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279

The Geological Society of America Special Paper 436

2008

The Guerrero Composite Terrane of western Mexico: Collision and subsequent rifting in a supra-subduction zone

E. Centeno-García*Instituto de Geología, Universidad Nacional Autónoma de México, Ciudad Universitaria, México D.F. 04510, México

M. Guerrero-SuasteguiO. Talavera-Mendoza

Unidad Académica de Ciencias de la Tierra, Universidad Autónoma de Guerrero, AP 197, Taxco el Viejo, Guerrero, México

ABSTRACT

The Guerrero Composite Terrane of western Mexico is the second largest ter-rane in North America. Mostly characterized by submarine volcanism and formed by five terranes, the Guerrero records vast and complex subduction-related processes influenced by major translation and rifting. It is composed of the Teloloapan, Gua-najuato, Arcelia, Tahue, and Zihuatanejo Terranes. The Teloloapan Terrane is made up of Lower Cretaceous island-arc (IA) andesitic to basaltic submarine lava flows, interbedded with limestone and shallow-marine volcaniclastic rocks. The Guanajuato and Arcelia Terranes are characterized by Lower Cretaceous supra-subduction ophiolite successions formed by deep-marine volcanic and sedimentary rocks with mid-oceanic-ridge basalt (MORB), oceanic-island basalt (OIB), and island-arc basalt (IAB) signatures. These two terranes are placed between the continent and the more evolved arc assemblages of the Zihuatanejo Terrane. The Tahue Terrane is composed of Paleozoic accreted arc and eugeoclinal sedimentary rocks, Triassic rift-related metaigneous rocks, and overlain unconformably by pillow basalts, limestone, and volcaniclastic rocks. The Zihuatanejo Terrane was formed by Triassic ocean-flank to ocean-floor assemblages accreted in Early Jurassic time (subduction complexes). The subduction complexes are overlain by Middle Jurassic–evolved volcanic arc rocks, which are in turn unconformably overlain by Early and Late Cretaceous subaerial and marine arc-related volcano-sedimentary assemblages.

Mesozoic stratigraphy at the paleocontinental margin of Mexico (Oaxaquia and Mixteca Terranes) is formed by Triassic submarine fan turbidites accreted during Early Jurassic time; Middle Jurassic–evolved volcanic arc rocks are unconformably covered by a Late Jurassic to Cretaceous calcareous platform.

Six stages in the tectonic evolution are proposed on the basis of the stratigraphic and deformational events recorded in western Mexico: (1) A passive or rifting margin developed along the western margin of continental Mexico throughout the Triassic. A

*[email protected]

Centeno-García, E., Guerrero-Suastegui, M., and Talavera-Mendoza, O., 2008, The Guerrero Composite Terrane of western Mexico: Collision and subsequent rifting in a supra-subduction zone, in Draut, A., Clift, P.D., and Scholl, D.W., eds., Formation and Applications of the Sedimentary Record in Arc Collision Zones: Geological Society of America Special Paper 436, p. 279–308, doi: 10.1130/2008.2436(13). For permission to copy, contact [email protected]. ©2008 The Geological Society of America. All rights reserved.

280 Centeno-García et al.

INTRODUCTION

The present configuration of continental Mexico was built after accretion of basement remnants and oceanic terranes. Dur-ing most of their Mesozoic history, Proterozoic to Paleozoic accreted terranes formed a relatively narrow neck of land adja-cent to the North American craton. This was bordered on its eastern side by rifting and on its western side by active subduc-tion. Thus Mexico is probably one of the most suitable regions in North America for studying the interaction between these two differing tectonic scenarios. We suggest in this paper, based on evidence recorded in the stratigraphy of the Guerrero Composite Terrane and surrounding terranes, that the almost continuously subducting Pacific margin of Mexico was directly influenced by extensional tectonics associated with the breakup of Pangea and the formation of the Gulf of Mexico.

The Guerrero Composite Terrane (Campa and Coney, 1983) constitutes approximately one-third of Mexico. As originally described, it is the largest of all the Mexican terranes and probably the second largest of the North America Cordillera after Wrangel-lia (Campa and Coney, 1983; Centeno-García et al., 1993a). The Guerrero Composite Terrane is characterized mostly by subma-rine and locally subaerial volcanic and sedimentary successions that range in age from Jurassic (Tithonian) to middle–Late Creta-ceous (Cenomanian), and scarce exposures of older rocks. A wide variety of models has been proposed for the origin of the Guerrero Composite Terrane. Like other terranes of the North America Cor-dillera, it was first interpreted as an exotic terrane formed by a far-traveled Cretaceous oceanic arc. Some authors have suggested that it was an oceanic arc terrane that was accreted to nuclear Mexico in Late Cretaceous time via a westward-dipping subduction zone that closed a major ocean basin (Lapierre et. al., 1992; Tardy et al., 1994; Dickinson and Lawton, 2001, etc.). Other authors have

suggested that the Guerrero Composite Terrane might represent one or more complex systems of two or three peripheral arcs that developed relatively close to the continent (Campa and Ramírez, 1979; Ramírez-Espinosa et al., 1991; Mendoza and Suastegui, 2000; Centeno-García et al., 2003; Centeno-García, 2005). Some models even proposed that the arc was autochthonous and was built upon Proterozoic continental crust of nuclear Mexico (de Cserna, 1978; Elías-Herrera and Sánchez-Zavala, 1990). In other words, there is a model for each likely possibility, but each lacks strong supporting evidence.

New findings on the stratigraphy, discussed in this paper, suggest a more complex evolution, implying a series of accre-tions to the continent followed by rifting, and later by collision. In this paper we attempt to present our insights into the evolu-tion of western Mexico gained from examining the stratigraphy and structure, and the geochemical and geochronological data, of such a vast area. However, we discuss in this paper only strati-graphic units and localities that are keys for reconstructing the tectonic evolution. This paper synthesizes the work done by many authors. Although there is the need for more geochrono-logical and detailed field work, we consider that the preliminary tectonic model presented in this paper is consistent with the evi-dence collected to date.

OVERVIEW OF THE GUERRERO COMPOSITE AND NEIGHBORING TERRANES

The stratigraphy of western Mexico is synthesized in this paper under the framework of tectono-stratigraphic terranes, which are regions that share the same geological history and are bounded by major faults. As mentioned before, by the early Mesozoic, the Paleozoic and Proterozoic terranes were already accreted to the southern part of the North American craton.

thick siliciclastic turbiditic succession of the Potosi Submarine Fan was accumulated on the paleo-continental shelf-slope and extended to the west in a marginal oceanic basin. (2) Subduction began in the Early Jurassic, and the turbidites of the Potosi Fan with slivers of the oceanic crust were accreted, forming a wide subduction prism. (3) Exhumation of the accretionary prism and development of a Middle Jurassic con-tinental arc onto the paleo-continental margin (Oaxaquia and Mixteca Terrane) took place, and also in the Zihuatanejo Terrane. (4) Intra-arc strike-slip faulting and rift-ing of the Middle Jurassic continental arc took place along with migration of the subduction toward the west and development of a calcareous platform in Oaxaquia and the Mixteca Terrane (continental Mexico). (5) Drifting of the previously accreted Tahue and Zihuatanejo Terranes formed a series of marginal arc-backarc systems, or one continuously drifting arc with intra-arc and backarc basins during Early to middle Cretaceous time. (6) Deformation of the arc assemblages, and development of Santonian to Maastrichtian foreland and other basins, date the final amalgamation of the Guerrero Composite Terrane with the continental margin.

Keywords: Guerrero Terrane, Mexico, tectonics, Triassic subduction complex, Creta-ceous arc volcanism.

Guerrero Composite Terrane of western Mexico 281

Those that already formed part of the continental margin dur-ing the Mesozoic were Oaxaquia and the Mixteca, Parral, and Cortes Terranes (Fig. 1). Terranes accreted or displaced during the Mesozoic were those of the Guerrero Composite, the Central, as well as terranes of the western Baja California Peninsula. The latter will not be reviewed in this paper. A brief summary of the stratigraphy is described as follows; more detailed descriptions of key areas and events are discussed later.

OAXAQUIA

At the end of the Paleozoic, Proterozoic basement terranes of Gondwanan affinity were already accreted to the southern part of the North American craton. The largest of these is the Oaxa-quia block (Fig. 1), a crustal fragment, subcontinent in size, of Grenville affinity (Ortega-Gutiérrez et al., 1995). This crustal block forms the backbone of eastern Mexico and is referred to herein as continental Mexico for the Mesozoic. Oaxaquia has a Precambrian (1157–900 Ma) crystalline basement (gneisses and anorthosites; Patchett and Ruíz, 1987; Ortega-Gutiérrez et

al., 1995; Ramírez-Ramírez, 1992; Lawlor et al., 1999; Solari et al., 2003; Keppie et al., 2003). It is covered by Paleozoic sedimentary rocks (Fig. 2) that are capped by Permian volca-nic and volcaniclastic rocks (McKee et al., 1999; Stewart et al., 1999; Rosales-Lagarde et al., 2005). Triassic (Carnian–Norian) sedimentary rocks (La Ballena Formation) are exposed at the western margin of Oaxaquia (Labarthe et al., 1982; Silva-Romo, 1993; Tristán-Gonzalez and Torres-Hernández, 1994; Centeno-García and Silva-Romo, 1997; Barboza-Gudiño et al., 1998, 1999, 2004; Bartolini et al., 2002). These rocks are made up of a thick succession of turbidites (Fig. 2) deposited in a submarine fan environment named the Potosi Fan (Centeno-García, 2005).

Triassic rocks of the Potosi Fan were deformed prior to deposi-tion of Jurassic volcanic-volcaniclastic rocks (Centeno-García and Silva-Romo, 1997). They are interpreted as a Jurassic continental arc and rest unconformably on the Triassic Potosi Fan. Jurassic arc strata are made up of subaerial andesitic-rhyolitic lava flows, interbedded with volcaniclastic rocks (Silva-Romo, 1993). The arc sequence changes transitionally upsection to shallow-marine

Cortes TerraneOaxaquiaCenozoic volcanism

Parral TerraneCentral Terrane

Fig. 8

Oaxaquia

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Fig. 6

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AmericaLa Babia Fault

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Figure 1. Map showing main tectono-stratigraphic terranes, major faults mentioned in the text, and locations of Figures 4, 6, and 8.


volcaniclastic rocks, limestone, and some evaporites (Fig. 2; Silva-Romo, 1993; Tristán-González and Torres-Hernández, 1994; Barboza-Gudiño et al., 2004). Calcareous sedimentation in Oaxaquia ranges in age from late Oxfordian–Kimmeridgian to Turonian and is interpreted as the southern extension of the North

American seaway. A major change upsection from calcareous to clastic sedimentation occurred at the uppermost part of the Cre-taceous, forming a thick succession of sandstone, shale, and con-glomerate (Caracol Formation; Silva-Romo, 1993). Oaxaquia is overthrust by the Guerrero Composite Terrane (Fig. 1).

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rhyolitic-andesiticsubmarine volcanism

rhyolitic-andesiticcontinental volcanism

volcaniclastic rocks

metamorphic arc volcanic and sedimentary rocks

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GUERRERO COMPOSITE TERRANE

Maastrichtian

Campanian

Santonian

Coniacian

Turonian

Cenomanian

Albian

Aptian

Barremian

Hauterivian

Valanginian

Berriasian

Tithonian

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Oxfordian

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Figure 2. Simplified stratigraphic columns for Oaxaquia and terranes mentioned in the text. They show the age range (in Ma) of sedimentation and magmatism for western and central Mexico. Geochronological data are represented as follows: Black circles are U/Pb ages, and diamonds are Ar/Ar and K/Ar ages.


MIXTECA TERRANE

The basement of the eastern Mixteca Terrane is made up of pre-Mississippian polydeformed metamorphic rocks of the Acat-lán Complex (Ortega-Gutiérrez, 1981; Ruíz et al., 1988; Yañez et al., 1991). This complex is considered to be the result of complex interactions between Gondwana and Laurentia previous and dur-ing the assembling of Pangea (Ortega-Gutiérrez et al., 1999). It is unconformably overlain by Permian sedimentary rocks, which are in turn overlain unconformably by Middle Jurassic volcanic and sedimentary rocks (Fig. 2; García-Díaz et al., 2004). At the western part of the terrane, near the limit with the Guerrero Com-posite Terrane, partly metamorphosed volcanic and volcaniclastic rocks are exposed (Taxco Schist and Chapolapa Formation; de Cserna and Fries, 1981; Talavera-Mendoza, 1993; Campa and Iri-ondo, 2004). The Taxco Schist is made up of andesitic to rhyolitic lavas and volcaniclastic rocks of Early Cretaceous age (Talavera-Mendoza, 1993; Campa and Iriondo, 2004). The Taxco Schist is unconformably overlain by a thick limestone succession of Albian to Cenomanian age and by Turonian–Maastrichtian clastic rocks (Mexcala Formation; Campa and Ramírez, 1979; Talavera-Mendoza et al., 1995). Contacts between the Mixteca Terrane and Oaxaquia, as well as between the Mixteca and Guerrero Com-posite Terranes, are partially exposed. The Mixteca Terrane is on strike-slip fault contact with Oaxaquia, and rocks of the Guerrero Composite Terrane are thrust over the Mixteca Terrane.

PARRAL TERRANE

The Parral Terrane (Figs. 1 and 2) was first defined by Pacheco et al. (1984) and Coney and Campa (1987) and was rede-fined by Centeno-García (2005). The basement of the Parral Ter-rane is formed by Devonian to Carboniferous metamorphic rocks (Pescadito Schist; Eguiluz and Campa, 1982; Araujo and Arenas, 1986; Zaldivar and Garduño, 1984). These Paleozoic metamor-phic rocks are unconformably overlain by red beds and volca-nic successions (Nazas Formation; Pantoja-Alor, 1963), which change transitionally to Tithonian limestone (Araujo and Arenas, 1986; Contreras-Montero et al., 1988). Cretaceous calcareous and clastic sedimentation of the Parral Terrane is laterally con-tinuous with the calcareous-clastic deposits that cover Oaxaquia and the Central Terrane. Relationships among the Parral, Cen-tral, and Cortes Terranes and the Parral Terrane and Oaxaquia are unknown, because the contacts are covered by Cretaceous lime-stone or by Cenozoic volcanic successions. Therefore, the exact locations of their boundaries are unknown but are inferred by the difference in styles of deformation of the Cretaceous rocks.

