Journal of South American Earth Sciences 26 (2008) 252–260

Home , Choiyoi Group, Gastre Fault, San Rafael orogeny, Uspallata

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Journal of South American Earth Sciences

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Permo-Triassic oblique extension in the Potrerillos-Uspallata area, western Argentina

Laura Giambiagi a,*, Amancay N. Martinez b a CONICET-IANIGLA Centro Regional de Investigaciones Cientíﬁcas y Tecnológicas, Parque San Martín s/n, 5500 Mendoza, Argentina b Laboratorio de Tectónica Andina, Universidad de Buenos Aires, Departamento de Ciencias Geológicas, Ciudad Universitaria, Pabellón II, 1428, Capital Federal, Argentina article info abstract

Keywords: The Permo-Triassic evolution of southwestern South America was characterized by the development of a Permo-Triassic volcanism great amount of volcanism under extensional conditions. Structural analyses of faults developed contem- Choiyoi Group poraneously with this volcanism in the key area of Potrerillos-Uspallata suggest the existence of an obli- Lithospheric anisotropy que extensional setting controlled by the presence of a pre-existing lithospheric anisotropy. A clear Oblique extension parallelism between the trace of an inferred Devonian suture zone, the Late Paleozoic San Rafael orogenic Andes belt and the Permo-Triassic rifting suggests that Early and Late Paleozoic tectonic inheritance permitted the reactivation of a NNW-trending zone of lithospheric weakness. The reactivation of this pre-existing weak zone during Late Permian to Early Triassic times has resulted in the generation of a new complex fault system, which concentrated the oblique-slip normal displacement related to a NNE–SSW stretching (N23°E). Ó 2008 Elsevier Ltd. All rights reserved.

1. Introduction zone, has been traced from the Atlantic coast to the San Rafael block (Urien et al., 1995; Jacques, 2003), and it represents a pre- Several major zones of intraplate deformation, also referred to existing network of structures that was formed or reactivated dur- as megafractures or megashears, occur as major structural aniso- ing a Late Paleozoic compressional event (Fig. 1A). This anisotropy tropies throughout the upper crustal rocks of southern South can be traced northward, along the tectonic depression between America (Coira et al., 1975; Rapela et al., 1991; Rapela and the Cordillera Frontal and Precordillera, running through the Pankhurst, 1992; Tickyj et al., 1997; Visser and Praekelt, 1998; Uspallata-Calingasta valley, or northwestward along the Río Blanco Franzese and Spalletti, 2001; Aceñolaza et al., 2002; Jacques, lineament (Fig. 1A and B). 2003). Several of these deformational zones are believed to be re- In this paper, we present evidence of the presence of the San lated to deep-crustal or even trans-lithospheric structures (Rapela Rafael anisotropy cross-cutting the Andes and its reactivation in and Pankhurst, 1992; Jacques, 2003). Although no clear proof of an oblique extensional mode during the development of the Late transcurrent movement along these anisotropies has been re- Permian to Middle Triassic magmatic event, the rocks of which ported, as was pointed out by von Gosen and Loske (2004) for are known as the Choiyoi Group. Detailed structural analysis, com- the Gastre fault system, they have been inferred to correspond to bined with stratigraphic, petrological and geochemical studies, was pre-existing heterogeneities in the continental lithosphere which carried out in an area where excellent exposures of Permo-Triassic act as planes of weakness during subsequent tectonic events. These faults are preserved. This area is located close to the towns of Potr- anisotropies have been traced from the Atlantic coast to the erillos and Uspallata, at 33°S latitude, and covers the eastern part Andean foothills with WNW to NNW trends, but the complex of the Cordillera Frontal and western part of the Precordillera structural history of the Andes makes them difficult to recognize (Fig. 1B). Our goals are (1) to define the Permo-Triassic structural across this orogen. Attempts to quantify pre-Cenozoic deformation framework of the area with field data and kinematic indicators, in the Andes face the difficulty of recognizing pre-Andean faults in- and (2) to discuss the role of a lithospheric inherited anisotropy side this Cenozoic orogeny, because ancient faults are often over- in the development of the Permo-Triassic extensional system. printed or obscured by intense Andean tectonics. One of these major anisotropies, the NW-trending San Rafael deformational 2. Geological setting

The study region is located between two parallel north–south * Corresponding author. E-mail addresses: [email protected] (L. Giambiagi), [email protected] trending mountain ranges of the Andes formed during the Neogene (A.N. Martinez). Andean orogeny: the Cordillera Frontal, composed of pre-Jurassic

