Paleogeography of the northern portion of the Mixteca terrain, southern Mexico, during the Middle Jurassic

Journal of South American Earth Sciences, Vol. 3, No. 4, pp. 195-211, 1990 Printed in Great Britain

0895-9811/90 $3.00+ 0.00 © 1991 Pergamon Press plc & Earth Sciencesand ResourcesInstitute

Paleogeography of the northern portion of the Mixteca terrain, southern Mexico, during the Middle Jurassic C. CABAI.I,ZRO-MIRANDA I, D. J. M O R A N - Z E N T E N O I,J. U R R U T I A - F U C U G A U C H I I, G. S I L V A - R O M O 2, H. B O H N E L ~,Z. J U R A D O - C H I C H A Y I,and E. C A B R A L - C A N O ~ tLaborateriode Paleomagnetismo y GeoflsicaNuclear,Institutede Geoflsica,Universidad Nacional Autrnoma de Mrxico,D. Coyoacan 04510 DF, M~xico; 2Divisi6nde Ingenieriaen Cieneiasde la Tierra, Facultad de Ingenieria,Universidad NacionalAutdnoma de Mrxico,D. Coyoac~n 04510 DF, Mrxico (received October 1988; accepted September 1990) Abstra©t---A preliminary paleogeographic reconstruction of the northern Mixteca terrain in southern Mexico is presented for the Middle Jurassic. The reconstruction is derived from combined analyses of spatial distribution of marine-continental Jurassic sedimentary units, identification of sediment source, and observations based on sedimentary indicators of environment and transport directions, as well as paleomagnetic and anisotropy of magnetic susceptibility (AMS) results. There is an overall agreement between the AMS magnetic fabric results and the sedimentary indicators of current directions and paleogeographic elements. The results suggest a coastline at the south-southwest portion of this terrain, a general transport of fluvial sediments to the south and southwest, and marine influxes from the south. A Pacific margin provenance is supported by the paleomagnetic results for the northern portion ofthe Mixteca terrain. Resumen---Se presonta una reconstrucci6n paleogeogr~ica preliminar, de la porci6n norte del terreno Mizteca, sur de Mrxico, para el Jur6sico Medio. La reconstrucci6n se deriva de an~klisis combinados que ineluyen, la dif~xibuei6n espacial de las unidades sedimentarias jur6sieas y el reconocimiento del carlctar marine o continental de los dep(mitos; identificaci6n de las fuentes de aporte de los sedimentos y observaciones de los indicadores sedimentarios que reflejan el ambiente y las direcciones de transporte; estudios paleomagnrticos y de la anisotropia de susceptibilidad magnrtica (AMS). En general se observa una concordancia entre la flbrica magnrtica rosultante de la AMS y las observacionos de los indicadores sedimentarios de direcciones de corriente y elementes paleogeogrrficos. El conjunte de los resultados sugiore una Hnea de costa en la porci6n sur-~suroeste de este terrenoo un transporte de sedimentos fluviales hacia el sur y suroeste preponderantemente e influjos marines provenientes del sur. Lo8 rosultados paleomagnrticos del norte del terreno Mixteca, sustentan una precedencia asociada con la margen del Oceano Pacifico.

INTRODUCTION AS PART of a long-term project to study the tectonic

evolution of southern Mexico, we have attempted a Middle Jurassic paleogeographic reconstruction of the northern Mixteca area (Fig. 1). This reconstruction is based on combined analyses of spatial distribution and characteristicsof continental and marine-continental Jurassic sedimentary units, identification of sediment source, observations of sedimentary indicators of environment and transport direction, and studies of paleomagnetism and magnetic fabrics. Petrofabric and magnetic fabric studies were the main paleocurrent indicators employed and were concentrated in the Middle Jurassic Tecomazflchil Formation. Southern Mexico is dommposed of several terrains with contrasting tectonicand stratigraphic histories (Fig. 1). The study area is situated in the Mixteca terrain, which is characterized by a Paleozoic metamorphic basement. At the eastern margin of the area, this basement is in tectoniccontact with the Precambrian metamorphic complex of the Oaxaca terrain (Figs. 1 and 2). To the south and southwest, the Mixteca terrain is limited by the Xolapa terrain (Fig. I), which is interpreted as the exposed roots of an eroded volcanic arc active during the Mesozoic. 195

Jurassic sedimentary units are well exposed over the study area, but considerable uncertainty exists concerning the major paleogeographic elements and their significance for the tectonic evolution of southern Mexico. For instance, the possibility exists that the coastline and polarity of sedimentation was to the north or northeast, toward the then developing Gulf of Mexico. However, t h e continental-marine distribution of deposits s u g gests that a Middle Jurassic marine connection may have existed to the south or southwest, toward the paleo-Pacific Ocean. This latter arrangement and the lack of volcanic influence in the Middle Jurassic sequences conflict with petrologic and tectonic characteristics of the Xolapa terrain exposed to the south and southwest (Fig. 1). These hypotheses have profound implications for understanding the paleogeographic and tectonic evolution of southern Mexico.

