaniclastic Homocline east of the Chiapas Massif, we col- calculated assuming the distribution of directions deposits, and volcanic rocks. The sequence was lected a total of 34 paleomagnetic sites in rocks is reasonably described by Fisher statistics.

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Matías Romero Miocene volcanic arc B 95°10′W 95°00′W 52-56 Pyroclastic-Volcanoclastic

Monzonite A 16°50′N Allochthonous (Upper Cretaceous) 92°55′W 92°50′W Greenstone La Angostura Sierra Madre Limestone Marble La Soledad San Ricardo Formation Lagunas Siliciclastic phyllite Volcanoclastic phyllite Jerico Member Autochthonous (Maya Block) vc1 El Diamante Member Sierra Madre Limestone vc2 Santos Fm. Todos Todos Santos Formation La Silla volcanic rocks 16°10′N Almoloya vc9 SAMPLING SITES vc20 vc3 vc21-22 Permian- Chiapas Massif vc14 37-38 vc17-18 vc26-28 vc10 Concordia Chivela

El Diamante vc9 La Angostura MEX vc4 185 vc5-8 Santiago ′ 16°05 N Independencia

N N vc12 B.Juárez vc15 vc16 vc30 10 km 5 km vc31 vc11 16°35′N Ixtepec La Ventosa

Gulf of 93°W 90°W 87°W Mexico Veracruz Yucatán block Caribbean sea basin Mexico 18°N 18°N Acatlán Mixtequita Guatemala vf30 B Maya Mts. yman trough Ca Oaxaca Chia Cuicateco 150 km 16°N–93°W pas A Xolapa massi gua f Polochic-Mota Gulf of Tehuantepec C

Hondu 15°N Chicomuselo ras de vf32–35

uplfit p

r Me e soam ssio eric vf36–40 Pacific a n n t ren ch Ni Ocean cara gua de pressi 96°W 93°W 90°W 87°W on N vf42–45

Custepec 3 km C

Figure 2. General outcrop distribution of the Todos Santos Formation (dark gray pattern) and basement terranes of southern Mexico and Central America. (A) Detailed geologic map with sampling sites in the Angostura Lake area. (B) Detailed geologic map with sampling sites in the Tehuantepec region (after Pérez-Gutiérrez et al., 2009). (C) Sites collected in the Custepec dikes.

PALEOMAGNETIC RESULTS over a narrow range of laboratory unblocking (high laboratory unblocking temperature) shows temperatures typically above 650 °C; demag- coercivities in excess of 120 mT, but can be well The magnetization in rocks of the Todos netization reveals a northwest-directed magne- determined by stable endpoints (Fig. 3C). Santos Formation is relatively complex, as the tization of negative inclination. Other volcanic In sedimentary rocks interbedded with NRM is generally multivectorial. Typically, rocks from the Angostura region have two- volcanic ﬂ ows, a low stability magnetization stable endpoint behavior was observed in vol- component magnetizations, one component is is north directed and of moderate positive canic rocks (Fig. 3A) and some of the dikes. north directed and of low coercivity, the second inclination. This component is removed after Volcanic rocks generally display nearly univec- one is west directed and of negative inclination heating to ~400 °C (Fig. 3B). The high sta- torial behavior, and the magnetization unblocks (in situ). The high magnetic stability component bility magnetization is of similar direction to

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A-Site vc20 B-Site vc31 N, Up C-Site vc19 665 N, Up N, Up E N, Up 19f 635 20e 31d 590 530

Nrm 400 200 320 500 550 5a 240

590 Nrm 600 160 E 50 70 550 40 D-Site vc5 635 25 120 650 90 665 20 30 Figure 3. Examples of orthogo- 670 E 80 675 15 nal demagnetization diagrams 680 E 10 for the Todos Santos and La Jo=0.81 e-3A/m Jo=1.79 e-4A/m Jo=0.51 e-4A/m 5 Silla formations, as well as Nrm Jurassic dikes from the Custe- Jo=2.35 e-5A/m Nrm W, Up G-Site vf52 pec region. Solid (open) sym- W, Up H-Site vf32 bols are projections on the N, Up 558 E-F Site vc6 520 32h 565 horizontal (vertical) plane. All N, Up 6b 6f N diagrams in geographic coor- 52c dinates. Demagnetization steps Jo=1.55 e-4A/m 425 (in milliTesla, mT, or °C) given E 560 400 E along projections on the verti- 240 160 320 320 240 Jo=5.21 e-5A/m cal plane. 160 N 400 480 560 673 535 325 100 668 545 Jo=3.79 e-4A/m 200 480 663 Jo=51.64 e-3A/m 80 655 645 635 Nrm 80 620 580 605 540 500 475 Nrm 425 325 200 Nrm 100 Nrm

