versión impresa ISSN 2007-2902
Rev. mex. cienc. geol vol.26 no.1 México abr. 2009
Kinematics and UPb dating of detrital zircons from the Sierra de Zacatecas, Mexico
Cinemática y fechamiento UPb de circones detríticos de la Sierra de Zacatecas, México
Felipe de Jesús EscalonaAlcázar1, 2, 3, *, Luis Alberto DelgadoArgote2, Bodo Weber2, Ernesto Patricio NúñezPeña4, Víctor A. Valencia5, and Olivia OrtizAcevedo1
1 Departamento de Ordenamiento Ecológico, Instituto de Ecología y Medio Ambiente de Zacatecas, Av. México 151, Col. La Florida, 98600 Guadalupe, Zacatecas, Mexico. * firstname.lastname@example.org
2 División de Ciencias de la Tierra, Centro de Investigación Científica y de Educación Superior de Ensenada (CICESE), Km. 107 Carr. TijuanaEnsenada, 22860, Ensenada, Baja California, Mexico.
3 Faculty of Earth and Life Sciences, Vrije Universiteit Amsterdam, De Boelelaan 1085, 1081 HV Amsterdam, The Netherlands.
4 Unidad de Ciencias de la Tierra, Universidad Autónoma de Zacatecas, Calzada de la Universidad 108, Col. Progreso, 98058, Zacatecas, Zacatecas, Mexico.
5 Department of Geosciences, University of Arizona, 1040 East Fourth St., Tucson, AZ, 857210077, United States of America.
Manuscript received: November 21, 2007
Corrected manuscript received: November 3, 2008
Manuscript accepted: November 9, 2008
The Mesozoic succession of the Sierra de Zacatecas (SZ) was originally described in two sequences. The older is the La Pimienta Phyllite or sometimes termed Zacatecas Formation (ZF) of presumably Late Triassic age based on paleontological constraints obtained at the beginning of the twentieth Century. Overlying the ZF is Las Pilas Volcanosedimentary Complex (LPVC) of unknown age, whose contact has been interpreted to be tectonic. We have studied these sequences in detail in order to constrain their stratigraphic relationships, deformation style, composition, and maximum depositional age.
The ZF has been metamorphosed under greenschist facies conditions and its protolith is composed of sandstone, mudstone, limestone and rare conglomerates interlayered with lava flows and tuffs. This unit is intruded by dikes, sills, laccolithic bodies and we interpret the presence of hydrothermal vents at the type locality in the Arroyo El Bote. The ZF is in our interpretation in gradual contact with the LPVC. The LPVC is an igneous assemblage mainly made up of mafic pillowed to massive flows, commonly foliated and/or deformed. Contemporaneous laccolithiclike bodies, dikes and sills are observed. Feldespatic greywackes and wackes, mudstone and minor tuffs and limestone are interbedded within the lava flows. UPb dating of detrital zircons from ZF and LPVC from the Arroyo El Bote and the Sauceda de la Borda areas, respectively, yielded ~132 Ma and ~160 Ma old ages. Approximately 30% of the zircons from the ZF represent Paleozoic and Proterozoic inheritance, which is lacking in the LPVC, and suggests a change in the sedimentary source. The maximum depositional age for both sequences in this area is therefore interpreted as Early Cretaceous (Hauterivian), in contrast to the previous proposed Late Triassic age. According to the distribution and types of rocks, the volcanoplutonic and sedimentary assemblage may have been emplaced in a volcanic field that developed in an intraarc or backarc basin with sedimentary supply derived from a nearby continental block and a magmatic arc.
The geometric analysis of foliation shows that the average poles of the ZF and LPVC are 86°/235° and 86°/244°, respectively. The paleostress analysis yield an average σ1 for the thrust faults at 01°/219° suggesting that foliation and reverse faulting should have been developed contemporaneously during the Laramide Orogeny. Normal faults formed during a later extensional regime are associated to an average σ3 vector oriented 00°/029°. Since normal faults always cut the thrust faults, we interpret that normal faults developed in a postorogenic event.
Key words: stratigraphy, detrital zircon geochronology, paleostress, Hauterivian, Zacatecas, Mexico.
Desde las primeras descripciones, las rocas mesozoicas de la Sierra de Zacatecas se han descrito en dos secuencias. La más antigua corresponde a la Filita La Pimienta o Fm. Zacatecas (ZF), cuya edad paleontológica es del Triásico Tardío. También se ha descrito que está en contacto tectónico con el Complejo Volcanosedimentario Las Pilas (LPVC), cuya edad se desconoce. En este trabajo se estudiaron con detalle las composiciones, relaciones de contacto, estilos de deformación y edades de depósito de dichas secuencias litológicas en la parte central y norte de la Sierra de Zacatecas.
La ZF presenta metamorfismo en facies de esquistos verdes y está compuesta por areniscas, lodolitas, calizas y, en menor medida, conglomerados interestratificados con derrames lávicos y tobas. La secuencia está intrudida por diques, sills, lacolitos dioríticos, e interpretamos la presencia de ventilas hidrotermales en la localidad tipo. El contacto con la LPVC es gradual. El LPVC está formado por series de lavas máficas cuyas texturas varían de almohadilladas a masivas, comúnmente foliadas y deformadas. La secuencia volcánica puede estar interestratificada con wacka, grauvaca, lodolita y, en menor cantidad, tobas y caliza. También hay cuerpos lacolíticos, diques y sills asociados a las rocas volcánicas. Las edades isotópicas UPb de zircones detríticos de la ZF y LPVC de una muestra obtenida en el Arroyo El Bote y otra en el área de Sauceda de la Borda, respectivamente, indican que provienen de una fuente cuya edad es de ~132 Ma y ~160 Ma. Aproximadamente 30% de los granos analizados de la ZF tienen edades más antiguas, del Paleozoico al Arqueano. En cambio en la muestra de la LPVC los zircones más antiguos son del Jurásico Tardío. La edad máxima de depósito de ambas secuencias se define como del Cretácico Temprano (Hauteviriano), en contraste con la edad del Triásico Tardío asignada en trabajos anteriores. De acuerdo con la distribución y tipos de roca, la asociación volcanoplutónica y sedimentaria definiría un campo volcánico que pudo desarrollarse en un ambiente interior de arco o de trasarco, con sedimentos provenientes de un bloque continental y de un arco magmático.
