Latest Paleozoic–early Mesozoic structures in the central Oaxaca Terrane of southern Mexico: deformation near a triple junction

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Tectonophysics 301 (1999) 231–242

Latest Paleozoic–early Mesozoic structures in the central Oaxaca Terrane of southern Mexico: deformation near a triple junction E. Centeno-Garcia Ł , J. Duncan Keppie Instituto de Geologia, Universidad Nacional Autonoma de Mexico, Ciudad Universitaria, 04510 Coyoacan, D.F., Mexico Received 25 March 1997; accepted 12 August 1998

Abstract Paleozoic rocks in the Oaxaca Terrane of southern Mexico occur as two outliers (Rio Salinas and Santiago Ixtaltepec) unconformably overlying the 1-Ga Oaxaca Complex. They consist of the Upper Cambrian–Lower Ordovician Tin˜u Formation, Mississippian Santiago Formation, lower–middle Pennsylvanian Ixtaltepec Formation and the unfossiliferous Yododen˜e Formation of presumed Permian or younger age. These Paleozoic rocks have been deformed by several sets of structures. Three moderately westerly-dipping, bedding-parallel shear zones displaying dextral kinematic indicators (C–S fabrics, curvilinear isoclinal folds, en-echelon boudinage, en-echelon veins, and strained amygdales) occur along the boundaries between the Tin˜u and Santiago formations, between the Santiago and Ixtaltepec formations, and within the Ixtaltepec Formation in the Santiago Ixtaltepec outlier. The bedding and these bedding-parallel shear zones are deformed by N–S, upright-asymmetric, subhorizontal folds accompanied by a slaty or spaced cleavage, that are in turn deformed by several sets of kink bands: subhorizontal and steeply dipping E–W, NW–SE and NE–SW. In the northern part of the Santiago Ixtaltepec outlier, the stratigraphy, bedding-parallel shear zones and slaty cleavage are displaced by NW–SE normal faults, all of which are truncated by the angular unconformity at the base of the Cretaceous, which brackets their age as post-Early Permian and pre-Cretaceous. Geometric correlation of the major N–S folds with E-vergent thrusting of the Oaxacan Complex over the Juarez Terrane suggests that they are older than Middle Jurassic. No age constraints are available for the kink and chevron folds; however, most may be related to Laramide structures in the overlying Cretaceous rocks. Unfolding the major structures in the Paleozoic rocks reorients the bedding-parallel shear zones to subhorizontal detachment shear zones or faults with a top-to-the-north sense of displacement. They may be related to either gravity sliding following amalgamation of Pangea, extensional rifting of the Yucata´n Peninsula from southern Laurentia or intra-arc rifting associated with a Permo–Triassic arc within southern Mexico. The major N–S fold structures and E-vergent thrusting of the Oaxacan Complex over the Jua´rez Terrane may represent a response to (1) convergent subduction of the Kula and=or Farallon plates beneath western Mexico, (2) migration of a Gulf of Mexico–Yucatan–South America triple junction.  1999 Elsevier Science B.V. All rights reserved. Keywords: Paleozoic; stratigraphy; Oaxaca; Mexico; tectonics; structural geology

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0040-1951/99/$ – see front matter  1999 Elsevier Science B.V. All rights reserved. PII: S 0 0 4 0 - 1 9 5 1 ( 9 8 ) 0 0 2 1 3 - 3

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1. Introduction Paleozoic rocks in the Oaxaca terrane of southern Mexico occur as small outliers unconformably overlying the 1 Ga Oaxacan Complex (Fig. 1). The Paleozoic rocks consist of uppermost Cambrian– lowermost Ordovician, Carboniferous and originally interpreted as Permian rocks that were believed to have been gently tilted prior to being unconformably overlain by Cretaceous rocks (Robison and PantojaAlor, 1968). Given the intense Devonian deformation recorded along the boundary between the Oaxaca and Mixteco terranes some 50 km to the west, and the Laramide folding and thrusting exhibited by the overlying Cretaceous rocks (Ortega-Gutı´errez, 1978), the absence of deformation in the Paleozoic rocks would appear to be anomalous. This absence might be explained if the Oaxacan Complex formed a buttress that shielded the Paleozoic rocks from deformation. This paradox prompted us to reexamine the Paleozoic rocks of the Oaxaca Terrane to see if they recorded some evidence of the Devonian and=or Laramide deformational events in their sedimentol-