CABORCA AND CORTES TERRANES

The Caborca Terrane has a Proterozoic basement older than 1.7 Ga (Anderson and Silver, 1981), covered by a thick Paleozoic sedimentary succession. It has been interpreted to be a displaced block of continental North America, originally located along the

Paleozoic “miogeosyncline” of Nevada. It was transferred toward the south by Middle to Late Jurassic time via the Mohave-Sonora megashear (Anderson and Silver, 1979, 2005; Stewart et al., 1990). The Cortes Terrane is interpreted as an autochthonous ter-rane to North America, which probably evolved at the margin of the Caborca Terrane (Stewart et al., 1990). It is made up of a thick succession of Paleozoic deep-marine turbidites that were thrust over platform limestone of the Caborca Terrane (Figs. 1 and 2). The Cortes Terrane is interpreted as continental-slope depos-its, and it is considered the southern extension of the Paleozoic Cordilleran “eugeoclinal” deposits from Nevada and California (Poole and Madrid, 1988; Coney and Campa, 1987; Stewart et al., 1990). The previously deformed Paleozoic deep-marine rocks of the Cortes Terrane are overlain by Triassic (Carnian–Norian) ter-restrial and marine sedimentary rocks (Stewart et al., 1990; Stew-art and Roldán-Quintana, 1991). The Triassic rocks are overlain by Cretaceous red beds and volcanic rocks (Stewart and Roldán-Quintana, 1991). Contact relationships between the Cortes and Guerrero Composite Terranes have not been well constrained, but the contact is inferred to be a Late Cretaceous thrust fault.

CENTRAL TERRANE

The nature of the basement of the Central Terrane is unknown, but it is assumed to be different from the Proterozoic basement of Oaxaquia because its oldest exposed rocks near its contact are a subduction-related accretionary complex (Taray Formation; Anderson et al., 1990; Diaz-Salgado et al., 2003; Anderson et al., 2005; Centeno-García, 2005). The subduction zone on which the Taray Formation was deformed was probably constructed along the Oaxaquia continental margin between Late Permian and Early Jurassic time (Diaz-Salgado et al., 2003; Anderson et al., 2005). The complex is unconformably overlain by Oxfordian subaerial rhyolitic to andesitic volcanic rocks and red beds (Jones et al., 1995). These rocks change transitionally to shallow-marine lime-stone that ranges in age from Late Jurassic to Late Cretaceous (Córdoba-Méndez, 1964). The location of the northern and east-ern contact between the Central Terrane and Oaxaquia is inferred on the basis of the location of the last exposures of Paleozoic–early Mesozoic rocks, and a contrast in deformation styles of Cre-taceous rocks in both (Fig. 1). The contact between the Central and Guerrero Composite Terranes has not been studied in detail but is inferred on the basis of the distribution of the northernmost exposures of Cretaceous marine volcanic rocks that belong to the Guerrero Composite Terrane. Structural trends on both sides of the contact suggest that the Central Terrane is overthrust by the Guerrero Composite Terrane to the south (Fig. 1). The thrusting is inferred to have occurred about Late Cretaceous time.

GUERRERO COMPOSITE TERRANE

Areas with large volumes of Lower Cretaceous volcanic and volcaniclastic rocks, located toward the west of Oaxa-quia and the Mixteca Terrane, were originally grouped as the


Guerrero Terrane by Campa and Coney (1983) and were, 10 years later, divided into the Tahue, Nahuatl, and Tepehuano Terranes by Sedlock et al. (1993). Subsequent regional map-ping has shown that the divisions proposed by Campa and Coney (1983) are closer to the field locations of faults delim-iting the terranes than those of Sedlock et al. (1993). There-fore, more recent reviews of the terrane distribution of Mexico (e.g., Centeno-García, 2005) have been based on Campa and Coney (1983). The Guerrero is a composite terrane, formed by at least five terranes: Tahue, Zihuatanejo, Guanajuato, Arcelia, and Teloloapan (Figs. 1 and 2; Talavera-Mendoza et al., 1995; Mendoza and Suastegui, 2000; Centeno-García et al., 2003; Centeno-García, 2005). Their stratigraphy is briefly described from NNW to ESE (Fig. 1):

Tahue Terrane

The Tahue Terrane contains the oldest rocks found so far within the Guerrero Composite Terrane (Fig. 2; Centeno-García, 2005). These rocks comprise Ordovician marine rhyolitic-andesitic lavas and clastic and calcareous rocks, all deformed and metamorphosed to low-greenschist facies (El Fuerte Com-plex; Mullan, 1978; Roldán-Quintana et al., 1993; Poole and Perry, 1998). These rocks may have originated as an oceanic arc that apparently was accreted previous to the deposition of Pennsylvanian–Permian deep-marine sedimentary rocks (San José de Gracia Formation; Carrillo-Martínez, 1971; Gastil et al., 1991; Arredondo-Guerrero and Centeno-García, 2003; Centeno-García, 2005). These deep-marine turbidites are strongly deformed but do not show the metamorphism of the El Fuerte Complex; thus an unconformable contact relationship between these two units is inferred. Paleozoic rocks of the Tahue Terrane are unconformably overlain by Cretaceous marine arc volcanic rocks and are interpreted as part of the Guerrero Arc (Ortega-Gutiérrez et al., 1979; Henry and Fredrikson, 1987; Roldán-Quintana et al., 1993; Freydier et al., 1995). These rocks are also cut by mafic and ultramafic intrusions that are part of the same Cretaceous arc magmatism (Henry and Fredrikson, 1987; Gas-til et al., 1999; Arredondo-Guerrero and Centeno-García, 2003). Therefore, the Paleozoic units form the basement upon which the arc was built. The Tahue Terrane also contains metamorphic rocks of Triassic age (Keppie et al., 2006). The contact relation-ship between the Cortes and Tahue Terranes has not been studied in detail, but it is inferred to be a thrust (Fig. 1; Roldán-Quintana et al., 1993). The contact between the Tahue and Zihuatanejo Ter-ranes is not exposed.

Zihuatanejo Terrane

The Zihuatanejo Terrane is the largest of all terranes that form the Guerrero Composite Terrane (Fig. 1). It extends north of the Mexican Volcanic Belt and along the Pacific Coast of Mex-ico (Centeno-García et al., 1993a, 1993b; Talavera-Mendoza et al., 1995; Mendoza and Suastegui, 2000). Its basement is made

up of large volumes of Triassic (Norian) quartz-rich turbidites (sandstone and shale) that are tectonically imbricated (Campa et al., 1982; Centeno-García et al., 1993a, 1993b). The turbi-dites form a matrix within which are blocks and slabs of pillow basalts, diabase, banded gabbros, chert, and limestone (Fig. 2). These rocks have received different names at different outcrops: Zacatecas Formation, Arteaga Complex, and Las Ollas Complex (Burckhardt and Scalia, 1906; Ranson et al., 1982; Cuevas-Pérez, 1983; Monod and Calvet, 1991; Centeno-García and Silva-Romo, 1997; Talavera-Mendoza, 2000; Centeno-García et al., 2003). The deformation of these rocks varies from gently folded strata to highly sheared block-in-matrix textures, and their meta-morphism ranges from none to high-greenschist–amphibolite facies (Centeno-García et al., 2003). Blueschist facies have been reported only in one locality (Las Ollas Complex; Talavera-Mendoza, 2000). These lithologies are interpreted to constitute an Upper Triassic(?)–Lower Jurassic subduction-related accre-tionary complex.

Scattered exposures of rocks of Middle to Late Jurassic–evolved arc volcanism lie along the Pacific Coast of the Zihua-tanejo Terrane. These rocks are made up of submarine rhyo-litic lavas and volcaniclastic rocks, and granitoids that were emplaced in rocks of the accretionary complex (Bissig et al., 2003; Centeno-García et al., 2003). The Middle to Upper Juras-sic arc rocks were in turn deformed and exhumed previous to the deposition of uppermost Jurassic–Cretaceous arc-related strata (Centeno-García et al., 2003).

The Cretaceous arc succession ranges from Berriasian to Cenomanian in age, and it includes andesitic, basaltic, and some rhyolitic volcanic and volcaniclastic rocks, interbedded with limestone, evaporites, and some red beds (Grajales and López, 1984). The arc succession contains abundant fossils such as rud-ists, gastropods, microfossils, fossil logs, and vertebrates.

This arc succession was deformed prior to the intrusion of large granitoids of latest Cretaceous to Paleogene age (Schaaf et al., 2000). Also, uppermost Cretaceous (Santonian to Maas-trichtian) red beds and volcanic rocks rest unconformably on all previous units (Altamira Areyán, 2002; Benammi et al., 2005). The contact between the Zihuatanejo Terrane and Oaxaquia is exposed at its northern limit, where Cretaceous arc rocks of the Zihuatanejo Terrane are thrust over shallow-marine limestone of Oaxaquia. Its contact with the Arcelia and Guanajuato Terranes is inferred to be an east-verging thrust, but it is covered by upper-most Cretaceous and Cenozoic red beds and volcanic rocks.

Guanajuato Terrane

The Guanajuato Terrane has been interpreted as a com-plete crustal section through a primitive island arc that appears to lack an older basement (Ortiz-Hernandez et al., 1991; Ortiz-Hernandez, 1992). It has also been interpreted as the remains of an oceanic basin that lay between the Guerrero arc and the conti-nental margin (Freydier et al., 2000). This terrane was formed by a series of tectonic slivers that placed lower crust rocks (gabbro,


tonalite, serpentinite, wehrlite, and dike swarms) on pillow basalts, rhyolitic tuffs, volcanic turbidites, chert, and black detri-tal limestone (Quintero-Legorreta, 1992; Ortiz-Hernandez et al., 1992; Lapierre et al., 1992; Monod et al., 1990; Martínez-Reyes, 1992; Ortiz-Hernandez et al., 2003). These rocks were poorly dated as Tithonian–Hauterivian in age (Ortiz-Hernandez et al., 2003; Hall and Mortensen, 2003). Previously deformed volca-nic turbidites are unconformably overlain by Aptian–Albian limestone (Ortiz-Hernandez et al., 2003). This suggests that sedimentation and at least one phase of deformation occurred previous to the Aptian–Albian (Ortiz-Hernandez et al., 2003). At present the Guanajuato Terrane is thrust over the calcareous platform of Oaxaquia (Ortiz-Hernández et al., 2002). Contact relationships between the Guanajuato and Zihuatanejo Terranes have not been constrained.

Arcelia Terrane

The Arcelia Terrane is made up of basaltic pillow lavas and ultramafic bodies, black shale and chert, and volcanic turbidites, all intensively deformed and partly metamorphosed (Ramírez-Espinosa et al., 1991; Talavera-Mendoza et al., 1995). It is char-acterized by Early Cretaceous deep-marine primitive arc or arc-related oceanic facies and shows the least evolved magmatism of all the arc successions of the Guerrero Composite Terrane (Talavera-Mendoza et al., 1995; Mendoza and Suastegui, 2000). The Arcelia Terrane appears to lack an older basement (Talavera-Mendoza et al., 1995; Mendoza and Suastegui, 2000). Rocks of the Arcelia Terrane apparently were thrust over the assemblages of the Teloloapan Terrane, and were in turn overthrust by rocks of the Zihuatanejo Terrane. However, these contacts are inferred because they are covered by younger red beds.

Teloloapan Terrane

The Teloloapan Terrane consists of two distinct regions: the eastern region is characterized by shallow-marine volcanic and sedimentary deposits (Fig. 2), and the western region by deeper volcanic and sedimentary facies (Guerrero-Suastegui et al., 1991; Ramírez-Espinoza et al., 1991; Talavera-Mendoza et al., 1995; Mendoza and Suastegui, 2000; Guerrero-Suastegui, 2004). Both are marine arc assemblages, which vary in composition from basalt-andesite to scarce dacite-rhyolite (Talavera-Mendoza et al., 1995). This unit contains microfossils (radiolarians and coccoliths), gastropods, and bivalves that range in age from Hauterivian to Aptian; these rocks change transitionally upsec-tion to Aptian–Albian island-arc carbonates (Guerrero-Suastegui et al., 1991; Ramírez-Espinoza et al., 1991; Talavera-Mendoza et al., 1995). The Teloloapan Terrane (Fig. 1) is exposed in the east-ernmost parts of the Guerrero Composite Terrane. It is charac-terized structurally by a complex thrust-fault system that verges eastward. Its Lower Cretaceous rocks are severely deformed and metamorphosed in low-grade greenschist facies. The Teloloapan Terrane overrides either Lower to Middle Cretaceous platform

carbonates or Upper Cretaceous clastic sediments that belong to the Mixteca Terrane (Fig. 1; Campa and Ramirez, 1979). The nature of its basement remains unknown. Metamorphic rocks that are exposed near the northwestern boundary of the Teloloa-pan Terrane with the Arcelia Terrane have been interpreted as a possible basement for the former (Elías-Herrera and Sánchez-Zavala, 1990; Sanchez-Zavala, 1993). The rocks in this area are of uncertain age and origin.

TECTONIC MODEL

The most abundant rocks of the Guerrero Composite Terrane are marine, and rarely subaerial, arc volcanic and sedimentary successions that range in age from latest Jurassic (Tithonian) to middle Late Cretaceous (Cenomanian). The composition of the few scattered exposures of older units suggests a complex earlier tectonic evolution. These older rocks were not taken into con-sideration for the tectonic models proposed by previous authors (de Cserna, 1978; Campa and Ramirez, 1979; Elías-Herrera and Sánchez-Zavala, 1990; Tardy et al., 1994; Lapierre, et. al., 1992; Dickinson and Lawton, 2001, etc.). Based on the available infor-mation, we identified six main tectonic stages in the evolution of the Guerrero Composite Terrane. These stages are represented in Figure 3 and are briefly described in this section. Detailed discus-sion of the data that support the reconstruction of each stage is presented in the following section.

Stage I: Collision of a Paleozoic Oceanic Arc?—Basement of the Tahue Terrane

The basement of the Tahue Terrane (Fig. 3) is composed of the early Paleozoic accreted volcanic-sedimentary rocks of the El Fuerte Metamorphic Complex. There are not enough data available to constrain the origin of this complex. Preliminary interpretations considered these rocks as remnants of Gondwa-nan crust accreted during the formation of Pangea (Poole et al., 2005). In this model, metamorphic rocks of El Fuerte could be the western continuation of basement rocks of the Parral Terrane (Figs. 1 and 2). An alternative interpretation is that the El Fuerte Complex may be a displaced fragment of the early Paleozoic arc (Antler Arc) that collided with the western continental margin of North America during late Paleozoic time (Burchfiel et al., 1992; Sánchez-Zavala et al., 1999; Dickinson, 2004; Centeno-García, 2005). Carboniferous deep-marine turbidites (San José de Gracia Formation) that apparently cover the lower Paleozoic arc rocks unconformably may be correlative with deep-marine sedimen-tary rocks exposed in the eastern peninsular ranges of Baja Cali-fornia and the southwestern Cordillera of North America (Gastil et al., 1991; Centeno-García, 2005).

In either of the two scenarios, deformed Paleozoic rocks of the Tahue Terrane are the basement upon which Cretaceous vol-canism was built, indicating an earlier history of accretion of the Guerrero Composite Terrane than was previously interpreted by other authors.

Potosí Submarine Fan

Continent margin

Vizcaino?

?

pre 180-163 Ma

Arteaga ComplexTaray Complex

?

