The Late Cretaceous fold-thrust belt in the Peña de Bernal— Tamazunchale area and its possible relationship to the accretion of the Guerrero Terrane
Published:January 01, 2012
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Michelangelo Martini, Elisa Fitz, Luigi Solari, Antoni Camprubi, Peter J. Hudleston, Timothy F. Lawton, Gustavo Tolson, Elena Centeno-García, 2012. "The Late Cretaceous fold-thrust belt in the Peña de Bernal— Tamazunchale area and its possible relationship to the accretion of the Guerrero Terrane", The Southern Cordillera and Beyond, José Jorge Aranda-Gómez, Gustavo Tolson, Roberto S. Molina-Garza
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The continental interior of Mexico is characterized by a Late Cretaceous prominent fold-thrust belt that shows characteristics of an eastward-tapering orogenic wedge. According to structural data and geothermometry of the deformation, this wedge is the result of horizontal stresses directed from the west (Pacific domain). The orogenic wedge is bounded to the west by the Guerrero Terrane, which is the second largest juvenile terrane accreted to the North American Cordillera. The possible linkage between the accretion of the Guerrero Terrane and the regional shortening in the Mexican interior is examined in detail in the region comprised between the Sierra de Guanajuato and the Peña de Bernal—Tamazunchale areas. In order to test the accretion hypothesis, we present key stratigraphic, structural, and geochronologic data from the Mexican Cordillera in central Mexico, and discuss the problems that exist in connecting the accretion of the Guerrero Terrane to the orogenic deformation of the Mexican continental interior.
Terrane accretion, flat slab subduction, and convergence rate variations were recognized as major tectonic processes that shaped the North American Pacific edge during the Mesozoic and the early Cenozoic (e.g., Coney et al., 1980; Bird, 1988; Saleeby, 2003; Dickinson, 2009; Liu et al., 2010). The Mexican segment of the Northern Cordillera is characterized by a prominent Late Cretaceous—Paleogene fold-thrust belt, the Sierra Madre Oriental or Mexican Fold-Thrust Belt, which is oriented NW-SE to E-W (Fig. 1A) (Fries, 1960; Campa-Uranga et al., 1976; Suter, 1984). This belt is bounded to the west by the Guerrero Terrane (Fig. 1A), which is the second largest juvenile terrane accreted to the North American Cordillera (Campa and Coney, 1983). The juxtaposition of such an allochthonous lithospheric block to the Mexican portion of the North American continent has motivated many authors to propose a linkage between the accretion of the Guerrero Terrane and the regional shortening in the Mexican Fold-Thrust Belt (Campa-Uranga, 1985; Campa and Coney, 1983; Salinas-Prieto et al., 2000; Keppie, 2004; Talavera-Mendoza et al., 2007). Such a linkage implies that the Mexican Fold-Thrust Belt was built during a single long-standing orogenic pulse that occurred during Late Cretaceous to Paleogene. This scenario deserves to be examined in detail, and until it is, the origin of the driving force that produced the Mexican Fold-Thrust Belt will remain in doubt.
In this work, we explore the hypothesis that accretion of Guerrero Terrane could have been the possible cause of the Late Cretaceous—Paleogene shortening of the Mexican continental interior. To do this, we use structural evidence, geochronological data, and stratigraphic constraints in key outcrops along two cross sections crosscutting the Mexican Fold-Thrust Belt in the Peña de Bernal—Tamazunchale area, and the Guerrero Terrane suture in the Sierra de Guanajuato (Fig. 1B).
The Mexican Fold-Thrust Belt
The Mexican Fold-Thrust Belt is a remarkable tectonic feature in the geology of Mexico because of its extent and prominent topographic expression in the Sierra Madre Oriental, which extends from the United States—Mexico border to the Tehuante-pec Gulf in southern Mexico (Fig. 1A). This tectonic feature is the result of considerable shortening that affected the Mesozoic sedimentary cover of the Mexican continental interior and, less intensely, its Precambrian crystalline basement. The Mexican Fold-Thrust Belt is interpreted as the southern continuation of the Rocky Mountains Fold-Thrust Belt in the United States (Campa-Uranga, 1985). In northern Mexico, the Fold-Thrust Belt apparently includes structures of both the Sevier and Laramide orogenic belts, as evidenced by thick-skinned structures cutting thin-skinned structures (Chávez-Cabello et al., 2007; Fitz-Díaz et al., 2011a). Farther south, in central Mexico, the deformation style in the Mexican Fold-Thrust Belt is dominantly thin-skinned with almost no participation of the basement rocks (Fitz-Díaz et al., 2011a).
The Mexican Fold-Thrust Belt in the Peña de Bernal— Tamazunchale Area
In the Peña de Bernal—Tamazunchale area, the Mexican Fold-Thrust Belt shows an eastward-tapering wedge shape on a cross-section oriented NE-SW (Figs. 1B and 2). Its evolution as an orogenic wedge is supported by: (1) a gradient in deformation increasing to the west (Fig. 2) (Fitz-Díaz et al., 2011a; Fitz-Díaz et al., 2012); (2) a gradient in the temperature of deformation also increasing to the west, suggesting a deeper burial of the younger thrust sheets incorporated into the wedge (Fitz-Díaz et al., 2011b); and (3) the ages of syn-tectonic sedimentary deposits which are progressively younger to the east.
Two generations of shortening structures, D1MFTB and D2MFTB, can be recognized within the Mexican Fold-Thrust Belt, both with a general direction of tectonic transport to the east (Fig. 3) (Fitz-Díaz et al., 2012). D1MFTB is the most intense deformation event in the area, responsible for the tightest folds, a pervasive S1MFTB cleavage at the mesoscopic scale and kilometer-scale displacement thrusts. D2MFTB structures are represented by meter-scale displacement thrusts, open folds, and a spaced S2MFTB cleavage. The pronounced horizontal shortening accommodated during D1MFTB (∼50% in average; Fitz-Díaz et al., 2011a) implies that a considerable part of the topography that we observe now in the Sierra Madre Oriental was produced during this event. The timing of the shortening in the Mexican Fold-Thrust Belt is constrained by the age of the infill of the foreland basins, which was later involved into the orogenic wedge. Based on available data, the shortening started at least at 91 Ma on the western side of the section (Hernández-Jáuregui, 1997) and ended at ca. 65 Ma to the front (López-Oliva et al., 1998) (Figs. 2 and 3).