Del análisis geométrico de la foliación de la ZF y del LPVC, se observa que las orientaciones promedio de sus polos son 86°/235° y 86°/244°, respectivamente. A partir del análisis de paleoesfuerzos se obtuvo un promedio de σ1 para fallas inversas de 01°/219°, lo que sugiere que la foliación y el fallamiento inverso pudieron haberse desarrollado de forma contemporánea durante la Orogenia Laramide. El fallamiento normal se desarrolló posteriormente en un régimen extensional con una orientación promedio del vector σ3 de 00°/029°. Debido a que las fallas normales cortan a las inversas, interpretamos que el fallamiento normal se desarrolló en un evento postorogénico.
Palabras clave: estratigrafía, geocronología de zircones detríticos, paleoesfuerzos, Hauteriviano, Zacatecas, México.
The Sierra de Zacatecas (SZ) is a topographic high located in the central part of the State of Zacatecas (Figure 1a). This is one of the few outcrops north of the TransMexican Volcanic Belt (TMVB) with a Mesozoic igneous and clastic sedimentary rock assemblage (Figure 1a). The origin and tectonic setting of the Mesozoic sequence of the SZ are controversial. However there is a general agreement in relating it to the Guerrero terrane (GT) (CentenoGarcía and SilvaRomo, 1997) or Guerrero Superterrane (Dickinson and Lawton, 2001) (Figure 1a). The GT is made up of an island arc, backarc and oceanfloor assembly of Triassic to Early Cretaceous age (CentenoGarcía and SilvaRomo, 1997). Although the terrane has been extensively studied south of the TMVB, the limited outcrops in the northern areas make it difficult to delineate its northeastern boundaries. The closest locations to SZ where the Guerrero terrane has been analyzed are Guanajuato (Tardy and Maury, 1973; Lapierre et al., 1992; OrtizHernández et al., 1992, 2003; Tardy et al., 1992, 1994; Freydier et al., 1996, 1997, 2000), the Baja California peninsula (Dickinson and Lawton, 2001; Busby et al., 2006), State of Mexico (DelgadoArgote et al., 1992), Guerrero (DelgadoArgote et al., 1992; TalaveraMendoza et al., 2007) and Sinaloa (OrtegaGutiérrez et al., 1979, Gastil et al., 1999) (Figure 1a). Under this scenario, the age and tectonic setting of the SZ is of great significance in defining the probable extension of the GT, and will improve the knowledge on the geologic evolution of central Mexico during the Mesozoic.
The Sierra de Zacatecas is located in the central part of Mexico (Figure 1a). It is subdivided in two major stratigraphic units (Burckhardt, 1906; GutiérrezAmador, 1908; McGehee, 1976; Ranson et al., 1982; Sedlock et al., 1993; CentenoGarcía and SilvaRomo, 1997; BarbozaGudino et al., 1999; SilvaRomo et al., 2000; Bartolini et al., 2001).
The lowermost stratigraphic unit is the Zacatecas Fm. (ZF) (GutiérrezAmador, 1908), sometimes also called La Pimienta Phyllite (Ranson et al., 1982), which crops out in the western part of the SZ. The ZF is overlain by a lithologic unit that has received several names: the Zacatecas greenstone (McGehee, 1976), Las Pilas Diorite (Ranson et al., 1982), Zacatecas Microdiorite (Ponce and Clark, 1988) or La Borda Fm. (CentenoGarcía and SilvaRomo, 1997). As named and described by Ransom et al. (1982), Las Pilas Volcanosedimentary Complex (LPVC) is the name given to the lithologic unit above ZF. The ZF and the LPVC are supposed to have formed, developed and evolved in different tectonic settings prior to their amalgamation to the North America craton during the Late Cretaceous (CentenoGarcía et al., 1993; Busby et al., 2006) or the Late Cretaceous Early Tertiary (ArandaGómez et al. 2000) Laramide Orogeny.
Some of the key questions arising from the revision of the literature and our geologic survey are: 1) Are the ZF and the LPVC two different facies of a single volcanosedimentary complex?; 2) What are their depositional ages?; 3) In what kind of tectonic environments the ZF was juxtaposed with the LPVC?. To address these issues we conducted a detailed geologic survey to define and describe the stratigraphy and the contact relationships of the ZF and LPVC in the SZ. In order to establish a maximum age of the sequences, we separated detrital zircons for LAICPMS dating from one sample of the ZF and one from the LPVC. Based on these results, we propose a new interpretation on the origin and evolution of the Sierra de Zacatecas.
PREVIOUS STRATIGRAPHY AND AGE CONSTRAINTS
The lithologic compositions of the ZF and the LPVC are controversial. The ZF has been described as a mainly detrital sedimentary package associated with coeval volcanic rocks (Burckhardt, 1906; GutiérrezAmador, 1908; McGehee, 1976; Ranson et al., 1982; Monod and Calvet, 1992; Sedlock et al., 1993; CentenoGarcía and SilvaRomo, 1997; Yta et al., 2003) or without them (BarbozaGudino et al., 1999; SilvaRomo et al., 2000; Bartolini et al., 2001). The ZF has been interpreted as a flyschlike deposit that formed a submarine fan close to the western passive margin of Pangea (Monod and Calvet, 1992; BarbozaGudino et al., (1999); SilvaRomo et al., 2000; Bartolini et al., 2001). In contrast GutiérrezAmador (1908) and Monod and Calvet (1992) suggested a shallow marine environment.