TECTONO-STRATIGRAPHIC TERRAINS OF SOUTHERN MEXICO The terrains of southern Mexico have distinct tectonic and stratigraphic histories (Campa and Coney, 1983). The documented tectonic boundaries and stratigraphic contrasts between some terrains suggest an allochtonous character.

C. CABALLERO-MIRANDA, D, J. MORAN-ZENTENO, J. URRUTIA-FUCUGAUCHI,

196

et ai.

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Mexico

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Fig. I. Schematic tectonic map showing the tectono-stratigraphic terrains in southern Mexico (after Campa and Coney, 1983): I, Guerrero; 2, Mixtoca; 3, Oaxaca; 4, Juarez; 5, Maya; and 6, Xolapa. The shaded square indicates the study area shown in Fig. 2.

The Oaxaca terrain has a Precambrian basement of Grenville affinity, with reported radiometric dates in the range 940-1080 Ma (Fries et al., 1962, 1966; Fries and Rinc6n-Orta, 1965). This basement consists of a basal anorthosite massif and a sequence of paragneisees, orthogneisses, charnockites, granulites, and banded gneisses (OrtegaGuti6rrez, 1981). Paleomagnetic results for the anorthosites, paragneisses, and charnockites give a paleolatitude compatible with a paleoposition close to the Grenville province in Ontario, Canada (Urrutia-Fucugauchi et al., 1986; Batlard et al., 1989). The Cambro-Ordovician sediments of the Paleozoic cover contain Tremadocian trilobites (Pantoja-Alor and Robinson, 1967) similar to those of the Olenid-Ceratopygid province of Gondwana, in contrast to those of the Highatetla province of North America (Whittinton and Hughes, 1974). Basement for the Mixteca terrain is the Paleozoic Acatl~n Complex, which consists of slates to migmatites, granites to ultramylonites, gabbros to eclogites, and a tectonized ophiolite (OrtegaGuti~rrez, 1981). Rb-Sr age determinations (RuizCastellanos, 1979, Cserna et al., 1980) point to Taconian and Acadian phases of deformation and metamorphism. The Oaxaca and Mixtuca terrains may have come into contact during Devonian or pre-Devonian times (Urrutia-Fucugauchi, 1984). Alternative interpretations place the time of accretion for these terrains as late as Early Cretaceous (e.g., RamirezEspinosa, 1984). Paleomagnetic analyses of the Acatltin Complex have not produced conclusive results, mainly because of its structural complexitiee and deformational/metamorphic history (Mor~m-Zenteno et al., 1986; Fang et al., 1989). To the south, the Oaxaca and Mixteca terrains are limited by the Xolapa terrain (see Fig. 1), which shows contrasting characteristics. This terrain has a WNW elongated shape, about 600 km long and 50-

150 km wide. Xolapa Complex rocks are mainly amphibolite facies orthogneisses, schists, migmatites, and some lower metamorphic-grade rocks (Cserna, 1965). The age of the Xolapa Complex has been a matter of controversy. Radiometric dates from Cambrian to Tertiary have been obtained from several materials analyzed with different methods (Csorna et al., 1962, 1974; Halpern et al., 1974; Guerrero-Garcia, 1975; Guerrero-Garcia et al., 1978; Murillo-Muf~ethn et al., 1986). Whole rock Rb-Sr and U-Pb determinations in widely d i s tributed orthogneisses have yielded both Jurassic (Guerrero-Garcia, 1975; Guerrero-Garcia et aL, 1978) and Early Cretaceous dates (Morlm-Zenteno et al., in prep.). The Xolapa Complex represents the roots of a deformed volcanic arc active during the Mesozoic - - most probably from Jurassic to Early Cretaceous times. The deformed rocks of this arc were intruded by some granite to granodiorite bodies during the rest of the Mesozoic and T e r ~ r y , and in places there are mafic dikes and pegmatitic veins. JURASSIC SYSTEM Description and Distribution of Rock Units