ogy or structure. This paper presents the results of this reexamination and documents at least four deformational phases in the Paleozoic rocks, one of which developed before deposition of the Permian or younger rocks and two before deposition of Cretaceous rocks. The other phase is of uncertain age but appears to post-date deposition of the Cretaceous rocks. The recognition of two bedding-parallel shear zones within what was formerly regarded as an intact stratigraphic section is important in view of the global importance of this Paleozoic section to pre-Pangean Laurentia–Gondwana tectonic interactions, since it represents the only relatively complete and significant Paleozoic section in southern Mexico (e.g., Pantoja-Alor and Robison, 1967; Rowley and Pindell, 1989; Ortega-Gutı´errez et al., 1995; Keppie and Ortega-Gutı´errez, 1995).

2. Geological setting Mexico may be divided into several tectonic regimes (Fig. 1, inset). In northern Mexico, cratonic

Fig. 1. Map of the locations of Paleozoic rocks in the Oaxaca Terrane of southern Mexico (inset shows the main tectonic subdivisions of Mexico).

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North America is bordered on its southern side by the Ouachita Orogen (Moreno et al., 1993; Stewart et al., 1993). Numerous terranes occur to the south of the Ouachita Orogen (Fig. 1; Campa-Uranga and Coney, 1983; Sedlock et al., 1993; Ortega-Gutı´errez et al., 1995; Keppie and Ortega-Gutı´errez, 1995). The composite Neoproterozoic–Paleozoic Maya Terrane occurs around the Gulf of Mexico (Sedlock et al., 1993), and the 1 Ga Oaxaquia (the inferred subsurface extension of the Oaxacan Complex) forms the central backbone of Mexico (Ortega-Gutı´errez et al., 1995). Oaxaquia is bounded by (1) the early Mesozoic Juarez Terrane on its eastern side, (2) the upper Paleozoic Juchatengo Terrane on its southern side, and (3) the lower Paleozoic Mixteco Terrane on its western side. The western part of Mexico consists of a collage of Mesozoic terranes derived from the Pacific. The Mixteco and Juchatengo terranes were accreted to Oaxaquia during the Devonian and late Paleozoic, which, together with the Maya Terrane, were accreted to Laurentia in the late Paleozoic during the terminal stages of collision between North and South America (Yanez et al., 1991; Stewart et al., 1993; Ortega-Gutı´errez et al., 1995). The Juarez Terrane is a mylonitic complex interpreted as a thrust zone reactivated by dextral shearing during the opening of the Gulf of Mexico followed by normal brittle–ductile fault reactivation during Cenozoic uplift of the mylonite belt (Delgado-Argote, 1988; Centeno-Garcı´a et al., 1990; Alaniz-Alvarez et al., 1996). In southern Mexico, the 1-Ga rocks of the Oaxacan Complex are overlain by outliers of Paleozoic rocks at the Santiago Ixtaltepec and Rio Salinas areas in central Oaxaca State (Fig. 1). The oldest sedimentary unit preserved in the outliers is the Tin˜u Formation, which rests unconformably upon granulitic gneisses of the 1-Ga Oaxacan Complex. It consists of interbedded, marine limestone and shale grading upwards into predominantly shale and siltstone (Robison and Pantoja-Alor, 1968; Pantoja-Alor, 1970) (Figs. 2 and 3). Trilobites, brachiopods and conodonts in the Tin˜u Formation are of Tremadocian age and have closest faunal affinities with Gondwana: specifically South America and the Rhenish– Bohemian fauna of western, southern and central Europe (Robison and Pantoja-Alor, 1968; Sour-Tovar, 1990). The total thickness of the Tin˜u Formation