Potosi Fan(deformed)

Stage IIMarginal oceanic basinwith active rift volcanism

Stage IIIAccretion via subductionwide accretionary prism

Stage IVContinental arc andcontemporaneous strike-slipand extension (roll-back ofthe subducting plate?)

Stage VI aDeformation and locallymetamorphism, cut by105 Ma granitoids in theZihuatanejo terrane

Stage VI bActive continental arc inthe west and syntectonicmarine turbidites (foreland)in the east and deformation

Stage VFormation of a multiple arcssystem or a single arc withintra-arc/back arc rifting

Cretaceous

Cretaceous

W E

158 Ma163-158 Ma

?

Berriasian-Cenomanian

transtension?

++

? ? Oaxaquia and Mixteca?

ArceliaGuanajuato

transtension?

Cenomanian?-pre Santonian

compressiongranitoids 105 Ma

Calcareous K platform

Siliciclastic turbidites share the same provenancen

Subduction

Early Jurassic

TriassicCarnian-Norian

Middle toLate Jurassic

Oaxaquia and Mixteca



Vizcaíno? Guerrero Central Oaxaquia

Vizcaíno? Zihuatanejo (Guerrero Composite) Oaxaquia

Guerrero Central Oaxaquia

Guerrero Composite Terrane Central Oaxaquia (north)

Tumbiscatío Granitoid

Santonian?-MaastrichtianCretaceous

TeloloapanZihuatanejo/Tahueand Mixteca

ArceliaGuanajuato

Guerrero Composite Terrane Central / Oaxaquia

TeloloapanZihuatanejo/Tahue and Mixteca (south)

Red BedsMarine turbidites

CortesTahue

(Guerrero Composite Terrane)Stage IArc collision and development of a passive marginPaleozoic


Caborca

Passive margin sediments

and Central terranes

Arteaga Basin

Subduction

Figure 3. Tectonic models for the evolution of western Mexico, showing the alternating stages of subduction-collision and rifting.


Stage II: Late Triassic Passive Margin—Deposition of the Potosi Fan

The paleo-continental edge of Mexico lay approximately at the western boundary of the Oaxaquia and Mixteca Terranes in the early Mesozoic (Fig. 1; Centeno-García, 2005). Thus the Central and Guerrero Composite Terranes (Fig. 1) were accreted or displaced to their present position during the Mesozoic. Sedi-mentation along the western continental margin of Oaxaquia was dominated by large volumes of siliciclastic turbidites (quartz-rich sandstone and shale) that were deposited in the distal continental shelf or at the continental slope at least during Carnian–Norian time (Fig. 3). Accretionary complexes that form the basement of the Central Terrane and parts of the Guerrero Composite Terrane (Zihuatanejo Terrane) are formed largely (up to 60% of the total area of exposures) by similar quartz-rich sandstone and shale turbidites that made up the matrix within which blocks of vari-able composition are embedded. These turbidites in the accreted terranes contain fossils of the same age as those from turbidites deposited at the continental slope of Oaxaquia.

Detrital zircon ages obtained from turbidites from all the localities of the Carnian–Norian turbidites, from Oaxaquia to basal accretionary complexes of the Central and Zihuatanejo Ter-ranes, show the same populations, which suggest that the fan tur-bidites spread into a marginal oceanic basin that was later accreted to the continental margin. These siliciclastic rocks are grouped as the Potosi Fan (Centeno-García, 2005) and are important because they can be traced from Oaxaquia to the present Pacific Coast of Mexico, and they tie together the Central Terrane, the west-ernmost part of the Guerrero Composite Terrane (Zihuatanejo Terrane), and the continental margin of southern North America (Oaxaquia) during Late Triassic time. Thus, the Potosi Subma-rine Fan may have been a large sedimentary feature, probably close to the dimensions of the present Bengal Fan.

There is no evidence of Triassic magmatism in continental Mexico, and detrital zircon geochronology of the fan turbidites show that the youngest age populations are much older than dep-ositional ages in all the studied localities of the Potosi Fan (Fig. 3; Centeno-García et al., 2005; Centeno-García, 2005). Therefore, the Potosi Fan probably was deposited across a passive margin, or at least a margin that had no active subduction along the length of the fan at the time of deposition.

Stage III: Accretion of the Potosi Fan to the Continental Margin via Subduction—Basement of the Central and Zihuatanejo Terranes

All the Triassic units of central and western Mexico are strongly deformed and partially metamorphosed, indicating that a major compressional event occurred during latest Triassic–Early Jurassic time. This event is characterized by tight folding, shear-ing, and axial cleavage in the continent-slope deposits of the Potosi Fan in Oaxaquia (La Ballena Formation), and block-in-matrix texture in the Taray Formation (Central Terrane),

in the Zacatecas Formation, and in the Arteaga and Las Ollas Complexes (Zihuatanejo Terrane). These last three units formed in the distal ocean-floor zone of the Potosi Fan. The presence of mélanges (Arteaga Complex and Taray Formation) as well as blueschist in the Las Ollas Complex (Zihuatanejo Terrane) indi-cates that deformation occurred in a subduction zone. During this deformational event the turbidites of the Potosi Submarine Fan, with slivers of the oceanic crust and its sedimentary cover, were accreted to the continent. This accretionary prism apparently was very wide, as suggested by the large areas that are floored by it. Whether the subducting slab was dipping toward the west (under an oceanic arc) or the east (under continental Mexico) has not been constrained. There are two isolated reports of dated Early Jurassic volcanic rocks in Oaxaquia (Barboza-Gudiño et al., 2004; Fastovsky et al., 2005), but whether they are part of a continental arc or not is not known. Evidence of contemporane-ous oceanic-arc magmatism is exposed in the Vizcaíno Penin-sula of Baja California (Kimbrough and Moore, 2003), where Triassic–Jurassic volcanic rocks have geochemical signatures of primitive arc affinity. It is possible that the rocks in the Vizcaíno Peninsula represent a displaced fragment of an oceanic arc that accreted to the Arteaga and Las Ollas Complexes of the western Guerrero Composite Terrane, which in turn accreted to the Taray, Zacatecas, and La Ballena Formations, but this model needs to be supported by more evidence.

Stage IV: Late Jurassic Continental Arc—Overlapping Assemblage for Guerrero Composite Terrane, Central Terrane, Oaxaquia, and Mixteca Terrane

Subaerial volcanic and sedimentary rocks, as well as shallow porphyritic intrusives, dikes, and sills, overlie or cut previously deformed Triassic sedimentary rocks in Oaxaquia and rocks of the accretionary prism in the Central Terrane. These rocks range in age from 174 to 158 Ma (Jones et al., 1995; Barboza-Gudiño et al., 2004). A common attribute of all the outcrops of these rocks is that they are mostly rhyolitic in composition, with minor dacitic-andesitic lava flows and tuffs, and show evolved-arc geo-chemical signatures (Centeno-García and Silva-Romo, 1997; Centeno-García, 2002; Centeno-García and Díaz-Salgado, 2002). Coeval volcanic rocks have been reported in the Mixteca Terrane as well, suggesting that arc volcanism was widespread in conti-nental Mexico at that time (García-Díaz et al., 2004). Rocks of similar age range and similar evolved-arc geochemical signatures are exposed in the western Zihuatanejo Terrane of the Guerrero Composite Terrane (Bissig et al., 2003; Centeno-García et al., 2003). This suggests that the Guerrero Composite Terrane may have been incorporated into the continental margin by that time.

Summarizing the data described above: (1) Triassic base-ment rocks of the Zihuatanejo Terrane (Guerrero Composite Ter-rane) share a provenance linkage with rocks of the same age in Oaxaquia and the Central Terrane; (2) all Triassic rocks, from those deposited on the paleo-continent’s margin of Mexico to those within the accreted terranes, were deformed previous to the


development of a Late Jurassic continental arc; and (3) evolved Upper Jurassic continental-arc volcanism was widespread among continental Mexico and accreted terranes (Central and Zihua-tanejo Terranes). On the basis of these facts, we propose in this paper that the first accretion of the Guerrero Terrane occurred during latest Triassic–Early Jurassic time instead of near the end of the Cretaceous, as previously proposed by other authors. Therefore, the Late Jurassic magmatic event represents an over-lapping assemblage that stitches all the terranes of central and western Mexico for that period.

Stage V: Late Jurassic–Early Cretaceous Intra-Arc Strike Slip(?)–Rifting of the Continental Arc—Drifting of the Guerrero Composite Terrane

It has been proposed that major lateral displacements occurred during the activity of the Jurassic continental arc of stage IV (Anderson and Silver, 2005). Therefore, the arc was originally in a more northerly position, and it was displaced, via the Mojave-Sonora Megashear, to its present position in central Mexico prior to, or at, the early stage of development of the cal-careous platform (Anderson and Silver, 2005).

Whether this major strike-slip system existed or not has been widely discussed (see GSA Special Paper 393). We consider that extensive geological evidence of major tectonism during and after arc volcanism exists (see following discussion). The cessa-tion of magmatism in the Central Terrane and Oaxaquia suggests a change in the location of the subduction zone. Then, a major regional calcareous platform developed over the arc and other older rocks. This major transgression initiated the deposition of limestone on Oaxaquia, and on the Mixteca and Central Ter-ranes. Calcareous sedimentation in central and eastern Mexico was characterized by high subsidence rates (Goldhammer, 1999). Arc magmatism continued only in a small area in the western Mixteca Terrane and became widespread in the Guerrero Com-posite Terrane. Although there is some overlap in age ranges of arc volcanism among the terranes that form the Guerrero Com-posite Terrane, there is a general trend from older ages in eastern Oaxaquia and the Central Terrane to younger ages in the western Guerrero Composite Terrane (Fig. 3). This suggests a possible W-SW migration of the subduction zone. We propose that dur-ing and after the continental arc activity (Late Jurassic–Early Cretaceous time), large amounts of extension and lateral trans-lations may have occurred (see inferred faults in Fig. 1). This extensional-transtensional(?) event split the continental arc, ini-tiating the drifting of parts of previously accreted oceanic rocks (basements of the Tahue and Zihuatanejo Terranes) and the gen-eration of new oceanic crust (Guanajuato and Arcelia Terranes).

With the data available, it seems that volcanic activity at the northern Zihuatanejo Terrane and at the Guanajuato and Teloloa-pan Terranes was restricted to latest Jurassic–Early Cretaceous time (Fig. 3). In contrast, in the Arcelia, Tahue, and southern Zihuatanejo Terranes, arc volcanism apparently continued up to Albian–Cenomanian time (Fig. 3). Geochemical and isotopic

compositions of most of the Upper Jurassic–Cretaceous igneous rocks of the different arc assemblages of the Guerrero Composite Terrane suggest primitive sources, with little or no influence on an evolved continental crust (e.g., Ortiz-Hernandez et al., 1991; Lapierre et al., 1992; Centeno-García et al., 1993a; Freydier et al., 1995; Gastil et al., 1999; Talavera-Mendoza et al., 1995; Men-doza and Suastegui, 2000, among others). Basalts with ocean-island (OI) and mid-oceanic-ridge basalt (MORB) signatures of the Arcelia and Guanajuato terranes (Lapierre et al., 1992; Ortiz-Hernandez et al., 2003; Mendoza and Suastegui, 2000) suggest the influence of a mantle source for the magmatism.

Regional differences in the strata suggest abrupt lateral changes in the depositional environments from shallow marine to deep marine. Also, lateral differences in thickness of the suc-cessions suggest that they may have been deposited in alternate subsiding basins and basement highs where the deposits draped thinly or were absent. These major geological differences suggest that intra-arc rifting was considerable and was probably associ-ated with a complex paleogeography of marginal arc and backarc systems in western Mexico. Whether or not the different terranes of the Guerrero Composite Terrane were formed in a single arc has not been constrained. Some authors proposed that the Guer-rero Terrane formed from a complex system of two or three arcs (Ramírez-Espinosa et al., 1991; Mendoza and Suastegui, 2000). However, no Cretaceous subduction-related accretionary prisms have been identified within any of the terranes of the Guerrero Composite Terrane.

Stage VI: Final Accretion of the Guerrero Composite Terrane, and Development of a New Continental Arc

A major Late Cretaceous–early Paleogene orogenic phase is recorded throughout Mexico, coeval to the Sevier and Laramide orogenies in western North America. This event is associated with the Mexican Fold and Thrust Belt of the Sierra Madre Oriental. Apparently, final amalgamation of the Guerrero Composite Ter-rane occurred during this orogenic event, and volcanic and sedi-mentary rocks of the Teloloapan, Guanajuato, Zihuatanejo, and Tahue Terranes were thrust over the calcareous platform rocks of Oaxaquia and the Central, Cortes, and Mixteca Terranes. The amount of tectonic transport apparently is significant, as xeno-liths of Precambrian continental crust were found in Cenozoic volcanic rocks that erupted onto accreted rocks of the Guanajuato Terrane (Urrutia-Fucugauchi and Uribe-Cifuentes, 1999). Sig-nificant tectonic transport is also suggested by the amount of shortening that produced tight folding and major thrusting within the northern Zihuatanejo Terrane and the Arcelia and Teloloapan Terranes (Salinas-Prieto et al., 2000). In contrast, deformation of Cretaceous rocks in the southern parts of the Zihuatanejo Terrane formed wide regional anticlines, and some overturned folds and minor thrust faults locally. The structures generally trend NW-SE, although locally some structures trend N-S and E-W.

Santonian terrestrial sedimentation covers unconformably the previously deformed arc assemblages of the Zihuatanejo


Terrane (Benammi et al., 2005). Synorogenic sedimentary basins (Caracol Formation in Oaxaquia, and Mexcala Formation in the Mixteca Terrane) containing clasts derived from the Guerrero Composite Terrane suggest that these terranes were deformed and exhumed by that time. In addition, synorogenic sedimenta-tion overlaps the Arcelia and Teloloapan terranes (Miahuatepec Formation), which suggests that these two terranes were also amalgamated during the same orogenic event (Mendoza and Sua-stegui, 2000; Guerrero-Suastegui, 2004). All these synorogenic basins range in age from Turonian to Maastrichtian. In addition, Paleocene granitoids along the coast cut the previously folded units of the Zihuatanejo Terrane and suggest a Late Cretaceous–early Paleogene deformation.

Therefore, final amalgamation of the Guerrero Composite Terrane occurred between Santonian and Turonian–Maastrich-tian time.

DISCUSSION

This section summarizes the stratigraphic, structural, and geochemical data that support the proposed stages for the tec-tonic evolution of western Mexico.

Stage I: Origin of the Basement of the Tahue Terrane

Exposures of pre-Cretaceous rocks in northwest Mexico are scattered; thus contact relationships among them can only be indirectly inferred (Figs. 1 and 4). Approximate distribution of the contacts among the terranes of western Mexico (Caborca, Cortes, and Tahue; Figs. 1 and 4) was outlined on the basis of the geographic distribution of pre-Cretaceous outcrops and lat-eral changes in the isotopic signatures of Cretaceous–Paleogene granitoids (Valencia-Moreno et al., 2001). Thus the nature of the contacts and the amount of displacement among different base-ments are unknown. In this section the main stratigraphic units that define the terranes are described following a NW to SE tran-sect throughout the Paleozoic rocks of the Caborca, Cortes, and Tahue Terranes (Guerrero Composite Terrane).

