This paper presents a detailed investigation of the structure and evolution of the Precordillera southern sector (Argentina). We document the development and successive reactivation of regional and discrete structural grain through time, and discuss the existence of a large-scale mechanical anisotropy present in the lithosphere. Our kinematic studies indicate that the Permian orogeny generated a doubly vergent fold-and-thrust belt of transpressive deformation, where strain was partitioned into two different types of deformation domains. The west-vergent western domain was characterized by partitioned transpression with shortening dominating, and a strike-slip–dominated subdomain. The east-vergent eastern domain was characterized by pure contractional deformation.
Our model for the Late Permian to Early Triassic evolution of the Precordillera involves a north-northwest–trending weakness zone affected by north-northeast–directed extension, generating an area with transtensional deformation during the Choiyoi volcanism development. Later, during the Triassic generation of the Cuyana rift basin, the northeast stretching direction was orthogonal to the rift trend, indicating pure extensional deformation. We propose a model where the clear parallelism between the distribution of an inferred early Paleozoic suture zone, a north-northwest–trending late Paleozoic belt, and Permian–Triassic rift-related magmatism indicates the reactivation of a north-northwest–trending long-lived lithospheric weakness zone.
The Andean Mountains compose a linear orogenic belt formed along the contact between the Nazca and South American plates. Although their actual morphology is the result of compressional strain at the convergent plate margin during the Cretaceous–Holocene Andean orogenic cycle, they preserve evidence of previous periods of deformation along the proto-Pacific margin of Gondwana. At the studied latitudes, the Argentine Andes include different north-south–trending morphostructural units: from west to east (Fig. 1), these are the Principal Cordillera, the Frontal Cordillera, and the Precordillera foreland fold-and-thrust belt. The study area covers the easternmost Frontal Cordillera and the southern sector of the Precordillera, between lat 32°20′S and 33°S (Fig. 1). The Precordillera forms a north-south–trending mountain chain, 450 km long and 80 km wide. North of 32°30′S, traditional models for the Precordillera fold-and-thrust belt have involved a thin-skinned tectonic style with complete detachment of a deformed sedimentary cover from a basement dipping gently to the west (Ramos et al., 1986; Allmendinger et al., 1990; von Gosen, 1992). However, in the southern sector, the belt has particular characteristics that reflect a thick-skinned tectonic style (von Gosen, 1995; Folguera et al., 2001; Vergés et al., 2007). In this work we focus our attention on this sector of the Precordillera that appears to have important geological and structural differences from the northern portion due to the imprint of previous deformational events.
This paper presents a detailed investigation of the Precordillera structure and evolution, focusing on the control of the early Paleozoic structural grain on the subsequent deformational events. We present evidence showing the existence of a Permian doubly verging fold-and-thrust belt, controlled by preexisting early Paleozoic discrete and regional fabrics, and the presence of a preexisting lithospheric weakness zone that controlled the location of the Permian–Triassic transtensional system. Our work is based on an integrated study combining field structural analysis, compilation of existing geological maps and data, and extensive fault kinematics analysis provided by fault-slip data from major and mesoscale structures.
The prolonged history of convergence against the Gondwana Pacific edge resulted in several episodes of contractional, extensional, and strike-slip deformation (Ramos, 1988; Mpodozis and Ramos, 1989). Overprinting relationships between different structures in the Precordillera and the Frontal Cordillera preserve evidence of at least four deformational events that took place in early Paleozoic, Early Permian, Late Permian–Middle Triassic, and Miocene–Holocene time.
The early Paleozoic tectonic history of the South American southwestern margin was mainly controlled by this subduction process and the accretion of exotic terranes (Ramos, 1988). At the studied latitudes, this deformation is characterized by two orogenic events; the first occurred during the Middle to Late Ordovician and is referred to as Ocloyic orogeny, and the second occurred during Devonian time, and is known as Chanic orogeny. The timing of deformation of the Chanic orogeny is evidenced by a 385 Ma 40Ar/39Ar date on neocrystalline white mica in a shear zone in the Bonilla area (Davis et al., 2000a).
The late Paleozoic tectonic cycle began with the subduction inception along the present continental margin after the collision of the exotic terranes. In Early Permian time, a widespread contractional event, known as San Rafael orogeny (Azcuy and Caminos, 1987; Ramos, 1988), generated a wide orogenic belt and crustal thickening (Llambías and Sato, 1990; Mpodozis and Kay, 1990). By the end of this compressional phase and before the Permian–Triassic extensional one, clockwise block rotations probably took place between 280 and 265 Ma, before the extrusion of the Choiyoi volcanics (Rapalini and Vilas, 1991). These crustal block rotations have been found in the Uspallata-Callingasta Valley and were attributed by Rapalini and Vilas (1991) to dextral strike-slip movements parallel or subparallel to the continental margin.
During Late Permian to Early Cretaceous time, rifting was widespread in the South American plate. This extensional tectonic regime was a forerunner to the Gondwana fragmentation and the South Atlantic Ocean opening (Uliana et al., 1989). During the beginning of this period, between the Late Permian and the Early Triassic, a widespread magmatic event occurred, represented by the Choiyoi Group volcanics and intrusives. The lower part of the Choiyoi Group represents compositions and geochemical signatures typical of arc magmatism, whereas the upper acidic magmatics indicate the change from a compressive tectonic regime to an extensional one (Llambías et al., 1993). The extensional regime continued during Triassic time and led to the formation of a series of rift systems, with an overall north-northwest trend, formed along the southwestern margin of Gondwana (Charrier, 1979; Uliana et al., 1989). Ancient major crustal boundaries among previously amalgamated terranes are inferred to have controlled the development of peripheral rift systems in southern South America (Ramos et al., 1986; Ramos and Kay, 1991).