Most of the rocks composing the Mexican Fold-Thrust Belt along the studied section are part of platformal and basinal Cretaceous carbonate successions (Suter, 1987) that are exposed in large portions of the Mexican territory. A stratigraphic synthesis of these successions is presented in Figure 3. Platform successions (El Doctor and Valles—San Luis platforms) are thickly bedded, very homogenous in composition (rich in calcite and dolomite) and are of typical Cretaceous platformal facies with abundant rudists, gastropods, and miliolids. On the other hand, basinal successions (Zimapan and Tampico-Misantla basins) are thinly bedded and more heterogeneous in composition, resulting from abundant shale, bentonite, and chert alternating with limestone beds rich in planktonic foraminifera and calcisferulids. The Lower Cretaceous calcareous rocks are conformably overlain by syn-tectonic turbidites, which vary in age from Turonian to Maastrichtian, from west to east (Hernández-Jáuregui, 1997; López-Oliva et al., 1998) (Fig. 3).
The rocks in both platforms and basins contain abundant syn-tectonic veins, evidence of fracturing and fluid flow during deformation (Fitz-Díaz et al., 2011b). The pervasive fracturing in the platforms allowed meteoric fluid infiltration, whereas in the basins, where the carbonates are interbedded with impermeable shale, there was less such infiltration. This interpretation is supported by an integrated study of folds and associated veins, involving field observations and stable isotope and fluid inclusion analysis (Fitz-Díaz et al., 2011b). The results of this study will be discussed on the outcrop.
The Tolimán succession consists of three lithologic units: El Chilar Complex and the San Juan De La Rosa and Peña Azul formations, which are exposed in the Tolimán area (Figs. 2A and 3). The latter two units were originally defined and mapped by Carrillo-Martínez (1989 and 1997, respectively). However, Carrillo-Martínez lumped parts of El Chilar Complex and San Juan De La Rosa Formation together. Recent sedimentological studies, supported by detrital zircon geochronology, demonstrated that these packages are of different origin and age, and therefore they are now assigned to two different formations (Dávila-Alcocer et al., 2009).
El Chilar Complex is the oldest unit in the Tolimán area and it is mostly composed of quartz-rich sandstone and phyllite containing an exotic block of radiolarite cut by mafic dikes (Dávila-Alcocer et al., 2009). This unit shows an intense pre-D1MFTB deformation expressed by a strong anastomosing fabric and a broken-formation texture. Based upon provenance analysis, the rocks of the El Chilar Complex have been interpreted as a part of the Potosí Fan, a huge turbiditic fan exposed discontinuously from the present-day Pacific coast to Matehuala (Fig. 1A). Petrogenetic evidence suggests that the Potosí Fan developed at the Pacific edge of the Mexican portion of the North American craton as the result of rivers draining an extensive portion of western equatorial Pangea during the Late Triassic (Silva-Romo et al., 2000; Barboza-Gudiño et al., 2010).
The San Juan De La Rosa Formation is made up of felsic lava flows and volcaniclastic turbidites that unconformably overlie the El Chilar Complex. The shapes (almost all euhedral) and the ages (a hundred measured) of detrital zircon indicate a strong influence of felsic magmatism at ca. 140 Ma ago. The felsic composition of the San Juan De La Rosa Formation volcanic rocks contrasts with the dominantly mafic volcanism that characterizes the Guerrero Terrane, although there is significant overlap in age of the San Juan De La Rosa Formation and the rocks of the Guerrero terrane. These rocks were affected by intense shortening, as evidenced by large-scale folds and associated pervasive axial plane cleavage, which presumably occurred during the D1MFTB phase of the Mexican Fold-Thrust Belt. The lack of pre-D1MFTB structures in the San Juan De La Rosa Formation, as well as its unconformable contact with the El Chilar Complex, suggest that the pre-D1MFTB event occurred at least before 140 Ma.
The Peña Azul Formation consists of detrital limestone interbedded with volcaniclastic sandstone. These rocks conformably overlie the San Juan De La Rosa Formation. The contact between these two units is locally sheared, probably during the Late Cretaceous shortening. Rocks of the Peña Azul Formation are isoclinally folded, and developed an axial plane sub-parallel to bedding and to the contact with the underlying San Juan De La Rosa Formation.
The Guerrero Terrane
The Guerrero Terrane is the second largest juvenile terrane of the North American Cordillera (Campa and Coney, 1983) and comprises approximately one-third of Mexico (Fig. 1A). It is made up of Late Triassic unmetamorphosed to low-grade amphibolite facies turbidites of the Potosí Fan, which are unconformably overlain by Middle Jurassic to Early Cretaceous volcano-sedimentary arc successions, and a felsic igneous province, which is exposed from Zacatecas to Teloloapan along the Guerrero Terrane boundary, and is constituted by 151−126 Ma rhyodacitic volcanic and hypabyssal rocks hosting volcanogenic massive sulfide ore deposits (see Centeno-García et al., 2008, and Mortensen et al., 2008, for a review). Different origins have been proposed for the Guerrero Terrane, which can be summarized in the following groups of paleogeographic reconstructions. Group 1 includes the allochthonous models, in which the Guerrero Terrane is considered as an exotic Pacific arc accreted to the Mexican portion of the North American craton by the consumption of the oceanic Mezcalera plate, which constituted the substrate of an extensive pre-Aptian basin, named Arperos Basin (Lapierre et al., 1992; Tardy et al., 1994; Dickinson and Lawton, 2001) (Figs. 4A and 4B). Group 2 considers the Guerrero Terrane as an allochthonous fringing multi-arc system, accreted by the closure of relatively small pre-Cretaceous oceanic basins at multiple subduction zones with varying polarities (Talavera-Mendoza et al., 2007) (Fig. 4C). Group 3 includes para-autochthonous models that interpret the volcano-sedimentary successions as part of a para-autochthonous west-facing arc, which drifted into the paleo-Pacific domain by the opening of the Cretaceous back-arc oceanic Arperos Basin, and then was subsequently accreted back to the Mexican continental core (Lang et al., 1996; Cabral-Cano et al., 2000; Elías-Herrera et al., 2000; Centeno-García et al., 2008; Martini et al., 2009; Martini et al., 2011) (Fig. 4D).