At the southern margin of the Caborca Terrane a thick shel-fal limestone succession is exposed that contains Carboniferous–Permian fusulinids and other shallow-marine fossil fauna (Stew-art et al., 1990). These rocks are overridden by a north-verging major thrust fault that places deeper marine sedimentary rocks of the Cortes Terrane on the shelfal rocks of the Caborca Ter-rane (Fig. 1; Coney and Campa, 1987; Poole and Madrid, 1988; Stewart et al., 1990).

Basal metamorphic rocks are not exposed in the Cortes Ter-rane, but its basement has been interpreted as thinned Proterozoic rocks, perhaps the same as in the Caborca Terrane, or else Pro-terozoic metamorphic rocks different from those of the Caborca Terrane (McDowell et al., 1999; Valencia-Moreno et al., 1999; Valencia-Moreno et al., 2001). The deep-marine sedimentary rocks of the Cortes Terrane are sandstone and shale turbidites, graptolitic shale, chert, and layered barite that range in age from

Ordovician to Devonian–Early Mississippian and were deformed during the Mississippian (Poole and Madrid, 1988; Stewart et al., 1990). These rocks are in turn overlain by Upper Carboniferous and Permian turbidites (Fig. 5; Poole and Madrid, 1988; Stew-art et al., 1990; Poole et al., 2005). They all were deposited in a deep-marine environment and are interpreted to be part of the Paleozoic continental slope-rise deposits of western North Amer-ica (Poole and Madrid, 1988; Stewart et al., 1990). All these units of the Cortes Terrane were deformed and thrust over the Caborca Terrane by Late Permian to Early Triassic time, and they are unconformably covered by Upper Triassic terrestrial and marine sedimentary rocks (Stewart et al., 1990). Therefore, the Caborca and Cortes Terranes were assembled by early Mesozoic time.

The nature of the contact between the Cortes and Tahue Ter-ranes (the latter belonging to the Guerrero Composite Terrane) has not been mapped in detail. It is inferred to be a thrust that verges toward the north, and it is probably north of El Fuerte town in Sinaloa State (Fig. 5), based on the northernmost expo-sures of Cretaceous marine volcanic rocks of the Guerrero Ter-rane (Servais et al., 1982; Henry and Fredrikson, 1987; Roldán-Quintana et al., 1993; Freydier et al., 1995).

The oldest Paleozoic rocks of the Tahue Terrane (Guer-rero Composite Terrane) are exposed in the area of El Fuerte (El Fuerte Complex; Figs. 4 and 5). The El Fuerte Complex is formed by marine rhyolitic to andesitic lava flows and volca-niclastic rocks, interbedded with quartz-rich sandstone, shale, and thin-bedded limestone (Mullan, 1978; Roldán-Quintana et al., 1993; Poole and Perry, 1998). All these various components are deformed and metamorphosed to greenschist facies (Mul-lan, 1978; Roldán-Quintana et al., 1993). Sedimentary rocks of the El Fuerte Complex contain Ordovician conodonts (Poole and Perry, 1998). Preliminary geochemical analyses indicate a calc-alkaline island-arc affinity for the volcanic rocks of the El Fuerte Complex, and are similar to those from coeval Paleo-zoic arc rocks in the Klamath Mountains of Northern California (Lapierre et al., 1987). However, more detailed geochemical and geochronological work needs to be done to constrain their origin and relationships.

Upper Paleozoic deep-marine sedimentary rocks are ex-posed south of El Fuerte, in San José de Gracia town, Mazat-lán City, and other scattered localities in Sinaloa State (Figs. 4 and 5). These rocks belong to the San José de Gracia Formation (Carrillo-Martínez, 1971; Gastil et al., 1991; Arredondo-Guerrero and Centeno-García, 2003) and are made up of quartz-rich sand-stone and shale turbidites, thin-bedded calcareous debris flows, black shale, and chert. The turbidites contain olistoliths of lime-stone with chert nodules, which in turn contain Middle Pennsyl-vanian to Early Permian fossils at the San José de Gracia locality (Carrillo-Martínez, 1971; Gastil et al., 1991). The San José de Gracia Formation has been interpreted as deposits in a deep-marine environment (Gastil et al., 1991). The contact between the El Fuerte Complex and the San José de Gracia Formation is not exposed. However, major differences in deformation and metamorphism (turbidites of the San José de Gracia Formation

Granitoids (Upper Jurassic-Lower Cretaceous)Cretaceous Guerrero Arc Marine volcanic and sedimentary rocks

San Francisco Gneiss (Triassic)

El Fuerte Complex (Ordovician)

San Jose de Gracia Fm. (Carboniferous)

106°107°108°109°

24°

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26°

27°

23°

R A N

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0 25 50 100km

MAZATLÁN

CULIACÁNRecent sediments

Recent basalts

Miocene-PlioceneVolcanic and sedimentary (conglomerate-sandstone)

Granitoids (Paleocene-Oligocene)

Upper Cretaceous-OligoceneTarahumara Formation and Lower and UpperVolcanic Supergroup

Tahue Terrane (Guerrero Composite Terrane)

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Terrestrial and marinesedimentary rocks (Triassic)

Caborca Terrane

N

Cortes Terrane

Caborca Terrane

Cortes Terrane

Figure 4. Geologic map of Sinaloa and the southern Sonora states, showing the geology of Caborca, Cortes, and Tahue Terranes (after Carrillo-Martínez, 1971; Mullan, 1978; Gastil et al., 1978; Henry and Fredrikson, 1987; Stewart and Roldán-Quintana, 1991; Ortega et al., 1992; and our own work).

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ity

Pale

ozoi

c

Earl

y Tr

iass

ic

Maa

stric

htia

n

Cam

pani

an

San

toni

an

Con

iaci

an

Turo

nian

Cen

oman

ian

Alb

ian

Apt

ian

Bar

rem

ian

Haute

rivia

n

Vala

ngin

ian

Ber

riasi

an

Tith

onia

nKi

mmer

idgian

Oxf

ordi

an

Cal

lovia

n B

atho

nian

Baj

ocia

nAa

leni

an

Toar

cian

Plien

sbac

hian

Sin

emur

ian

Het

tang

ian

Rha

etia

n

Noria

n

Carni

anLa

dinian

GUER

RERO

COMP

OSITE

TERR

ANE

Felsic

lavas

and

volcan

iclastic

rocks

189-17

2 Ma


are strongly deformed but not metamorphosed) indicate that the contact is probably an angular unconformity.

Both units of the Tahue Terrane (El Fuerte Complex and San José de Gracia Formation) are important because they can constrain the paleogeography of the northern Guerrero Terrane. Preliminary single-grain, detrital zircon geochronology from the quartz-rich sandstone from the turbidites of the San José de Gracia Formation shows populations that have a North American affinity (Centeno-García et al., unpublished data) and are similar to those from Paleozoic rocks in Baja California, in the Cortes Terrane, and in Nevada (Gehrels et al., 2002).

The stratigraphy, geochemistry, and provenance of the Paleo-zoic rocks suggest that the Tahue Terrane (Guerrero Composite Terrane) was linked to the tectonic evolution of the western conti-nental margin of North America, probably up to Permian–Triassic time. After that, there were major differences in the composition of the Mesozoic sedimentary cover of the Caborca-Cortes Ter-ranes with respect to that of the Tahue Terrane. Therefore, it is likely that a fragment of previously accreted island-arc and con-tinent-margin assemblages drifted from the continental margin sometime in the early Mesozoic.

Contact relationships between the Paleozoic sedimentary rocks of the San José de Gracia Formation (Tahue Terrane) and the Triassic subduction-related complex of the Zihuatanejo Ter-rane are unknown because the contact is covered by younger rocks. However, the Tahue and Zihuatanejo Terranes share simi-lar Cretaceous volcanic and sedimentary cover.

Stages II and III: Triassic Potosi Fan and Its Accretion to the Continental Margin

There are few exposures of Triassic rocks in Mexico, and they are limited to the Caborca and Cortes Terranes, western Oaxaquia, the Central and Zihuatanejo Terranes, and a small out-crop in the Vizcaíno Peninsula in Baja California. Triassic rocks have not been found in the Mixteca Terrane or in other terranes of Mexico. In this section we briefly describe Triassic rocks of the Cortes and Tahue Terranes (Barranca Group and Francisco Gneiss) and focus on the marine Triassic rocks of Oaxaquia (La Ballena Formation), the Central Terrane (Taray Formation), and the Guerrero Composite Terrane (Zacatecas Formation, and the Arteaga and Las Ollas Complexes).

Barranca Group and Francisco GneissTriassic (Carnian–Norian) sedimentary rocks of the Cortes

Terrane are made up of fluvial sandstone and shale that contain abundant coal layers (Barranca Group; Stewart and Roldán-Quintana, 1991). These sediments were deposited unconform-ably on previously deformed Paleozoic deep-marine rocks. The Triassic fluvial deposits change transitionally up the column to shallow-marine siliciclastic deposits. These rocks have no evi-dence of contemporaneous volcanism. In contrast, Triassic rocks of the Tahue Terrane (Guerrero Composite Terrane) are made up of metamorphosed igneous rocks of the Francisco Gneiss near

Sonobari (Figs. 4 and 5; Mullan, 1978; Keppie et al., 2006). The Francisco Gneiss is made up of migmatized gneisses and amphibolites that have within-plate and continental tholeiite geo-chemical signatures (Keppie et al., 2006). This suggests that the Tahue and Cortes terranes may have been geographically sepa-rated by that time.

La Ballena FormationTriassic rocks of Oaxaquia crop out on its western margin,

near its boundary with the Guerrero Composite Terrane (Figs. 1 and 6). They are grouped as the La Ballena Formation (Silva-Romo, 1993; Silva-Romo et al., 2000), and their largest expo-sures are in the Peñón Blanco, Charcas, and Real de Catorce areas (Fig. 6; Silva-Romo, 1993; Tristán-González and Torres-Hernández, 1994; Barboza-Gudiño et al., 2004). The La Ballena Formation is made up of quartz-rich sandstone and shale, and scarce conglomerates deposited as small channel-fill lenses. The sedimentary structures of these Triassic rocks indicate deposi-tion mostly by turbidity currents, although some debris flows and large slumps are present. This sequence contains abundant trace fossils and ammonites and bivalves of Late Triassic (Car-nian) age at the Peñón Blanco and Charcas areas (Cantu-Chapa, 1969; Silva-Romo et al., 2000; Bartolini et al., 2002). Sedimen-tary structures and fossil fauna suggest that the deposition of this unit occurred in a submarine fan that developed on an external platform or continental slope setting. These rocks form part of the Potosi Submarine Fan (Centeno-García, 2005). The origi-nal thickness is unknown, but up to 4640 m was penetrated by exploration drilling without reaching the base of the succession (López-Infanzón, 1986).

Taray FormationSimilar marine siliciclastic rocks crop out at the Pico de

Teyra region in the Central Terrane (Figs. 5 and 6). They belong to the Taray Formation, made up of highly deformed quartz-rich turbidites (sandstone and shale) interbedded with some black chert and scarce detrital limestone that contains fragments of cri-noids, gastropods, corals, bivalves, and bryozoans (Diaz-Salgado et al., 2003). The Taray siliciclastic turbidites form a matrix within which blocks of black and green chert, pillow basalts, ser-pentinite, and crystallized limestone can be found (Figs. 5 and 6; Diaz-Salgado et al., 2003). The age of this unit remains undeter-mined; however, there are reports of fusulinids from one of the limestone blocks (Anderson et al., 1990). The youngest detrital zircons collected from the sedimentary matrix are Late Permian in age (Diaz-Salgado et al., 2003). There is also a report of molds of bivalves of possible Carnian age (Barboza-Gudiño et al., 1999; Bartolini et al., 2002). Thus deposition of the sedimentary matrix should have occurred between the Late Permian to the Late Trias-sic. The Taray Formation has a block-in-matrix structural style, formed by centimeter-size blocks to blocks of hundreds of meters in size, all in a highly sheared sedimentary matrix. This charac-teristic is typical of a subduction accretionary complex (Ander-son et al., 1990, 2005; Diaz-Salgado et al., 2003).


Zacatecas FormationThe oldest rocks of the Zihuatanejo Terrane in its northern-

most exposure are Triassic in age as well (Fig. 6). They make up the Zacatecas Formation, which crops out in a small tectonic window at the western margin of Zacatecas City (Fig. 6; Burck-hardt and Scalia, 1906; Ranson et al., 1982; Cuevas-Pérez, 1983; Monod and Calvet, 1991). This formation is made up of quartz-rich turbidites (sandstone and shale) that contain blocks of pil-low basalts that have MORB geochemical signatures (Fig. 7; Centeno-García and Silva-Romo, 1997). The Zacatecas Forma-tion contains fossil ammonites and bivalves of Late Triassic (Car-nian) age (Burckhardt and Scalia, 1906; Bartolini et al., 2002). Its contact with the La Borda Formation of Late Jurassic(?)–Cre-taceous age is inferred to have been originally an unconformity, but it was sheared and detached during Late Cretaceous thrust-ing and folding (Fig. 3). Rocks of the Zacatecas Formation show

structures associated with two distinct deformational events, one of them prior to the deformation that is recorded in the Creta-ceous rocks as well. The small size of the outcrop prohibits con-straints on the tectonic origin of the Zacatecas Formation, but its lava flows and siliciclastic turbidites are similar to those from the Arteaga Accretionary Complex, which is exposed in the southern part of the Zihuatanejo Terrane.