When the Mesozoic extensional period was over, by the late Early Cretaceous, a major plate tectonic reorganization took place. By this time, the compressive Andean cycle started (Mpodozis and Ramos, 1989). There is, however, no evidence of this early compression in the studied area, where shortening started during the Middle to Late Miocene (Irigoyen et al., 2000; Giambiagi et al., 2003). The active thrust front is now located in the eastern border of the Precordillera, near the city of Mendoza, where the Quaternary strata are involved in the Andean deformation.
Rocks ranging from Cambrian to Pleistocene age crop out in the study area (Figs. 2 and 3). Proterozoic rocks do not crop out in the Precordillera, but U-Pb ages of gneissic xenoliths in Miocene intrusions have shown that the basement corresponds to metamorphic rocks of Grenvillian ages (1.1 ± 0.1 Ga; Kay et al., 1996). The Cambrian–Ordovician rocks correspond to metacarbonates and metashales that crop out in the western and eastern sectors, respectively. The first are grouped into the Puntilla de Uspallata Formation and the Bonilla Group. They consist of schists, phyllites, quartzites, and greenschists, with lenses and layers of marbles, dolomites, metagabbros, and serpentinites (Keidel, 1939; De Römer, 1964), and the age is assumed to be Cambrian to Ordovician (Varela, 1973; von Gosen, 1995). A greenschist facies metamorphism was deduced by von Gosen (1995), due to incipient to pronounced quartz recrystallization in the Puntilla de Uspallata Formation and the Bonilla Group, respectively. Von Gosen assumed that the Puntilla de Uspallata Formation and the Bonilla Group were parts of the same sequences, and proposed that the Bonilla Group may have represented a deeper crustal sequence below the Puntilla de Uspallata Formation and that differences in structural geometries and metamorphism seem to be the effect of different strain and temperature states related to different crustal positions. Mafic and ultramafic rock complexes are interlayered in these metasedimentary rocks north of the study area (Haller and Ramos, 1984; Gerbi et al., 2002). These rocks range in age from Neoproterozoic to Silurian (Davis et al., 2000b). U-Pb zircon ages and Nd isotope data allowed Davis et al. (2000b) to interpret the ultramafic complex as deep continental crust underplated by ultramafic and mafic rocks that underwent granulite facies metamorphism; the mafic rocks are characterized by shallow-level intrusions and flows that underwent lower greenschist facies metamorphism, similar to the Bonilla and Puntilla de Uspallata units. A metasedimentary unit that crops out in the eastern sector of the Cordón del Plata (Heredia et al., 2009) could be tentatively assigned to the Late Ordovician Las Lagunitas Formation that crops out to the south, in the Cordón del Carrizalito range (Tickyj et al., 2009).
In the eastern sector of the Precordillera a passive-margin carbonate succession of Early Cambrian to Middle Ordovician age (Empozada and Cerro Pelado Formations) documents the transition from shallow-marine platform carbonate facies to deep-water black shales with olistoliths of intrabasinal carbonates (Astini et al., 1995; Heredia and Gallardo, 1996). Overlying these rocks, a sequence of clastic sediments has been grouped into the Villavicencio Formation and dated as Silurian to Devonian, although its precise age is uncertain (Cuerda et al., 1993).
The early Paleozoic succession is separated from the Carboniferous deposits by an angular unconformity attributed to the Middle to Late Devonian Chañic diastrophism (Furque and Cuerda, 1979; Azcuy and Caminos, 1987). Pennsylvanian to Early Permian marine clastic deposits were grouped in the Santa Máxima, Loma de los Morteritos, Jarillal, and El Plata Formations (Rolleri and Criado Roqué, 1969; Caminos, 1965). Carboniferous and Permian granitoids intrude the Carboniferous successions and the Villavicencio Formation (Polanski, 1958).
A thick succession of Permian–Triassic volcanic rocks and shallow-level batholiths of the Choiyoi Group unconformably overlies the previously deformed rocks. The lower half of the succession is dominated by andesitic lavas and breccias, and the upper half consists of rhyodacitic and lithic pyroclastics and rhyolite flows (Martinez, 2005; Giambiagi and Martinez, 2008). The lower half comprises typical arc rocks, while upper half consists of subalkaline to alkaline rocks that have geochemical characteristics attributed to a magmatic arc of postorogenic character (Martínez, 2005). This magmatism has been associated with an extensional regime, probably related to the final stage of a subduction process (Llambías and Sato, 1995; Llambías, 1999). The injection of basalt into the lower crust, creating a mafic underplating, contributed to the high volumes of melt and generation of bimodal volcanism (Kay et al., 1989; Llambías et al., 2003).
Early to Late Triassic nonmarine sediments and subordinate volcanic rocks of the Uspallata Group and the Siete Colores Formation were deposited in the Cuyo rift basin. During the early depositional phase, during the Early to early-Late Triassic (Ávila et al., 2006; Zavattieri and Prámparo, 2006; Spalletti et al., 2008), the sediments were restricted to a series of partially isolated depressions controlled by fault-driven subsidence linked to rifting. This phase was followed by a period of regional subsidence attributed to thermal decay (Kokogian and Mancilla, 1989; Dellapé and Hegedus, 1995).