that observed in volcanic rocks, but of lower Near univectorial decay of the NRM is The paleomagnetic results for three sites unblocking temperatures (<600 °C). The anti- observed in sample vf52 (from the Matías in the red beds of the Todos Santos Forma- pode of the northwest-directed magnetization Romero region, Fig. 3G) and in most samples tion were lost, and three additional sites did of negative inclination is observed in other from this region. The magnetization is of dis- not respond to demagnetization producing samples, after removal of a north-directed tributed laboratory unblocking temperatures uninterpretable data. A ChRM was obtained overprint (Fig. 3D). A more complex behavior between ~200 °C and 660 °C, and it decays for all remaining sites (Table 1). Of the sites is observed in some of the red bed samples. abruptly between 660 °C and 670 °C. The in dikes along the Tablón River, four did not For instance, samples from site vc6 are char- Custepec dikes (Fig. 3H) have high NRM yield useful data. As mentioned above, the few acterized by a low stability, north-directed, and intensities (typically ~0.2–1.3 A/m), but their sites collected in the coarse sandstones of the moderate positive inclination (Fig. 3E). An behavior is multicomponent. A spurious low- Concordia facies of the Jerico Member, high in intermediate laboratory unblocking tempera- stability magnetization is removed after heat- the Todos Santos succession, failed to produce ture magnetization is southeast directed and ing to 200 °C; further treatment removes a interpretable data. of negative inclination. The highest magnetic prominent north-directed and positive inclina- Site vc18 in La Silla Formation is of partic- stability magnetization is southeast directed tion magnetization ﬁ nally revealing a small, ular interest, because samples were collected and of moderately steep positive inclination. but well-deﬁ ned, southeast-directed magne- from discrete volcanic clasts in a volcani- Demagnetization data show that the interme- tization of negative inclination. We note that clastic deposit. The clasts are angular, and diate and high unblocking temperature mag- the host rock to the dikes yields characteristic supported in a sandy matrix of volcaniclastic netizations can be removed simultaneously magnetizations (ChRM) that resemble previ- material; the deposit is interpreted as a lahar. (Fig. 3F, from the same site vc6). The com- ous results for the Chiapas Massif (Molina- The samples yield univectorial demagnetiza- posite magnetization is of shallow inclination; Garza et al., 1992), suggesting that regional tion behavior, with magnetizations of dis- it is an artifact of removing two magnetizations remagnetization of the dikes and host rocks tributed coercivity and distributed laboratory with opposite inclinations. has not occurred. unblocking temperatures. The direction of the

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TABLE 1. PALEOMAGNETIC DATA AND STATISTICAL PARAMETERS FOR THE TODOS SANTOS AND LA SILLA FORMATIONS, SOUTHERN MEXICO Dec (°) α Site Coord. (°N, °W) N in situ Inc (°) k 95 strike, dip Tilt corrected verad La Angostura vc1 * 16.178636,92.952886 10\10 50.9 49.7 19.6 11.2 345, 23 57.6, 28.1 andesite vc2 * 16.176964,92.953439 8\9 199.3 33.1 317 3.1 345, 23 187.6, 43.4 andesite vc3 16.148522,92.947142 8\8 346.8 –1.2 207.1 3.9 345, 23 346.2, –1.8 andesite vc4 16.100836,92.903708 6\8 168.4 55.2 329.9 3.7 41, 73 151.7, –11.2 andesite vc5 16.099994,92.899175 7\10 132.4 65.6 31.2 11 41, 73 131.6, –7.4 red bed vc6 16.099994,92.899175 4\8 165.7 31.1 28.8 17.4 41, 73 165.9, –31.5 red bed vc7 16.100166,92.899175 6\10 208.9 40.4 18.1 21.9 41, 73 168.5, 5.4 red bed vc8 16.100166,92.899175 4\7 184.0 42.1 70.5 11 41, 73 168.5, –13.4 red bed vc9 16.156542,92.816653 4\11 317.0 –0.4 24.1 19.1 307, 5 316.5, 3.0 red bed vc10 * 16.124722,92.831181 8\8 185.8 –12.2 533.2 2,4 310, 6? 186.4, –7.3 andesite vc11 * 16.004722,92.911147 7\7 350.5 16.6 104.0 6.6 7, 54? 11.5, 22.8 andesite vc12 16.027610,92.882732 3\5 289.4 –49.2 144.3 10.3 7, 54 285.1, 4.1 red bed vc15 16.142441,92.883291 5\7 326.3 –39.8 27.3 14.9 7, 54 312.7, 1.6 red bed vc16 16.027384,92.883386 5\8 358.6 –29.8 19.6 17.7 7, 54 338.0, –11.0 red bed vc19 16.139760,92.874847 4\6 318.8 –23.0 96.5 353, 41 308.8, 17.9 red bed vc20 16.144285,92.884132 10\10 336.0 –22.0 441.7 2.3 347, 50 323.3, –6.0 andesite vc21 16.140303,92.876595 3\9 330.0 –9.3 14.9 10.5 19, 40 332.8, 20.8 red bed vc22 * 16.140674,92.876501 4\6 19.9 51.8 88.5 9.8 19, 40 58.4, 36.8 andesite vc26 16.140130,92.874416 3\10 172.9 13.1 12.3 16.7 7, 37 168.0, 2.2 red bed vc27 16.139281,92.874763 4\8 337.6 –10.7 43.4 14.1 7, 37 336.9, 8.2 red bed vc28 16.139281,92.874763 8\10 298.3 –17.9 50.1 7.9 8, 38 298.3, 17.6 red bed vc29 * 16.100304,92.901532 8\8 227.6 –18.9 316.2 3.1 41, 73 241.0, 0.5 red bed vc30 16.027384,92.883386 7\7 311.5 –55.8 67.7 7.4 7, 50 295.9, –10.2 red bed vc31 16.027384,92.883386 4\7 323.2 –49.9 42.2 14.3 7, 50 305.1, –8.6 red bed Mean 17 334.4 –32.5 8.1 13.3 325.0, 4.6 11.9, 10.8 Normal 317.4, 1.7 k = 13.2, 13.0, n = 11 Reverse 159.5, –9.5 k = 17.1, 16.7, n = 6 Paleopole 52.9°N, 158.9°E dp = 5.2, dm = 10.4 Overprint vc5ov 10\10 173.4 –27.3 12.5 15.2 vc6ov 6\8 157.1 –34.1 39.2 10.8 vc8ov 7\7 189.8 –28.8 26.7 6.2 vc37–39ov 4\16 174.8 –29.8 8.3 17.9 Mean 4 174 –34.5 47.2 13.5 Custepec dikes vf30 15.7796, –92.9467 8 149.6 –21.8 21.7 11.4 vf31 15.7796, –92.9467 2 148.1 –12.4 vf32 15.7796, –92.9467 7 161.4 –17.3 53.1 7.0 vf33 15.7796, –92.9467 6 156.7 –17.3 39.8 8.7 vf35 15.7796, –92.9467 7 167.6 –36.5 31.7 10.9 vf40 15.7796, –92.9467 11 311.1 14.6 11.1 27.6 Mean 6 332.0 20.2 30.7 12.3 335.0, 5.0 Paleopole 62.2°N, 169.4°E dp = 6.8, dm = 12.8 62.0°N, 151.2°E Host rock vf42 host 15.7796, –92.9467 16 269.4 26.3 19.2 8.1 vf44 host 15.7796, –92.9467 8 264.1 24.5 33.5 14.7 vf45 old dike 15.7796, –92.9467 6 253.3 0.3 9.6 27.1 El Tablón§ 18 249.2 –6.5 7.1 13.9 # α Combined 16 257.7 –3.7 Paleopole 12.3°S, 178.6°E ( 95 = 9; k = 17.9) Matias Romero region vf52 16.892819, –95.011744 6 331.7 32.6 10.4 21.8 251, 36 333.2, –3.0 vf53 16.892819, –95.011744 3 296.2 8.9 25.3 25 251, 36 294.4, –16.7 vf54 16.892819, –95.011744 8 354.5 29.6 88.1 5.9 251, 36 351.4, 4.0 vf55 16.892819, –95.011744 9 340.2 14.6 32.9 9.1 251, 36 340.2, –21.4 mg37† 5 283.2 0.6 9.6 18.5 285.5, 15.9 mg38† 3 309.1 50 11 20.2 286, –1 mg39† 6 127.2 10.8 14.6 23.7 126.3, –2.8 Mean 7 316.6 22.7 7.8 23 312.9, 3.2 k = 7.5, α = 23.6 Paleopole 41.3, 163.5 dp = 11.8, dm = 23.6 *Sites excluded from ﬁ nal calculations, see text for explanations. †Sites reported by Molina-Garza et al. (1992). §Site reported by Molina-Garza et al. (2009). #Combined with 12 VGPs (in situ) reported by Molina-Garza et al. (1992).