Jurassic sedimentary rock bodies are exposed in several isolatedlocalitieswithin the e~tern portion of the Mixteca terrain. In the southern and western portion of the study area (Tlaxiaco, TezoatIAn, and Olinala regions; see Fig. 2), the Middle Jurassic sequences comprise the upper part of the Cormuelo Group and the overlying Tecocoyun~ Group (Fig. 3). In the northeastern sector, Middle Jurassic rocks are represented by the Tecomazaehil Formation (Petlalcingo and Hugjuapan areas; see Fig, 2) and by other rock units of similar characteristics (Tecomatl#in and Totoltepec areas; see Figs. 2 and 4)~

The Mixteca terrain, southern Mexico: Middle Jurassic paleogeography 99 °

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Fig. 2. Distribution of Jurassic sedimentary units in the northern Mixtoca terrain• Note that predominantly continental sediments are exposed to the northeast (TecomazCtchilFormation, JmTz), whereas marine sediments crop out mainly to the SSW (Tecocoyunca Group, JmTy). Numbers correspond to the stratigraphic columns shown in Figs. 3 and 4; the boxed area is enlarged in Fig. 5.

The Upper Consuelo Group comprises the Cualac Conglomerate (Erben, 1956), of Bajocian age, which is a coarse oligomictic conglomerate that consists mainly of subrounded metamorphic-quartz cobbles and boulders. This conglomerate changes laterally to finer granulometry and consequently shows a lens-shaped geometry. The unit conformably overlies the Rosario Formation, from the Early Jurassic lower Consuelo Group, which in turn overlies the metamorphic Acatl&n Complex and, at Tezoatl4n, covers a volcanic unit possibly of early Mesozoic age (Mor4n-Zenteno, 1987)• In the area to the east of Olinal4, the Consuelo Group also covers a Mesozoic volcanic unit (Las Lluvias Ignimbrite), which in turn covers the late Paleozoic Olinai~

Formation (Fig. 4; Corona-Esquivel, 1981; Flores de Dios and Buitr6n, 1982). The overlying Tecocoyunca Group (Erben, 1956), which consists of the Zorrillo, Taberna, Sim6n, Otatera, and Yucuf~uti Formations, contains fluvial and marine sediments of early Bajocian to Callovian age. Dominant lithologies in the Tecoco-yunca Group are litharenitic sandstones and silt-stones, with interbedded levels of coal and shales; a thin conglomerate unit at the middle of the sequence (Sim6n Formation); and marine intervals of limestone and coquinas, especially in the upper part of the group. Most lithic components in the sandstones are metamorphic and seem to have been derived from the Acatl4n Complex. The average thickness

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Fig. 3. Stratigraphic columns for some areas in the Mixteca terrain where the Tecocoyunca Group is exposed (see Fig. 2): I~west of the OlinalA area; 2 east of the OlinalA area; 3, TezoatlA~ area; at~d 4, Tlaxiaco area. Values on the leftare stratigraphic thicknesses in meters.

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Fig. 4. Stratigraphic columns for some areas in the Mixteca terrain where the Tecomaztlchil Formation is exposed (see Fig. 2): 5, Tecomatl~n area; 6, Ixcaquixtla-Totoltepec area; 7, Petlalcingo area; 8, San Jos6 Ayuguila area; and 9, Huajuapan de Le6n-Yosocuta area. Values on the left are stratigraphic thicknesses in meters. Relative stratigraphic positions ofpetrofabric and/or magnetic fabric studies within the Tecomaz~chil Formation are shown in columns 7-9.