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ranges from 60–23 m in the Santiago Ixtaltepec outlier to 100 m in the type section at Rio Salinas (Fig. 2a and Fig. 3a; Pantoja-Alor, 1970; Sour-Tovar and Quiroz-Barroso, 1989). The upper clastic (siltstone and shale) member is thicker in the type section in Rio Salinas than in the Santiago Ixtaltepec area. In the Santiago Ixtaltepec outlier (Fig. 2a), the Tin˜u Formation is overlain by the 80-m-thick Santiago Formation comprised of a basal quartz-rich calcareous sandstone and some conglomerate, overlain by marine limestone, calcareous siltstone and shale containing brachiopods and crinoids of Mississippian age (Pantoja-Alor, 1970; Navarro-Santillan and Sour-Tovar, 1995). This unit is, in turn, overlain by the 500-m-thick Ixtaltepec Formation made up of shale, siltstone, sandstone, and minor limestone containing brachiopods, gastropods, molluscs, bryozoans, corals, trilobites, crinoids and trace fossils of early–middle Pennsylvanian age (PantojaAlor, 1970; Morales-Soto, 1984; Sour-Tovar and Quiroz-Barroso, 1989; Quiroz-Barroso and SourTovar, 1995). The contacts between the Tin˜u, Santiago, and Ixtaltepec formations were inferred to be unconformities by Pantoja-Alor (1970); however, this study indicates that they are both tectonic contacts (see below). The Ixtaltepec Formation is unconformably overlain by the >500-m-thick Yododen˜e Formation consisting of conglomerate, sandstone, siltstone, and minor shale which is unfossiliferous. Pantoja-Alor (1993) inferred that it ranges in age from late Pennsylvanian to middle Permian based on correlation with the Matzitzi, Los Arcos and Patlanoaya formations which extend into the Early Permian (Leonardian) (Corona-Esquivel, 1983; Villasen˜or-Martinez, 1987; Weber and Cevallos-Ferriz, 1994). However, clasts of limestone in the conglomerate contain fusullinids of Lower Permian age, suggesting that the formation might be Mesozoic (A. Flores de Dios, pers. commun.). Lower Cretaceous marine limestone and Tertiary red beds unconformably overlie the Paleozoic rocks in the Santiago Ixtaltepec area (Fig. 2a). Sills and dykes intrude the four formations but not the Cretaceous marine rocks, and only the pre-Yododen˜e sills have been affected by deformation. In the Rio Salinas outlier (Fig. 3a), the Tin˜u Formation is also overlain by Cretaceous and Tertiary rocks along the western edge of the outlier.

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Fig. 2. Map, cross-sections and structural data (plotted on lower hemisphere stereographic projections) of the Santiago Ixtaltepec Paleozoic outlier: (a) geological map (modified from Pantoja-Alor, 1970) and cross-sections of the outlier; (b) bedding plane poles (dots), and π-circles derived from NE and central and south sections; (c) minor structures associated with bedding-parallel shear zones: great circles represent average orientations of shear zones, dots are isoclinal fold axes, open circles with arrows are slip directions derived by plotting the lines in the shear planes that are perpendicular to the intersection between the C and S planes: arrows indicate relative motion viewed from above (see Fig. 4a), cross D mean long axis of amygdules in deformed sill in foliation relative to sill margins and bedding; (d) poles to slaty and spaced cleavage (ð) and slaty cleavage-bedding intersection lineations (dots); (e) mean kink band axial planes (great circles with asymmetry indicated by Z and S) and kink band fold axes (dots); (f) bedding plane poles in unconformably overlying Cretaceous rocks.

3. Structure Reexamination of the Paleozoic units in the two outliers reveals that they have been deformed by several sets of structures prior to the deposition of the Cretaceous rocks: (1) bedding-parallel shear zones; (2) N–S upright folding associated with a slaty or spaced cleavage; and (3) NW–SE faulting.

Several sets of later kink bands and chevron folds of uncertain relative age also occur. 3.1. Shear zones In the Santiago Ixtaltepec outlier, two beddingparallel shear zones are preferentially developed in the shaly horizons along the boundaries between the

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Fig. 3. Map, cross-section and structural data (plotted on lower-hemisphere stereographic projections) of the Rio Salinas Paleozoic outlier. (a) Geological map (modified from Pantoja-Alor, 1970). (b) W–E cross-section. (c) Bedding plane poles (dots) and π-circles derived from minor folds and from cross-section. (d) Poles to slaty and spaced cleavage (ð), bedding-cleavage intersections and associated minor folds (dots), cleavage fans (dashed lines), and mean fold axial planes (solid lines). (e) Mean kink band axial planes (great circle lines) and kink fold axes (dots). (f) Poles to faults (dots) and dikes (ð).