The first study that interpreted an Upper Triassic age for the ZF, based on fossil evidence, was made by Burckhardt (1906). Such interpretation has been widely adopted or even reinterpreted as Middle to Upper Triassic age (Figure 1b) (GutiérrezAmador, 1908; McGehee, 1976; Ranson et al., 1982; Sedlock et al., 1993; BarbozaGudino et al., 1999; SilvaRomo et al., 2000; Bartolini et al., 2001). SilvaRomo et al. (2000) considered the upper part of the ZF as Middle Jurassic. Yta et al. (2003) further considered the possibility of a late Paleozoic age, as was originally suggested by Burckhardt (1906). Reported ages for the LPVC, based on stratigraphic correlation, are also controversial and vary from Early Cretaceous (Monod and Calvet, 1992; CentenoGarcía and SilvaRomo, 1997; Yta et al., 2003) to Late Jurassic (Sedlock et al., 1993) (Figure 1b). On the basis of geochemical data, the tectonic setting for the ZF has been interpreted as an oceanbasin or primitive island arc (CentenoGarcía y SilvaRomo 1997), while for the LPVC is interpreted as an oceanic island arc (Lapierre et al., 1992).
The LPVC has been barely studied and its stratigraphic relationships with the ZF have been interpreted in several ways. The contact has been described to be tectonic (Monod and Calvet, 1992; CentenoGarcía and SilvaRomo, 1997; SilvaRomo et al., 2000; Bartolini et al., 2001), discordant (McGehee, 1976), obscure (Sedlock et al., 1993) or continuous (Burckhardt, 1906; GutiérrezAmador, 1908; McGehee, 1976; Ranson et al., 1982).
The LPVC has been described as a laccolithlike or hypabyssal intrusion of andesitic composition (McGehee, 1976; Ranson et al., 1982). Some igneous rocks were generically described as intrusive and other as extrusive (Ranson et al., 1982), or as pillows and massive basaltic to andesitic lava flows interlayered with volcanoclastics, limolite and radiolarite (Lapierre et al., 1992; Centeno García and SilvaRomo, 1997; Yta et al., 2003). According to CentenoGarcía and SilvaRomo (1997), and Mortensen et al. (2008), the depositional facies of the LPVC could correspond to a deep water marine environment that developed in an intraarc basin or backarc basin.
The ZF has experienced greenschist facies metamorphism and ductile deformation that promoted the formation of a foliation that varies in intensity from moderately to well developed, sometimes exhibiting crenulation cleavage, locally tightly folded. The style of deformation, age and duration have been poorly studied. Research works by McGehee (1976) and Ranson et al. (1982) indicate at least two deformation stages for the ZF. However, because of the limited amount of data, these results are inconclusive. Most authors agree that at least two deformation phases are responsible for the deformation of the Mesozoic sequence of the SZ: the first one is preEarly Cretaceous age and the second one is Late Cretaceousearly Tertiary (Monod and Calvet, 1992; Sedlock et al., 1993; CentenoGarcía and SilvaRomo, 1997; Goldhammer, 1999; SilvaRomo et al., 2000; Dickinson and Lawton, 2001; Yta et al., 2003). In addition, Bartolini et al. (2001) also suggested a third deformation phase that occurred during the Late Triassic Early Jurassic (?) time.
By correlation, the origin and evolution of the LPVC has been interpreted to belong to the Guerrero terrane of Late JurassicEarly Cretaceous age of southern and central Mexico (De Cserna, 1976; Campa and Coney, 1983; Sedlock et al., 1993; CentenoGarcía and SilvaRomo, 1997; SilvaRomo et al., 2000; Dickinson and Lawton, 2001). The Guerrero terrane has been described in southern Mexico as a composite terrane comprising island arc and backarc basins, with or without oceanic crust, of Triassic to Early Cretaceous age (Campa and Coney, 1983; CentenoGarcía et al., 1993, 2008; Bissig et al., 2008; Mortensen et al., 2008). This terrane was accreted to central Mexico during the Late Cretaceous (CentenoGarcía et al., 1993; Busby et al. 2006) or during Late Cretaceousearly Tertiary Laramide Orogeny (Sedlock et al., 1993; Dickinson and Lawton, 2001; CentenoGarcía et al., 2008). The accretion produced lowgrade metamorphism and some discrete folding (Campa and Coney, 1983; CentenoGarcía et al., 1993).
METHODOLOGY AND ANALYTICAL TECHNIQUES
In this work we present the results of our geologic mapping of the Mesozoic of the Sierra de Zacatecas. Initial survey was made on synthetic stereopars made from a LandsatTM image (October 23rd, 2001) P029, R044 scale 1:150,000. The main structural features were recognized, and together with the geologic maps Zacatecas and Guadalupe, scale 1:50,000 (CETENAL, 1979a, 1979b), they were the bases for planning the survey. Detailed geologic mapping was made along many roads and creeks, with verification points each ~150 m or less when lithologic or structural changes were observed. We collected structural fault and foliation data throughout the stratigraphic section. Planes were measured using the righthand rule. The notation used for planes is azimuth/dip/rake and for lines is plunge/trend (Marshak and Mitra, 1988). Fault planes show well developed striae, sometimes with precipitation of calcite and chlorite. Cataclastic bands or gouge developments are also common. The largest observed faults show displacements of less than a few meters; most of the faults show minor displacements. Kinematic indicators used were the ones described by Petit (1987). The analysis of foliation was carried out by separating the data by area.