remanence is widely scattered among clasts, DISCUSSION OF tics. Precision parameter values range between but well grouped within single clasts (Fig. 4). PALEOMAGNETIC RESULTS 12 and 533, and most of the sites show low This result indicates that acquisition of the within site dispersion. Nonetheless, four sites magnetization preceded incorporation in the Of the 24 sites in the Todos Santos and La Silla were excluded from the overall mean because deposit. This is interpreted as a positive intra- Formations for which we report an interpretable we are uncertain of their structural attitude. Sites formational conglomerate test. site mean direction, most have acceptable statis- vc1 and vc2, in the northern part of the area, are

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A B by the presence of dual polarity magnetizations. N, Up vc18g vc18h N, Up C The best estimate of the age of the magnetiza- NRM N tion characteristic of these rocks is provided by 400 the Pliensbachian date of andesite fl ows at site vc4 (ca. 191 Ma), and the range of dates in detrital zircons from the Todos Santos assemblage (ca. 196–161 Ma; Godínez-Urban et al., 2011). 500 The volcanic rocks are unquestionably of Early Jurassic age. The sites in detrital sedimentary Jo=0.72A/m rocks have maximum ages of deposition of ca. 580 161 Ma, but this age estimate is only based on Jo=0.36A/m a single zircon grain; a more conservative maxi- 620 E mum deposition age is provided by a cluster of E 620 six concordant zircon dates around 171 Ma. 580 The accepted site collection from La Ango- 500 400 stura Lake includes six sites of reverse polar- NRM ity and eleven of normal polarity. Normal and reverse polarity magnetizations are ~19° from Figure 4. Orthogonal demagnetization diagrams and equal area projection of sample direc- antipodality, but yield a positive (rank C) rever- tions from a volcaniclastic deposit, used for a conglomerate test. Symbols in demagnetization sal test (McFadden and McElhinny, 1990). diagrams as in Figure 3. For the equal area projection, solid (open) symbols are projections The data set for the collection of sites from La in the lower (upper) hemisphere. Angostura exhibits relatively high between-site dispersion (for the distribution of site mean directions, the angular standard deviation is 23.4°). in isolated exposures of andesite fl ows. AMS the increase in the precision parameter from 4.8 The dispersion may be attributed to the coarse data did not defi ne well-developed foliation (in situ) to 10 (tilt corrected) provides a positive scale of the structural correction, to uncertain- planes, thus we assumed that these fl ows have an regional tilt test. In addition, a primary origin of ties in the structural correction (including the attitude similar to that of the Cretaceous rocks of the remanence is supported by a positive con- presence of primary dips in volcanic rocks), and the San Ricardo Formation exposed to the east. glomerate test (site vc18 in lahar deposits), and also to tectonism (rapid plate motion or rotation This correction, however, fails to bring these sites into agreement with others to the south and N N N it is likely that our assumption is incorrect. A ABC similar situation occurred with sites vc10 and vc11, where we had assumed they have an orientation similar to the nearest site. Sites vc22 and vc29 have reasonably well-determined attitudes, but the data from them are interpreted as out liers possibly recording anomalous fi eld behavior (transitional fi elds or excursions). All but one of the sites in the Jericó member did not yield interpretable data. The remaining 17 estimated site mean directions defi ne, in situ, two principal groups of N N N directions. One group falls in the northwest DE F quadrant with negative shallow to steep inclinations, and the other in the southeast quadrant with positive inclinations. It is evident that this magnetization was acquired prior to folding of the sequence because the in situ directions are too steep for Jurassic or younger time (Fig. 5). A prefolding age of magnetization acquisition is supported, but not confi rmed, by a small increase in the precision parameter of the distribution of site means from 8.1 to 12 (not statistically signifi cant), and also by tilt-corrected shallow inclinations (~5°) consistent with the inferred near equatorial setting of the Gulf in Figure 5. Stereographic projection of site mean directions in the Todos Santos and La Silla For- Early to Middle Jurassic time. When the data set mations. (A) All sites in situ, (B) selected sites in situ, and (C) selected sites tilt-corrected. (D) and for the Todos Santos Formation in the Matías (E) Sites in the Matías Romero region (in situ and tilt-corrected), and (F) the Custepec dikes. Romero region is included in a regional tilt test, Open (closed) symbols are projections on the upper (lower) hemisphere.