Tin˜u and Santiago formations, and between the Santiago and Ixtaltepec formations (Fig. 2a). The lower shear zone is located in shale in the uppermost part of the Tin˜u Formation and is bounded at the top by quartz-rich arenaceous limestone of the Santiago Formation. The middle shear zone deforms shale containing thin limestone and sandstone horizons intruded by a sill in the upper part of the Santiago Formation and is capped by shale and sandstone of the Ixtaltepec Formation. At the southern end of the outlier, the middle shear zone cuts downwards across the Santiago Formation and places the Ixtaltepec Formation directly upon the Tin˜u Formation. A minor shear zone occurs within the upper part of the Ixtaltepec

Formation. All shear zones place younger rocks over older, and may partly explain missing stratigraphy between the various formations, that was previously attributed to hiatuses (Pantoja-Alor, 1970). Shear zones were not found in the Yododen˜e Formation. In the Rio Salinas outlier (Fig. 3a), the formation is absent, but no shear zone fabrics were observed. These shear zones are characterized by a variety of minor structures. Anastomosing shear zones and C–S fabrics are well developed in the shaly lithologies and indicate subhorizontal dextral movement in moderately westward-dipping shear zones (Fig. 2c, Fig. 4a and Fig. 5a). In the middle shear zone, minor isoclinal folds are developed in the shale, thin

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Fig. 4. Minor structures observed in the Paleozoic outliers. (a) Diagram of bedding-parallel shear zones containing isoclinal folds, sill with deformed amygdales, and C–S fabrics showing C–S intersections and method of deriving and stereographic plotting slip direction on the shear plane. (b) Upright, subhorizontal folds associated with a refracted slaty and spaced cleavage.

Fig. 5. Photograph of minor structures within a bedding-parallel shear zone in the Paleozoic rocks.

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limestone and sandstone layers (Fig. 4a). Their axial planes lie parallel to the shear zone, and although their folds axes vary in orientation within the shear zone plane most are subhorizontal suggesting that they have been rotated towards the slip direction (Fig. 2c). A rhyolitic sill within the middle shear zone is cleaved indicating that it was intruded before the shear zone deformation. This cleavage is oriented anticlockwise of the sill margins and the beddingparallel shear zones indicating dextral shearing also (Fig. 4a). The sill contains deformed amygdales that are flattened in the plane of the cleavage (Fig. 4a). On the cleavage, the orientation of the amygdale long axes plunges gently to the north roughly parallel to the slip direction in the host shear zone (Fig. 2c). In a plane perpendicular to the cleavage but containing the mean amygdale long axis, they have a mean aspect ratio of 4 : 1. Locally, en-echelon veins and en-echelon boudinage of relatively competent limestone and sandstone horizons were observed (Fig. 4a), both of which confirm the dextral sense of displacement. 3.2. Major folds and associated structures The shear zones and the bedding in the Paleozoic rocks are folded by generally N–S, subhorizontal, steeply inclined folds associated with an axial planar slaty or spaced cleavage (Fig. 2b, d, and Fig. 3b– d). This folding produced the major structure in the Paleozoic rocks. Slaty cleavage is also present in the Yododen˜e Formation. In the Santiago Ixtaltepec outlier, they represent the western limb of a major anticline (Fig. 2a); the eastern limb is not exposed. In the Rio Salinas outlier, the Paleozoic rocks are folded into a major syncline (Fig. 3a, b). The slaty cleavage is moderately to steeply inclined to the west (Santiago Ixtaltepec: Fig. 2d) or to both the east and west (Rio Salinas: Fig. 3d). Cleavage refraction is present where sandstone and limestone horizons are interbedded with the shales, and produces cleavage fans about generally subhorizontal axes (Fig. 2d, Fig. 3d, Fig. 4b and Fig. 5d). Bedding=cleavage intersections are parallel to associated minor fold axes and are oriented generally subhorizontal to gently plunging to the NW–SE to NNE–SSW in the Santiago Ixtaltepec outlier (Fig. 2d), or NNW to NE in the Rio Salinas outlier (Fig. 3d).