Otlolpec

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C. CABALLERO-MIRANDA, D. J~ MORAN-ZENTENO, J. URRUTIA-FUCUGAUCHI,

of the group is about 600 meters. The fossil record of the Tecocoyunca includes C y c a d o p h y t a plant remains (Wieland, 1913, 1914; Person, 1976; SilvaPineda, 1984) and marine pelecypods, gastropods, brachiopods, and ammonoids (Burckhardt, 1930; Erben, 1956; Alencaster, 1963). The ammonoid fauna contains, among other genera, Neuqueniceras, Eurycephalites, and Xenocephalites, which have Pacific affinities (Imlay, 1980; Westermann et al., 1984)~ The Tecomazfichil Formation is characterized by a sequence of continental sediments of Bajoclan(?) to Callovian age, composed predominantly of sandstones, silstones, and conglomerates (PerezIbarguengoita et al., 1965). The sandstones are mainly inmature litharenites and arkoses; they contain conglomeratic components and elongated quartz clasts and show cross stratification. Field and thin-section observations indicate that most lithic and quartz components are derived from metamorphic rocks, apparently from the Acatl~n Complex; other lithic components are derived from previous detritic sediments with a smaller proportion (<5%) from volcanic rocks. There is a coarse poorly sorted conglomerate unit at the base, in the area close to Petlalcingo, and several conglomerate beds in intermediate levels of the sequence. The conglomeratic member at the base is composed mainly of subangular quartz cobbles and boulders in a matrix-supported framework, while the intermediate conglomerate levels includes some clastsupported beds composed mainly of subrounded quartz. The 500 to 2000 meter thick Tecomazfichil Formation overlies the metamorphic Acatl~n Complex and a trondjhemitic intrusive in the Totoltepec area. This unit is followed conformably by the marine Oxfordian Chimeco Formation in Petlalcingo, and disconformably by other Jurassic or Cretaceous units in other places. The variable granulometry and poorly sorted nature of the Tecomaz~chil Formation suggest deposition by a fluvial system in a high relief area. The basal conglomerate in Petlalcingo corresponds to proximal alluvial fan facies, and the intermediate coarse sandstone members indicate channel and point bar deposits of a meandering fluvial system. The fine-grained sandstone and silstone intervals may be interpreted as flood plain deposits. In contrast, the finer granulometry of the Tecocoyunca Group reflects lower energy environments. The coal beds and marine levels indicate extensive palustrine deposits at plains close to sea level. Lowenergy conditions for the southern region may be also interpreted for Toarcian-Aalenian time (Consuelo Group), with a short interval of high energy represented by the Cualac Conglomerate, deposited a s alluvial fans over a contrasted relief.

Petrofabric Studies Petrofabric studies have been concentrated in the Tecomazfichil Formation in the area of Petlal-

et ai.

cingo/Huajuapan (Figs. 2 and 5). Sampling and field observations were carried out at different stratigraphic levels along three sections to the west of Huajuapan (one in the San Francisco Yosocuta area, and two in the San JerSnimo Silacoyoapilla area) and at other isolated localities. In all localities, megascopic measurements were made of long-axis orientation azimuths of clasts at the upper plane of stratification. Only at the San Francisco Yosocuta section and at localities 19 and 20-4 were thin-section slides parallel to stratification prepared for microscopic measurements and petrographic observations. Directions of clast imbrication and cross stratification were considered only in limited cases because of the restricted number of appropiate outcrops for such observations. Megascopic petrofabric measurements correspond to conglomeratic sandstone and conglomerate beds, and microscopic measurements correspond to coarse and fine-grained sandstone beds. Long-axis azimuths of clasts and grains were rotated to the horizontal and then plotted in rosette diagrams. Megascopic analyses were made in 24 localities for a total of 1993 measurements. Several localities were rejected because of the large dispersion of orientation data, thus leaving a total of 1030 useful measurements from 15 localities. Microscopic analyses were made on 16 samples from 8 localities, with 514 useful measurements after rejecting 2 samples that showed a large dispersion of orientation data. Megascopic and microscopic observations agree well at given stratigraphic levels. Clast and grain orientations are either subparallel at some sites (e.g., sites 6 and 19) and almost perpendicular at other sites (like site 5; Fig. 6). This fact may reflect different transport conditions of clasts and grains within the same current system: alignments of long axes parallel in a density flow and at right angles due to rolling (Collinson and Thompson, 1982). Cross stratification observations at sites 10 and 107 also agree with outcrop clast orientations. The results from the Tecomazflchil Formation document strong variations at different stratigraphic levels, as shown at sites 5, 19, 11, 9 and 15 (Figs. 6, 7, and 9), whose locations are shown in Fig. 4. Obviously, there is not just one preferential direction throughout the Tecomazfichil Formation, which would suggest the orientation of a stable current system. Instead, changes in long-axis alignments along different stratigraphic levels suggest a real variability in the paleocurrent system.

Magnetic Fabrics Study Anisotropy of magnetic susceptibility (AMS) measurements were made on 150 specimens along two sectionsof the Tecomaz~chil Formation in order to document the magnetic fabric. One 20-meterthick section near Petlalcingo was sampled in the upper portion of the sequence (column 7 in Fig. 4),

The Mixteca terrain, southern Mexico: Middle Jurassic paleogeography

201

® LOCALITIES WrTH FABRIC ANALYSIS POST,-TECOMAZUCHIL COVER TECOMAZUCHIL F.