Fig. 1. (A) Location map of the study area. The San Rafael anisotropy was outlined. (B) Regional map of the study area, showing the morphostructural units, the inferred boundary between Chilenia and Cuyania terranes (from Ramos, 2004), and the boundary between Cuyania and western Gondwana.

rocks uplifted by high-angle faults, and the Precordillera fold and (Ramos, 1988). In Early Permian times a widespread compressive thrust belt characterized by west- and east-vergent basement-in- event known as the San Rafael phase (Azcuy and Caminos, 1987; volved faults affecting Proterozoic to Neogene metamorphic and Ramos, 1988), which occurred between 280 and 270 Ma (Sato sedimentary rocks. The pre-Permian rocks correspond to metamor- et al., 1990; Martinez, 2005), generated a NNW-trending orogenic phic, igneous and sedimentary rocks of Proterozoic to Early Car- belt and important crustal thickening in the southwestern part of boniferous age. They are thought to belong to at least two the Gondwana supercontinent (Llambías and Sato, 1995; Mpodozis different terranes, Cuyania and Chilenia, suspected to have collided and Kay, 1990). Following the dominantly compressional setting, against the old cratonic basement of western Gondwana during the regional tectonic regime underwent a major change towards Early Paleozoic times (Ramos et al., 1986; Ramos, 2004). Although an extensional regime that was reflected by strong magmatic activ- the precise extent of both terranes remains unclear, the suspected ity, known as the Choiyoi Group (Caminos, 1965; Stipanicic et al., suture zone between them has been located roughly between the 1968; Rolleri and Criado Roque, 1969; Kay et al., 1989; Mpodozis Cordillera Frontal and Precordillera with a NNW–SSE trend in the and Kay, 1992; Llambías and Sato, 1995; Llambías et al., 2003), south, where the study area is located, and a N–S to NNE–SSW during the Late Permian to Middle Triassic (270–230 Ma, Martinez, trend in the north (Fig. 1B) (Haller and Ramos, 1984; Ramos 2005). In the study area and southward, the Choiyoi rocks are ex- et al., 1986; Davis et al., 2000). This suture zone is marked by out- posed in a narrow NNW-trending zone (Fig. 1B) and have a strong crops of mafic and ultramafic rocks, with geochemistry and isoto- spatial relationship with the San Rafael orogenic belt, suggesting pic signatures indicating an oceanic crust obducted during that the latter influenced the location of the Permo-Triassic Devonian times (Ramos et al., 1986). Toward the south, the San extension. Rafael block has been assigned as the southwards extension of the Cuyania terrane (Cingolani et al., 2003). Aeromagnetic studies 2.1. Choiyoi Group carried out in the San Rafael area by Chernicoff and Zappettini (2003) depict a NNW-oriented outstanding discontinuity which In southern South America, the Permo-Triassic magmatism of has been inferred to run parallel and adjacent to the Chilenia- the Choiyoi Group extends along a NNW- to NW-trending belt Cuyania boundary. from at least 28°S in the Cordillera Frontal to 41°S in the North Pat- The Late Paleozoic tectonic cycle began with the inception of agonian Massif. This magmatism is characterized by extrusive subduction along the western continental margin of Gondwana rocks and shallow level batholiths. The bottom of this sequence 254 L. Giambiagi, A.N. Martinez / Journal of South American Earth Sciences 26 (2008) 252–260 is composed of andesitic lavas and breccias, while the upper part to alkaline and have geochemical characteristics attributed to a corresponds to acid pyroclastics flows. These rocks are subalkaline magmatic arc of post-orogenic character. This magmatism has

Fig. 2. Geological map of the Potrerillos-Uspallata area where distinction between non-reactivated and reactivated Permo-Triassic faults is made. Location shown on Fig. 1. L. Giambiagi, A.N. Martinez / Journal of South American Earth Sciences 26 (2008) 252–260 255