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contemporaneous with deposition and magne- Equal Area tization acquisition). We note that there is more scatter in declination than in inclination (the average of inclinations is I = 4.3° ± 6.2, whereas declinations vary from 285.1° to 346.2°). This observation suggests the possibility of apparent tectonic rotation due to plunging fold axes, but structural data do not indicate appreciable plunge to any of the folds investigated. These complica- tions contribute to an increase in the scatter of site means, but probably not in a systematic way. We calculate a grand, structurally corrected mean for the Todos Santos and La Silla formations in the Angostura area of declination (Dec) = 325.0°, inclination (Inc) = 4.6° (Table 1; Fig. 5), which we interpret to refl ect (with small uncertainty; α 95 = 10.8°) the average Early to Middle Jurassic fi eld direction for the southwestern region of the Maya Block (i.e., the Chiapas Massif). The data set for the Custepec dikes is small. The overall mean is Dec = 332.0°, Inc = 20.2° (n = 6 sites, Table 1). Because the dikes intrude granitic rocks of the Chiapas Massif, there is no accurate reference to the paleohorizontal and we consider this result a less reliable estimate of the Jurassic fi eld direction for the region. We note, however, that the dikes at Custepec and along the Tablón River strike mostly SE (or NE) and dip steeply to the SW (or NW); dips are ~75° (Fig. 6). The deviation from vertical is consistent with tilt to the NNE, in a direction similar to that for nearby Cretaceous limestones of the Sierra Homocline. Using a cylindrical best-fi t Figure 6. Beta stereographic projection of dike orientation (black lines and red dots marking approach to all dike orientations, we calculated the corresponding pole). The best-fi t plane to poles is shown as a red solid line. The correction the correction that brings the dikes closest to required to bring the best-fi t plane to the vertical (pink line) is applied as structural correc- the vertical. The tilt correction of 108/0 (trend/ tion to the Custepec dike paleomagnetic mean (large circles, before and after correction). plunge) with 22° of tilt on the dike data brings the observed mean paleomagnetic direction to Dec = 335° and Inc = 5° (Table 1), which is closer to Maya Block is assumed to have shared plate ent polar wander. The fi rst issue arises from the observed mean of the Todos Santos red beds boundaries with North America, we compare in the contrasting paleomagnetic results from and El Diamante volcanic rocks. Finally, the Figure 7 the observed paleomagnetic poles with northeast and southwest North America (e.g., small data set for the Todos Santos Formation the North America reference apparent polar Van Fossen and Kent, 1990; Bazard and Butler, in Matías Romero, combined with previously wander path (APWP), after Besse and Courtillot 1991; Steiner, 2003). Middle Jurassic poles from published data (Molina-Garza et al., 1992), pro- (2002). This comparison allows us to estimate the northeast part of the continent lie at high lati- vides the means to test for the regional integrity relative motion between these crustal blocks. tudes, near the present-day North Pole, which in of paleomagnetic data. The mean for the Todos Actual calculations of rotation (R) and latitudi- turn predicts higher paleolatitudes for the NA Santos Formation in the Tehuantepec region is nal displacement (F)—and corresponding errors plate. In contrast, Early to Middle Jurassic poles Dec = 312.9°, Inc = 3.2° (n = 7 sites, Table 1). ΔF and ΔR—are based on Butler (1992). from the American southwest fall at latitudes The mean inclination in the Tehuantepec region An important caveat in our analysis is that any near 60°N, and in turn predict lower paleolati- is indistinguishable from the result obtained in comparison with the Jurassic APWP for North tudes. The effect of the reference poles from both the La Angostura area, and declinations differ America is complicated by continued contro- regions is smaller in the relative orientation of by only 12.1°. The data sets from La Angostura, versy over the reliability of Jurassic results for the plate with respect to the paleomeridian (and Custepec, and Matías Romero yield a mean the North American craton, in particular dur- thus expected declinations and rotation esti- paleolatitude of 2.1°N ± 3.4°. ing the Middle Jurassic, as well as controversy mates). The second point relates to contrasting Corrected locality mean directions were concerning the magnitude of rotation of the estimates of clockwise rotation of the Colorado used to calculate paleopoles and their associ- Colorado plateau, where many “reference” Plateau with respect to the craton (Molina-Garza ated errors (dp, dm), assuming a central sam- paleomagnetic data have been obtained. To et al., 1998; Kent and Witte, 1993; Steiner, pling location at 16.1°N–92.9°W (Angostura further complicate this analysis, the Jurassic is 2003; Kent and Olsen, 2008). These estimates area), 16.55°N–93.65°W (Matías Romero), and a time when there is rapid motion of the North range from ~4° to over 15°. The plateau has the 15.8°N–92.95°W (Custepec dikes). Because the American (NA) plate and a fast rate of appar- most complete stratigraphic record of the early