After a hiatus of several million years, thrusting and uplift of the Principal and Frontal Cordilleras resulted in the development of a foreland basin with the accumulation of a 4000-m-thick nonmarine succession during Miocene–Pliocene time (Irigoyen et al., 2000).
METHODOLOGY AND DATA COLLECTION
Field analysis consisted of detailed geological mapping and structural analysis of the southern sector of the Precordillera. Three geological transects are presented to illustrate the major structures of the upper crust (Fig. 4). The cross sections were originally constructed at a scale of 1:25,000, integrating surface geological and structural data resulting from extensive field surveys. The age of fault activity was bracketed by taking into account the ages of rock units affected and unaffected by faulting. In some cases, geometric links between structures allowed the reconstruction of a relative chronology. Palinspastic reconstructions of Andean structures are an important tool for examining and understanding pre-Andean deformation. We systematically back-tilted the Permian–Triassic volcanic rocks and the Triassic continental strata to the horizontal position and reconstructed the pre-Andean orientation of structures in order to determine and separate Andean and pre-Andean structures.
To unravel the complex history of deformation of the Precordillera during the pre-Andean period, we investigated the structural grain developed in early Paleozoic rocks. Throughout the study area brittle and ductile deformation features were observed and recorded at outcrop (meso) and map (macro) scales. A detailed structural analysis of the early Paleozoic structural grain within the Precordillera was based on 1009 structural measurements of bedding, cleavage, lineations, shear zones, and mesoscopic folds, taken at 38 stations.
For late Paleozoic to Holocene brittle structures, we measured fault orientation, slip direction, and amount and sense of displacement. We selected 33 sites from the study area for fault kinematics analyses; 735 major (displacement >50 m) and mesoscale (displacement <50 m) faults were measured. Slips on individual faults were integrated to determine the orientation of the principal axes of incremental strain within the area. This technique employs the orientation of the shortening and extension axes (P and T axes) to estimate principal directions of brittle strain (Marrett and Allmendinger, 1990), and provides an approximate orientation of the principal strain axes (Twiss and Unruh, 1998). Principal strain axes have been computed using the moment tensor summation method as implemented in the FaultKin 4.3 stereonet program of Allmendinger (2001). We calculated the principal axes of shortening (λ3) and extension (λ1) for the incremental strain tensor associated with each fault using measurements of fault-plane attitude, slip vector, and sense of displacement. An average kinematic solution for each fault population is calculated using linked Bingham distribution statistics to determine the directional maxima of the λ3 and λ1 strain axes for the fault arrays (Marrett and Allmendinger, 1990).
In order to avoid errors when assigning a time for the major structures, a Permian age was assigned for faults affecting the Carboniferous strata but not the Permian–Triassic volcanics. In the same way, a Permian–Triassic age was assigned for faults affecting the Choiyoi and Uspallata Groups but not the Neogene strata. Other criteria used to establish a relative age relationship among structures include crosscutting relationships and successive striae observed on fault surfaces.
At the study latitudes, the Precordillera corresponds to a bivergent fault system that can be structurally divided into western and eastern structural domains. The Villavicencio fault is considered to represent the major tectonic boundary between the domains (Fig. 3).
The western domain displays a complex structural framework with orientations ranging from west-northwest to north-northeast and very complex crosscutting relationships. The regional structure of this zone consists of several main faults, the Uspallata, Puntilla de Uspallata, Canteras, Bonilla, Burro, Siete Colores, Pampa de Canota, La Polcura, La Manga, and La Carrera fault system (Fig. 3). The predominant vergence of the faults and folds in this area is toward the west, except faults in the eastern sector of the Cordón del Plata range (Fig. 4E).
The Uspallata fault is an inferred structure, the trace of which is interpreted to extend subparallel to the Uspallata Valley with a north-northeast trend. It is inferred to be a west-vergent Cenozoic reverse fault uplifting early Paleozoic metasedimentary rocks and Permian–Triassic volcanics. The north-trending east-vergent Puntilla de Uspallata fault uplifts the metasedimentary rocks on top of the Carboniferous marine sequences. It is a Permian structure unconformably covered by Permian–Triassic volcanics, indicating that it was not reactivated during the Andean compression, except in its northernmost sector where it uplifts Carboniferous strata on top of Permian–Triassic deposits. A splay of this fault, the Canteras fault, extends parallel to it and uplifts Carboniferous sequences on top of the Triassic deposits.
The Bonilla fault is a reverse structure with north-south trend and vergence toward the west. The fact that it juxtaposes Cambrian–Ordovician deeper crustal rocks (Bonilla Group) on top of shallower Cambrian–Ordovician rocks (Puntilla de Uspallata Formation), and that no Silurian–Devonian rocks have been preserved in the western sector of the Precordillera, allows us to consider this fault as an early Paleozoic structure. Its late Paleozoic movement was determined by von Gosen (1995), who documented that this structure uplifts the Cambrian–Ordovician rocks on top of the Carboniferous sediments and is unconformably covered by the Permian–Triassic volcanics north of the study area. The north-trending, west-vergent Burro fault separates contrasting pre-Carboniferous basement units from the Carboniferous marine deposits. The Permian–Triassic volcanics unconformably cover this fault, revealing its Permian age.