Arteaga ComplexMore exposures of Triassic(?) rocks are found in the south-

ern part of the Zihuatanejo Terrane (Fig. 7). Their largest outcrops are located in the Arteaga, Placeres del Oro, and Tiquicheo areas (Arteaga Complex) and near Zihuatanejo City (Las Ollas Complex) (Fig. 8; Centeno-García et al., 1993a, 1993b; Talavera-Mendoza et al., 1995; Mendoza and Suastegui, 2000). The Arteaga Com-plex is made up of quartz-rich turbidites (sandstone and shale),

100 km

Zacatecas

Guanajuato

Peñón Blanco

Charcas

Pico de Teyra

Caopas

Huizachal

HuiznopalaGranjeno

Real de Catorce

U/Pb 158 Ma

U/Pb190 Ma

U/Pb 147-150 Ma

Centralterrane

Zihuatanejoterrane

Guanajuato

terrane

San Luis Potosí

Concepción del Oro

Matehuala

24°

23°

22°

101° 100°102°

Cenozoic cover

Ju-K Calcareous Platform

Ju-K pillow basalts, deep marine volcanic turbidites, chert, and mafic and ultramafic plutons

Ju-K pillow basalts, volcanic turbidites, detrital limestone

Ju felsic and intermediate lavaflows, epiclastic rocks and redbeds

Oaxaquia and Central Terrane

Proterozoic and Paleozoic metamorphic and sedimentary rocks

Tr Quartz-rich shale and sandstoneturbidites (Potosí Fan)Tr(?) Accretionary complexblocks of pillow basalt, ultramafic,chert, and marble in a sedimentary quartz-rich matrix

Guanajuato Terrane

Zihuatanejo Terrane

Tr Accretionary complexblocks of pillow basalt, in a sedimentary quartz-rich matrix

U/Pb146 Ma

O a x a q u i a

Figure 6. Geologic map of central Mexico, showing main stratigraphic units of Oaxaquia and the Central, Zihuatanejo, and Guanajuato Terranes (modified from Ortega et al., 1992). Tr—Triassic; Ju—Jurassic; K—Cretaceous.

subaer

ial to

marin

e sha

le, san

dstone

, eva

porites

, limest

one,

basalti

c-ande

sitic

lava f

lows

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anian

Com

burin

dio

and

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aso

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shallow

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e lim

estone

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ticroc

ks, fe

w ande

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a flow

sAp

tian Al

bian

Arte

aga C

ompl

exNo

rian-

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ianma

trix is

quartz

-rich t

urbidit

esblo

cks of

pillow

lavas,

chert

,ser

pentini

te and

limest

oneacc

retion

ary co

mplex

Anga

o an

d Sa

n Lu

cas F

ms.

volcan

ic turb

idites,

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felsic a

nd bas

ic lava

s sha

llowing

upwa

rdNe

ocomia

n

Villa

de A

yala

Fm

.MO

RB ba

saltic

pillow

lavas,

IA lav

as and

dik

es, ch

ert, d

eep

marin

e volc

anicla

stictur

bidites

Early

Cretac

eous

Cuale

fels

ic lava

s and

Tumbs

catío g

ranitoi

ds 16

3-155

Ma

Lim

esto

neAp

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an

felsic a

nd and

esitic

lavas

and

volcan

iclastic

ssha

llow ma

rine in

the

east, d

eep ma

rine in

the

west

145-13

7 Ma

Cuau

tla an

d Mo

relo

s Fm

s.Lim

estone

Aptian

-Albia

nTa

xco

Schi

st,

Zica

pa an

d Ch

apol

apa

felsic l

avas a

nd vol

canicla

stics

contine

ntal a

ffinity

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lan C

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exm

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poly d

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ed sed

imenta

ry and

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lision

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le and

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eland-

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illArc

elia Te

rrane

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apanTe

rrane

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ca Te

rrane

(wes

tern C

ontin

ent-m

argin)

subaer

ial few

marin

e vol

canicla

stic ro

cks,

felsic l

avas

Aptian

unconf

ormity

unconf

ormity

unconf

ormity

127-13

3 Ma

volca

niclas

tic ro

cks

and l

imes

tone

age n

ot we

ll co

nstra

ined

Coast

al regi

onHu

etamo

regio

n

WES

TEA

ST

Pale

ozoi

cEa

rly

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ssic

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stric

htia

n

Cam

pani

an

San

toni

an

Con

iaci

an

Turo

nian

Cen

oman

ian

Alb

ian

Apt

ian

Bar

rem

ian

Haute

rivia

n

Vala

ngin

ian

Ber

riasi

an

Tith

onia

nKi

mmer

idgian

Oxf

ordi

an

Cal

lovia

n B

atho

nian

Baj

ocia

nAa

leni

an

Toar

cian

Plien

sbac

hian

Sin

emur

ian

Het

tang

ian

Rha

etia

n

Noria

n

Carni

anLa

dinian

GUER

RERO

COMP

OSITE

TERR

ANE

Las Llu

vias

Ignimb

rite168

-179 M

a

Figu

re 7

. Sim

plifi

ed s

trat

igra

phic

col

umns

for

the

terr

anes

des

crib

ed in

the

text

that

are

sou

th o

f th

e T

rans

mex

ican

Vol

cani

c B

elt,

exce

pt f

or t

he G

uana

juat

o Te

rran

e, a

nd in

clud

e th

e M

ixte

ca T

erra

ne a

nd th

e Te

lolo

apan

, Arc

elia

, and

Zih

uata

nejo

Ter

rane

s (G

uerr

ero

Com

posi

te T

erra

ne).

Ver

tical

sca

le s

how

s th

e ag

e ra

nge

(in

Ma)

. MO

RB

—m

id-o

cean

ic-r

idge

bas

alt;

IA—

isla

nd a

rc.

ZIH

UA

TA

NE

JO

TE

RR

AN

E

040

1020

Zap

otitl

ánN

evad

o de

Col

ima

Min

atitl

án

Telo

loap

anTaxc

o

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la

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sas

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er

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lpan

cing

oZ

ihua

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tam

o

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cuar

o

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apa

Cha

pola

pa

deep marine

shallow marine

Arc

elia

Pla

ya A

zul

Nue

vaIta

lia

103°

ARCELIA TERRANE

N

TELOLOAPAN TERRANE

104°

102°

101°

Col

ima

Láza

ro C

árde

nas

19°

100°

18°

PA

CI F

I C O

CE

AN

km

MIX

TEC

O T

ERR

AN

E

XOLA

P A T

ERR

AN

E

Mix

teca

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rane

Xola

pa T

erra

ne

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an T

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ne

Arc

elia

Ter

rane

Zihu

atan

ejo

Terr

ane

Arte

aga

Sub

duct

ion

Com

plex

(T

riass

ic)

Las

Olla

s S

ubdu

ctio

n C

ompl

ex

(age

unk

now

n)

Mid

dle

to U

pper

Jur

assi

c pl

uton

s

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stal

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tace

ous

arc

asse

mbl

age

shal

low

mar

ine

and

terre

stria

l rhy

olite

, an

desi

te a

nd s

ome

basa

lt, lim

esto

ne,

volc

anic

last

ic a

nd b

asem

ent-d

eriv

ed

clas

tic ro

cks

Hue

tam

o ar

ea, C

reta

ceou

s ar

c as

sem

blag

e, m

ostly

mar

ine

volc

anic

last

ic ro

cks,

limes

tone

and

so

me

pryr

ocla

stic

and

lava

flow

s

Low

er C

reta

ceou

s ar

c as

sem

blag

essh

allo

w m

arin

e an

desi

tic to

bas

altic

la

va f

low

s an

d vo

lcan

icla

stic

rock

s,

mas

sive

and

reef

al lim

esto

ne (e

ast)

deep

mar

ine

lava

flow

s, c

alca

reou

s de

bris

flow

s an

d vo

lcan

ic tu

rbid

ites

(wes

t)

Mia

huat

epec

For

mat

ion

Upp

er C

reta

ceou

s cl

astic

rock

sfo

rela

nd b

asin

-fill (

over

laps

A

rcel

ia a

nd T

elol

oapa

n te

rrane

s)

Cre

tace

ous

IAB

pillo

w la

vas,

fine-

grai

ned

volc

anic

turb

idite

san

d ch

ert (

deep

mar

ine)

Cre

tace

ous

MO

RB

pillo

w la

vas,

fine-

grai

ned

volc

anic

turb

idite

san

d ch

ert (

deep

mar

ine)

Mex

cala

For

mat

ion

Turo

nian

-Maa

stric

htia

n cl

astic

rock

sfo

rela

nd b

asin

-fill (

over

laps

M

ixte

co a

nd T

elol

oapa

n te

rrane

s)

Apt

ian-

Alb

ian

Cal

care

ous

Pla

tform

Low

er C

reta

ceou

s co

ntin

enta

lar

c as

sem

blag

es, m

arin

e an

d te

rrest

rial r

hyol

ite to

and

esite

lava

flo

ws

and

epic

last

ic ro

cks,

qu

artz

-rich

cla

stic

rock

s

Jura

ssic

to C

reta

ceou

s m

igm

atite

s, g

neis

ses,

and

plu

tons

Pal

eozo

ic A

catlá

n C

ompl

ex

Ove

rlapp

ing

asse

mbl

ages

Cut

zam

alaF

orm

atio

nS

anto

nian

-Maa

stric

htia

n re

d be

ds(o

verla

ps Z

ihua

tane

jo a

nd A

rcel

ia

terra

nes)

Cen

ozoi

c vo

lcan

ic a

ndse

dim

enta

ry ro

cks

Upp

er C

reta

ceou

s to

Pal

eoge

negr

anito

ids

Obs

erve

d te

rrane

bou

ndar

y (th

rust

faul

t)

infe

rred

terra

ne b

ound

ary

(thru

st fa

ult)

Riv

ers

Roa

ds

Sta

te lim

it

infe

rred

terra

ne b

ound

ary

(stri

ke-s

lip fa

ult)

Jilo

tlán

Teca

litlá

n

Piju

amo

Man

zani

llo

Teco

man

Coa

lcom

anA

guili

lla

U/P

b 16

3+3

Ma

Aqu

ila

Tepa

lcat

epec

Art

eaga

Tum

bisc

atío

Ar/A

r 152

-158

Ma

Bal

sas

Riv

er

Infie

rnill

o D

am

Teju

pilc

o

Figu

re 8

. G

eolo

gic

map

of

sout

hwes

tern

Mex

ico,

sho

win

g th

e si

mpl

ified

geo

logy

of

the

Mix

teca

, Tel

oloa

pan,

Arc

elia

, an

d Z

ihua

tane

jo T

erra

nes

(aft

er C

ampa

and

Ram

irez

, 19

79;

Ort

ega

et a

l., 1

992;

Tal

aver

a-M

endo

za e

t al.,

199

5; C

oron

a-C

háve

z an

d Is

rade

-Alc

ánta

ra, 1

999;

Men

doza

and

Sua

steg

ui, 2

000;

Cen

teno

-Gar

cía

et a

l., 2

003)

. IA

B—

isla

nd-a

rc b

asal

t; M

OR

B—

mid

-oce

anic

-rid

ge b

asal

t.


black and green chert, and mafic tuff that form a matrix that con-tains blocks and slabs of pillow basalts, diabase, banded gabbros, chert, and limestone, all deformed in a block-in-matrix structural style (Centeno-García et al., 2003). Chert layers contain radio-larians of Triassic (Ladinian–Carnian) age (Campa et al., 1982). Pillow basalts and gabbros have oceanic geochemical signatures (MORB; Centeno-García et al., 1993a; Centeno-García et al., 2003). Sedimentary structures preserved in some exposures of unmetamorphosed turbidites, along with the affinity of the few fossils found in the sedimentary rocks of the matrix, suggest that the sequence was deposited in a deep-ocean environment. Appar-ently the quartz-rich turbidites were contemporaneous with oce-anic magmatic activity, as they are interbedded with volcaniclas-tic rocks (Centeno-García et al., 2003). The block-in-matrix style of deformation of the Arteaga Complex, as well as its lithological associations, indicate that it was formed in a subduction accre-tionary prism. Metamorphism ranges from none to amphibolite facies; blueschist facies has not been found in the area.

Las Ollas ComplexThe Las Ollas Complex forms part of the Zihuatanejo

Terrane and is exposed near Zihuatanejo City (Figs. 7 and 8; Talavera-Mendoza, 2000). This complex is a tectonic mélange formed by highly sheared blocks of metabasalt, banded and mas-sive gabbro, metadolerite, ultramafic rocks, and shale and quartz-rich sandstone (Talavera-Mendoza, 2000). These blocks are enveloped in a highly sheared clastic (quartz-rich sandstone) or serpentinitic matrix (Talavera-Mendoza, 2000). Blueschist facies were reported by Talavera-Mendoza (1993, 2000). Geochemical compositions of the basalts are typical of MORB and primitive oceanic-arc magmas (Talavera-Mendoza, 2000). 40Ar/39Ar and K/Ar ages obtained from amphibole from several metagabbro blocks range from 223 Ma to 96 Ma (Permian to early Ceno-manian) (Delgado, 1982; A. Iriondo, 2003, personal commun.). This has been interpreted to be the subduction complex of the Cretaceous arc (Vidal-Serratos, 1991; Talavera-Mendoza, 1993); however, its contact relationships with Cretaceous arc-related rocks, and similarities with the Arteaga Complex, suggest an ear-lier origin.

Quartz-rich turbidites from the La Ballena Formation of Oaxaquia (continental Mexico), the matrix of the Taray Forma-tion of the Central Terrane, and the Arteaga and Las Ollas Com-plexes and the Zacatecas Formation of the Zihuatanejo Terrane (Guerrero Composite Terrane) have similar and distinctive com-positions and detrital-zircon age populations (Centeno-García et al., 2005; Talavera-Mendoza et al., 2007). Therefore, Triassic sedimentation of the central and western terranes of Mexico is linked by provenance. The youngest zircon age populations from all the samples (latest Permian) are much older than the depo-sitional ages of the turbidites (Carnian–Norian), which means that there was no active volcanism at that time. In other words, there is no evidence of Late Triassic continental arc volcanism in Mexico. Zircon age populations of the Potosi Fan are different from those of Triassic quartz-rich sandstone from the Caborca

and Cortes Terranes (González-León et al., 2005) but are simi-lar to those from Triassic fluvial sedimentary rocks of Arizona (Anderson, 2006). This suggests that at the end of the Triassic the terranes of central and western Mexico may have been to the north of their present locations.

Based on this evidence, we propose that the margin of the western paleo-continent of Mexico was passive or rifting at the end of the Triassic. This passive margin received abundant clastic sedimentation, forming the large Potosi Fan. Sediments of this fan were deposited on oceanic crust (Arteaga Basin in Fig. 3). When subduction started, slivers of the ocean floor were tectoni-cally mixed with the already existing passive-margin quartz-rich turbidites that were forming the Taray and Zacatecas Formations as well as the Arteaga and Las Ollas Complexes. Whether the ocean basin that was covered by sediments of the Potosi Fan was an active marginal oceanic basin, a marginal backarc basin, or an open ocean–continent flank is still uncertain. The only potential evidence of association of the Potosi Fan sediments with tholei-itic oceanic volcanism is the volcaniclastic rocks interbedded with the siliciclastic turbidites in the Arteaga Complex, as the volcaniclastic rocks have geochemical signatures between primi-tive island arc and MORB (Centeno-García et al., 2003).

At least two phases of deformation are found in all the Trias-sic rocks of Oaxaquia and the Central and Zihuatanejo Terranes. The first event comprised strong shearing and tight folding, and the block-in-matrix structures. A second event was recorded only in the Arteaga Complex. This event deformed the Jurassic granitoids as well, and it is characterized by a mylonitic fabric. The third event was common to all the Triassic units and is also recorded in the Jurassic and Cretaceous cover sediments, and it is characterized by axial cleavage, open to tight folding, reverse faulting, and thrusting.

The time of accretion of the Central Terrane with Oaxaquia is assumed to have been prior to the Middle Jurassic, because the La Ballena Formation of Oaxaquia and the Taray Formation of the Central Terrane were deformed and locally metamorphosed prior to deposition of Upper Jurassic terrestrial volcanic and clas-tic formations (Tristán-González and Torres-Hernández, 1994; Jones et al., 1995; Silva-Romo et al., 2000). The Zihuatanejo Terrane (Guerrero Composite Terrane) was also accreted at that time, because the Arteaga Complex is cut by granitoids of Middle Jurassic age as well (Centeno-García et al., 2003).