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Hall and Najmudin (1994) development of additional structures responsible 24°N 241.5°E Mv for shortening during the Late Cretaceous to Best fitting small circle 190 19°N 265°E 200 30 early Tertiary. It was found that strain estimates PH 170 60 ky 160 north and east of the plateau are consistent with 35° 90 60° 30° 35° 150 some 5°–8° of plateau rotation about a proximal 215 140 su 20° 25° Euler pole, or possibly a much smaller rotation 235 SR about a more distal Euler pole. Again, although CU a rotation appears to have affected the Colorado 250 sampling site Plateau, the magnitude of rotation is modest and TS generally difﬁ cult to document at a high level of conﬁ dence with paleomagnetic methods. 30°N MR The estimates of rotation for the Jurassic rocks we studied are summarized in Table 2. For the Angostura area, they range from ~7° to 240°E 38° (counterclockwise); only the latest Middle Jurassic pole for the Summerville provides a small estimate, but most of the sites in the Maya

210°E Block are in rocks older than middle Callovian. 120°E The Matías Romero result indicates, on aver- 180°E 150°E age, ~12° greater magnitude of rotation than the Angostura area result (range from 19° to 50°, but CM2 with greater uncertainty because the data set is CM smaller; Table 2). The observation of apprecia- MM ble counterclockwise rotation of Todos Santos Formation strata in the Tehuantepec region is most important because it implies that the rotation observed at Angostura is not a local effect of upper Mesozoic or Cenozoic tectonism, but Figure 7. Orthographic projection of paleomagnetic pole data for the Maya Block compared reﬂ ects broader scale regional motion of the with the North American apparent polar wander path (APWP) (heavy black solid line, with western Maya Block (Chiapas and the Tehuan- average pole estimates every 15 or 10 Ma) (Besse and Courtillot, 2002), and selected refer- tepec region). The ~35° counterclockwise rota- ence poles. Paleomagnetic pole data are summarized in Table 3. Dotted and dashed lines tion estimate is smaller than that derived from indicate the trajectory of rotated poles according to selected Euler poles. The trajectory of the paleomagnetic data for the Chiapas Massif Todos Santos pole is shown using as an Euler pole the best-ﬁ tting small circle to the Chiapas (Molina-Garza et al., 1992), suggesting that data. The position of the rotated pole is shown for rotations of 20°, 25°, 30°, and 35°. either some motion occurred between the Maya Block and North America prior to opening of the Gulf of Mexico (possibly during a Triassic rift phase), the Chiapas Massif moved indepen- Mesozoic in North America, therefore a large Besse and Courtillot (2002). The Kayenta paleo- dently of the Maya Block prior to Early Jurassic number of reference poles have been derived pole was restored assuming 7.4° rotation of the time, or a combination of these processes took from rocks from this region. Taking these facts Colorado plateau (Molina-Garza et al., 1998). It place. We explore these interpretations below. into consideration, we calculated estimates of R should also be noted here that Wawrzyniec et al. Smaller estimates of post–Early Jurassic and F based on the Pliensbachian to Callovian (2007) modeled the rotation of the plateau in an rotation of the Chiapas Massif area are obtained age of the magnetization in the Todos Santos evaluation of a hypothesis that requires exces- based on the mean direction of magnetization and La Silla formations. We make direct com- sively large right-lateral strike-slip displace- obtained from the Custepec dikes (range from parison with poles for the Pliensbachian Kayenta ment on the eastern margin of the plateau during 19.4° to 25.7°; a smaller value is obtained com- Formation (Bazard and Butler, 1991), late Callo- the Late Cretaceous to early Tertiary Laramide paring with the Summerville Formation). Sev- vian Summerville Formation (Steiner, 2003), Orogeny. Such offset, although unreasonably eral explanations are possible. One is that the the ca. 169 Ma Moat Volcanics from the White large (Woodward, 2000) was assumed to bal- dikes and thus their characteristic magnetiza- Mountains (Van Fossen and Kent, 1990), and the ance the concurrent shortening along the north- tions are younger than Callovian; for several of synthetic pole for North America for 170 Ma of ern margin of the plateau in association with the dikes only minimum ages can be determined

TABLE 2. ROTATION ESTIMATES FOR THE ANGOSTURA AND MATÍAS ROMERO REGIONS Rotations/Reference Pole R ΔRFΔFRΔRFΔF Angostura Matías Romero Van Fossen and Kent (1990) 78.7°N–90.3°E −34.4 12.9 −5.2 12 −46.3 26 −7.3 16 Besse and Courtillot (2002) 75.5°N–110.1°E −29.4 12.7 −0.8 11.6 −41.3 25 −3.2 15 Bazard and Butler (1991) 58.8°N–81.7°E −37.9 11.1 32.7 10.9 −49.6 24 31 14 Steiner (2003) 57.3°N–148.2°E −7.2 12.4 7.9 11.1 −18.7 24 3.7 14