Some of these east- and west-vergent faults are cut by northwest- to north-northwest–trending strike-slip faults throughout the western sector of the Precordillera. They were interpreted by von Gosen (1995) as older structures that were reutilized as strike-slip faults during the Andean deformation. Kinematic analyses of shear planes associated with these faults, and facies and thickness variations across them in the Permian–Triassic volcanics, allow us to infer that some of them were developed during the Permian–Triassic extensional event and others correspond to reactivated structures, developed during the Permian orogeny (Giambiagi and Martínez, 2008). However, late Paleozoic deposits are crosscut by dextral north-northwest– and sinistral north-northeast–trending strike-slip faults, which cannot be traced into the Permian–Triassic sequences (von Gosen, 1995). Mutual crosscutting relationships between oppositely dipping faults suggest that these faults developed as a conjugate set. Some of these strike-slip faults cut the thrust faults, and some do not, but they predate the Permian–Triassic volcanic rocks. These observations show that the strike-slip faults are most likely to have developed synchronously with the late Paleozoic thrust faults and they are kinematically related to them, while others are genetically related to the Permian–Triassic extension.
The two most prominent Permian–Triassic identified faults correspond to the La Polcura and La Manga faults (Fig. 3). The La Polcura fault extends for >30 km, has a northwest trend, and a vertical displacement of at least 500 m. Slickensides on exposed fault surfaces indicate two sets of striae (Fig. 5). The oldest set shows a sinistral oblique-slip normal movement and the youngest has a sinistral strike slip. The youngest set is inferred to represent the Andean reactivation of the structure. The La Manga fault is subparallel to La Polcura fault and merges toward this structure southward. A detailed analysis of its southeastern termination shows that the fault was reactivated during the Andean shortening. Both faults, with a parallel trend but dipping one against the other, delineate a graben structure that probably has controlled the localization of the andesitic breccias and lavas and the lithic ignimbrites of the middle and upper sections, respectively, of the Choiyoi Group (Fig. 5).
The late Paleozoic and Andean deformational events strongly affected the Cordón del Plata range, with the development of north-northeast– to north-south–trending, east-vergent, basement-involved reverse faults. Basement blocks are bounded by reverse faults and are crossed by sinistral strike-slip faults with northwest orientation (Fig. 3). The La Carrera fault system has been traditionally recognized as responsible for the Andean uplift of the Cordón del Plata range, which reaches an altitude of 6000 m in the Cerro El Plata (Caminos, 1965; Polanski, 1972; Cortés, 1993). The Cerro Arenal fault is the westernmost of these foreland-directed structures (Figs. 3 and 4). It places the Carboniferous to Permian black shales and granitoids on top of the Permian–Triassic volcanics. The Hoyada fault uplifts a thin sheet of the early Paleozoic phyllites over the Carboniferous sequences, but it is unconformably covered by the Permian–Triassic volcanics, indicating a Permian age without subsequent reactivation. Toward the east, a major sheet of Carboniferous marine deposits has been placed over the Neogene synorogenic deposits by activity along the Cerro Médanos fault. This fault trends north-northeast north of La Polcura creek, and has a north-south trend south of it. The change in trend is related to the presence of the Cerro Médanos granitoid. The easternmost faults of the La Carrera fault system, the Río Blanco and El Salto faults, affect the Neogene–Quaternary synorogenic deposits (Folguera et al., 2001; Casa, 2005), and show evidence of Quaternary activity (Fauqué et al., 2000; Casa, 2005).
The eastern domain corresponds to a doubly vergent tectonic range, composed of early Paleozoic–Triassic sedimentary rocks and the Permian–Triassic volcanics. The structures within this zone have north-south to north-northeast trends, and compose the Villavicencio, Cerro Pelado, Yaretas, Canota, Del Toro, Guamparito, San Isidro, Melocotón, Divisadero Largo, and La Cal faults (Fig. 4), and two pre-Andean fault systems, named Caracoles and Rincón. The Villavicencio fault separates two areas with contrasting Cambrian–Ordovician history, suggesting activity of this fault since the early Paleozoic. The early Paleozoic geology west of this fault is composed of metamorphic rocks. East of the fault, the early Paleozoic stratigraphy is characterized by carbonates and clastic metasedimentary rocks.
The Cerro Pelado fault uplifts the Cambrian–Ordovician carbonates and mudstones and the Silurian–Devonian metasedimentary rocks of the Villavicencio Formation on top of Carboniferous rocks (Fig. 6). This Andean brittle fault is associated with an early Paleozoic ductile shear zone parallel to its actual trace. Because the Carboniferous rocks are not affected by this ductile deformation, we interpreted this shear zone to have developed during the early Paleozoic orogeny.
In the Yaretas area, the main style of deformation consists of a series of north-northeast–trending thrust faults and fault-propagation folds with a basal detachment in the Silurian–Devonian rocks. Crosscutting relationships indicate three stages of deformation: the first occurred during early Paleozoic time before the deposition of the Carboniferous strata. The second stage occurred during the late Paleozoic by formation of an east-southeast–vergent thrust fault that could not be traced into the Permian–Triassic volcanics. The third stage corresponds to Cenozoic reactivation. The Silurian–Devonian and Pennsylvanian sedimentary rocks are cut by east-vergent low- to medium-angle faults that could not be traced into the Permian–Triassic sequence. These faults belong to the Rincón fault system and are interpreted as representing a Permian thin-skinned fold-and-thrust belt with detachment within the Silurian–Devonian sedimentary rocks.