Whereas the kinematic analysis was made accordingly with the methodology proposed by Marret and Allmendinger (1990), fault data were separated depending on their stratigraphic position from bottom to top, the crosscutting relationships (thrust faults are cut by normal faults), the quality of the data (well, moderately and poorly preserved), the rock type, and kinematic compatibility. The final results are shown by area.
Detrital zircons from one sample from the ZF (feldspathic greywacke) and one from LPVC (feldspathic wacke) were analyzed. The zircons were separated at the facilities of the Department of Geology, CICESE. The original rock samples, ~20 kg each one, were crushed and powdered. The 80 mesh fraction (≈ 200 µm) was selected and the zircons separated using a Wilfley table, a Frantz isodynamic separator, heavy liquids (methylene iodide) and nitric acid. The final separation was made by hand picking.
UPb geochronology of zircons was conducted by laser ablation multicollector inductively coupled plasma mass spectrometry (LAMCICPMS) at the Arizona LaserChron Center. The analyses involved ablation of zircon with a New Wave/Lambda Physik DUV193 Excimer laser (operating at a wavelength of 193 nm) using a spot diameter of 35 microns. The collector configuration allows measurement of 204Pb with an ioncounting channel, whereas 206Pb, 207Pb, 208Pb, 232Th and 238U are measured simultaneously with Faraday detectors. All analyses were made in static mode with a laser beam diameter of 35 µm, operated with an output energy of 60 mJ at 23 kV and a pulse rate of 8 Hz. Data measurements, correction and error determination were performed as described by Gehrels et al. (2006). The results of the measurements for the ZF and LPVC are shown in Tables 1 and 2, respectively.
In this section we describe the stratigraphic sequences and structural data of the ZF and LPVC, which, together with the UPb dates of detrital zircons, will allow us to propose a tectonic scenario for both sequences.
As shown in Figures 2, and 3, the lowermost stratigraphic unit is the Zacatecas Formation (ZF), which crops out in the western part of the Sierra de Zacatecas (Figure 2). The ZF consists of phyllite, schist and slate showing significant variations in color, grain size and mineralogy. These differences are most likely related to changes in composition of the protolith rather than to changes in the degree of metamorphism. On a local scale, the ZF can be tightly folded. The original fabric can be obscured because of greenschists facies metamorphism and/or Tertiary hydrothermal alteration.
Primary sedimentary textures and composition of the ZF range from medium to finegrained sandstone and mudstone that contain a few conglomeratic layers and limestone lenses. Sandstones are quartz poor; feldspars and lithic fragments (plutonic, aphanitic and/or porphyritic volcanics) are rare to moderately abundant, and the matrix constitutes 26 to 70% of the rock. The shape of detrital components of the sandstones varies from subrounded to subangular. These rocks show a welldeveloped foliation defined by sericite. The thickness of individual strata is ≤50 cm, although the real thickness is difficult to define because the contact between sandstone and mudstone is commonly obliterated by foliation and/or weathering. The sedimentary succession of the ZF includes rare conglomeratic or conglomeratic sandstone layers that can be matrix or clast supported, with maximum thickness of ~40 m. The clasts are composed of quartzite and oxidized and/or silicified volcanic rocks(?). The clasts range in size from 1 cm up to ~5 cm, but occasionally can be as large as 30 cm in diameter. Limestone, sometimes folded, has also been identified in the ZF. Intercalated lava flows and probably also tuffs can be observed within the ZF. The presence of tuffs is not clear because the strong foliation and hydrothermal alteration obscure the primary structure and texture of the rock. However, we infer its presence from the existence of some broken euhedral to subhedral feldspar crystals in the sandstones. The lava flows are less foliated than the sedimentary rocks because of to their higher competence. The thickness of the lava flows can not be clearly defined because of the foliation, but it appears to be up to ~50 cm in some places. The lavas are porphyritic with argillitized plagioclase phenocrysts less than 1 mm in length. Chlorite is concentrated in the mafic minerals (clinopyroxene and olivine ?) and in the mafic phases(?) in the matrix. Hydrothermal epidote is also present. Pillow structures in lava flows, enclosing clastic sediments between the individual tubes are still present, indicating a submarine extrusion. Along the Arroyo El Bote, close to the type locality, the presence of a hydrothermal vent (see location in Figure 2) is suggested by horizons of brecciated quartz, lava blocks and sediment fragments in an aphanitic matrix (Figure 4). Plagioclase phenocrysts are also present in the matrix. Pervasive silicification and intense fracturing is observed around the vent, and some fractures are filled with hematite and magnetite. The intensity of fracturing and alteration decreases with increasing distance from the vent. Close to the vent there is a lava flow concordant to the sedimentary units of the ZF, which we interpret as related to the vent, suggesting that the volcanic activity took place during the deposition of the basin sediments. The vents are associated with the lava flows interbedded with the sediments. The lavas (Figure 4) are thin and some of them show pillow structures.
Las Pilas Volcanosedimentary Complex
The LPVC is exposed in the central and northern part of the SZ (Figure 2). Towards the dominant igneous LPVC is a transition characterized by an increase in the abundance of mafic lava flows, scarce tuffs, dikes, sills and laccolithlike bodies (Figure 3). Interlayered with the igneous rocks, there are a few feldspathic greywacke and wacke, mudstone, and limestone lenses; the sediment abundance increases upward the stratigraphic section.
Lava flows are the most abundant rock type in the LPVC. Based on the textural variations, we divided the extrusive sequence in massive and pillowtype, commonly foliated and/or deformed (Figures 2 and 3).