been associated with an extensional regime, probably related to tion have high levels of K2O and SiO2 and are peraluminous with the final stage of a subduction process (Llambías and Sato, 1995; a calc-alkaline to alkaline imprinter. This trace-element pattern Llambías, 1999). The heat source has been proposed to be injection could indicate a low-pressure with a plagioclase-bearing source. of extension-related basalt into the lower crust (Kay et al., 1989), These rocks present medium La/Yb ratios and can be related to creating a mafic underplating which would have contributed to an extensional environment. The granitic rocks of the upper sec- the high volumes of melt and generation of bimodal volcanism tion are mildly peraluminous and have high SiO2 with relatively (Llambías et al., 2003). high K2O contents. These characteristics, together with high Ce In the study area (Fig. 2), Martinez (2005) grouped these rocks and Ga values, determine them to be alkaline A-type granitoids into three sections (Fig. 3). The lower section is composed of con- formed in a post-orogenic setting (Martinez, 2005). Their large tinental sandstones and conglomerates, as well as andesitic breccia negative Eu and Sr anomalies indicate a shallow level of emplace- and lava flows with hornblende phenocrystals. It is separated from ment into a normal or thinned crust. Fig. 4 illustrates the La/Yb vs. the Early to Late Paleozoic rocks by a high angular unconformity. Fe2O3 relationships for the three sections of the Choiyoi Group. The The middle section groups rhyodacitic ignimbrite and lavas. The lower section rocks show high Fe2O3 contents and a medium La/Yb upper section corresponds to lithic rhyodacitic ignimbrites as well ratio, whereas the middle section rocks have less Fe2O3 and a sim- as granitic plutonic and subvolcanic rocks. The outcrops of these ilar La/Yb ratio, suggesting that both sections developed along a rocks are highly controlled by the presence of WNW to NNW- normal crust. The upper section rocks have a lower La/Yb ratio, trending normal and sinistral oblique-slip faults (Fig. 2). indicating a thinner crust than the one present during the extru- Trace-element characteristics of the Choiyoi rocks are useful in sion of the lower and middle section rocks. understanding their tectonic evolution. Particularly important to determine an extensional environment are their flat HREE pattern, 3. Regional structures steep LREE levels and negative Eu and Sr anomalies (Martinez, 2005). Rocks of the lower section have low SiO2 and K2O contents The prolonged history of convergence of oceanic crust against with calc-alkaline signatures. Trace-element characteristics of the Pacific edge of Gondwana resulted in several episodes of defor- these rocks are flat HREE patterns and variable LREE levels, with mation during Paleozoic to Cenozoic times (Ramos, 1988; von no Eu anomaly and a medium La/Yb ratio, suggesting an incipient Gosen, 1995; Cortés et al., 1997; Davis et al., 2000). Overprinting thinner crust (Fig. 3). On the other hand, rocks of the middle sec- relationships between different structures in the Precordillera

1000 Upper Section noitcesreppU

etirdnohc/kcoR 100

noitceselddiM 1000 Middle Section

etirdnohc/kcoR 100

Granitic intrusives and porphyries Rhyolitic 1 ignimbrites

1000 Lithic ignimbrites Lower Section noitcesrewoL Rhyodacitic ignimbrites and lavas

Andesitic etirdnohc/kcoR 100 breccia

Andesitic lavas

10 Conglomerates

Sandstones

1 San Rafael event

Fig. 3. Schematic columnar section of the Choiyoi Group rocks, not on scale, cropping out in the Potrerillos area and trace-element diagrams for rocks of the lower, middle and upper sections of the Choiyoi Group, normalizad to the Leedey chondrite of Masuda et al. (1973). 256 L. Giambiagi, A.N. Martinez / Journal of South American Earth Sciences 26 (2008) 252–260

20 Andesitic lavas LS 18 Middle basic dykes section Rhyodacitic lavas 16 MS acid dykes rhyodacitic ignimbrites 14 US Granitic intrusives 12 rhyolitic ignimbrites

bY/aL 10 Lower 8 section 6 4 Upper 2 section 0 0246810

Fe23 O

Fig. 4. La/Yb vs. Fe2O3 relationships for the lower, middle and upper sections of the Choiyoi Group.