Early Paleozoic Deformation
In order to evaluate subsequent reactivations of the early Paleozoic structures during the Early Permian, Permian–Triassic, and Cenozoic deformational events, we studied the structural grain of these rocks. Two phases of deformation were recognized in the early Paleozoic rocks of the Precordillera western domain, labeled D1 and D2, which were first studied by De Römer (1964), von Gosen (1995), and Cortés et al. (1997). The first phase, D1, was accompanied by low-grade regional metamorphism (M1) and a pervasive foliation (S1), and produced the dominant structural grain within the Cambrian–Ordovician metasedimentary rocks (Fig. 7). The Puntilla de Uspallata Formation exhibits tight to isoclinal folds (F1) with dimensions from several centimeters (Fig. 8A) to hundreds of meters, and axial planes striking north-south to north-northeast and dips toward the east. The Bonilla Group rocks are affected by penetrative cleavage planes oriented subparallel to the bedding, striking north-south and dipping to the east (Fig. 7). F1 folds have, at present, north-south to north-northwest trends and are associated with penetrative axial plane foliation (S1) (Fig. 8B) dipping toward the east. An associated mineral stretching lineation (L1) is marked by aligned sericite and clastic muscovite (von Gosen, 1995). F1 folds are generally asymmetric with a vergence of top up to the west. On the stereonet, the cleavage and axial planes are subparallel. Both planes define moderately to steeply east dipping surfaces that are parallel to the mean bedding orientation. The D1 structures in the western domain provide evidence for significant northwest-southeast shortening, with tectonic transport toward the northwest.
The second phase, D2, is much less conspicuous than D1. It includes a pervasive low-temperature foliation (S2), especially in pelitic layers, as well as reverse faults. This phase was previously recognized by von Gosen (1995) in the Uspallata area affecting the Cambrian–Ordovician metasedimentary rocks. The S2 cleavage is developed in the metashales and metasiltstones as a continuous slaty to phyllitic cleavage, and as a spaced cleavage in metasandstones. It is represented by northeast-trending folds (F2) on scales of decimeters to tens of meters with vergence toward the northwest and southeast, and axial-plane crenulation cleavage (von Gosen, 1995) (Fig. 8C).
In the eastern sector of the Precordillera, the Silurian–Devonian turbiditic rocks were suitable for studying the tectonic vergence, because the metamorphic processes produced penetrative foliations mainly visible in the metapelitic beds and the stratigraphic polarity was easily recognized. The S1 cleavage is present in pelitic lithotypes, while in metasandstones no visible cleavage is developed (Fig. 8D). These metasedimentary rocks have undergone variable degrees and styles of deformation throughout the unit, with intensity increasing westward. In the eastern sector, they present penetrative foliation in the fine-grained layers that is commonly oblique to bedding. In the central sector, the unit is moderately deformed and well foliated, but there are distinct zones of greater and lesser strain. One of the areas of higher strain corresponds to the westernmost outcrop of the unit, uplifted by the Villavicencio fault, where the rocks have highly planar fabric and isoclinal folds. The foliation within these metasedimentary rocks predominantly strikes northeast and dips to the west.
The timing of D1 and D2 deformations is not precisely defined. These deformational events were assumed to be related to the Silurian to Early Devonian thermal event deduced by Buggisch et al. (1994) from radiometric K/Ar dating from fine-grained mica fractions of the Cambrian–Ordovician metasedimentary rocks (von Gosen, 1995). D1 and D2 seem to have been continuous and are attributed to a single tectonic event.
Late Paleozoic Deformation
The late Paleozoic rocks are affected by thrust faults and folds with wavelengths and amplitudes from several centimeters to kilometers (Fig. 9). This brittle deformational event affects the Pennsylvanian to Early Permian marine deposits, but it does not affect the Late Permian to Early Triassic rocks (Fig. 9C), indicating an Early to Middle Permian age. Within the early Paleozoic rocks this deformation is documented by brittle structures that are difficult to distinguish from Andean structures, except when they are unconformably covered by the Permian–Triassic volcanics. The strike of late Paleozoic faults is north-south to north-northwest in the western domain, and north-northeast to northeast in the eastern domain (Fig. 10). Folds trend from north-northwest to northeast, and their axial planes dip either to the west or the east.
The kinematic analysis permits us to estimate the shortening direction for the Permian compressive event (Fig. 10). Fault-slip data along the western sector indicate a contractional regime with a west-northwest–trending λ3. In the eastern sector, kinematic analyses indicate a contractional regime with a northwest-trending λ3. Although the Permian structure results primarily from orogen-normal contraction, the strike-slip fault systems affecting the belt indicate a left-lateral strike-slip regime associated with a north-south–trending λ3 and an east-west–trending λ1. In order to reconstruct the Early Permian contractional regime, these shortening directions should be rotated counterclockwise, up to 80° (Rapalini and Vilas, 1991). By doing this, the Permian contraction should have been southwest directed.
We reconstructed the geometry of the late Paleozoic structure by a palinspastic restoration of Andean structures (Fig. 11A). The reconstruction of Permian thrust and strike-slip faults suggests the development of a doubly vergent fold and thrust belt during the San Rafael orogeny (Fig. 11B). Both western and eastern domain faults were oriented parallel to the early Paleozoic tectonic grain. This parallelism between basement grain and late Paleozoic structures as well as the contractional and strike-slip regimes being contemporaneous are evidence of regional strain partitioning. We discuss this in the following section.