The Arperos Basin consists of an association of deep-water sedimentary rocks and pillow basalts (Tardy et al., 1994). Exposures of these have been reported at Porohui, Sierra de Guanajuato, Valle de Bravo, and Arcelia, along an ∼70-km-wide NNW-SSE polydeformed belt that has been interpreted as the suture of the Guerrero Terrane (Tardy et al., 1994) (Fig. 1A). Considering that the Arperos Basin would have played a different role in each paleogeographic scenario, the study of these rocks is key to reconstructing the Mesozoic tectonic evolution of the Guerrero Terrane, as well as the effects of its accretion across the Mexican interior.
Stratigraphy of the Sierra de Guanajuato
The Sierra de Guanajuato is located in central Mexico, along the inferred boundary of the Guerrero Terrane (Fig. 1A). It is constituted by a pile of tectonic thrust sheets, each of them characterized by distinct lithostratigraphic successions. The lower structural levels are composed of the Esperanza and the Arperos assemblages (Figs. 5–7). The Esperanza assemblage is made up of Tithonian quartz-rich turbidites and rhyodacitic crustal melts (Esperanza Formation), which grade upward to Lower Cretaceous micrite (Valenciana Formation in Martini et al., 2011) (Figs. 5–7).
The Arperos assemblage is made up of the Arperos Formation, which consists of Lower Cretaceous pillow basalt and hyaloclastite, radiolarian chert, cherty shale, and scarce volcaniclastic slump deposits (Fig. 7A and 7B). These rocks were previously assumed to grade upward to an overlying ∼700-m-thick volcani-clastic turbidite unit, which was described as the upper part of the Arperos Formation (Ortíz-Hernández et al., 1992; Tardy et al., 1994; Martini et al., 2011). However, new field observations indicate that the volcaniclastic turbidites are tectonically emplaced on the previously folded and sheared rocks of the Arperos Formation. Based on these new field observations, we upgrade the stratigraphy proposed by Martini et al. (2011) and group the turbidites in a new informal unit named Cuestecita formation (Figs. 6A and 7A).
The Esperanza and Arperos assemblages are overthrust by Upper Jurassic—Lower Cretaceous rocks of the El Paxtle assemblage, which consists of the El Paxtle Formation and the Tuna Manza Intrusive Complex (Martini et al., 2011) (Fig. 7A). The El Paxtle Formation is composed of pillow and massive basaltic flows, basaltic tuff, volcaniclastic sandstone, and chert (Lapierre et al., 1992; Ortíz-Hernández et al., 1992). The Tuna Manza Intrusive Complex is composed of gabbro, diorite, and tonalite, locally intruded by basaltic and doleritic dike swarms, scarce wehrlite, and olivine clinopyroxenite, grading transitionally into layered clinopyroxene metagabbro (Lapierre et al., 1992; Ortíz-Hernández et al., 1992). Geochemical evidence indicates that the intrusive complex and the eruptive succession are cogenetic, representing the geologic record of a Late Jurassic—Early Cretaceous magmatic arc built on a suprasubduction tectonic setting (Lapierre et al., 1992). A Late Jurassic—Early Cretaceous dominantly mafic arc, with a stratigraphy comparable with the El Paxtle Formation, was described ∼200 km south of the Sierra de Guanajuato, in the Teloloapan area (Talavera-Mendoza and Guerrero-Suástegui, 2000) (Fig. 1A). Based on the chronologic and stratigraphic correspondence, Martini et al. (2011) proposed that rocks of the Teloloapan and El Paxtle assemblages might represent the vestiges of a single N-S to NNW-SSE magmatic arc belonging to the Guerrero Terrane.
The deformation and tectonic emplacement of these Upper Jurassic—Lower Cretaceous assemblages occurred during a complex D1SG deformation event, which is post-dated by the unconformable deposition of Albian neritic limestone of the La Perlita Formation on top of the tectonic pile (Fig. 7A) (Chiodi et al., 1988). D1SG structures, as well as bedding of the La Perlita limestone, were slightly folded during a subsequent D2SG shortening phase, which is constrained in the Late Cretaceous by the cross-cutting Paleocene Comanja de Corona granite (Ortíz-Hernández et al., 1992).
Sandstone Composition and Provenance
Sandstone composition from the Esperanza, Cuestecita, and El Paxtle formations was determined in detail by Martini et al. (2011). According to these authors, at least three different sandstone compositions can be identified in the Sierra de Guanajuato. Sandstones from the Esperanza Formation are recycled-orogen well-sorted sublitharenites (Fig. 7B), principally fed by Upper Triassic rocks of the Potosí Fan and Lower Jurassic felsic volcanic rocks of the Nazas Formation (Martini et al., 2011) (Fig. 8). Sandstones from the El Paxtle Formation are transitional-arc, poorly sorted, lithic arkose and arkosic litharenite (Fig. 7B) fed by a proximal mafic volcanic source, likely represented by the basaltic flows of the El Paxtle Formation (Martini et al., 2011). Sandstones from the Cuestecita formation are transitional-arc, moderately sorted, lithic arkose, and arkosic litharenite (Fig. 7B) that were fed by two different groups of sources: (1) proximal sources distributed along the Guerrero Terrane boundary, which are represented by mafic volcano-sedimentary rocks of the El Paxtle Formation and the 151–126 Ma Zacatecas-Teloloapan felsic province (Fig. 8); and (2) distal sources from the Guerrero Terrane inland, which are likely represented by the Potosí Fan, the 157–154 Ma felsic arc rocks of the Cuale province (Mortensen et al., 2008), the ca. 120 Ma intermediate Huetamo igneous rocks (Martini et al., 2009), and the ca. 163 Ma Tumbiscatio felsic granitoids (Centeno-García et al., 2008) (Fig. 8).