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Fig. 5. Localities with petrofabric and magnetic fabric analyses within the Tecomaztlchil Formation. Sratigraphie positions are shown on Fig. 4.

with 55 specimens (Urrutia-Fucugauchi, 1982). Another section was sampled near San Francisco Yosecuta (column 9 in Fig. 4); 15 block samples were collected at different stratigraphic levels spanning the whole Tecomazt~chil Formation and yielding 95 specimens (Caballero--Miranda, 1990).

The specimens consisted of cylinders 2.54 cm in diameter and 2.2-2.3 cm in height (Seriba and Heller, 1978; Urrutia-Fucugauchi, 1980a). The AMS of the Petlalcingo section was measured at room temperature with an anisotropy attachment to a Digico spinner magnetometer, and specimens of

The Mixteca terrain, southern Mexico: Middle Jurassic paleogeography

203

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Fig. 7. Example of fabric variation a t one site. From bottom to top, the petrofabric rose diagrams are separated a t 8 m e t e r intervals. For i a m p l e 2, results of the magnetic fabric studies are plotted in an equal-area polar projection: numbers 1,2, and 3 correspond to the k l , k2, a n d k3 axes of each specimen (the square, triangle, and circle are, respectively, the m e a n directions of each k axis; n = n u m b e r ofobservations). The difference between mean petrofabric orientation and mean k2 orientation is 15 degrees.

204

C. CABALLERO-MIRANDA, D. J. MORAN-ZENTENO, J. URRUTIA-FUCUGAUCHI,

et ai

the Huajuapan section were measured with a Molspin anisotropy system after heating them to 610°C in order to enhance the bulk susceptibility and anisotropy degree (Urrutia-Fucugauchi, 1981; ShultzKrutisch and Heller, 1985). In both cases, the axial low-field susceptibilities were measured on a susceptibility bridge. Magnitudes and directions of the three principal susceptibilities (kl > k2 > k3) were computed (Khan, 1962). For the analysis, directions were referenced to the present horizontal and then to the paleo-horizontal and plotted on an equal-area polar projection (Fig. 9). AMS magnitudes are analysed in terms of four parameters: degree ofanisotropy is given by: P = kl/k3

(1)

where the degree increases as P increases (Nagata, 1953). Magnetic foliation is estimated by: F -- k2 / k3

(2)

where a high value of F indicates a high degree of planar-parallel magnetic fabrics (Stacey et al., 1960). The magnetic lineation is defined by: L = kl / k2

1.

SITE 20-4 (ofter Urrutio- Fuc~Oo, chi, 19621

(3)

where a high value of L indicates a high degree of linear-parallel structure (Balsey and Buddington, 1960). The shape of susceptibility ellipsoid is estimated by E = F/L = k2.K2/kl.k3

N

meon

(4)

so that if E > 1 the ellipsoid is predominantly oblate and if E < 1 the ellipsoid is predominantly prolate (Hrouda and J~nak, 1971; Hrouda et al., 1971). Several studies (e.g., Rees, 1965; Crimes and Oldershaw, 1967; Hamilton and Rees, 1970) have documented the fabrics of sedimentary rocks. The preferred orientation of kl (maximum susceptibility axis) gives magnetic lineations parallel or perpendicular to water current directions (Rees, 1965). In the specimens studied, the susceptibility ellipsoids tend to be more oblate than prolate. The k3 (minimum susceptibility axis) directions are almost normal to bedding, and kl and k2 axes lie almost in the bedding plane in two orthogonal directions. Specimens from the Petlalcingo section show kl directions forming two clusters which lie almost 90 degrees apart on the bedding plane, one toward the northeast and the other to the northwest. So the magnetic fabric appears with foliation planes parallel or subparallel to bedding and one or two magnetic lineations, which should correspond to a depositional fabric (e.g., Rees, 1965). These magnetic lineations may suggest the dominant current directions. In the Petlalcingo area, limited observations in thin sections of long-grain orientations in samples used for the AMS study (site 20-4; see column 7 in Fig. 4), agree with the NE cluster of kl directions, which has a shallower angle to bedding than the NW kl cluster (Urrutia-Fucugauchi, 1982). On the

n= 41

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Fig. 8. Petrofabric and magnetic fabric r e . i r a from the P e t lalcingo section (see P i p . 4 and 5). The magnetic fabric (from Urrutia-Fucugauchi, 1982) was plotted in a ~mrvographi¢ polar projection; the rose diagram in the upper right corner Ihowo m a g netic lineation.