and Cordillera Frontal preserve evidence for at least four deforma- NNW trend, formed along the western margin of Gondwana tional events that occurred since the Early Paleozoic: (1) a Middle (Charrier, 1979; Uliana et al., 1989). One of these basins, the Cuyo to Late Devonian collisional event; (2) the Late Paleozoic San Rafael basin, corresponds to a NNW-trending narrow basin parallel to the orogeny; (3) the Permo-Triassic extension; and (4) the Neogene inferred suture zone between the Chilenia and Cuyania terranes Andean orogeny. In the western part of the Precordillera, the lower (Fig. 1B). Paleozoic metamorphic rocks are affected by N–S to NE–SW trend- The present morphology of the Precordillera and Cordillera ing folds verging toward the west and northwest (von Gosen, Frontal is the result of Miocene to present Andean shortening 1995) and high-angle NNE-trending west-vergent faults (Fig. 2; and uplift. The tectonic style of the western part of the Precordill- Cortés et al., 1997; Folguera et al., 2001; Folguera and Giambiagi, era and the eastern flank of the Cordillera Frontal is strongly influ- 2002). These structures developed during Middle to Late Devonian enced by pre-Andean paleogeographic features and structures, time (von Gosen, 1995; Davis et al., 2000) and are thought to be re- indicating that the resulting structural styles were predetermined lated to the collision between the Cuyania and Chilenia terranes by conditions established in earlier tectonic events as was previ- (Ramos et al., 1986; Davis et al., 2000). ously pointed out by von Gosen (1995). The Late Paleozoic orogenic belt has a NNW-trend, and its location coincides, in the study area and southwards, with the inferred 3.1. Analysis of Permo-Triassic faults Early Paleozoic suture between the Cuyania and Chilenia terranes. During this time a thin-skinned thrust belt developed in the fore- Field analysis consisted of detailed geological mapping and land, located immediately towards the east of the study area, in structural analysis of the Potrerillos-Uspallata area (Fig. 2A). We the eastern part of the Precordillera, where sheets composed of chose this area because it has preserved a great amount of Per- Silurian to Carboniferous strata were thrust eastward by low-angle mo-Triassic structures, and it has only been affected by large NE- to NNE-trending faults (Folguera et al., 2001; Folguera and high-angle Cenozoic faults, the movement of which can be easily Giambiagi, 2002). In the internal zone of the orogen, in the eastern reconstructed. In this area, the volcanic rocks of the Choiyoi Group flank of the Cordillera Frontal, a series of N- to NNE-trending re- and older rocks are affected by normal and oblique-slip normal verse high-angle faults were involved in what is known as the La faults with WNW to NW trends. The great thickness and facies vari- Carrera fault system (Caminos, 1965, 1979; Polanski, 1972; ations of these volcanic rocks near these structures, and the rela- Fig. 2). Locally, the inconsistency between NNW-trending regional tionship between structures and the intrusion of plutonic rocks and NNE- to NE-trending local Late Paleozoic structures can be ex- allow us to consider them as synmagmatic Permo-Triassic struc- plained by clockwise block rotations inferred to have taken place tures. The two most prominent Permo-Triassic faults identified cor- between 280 and 265 Ma, before the extrusion of the Choiyoi vol- respond to the Polcura and La Manga faults (Figs. 2A and 4). The canics (Rapalini and Vilas, 1991). These crustal block rotations Polcura fault extends for more than 30 km, has a NW trend and a have been found in the Uspallata-Calingasta valley and have been vertical displacement of at least 500 m. Slickensides on exposed slip attributed by Rapalini and Vilas (1991) to dextral strike-slip move- surfaces indicate two sets of striae (Fig. 2B). The older set shows a ment parallel or subparallel to the continental margin. Dextral sinistral oblique-slip normal movement and the younger a sinistral movements along NNW-trending San Rafael structures have also strike-slip one. The fault does not show constant strike trends, but been found in the San Rafael block area by Japas and Kleiman varies between 305° and 345° in a stair mode. The La Manga fault is (2004). In accordance with Rapalini and Vilas (1991), Japas and parallel to the Polcura fault and merges toward this structure by a Kleiman (2004) proposed the existence of a NE–SW oriented trans- series of en echelon strike-slip faults. A detailed analysis of its pressional regimen during the Early Permian, suggesting the reac- southeastern termination shows that the fault was reactivated dur- tivation of a pre-existing crustal fabric. ing the Andean shortening. The La Polcura and La Manga faults, Following the dominantly compressional setting that led to the trending parallel but dipping one against the other, delineate a gra- San Rafael orogeny, the regional tectonic regime underwent, dur- ben structure that apparently controlled the localization of the ing the Late Permian to Middle Triassic, a major change towards andesitic breccias and the lithic ignimbrites of the middle and an extensional regime that was reflected by the magmatic activity upper sections of the Choiyoi Group, respectively (Figs. 2 and 5). and structural development that will be analysed and discussed in We measured fault orientation and sense of displacement of the this paper. The extensional regime continued during the Triassic Permo-Triassic structures, the criteria of which include: offset of local and led to the formation of a series of rift systems, with overall horizons, growth direction of mineral fibres (quartz-slickenfibres), L. Giambiagi, A.N. Martinez / Journal of South American Earth Sciences 26 (2008) 252–260 257