Massive flows are abundant in the SZ. Fracturing can be intense or absent, while foliation can vary from poorly to moderately developed. The massive flows, when foliated, always show a well developed foliation defined by chlorite. Foliated flows are found next to dioritic intrusions, which suggests that the competent dioritic bodies acted as stress barriers during the deformation phase. The deformed flows exhibit poorly to moderately developed foliation, brecciation and/or intense fracturing that makes difficult to recognize any internal structure. The pillowed lavas, by contrast, have a well developed pillow structure, lava tubes and sometimes fine grained sediments between the pillows. The size of the pillows varies from ~30 cm to ~1 m. Over the lava tubes it is sometimes possible to distinguish striae presumably formed by the overriding flow. No specific stratigraphic distribution is observed for each extrusive igneous type. However, towards the top of the sequence massive flows covered by pillowed lavas are conspicuous.
The mineralogy of the flows consists of ≤1%, quartz and 0 to 30% plagioclase phenocrysts (An20 to An45), clinopyroxene phenocrysts varying from 0 to 34%. Olivine, when present, can be as much as 3%. Plagioclase and clinopyroxene can be also found as aggregates forming a glomerophyric texture. Matrix varies from microcrystalline to cryptocrystalline. Lava flows have intense hydrothermal and/or deuteric alteration, which promoted the formation of sericite, calcite, epidote and chlorite.
In some places, the boundaries between lava flows and intrusive bodies are not clear and both kinds of units show the same textural relationships in thin section. These observations together with the crystal abundance suggest that some magma could have been emplaced in a meltimpregnated regime involving subsurface crystallization that promoted the formation of dikes and sills as proposed in the model of Reid (2003).
Most intrusive bodies are dikes and sills and may evolve to laccolithlike plutons (Figure 2, sections AA' and BB'). The laccolithic intrusions are almost elliptical in plan view. Their size is less than 2 km in diameter. The contacts between the intrusive and sedimentary rocks are normally sharp but, when the intrusives are in contact with the lava flows, the boundaries are not easily recognized. The intrusions can be highly fractured, faulted (brittle faulting) and/or hydrothermally altered. Essential minerals in laccoliths, dikes and sills are similar both in abundance and composition with respect to lava flows: quartz (≤1%), plagioclase from An20 to An45 and abundances that varies from 25 to 50%; clinopyroxene varies from 3 to 25%. The olivine, when present, is less than 3%. Mafic minerals are alterated to chlorite, whereas plagioclase is altered to calcite and sericite. The textures are phaneritic, porphyritic, traquitoid and poikilitic, in this order of decreasing abundance. In the last one the plagioclase is the oikocryst.
Apparently, dikes and sills have developed from the laccoliths. The thickness of the dikes can reach 10 m and sometimes the dikes change into sills following the bedding planes. The sills can have a thickness near to 2 m and extend for about ~1.5 km. It seems that the dikes were emplaced as meltimpregnated bodies in a shallow environment, more or less contemporaneous with the extrusion of the lava flows. Locally, the dikes can be abundant, but thin (~1 m in thickness). These dikes are close to laccolithic intrusions from which they probably derived.
A sequence made up by greywacke, wacke, mudstone, and scarce sandstone, tuff and limestone is interlayered and covering the volcanoplutonic suite of the LPVC. Near the boundary between the ZF and LPVC, these sediments show a well developed foliation. Moving upward in the stratigraphic column, foliation becomes weaker and is indistinguishable from bedding at the top. Detritic sedimentary rocks are matrix rich (>20%). Detrital components include quartz, either mono or polycrystalline, and plagioclase; the abundance of lithic fragments vary considerably. The individual layers of sediments are < 50 cm in thickness, and show a rhythmic interbedding of greywacke, wacke and mudstone. Upsection, the amount of sediments increase in abundance, and near the top of the sequence the thickness can be >50 m. The interlayering of the different detrital sediments defines a flyschlike deposit. Limestone occurs as lenses less than 4 m thick. The limestone outcrops, which are mostly interlayered with the deformed or pillowed lava flows, are not always associated with the detrital record. Owing to the recrystallization and weathering, it was not possible to identify the internal characteristics of the limestone. The sedimentary assemblages are interlayered with the lava flows and are cut by the intrusive suite in sharp contact.
An interpretation of the internal distribution of the LPVC and ZF is shown in the crosssections AA' and BB' (Figure 2). As will be discussed later, the distribution of igneous rocks and sediments suggests the existence of a volcanic field in an island arc or in its backarc basin.
The Mesozoic stratigraphic sequence of the SZ is unconformably overlain by the Zacatecas Red Conglomerate (ZRC) (Figure 3). The age of the ZRC has been previously reported as Paleocene (Ponce y Clark, 1988). The conglomerate is supported by a sandy matrix and has a distinctive red color. The ZRC is well to moderately consolidated; their clasts are subangular to subrounded varying in size from 2 mm to about 5 cm, occasionally they can be up to 30 cm in diameter. Clasts fragments are made of quartzite, phyllite, schist, diorite and mafic rocks. Although no granitic bodies have been recognized in the SZ, a few layers of the ZRC are granitoid rich. Most of the clasts originated from the Mesozoic rocks described before. Strata are irregular in shape, structureless, massive, and in channels, belonging to fluvial or alluvial sediments deposited in a faultbounded basin. The ZRC crop out only in the Zacatecas and Guadalupe urban areas (Figure 2), filling a roughly EW trending basin bounded to the north by the south dipping Veta La Cantera normal fault and to the south by several small faults dipping north. The upper part of the ZRC is interbedded with thin layers, less than 50cm thick, of airfall tuffs. The EoceneOligocene geologic record of the SZ consists of airfall and ashflow tuffs, volcanic breccias, rhyolitic flows and a few and thin conglomeratic red beds interlayered with the volcanics. Rhyolitic to dacitic dikes cut the entire sequence. Most of the Tertiary volcanic outcrops are located south of Zacatecas and Guadalupe cities in the Cerro La Virgen (Figure 2).