other hand, the NW cluster agrees with the outcrop long--axis clast orientations observed at site 20-2 of the same section. In the San Francisco Yosocuta area, the k2 mean direction generally agrees with t h e principal direction of the microscopic or even m e g u e o p i c petrofabric (Figs. 7b and 9a), but sometimes the k l direction is the one that agrees with the petrofabric orientation (Fig. 9b). Our AMS study shows a depositional magnetic fabric with kl or k2 orientations in the bedding plane. Petrofabric observations of exactly the same stratigraphic level agree with e i t h e r k l or k2 orientation. In some cases, the spatial distribution

2"/t~ .

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of kl and k2 susceptibility axes, which suggest a well defined imbrication, allowed us to determine the paleocurrent sense. For instance, at site 15 (Fig. 9) the sense of current lies between SSW and Sl 6W and corresponds to the k2 axis; at site 9 (Fig. 9), it lies between $98E and S120E, corresponding to the kl axis. In some cases, cross stratification aided in interpreting the paleocurrent direction. As in the petrofabric studies, there is no one preferred orientation of magnetic fabric for the entire Tecomazfichil Formation; instead, there are strong variations at different stratigraphic levels, suggesting a real variability of the paleocurrent system. Petrofabric and magnetic fabric studies of the Tecomazhchil Formation, as well as its geological characteristics (Caballero-Miranda, 1990), led us to conclude that the fluvial system that generated Tecomazfichil deposits was a meadering type. These results, and spatial distribution of the continental and marine Jurassic sedimentary units (Tecomaz6chil Formation and Tecocoyunca Group, see Fig. 2), suggest that this fluvial system flowed generally SSW. The paleocurrents of this system could be enclosed in valleys with the influence, during some intervals, of alluvial fans. In the San Jos6 Ayuquilafrexcalapa area, for instance, spatial distribution of the coarsest conglomerate beds of the Tecomazuchil Formation (Ortega-Guti6rrez, 1978; Caballero-Miranda, 1990) suggests a NNW-trending alluvial fan belt with sedimentation polarity to the SSE. Paleomagnetic Studies

Paleomagnetic studies were conducted for most of the Jurassic units in the study area in order to document the tectonic evolution and to provide data on paleo-latitudes. Results were obtained for the Tecomazhchil Formation ( U r r u t i a - F u c u g a u c h i , 1980b; BShnel, 1985), and for the Zorrillo, Taberna, Simbn, and Yucufiuti Formations of the Tecocoyunca Group (BShnel, 1985; Mor~n-Zenteno, 1987). Additional studies have been completed on the Oxfordian Caliza con Cidaris Formation (Mor~nZenteno, 1987) and the Toarcian Rosario Formation (B6hnel, 1985). Results will be reported in detail elsewhere. Mean directions and pole positions for the Tecomazfichil, Yucufiuti, and Caliza con Cidaris Formations are summarized in Table 1 (MortlnZenteno, 1987). All mean site directions diverge from the expected directions estimated for the corresponding segment of the North American apparent polar wander path (APWP). The declination values for the three formations are different, which may suggest relative rotations among the sampling sites. The inclinations, on the other hand, are very consistent and give values higher than those expected for the area. These data may indicate paleo-latitude displacements of the Mixteca terrain with respect to the North America craton. The simplest movements