The subduction zone that formed the Taray and Zacatecas Formations, and the Arteaga and Las Ollas Complexes, was probably constructed along the continental margin of Oaxaquia in Early Jurassic time. Whether the subducting slab was dipping to the east or to the west has not been determined.

Stage IV: Jurassic Continental Arc of Western Mexico

Erosion and exhumation of the accreted continental slope sediments and the accretionary complexes occurred prior to the initiation of Middle to Late Jurassic magmatism. This is indicated by the major angular unconformity that separates the Jurassic arc


succession from the deformed Triassic rocks of Oaxaquia and the Central and Zihuatanejo Terranes. Jurassic arc magmatism has also been identified in the Mixteca Terrane. The Jurassic arc rocks have different names at different locations; they are hereby described by their occurrence in different terranes:

Nazas, Huizachal, and La Joya Formations in OaxaquiaThe La Ballena Formation (Oaxaquia) is unconformably

overlain by the volcanic rocks and red beds of the Nazas Forma-tion in the Peñón Blanco, Charcas, and Real de Catorce areas (Figs. 5 and 6; Silva-Romo, 1993; Tristán-González and Torres-Hernández, 1994; Barboza-Gudiño et al., 2004). The Nazas For-mation is made up of dacitic and minor rhyolitic and andesitic lava flows and pyroclastic flows, dikes, and porphyritic shallow intrusives. The volcanic rocks are interbedded with conglomer-ate, sandstone, and scarce paleosols. Conglomerate is formed mostly by volcanic clasts and a few clasts of sandstone and shale derived from the underlying La Ballena Formation. The volca-niclastic conglomerate and sandstone form lens-shaped bedding with low-angle cross-bedding, interbedded with some debris flows, suggesting that they were deposited in a terrestrial (fluvial and alluvial fan) environment.

Although their age has not been well constrained at all the exposures, there is a report of U/Pb ages as old as 189 Ma at a subaerial volcanic-sedimentary succession in Huizachal (Huizachal Formation; Fastovsky et al., 2005), which might not belong to the same volcanic arc event (Figs. 5 and 6). Rocks of the Nazas Formation at Real de Catorce yielded U/Pb ages of 172 ± 5 Ma (Barboza-Gudiño et al., 2004). The Nazas Forma-tion changes transitionally upward to shallow-marine volcanicla-stic rocks, some evaporites, and thin-bedded limestone, which in turn become a thick limestone succession in the Peñón Blanco and Charcas areas (Fig. 6). The basal part of this limestone suc-cession contains late Oxfordian–Kimmeridgian fossil faunas (Centeno-García and Silva-Romo, 1997). In contrast, there is an internal angular unconformity in the Real de Catorce local-ity (Fig. 6), which separates in two units, the terrestrial volcanic and sedimentary successions (Nazas and La Joya Formations; Barboza-Gudiño et al., 2004). The upper La Joya Formation changes transitionally upward to shallow-marine volcanic sand-stone and shale interbedded with thin limestone strata. The oldest fossils reported from the base of the limestone succession in Real de Catorce are Oxfordian in age (Barboza-Gudiño et al., 2004).

Caopas, Rodeo, and Nazas Formations of the Central TerraneThe volcanic cover of the Taray Formation (Central Terrane;

Figs. 5 and 6) belongs to the Caopas, Rodeo, and Nazas Forma-tions (Córdoba-Méndez, 1964; López-Infanzón, 1986; Jones et al., 1995). The Rodeo and Nazas Formations are lateral equiva-lents of the same rocks but named differently in separate out-crops (Díaz-Salgado, 2004). Both units are made up of rhyolitic to andesitic lava flows and dikes, and pyroclastic deposits that are interbedded with fluvial sedimentary rocks, mostly sandstone and conglomerate (Anderson et al., 1990, 1991; Jones et al., 1995;

Díaz-Salgado, 2004). The Caopas Formation was formed by shallow porphyritic intrusives. Felsic volcanic rocks of the Rodeo Formation yielded a K-Ar age of 183 Ma (López-Infanzón, 1986), and the Caopas Formation a U/Pb age of 158 Ma (Jones et al., 1995). Terrestrial volcaniclastic rocks of the Rodeo Formation are interpreted to have been deformed previous to the deposition of late Oxfordian limestone (Anderson et al., 1991; Bartolini et al., 2002). However, in another locality nearby the Nazas Forma-tion changes transitionally upward to shallow-marine calcareous rocks that range in age from Late Jurassic to Late Cretaceous (Córdoba-Méndez, 1964; Díaz-Salgado, 2004).

All these volcanic-sedimentary units are interpreted in this work as the first overlapping succession that stitches the Central Terrane with Oaxaquia. The Caopas and Rodeo Formations, as well as the Nazas Formation, are interpreted as continental intra-arc assemblages (Jones et al., 1995).

Las Lluvias Ignimbrite of the Mixteca TerraneJurassic arc volcanism was also recorded in the Mixteca

Terrane in which ignimbrites, interbedded with fluvial and shallow-marine siliciclastic deposits, yielded U/Pb ages of 168.2 ± 1.2 Ma, 177.3 ± 1.5 Ma, and 179.1 ± 1.5 Ma (Campa and Iri-ondo, 2003).

Cuale Assemblage and Tumbiscatio Granitoids of the Zihuatanejo Terrane

Evidence of coeval Jurassic magmatism has been found in two localities in the Zihuatanejo Terrane (Guerrero Com-posite Terrane). One of the exposures is NE of Puerto Vallarta City, in the Cuale mining district, and the other locality is in the Tumbiscatio region, both along the Pacific Coast (Figs. 7 and 8). Rocks at Cuale contain volcanogenic massive sulfide (VMS) deposits and are composed of submarine rhyolitic lavas and tuffs, volcanic sandstone with evolved-arc geochemical affinity (Bis-sig et al., 2003), and shale that yielded U/Pb ages of 162.4 and 155.9 Ma (Bissig et al., 2003). These rocks are strongly deformed and partially metamorphosed, and their contact with Cretaceous unmetamorphosed marine volcanic and sedimentary successions has not been determined.

Two Jurassic granitoids crop out in the Tumbiscatio region. They were emplaced in previously deformed sedimentary rocks of the Arteaga Complex, and vary in composition from grano-diorite to granite to quartz monzonite. Their geochemical com-positions are typical of calc-alkaline subduction-related gran-ites, which are more evolved than granitoids of Cretaceous and Cenozoic ages from the same area. Both granitoids show intense shearing and internal deformation. Grajales and López (1984) obtained one K/Ar date of Late Jurassic age (158 Ma). U/Pb isotopic analysis yielded a 163 Ma age, and Ar/Ar ages are 158 and 152.4 Ma (Centeno-García et al., 2003). The igneous rocks of the Cuale and Tumbiscatio regions have strong similarities in geochemical composition and age with volcanic rocks of the Central Terrane (Caopas, Rodeo, and Nazas); thus we suggest that they probably originated in the same volcanic arc. Therefore,


the Arteaga Complex was probably accreted to the continental margin, either near or along the strike from central Mexico.

The Jurassic volcanic event did not produce a thick strati-graphic column and apparently did not have large volumes of volcanic products. The column changes transitionally upward to shallow-marine calcareous rocks. Therefore, the lithologic asso-ciations and vertical facies changes of this volcanic-sedimentary event are similar to those of a continental rift. However, the scarce geochemical analyses from its volcanic rocks suggest an arc set-ting (Jones et al., 1995). These rocks have been interpreted as the southern continuation of the Jurassic continental arc that devel-oped along the southwestern margin of North America (Jones et al., 1995).

It has been proposed that major strike-slip faults were prob-ably active during the arc activity (Mojave-Sonora Megashear; Jones et al., 1995). This could explain the fact that the Potosi Fan is south of its possible continental fluvial correlative in Arizona, as well as the southward displacement of the Tahue Terrane.

Whether or not the Jurassic volcanic event was coeval with a major transform fault has not been well documented. The evidence in favor of an important synsedimentary deformation involving major extension is as follows: (1) Minor synsedimen-tary normal faults and local angular unconformities are present within the Jurassic arc volcanic and sedimentary successions, and pre-Cretaceous mylonitic shearing is recorded in the Juras-sic granitoids of the Tumbiscatio region (Zihuatanejo Ter-rane). (2) Arc magmatism suddenly ceased in the Central Ter-rane and Oaxaquia, followed by a rapid transgression recorded in a few meters of transitional sedimentation. (3) Subsidence rates apparently were significant during the early stages of the Oxfordian–Kimmeridgian marine sedimentation, because the calcareous rocks show evidence of deeper sedimentation at higher stratigraphic levels as well as overall rapid sedimenta-tion. (4) Although fault planes have been obliterated by younger deformational events, they have been inferred by the rapid lat-eral changes in thickness and facies of the calcareous succession through the interval from the end of the Jurassic to the Early Cretaceous. (5) In addition, major regional lineaments have been identified in central and eastern Oaxaquia, including the San Marcos and La Babia Faults (Fig. 1) (Goldhammer, 1999; Chávez-Cabello et al., 2005).

Stage V: Rifting of the Guerrero Terranes and Formation of a Complex Arc System

In this section we list the main stratigraphic features of the volcanic-sedimentary successions of the Guerrero Composite Terrane and the Mixteca Terrane. Arc volcanism was absent in Oaxaquia and the Central Terrane through the end of the Juras-sic and the Cretaceous. During this period, oceanic crust was emplaced toward the east of Oaxaquia in the Gulf of Mexico, and continuous subsidence prevailed throughout the Early Creta-ceous, resulting in a thick calcareous platform that covered all the Central Terrane and Oaxaquia.

Although much detailed work needs to be done in order to reconstruct the paleogeography of western Mexico during the Cre-taceous, the available evidence indicates three important features:

1. Magmatism prograded generally east to west through time, from the oldest ages in the Oaxaquia and Mixteca Terranes to the youngest ages in the coastal Zihuatanejo Terrane. There is some overlap of age ranges for the vol-canism among the different terranes, e.g., volcanism of the Mixteca Terrane overlaps in age with part of the vol-canism of the Teloloapan Terrane (Guerrero Composite Terrane). However, on a large scale, Albian–Cenomanian volcanism is absent in the Mixteca and the Teloloapan Terranes, and it is widespread in the coastal region of the Zihuatanejo and Arcelia Terranes.

2. Magma chemistry changed through time toward a more primitive melt. The Middle Jurassic volcanic and intru-sive rocks in all the terranes show mostly felsic-evolved continental-arc geochemical signatures, including the Mixteca Terrane and Oaxaquia. In contrast the Cretaceous volcanic rocks of the Guerrero Composite Terrane range from tholeiitic basalts to andesites, with few rhyolites. They show more primitive island-arc (IA) geochemical signatures overall, and some even have MORB to oceanic-island basalt (OIB) signatures. The Mixteca Terrane is the exception to this trend; its magmatism remained evolved, with continental arc signatures, into the Cretaceous.

3. Within different assemblages of the Guerrero Composite Terrane there are major differences in the stratigraphy, sediment composition, and depositional environments. And the Guerrero Composite Terrane overall is different from the volcanic-sedimentary rocks of the Mixteca Ter-rane to the east. In their present distribution, areas with shallow-marine and terrestrial volcanic-sedimentary suc-cessions alternate with areas with deep-marine volcanic-sedimentary successions, and suggest a complex paleo-geography for that time.

These three features are hereby interpreted as evidence of intra-arc rifting-translation. We propose, as a hypothesis to be tested, that the subduction zone might have migrated to the west. This would have produced thinning of the crust, which in turn would have originated more primitive IA geochemical signatures of the magmas and promoted the development of deep basins. Whether the amount of extension was large enough to develop oceanic basins and several parallel subduction zones has not been determined.

The stratigraphy, depositional environments, age, and geo-chemical affinities of the main units are summarized by terrane. First, those of southern Mexico are described, following a section from east to west. Next, Cretaceous rocks of the northern terranes are described from east to west as well.

Mixteca TerraneThree localities with Early Cretaceous volcanism have been

identified in the western Mixteca Terrane near the contact with


the Guerrero Composite Terrane: the Taxco Schist and the Chapo-lapa and Zicapa Formations (Fries, 1960; de Cserna and Fries, 1981; Talavera-Mendoza, 1993; Campa and Iriondo, 2003; Fitz et al., 2002). The Taxco Schist is made up of submarine andesitic to rhyolitic lava flows and tuffs interbedded with epiclastic rocks and quartz-rich sandstone and shale (de Cserna and Fries, 1981; Talavera-Mendoza, 1993). Its volcanic rocks have a continental-arc geochemical affinity, more evolved than contemporaneous magmatism from the Guerrero Composite Terrane (Talavera-Mendoza; 1993; Centeno-García et al., 1993a). The Zicapa For-mation is made up of dacitic to rhyolitic lava flows interbedded with fluvial deposits (Fitz et al., 2002). The Chapolapa Formation is composed mostly of marine lava flows and epiclastic rocks. The abundance of quartzites within the volcanic-sedimentary successions of the Taxco Schist and Zicapa Formation suggests that a crystalline basement was exposed during the arc activity.

U/Pb dating of lavas from the Taxco Schist by sensitive high-resolution ion microprobe (SHRIMP) methods yielded 130–131 Ma ages (Campa and Iriondo, 2004), and from the volcanic-volcaniclastic rocks of the Zicapa Formation, 127 Ma (Fitz et al., 2002). Lava flows from the Chapolapa Formation have 129–133 Ma SHRIMP U/Pb ages. The Taxco Schist shows one phase of deformation and metamorphism prior to the deposi-tion of Aptian–Albian carbonates. Thus, Early Cretaceous vol-canic rocks of the Mixteca Terrane are unconformably covered by a carbonate platform that ranges in age from Early to middle Cretaceous (Fries, 1960).

The limestone succession in the western Mixteca Terrane changes upward to a thick clastic succession (Mexcala Forma-tion) of Turonian to Maastrichtian age (Guerrero-Suastegui, 2004). The Mexcala Formation is made up of alternating sand-stone, shale, and conglomerate, deposited in deltaic and subma-rine-fan environments (Figs. 7 and 8). It is a synorogenic deposit (foreland basin-fill) associated with regional thrusting and fold-ing of both the Guerrero Composite and Mixteca Terranes at the end of the Cretaceous. Therefore, the Mexcala Formation is the first overlapping assemblage that stitches the Guerrero Composite Terrane and the Mixteca Terrane, and marks the final amalgama-tion of the Guerrero Composite Terrane to continental Mexico.