Km SW NE 5 La Manga Polcura 4 Fault Fault

0 4km

Fig. 5. SW–NE oriented geological cross-section of the meso-scale area. Location on Fig. 2. Note the graben-like geometry formed by the La Polcura and La Manga normal faults which controlled the deposition of breccias and ignimbrites of the Choiyoi Group. secondary striated and tensile fractures, and associated steps, (Fig. 6). Slips on individual faults are integrated to determine the grooves on the fault surface, and drag folds. Our data set consists principal directions of brittle strain within the area. The heteroge- of 72 measures on faults with sense of displacement indicators neous population of fault-slip data (Fig. 6A) was divided into three

n: 19 LINKED EINGHAM AXES B Eigenvalue Eigenvector (T&P) 1 1. 0.4173 18.3º 3.5º 2. 0.0031 288.5º 1.7º 3. -.4204 173.0º 86.1º

Total population Mean strike vector: 295º LINKED EINGHAM AXES Eigenvalue Eigenvector (T&P) n: 72 C n: 22 1. 0.4597º 205.7º 2.8º 2. -.0856º 295.9º 4.5º 3. -.3741º 83.9º 84.8º

1 Mean strike vector: 316º LINKED EINGHAM AXES D n: 31 Eigenvalue Eigenvector (T&P) 1. 0.2574º 182.6º 25.7º 2. 0.1417º 352.2º 63.9º 3. -.3992º 90.6º 4.1º

Fig. 6. Equal-area stereographic projections of fault-slip data of the meso-scale area. n is the number of data. Great circle represents fault plane, ball and arrow represents trend and plunge of striae and relative movement of the hanging wall. We used the orientation of P (black dots) and T axes (white squares) to estimate principal strain rate axes using Richard Allmendinger’s FaultKinWin program. (A) Heterogeneous population of all fault slip data obtained. The whole set was divided into three subsets. (B) Homogeneous data subset integrating faults with normal offsets. (C) Homogeneous data subset of faults with sinistral oblique-normal offset. and (D) Oblique-slip compressional faults. 258 L. Giambiagi, A.N. Martinez / Journal of South American Earth Sciences 26 (2008) 252–260 homogeneous subsets on the basis of orientation and sense of dis- direction and almost vertical shortening (Fig. 6B and C). The varia- placement. The first population strikes WNW (295°) and displays tion in the orientation of the extension axis between the two do- normal offsets with minor or no component of strike-slip (Fig. 6B). mains is small (7°) and is not significant in terms of kinematic The second population corresponds to NW-trending oblique-slip analysis. The kinematic analysis of the third population shows normal faults (316°), with sinistral offset (Fig. 6C). Because An- great diversity. The reactivation of Permo-Triassic structures of dif- dean deformation reactivated some of these structures in a ferent orientations is interpreted to be the cause of this diversity. strike-slip mode, we identified superposition of striations on fault Its solution indicates strike-slip mode with a small component of planes whenever possible. The third population corresponds to east–west shortening. We combined the first and second popula- strike-slip faults with few compressive dip–slip components and tions which are interpreted to reflect the same deformation event sinistral offsets (Fig. 6D). Some of these faults are cutting the An- and obtained the direction of 22.9° and 0.2° for the extension axis, dean dip–slip compressional structures, indicating that they are suggesting that the NW- and WNW-trending structures are the re- Andean in age. sult of an oblique extensional regime. With the assumption that the stress field was homogeneous during deformation affecting the study area, and that faults did 4. Relative timing of the extension not interact mechanically, we used the orientation of the shortening and extension axes (P and T axes) to estimate principal direc- In previous sections, we used kinematic and geometric studies tions of brittle strain (Marrett and Allmendinger, 1990) which of faults developed synchronously with the Choiyoi rocks to prove provide an approximate orientation of the principal strain rate axes the presence of an oblique extensional setting during Permo-Trias- (Twiss and Unruh, 1998). The kinematic axes of the two popula- sic times. In this section, we try to assign a relative time for the tions of faults indicate that the Permo-Triassic deformational phase duration of this deformational event by: (i) relationships between was characterized by NNE–SSW oriented, subhorizontal extension different extensional structures; and (ii) relationships between the