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(Godínez-Urban et al., 2011). A second expla- ways. If regional tilt has affected the Massif, the 6.2°N, which are in excellent agreement with nation is that the area sampled along the Custe- magnitude is relatively small and it has a minor paleolatitudes observed in western Chiapas. pec road was affected by a more complex tilt effect on the pole position. If internal deforma- Steiner (2005) reported a paleopole for than that inferred from our analysis of dike ori- tion has affected the Massif this could explain Silurian intrusions in the Maya Mountains entations. A third explanation is that the data for the relatively high dispersion of site mean direc- that were affected by a major thermal event in the dikes are insuffi cient to average paleosecular tions (k = 17.9). Such deformation is unlikely the Early Triassic. The paleopole, located at variation. to bias the pole position in a systematic way 16.8°S–186.2°E, is statistically indistinguish- Four sites established in host rock sampled because the paleomagnetic sites are widespread able from the Chiapas Massif pole. We interpret in the same general locality as the Custepec over nearly the entire length of the Massif. Fig- this observation to suggest that the Massif and dikes, and the results from a single site reported ure 7 shows the Massif pole positions with and the Maya Mountains were already part of the by Molina-Garza et al. (2009), provide inter- without tilt correction. Maya Block and that some motion of the entire pretable data that allow us to revise the mean An additional consideration in interpreting Maya Block with respect to North America took Permian age paleomagnetic pole for the Chiapas the paleomagnetic data for the Chiapas Massif place during the protracted rifting phase, prior Massif (Table 3). The revised pole is located at is the possibility of a component of rotation that to the Late Jurassic drift phase and formation of α 12.3°S–178.6°E, with an 95 of 9.2°. Molina- may have occurred in late Cenozoic time. Defor- ocean fl oor in the deep Gulf. This motion has Garza et al. (1992) argued that paleomagnetic mation associated with two structural elements been suggested by other authors (Mickus et al., directions from the Chiapas Massif should be adjacent to the Massif may have contributed to a 2009), and would account for the inferred dif- corrected for the regional tilt of Todos Santos small Cenozoic rotation. These are the Polochic- ferential rotation with respect to North America and San Ricardo strata in the Sierra Homocline Motagua sinistral strike-slip fault system to the for the Chiapas Massif (~70°) and for the Todos province. Their argument is based on the fact south and the Chiapas foldbelt to the east. In Santos-La Silla formations (~35°). that Mesozoic strata directly overlying the gra- particular, shortening within the foldbelt may be The possibility of post–latest Paleozoic– nitic rocks of the Massif are tilted by some 12° attributed to a small counterclockwise rotation of Triassic relative motion between the Chiapas to the northeast. This correction is supported by the Massif about a local vertical axis that pro- Massif and the rest of the Maya Block is not our analysis of dike orientations in the Custe- duced northeast-directed contraction. This rota- fully resolved. Above we noted that the con- pec and Tablón river areas, but for consistency tion may account for part of the difference in the cordance of paleomagnetic pole positions for we used the regional tilt of Cretaceous strata for magnitude of rotation between the Massif and the Maya Mountains and the Chiapas Massif the Homocline. Correction for this regional tilt the Jurassic rocks studied, but only if this rota- could be interpreted to suggest minimal dis- brings the pole to 10.6°S–184.2°E. The revised tion was accommodated by faults that did not placement between these crystalline terranes pole is not signifi cantly different from the paleo- affect the Sierra Homocline. Additional paleo- that today are included as parts of the Maya pole previously published, but the larger data magnetic data for younger rocks in the region are Block. However, we identify two lines of evi- set (n = 16 sites) improves the precision of the necessary to evaluate this hypothesis. dence that support appreciable relative displace- pole determination. Our fi eld observations in Latitudinal displacement between the Maya ment between the Chiapas Massif and the rest the Angostura area suggest that internal defor- Block and North America is more diffi cult to of the Maya Block in pre-Jurassic time. One is mation of the Massif may be more complex evaluate, because unlike declination anoma- the possibility of a tighter fi t of the Maya Block than a modest regional tilt to the northeast. This lies, which are similar for the different refer- against the northern Gulf margin if the Massif simple relationship was observed in the Cin- ence poles used, inclination anomalies vary was located elsewhere. As mentioned earlier, talapa region, but in the Angostura region we signifi cantly using different cratonic reference other authors have proposed a different location observed depositional contacts between crystal- poles. Nonetheless, it is useful to compare the for the Massif. The other considers the relation- line rocks of the Massif and Jurassic stratifi ed observed paleolatitudes for Jurassic rocks of the ship between the Massif and the East Mexico rocks, a normal-fault contact between rocks of Maya Block with those observed in northeast arc (Dickinson and Lawton, 2001). Restora- the Massif and the Jericó Member of the Todos Mexico. The available data for the Jurassic La tion of the Maya Block, into the northern Gulf Santos Formation, a strike-slip fault contact Boca Formation of the Huizachal Group in the region, results in the position of the arc rocks of between rocks of the Massif and folded strata Sierra Madre Oriental (Gose et al., 1982) indi- the Chiapas Massif far removed from an associ- of the Todos Santos Formation, as well as an cate signifi cant vertical-axis rotation between ated trench. As Figure 1 shows, this is evident outlier of the Massif along the strike-slip fault localities along the Huizachal-Peregrina uplift. in models proposed by Ross and Scotese (1988) parallel to the Angostura Lake. Because the declinations observed at some and Pindell and Kennan (2009) but only if the We are cautious about the level of certainty localities are nearly east-west and inclinations Massif is attached to the Maya Block (some- in our interpretation of the paleopole for the are shallow, polarity interpretations are ambigu- thing not assumed in either of those reconstruc- Chiapas Massif, but the paleomagnetic result ous. Using different polarity interpretations we tions). The relationship of the Massif having an from these rocks is still important in several calculated paleolatitudes between 2.3°N and east-dipping subduction system under western