Teloloapan TerraneThe Teloloapan Terrane (Figs. 1 and 8) is exposed in the east-

ernmost parts of the Guerrero Composite Terrane. This terrane is characterized structurally by a complex east-vergent thrust-fault system. Its rocks are severely deformed and metamorphosed to low-grade greenschist facies. The Teloloapan Terrane overrides either Cretaceous platform carbonates or Upper Cretaceous silic-iclastic rocks that belong to the Mixteca Terrane (Figs. 7 and 8; Talavera-Mendoza et al., 1995).

The nature of the basement of the Teloloapan Terrane remains unknown. Metamorphic rocks of the Tejupilco area (Fig. 8) were interpreted as a possible basement for the Telo-loapan Terrane by Elías-Herrera and Sánchez-Zavala (1990), and Sanchez-Zavala (1993). These authors suggested that the

Tejupilco volcanic-sedimentary sequence might represent an arc assemblage older than the rest of the Guerrero Terrane magma-tism. They based this conclusion on U-Pb dates from associ-ated sulfide deposits. The ages they obtained vary broadly from Carnian (227 Ma) to Oxfordian (156 Ma). However, the same volcanic-sedimentary rocks were considered a part of the Cre-taceous arc assemblage by other authors (Campa and Ramirez, 1979; Talavera-Mendoza et al., 1995).

The arc assemblage of the Teloloapan Terrane consists of two distinct regions with different volcanic and sedimentary rocks. The eastern region is characterized by shallow-marine deposits, and the western region is composed of deeper facies (Guerrero-Suastegui et al., 1991; Ramírez-Espinoza et al., 1991; Talavera-Mendoza et al., 1995; Mendoza and Suastegui, 2000; Guerrero-Suastegui, 2004).The stratigraphy of the eastern region, from base to top, is made up of basaltic to andesitic pillow and massive lava flows, volcanic breccias, and pyroclastic flow depos-its (Villa de Ayala Formation; Talavera-Mendoza et al., 1995). These deposits are interbedded with epiclastic sandstone and conglomerate. Primary structures in the volcaniclastic rocks sug-gest a marine depositional environment (Guerrero-Suastegui et al., 1991; Guerrero-Suastegui, 2004). Storm deposits, coral frag-ments, and other fossils suggest shallow and warm waters. This unit contains fossil gastropods and bivalves that range in age from Hauterivian to Aptian (Guerrero-Suastegui et al., 1991; Ramírez-Espinoza et al., 1991; Talavera-Mendoza et al., 1995).

Geochemical analyses of volcanic rocks of the Villa de Ayala Formation of the Teloloapan Terrane indicate that the magma-tism is calc-alkaline and similar to that of active intraoceanic arcs (Talavera-Mendoza, 1993; Talavera-Mendoza et al., 1995; Lapi-erre et al., 1992; Mendoza and Suastegui, 2000; Centeno-García et al., 1993a). The base of the Villa de Ayala Formation is not exposed. The maximum thickness is considered to be ~3000 m (Guerrero-Suastegui, 2004). The volcanic succession of this for-mation changes transitionally upward to thick, massive reefal limestone of the Teloloapan Formation. At the base the Teloloa-pan Formation is composed of intertidal limestone interbedded with volcaniclastic rocks containing rudists and nerineas of late Aptian–early Albian age (Guerrero-Suastegui et al., 1991, 1993; Guerrero-Suastegui, 2004). Thus magmatism ceased prior to the late Aptian (Guerrero-Suastegui et al., 1991; Mendoza and Sua-stegui, 2000; Guerrero-Suastegui, 2004). The Teloloapan Forma-tion grades upward into the Pachivia Formation of Turonian age, which is made up of shale and fine-grained sandstone and shale. The Pachivia Formation is the western equivalent of the Mexcala Formation of the Mixteca Terrane and indicates that the Telo-loapan and Mixteca Terranes were already in close proximity (Guerrero-Suastegui et al., 1991; Talavera-Mendoza et al., 1995; Guerrero-Suastegui, 2004).

The stratigraphy of the western part of the Teloloapan Ter-rane comprises submarine basaltic, andesitic, and felsic lava flows and volcaniclastic rocks (Villa de Ayala Formation) depos-ited in deeper water conditions than the sediments of the eastern Teloloapan Terrane. It is in transitional contact upsection with


the Acapetlahuaya Formation, composed of thin-bedded volca-nic shale and sandstone at the base, at some localities interbed-ded with dark, thinly laminated limestone. It changes transition-ally upward to shale, with little or no volcanic material at the top (Campa and Ramirez, 1979; Guerrero-Suastegui, 2004). This unit has been highly tectonized, making it difficult to calculate its original thickness and contact relationships. Apparently, the Acapetlahuaya Formation changes laterally toward the west and overlies transitionally the volcaniclastic deposits of the Villa de Ayala Formation. Its upper contact with the Amatepec Forma-tion is highly tectonized. The Acapetlahuaya Formation contains ammonoids, and radiolarians that are late Aptian in age (Campa et al., 1974; Guerrero-Suastegui et al., 1993; Talavera-Mendoza et al., 1995; Guerrero-Suastegui, 2004).

The Amatepec Formation is made up of thin-bedded black detrital limestone and is devoid of volcanic material. It is inter-preted as deep-basin–slope deposits. This formation is tightly folded and overlies either the Villa de Ayala or the Acapetlahuaya Formation. It is late Albian to early Cenomanian in age, based on calcispherulids, planktonic foraminifers, and radiolarians (Campa and Ramirez, 1979; Guerrero-Suastegui et al., 1991, 1993; Talavera-Mendoza, 1993; Talavera-Mendoza et al., 1995). The deep-marine limestone is overlain by turbiditic sandstone-shale successions of the Miahuatepec Formation (Talavera-Mendoza et al., 1995). Fossils have not been found, but it is at least post–early Cenomanian because of its stratigraphic position. The Miahuate-pec Formation was deposited, during the amalgamation of the Zihuatanejo, Arcelia, and Teloloapan Terranes, in a thrust-related basin (Guerrero-Suastegui et al., 1991; Ramírez-Espinoza et al., 1991; Talavera-Mendoza et al., 1995; Mendoza and Suastegui, 2000; Guerrero-Suastegui, 2004).

The ages of magmatism of the Teloloapan Terrane have been poorly constrained by the limited fossils found in the vol-caniclastic levels. A few U/Pb isotopic ages from felsic lavas at the base of the succession range in age from 137.4 to 145.9 Ma (Tithonian–Hauterivian; Mortensen et al., 2003). Thus, magma-tism of the Teloloapan Terrane is in part contemporaneous with that of the Mixteca, Guanajuato, and Zihuatanejo Terranes.

There are three distinctive differences in the Cretaceous stra-tigraphy between the Mixteca and Teloloapan Terranes: (1) Vol-canism of the Mixteca Terrane is more evolved, and its isotopic signatures show influence of old continental crust in the magma generation. In contrast, volcanism of the Teloloapan Terrane is more primitive and has no traces of contamination by old con-tinental crust (Centeno-García et al., 1993a; Talavera-Mendoza et al., 1995; Mendoza and Suastegui, 2000). (2) Metamorphic and quartz clasts are abundant (up to 70%) in the sandstones that are interbedded with volcanic rocks in the Mixteca Terrane but are absent throughout the stratigraphic column of the Teloloapan Terrane. (3) Magmatism ceased in the Mixteca Terrane before the Aptian, and part of the volcanic-sedimentary succession was deformed and metamorphosed (Taxco Schist). In contrast, vol-canism continued in the Teloloapan Terrane until Aptian–Albian time, and no internal deformation has been identified.

The arc volcanism of the Mixteca and Teloloapan Terranes has been interpreted as part of a single arc-backarc system in which volcanism of the Mixteca Terrane would be the backarc basin (Cabral-Cano et al., 2000; Monod et al., 1994). An alter-native interpretation is that these two terranes belong to differ-ent arcs, separated by a double-dipping subduction of an oceanic basin (Guerrero-Suastegui, 2004).

Arcelia TerraneThrust over the Teloloapan Terrane is the Arcelia Terrane

(Guerrero Composite Terrane), which shows deeper marine facies and less evolved magmatism than the rest of the arc suc-cessions of the Guerrero Composite Terrane (Talavera-Mendoza et al., 1995; Mendoza and Suastegui, 2000). This terrane is made up of basaltic pillow lavas and ultramafic bodies, black shale and chert, and fine-grained volcanic turbidites (Fig. 7), all intensively deformed and partly metamorphosed (Ramírez-Espinoza et al., 1991; Talavera-Mendoza et al., 1995). The chert layers contain radiolarians reported as Albian–Cenomanian in age (Dávila and Guerrero, 1990; Ramírez-Espinoza et al., 1991). Ar/Ar and K/Ar ages (93.4–105 Ma; Delgado et al., 1990; Ortiz and Lapierre, 1991; Elías-Herrera, 1993) are compatible with biochronology, but detrital zircon ages from volcanic turbidites are older (mean age, 130 Ma; Talavera-Mendoza et al., 2007). Geochemical sig-natures of the Arcelia magmas are similar to those in recent prim-itive IAs and oceanic basins (MORB) (Talavera-Mendoza, 1993; Talavera-Mendoza et al., 1995; Mendoza and Suastegui, 2000). There are no exposures of older rocks in the Arcelia Terrane, and no clasts of older metamorphic or sedimentary rocks have been found in its sedimentary strata. Mendoza and Suastegui (2000) suggest that this terrane is entirely oceanic, that it may have origi-nated as an independent oceanic arc and backarc basin, and that it represents partly developed oceanic crust. An alternative inter-pretation is that the Arcelia Terrane could also be a backarc basin of the Zihuatanejo Terrane (Centeno-García et al., 2003a).

Southern Part of the Zihuatanejo TerraneUppermost Jurassic–Cretaceous volcanic-sedimentary assem-

blages of the Zihuatanejo Terrane can be grouped in three main regions: northern Zihuatanejo Terrane (Zacatecas area), Huetamo area, and coastal Zihuatanejo-Colima region (Figs. 7 and 8). The uppermost Jurassic to Cretaceous strata of the southern Zihuatanejo Terrane are not as strongly deformed as those in other terranes, and original contact relationships and complete stratigraphic columns are well preserved. The strata are characterized by numerous lat-eral facies changes and internal erosional and angular unconformi-ties. The geographic distribution of the facies is highly irregular, and it has not yet been determined in detail. Therefore, compiling, correlating, and synthesizing the stratigraphy of the area is difficult because it varies considerably from one locality to another. The stratigraphy of the northern Zihuatanejo Terrane (Zacatecas area) is described later.

The stratigraphic column of the southern Zihuatanejo Ter-rane in the Huetamo area is made up of Triassic basement rocks


of the Arteaga Complex, overlain by uppermost Jurassic to Cre-taceous volcanic and sedimentary cover. These rocks are thrust over the Arcelia Terrane (Figs. 7 and 8). Arc-related rocks of the Huetamo region (Figs. 7 and 8) overall have been formed by a thick succession of alternating shale, sandstone, and conglom-erate, with scattered basaltic pillow lavas, submarine ignimbrite flows, and other intermediate pyroclastic and epiclastic flows in the lower parts of the succession (Angao and San Lucas For-mations; Pantoja, 1959; Guerrero-Suastegui, 1997). These arc-related rocks lie unconformably on the Arteaga Complex (Figs. 7 and 8). Fossils of Late Jurassic age have been reported from the Angao Formation (Pantoja, 1959), although the major exposures of volcaniclastic rhythmic sedimentary rocks are Berriasian to upper Aptian (Guerrero-Suastegui, 1997). Their depositional environment changes upsection from deep to shallow marine.

The volcaniclastic rocks of the San Lucas Formation change toward the top to thick limestone zones with fossil ammonites, orbitolinids, and rudists of late Aptian–early Albian age (El Cajon and Mal Paso Formations; Guerrero-Suastegui, 1997; Pantoja-Alor and Caballero, 2003). This sequence alternates with or changes laterally into marine and terrestrial volcanic sandstone and conglomerate (Comburindio Formation; Guerrero-Suastegui, 1997; Pantoja-Alor and Caballero, 2003). The conglomerate is covered by massive, thick packets of limestone (Huetamo For-mation) that contain fossils of late Albian–Cenomanian age. This unit is found only in the central parts of the Huetamo region (Pan-toja, 1990).

The arc succession of the Zihuatanejo Terrane in the Huetamo area was deformed prior to the deposition of a thick, subaerial red-bed succession that is interbedded with volcanic rocks (Cut-zamala Formation of Campa and Ramirez, 1979) and is related to a continental arc of Santonian–Maastrichtian age (Altamira-Areyán, 2002; Benammi et al., 2005).

The oldest Cretaceous rocks of the Zihuatanejo Terrane in the Zihuatanejo-Colima region of coastal Mexico that have been penetrated by drilling are Berriasian–Hauterivian in age (Alberca Formation; Cuevas, 1981). The lower member of the Alberca Formation is made up of interbedded black shale, sandstone, and limestone, and some tuff. The upper member is composed mostly of andesitic-basaltic lava flows interbedded with limestone and shale. The Alberca Formation changes transitionally upward to andesitic and basaltic lava flows, with some rhyolitic flows, inter-bedded with pyroclastic (intermediate tuffs and ignimbrites) and epiclastic deposits. It contains limestone packets interbedded with subaerial conglomerate and sandstone, red siltstone, and some evaporites, and continues into limestone with scarce basaltic pil-low lavas at the top (Tecalitlán, Tepalcatepec, and Madrid Forma-tions). The age range of these units, based on their fossil content, is Barremian to Cenomanian (Grajales and López, 1984).

Along the west coast between the cities of Colima and Zihuatanejo are exposures of an important succession of red beds, alternating with lesser amounts of limestone in compari-son with other areas of the Guerrero Composite Terrane. The assemblage is made up of rhyolitic lavas (lava flows, breccias,

and ignimbrites) and minor andesitic and dacitic lavas (Tecalitlán Formation, Titzupa-La Unión assemblage, Playitas Formation, etc.; Ferrusquía et al., 1978; Grajales and López, 1984; Pantoja and Estrada, 1986; Centeno-García et al., 2003). These units are interbedded with epiclastic deposits such as tuff, volcanic shale, and sandstone, and some conglomerate. The assemblage also contains thin beds of limestone containing orbitolinids, gastro-pods, and some pelecypods of late Albian–Cenomanian age (Fer-rusquía et al., 1978; Grajales and López, 1984). Raindrop marks, desiccation polygons, and dinosaur footprints can be found in this succession (Ferrusquía et al., 1978). The lower parts of the Cre-taceous succession are missing in the Arteaga region, where non-marine and shallow-marine volcanic and volcaniclastic rocks of Aptian–Albian age rest unconformably on the Arteaga Complex.

Overall, Cretaceous volcanic rocks of the southern Zihua-tanejo Terrane show geochemical and isotopic signatures that suggest a transitional composition between oceanic island arcs and active continental margins (Centeno-García, 1994; Freydier et al., 1997; Mendoza and Suastegui, 2000). The high potassium content, abundance of felsic lavas, and trace element abundances of these volcanic rocks are similar to those observed in IAs where the crust is thick (>~20 km), allowing magmatic differentiation (Centeno-García, 1994).