result from considering a magnetic polarity con-o sistent with a northern hemisphere position. At that time the Mixteca terrain was located to the northwest of its present--day position and may have moved along the left-lateral Mojave-Sonora and Trans-Mexican volcanic belt m e g a s h e a r s (e.g., Pilger, 1978; Scotese et al., 1979; Anderson and Schmidt, 1983; Morhn-Zenteno, 1987; O r t e g a Guerrero, 1989). The mean site directions obtained for the Tecomaz6chil and Caliza con Cidaris Formations, lie close to the present-day direction and to the axial dipole direction in Oaxaca. Nevertheless, we believe that these directions are primary and do not represent a recent remagnetization. First, without tectonic corrections the directions clearly deviate from the current one (Urrutia-Fucugauchi, 1980b; B6hnel, 1985; Mortin-Zenteno, 1987). Second, in the Tecomazflchil Formation we found samples with different magnetic polarities. The mean directions of different polarities are antipodal, pointing to a primary origin. Alternatively, a southern hemisphere position for the Mixteca terrain has been suggested on the basis of affinity of the Middle Jurassic ammonite fossil assemblage (Taylor et al., 1984; Westermann et al., 1984). The paleo-position proposed for the Mixteca terrain lies close to the Central Andes, but this position requires a large rotation (about 150 degrees). Paleomagnetic data for the Cretaceous of the Mixteca terrain are summarized in Table 3. Paleomagnetic results for the latest Cretaceous-early Tertiary of the Xolapa terrain are also included. The agreement between mean directions and pole positions, with their corresponding data for cratonic North America and northern Mexico, indicates that the Mixteca and Oaxaca terrains have maintained a similar relative position with respect to North America since the Albian-Cenomanian ( U r r u t i a Fucugauchi and Van der Voo, 1983; B~hnel, 1985; Urrutia-Fucugauchi, 1988). The Xolapa terrain also maintained a paleo-position in the Pacific margin at least since the Late Cretaceous-early Tertiary (Urrutia-Fucugauchi, 1983; Bfhnel et al., 1988). Paleomagnetic data are needed for the interval Middle Jurassic to mid-Cretaceous for the Mixteca and Oaxaca terrains. For the Xolapa terrain, efforts to obtain paleomagnetic data for pre-latest Cretaceous units have been unsuccessful because of metamorphism and structural complexities. In summary, paleomagnetic results indicate that the Mixteca and Oaxaca terrains were in a displaced position during the Middle Jurassic and arrived at their present relative position before or by the Albian-Cenomanian. The only paleomagnetic results for the Xolapa terrain are for the latest Cretaceous--early Tertiary intrusive complex of Acapulco (BShnel et al., 1988), and they indicate that the terrain was already in place by that time.

The Mixteca terrain, southern Mexico: Middle Jurassic paleogeography

207

BATHONIAN-BAJOClAN FLUVIAL DEPOSITS OF NORTHERNOAXACA, MEXK~O Acatlan Terrain .:'::::.".;~.;.:.::;.?.'..:.~:... ee

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The paleogeographic reconstruction of Fig. 10 represents in a simplified way the broad major characteristics of the data analyzed: the spatial distribution and m a r i n e - c o n t i n e n t a l n a t u r e of Middle Jurassic sedimentary units, identification of sediment source, observations of sedimentary indicaters of transport directions, magnetic fabrics, and paleomagnetic data. The reconstruction represents the simple solution to the data analyzed and provides us with an opportunity to speculate on the tectonic evolution of southern Mexico - - speculations we offer as a working hypothesis with particular requirements for further work. The Mixteca terrain may have undergone a complex history of tectonic motions. For the J u r a s sic, the paleomagnetic data available indicate possible large-scale translations. A paleo-position in the northwestern margin of North America seems to require a smaller amount of transport and is apparently consistent with the direction (if not the magnitude) of the expected movement required by the Mojave-Sonora and Trans-Mexican volcanic belt megashears (Scotese et al., 1979; Anderson and Schmidt, 1983; Table 2). This hypothesis implies a 1000 km southeastward motion for the Mixteca terrain, which may have already joined the Oaxaca terrain to forming a composite terrain ( U r r u t i a Fucugauchi, 1984; Urrutia-Fucugauchi et al., 1988). An alternative interpretation is to consider a southern hemisphere paleolatitude. This has already been proposed from other analyses of paleomagnetic data for northwestern North American terrains (e.g., Hillhouse, 1977; Jones et al., 1977; Yole and Irving, 1980; Panuska and Stone, 1981; Stone et al., 1982; Gehrels and Saleeby, 1987). The Mixteca terrain contains a well documented late Bajocian-early Callovian ammonite fauna (Wester-

208

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Table 2. Expected directions for the Mixteca terrain during the Middle Jurassic~ calculated assuming proposed alternative tectonic models. Rotation

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Pole Rotation

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............................................. N E N E

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Notes: Reference paleomagnetic pole is for 172 M a (May and Butler, 1986). Rotation models are: A, rotationof 15 ° along the Mojave-Senora megashear (Anderson and Schmidt, 1983); B, rotation of 30 ° along fault systems in northern Mexico (see summary in Urrutia-Fucugauchi, 1984); C, rotationof 3 ° along the Trans-Mexican volcanic belt and of 15° along the Mojave-Senora megashear; D, rotation of 264 along a lateral fault in northern Mexico (Scotese et al., 1979); and E, no rotation considering a similar relativepositionwith respectto North America.