UPb ages of detrital zircons
For this study, we separate detrital zircons for UPb dating from a sandstone layer of the ZF (Figures 2 and 3) close to the place were Bruckhardt (1906) described fossils of Late Triassic age. The results of UPb in situ dating analyses of the 43 detrital zircons obtained from the sample 88 are shown in Table 1 and in Figure 5b. The detrital zircons from the ZF gave two main picks at 132 and 160 Ma, followed by minor Paleozoic and Precambrian clusters which involve about 30% of the data (Figure 5b). The ages obtained for the ZF clearly indicate a maximum depositional age of this sequence in the Early Cretaceous (Hauterivian), which is significantly younger than the formerly proposed Late Triassic age.
From the LPVC we obtained a sample, close to the Sauceda de la Borda town (Figure 2), from a feldspathic wacke to separate detrital zircons. The UPb ages obtained from the LAICPMS measurements of 20 grains (Table 2) cluster at ~132 and ~160 Ma (Figure 5a). As in the previous case, these ages suggest a maximum depositional age of the Early Cretaceous (Hauterivian). The wacke strata come from La Borda Formation, inferred to be Cretaceous by Miriam Yta (in CentenoGarcía and SilvaRomo, 1997).
The upper part of the Mesozoic stratigraphic section shows a change in deformation style when compared with the lower part. The ZF has mostly experienced ductile deformation; whereas the LPVC experienced brittle deformation. In the ZF, the foliation obliterates most of the striae, resulting in only limited fault data.
The intensity of foliation within the ZF varies with the mechanical strength of the rock; the fine grained sedimentary layers are more intensely foliated then the coarse grained rocks or lava flows. Locally, the layers of the ZF can be tightly folded, although no large folds have been recognized. At microscopic scale, the ZF has slaty cleavage, "beards" and a few sigma porphyroclasts; the foliation is defined mainly by sericite.
In the LPVC the foliation development decreases towards the upper part of the stratigraphic section. The sedimentary layers of the lower part can have pencillike structure, which is absent in the upper strata. The crosscutting fault relationships indicate that thrust faults are older than the normal ones.
The foliation data are shown together with the kinematic analysis of thrust faulting in areas of kinematic compatibility (Figure 6). In the area 1, fault trending is mainly directed towards N 4070° W, S 2040° E and S 6070° E, in that order of abundance; other structures are perpendicular to these orientations (Figure 6, area 1, "a"). For this area, the shortening axes show a wide dispersion in the stereogram (Figure 6, area 1, "b"), with a significant grouping in the NE and SW quadrants. Their average compression axis (σ1) is 03°/041°. The poles to foliation are located at the centerright of the stereogram, showing a barely lineal trend directed to the NESW (Figure 6, area 1, "c"). In this area, the normal faults have three dominant orientations at N 4050° W, N 7080° E and N 2030°W, in that order of abundance (Figure 7, area 1, "a"). Their extension axes show some dispersion in the stereogram, with a concentration in the NE and SW quadrants. Their average extension axis (σ3) is located at 01°/025° (Figure 7, Area 1, "b"). In the area 1, the σ1 orientation for thrust faults is approximately perpendicular to the trend of the main thrust and normal faults; and to the orientation of the foliation planes.
In the area 2, thrust faulting has three preferred orientations: S 7080° E, S 2030° E and N 1020° W, with similar abundance (Figure 6, area 2, "a"). Their shortening axes are mainly located in the NE quadrant, with the average compression axis (σ1) at 06°/091°. In the same area, the normal faults are mostly directed towards N 2040° W, N 5060° W and N 8090° W (Figure 7, area 2, "a"); they have less dispersion than the thrust faults. Their extension axes are located dominantly in the NE and SW quadrants with the average extension axis (σ3) located at 15°/045° (Figure 7, area 2, "b"). The normal and thrust faults show some parallelism; however, thrust faulting is more randomly oriented than normal faults. The shortening and extension axes of both kinds of faulting are largely located in the NE and SW quadrants. The paleostress axes for both types of faulting are located in the NE quadrant, as in area 1 (Figures 6 and 7), suggesting that normal faulting developed by extension due to residual stresses after the compressive stage.
The thrust faults in area 3 are mainly directed towards N 010° E and S 1020° E (Figure 6, area 3, "a"). Resulting shortening axes are chiefly located in the NE and SW quadrants with their compression axis (σ1) located at 02°/232° (Figure 6, area 3, "b"). In this area, the poles to foliation are dispersed throughout the central part of the stereogram, although a minor NNWSSE and NESW trends can be observed (Figure 6, area 3, "c"). The NNWSSE trend is perpendicular to the σ1 orientation, which suggests a genetic relationship.
In El Orito area (Figure 6, area 4), the trend of thrust faults is N 2030° W, S 6070° E, and N 010° E, in this order of abundance (Figure 6, area 4, "a"). The shortening axes show some dispersion but can be grouped in the NE and SW quadrants. The average compression axis (σ1) is located at 05°/086° (Figure 6, area 4, "b"). The poles to foliation are located at the centerleft of the stereogram (Figure 6, area 4, "c"), with a trend roughly perpendicular to the orientation of σ1. The normal faults trends are mostly S 6070° E, N 010° W, N 2050° W and N 6070° E, in this order of abundance (Figure 7, area 4, "a"). Their extension axes are mostly grouped in the NE and SW quadrants, with the average at 03°/039° (Figure 7, area 4, "b"). The thrust and normal faults trends are mainly located in the NW and SE quadrants; as well as their shortening and extension axes, respectively.
In the area 5, thrust faults are mainly oriented towards the NW and SE quadrants, with a few data in the other ones (Figure 6, area 5, "a"). Their shortening axes are largely located in the NE and SW quadrants with an average compression axis (σ1) located at 01°/195° (Figure 6, area 5, "b"). The foliation poles show certain grouping at centerright of the stereogram (Figure 6, area 5, "c"). Normal faulting is mostly directed towards S 4070° E (Figure 7, area 5, "a"). Their extension axes tend to group in the NE and SW quadrants with σ3 located at 00°/198° (Figure 7, area 5, "b"). In area 5, the thrust and normal faults trends are mostly parallel, as well as their shortening and extension axes. Some normal faults are parallel to the orientation of the foliation.