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3.3. Kink bands The Tin˜u, Santiago, and Ixtaltepec formations are also deformed by several sets of kink bands. In the Santiago Ixtaltepec outlier, steeply inclined, Z-shaped NE-trending and S-shaped NW-trending kink bands (Fig. 2e) appear to be conjugate because they form chevron folds where they intersect (Fig. 5e). Their combined orientation indicates that they were produced during N–S shortening. On the other hand, steeply inclined kink bands in the Rio Salinas outlier are NW- and W-trending: their relative age relationships were not observed (Fig. 3e). Also present in both outliers are a set of subhorizontal kink bands that were not observed intersecting the steeply dipping kink bands (Fig. 2e and Fig. 3e). Kink bands were not observed in the Yododen˜e Formation. 3.4. Faults At the northern end of the Santiago Ixtaltepec outlier, the stratigraphy, the two shear zones and the slaty cleavage are cut by three NW-trending normal faults (Fig. 2a). A NE-trending normal fault also occurs between the southern two NW-trending faults (see cross-section A–B in Fig. 2a). The NW-trending faults, along with the shear zones, the western limb of the major anticline, and the slaty cleavage are overlain by an angular unconformity at the base of the Cretaceous rocks. In the Rio Salinas outlier, several N–S faults displace the Paleozoic rocks and obliquely cut across the major syncline (Fig. 3a and b). Mafic dikes have been intruded along these faults and they display both intrusive and faulted contacts. No minor structures were recorded in these faults. 3.5. Folds in unconformably overlying Cretaceous rocks Cretaceous calcareous shale and massive limestone overlie the Paleozoic rocks with angular unconformity (Fig. 2a and Fig. 3a). Bedding measurements in these rocks adjacent to the Santiago Ixtaltepec Paleozoic outlier indicate that they have been gently folded about upright N- and E-trending folds (Fig. 2g). The parallelism of the NW folds in

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the Paleozoic and Cretaceous rocks suggests that the anticline in the Paleozoic rocks may be made up of two components: pre- and post-Cretaceous. 3.6. Unfolding In order to understand the kinematics of the three shear zones, it is necessary to unfold the major N–S fold in the Paleozoic rocks of the Santiago Ixtaltepec outlier because it post-dates the shear zone development. This was accomplished in a two-stage process: unfolding N–S folds in the Cretaceous rocks which decreased the westward dip of the Paleozoic rocks, followed by flexural unfolding about the major N–S folds to reduce the bedding to horizontal. As the three shear zones are essentially parallel to the bedding, this results in horizontal shear zones with top-to-the-north kinematics. In this light, it appears that the three shear zones become detachment shear zones or faults. The younger-over-older structural sequence is consistent with a north-dipping extensional faulting transporting rocks northwards. 3.7. Correlation and age of structures A younger limit on the age of the three shear zones, the major N–S folds and associated cleavage, and the NW-trending normal faults is provided by the fact that they are all truncated by the angular unconformity at the base of the Cretaceous rocks. The absence of shear zones in the Yododen˜e Formation suggest at least a Late Permian age for the detachment faults. An older limit for the cleavage is provided by the age of the youngest deformed unit, the Yododen˜e Formation which may be Permian to Jurassic–Cretaceous. Further constraints on the age of the structures may be provided by their regional context. Although the structures to the west along the boundary between the Oaxaca and Mixteco terranes are inferred to be Devonian (Sedlock et al., 1993), similar Permian–Cretaceous structures have been reported to the east along the Oaxaca and Juarez terrane boundary (Alaniz-Alvarez et al., 1996). These authors reported that this latter terrane boundary represents a reactivated shear zone that was initially an E-vergent thrust, which was reactivated by ductile dextral shear and then by brittle normal fault-