TABLE 3. PALEOMAGNETIC POLES FOR THE MAYA BLOCK α Unit Symbol Age Pole N(n) 95 Reference San Ricardo Fm. SR Upper Jurassic 69.8°N–160.0°E (84), 3.0° Guerrero et al. (1990) Paso Hondo-Grupera PH Middle Jurassic? 74.2°N–95.4°E 4, 13° Gose and Sanchez (1981) Todos Santos-La Silla TS Early-Mid Jurassic 52.9°N–158.9°E 17, 9.0° This study Todos Santos-Matias Romero MR Early-Mid Jurassic 41.3°–163.5°E 7, 23.4° Molina Garza et al. (1992) combined with this study Maya Mountains plutons MM Triassic? 16.8°S–186.2°E (42), 4.3° Steiner (2005) Chiapas massif CM Permian 12.3°S–178.6°E 16, 9° Molina Garza et al. (1992) combined with this study

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Pangea favors a trench confi guration similar San Ricardo Formation (Guerrero-García fi t small circle, however, includes the poles to that implied in the model of Dickinson and et al., 1990). This unit has been assigned a proposed by Hall and Najmuddin (1994) and Lawton (2001); that is, a trench closer to the Late Jurassic–Early Cretaceous age. However, Pindell and Kennan (2009). eastern Mexico Tampico Block—even if other Guerrero-García et al. (1990) centered their An alternative way of considering the evo- details of the reconstruction by Dickinson and paleomagnetic sampling on the lower carbon- lution of the Gulf region is by keeping North Lawton (2001), including the position of south- ate or Palo Grande Member of the San Ricardo America fi xed (Fig. 8). We place a trench with a ern Mexico, remain unsolved. Formation (Quezada-Muñetón, 1983). Accord- confi guration similar to that proposed by Dickin- Restoration of some 35° of counterclockwise ing to Alencaster (1977), San Ricardo strata son and Lawton (2001), which closely parallels rotation of the Maya Block—a result derived contain Kimmeridgian-Portlandian molluscs the trend of the Nazas arc of northern Mexico, from averaging rotations estimated from the and brachiopods at a locality 20 km east of Cin- and then turns abruptly south. We place the Custepec, La Silla, and Todos Santos mean talapa. According to Quezada-Muñetón (1983), northern margin of South America according to directions—about an Euler pole on the eastern the Palo Grande Member may extend into the Pindell and Kennan (2009). We then reconstruct Gulf (24°N–81.5°E; Hall and Najmuddin, 1994) Oxfordian. Buitrón (1978) reports echinoderms the Maya Block according to different Euler brings into alignment volcanic rocks of the La known from the Kimmeridgian in Europe. poles. Although a defi nitive and robust single Silla Formation with the WNW trend of the Finally, Michaud (1988) reports Kimmeridgian Euler pole is not attainable, it appears that Euler Nazas arc of northern Mexico (Godínez-Urban dasycladales from limestones of the San poles in the eastern Gulf—near western Cuba— et al., 2011). A similar rotation brings the La Ricardo Formation. Therefore paleomagnetic best fi t the requirement of reconstructing the Silla-Todos Santos paleopole for the Angostura and biostratigraphic data suggest that rotation Massif at a reasonable distance from the trench, region into coincidence with the Early-Middle ended by ca. 151 Ma (end Kimmeridgian) and and the apparent polar wander path defi ned by Jurassic segment of the North American APWP thus the episode of rotation is estimated to be ca. data for the Maya Block. This path requires an (Fig. 7). The currently available paleomagnetic 10 Ma in duration. If the Massif and its Juras- Euler pole close to the Maya Block. data suggest that ~35° of counterclockwise rota- sic cover are restored to offshore Tamaulipas, tion is an accurate estimate of the rotational displacing volcanic rocks of La Silla Formation CONCLUSIONS component of motion of the Maya Block during ~800 km, the rate of slip along the Tamaulipas- the Late Jurassic drift phase of the Gulf opening. Golden Lane-Chiapas transform is ~8 cm/yr. Volcanic rocks of the Lower Jurassic La Silla Greater precision in reconstructing the Maya This value is somewhat rapid, suggesting that Formation and detrital sedimentary rocks of Block is diffi cult to afford with the available rotation may have initiated slightly earlier or the overlying Todos Santos Formation in west- paleomagnetic data from the Chiapas area, as fi nished slightly later. central Chiapas were deposited on Permian crys- well as the uncertainty in the North America ref- The consequence of employing different talline rocks of the Chiapas Massif. The ChRM erence poles. Our database provided by the most Euler poles for rotation of the Maya Block is in these rocks is northwest directed and of shal- robust results includes mean directions from 3 explored in Figures 7 and 8. Rotation about low inclination (~5°). Dual polarity magnetiza- sites in volcanic rocks, and 14 in the overlying an Euler pole describes the relative motion tions, a conglomerate test, and a regional tilt test red beds assigned to the Todos Santos Forma- between two areas on a sphere—North Amer- all suggest that the characteristic magnetization tion; one of those sites is in the Concordia facies ica and the Maya Block in this case. The motion of these rocks is primary. Both volcanic rocks (uppermost Todos Santos; Godinez-Urban et al., of the Maya Block with respect to North Amer- and red beds have similar shallow inclinations. 2011). Given that the data set is of insuffi cient ica is equally expressed by the motion of the The ChRM of Todos Santos strata in the Tehuan- quality to separate results from volcanic and paleomagnetic poles for the Maya Block with tepec Isthmus and mid-Jurassic dikes that intrude sedimentary rocks, we attempted a parametric respect to the North American apparent polar crystalline rocks of the Chiapas Massif are also simulation. The inclinations of volcanic rocks wander path, and this motion describes a small northwest directed and shallow. The paleomag- and sedimentary rocks are indistinguishable, but circle on the sphere. The Euler pole proposed netic results from these rocks are discordant declinations of the sedimentary rocks (~322°) by Hall and Najmuddin (1994), for example, with respect to the North America APWP, and indicate a slightly greater rotation than the simu- restores the Todos Santos-La Silla pole along indicate that the Maya Block has rotated in a lated declination from volcanic rocks (~330°). a circle in the direction of the low-latitude counter clockwise sense; our best estimate is Based on maximum deposition ages for the Jurassic segment of the APWP, toward the ~35° since ca. 161 Ma, and it is likely that most red beds, we interpret this as an indication that Kayenta cratonic reference pole. The Euler of this rotation occurred over a time span of ca. most of the rotation occurred after ca. 171 Ma, pole suggested by Ross and Scotese (1988) 10 Ma. Some of the uncertainty in the estimate of or 161 Ma if a single zircon age determination is rotates the Todos Santos-La Silla pole into a the magnitude of rotation is due to the poor defi - used as an age estimate (Godínez-Urban et al., position intermediate between the low-latitude nition of the Early to Middle Jurassic segment 2011). This interpretation is consistent with and high-latitude reference pole alternatives. of the North America APWP. Regardless, the stratigraphic constraints (Maya Block rotation We note that the paleomagnetic data from the timing and magnitude of rotation are both con- postdates Callovian salt deposition; Pindell and Chiapas Massif area, Matías Romero area, and sistent with the proposed post-Callovian opening Dewey, 1982; Pindell, 1985; Pindell et al., 2006; Todos Santos-La Silla formations approximate of the Gulf of Mexico, and a rotational origin for Salvador, 1987). Based on the available data, we a small-circle, the best fi t of which defi nes an the Gulf. Rocks of the Chiapas Massif and the cannot assess if some Todos Santos strata, the Euler pole at 19°N–265°E. This Euler pole, Maya Mountains record a greater magnitude of upper Jericó Member for example, were depos- however, does not provide a satisfactory fi t counterclockwise rotation (~70°), which in turn ited when rotation was already under way. An for the Maya Block in the interior of the Gulf suggests that prior to the drift phase of the Gulf end-Callovian age of 161 Ma is used in Walker region because it places the Maya Block too of Mexico opening, relative motion between the and Geissman (2009). far to the south in the Gulf region, in an area Maya Block and North America (or between Rotation, however, was apparently com- that was occupied by South America. The con- Chiapas and North America) occurred during a pleted by the time of deposition of the lower fi dence error in the determination of the best- protracted rifting phase in the Triassic.