Late Permian–Late Triassic Deformation
The Late Permian–Late Triassic represents two extensional events, one concomitant to an important volume of bimodal volcanism during the Late Permian to Early Triassic, and the other characterized by the development of a continental rift basin during the late-Early to early-Late Triassic. The volcanic and volcaniclastic rocks of the Choiyoi Group and older rocks are affected by normal- and oblique-slip normal faults with west-northwest to northwest trends (Fig. 12). The great thickness and facies variations of these volcanic rocks near these structures (Figs. 13A–13C), and the relationships between structures and the intrusion of plutonic rocks indicate that they are synmagmatic Permian–Triassic structures (Giambiagi and Martinez, 2008).
The interpretation of the fault-slip data from major and mesoscale faults affecting the Permian–Triassic volcanics indicates a homogeneous orientation of north-northeast maximum stretching direction (Fig. 12). During this deformational event, two populations of major and mesoscale faults developed. The first population strikes west-northwest to northwest and displays normal offset with a minor or no component of strike-slip. The second population corresponds to north-northwest–trending oblique-slip normal faults, with sinistral offsets, such as the La Manga fault (Fig. 12). We interpreted both populations of faults to be newly created faults. Although the major north-northwest-trending oblique-slip faults could be interpreted as reactivation of late Paleozoic structures, the mesoscale oblique faults cannot be assigned to reactivated structures because they only affect the Permian–Triassic volcanics. This suggests that both major and mesoscale oblique slip faults were developed during the Permian–Triassic extension.
Concomitant to the Triassic synrift continental sedimentation and volcanism, west-northwest to north-northwest extensional faults developed (Figs, 13D, 13E). The faults have a dip-slip offset with a minor strike-slip component (Fig. 14), indicating that they are newly created Triassic normal faults. Two north-northwest–striking normal faults, the Siete Colores and Pampa de Canota, controlled the deposition of Triassic synrift sedimentary and volcaniclastic rocks. Subordinate north-south–trending dextral strike-slip faults were observed. Fault-slip data for this event indicate a northeast stretching direction. This northeast direction rotates to east-northeast close to the city of Mendoza, where a transfer zone between a northern half-graben with west polarity and a southern one with east polarity was previously interpreted (Legarreta et al., 1992).
Andean deformation and uplift are the causes of the present elevation of the Precordillera and the Frontal Cordillera. At the latitudes of the study area, this Andean orogenic phase started in Early Miocene time in the Principal Cordillera, reaching the study area during the Late Miocene to Early Pliocene (Irigoyen et al., 2000; Giambiagi et al., 2003). Early and late Paleozoic faults with a favorable orientation with respect to Andean compressional stresses were reactivated as reverse faults, whereas reactivated Permian–Triassic faults show sinistral displacements (Cortés et al., 2006; Terrizzano et al., 2009). In the Precordillera, many of the faults for which we have documented pre-Andean activity affect at present the Permian–Triassic rocks, proving that the Neogene compression reactivated these preexisting structures. These data suggest that the doubly vergent character of the Andean Precordillera fold-and-thrust belt is mainly the result of the reactivation of Paleozoic structures.
Late Paleozoic Partitioned Transpression
Deformation in transpressional zones is often compartmentalized into domains where the different components of bulk strain are segregated (Molnar, 1992; Tikoff and Teyssier, 1994). Structural and geophysical studies of active convergent margins suggest that strain partitioning is so ubiquitous that it must be considered as the normal way for noncoaxial nonplane strain to be accommodated (Fossen et al., 1994; Jones and Tanner, 1995; Holdsworth et al., 1998). During oblique transpressional shortening, a noncoaxial strain component is usually partitioned into discrete narrow zones dominated by simple-shear wrenching that delineate broader zones of distributed, largely coaxial, pure-shear deformation (Molnar, 1992; Jones and Tanner, 1995).
Based on structural geometry and overprinting links, two main domains of structures related to the Late Permian orogeny have been recognized (Figs. 15A, 15B). Crosscutting of late Paleozoic structures in the western domain indicates that contractional and strike-slip events occurred synchronously and that their different kinematics are probably joined. This domain is characterized by partitioned transpression with a shortening-dominated subdomain, plus one strike slip-dominated, in the western and eastern regions, respectively. Close to the Villavicencio fault, the mesoscale faults display a strike-slip regime with a north-south principal shortening direction that tends to be parallel to the fault. In contrast, western and eastern sectors show a west to west-northwest principal shortening direction, generally oriented orthogonally to the late Paleozoic faults.
Our kinematic analysis indicates that strain was partitioned into two different types of deformation domains. The western domain was deformed by heterogeneous simple shear. In this sector, the bulk strain compartmentalization into predominantly strike-slip and dip-slip deformation zones is not complete. Instead, this domain is characterized by strain heterogeneity, with mutually crosscutting strike-slip and dip-slip faults indicating synchronous shortening and wrenching. The eastern domain, however, was deformed by homogeneous pure shear. The strain compartmentalization model requires the preexistence of one zone of weakness, along which a component of simple shear can be accommodated (Jones and Tanner, 1995). Such weakness might include the Villavicencio shear zone, interpreted to represent a rheological anisotropy (Fig. 11).
Reactivation of Regional and Discrete Structural Grain
Fault reactivation processes play an important role in upper crustal dynamics. Structural reactivation of discrete structures occurs when displacements are repeatedly focused along well-defined, preexisting features such as faults or shear zones (Holdsworth et al., 1997). Theoretical, experimental, and microstructural studies have shown that there are numerous fault and shear zone processes that may lead to weakening of preexisting structures (Handy, 1989; Rutter et al., 2001). Fault zones in the brittle regime are likely to have lower cohesive strength and sliding friction than host rock (Donath and Cranwell, 1981).