ing. A stratigraphic younger limit on the ductile mylonitiziation is provided by the presence of mylonite clasts in Valanginian (Lower Cretaceous) marine sediments (Alaniz-Alvarez et al., 1996). They dated the San Felipe granite and associated granitic strips, which were inferred to have been intruded during the dextral shear deformation, at 165 š 20 Ma (U–Pb lower intercept on zircon) and 172 š 2 Ma (40 Ar=39 Ar plateau age of muscovite defining the mylonitic foliation). These data place the time of ductile dextral deformation above 400ºC as close to the Lower–Middle Jurassic boundary, which also provides a younger limit on the time of eastward thrusting. However, more accurate dating in the area is needed to better constrain the age of the different deformational events. Comparison of the structures in the Paleozoic outliers with those recorded along the boundary between the Oaxacan Complex and the Juarez Terrane provides just one obvious correlation: the major N–S folds and associated cleavage development may have been synchronous with the E-vergent thrusting because both indicate E–W shortening. Such a correlation would further constrain the time of N–S folding between the Early Permian and Middle Jurassic. If this correlation is correct, then the NW- and NE-trending normal faulting in the Santiago Ixtaltepec outlier may be related to the Early–Middle Jurassic dextral movements in the Juarez Terrane. These constraints also apply to the age of the detachment faults. Definitive age brackets for the various kink bands are not available. The NW-trending kink bands in the Paleozoic rocks are geometrically parallel to NW folds in the overlying Cretaceous rocks (cf. Fig. 2f and Fig. 3e). Also the N–S shortening inferred to have produced the conjugate E–W-trending kink bands in the Paleozoic rocks may also have produced the E–W folds in the Cretaceous rocks. The subhorizontal kink bands in the Paleozoic rocks were not observed in surrounding areas; however, they may represent a response to Laramide thrusting that is developed elsewhere in the region (Ortega-Gutı´errez et al., 1992). It may be significant that structural evidence associated with the inferred Devonian collision between the Mixteco and Oaxaca terranes was not found. However, the present study cannot rule out the possibility of an hiatus in the Paleozoic outliers. Future

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work may clarify the boundary relationships between these two terranes.

4. Tectonic context 4.1. Palinspastic reconstruction To understand the significance of the Upper Permian–Lower Jurassic structures in the Paleozoic outliers on the Oaxacan Complex, it is necessary to locate them on palinspastic reconstructions in the extant tectonic context (Fig. 6). The location of southern Mexico in a Late Permian–Triassic Pangea reconstruction is uncertain due to the overlap of South America with southern Mexico and Central America (e.g. Bullard et al., 1965). This has generally been accommodated by either: (1) sliding southern Mexico and the Chortis block westwards along the Trans-Mexican Volcanic Lineament and the Motagua–Polochic fault zone, respectively (e.g. Pindell, 1985; Golonka et al., 1995), and rotating the Yucatan Peninsula and the Chiapas Massif into the Gulf of Mexico (Fig. 6) (Molina-Garza et al., 1992; Schouten and Klitgord, 1994); or (2) assuming that southern Mexico and the Chortis block were not accreted until the Mesozoic (e.g. Ortega-Gutı´errez et al., 1994). Paleomagnetic data appear to support the first hypothesis (McCabe et al., 1988). Thus, paleomagnetic data from the Paleozoic rocks of the Oaxaca Terrane indicate Late Permian to Early Cretaceous remagnetization, but at paleolatitudes compatible with its present location relative to North America, although between zero and 28º of subsequent anticlockwise rotation may have occurred (McCabe et al., 1988). This is consistent with Jurassic paleomagnetic data from the neighbouring Mixteco Terrane which indicate a similar paleolatitude but with a subsequent <30º clockwise rotation relative to North America (Fang et al., 1989; Ortega-Guerrero, 1989). The recent extension of the 1-Ga basement (named Oaxaquia) across the Trans-Mexican Volcanic Belt into northern Mexico possibly as far as the western extension of the Ouachita Orogen (Ortega-Gutı´errez et al., 1995; Keppie and Ortega-Gutı´errez, 1995) is consistent with Late Paleozoic accretion of Oaxaquia and Laurentia and with Carboniferous faunal provinciality data (Stewart et al., 1993). The 1-Ga