a b C B B

A A

c d B B

A A

e C ff B B A

Fig. 7. Normal and oblique-slip faulting contemporaneous with the extrusion of the Choiyoi volcanics. (a) Normal and oblique-slip NW-trending faults active during the deposition of sedimentary rocks (A) and andesitic breccias (B). (b) Angular unconformity between the sedimentary rocks (A) and andesitic lavas (B) of the lower part of the Choiyoi Group with the rhyodacitic lavas (C) of the middle part of the Choiyoi Group. Note that the sedimentary rocks are affected both by WNW-trending normal faults and NW-trending oblique-slip normal faults with thickness variations across them, indicating that faulting was synchronic with the sedimentation and extrusion of the lower Choiyoi Group but occurred before the extrusion of the rhyodacitic lavas. (c) Several WNW-trending normal faults with different relative times of movement related to the andesitic lavas. (d) Normal and sinistral strike-slip normal faults contemporaneous with the rhyodacitic ignimbrites (A) and rhyodacitic lavas (B) of the middle part of the Choiyoi Group. Several small normal faults are constrained during the time of the effusion of the ignimbrites. (e) WNW-trending normal fault active during the sedimentation of the rhyodacitic ignimbrites (A) and lower part of the rhyodacitic lavas (B). The upper part of the rhyodacitic lavas (C) unconformably covers the fault. (f). Several normal faults developed during the extrusion of the lithic ignimbrites of the upper part of the Choiyoi Group. L. Giambiagi, A.N. Martinez / Journal of South American Earth Sciences 26 (2008) 252–260 259

Choiyoi stratigraphical units and extensional structures. As was structural grains, developed during Early and Late Paleozoic defor- pointed out above, the kinematic structural analysis suggests that mational events respectively, strike N–S to NE–SW, suggesting that sinistral oblique-slip and normal faults evolved contemporane- the development of the WNW- to NW-trending Permo-Triassic ously under a NNE–SSW oriented maximum extension previous structures was not influenced by the upper crust fabric. Instead, to the Andean horizontal shortening. No clear cross-cutting rela- the parallelism between the NNW-trending Devonian deforma- tionships can be observed for faults of the first and second popula- tional zone, related to the inferred suture zone between the Chile- tions, but the compartmentalization of smaller displacement nia and Cuyania terranes, the Late Paleozoic San Rafael orogenic WNW-trending faults between larger displacement NW-trending belt and the Permo-Triassic Choiyoi Group outcrops suggests the faults is consistent with a similar age. presence of a lithospheric weakness zone, which induces strain Further evidence for the relative time of the oblique extensional localization and guided lithospheric reworking during subsequent deformation is derived from detailed stratigraphical studies of the deformational events. Choiyoi rocks. These rocks show regional-scale lateral changes in In the tectonic model presented here we propose that the NNW- thickness and facies which have a tectonic origin and sometimes trending inferred suture zone and the Late Paleozoic orogenic belt produce the total omission of the units, indicating syntectonic acted as a lithospheric anisotropy which accounts for the genera- magmatism. The complex relationship between faults and the dif- tion of an oblique extensional regime during Permo-Triassic times ferent units of the Choiyoi Group can be seen in Fig. 7. The oblique- and the presence of permeable structures which favored magma slip and normal faults mark some of the contacts between sedi- ascent. The reactivation of this pre-existing weakness has resulted mentary and volcanic rocks and are interpreted as being synerup- in the generation of a new complex fault system under oblique tive with: (i) sedimentary rocks and andesitic lavas and breccia of extensional conditions. The remarkable consistency between the the lower section of the Choiyoi Group (Fig. 7a–c); (ii) rhyodacitic NW- and WNW-trending orientations and their associated stress lavas and associated ignimbrites of the middle section (Fig. 7d and field permits us to consider that these extensional structures were e); and (iii) rhyolitic ignimbrites of the upper section (Fig. 7f). Sim- newly created faults, the orientations of which were determined by ilar field observations have been carried out by von Gosen (1995) the regional stress field. This suggests that the fault array devel- in the Uspallata area. This author studied several N to NW-trending oped synchronously as one largely heterogeneous system. These faults associated with slump folds, breccia, and breccia horizons conclusions match those found for the Choiyoi Group in the San within the Permo-Triassic deposits and interpreted them as synse- Rafael block (Fig. 1B) by Japas and Kleiman (2004), who proposed dimentary normal faults. This allows us to postulate that the extru- a sinistral oblique-slip regime during the evolution of the upper sives and shallow plutonic rocks of the lower, middle and upper part of the Choiyoi Group as a result of a NNE–SSW directed sections of the Choiyoi Group were generated under a constant stretching direction. NNE–SSW stretching direction. 6. Conclusions