Rocks of the southern Zihuatanejo Terrane are distinctive from the rest of the terranes because they were deposited in shallow-marine and fluvial environments, contain fossil verte-brates, and show calc-alkaline volcanism more evolved than that of the Teloloapan and Arcelia Terranes. Sedimentary rocks inter-bedded with the volcanic flows contain clasts of their basement rocks, made up of sandstone, quartz, and mylonitic granite. Thus its stratigraphy is similar to that of arcs constructed on intermedi-ate crust with a previous history of accretions. The presence of fossil vertebrates suggests proximity to the continent.

Northern Guerrero Terrane

Following a section from east to west in the northern part of the Guerrero Terrane, the main stratigraphic characteristic is an absence of rocks similar to those of the Mixteca or Teloloapan Ter-rane. Instead, deep-marine volcanic-sedimentary successions of the Guanajuato Terrane were thrust directly over limestone of the calcareous platform of Oaxaquia. Contact relationships between the Guanajuato Terrane and the northern Zihuatanejo Terrane are unconstrained because the contact is covered by younger units. It is inferred that the Guanajuato Terrane is overthrust by the Zihua-tanejo Terrane on the basis of regional vergence of the structures. Contact relationships between the Tahue and Zihuatanejo Ter-ranes are unknown because the contact is covered by overlapping Cenozoic assemblages.

Guanajuato TerraneThe succession at the Guanajuato Terrane has been described

as a complete stratigraphic column of an accreted volcanic arc, as its assemblages vary from the roots of the arc (gabbros and


diabases, and dike swarms) to pillow basalts, interbedded with thin-bedded siltstone, shale, chert, and fine-grained volcanic sandstone (Figs. 6 and 7; Ortiz-Hernandez et al., 1991; Ortiz-Hernandez, 1992). However, all the different stratigraphic levels are in the form of tectonic slivers (Fig. 7), with the deepest mafic levels (gabbro, tonalite, serpentinite, wehrlite, dike swarms) thrust over the upper stratigraphic levels (pillow basalt and vol-canic turbidites).

The uppermost thrust sheet is made up of ultramafic-mafic rocks of the Cerro Pelón tonalite and the Tuna Mansa diorite. These ultramafic rocks are thrust over a succession incorporat-ing a diabasic feeder dike swarm, basaltic pillow lavas (La Luz basalts), rhyolitic tuffs (Cubilete Tuff), and a deep-marine vol-caniclastic succession made up of sandstone and shale turbi-dites, chert, and black detrital limestone (Esperanza Formation; Quintero-Legorreta, 1992; Ortiz-Hernandez et al., 1992; Ortiz-Hernandez et al., 2003). Basalts of this assemblage show geo-chemical signatures similar to present primitive volcanic island arcs (Ortiz-Hernandez, 1992).

The third and lowermost structural level (Fig. 7) is com-posed of a thick turbidite succession of volcanic graywackes, quartzites, micritic limestone, radiolarian chert, black shale, and rare conglomerate resting on basaltic pillow lavas (Arperos Formation; Ortiz-Hernandez et al., 1992; Lapierre et al., 1992; Quintero-Legorreta, 1992; Monod et al., 1990; Martínez-Reyes, 1992; Ortiz-Hernandez et al., 2003). Pillow basalts at the base of the Arperos Formation are more alkaline than the La Luz basalts and show OI geochemical signatures (Ortiz-Hernandez et al., 2003). The Arperos Formation is unconformably overlain by the Aptian–Albian La Perlita Limestone (Ortiz-Hernandez et al., 2003).

It is difficult to reconstruct the role of the Guanajuato Ter-rane in the tectonic evolution of western Mexico because of the lack of enough geochronological data. The only U/Pb zircon age reported from the area comes from the El Gordo volcano-genic massive sulfide ore deposit (Hall and Mortensen, 2003), which is considered part of the lowermost succession by Hall and Mortensen, (2003), but it is at the stratigraphic level of the second thrust sheet (Cubilete tuff?) in the stratigraphy proposed by Ortiz-Hernandez et al. (1992). The age of a rhyolite from El Gordo volcanogenic massive sulfide ore deposit reported by Hall and Mortensen (2003) yielded a 146.1 Ma U/Pb age. There are also reports of badly preserved radiolarians from the Arperos For-mation that are not in good enough condition to be age indicators (possibly Valanginian–Turonian in age), but a report of nanno-fossils suggests a Tithonian–Hauterivian age (Ortiz-Hernandez et al., 2003). Other ages reported from the Guanajuato area are from K/Ar analyses and seem to have been reset by later thermal events (Ortiz-Hernandez et al., 1992, 2003). The sedimentary rocks of the La Luz and Arperos Formations seem to be distal volcanic turbidite deposits, but the abundance of limestone asso-ciated with the pillow lavas suggests that deposition occurred above the carbonate compensation depth (Ortiz-Hernandez et al., 2003). Aptian–Albian limestone of the La Perlita Formation rests

unconformably on the Arperos Formation and suggests that sedi-mentation and at least one phase of deformation occurred prior to the Aptian–Albian (Ortiz-Hernandez et al., 2003). Whether or not this deformation is related to the accretion of the Guanajuato Terrane to the continental margin has not been determined. At present the Guanajuato Terrane is thrust over the calcareous platform of Oaxaquia in the San Miguel de Allende area (Ortiz-Hernández et al., 2002).

Rocks of the Guanajuato Terrane have been correlated with the Arcelia Terrane, and both were interpreted as having formed part of an oceanic arc independent of the Zihuatanejo and other arc terranes (Ortiz-Hernandez et al., 1992). Also, these rocks are considered relicts of an oceanic basin consumed by subduc-tion related to the arc of the Zihuatanejo Terrane (Lapierre et al., 1992; Tardy et al., 1994). An alternative preliminary interpreta-tion, based on provenance and stratigraphy, is that the Guanajuato Terrane may have been the backarc basin of the Zihuatanejo Ter-rane (Centeno-García et al., 2003).

Zihuatanejo TerraneThe Upper Jurassic–Cretaceous stratigraphy of the Zacate-

cas area in the northern Zihuatanejo Terrane is very different than the stratigraphy of the neighboring Central Terrane and Oaxa-quia (Figs. 5–7). Whereas the strata in the northern Zihuatanejo Terrane are mostly composed of volcanic and volcaniclastic rocks, northern Oaxaquia and the Central Terrane were covered by a thick, shallow-marine calcareous platform during the Late Jurassic–Cretaceous (Centeno-García and Silva-Romo, 1997). This suggests that the Zihuatanejo Terrane was probably under-going dislocation from the continental margin during that time.

The arc stratigraphy of the Zacatecas area is formed by the La Borda, Chilitos, and El Saucito Formations (de Cserna, 1976; Yta et al., 1990; Olvera-Carranza et al., 2001; Olvera-Carranza, 2002). These three formations are made up of pillow basalts and volcanic breccias, interbedded with thin-bedded siltstone, shale, chert, and volcanic sandstone and conglomerate, with scarce fel-sic tuff beds and detrital limestone (Centeno-García and Silva-Romo, 1997; Olvera-Carranza, 2002). The chert layers contain radiolarian fossils of Neocomian(?) to Aptian–Albian(?) age (Yta et al., 1990; Olvera-Carranza, 2002). However, older U/Pb ages have been reported (150–148 Ma) from the base of the succes-sion (Danielson, 2000; Mortensen et al., 2003). Lapierre et al. (1992) and Freydier et al. (1995) characterized this magmatism as primitive IA and OI basalts. Sedimentary structures and fos-sil content suggest that the La Borda, El Saucito, and Chilitos Formations were deposited as distal turbidites and grain flows in a volcaniclastic submarine apron (Centeno-García et al., 2003). These Jurassic–Cretaceous arc successions contain important volcanogenic, massive sulfide ore deposits (Yta et al., 1990; Dan-ielson, 2000; Mortensen et al., 2003).

Tahue TeraneCretaceous successions of the Tahue Terrane are exposed

mostly in the Sinaloa de Leyva–Porohui region (Fig. 4). They


were formed by submarine pillow lavas, volcaniclastic rocks, shale, and limestone. They contain Albian ammonites (Ortega-Gutiérrez et al., 1979; Freydier et al., 1995; Gastil et al., 1999), but Ar/Ar ages from the lavas are younger (86 Ma; Gastil et al., 1999), suggesting resetting. The basaltic lavas show MORB and OIB geochemical affinities, but the volcaniclastic rocks are more evolved and show IA geochemical signatures (Freydier et al., 1995; Gastil et al., 1999). Although this volcanic succession has been interpreted as the northern continuation of the Arperos For-mation of the Guanajuato Terrane, and part of a major oceanic basin that originally lay between the Guerrero arc and the con-tinent (Tardy et al., 1994; Lapierre, et. al., 1992; Dickinson and Lawton, 2001), the stratigraphy does not support such a tectonic scenario because (1) the Cretaceous volcanic rocks rest uncon-formably on a Paleozoic basement, (2) the stratigraphy and facies associations are not indicative of deep-pelagic sedimentation and oceanic-ridge volcanism, and (3) the Guanajuato successions apparently are older than the arc assemblages of the Tahue and other parts of the Guerrero Composite Terrane.

SUMMARY

• The stratigraphy of the Guerrero Composite Terrane of western Mexico is characterized by a series of terranes whose basements were formed by Paleozoic to Triassic fragments of oceanic arcs, continental slope sediments, and ocean floor assemblages that were accreted to the con-tinent and consecutively rifted and translated.

• Metamorphosed Ordovician volcanic and marine sedimen-tary rocks and a thick succession of deep-marine turbidites of the NW Guerrero Composite Terrane (Tahue Terrane) make up the record of a middle Paleozoic collision and development of a Carboniferous to Permian passive mar-gin. These rocks might be equivalent to the early Paleozoic Antler Arc and eugeoclinal sedimentation in the SW Cor-dillera of North America.

• The continental margin during the early Mesozoic was located in the middle of Mexico, approximately along the boundary between Oaxaquia and the Central–Guerrero Composite Terranes. This continental margin was active during the Permian–Carboniferous, when a continental arc developed in Oaxaquia.

• Permian–Carboniferous arc-related magmatism ceased, and a passive or rifted margin developed along the west-ern continental margin of Mexico, extending throughout the Triassic. This development is suggested by the thick submarine siliciclastic turbidite succession that accumu-lated on the western paleo-continental shelf–slope region (Potosi Submarine Fan). The siliciclastic fan turbidites are mostly continent-derived, quartz-rich sandstone, siltstone, and shale, containing fossils of Carnian–Norian age.

• The Potosi Fan is interpreted as passive-margin deposits, as there is no evidence of contemporaneous magmatism either in the stratigraphy or in the provenance.

• The siliciclastic rocks of the Potosi Fan extended to the west in a marginal oceanic basin (Arteaga Basin) that at present forms the basement of the Zihuatanejo Terrane of the Guerrero Composite Terrane.

• The first compressional event that deformed the Triassic rocks originated tight folding, shearing, and axial cleavage in the La Ballena Formation, and block-in-matrix texture in the Taray and Zacatecas Formations and the Arteaga Complex. This deformation was related to subduction along the early Mesozoic continental margin. It may have started sometime between the Late Triassic and Early Jurassic, accreting the turbidites of the Potosi Submarine Fan, with slivers of the oceanic crust, to the continent.

• Whether the subducting slab was dipping toward the west or the east is not well constrained, but the accretionary prism apparently was very wide. Evidence of contempo-raneous oceanic arc magmatism is found in the Vizcaíno Peninsula, where a volcanic sequence of primitive arc affin-ity is exposed. It is possible that the rocks in the Vizcaino Peninsula represent a displaced fragment of an oceanic arc that accreted to the Arteaga Complex of the Guerrero Com-posite Terrane, but this model needs more evidence.

• Arc-related volcanic and sedimentary rocks unconform-ably overlie the deformed Triassic rocks of Oaxaquia and the Central and Guerrero Composite Terranes. They are characterized by continental rhyolitic to andesitic lava flows, interbedded with fluvial and alluvial deposits. The succession shows minor angular unconformities, probably related to tilting. These rocks have been interpreted as the southern continuation of the Jurassic continental arc that developed along the southwestern margin of the United States. Magmatism was active from ca. 163 to 155 Ma (Callovian–Oxfordian), although older volcanic rocks have been reported for eastern Mexico (189 Ma). The Jurassic arc shows more evolved geochemical signatures than the subsequent volcanic events.

• During and after the continental arc activity (Late Jurassic–Early Cretaceous), large amounts of extension and lateral translations probably occurred, as suggested by the changes in the stratigraphy. It has been proposed that major strike-slip faults were probably active during the arc activity (Mojave-Sonora Megashear). Arc magmatism ceased in central Mexico, and considerable subsidence and extension is evidenced by the fast deepening of the calcareous platform that developed over the arc rocks.

• Major stratigraphic, geochemical, and isotopic differences are evident in the different Cretaceous stratigraphic assem-blages among the Guerrero terranes. They are, from east to west: Andesitic-basaltic submarine lava flows and tuff (IA geochemical signatures), interbedded with limestone and shallow-marine volcaniclastics (Teloloapan Terrane) that were thrust over contemporaneous but more evolved arc successions and the calcareous platform of southern con-tinental Mexico (Mixteca Terrane). Ophiolite successions,


with deep-marine volcanic and sedimentary rocks with MORB, OIB, and IA signatures (Guanajuato and Arcelia Terranes), are placed between the continent and the more evolved arc in the north (Zihuatanejo Terrane) and between the two shallow-marine arcs (Teloloapan and Zihuatanejo Terranes) in the south.

• These major geological differences suggest that intra-arc rifting was considerable and originated a series of marginal arc-backarc systems in western Mexico, with complex paleogeography. Two possible scenarios can be proposed for the Cretaceous paleogeography of western Mexico: (1) that there was one single rifting arc, with westward migration of the magmatism and development of deep-marine intra-arc and backarc basins (Guanajuato and Arcelia Terranes); and (2) that rifting during the end of the Jurassic was large enough to allow the formation of multiple marginal island arcs, separated by oceanic backarc basins.

• The proposed timing of the final amalgamation of the Guerrero terranes to the margin of older terranes that form the eastern part of Mexico is Turonian to Maastrichtian, as suggested by the age span of foreland basins associated with the deformation of the arc. Overlapping the previ-ously deformed Arcelia and Zihuatanejo Terranes, a new arc developed along the coast by Santonian time.

ACKNOWLEDGMENTS

This paper is a contribution to PAPIIT projects IN109605–3 and IN116599, funded by the Universidad Nacional Autónoma de México (UNAM), and to projects UC-MEXUS Exotic ver-sus Fringing Arc Models: Implications for the Growth of Con-tinents, and SEP/2003 C02 42642. Special thanks are due J. Stock, C. Busby, C. Vita-Finzi, and A.E. Draut for their reviews and comments, which greatly improved the paper.

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Manuscript accepted by the society 24 april 2007

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