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basement age in the Chortis block of Honduras supports extension of Oaxaquia southwards across the Motagua–Polochic fault zone (Manton, 1996). Most authors reconstruct the Yucata´n block by rotating it through 60º and into the space between North and South America along the Tamaulipas Fault–Jua´rez mylonitic complex–Chiapas Fault (Fig. 6) (Pindell, 1985; Padilla-Sanchez, 1986; Ross and Scotese, 1988; Rowley and Pindell, 1989; Molina-Garza et al., 1992; Schouten and Klitgord, 1994; Alaniz-Alvarez et al., 1996). Opening of the Gulf of Mexico is inferred to have begun with rifting in the Late Triassic followed by rotation of the Yucata´n block and active basin formation in the Gulf of Mexico during the Middle and Late Jurassic (Salvador, 1991). Such a rotation is consistent with Late Permian–Jurassic paleomagnetic data from the Chiapas area, which suggests that the rotation to the present position was essentially complete by Late Jurassic times (Molina-Garza et al., 1992). 4.2. Tectonic setting of Paleozoic outliers in the Oaxaca Terrane On these late Paleozoic to early Mesozoic reconstructions, Mexico occupies a position near the triple junction between the Kula and=or Farallon plate, the North American plate and the South American plate that is characterized by three major tectonic settings: amalgamation and breakup of Pangea, and supra-subduction zone (Fig. 6). Final amalgamation of Pangea appears to have occurred in Late Carboniferous–Early Permian times and is recorded by development of the Ouachita Orogen. However, deposition of Late Carboniferous–Early Permian red beds and shallow marine carbonates in the Maya Mountains and the Yucatan Peninsula, southern Mexico and the Central Cordillera and Maracaibo block of Colombia (Rowley and Pindell, 1989) suggest that these areas were low lying. Breakup of Pangea began with rifting in the Late Triassic and opening of the Gulf of Mexico beginning in the Middle Jurassic and ending in latest Jurassic time (Pindell, 1985; Moran-Zenteno et al., 1988; Ross and Scotese, 1988; Molina-Garza et al., 1992; Schouten and Klitgord, 1994). The opening of the Gulf of Mexico might have required the development of a triple junction, in or-

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der to accommodate the Yucatan block, North and South American plates. Approximately orthogonal rifting along the E–W-trending southern margin of Laurentia becomes mainly dextral strike-slip along the NNW-trending western margin of the Gulf of Mexico. Thus, the change from an extensional to a compressional=strike-slip scenario in southern Oaxaquia might be the result of the migration of this hypothetical triple junction toward the south as the Gulf continued to open. At least two periods of east-dipping subduction of the Kula and=or Farallon plates beneath Mexico are recorded from Late Permian to Jurassic time. The first was expressed in the Late Permian–Triassic through the backbone of Mexico as isolated granitic plutons (that pass close to the Paleozoic outliers in southern Mexico), and into the Central Cordillera and Santa Marta block of Colombia (Fig. 6) (Rowley and Pindell, 1989; Torres-Vargas et al., 1993). The second was recorded as Late Jurassic continental-arc volcanic and volcaniclastic rocks, but is only well defined in central-northern Mexico (Jones et al., 1995). In such a complex tectonic setting, the detachment faults may possibly be related to either gravity sliding following the formation of Pangea, rifting associated with the early stages of opening of the Gulf of Mexico, or during Permian–Triassic intra-arc rifting. On the other hand, the major N–S folds and associated cleavage in the Paleozoic outliers may be correlated with eastward overthrusting of the Oaxacan Complex over the Juarez Terrane and may be related to (1) contractional deformation between the Kula and=or Farallon plates and Pangea, or (2) migration of a triple junction associated to the opening of the Gulf of Mexico (Fig. 6).

Acknowledgements We are grateful for funding supplied by the DGAPA-Universidad Auto´noma de Me´xico (Project #IN101095), CONACYT Project 0255P-T9506 (El Complejo Oaxaquen˜o y el Bloque Chortis en las

Fig. 6. Paleogeographic reconstructions for (a) the Late Permian– Triassic and (b) the mid-Jurassic southern Laurentia, Mexico, Central and northwestern South America.

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reconstrucciones Paleogeograficas de Laurencia y Gondwana anteriores a Pangea) and the Institute of Geology, UNAM. Special thanks are due to Jose Luis Sanchez, Francisco Sour-Tovar, and Antonio Flores de Dios whose discussion, assistance in the field and with the paleontology greatly helped the paper. We also acknowledge Gabriel Valde´z Moreno for drafting one of the figures. This paper represents a contribution to International Geological Correlation Project #376 ‘Laurentia–Gondwana Connections before Pangea’ and the Institute of Geology, UNAM.

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