In order to understand the depositional architecture of the Arperos Basin and reconstruct the Late Mesozoic evolution of the Mexican Pacific margin, we report here new petrographic data and U-Pb zircon ages from the volcaniclastic slump deposits of the Arperos Formation. These sandstones are poorly to moderately sorted, medium to fine lithic arkose and arkosic litharenite (Fig. 9A) that are composed of plagioclase, mafic to felsic volcanic rocks, mono- and poly-crystalline quartz, fragments of quartz-muscovite schist, quartz-rich sandstone, K-feldspar, minor detrital muscovite, and zircon (see modal proportions in Table DR1 of the Data Repository1). These rocks plot in the magmatic arc field in the QmFL, QtFL, and QpLvmLsm composition diagram of Dickinson (1985), and are undistinguishable from arenites of the Cuestecita formation (Fig. 7B). Zircons separated from two rock samples of the Arperos Formation arenites were dated by laser ablation-inductively coupled plasma mass spectrometry (LA-ICPMS) U-Pb geochronology (methodology, analytical results, and sampling site coordinates are given in Table DR2 of the Data Repository). The two rock samples share similar zircon populations, which are also indistinguishable to populations from sandstones of the Cuestecita formation (Fig. 9B). Euhedral to subhedral zircons yielded concordant to slightly discordant ages between 118 and 166 Ma, with main peak ages at 133 Ma and 132 Ma (Fig. 9B). Well-rounded zircons yielded concordant ages from 216 to 2069 Ma, producing subordinate peaks between the Late Triassic and Mesoproterozoic (Fig. 9B). Petrographic data and detrital zircon ages suggest that sandstones of the Arperos Formation resulted from the erosion of the Cuestecita formation or sources that fed the Cuestecita formation and remained available during the Aptian.
Paleogeography of the Guerrero Terrane
In the last few decades many studies successfully demonstrated that there is a close correspondence between sand composition and tectonic setting (e.g., Dickinson et al., 1983; Packer and Ingersoll, 1986). We use the composition and inferred provenance of sandstones from the Sierra de Guanajuato to understand the depositional setting and tectonic evolution of the Arperos Basin. Tithonian sandstones of the Esperanza Formation record the recycling of sources from the Mexican continental interior, and a clear sedimentological disconnection from the volcano-sedimentary successions of the Guerrero Terrane (Fig. 10A). In fact, arenites of the Esperanza Formation lack abundant plagioclase and volcanic fragments as well as zircon grains younger than 184 Ma (Martini et al., 2011), which indicates that no igneous source from the Guerrero Terrane was able to supply detritus to these rocks. The continental provenance of the Esperanza assemblage is also supported by the early Lower Cretaceous micrites of the Valenciana Formation. In fact, siliciclastic sedimentation dominated the Guerrero Terrane since the Aptian-Albian, in contrast to the widespread calcareous sedimentation on the platforms developed on the Mexican continental interior during the Late Jurassic and Early Cretaceous (Carrillo-Bravo, 1971), which represent the most probable source for the Valenciana Formation (Fig. 10A and 10B). By contrast, the Upper Jurassic-Lower Cretaceous sandstone of the El Paxtle, Cuestecita, and Arperos formations were deposited on the western side of the Arperos Basin, and were fed abundantly by the volcanic rocks of the Guerrero Terrane (Fig. 10A and 10B). The asymmetric provenance distribution of the Arperos Basin closely fits with sedimentological models proposed for present-day continent-influenced back-arc basins (Harrold and Moore, 1975; Packer and Ingersoll, 1986), and it is considered as a discriminatory feature in the identification of back-arc settings in old and tectonized petrotectonic assemblages. The back-arc setting recognized for the Arperos Basin favor a Group 3 paleogeographic scenario, where the Guerrero Terrane is interpreted as detached slice of the Mexican leading-edge that drifted in the paleo-Pacific domain during Late Jurassic—Early Cretaceous back-arc extension, and subsequently accreted back to the Mexican interior before the deposition of the Albian La Perlita Formation, producing the D1SG shortening.
It is worth noticing that, although sandstones from the El Paxtle Formation are dominated by volcanic grains, these rocks show different composition and provenance from sandstones of the Cuestecita and Arperos formation. We explain such compositional differences in terms of changes in the drainage feeding the Arperos Basin during its progressive opening and deepening. During the early stage of extension, the Arperos Basin was a continentally floored restricted basin, receiving detritus from proximal sources situated across the basin margins: the El Paxtle— Teloloapan arc at the western side, and the Potosí Fan at the eastern side (Fig. 10A). Subsequently, the persistence of extension produced the deepening of the basin and the generation of an oceanic floor represented by mid-oceanic-ridge basalt (MORB) pillow basalts and hyaloclastite of the Arperos Formation. During this stage, the subsidence of the basin margins permitted the access of detritus from the interior of the Guerrero Terrane and the deposition of the Cuestecita formation at the western side of the basin (Fig. 10B). Finally, progressive spreading caused tectonic instability of the basin margins, which slumped and fed intermittently the deepest part of the basin, resulting in the Cuestecita-like volcaniclastic beds alternating with radiolarite of the Arperos Formation (Fig. 10B). The timing of the transition from a restricted continent-floored to an open oceanic-floored basin is still poorly defined. However, the 150–145 Ma felsic crustal melts of the Esperanza Formation suggest that the continental extension continued at least until the beginning of the Cretaceous, whereas the youngest U-Pb ages of zircons from the Cuestecita and Arperos sandstones suggest that the oceanic spreading started at least at 118 Ma.
The Guerrero Terrane Accretion and the Development of the Fold-Thrust Belt in the Peña de Bernal-Tamazunchale Area
Previous literature assumed a direct cause-effect linkage between the accretion of the Guerrero Terrane and the development of the Mexican Fold-Thrust Belt (Campa-Uranga, 1985; Campa and Coney, 1983; Salinas-Prieto et al., 2000; Keppie, 2004; Talavera-Mendoza et al., 2007). We explore and discuss here such a scenario, in order to understand if the “spreading-accretion tectonics” documented for the Mexican Pacific Margin can be invoked as the cause for the regional shortening in the Peña de Bernal—Tamazunchale area.