5. Discussion: geodynamic model for the Permo-Triassic Structural, stratigraphical and geochemical studies allow us to extension propose a consistent tectonic scheme for the development of an oblique extensional deformational event during Late Permian to In continental rift zones, much of the deformation from rifting is Early Triassic times in southwestern South America. During this controlled by pre-existing anisotropies in the underlying rocks time, a NNW-trending deformational zone developed along a (Dunbar and Sawyer, 1989; Butler et al., 1997; Holdsworth et al., pre-existing lithospheric anisotropy located along an inferred su- 1997; Tommasi and Vauchez, 2001). This control can be related ture zone between the Chilenia and Cuyania terranes. This zone to a reactivation of the upper crust discrete or pervasive fabric or corresponds to a dense network of normal and oblique-slip normal a reworking of the lithospheric-scale volume of rocks which corre- faults along which the Permo-Triassic intraplate sinistral exten- sponds to the repeated focusing of metamorphism, deformation sional movements, related to a NNE–SSW stretching (N23°E), were and magmatism at an orogenic scale (Holdsworth et al., 2001). Rift accommodated. It comprises NW-trending sinistral oblique-slip systems tend to follow pre-existing orogenic belts and suture normal faults and WNW-trending normal faults. Geometric, tem- zones containing long-lived zones of mechanical weakness, which poral and kinematic relationships indicate that these faults are can be preserved in the lithospheric crust but also in the litho- genetically linked to the formation of the Choiyoi volcanic rocks. spheric mantle, that undergo repeated deformation and failure (Tommasi and Vauchez, 2001). Mantle anisotropies, such as rheo- Acknowledgements logical heterogeneities and mechanical anisotropies, developed during orogenic events and subsequently frozen into the litho- This research was supported by Grants from the University of spheric mantle, can survive for a long period of time and play a role Buenos Aires (UBACYT TW87) and CONICET (PICT 07-10942). We in preferential rift localization several hundred million years after gratefully acknowledge the use of Richard Allmendinger’s Fault- orogenesis (Vauchez et al., 1997, 1998). These large-scale mechan- KinWin program and Ernesto Cristallini´s Estereografica GR pro- ical anisotropies of the lithospheric mantle are due to the forma- gram. Special thanks are due to Natalia Marchese, Guillermo tion of a pervasive crystallographic/tectonic fabric produced by Peralta, Cristian Guerra, Diego Márquez, Diego Di Carlo and Ezequ- the preferred alignment of olivine crystals during major tectonic iel García Morabito for their help in the field. episodes (Tommasi and Vauchez, 2001). The tectonic inheritance permitted the systematic reactivation of ancient tectonic zones References by strain localization even if the extension direction is not normal to the structural trend of the belt (Vauchez et al., 1997). Moreover, Aceñolaza, F.G., Miller, H., Toselli, A.J., 2002. Proterozoic-early Paleozoic evolution in major shear zones have been considered to represent important western South America – a discussion. Tectonophysics 354, 121–137. Azcuy, C.L., Caminos, R., 1987. Diastrofismo. In: Archangelsky, S. (Ed.), El sistema controlling features in both magma ascent and pluton emplace- Carbonífero en la República Argentina. Academia Nacional de Ciencias, Córdoba, ment processes (Vigneresse, 1995; Rosenberg, 2004). pp. 239–251. In the study area a pervasive fabric, related to schistosity of Butler, R.W.H., Holdsworth, R.E., Lloyd, G.E., 1997. The role of basement reactivation in continental deformation. Geological Society of London, Journal 154, 69–71. lower Paleozoic rocks, and a discrete fabrics, marked by upper Caminos, R., 1965. Geología de la vertiente oriental del Cordón del Plata, Cordillera Paleozoic faults and fault-related folds, have been identified. These Frontal de Mendoza. Asociación Geológica Argentina, Revista 20, 351–392. 260 L. Giambiagi, A.N. Martinez / Journal of South American Earth Sciences 26 (2008) 252–260

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