In area 6, the thrust faults are oriented to N 8090° E and N 7080° E (Figure 6, area 6, "a"). Their shortening axes are located in the NNE and SSW positions in the stereogram, so that their average compression axis is 00°/001° (Figure 6, area 6, "b"). The normal faults mainly trend N 7090° W and N 6090° E, in this order of abundance (Figure 7, area 6, "a"). Their extension axes are largely grouped at the northern and southern parts of the stereogram with an average extension axis located at 01°/003° (Figure 7, area 6, "b").
From the paleostress analysis of each region shown in Figures 6 and 7, the resulting principal paleostress directions were plotted in Figures 8a and 8b. For normal and thrust faulting, the average σ1 and σ3 are directed towards the SW and NE, respectively. For the analysis of the displacement and sense of slip we show the different rake, dip and sense of slip for all the faults of the SZ (Figure 8c). The lack of specific trends or groups of data for any region may be due to the small displacement shown by the faults, the lithologic changes and/or the subtle changes in the paleostress direction. Figure 8c shows that the normal faults have dip angles larger than 30°, whereas only a few thrust faults have angles lower than 30°, with rakes over 60°. Fault displacements and dips show a wide dispersion that could be related to local mechanical discontinuities produced by the interlayering of sediments in the lava flows, or the presence of the more rigid intrusive bodies. The mechanical effect can also be related to the thickness and extent of massive and pillowed lava flows. Another hypothesis for the dispersion of the fault data is that, since the basement in the Zacatecas Volcanic Field is unknown, the mapped stratigraphic sequence can be repeated downward, in such a way that the interlayering of ductile and brittle layers could promote the dispersion of fault data.
Based on field relationships, structural analysis and UPb dating of zircons, the more likely explanation for the geologic relationships between the Zacatecas Fm. and the Las Pilas Volcanosedimentary Complex is that they belong to the same stratigraphic unit. This study support earlier interpretations on the origin of the metamorphic and "green rocks" of the Sierra de Zacatecas, which suggested that they belong to the same stratigraphic entity (GutiérrezAmador, 1908; McGehee, 1976; Ranson et al., 1982). Our age dating analysis shows that detrital zircons of the ZF and LPVC have similar ages at ca. 132 and 160 Ma. The maximum depositional age of the detrital zircons indicate that the Mesozoic sequence of the SZ is of Early Cretaceous (Hauterivian) age.
The protoliths of the ZF are sandstone, mudstone, limestone, a few conglomerate and lava flows. Lava flows and some tuffaceous horizons interlayered with fine grained sediments are present towards the top of the ZF. Sandstone contains broken euhedral to subhedral feldspar crystals, together with subrounded feldspar crystals and volcanic lithics, which suggest a volcanic source close to the place where the ZF and LPVC were deposited and emplaced. The ZF outcrops are located only in the centralwestern part of the SZ. The limited distribution of the lava flows and volcaniclastics, as well as the lithologic assemblage, suggest a shallow marine environment and a marked volcanic influence, as previously proposed by GutiérrezAmador (1908) and Monod and Calvet (1992). Previous workers have interpreted that the ZF is a submarine fan formed in a deep marine environment later translated and deformed during Jurassic time (McGehee, 1976; Sedlock et al., 1993; CentenoGarcía and SilvaRomo, 1997; Bartolini et al., 2001). BarbozaGudino et al., (1999) suggested that this fan was deposited close to the presentday location. An alternative environment for the deposition of the ZF and the emplacement and deposition of the LPVC sequences is in either an immature arc as in Guanajuato (Lapierre et al., 1992; OrtizHernández et al., 1992) or in the vecinity of Zacatecas city (Mortensen et al., 2008), or in an ocean floor environment (CentenoGarcía and SilvaRomo, 1997; CentenoGarcía et al., 2008). In that kind of basins, subsidence could have led to deposition of alternating layers of sediments and volcaniclastic. As the volcanic complex evolved, lava flows became the dominant rock type. The distribution of the igneous and sedimentary rocks schematically represented in Figure 2 support the presence of a volcanic field of Early Cretaceous age in this part of Mexico. The basin could be related to the Late JurassicEarly Cretaceous island arc of the Guerrero terrane (Campa and Coney, 1983; Sedlock et al., 1993; CentenoGarcía and SilvaRomo, 1997; Dickinson and Lawton, 2001). The nearest area where the Guerrero terrane crops out is in Guanajuato (Figure 1), where several workers have described lithologic sequences interpreted to be part of a Late JurassicEarly Cretaceous island arc (Lapierre et al., 1992; Tardy et al., 1992, 1994; Freydier et al., 1996, 2000; OrtizHernández et al., 2000). An alternative scenario is the allochtonous origin of the ZF as proposed by other authors. During Middle Jurassic time, a considerable translation occurred associated to the MojaveSonora Megashear (Sedlock et al., 1993; Jones et al., 1995; Bartolini et al., 2001) and processes of accretion, subduction and/or collision (Sedlock et al., 1993; CentenoGarcía and SilvaRomo, 1997) prior to the emplacement of the LPVC. However, deformation of the ZF is characterized by greenschists facies metamorphism and faults with small displacements. Based on these observations, Ranson et al., (1982) and Monod and Calvet (1992) suggested the presence of major thrust in the ZF with small displacements. As expected, folding and the development of foliation in the ZF are related to the composition and mechanical strength of the layers. The ductile deformation of the ZF could be related to a shallow environment heated by the intrusions of magmatic bodies such as dikes, sills and associated volcanism in an island arc environment as shown in the Figure 2. Towards the east, the brittle deformation observed in places within the LPVC, could be related to emplacement forces of the igneous bodies and to the stronger mechanical resistance of the lava flows and subvolcanic diorites with respect to the sedimentary layers.