Structural and stratigraphic data, supported by geother-mometry of the deformation, indicate that the Mexican Fold-Thrust Belt in the Peña de Bernal—Tamazunchale area is an eastward-tapering orogenic wedge that developed over a westward-dipping detachment zone during the D1MFTB (Eguiluz de Antuñano et al., 2000; Fitz-Díaz et al., 2011a). According to the orogenic wedge mechanical model of Davis et al. (1983), this implies that horizontal stresses directed from the west caused most of the orogenic deformation. According to this scenario, we should observe a gradient of the D1MFTB deformation increasing to the west, from the Guerrero Terrane boundary to the exposed front of the Mexican Fold-Thrust Belt. Moreover, according to orogenic wedge mechanics, we should also expect an eastward progressive migration of the regional shortening and tectonic front from the Guerrero Terrane suture belt to the tip of the wedge. These two conditions are satisfied in the Peña de Bernal—Tamazunchale area. Available data document that shortening increased from the Fold-Thrust Belt front to the Guerrero Terrane boundary (Fitz-Díaz et al., 2008; Fitz-Díaz et al., 2011a). Moreover, paleontologic data from the syn-tectonic foreland deposits suggest that the regional shortening acted like a tectonic wave that migrated progressively eastward across central Mexico (Hernández-Jáuregui, 1997; López-Oliva et al., 1998), favoring the hypothesis of the Guerrero Terrane as the “bulldozer” for the orogenic deformation of the Mexican continental interior. However, problems of applying this model arise when we analyze in detail the timing of the D1SG shortening in the Sierra de Guanajuato area. In fact, rocks of the Esperanza, Arperos, and El Paxtle assemblages were intensely deformed and subsequently unconformably overlain by Albian neritic limestone of La Perlita Formation (Chiodi et al., 1988), highlighting the importance of pre-Albian tectonics at least in the Sierra de Guanajuato. Based on such a stratigraphic relationship, the D1SG deformation event relative to the closure of the Arperos Basin and the accretion of the Guerrero Terrane can be constrained here to have occurred between 118 Ma, age of the youngest zircon population from the Cuestecita and Arperos formation, and 112 Ma, the Aptian-Albian boundary. In summary, available stratigraphic and chronological data suggest that the Guerrero Terrane should have collided with the Mexican core at least ∼20 Ma before the inception of the fold-thrust belt in the Peña de Bernal—Tamazunchale area, favoring the hypothesis that at least two major Cretaceous tectonic events are recorded in this part of the Mexican Cordillera: (1) the D1SG shortening produced by the closure of the Arperos Basin and the accretion of the Guerrero Terrane, and (2) the subsequent D1MFTB, producing the orogenic wedge in the Mexican interior. In this scenario, the D1MFTB shortening phase could be better correlated with the Late Cretaceous D2SG rather than with the D1SG accretion event. Nevertheless, chronologic data are at the moment meager and scattered, and more information is needed to confirm that the Guerrero Terrane was already accreted and amalgamated with the Mexican mainland at the time of the oro-genic wedge formation.
During the first day of the field trip, we will visit outcrops in the western portion of the Mexican Fold-Thrust Belt in the El Doctor—Zimapan area (Fig. 2) in order to recognize the structural style, mechanisms, and timing of deformation in the Zimapan Basin and El Doctor Platform.
Stop 1. Panoramic View of the Sierra Madre Oriental in the Middle of the Zimapán Basin (UTM: 449600E, 2311220N)
We are standing near the core of a major D1MFTB structure in the Zimapán Basin, El Piñón Anticline (Carrillo-Martínez, 1997). From this point we observe the distribution of the two characteristic units of the Zimapán Basin (Trancas and Tamaulipas formations) (Fig. 2) on both limbs of El Piñón Anticline. We can also see the location of the outcrops that we will visit in Stops 2–4. The topographic relief in the orogenic belt in this area is ∼2 km.
Stop 2. Volcanic and Volcaniclastic Rocks at the Base of the Trancas Formation (UTM: 450500E, 2310540N)
At this outcrop we can observe the oldest exposed part of the Zimapan Basin, which is represented by the Trancas Formation (Fig. 2). This unit mostly consists of volcaniclastic sandstone, shale, and scarce felsic volcanics, which grade transitionally upward to calcareous sandstone, siltstone, shale, and limestone. The purpose of this stop is to observe one 6-m-thick volcanic horizon interbedded with sandstones of the Trancas Formation (Fig. A1 in the Data Repository; see footnote 1). These volcanic rocks are dacitic-rhyolitic in composition, and contain several lithic fragments and blocks up to 30 cm in size. Lithic blocks and fragments are of quartzite, quartz-rich sandstone, and limestone. Based on the Late Jurassic—Early Cretaceous depositional age and the volcano-sedimentary composition, the Trancas Formation was considered as the easternmost exposure of an arc belonging to the Guerrero Terrane (Carrillo-Martínez, 1989). We will take the opportunity here to debate the tectonic setting and origin of this succession, and clarify the role of these volcanic rocks in the Upper Jurassic—Lower Cretaceous evolution of the Guerrero Terrane boundary zone.
Stop 3. Shortening Structures in the Trancas Formation (UTM: 450070E, 2310850N)
At this outcrop we can observe excellent exposures of folds affecting the sedimentary rocks of the Trancas Formation. These structures are good examples of passive folding related to the D1MFTB shortening phase of the Mexican Fold-Thrust Belt. F1MFTB folds are cylindrical, and vary from the type 2-1C of Ramsay (1967). They are upright to moderately inclined close folds, and show a constant vergence toward the east. F1MFTB folds are accompanied by an S1MFTB axial planar cleavage, pervasive at the centimeter-scale (Fig. A2 in the Data Repository).
Stop 4. Train of Folds in the Tamaulipas Formation (UTM: 447540E, 2310850N)
Exposed here is a spectacular train of asymmetrical inclined folds developed in the Tamaulipas Formation of the Zimapan Basin (Fig. 2). These folds developed during the D1MFTB shortening phase. Like most of the folds in the Mexican Fold-Thrust Belt in the Peña de Bernal—Tamazunchale area, they show a general vergence toward the east and commonly possess strongly attenuated forelimbs. The geometry and the scale of these folds are strongly influenced by lithological variations. For instance, most of the folds in the limestone layers are of chevron type and have wavelengths of a few meters, while the chert layers developed folds with rounded hinges and wavelengths of less than a meter (Fig. A3 in the Data Repository). There are beautiful examples of buckle folds in chert bands at different scales across the outcrop. There are also nice examples of fold-related veins that were emplaced early, during, and after folding. These folds and those of Stop 5 (Fig. 2) are probably the most studied ones in the Sierra Madre Oriental. These studies include kinematic analysis (Bolaños-Rodríguez et al., 2007; Vázquez-Serrano, 2010; Fitz-Díaz et al., 2012), Rare earth elements (REE) and stable isotope analysis of fold-related veins and host rock (Nava-Urrego, 2008), fluid inclusions microther-mometry, and illite characterization (Fitz-Díaz et al., 2011a). Together, these sources of information allow us to determine a best-fit kinematic model to explain the evolution of these folds, the linkage of mesoscopic deformation with grain-scale deformation (pressure-solution and solution transfer) operating in different parts of the fold, fluid flow at the fold scale, and the possible source of water active during deformation. The latter was possible by comparing the data from this outcrop with data from 9 other outcrops analyzed along cross-section AA′ (Fitz-Díaz et al., 2011b).