In previous sections we documented the late Paleozoic partitioned transpression. To account for this transpression, we proposed a model with important reactivation of preexisting planes of weakness in the upper and lower crust. The model of an obliquely convergent margin, where convergence is decoupled into a component of convergence normal to the margin and a shear component taken up by strike-slip movement parallel to it, cannot account for our structural data and kinematic analysis. Del Castello et al. (2005) showed that variable unbalanced topography and overburden are responsible for highly variable stress orientation. They suggested that temporal evolution of the topography might be key in controlling the transition from generally favored strike slip to relatively more efficient pure slip. However, this is not consistent with the observed crosscutting relation between thrusts and strike-slip faults. Instead, we favor a model where thrusts and strike-slip faults develop simultaneously in response to the principal shortening direction that was not normal to the preexisting fabric, and we argue that the existence of an obliquely convergent margin is not necessary to explain the observed strain partitioning, though we do not discard it.
The strike and dip of the early Paleozoic foliation in the western domain (azimuth, Az, 1°–11°, dipping steeply toward the east) and in the eastern domain (Az 31°–46°, dipping steeply toward the west), coincides with the trend and dip of the late Paleozoic brittle structures (Figs. 7 and 10). This indicates that the preexisting planes of foliation represent weakness planes that had a strong influence on the strike of subsequent brittle structures, and also on the dip of these fracture planes. In this way, the double vergence of the late Paleozoic belt is inferred to be primarily controlled by the regional structural grain (Fig. 15). Moreover, field evidence and kinematic data show that some ductile shear zones are bounded by brittle contractional faults of late Paleozoic and Cenozoic time, suggesting that early Paleozoic ductile shear zones were reactivated many times during the successive brittle deformational events. Examples of repeatedly reactivated structures are the Bonilla and Villavicencio faults; field evidence indicates that a previous early Paleozoic shear zone was reactivated during the Permian brittle contractional event, and reutilized during the Andean contractional event. However, not all ductile shear zones reactivated subsequently, and some late Paleozoic contractional faults were controlled by the regional fabric and not by discrete structures. This is explained by the fact that reactivation of these shear zones depends primarily on the relative strengths of the shear zone and their host rock, and their orientation in the prevailing stress field (Angelier, 1984; White et al., 1986, and references therein).
In continental lithosphere, much of the deformation is controlled by preexisting anisotropies in the underlying rocks (Dunbar and Sawyer, 1989; Butler et al., 1997; Holdsworth et al., 1997; Tommasi and Vauchez, 2001). This control can be related to regional or discrete fabric reactivations (Coward, 1995; Holdsworth et al., 1997) or reworking of the lithospheric-scale volume of rocks, which corresponds to the repeated focusing of metamorphism, deformation, and magmatism at an orogenic scale (Vauchez et al., 1998; Tommasi and Vauchez, 2001; Holdsworth et al., 2001). Mantle anisotropies, such as rheological heterogeneities and mechanical anisotropies, that developed during orogenic events and were subsequently frozen in the lithospheric mantle, can survive for a long period of time and play a role in preferential inception of rifting several hundred million years after the orogenesis (Vauchez et al., 1998). These large-scale mechanical anisotropies of the lithospheric mantle are due to the formation of pervasive crystallographic and/or tectonic fabrics produced by the alignment of olivine crystals during major tectonic episodes (Tommasi and Vauchez, 2001). The tectonic inheritance permits the systematic reactivation of ancient tectonic zones by strain localization even if the extension direction is not normal to the structural trend of the belt (Vauchez et al., 1998).
In the previous sections we documented the development and successive reactivation of regional and discrete structural grain through time. Here we discuss the existence of a large-scale mechanical anisotropy present in the lithosphere. The Permian–Triassic evolution of southwestern South America, between lat 31°S and 34°S, was characterized by the development of a great amount of volcanism under oblique extensional conditions. This transtensional event resulted in the generation of a new complex fault system that concentrated the oblique-slip normal displacement related to a north-northeast–south-southwest stretching (Fig. 15C). On the surface, this fault system corresponds to a fault array of north-northwest–trending sinistral oblique-slip normal faults and north-northwest– to northwest-trending dip-slip normal faults. Pure normal faults are inferred here to represent newly created Permian–Triassic structures, without influence of the Paleozoic structural grain. Even though oblique-normal faults are usually related to reactivation of preexistence weakness planes (Bott, 1959), our structural study of the western domain indicates that only few north-northwest–trending faults could be related to reactivation of late Paleozoic strike-slip faults. This suggests that the oblique-normal Permian–Triassic faults are newly created structures, not related to the Paleozoic grain. However, a clear parallelism between the north-northwest–trending early Paleozoic deformational zone (related to the inferred suture zone between the Chilenia and Cuyania terranes), and the Permian–Triassic volcanic outcrops has been previously outlined (Ramos et al., 1986; Kay et al., 1989). In the tectonic model presented here we propose that the inferred early Paleozoic suture zone acted as a lithospheric weakness zone, which induced strain localization and guided lithospheric reworking during the extensional event. This explains the transtensional deformation along a north-northwest weakness zone during Permian–Triassic time with a north-northeast–oriented extensional direction, and the development of newly created oblique-normal faults.