The results of the paleostress analysis (Figure 8) indicate a NESW oriented trend for compression and extension, parallel to the stress orientation during the Laramide Orogeny of Late CretaceousEarly Tertiary (Monod and Calvet, 1992; Sedlock et al., 1993; CentenoGarcía and SilvaRomo, 1997; Bartolini et al., 2001). The amount of shortening during the Laramide Orogeny has been estimated in ~90 km or 40% in a section of the frontal part of the chain in San Luis Potosí (Suter, 1987), whereas other authors suggests ~150 km or 30% of shortening in other areas (Sedlock et al., 1993). Whatever the distance of transport or the amount of shortening, most of the deformation has been accommodated in the ZF in a ductile mode, and by dominantly brittle deformation in the LPVC. A tentative hypothesis is that these differences, together with the location of the laccoliths, could have promoted the dispersion of the data shown in the Figures 6, 7 and 8c.
The LPVC has been correlated to the origin and evolution of the Guerrero terrane or superterrane, which is well documented in southern Mexico and has been interpreted as an island arc of Late JurassicEarly Cretaceous time (Campa and Coney, 1983; DelgadoArgote et al., 1992; Sedlock et al., 1993; CentenoGarcía and SilvaRomo, 1997; Dickinson and Lawton, 2001; TalaveraMendoza et al., 2007).
The lithologic and geochronologic data presented in this paper indicate that the Mesozoic sequence of the Sierra de Zacatecas can be correlated with the Guerrero terrane. The most probable tectonic setting where the Mesozoic Zacatecas Volcanic Field could have been developed is presented in Figure 9. The sedimentary supply, as shown by the detrital zircon age distribution, indicates Precambrian and Mesozoic sources for the ZF. As the volcanic field evolved and the back arc developed, a change in the sedimentary supply occurred, as observed in the detrital zircon ages for the LPVC. In this scenario, an intraarc or backarc basin with sedimentary input from the arc and from the continent is a suitable model for the development of the Mesozoic sequence of the SZ. This model allows the correlation of the Mesozoic Zacatecas Volcanic Field with the island arc and backarc basin of Late JurassicEarly Cretaceous that outcrops in Guanajuato.
In the Arroyo El Bote and in Sauceda de la Borda areas in the Sierra de Zacatecas, the contact between the Zacatecas Fm. and the overlying Las Pilas Volcanosedimentary Complex is interpreted to be gradual since no visual hiatus or major tectonic overthrusting has been recognized. The stratigraphic continuity of both formations is supported by similar peak ages of detrital zircons in two distinct sedimentary horizons (ca. 132 and 160 Ma) that point to a similar provenance. The occurrence of both Paleozoic and Proterozoic zircon grains in the lower ZF indicates a continental provenance of the sediments. With respect to the paleogeographic setting; the zircons were most likely derived from the North American Craton and an island arc. The relationships between sedimentary and igneous rocks suggest the presence of a volcanic arc providing sediments to an intraarc or backarc setting, similar to other sequences of the Guerrero terrane. In combination with the North American cratonic provenance of the zircons, the depositional facies can be assumed to be part of an active continental margin setting.
The maximum depositional age for the sequence, indicated by the ages of youngest detrital zircons (ca. 132 Ma), is a relevant information for the reconstruction of the Mesozoic terranes of Mexico. Despite the amount of zircons analyzed, the data included in this work are the first isotopic ages published for the Sierra de Zacatecas; this Early Cretaceous age (Hauterivian) is the first isotopic time marker for the sequence and is significantly younger than most previously suggested ages.
The deformation style of the Mesozoic SZ shows a NESW trend for both normal and thrust faulting. The SZ has ductile deformation at the bottom, and the stratigraphic section changes upward to a brittle style. All the poles of the foliation planes are close to the vertical, suggesting folds with large amplitude. On the basis of crosscutting relationships of the faults we propose that the compressive stage was followed by extension after the Laramide orogeny. The stress applied produced small displacements on the faults, perhaps due to the brittleductile coupling of the Mesozoic of the SZ and the small amount of shortening that occurred during the Laramide orogeny. All stresses applied during the Mesozoic evolution of the SZ were directed towards the northeast.
This work was made possible by the support of the Lic. Amalia Dolores García Medina, Governor of the State of Zacatecas, Mexico; support from Lic. Rafael Medina Briones, M. Sc. Jesús Patricio Tavizón García and Dra. Gema Mercado Sánchez is also acknowledged. We thank Víctor Pérez and Susana Rosas from CICESE for their help during the zircon separation. We acknowledge the help during field work of Luis Navarro and several geology students of the Unidad de Ciencias de la Tierra, Universidad de Zacatecas. Also thanks to Joaquín Ruiz and George Gehrels for allowing the use of the LAICPMS facilities at Department of Geosciences of the University of Arizona at Tucson. We acknowledge Tomas Peña for his help during the LAICPMS measurements. We also thank Suzanne Beglinger, Fred Beekman, Sierd Cloetingh, Maarten Corver, Karen Leever, Jesús Nájera, Oliver Nebel and Ernst Willingshofer for their comments and discussions to this manuscript. This work was partially supported by the project CONACYT 45817 granted to Luis Delgado. Also, we acknowledge the support of the Netherlands Research Centre for Integrated Solid Earth Sciences (ISES). Field work was supported by the Institute of Ecology and Environment of Zacatecas and CONACYT. The Arizona LaserChron Center is supported by the National Science Foundation Grant EAR0443387. We thank the comments by Elena CentenoGarcía, Luca Ferrari and Rafael BarbozaGudino, which greatly improved this paper.
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