Stop 5. “The Icon” Fold (UTM: 443315E, 2312330N)
This is a tight recumbent F1MFTB fold affecting limestone interbedded with shale, bentonite and chert layers from the upper reaches of the Tamaulipas Formation (Fig. 2 and Figure A4 in the Data Repository). We refer to this fold as the icon because it is an excellent structure representative of Zimapán Basin deformation including buckling, later flattening, as well as pre-, syn- and post-folding veins. REE and stable isotope analyses (δ13C and δ18O) in calcite from the different generations of fold-related veins, and δ13C and δ18O from water extracted from illite and fluid inclusions, indicate that the water active early and during deformation was locally derived, while late veins were strongly influenced by meteoric fluids (Fitz-Díaz et al., 2011b). K-Ar ages of the <2 μm illite size fraction extracted from a bentonite folded layer, indicate that the hinge (85 ± 1 Ma) is older than the limbs (77 ± 1.5 Ma) of this fold (Fitz-Díaz et al., 2011b). These data have been used to interpret the detailed kinematics of this fold in terms of initial buckling followed by simple shear.
Stop 6. Western Boundary of the Zimapán Basin, El Doctor Thrust (UTM: 439070E, 2306406N)
At this stop, we will observe rocks of the Aptian-Albian El Doctor Platform overriding Turonian syn-orogenic sedimentary rocks of the Soyatal Formation along a mayor D1MFTB thrust (Fig. 2 and Fig. A5 in the Data Repository). A detailed micro-paleontological study of these syn-orogenic turbidites allowed Hernández-Jáuregui (1997) to constrain the activity of this fault between 91 and 85 Ma. δ13C and δ18O isotopes in calcite from fault-related rocks and veins and from undeformed host rocks taken along a transect across this fault are discussed in (Fitz-Díaz et al., 2011b). These studies show that the calcite in the fault zone is depleted in S13C with respect to the undeformed rocks in the hangingwall and footwall, which have typical values of Cretaceous carbonates. This may have been caused by the passage of hydrocarbons or the mobilization of 13C-rich calcite by pressure-solution localized along the shear zone.
Stop 7. Upper Contact of the El Doctor Platform with the Soyatal Formation (UTM: 427670E, 2310020N)
At this site we observe the nature of the upper contact of the Aptian-Albian El Doctor Platform rocks with the Late Cretaceous syn-orogenic turbidites of the Soyatal Formation (Fig. 2). Despite the fact that this contact localized considerable amounts of shear during the D1MFTB phase, it still preserves some primary features, such as paleo-karst structures that were filled with terra rossa and calcareous shale. These paleo-karstic features indicate uplift above sea level before turbidite sedimentation on El Doctor Platform. The Turonian sea level drop is observed in most of the Cretaceous platforms in Mexico. It is not clear, however, if it is due to eustatic sea-level changes or due to regional tectonic activity. δ13C and δ18O isotopes in calcite from samples take along a transect across this contact show depleted values (Fitz-Díaz et al., 2011b) in δ13C in all the samples with respect to the undeformed Cretaceous carbonates. The δ13C anomaly is accentuated in the sheared rocks. Depletion in δ18O is also noted along the sheared contact between these two units. The depletion in δ13C could have been caused by organic matter and meteoric fluids deeply interacting with the carbonate succession during platform drowning.
Stop 8. Peña de Bernal (UTM: 401390E, 2294507N)
The Peña de Bernal is a monolith with a remarkable topographical expression that has been interpreted as a volcanic neck (Aguirre-Díaz et al., 2005) (Fig. 2 and Fig. A6 in the Data Repository). The color of the Peña is pink, very homogenous in composition, and displays a porphyritic texture with phenocrysts of quartz, sanidine and amphibole set in a microcrystalline matrix. Aguirre-Díaz et al. (2005) determined an age of 7.5 Ma for the rock (40Ar-39Ar-plagioclase) and interpreted this feature as an endogenous rhyolitic dome contemporaneous with other domes in the area, such as the domes around the rim of the Amazcala Caldera (Aguirre-Díaz and López-Martínez, 2001). Economically relevant epithermal veins occur in the neighboring San Martín active mine 5 km south of Peña de Bernal, although their age has been interpreted to be Oligocene (ca. 27.6 Ma) by Cam-prubí et al. (2003) and Camprubí (2009), based on K-Ar ages of host andesite-dacite. These deposits belong to the low-sulfidation subtype of epithermal deposits and formed essentially through recurrent shallow boiling, which was associated with extensive hydrothermal brecciation and re-brecciation in no less than four stages of mineralization (Núñez-Miranda, 2007). This is one of the few low sulfidation epithermal deposits known in Mexico with no intermediate sulfidation roots or preexisting stages of mineralization (Camprubí and Albinson, 2007). The main ore minerals (Núñez-Miranda, 2007) are electrum, naumannite, and tetrahedrite-tennantite (hypogene), nearly pure gold and chlorar-gyrite (supergene), and the latter two formed after electrum and naumannite. Temperatures of homogenization of fluid inclusions are generally up to 190 °C, though in the ore-bearing stage such temperatures are up to ∼300 °C, and salinities range from 0.5 to 2.6 wt% NaCl equivalent with only slight variations from stage to stage (Núñez-Miranda, 2007).
Stop 9. Shortening Structures in the Peña Azul Formation (UTM: 401250E, 2313860N)
This Barremian-Cenomanian unit is found in the Tolimán area to the west of El Doctor Platform (Fig. 2A and 3), where it covers the San Juan De La Rosa Formation and El Chilar Complex rocks. Rocks of the Peña Azul Formation experienced intense internal deformation, evidenced by isoclinal recumbent folds with a general vergence toward the east (probably associated with the D1MFTB phase of the Mexican Fold-Thrust Belt), and locally refolded into open folds (probably during D2MFTB). At this stop, we observe isoclinal folds developed in the basal layers of the Peña Azul Formation. Notice the considerable thickening of the sequence caused by folding in this area.