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105° Paleo-Pacific Ocean Paleo-Pacific Paleo-Pacific Ocean Paleo-Pacific in Chiapas (arrow marked +2°) and the range of paleolatitudes for the Huizachal group in Tamaulipas (arrow marked +2° to +6°). (arrow Tamaulipas in the Huizachal group marked +2°) and the range of paleolatitudes for in Chiapas (arrow additional explanations. text for on the reference pole of Besse and Courtillot (2002). Euler poles are indicated as squares. In all frames, we show the paleolat indicated as squares. poles are pole of Besse and Courtillot (2002). Euler on the reference Figure 8. Reconstructions of the Maya Block according to different authors. (A) Best-ﬁ 8. Reconstructions of the Maya Block according to different Figure Najmuddin (1994); (C) Marton and Bufﬂ

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ACKNOWLEDGMENTS in Bartolini, C., Wilson, J.L., and Lawton, T.F., eds., Molina-Garza, R.S., Van der Voo, R., and Urrutia-Fucugauchi, Mesozoic sedimentary and tectonic history of north- J., 1992, Paleomagnetism of the Chiapas Massif, This research was supported by a grant of the central Mexico: Geological Society of America Special southern Mexico; Evidence for rotation of the Maya Petroleum Research Fund of the American Chemical Paper 340, p. 1–58. Block and implications for the opening of the Gulf Society to Wawrzyniec and also by grant IN121002 Gose, W.A., and Sánchez-Barreda, A., 1981, Paleomagnetic of Mexico: Geological Society of America Bulletin, to Molina-Garza from the PAPIIT-UNAM program. results from southern Mexico: Geofísica Internacional, v. 104, p. 1156–1168, doi: 10.1130/0016-7606(1992)104 v. 20, p. 163–176. <1156:POTCMS>2.3.CO;2. We also thank the assistance in the fi eld by Linda Gose, W.A., Belcher, R.C., and Scott, G.R., 1982, Paleomag- Molina Garza, R.S., Acton, G.D., and Geissman, J.W., 1998, Donohoo-Hurley, as well as the Associate Editor and netic results from northeastern Mexico: Evidence for Carboniferous through Jurassic paleomagnetic data reviewers, whose comments improved the manuscript. large Mesozoic rotations: Geology, v. 10, p. 50–54, doi: and their bearing on rotation of the Colorado Plateau: 10.1130/0091-7613(1982)10<50:PRFNME>2.0.CO;2. Journal of Geophysical Research, v. 103, p. 24,179– REFERENCES CITED Guerrero-García, J.C., Herrero-Bervera, E., and Helsley, 24,188, doi: 10.1029/98JB02053. 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