mann et al., 1984; Sandoval and W e s t e r m a n n , 1986). The faunas correlate well with fossil assemblages of the Central Andes region of South America (Westermann et al., 1984). Taylor e t a [ . (1984) have suggested that this ammonite fauna could indicate a southern hemisphere paleo-position of the Mixteca terrain. However, these faunal affinities are not incompatible with a n o r t h e r n hemimphere position, considering the possible disporsal effects of paleo-oceanic circulation. For both interpretations, the Mixteca terrain should have arrived at southern Mexico before the mid--Cretaceous, as indicated by paleomagnetic data for the Albian--Cenomanian limestone units exposed on the Mixteca terrain (Urrutia-Fucugauchi and Van der Voo, 1983; B6hnel, 1985; Trevifio-Rodriguez, 1986; Urrutia-Fucugauchi, 1988). Results are sumrn~__rbagiin Table 3. The paleogeography proposed for the northern area of the Mixteea terrain d u r i n g the Middle J u r a u i c (Fig. 10) has implications for understanding the tectonic evolution of southern Mexico, particularly for the Xolapa terrain (see Fig. 1). The Xolapa terrain represents a low pressurehigh temperature belt, shown by its characteristic mineral assemblages in calcareous (wollastonite), marie (eummingtonite), and politic (cordierite/sillimanite/andalusits) rock units (Ortega-Guti~rrez,

1981). Thus, the Xolapa Complex, which contains abundant orthogneisses and migmatites, represents the roots of an ancient Mesozoic volcanic arc (Halpern et al., 1974; Ortega-Guti6rrez, 1981). Guerrero-Garcia et aI. (1978) reported concordant U-Pb dates on zircons of 165+3 Ma and an isochron Rb--Sr age of 180 +84 Ma for a quartz feldspar orthogneiss with an initial Sr ratio of 0.7056 + 0.0025. If the Xolapa Complex represents the roots of a volcanic arc active d u r i n g the early Mesozoic (Jurassic-Early Cretaceous?), then it could constitute a potential source of volcaniclastic materials to the Mixteca terrain. Nevertheless, the observed polarity of sedimentation of Middle and Upper Jurassic deposits in the Mixteca terrain and the lack of important volcanic components in these deposits do not support the previous contention. Accordingly, we offer the suggestion t h a t the Xolapa terrain arrived at its relative position in southern Mexico after Middle Jurassic times and before the early Tertiary. There is no documented evidence of a collision of the Xolapa terrain and its movement m a y have involved lateral transport along the paleo-continental margin, in a tectonic situation similar to that of present-day Baja California and to that suggested for the terrains of the western margin of North America.

The Mixteca terrain, southern Mexico: Middle Jurassic paleogeography

209

Acknowledgements---Critical comments by L. S~nchez--Barreda and Juan Francisco A. Vilas were helpful in the preparation of the manuscript. This project was partially funded by the Centro lnternacionalde Paleomagnetismo y Paleogeoflsica (CIPP).

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-tq

+t +t +t t~

I

i

eg ~

o

+1

~

Anderson, T. H., and Schmidt, V. A., 1983. The evolution of Middle America and the Gulf of Mexico-Caribbean Sea region during Mesozoic time. Bulletin of the Geological Society o f America 94,947-966.

~ -t-t

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Ballard, M. M., Van der Voo, R., and Urrutia-Fucugauchi, J., 1989. Paleomagnetic results from Grenvillian-aged rocks from Oaxaca, Mexico: Evidence for a displaced terrane. Precambrian Research 42,343-352.

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Baisey, J. P~, and Buddington, A. F., 1960. Magnetic susceptibility anisotropy and fabric of some Aderondack granites and orthogneisses. AmerieanJournalofScienee 258-A, 6-20.

e~ o

Beck, M. E., 1980. Paleomagnetic record of plate-margin tectonic processes along the western edge of North America. Journal of GeophysiealResearch 85, 7115-7131.

~, .It

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0

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~ oo

P t~

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P t~

Collinson, I. D., and Thompson, D. B., 1982. Sedimentary Structures. George Allen and Unwin Publishers, London, UK, 194 p.

"~ "to ',~

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3 I_

8 e~

o 0

g

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J. U R R U T I A - F U C U G A U C H I .

et ~

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