Stop 10. El Chilar Complex (UTM: 415300E, 2300460N)
At this locality (Fig. 2), this unit is composed of quartzite, phyllite, and scarce blocks of radiolarite and basaltic dikes, all tectonically mixed in a block-in-matrix fabric, commonly observed in a mélange. However, elsewhere El Chilar Complex mainly consists of quartzite and slate (Fig. A7 in the Data Repository). These features, together with U-Pb detrital zircon ages obtained from the unit (youngest zircon = 269 Ma) and the existence of other siliciclastic units of the same age and characteristics in México lead us to interpret El Chilar Complex as part of the Late Triassic Potosí Fan.
Stop 11. Type Locality of the San Juan De La Rosa Formation (UTM: 419850E, 2297500N)
We will walk a section that crosses the contact between the El Chilar Complex and San Juan De La Rosa Formation. The El Chilar Complex here mainly consists of 30-cm- to 1-m-thick layers of quartz sandstone with abundant detrital muscovite interbedded with thinly bedded and strongly foliated phyllite. The El Chilar Complex is unconformably overlain by rocks of the San Juan De La Rosa Formation. The contact is locally sheared probably during the Late Cretaceous D1MFTB shortening. The lower stratigraphic levels of the San Juan De La Rosa Formation are composed here of intensely foliated felsic tuff and less deformed volcaniclastic turbidites and conglomerate (Fig. A8 in the Data Repository).
Stop 12. Transitional Contact between the Peña Azul and San Juan De La Rosa Formations (UTM: 418310E, 2306950N)
At this stop, we can see the transition between the San Juan De La Rosa and Peña Azul formations (Fig. 2). We observe thinly bedded layers of volcaniclastic sandstone interbedded with calcarenite, covered by a thickly bedded breccia with abundant clasts of limestone, shale, and chert embedded in a calcareous matrix (Fig. A9 in the Data Repository).
Stop 13. Soyatal Formation (UTM: 418670E, 2307190N)
At this outcrop, the Soyatal Formation consists of alternating calcarenite and calcareous shale (Fig. A10 in the Data Repository). Sedimentary structures are typical of the Bouma sequence (e.g., graded sandstone, parallel lamination in fine grain sandstone and ripple marks), indicating that these rocks were deposited by turbidity flows. Detailed stratigraphic data suggest that the Soyatal Formation represents the foreland infill related to the early development of the Mexican Fold-Thrust Belt (Hernández-Jáuregui, 1997). According to this interpretation, the 91–85 Ma depositional age range obtained for turbidites of the Soyatal Formation constrain the shortening of the westernmost Mexican Fold-Thrust Belt (Hernández-Jáuregui, 1997). In this locality, rocks of the Soyatal Formation are overthrust by detrital limestone of the Peña Azul Formation.
This day will be devoted to a comprehensive overview of the stratigraphy and structural style of the Esperanza, Arperos, and El Paxtle assemblages. Key accessible exposures at the Sierra de Guanajuato will be visited in order to observe the structural architecture of the tectonic pile and allow reconstruction of the depositional and tectonic evolution of the Arperos Basin.
Stop 14. Stratigraphic Contact between the Esperanza and Valenciana Formations (UTM: 266255E, 2328677N)
The goal of this stop is to observe an excellent exposure of the stratigraphic relationship between the Esperanza and Valenciana formations of the Esperanza tectonostratigraphic assemblage (Figs. 6B and 7A). The Valenciana Formation is composed here of a polydeformed rhythmic succession of alternating thin-bedded micrite and shale that sit conformably on rhyo-dacitic lava and scarce related epiclastics. U-Pb zircons from a rhyolite at this outcrop yielded a 206Pb/238U weighted average age of 145.4 ± 1.1 Ma (Martini et al., 2011). Nine grains yielded concordant to slight discordant ages from 151 to 663 Ma, which define a discordia with a late Neoprotero-zoic upper intercept. This is interpreted as representing zircon inheritance from an older basement. The U-Pb magmatic age obtained by Martini et al. (2011) places the base of the Valen-ciana Formation in the lower Early Cretaceous, and suggests a provenance from the Mexican continent interior for this formation, as calcareous sedimentation occurred within the Guerrero Terrane since the Aptian-Albian.
Stop 15. Gabbro and Diorite of the Tuna Manza Intrusive Complex (UTM: 264138E, 2329920N)
Stops 15–17 are located along La Calera creek, ∼2 km northwest of Guanajuato (Fig. 6B), where we can visit almost continuous exposures of the Tuna Manza Intrusive Complex and the Esperanza assemblage. The purpose of this stop is to observe gabbro and diorite of the Tuna Manza Intrusive Complex. They are composed of calcic plagioclase with normal oscillatory zoning + green hornblende ± potassium feldspar ± quartz and accessory magnetite, pyrite, epidote, and apatite. These rocks are cut by porphyritic basalt and doleritic dikes. In some cases, dikes display necking along their lengths and lobate contacts with the host rocks (Fig. A11 in the Data Repository). Rocks of the Tuna Manza Intrusive Complex are interpreted as representing the stratified magmatic chamber of the Late Jurassic—Early Cretaceous El Paxtle arc of the Guerrero Terrane (Lapierre et al., 1992).
Stop 16. Santa Ana Thrust (UTM: 264538E, 2330155N)
We can observe here one of the major D1SG thrusts of the tectonic pile that makes up the Sierra de Guanajuato. This thrust is expressed as a ∼300-m-thick mylonitic shear zone along which the Tithonian rocks of the Esperanza Formation were thrust over the Valenciana Formation. At this locality, the Esperanza and Valenciana formations are highly sheared and recrystallized (Fig. A12 in the Data Repository). Locally, sandstone beds of the Esperanza Formation acted as rigid layers and experienced less amounts of deformation. Relics of primary texture and composition indicate that sandstones from the Esperanza Formation are continent-recycled sublitharenites, composed mainly of mono-and poly-crystalline quartz, and minor detrital muscovite, zircon, tourmaline, and lithic grains of quartz-arenite, felsic volcanics, quartz-muscovite schist, and phyllite. This composition and the detrital zircon U-Pb ages obtained from sandstones of the Esper-anza Formation support a continental provenance for this unit. In fact, the lack of abundant plagioclase and volcanic fragments as well as zircon grains younger than 184 Ma indicate that no igneous source from the Guerrero Terrane was able to supply detritus during the deposition of the Esperanza Formation (Fig. 10).