The western margin of the Triassic Cuyo basin extends for >700 km and is located above the presumed early Paleozoic boundary between the Cuyania and Chilenia terranes (Ramos and Kay, 1991). The results of our kinematic analysis along the Precordillera southern sector have implications for the tectonic development of Triassic basins along the southwestern margin of Gondwana. Our geological and kinematic analyses of the Triassic normal faults indicate that these structures were not reactivations of preexisting structures. Instead, they were newly created dip-slip normal faults perpendicular to the stretching direction, thus supporting the view that these basins were generated under a pure extensional event (Fig. 16B). Results show that a Late Permian to Early Triassic north-northeast transtensional event was followed by a late-Early to early-Late Triassic northeast pure extensional event (Fig. 16). This change in the extensional direction occurred at the time when an eastward shifting of the locus of deformation took place. In this way, the western border of the Cuyo rift basin represents the easternmost extension of the Permian–Triassic volcanic outcrops. This indicates that the lithospheric strength in the reworking zone increased during the development of the Permian–Triassic magmatic event, and subsequent extension along the Permian–Triassic volcanic outcrops was not favorable.
Among the principal controls that can increase the lithospheric strength are thinning of the crust, changing of the lower crust composition, and cooling of the lithosphere (England, 1983; Kusznir and Park, 1984; Sonder and England, 1989; Ranalli, 2000). For our study case, strengthening of the lithosphere could be related to different factors, such as change of the crust composition and thinning of the crust. The Permian–Triassic rhyolite province is on the outer margins of the Gondwana supercontinent and represents remelting and reworking of crust that was accreted several hundred million years earlier (Kay et al., 1989). This province appears to be related to a relatively hot mantle that yielded basalts that underplated and melted the preexisting crust (Mpodozis and Kay, 1992). Basalt formed in the mantle rises and becomes trapped in the lower crust, resulting in substantial crustal melting. This pre–Middle Triassic mafic crustal underplating could have had a significant effect during the subsequent Middle to Late Triassic extensional event in changing the lower crust composition, and thus increasing the lithospheric strength. The geochemical analysis of Permian–Triassic volcanism suggests progressive crustal thinning (Kay et al., 1989; Llambías et al., 2003; Martínez, 2005; Kleiman and Japas, 2009), consistent with the transtensional setting (Giambiagi and Martínez, 2008), which could have increased the lithospheric strength.
In this paper we described ductile discrete and regional structural fabrics and brittle faults that were repeatedly reactivated with different kinematic characteristics during successive deformation episodes affecting the southern sector of the Precordillera in the southern Central Andes. Based on structural geometry, overprinting relationships, and kinematic analysis, we recognize four main groups of structures related to the different stages of the tectonic evolution of this portion of the Andes. The Cambrian–Quaternary units of the Precordillera contain complex patterns of both superimposed and reactivated structures that allow us to unravel the deformational histories of the Andes at lat 32°S–33°S. Considerable variations in structural styles in the southern sector of the Precordillera are the results of a series of superimposed deformational events from the early Paleozoic to the Holocene. The tectonic inheritance of these older structures accounts for contrasting structural styles along strikes and between the Cordillera Frontal and eastern and western domains of the Precordillera.
The first deformation episode is related to west-east to northwest-southeast crustal shortening that controlled the development of regional and discrete structural fabric during the early Paleozoic. The regional fabric corresponds to north to north-northeast foliation dipping east in the western domain of the Precordillera, and north-northeast to northeast foliation dipping west in the eastern domain. The north-south–trending, west-dipping shear zones present in the western domain represent the discrete fabric, while the regional foliation represents the regional fabric. These early Paleozoic basement anisotropies have played a significant role in subsequent structural development of the area.
The Early Permian San Rafael orogeny was responsible for the west-northwest to northwest noncoaxial contractional deformation and the generation of a doubly vergent fold-and-thrust belt. The reactivation of early Paleozoic regional and discrete fabrics during the late Paleozoic orogeny generated a partitioned transpressional system. Our study shows that both the regional and discrete early Paleozoic ductile fabrics localized subsequent brittle deformation. The fabric with steep north-trending foliation in the western domain and steep northeast-trending foliation in the eastern domain controlled the trend and dip of late Paleozoic faults; some of the early Paleozoic shear zones were reactivated during this brittle deformational event.
The Permian–Triassic evolution in this sector of the Andes was characterized by the development of a great amount of volcanism under oblique extensional conditions. Kinematic analyses of faults developed during the extrusion of this volcanism suggest that the oblique-rifting setting was primarily controlled by the presence of a preexisting anisotropy within the lithosphere. A clear parallelism between the distribution of an inferred suture zone between two terranes accreted against South America during the early Paleozoic, a north-northwest–trending late Paleozoic belt developed during the San Rafael orogenic phase, and Permian–Triassic rifting, suggests that tectonic inheritance permitted the reactivation of a north-northwest–trending long-lived lithospheric weakness zone.
This research was supported by grants from the Agencia Nacional de Promoción Científica y Tecnológica (PICT 07-10942) and CONICET (Consejo Nacional de Investigaciones Científicas y Técnicas; PIP 5843 and PIP 638) to Giambiagi. We thank M. Etcheverría, M. Tunik, S. Barredo, V. Ramos, N. Heredia, J.L. Alonso, G. Gallastegui, J. García Sansegundo, P. Farías, R. Rodríguez, and A. Zavattieri for helpful discussions and comments, and two anonymous reviewers for their critical and helpful suggestions.