Stop 17. Peperites in the Esperanza Formation (UTM: 264959E, 2330839N)
At this locality, we can observe the clastic rocks of the Esperanza Formation cut by a dacitic dike. Peperites at the margins of the dike suggest that magma intrusion was contemporaneous with the sedimentation of the Esperanza Formation. Peperite domains are broadly parallel to the contacts of the parent intrusion. They are closed to moderately packed (Skilling et al., 2002) at the dike boundaries and grade to dispersal 3–4 m from the igneous contacts. Bedding and lamination in the host rocks are destroyed close to the contact with the intrusions, and millimeter-scale mutual injection of magma into sediment and vice versa can be observed (Fig. A13 in the Data Repository). U-Pb zircons from a dacitic rock sample from this outcrop yielded a 206Pb/238U weighted mean age of 150.7 ± 0.8 Ma (Martini et al., 2011), which constrains the deposition of the Esperanza Formation to have occurred in the Tithonian.
Stop 18. Volcanogenic Massive Sulfide Deposits in the Felsic Volcanics of the Esperanza Formation (UTM: 268610E, 2330525N)
In the Sierra de Guanajuato, scarce volcanogenic massive sulfide (VMS) deposits are hosted by the felsic volcanic rocks of the Esperanza Formation. The main VMS deposits in the area are found in the El Guapillo, La Virgen, and San Ignacio (Los Mexicanos) abandoned mines. These deposits have been traditionally included in the Guerrero Terrane. However, provenance analyses indicate that the clastic rocks of the Esperanza Formation were deposited far from the sedimentary influence of the arc successions of the Guerrero Terrane, and that these rocks were fed by sources from the Mexican continental interior (Martini et al., 2011). These data indicate that at least some of the VMS deposits of the region belong actually to the western part of the Mexican continental interior. The VMS deposits in Guanajuato usually have a rather disseminated character (Martínez-Reyes et al., 1995). The San Ignacio (Los Mexicanos) deposit (Fig. 6B) is constituted by pyrite, quartz, chalcopyrite, sphalerite, bornite, and pyrrhotite with associated phyllic alteration, and late stock-work-like veinlet arrays at their base constituted by quartz and chalcopyrite with associated strong silicification of host rocks (Mengelle-López et al., 2006). Microthermometric studies of fluid inclusions yielded temperatures of homogenization that range from 111 °C to 321 °C and salinities that range from 11.5 to 18.1 wt% NaCl equiv., whereas sulfur isotope compositions (δ34S) range from −3.4–17.4‰, thus supporting the occurrence of both magmatic and marine sources for sulfur (Mengelle-López, 2009). Host volcanic rocks are classified as andesites to dacites (Mengelle-López et al., 2006).
Stop 19: Concepción Thrust (UTM: 265117E, 2329980N)
Here we will observe an ∼120-m-thick D1SG mylonitic shear zone developed during the tectonic emplacement of tonalite of the Tuna Manza Intrusive Complex over the felsic volcanics of the Esperanza Formation (Fig. 6B). The shear deformation produced a mylonitic foliation that varies from a spaced disjunctive smooth to a continuous coarse fabric, pervasive at the submillimeter-scale. This foliation is defined by iso-oriented stretched grains of quartz and neoblasts of white mica, which are currently being dated. Mineral lineations are well preserved on foliation planes, and display an ESE average direction. Submillimeter- to centimeter-scale S-C′ structures, pyrite-porphyroclasts, mica fish, and asymmetric folds are observed on cuts subparallel to the XZ-plane of the finite strain ellipsoid, and indicate top-to-the-SE tectonic transport.
Stop 20. Pillow Basalt, Radiolarite, and Volcaniclastic Slump Deposits of the Arperos Formation (UTM: 250635E, 2333808N)
Beautiful exposures representative of all the lithologic variety that comprises the Arperos Formation will be visited along the Arperos creek, ∼15 km from the city of Silao (Fig. 6A). Here we have the opportunity to observe an almost continuous section comprising N-MORB pillow basalt (Fig. A14 in the Data Repository), radiolarite, and volcaniclastic slump deposits, which are interpreted to record the oceanic-stage of the Arperos Basin opening.
Composition and U-Pb detrital zircon ages from the volcani-clastic slump deposits of the Arperos Formation suggest that these rocks were deposited at the western side of the Arperos Basin, and were fed mainly by the Late Jurassic—Early Cretaceous arcs of the Guerrero Terrane (Fig. 10). Rocks of the Arperos Formations are intensely sheared during the D1SG with a top-to-the-SW tectonic transport, and are tectonically overlain by turbidites of the Cuestecita formation and plutonic rocks of the Tuna Manza Intrusive Complex.
The research was funded by PAPIIT (Programa de Apoyo a Proyectos de Investigación e Innovación Tecnológica) grants IN110810-3 to Antoni Camprubí. Joaquín Aparicio prepared thin sections. Carlos Ortega-Obregón (Laboratorio de Estudios Isotópicos, Centro de Geociencias, UNAM) performed isotopic analyses by LA-ICPMS on dated zircon samples.
Figures & Tables
The Southern Cordillera and Beyond
Prepared in conjunction with the 2012 GSA Cordilleran Section Meeting, Querétaro, Mexico, this volume's eight field guides showcase three aspects of the geology of the southern end of the North America cordillera: Mid-Tertiary and Quaternary volcanology, environmental geology, and Mesozoic tectonics. Field Guide 25 explores the Cenozoic stratigraphy of Sierra de Guanajuato, one of the most important Mexican mining districts, and addresses a controversial topic, the accretion of the Guerrero terrane and its possible role in the Late Cretaceous—Early Tertiary orogeny. Three guides related to the Trans-Mexican Volcanic Belt, an active magmatic arc related to subduction of the Rivera and Cocos plates, include new data about the recent volcanic history, physical volcanology, and volcanic hazards in Mexico's most densely populated area. Bringing the geosciences into societal problems, one guide presents data on ground deformation related to water extraction in urbanized areas of the Mexico City basin, and another explores the ghost town of the Mineral de Pozos mining district and the effect of mine tailings on groundwater.