journal of south american earth sciences · 2008. 12. 5. · l. giambiagi, a.n. martinez/journal of...

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

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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, Argentinab Laboratorio de Tectónica Andina, Universidad de Buenos Aires, Departamento de Ciencias Geológicas, Ciudad Universitaria, Pabellón II, 1428, Capital Federal, Argentina

a r t i c l e i n f o

Keywords:Permo-Triassic volcanismChoiyoi GroupLithospheric anisotropyOblique extensionAndes

0895-9811/$ - see front matter � 2008 Elsevier Ltd. Adoi:10.1016/j.jsames.2008.08.008

* Corresponding author.E-mail addresses: [email protected] (L. G

(A.N. Martinez).

a b s t r a c t

The Permo-Triassic evolution of southwestern South America was characterized by the development of agreat amount of volcanism under extensional conditions. Structural analyses of faults developed contem-poraneously with this volcanism in the key area of Potrerillos-Uspallata suggest the existence of an obli-que extensional setting controlled by the presence of a pre-existing lithospheric anisotropy. A clearparallelism between the trace of an inferred Devonian suture zone, the Late Paleozoic San Rafael orogenicbelt and the Permo-Triassic rifting suggests that Early and Late Paleozoic tectonic inheritance permittedthe reactivation of a NNW-trending zone of lithospheric weakness. The reactivation of this pre-existingweak zone during Late Permian to Early Triassic times has resulted in the generation of a new complexfault system, which concentrated the oblique-slip normal displacement related to a NNE–SSW stretching(N23�E).

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1. Introduction

Several major zones of intraplate deformation, also referred toas megafractures or megashears, occur as major structural aniso-tropies throughout the upper crustal rocks of southern SouthAmerica (Coira et al., 1975; Rapela et al., 1991; Rapela andPankhurst, 1992; Tickyj et al., 1997; Visser and Praekelt, 1998;Franzese and Spalletti, 2001; Aceñolaza et al., 2002; Jacques,2003). Several of these deformational zones are believed to be re-lated to deep-crustal or even trans-lithospheric structures (Rapelaand Pankhurst, 1992; Jacques, 2003). Although no clear proof oftranscurrent movement along these anisotropies has been re-ported, as was pointed out by von Gosen and Loske (2004) forthe Gastre fault system, they have been inferred to correspond topre-existing heterogeneities in the continental lithosphere whichact as planes of weakness during subsequent tectonic events. Theseanisotropies have been traced from the Atlantic coast to theAndean foothills with WNW to NNW trends, but the complexstructural history of the Andes makes them difficult to recognizeacross this orogen. Attempts to quantify pre-Cenozoic deformationin the Andes face the difficulty of recognizing pre-Andean faults in-side this Cenozoic orogeny, because ancient faults are often over-printed or obscured by intense Andean tectonics. One of thesemajor anisotropies, the NW-trending San Rafael deformational

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zone, has been traced from the Atlantic coast to the San Rafaelblock (Urien et al., 1995; Jacques, 2003), and it represents a pre-existing network of structures that was formed or reactivated dur-ing a Late Paleozoic compressional event (Fig. 1A). This anisotropycan be traced northward, along the tectonic depression betweenthe Cordillera Frontal and Precordillera, running through theUspallata-Calingasta valley, or northwestward along the Río Blancolineament (Fig. 1A and B).

In this paper, we present evidence of the presence of the SanRafael anisotropy cross-cutting the Andes and its reactivation inan oblique extensional mode during the development of the LatePermian to Middle Triassic magmatic event, the rocks of whichare known as the Choiyoi Group. Detailed structural analysis, com-bined with stratigraphic, petrological and geochemical studies, wascarried out in an area where excellent exposures of Permo-Triassicfaults are preserved. This area is located close to the towns of Potr-erillos and Uspallata, at 33�S latitude, and covers the eastern partof the Cordillera Frontal and western part of the Precordillera(Fig. 1B). Our goals are (1) to define the Permo-Triassic structuralframework of the area with field data and kinematic indicators,and (2) to discuss the role of a lithospheric inherited anisotropyin the development of the Permo-Triassic extensional system.

2. Geological setting

The study region is located between two parallel north–southtrending mountain ranges of the Andes formed during the NeogeneAndean orogeny: the Cordillera Frontal, composed of pre-Jurassic

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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 inferredboundary between Chilenia and Cuyania terranes (from Ramos, 2004), and the boundary between Cuyania and western Gondwana.

L. Giambiagi, A.N. Martinez / Journal of South American Earth Sciences 26 (2008) 252–260 253

rocks uplifted by high-angle faults, and the Precordillera fold andthrust belt characterized by west- and east-vergent basement-in-volved faults affecting Proterozoic to Neogene metamorphic andsedimentary rocks. The pre-Permian rocks correspond to metamor-phic, igneous and sedimentary rocks of Proterozoic to Early Car-boniferous age. They are thought to belong to at least twodifferent terranes, Cuyania and Chilenia, suspected to have collidedagainst the old cratonic basement of western Gondwana duringEarly Paleozoic times (Ramos et al., 1986; Ramos, 2004). Althoughthe precise extent of both terranes remains unclear, the suspectedsuture zone between them has been located roughly between theCordillera Frontal and Precordillera with a NNW–SSE trend in thesouth, where the study area is located, and a N–S to NNE–SSWtrend in the north (Fig. 1B) (Haller and Ramos, 1984; Ramoset al., 1986; Davis et al., 2000). This suture zone is marked by out-crops of mafic and ultramafic rocks, with geochemistry and isoto-pic signatures indicating an oceanic crust obducted duringDevonian times (Ramos et al., 1986). Toward the south, the SanRafael block has been assigned as the southwards extension ofthe Cuyania terrane (Cingolani et al., 2003). Aeromagnetic studiescarried out in the San Rafael area by Chernicoff and Zappettini(2003) depict a NNW-oriented outstanding discontinuity whichhas been inferred to run parallel and adjacent to the Chilenia-Cuyania boundary.

The Late Paleozoic tectonic cycle began with the inception ofsubduction along the western continental margin of Gondwana

(Ramos, 1988). In Early Permian times a widespread compressiveevent known as the San Rafael phase (Azcuy and Caminos, 1987;Ramos, 1988), which occurred between 280 and 270 Ma (Satoet al., 1990; Martinez, 2005), generated a NNW-trending orogenicbelt and important crustal thickening in the southwestern part ofthe Gondwana supercontinent (Llambías and Sato, 1995; Mpodozisand Kay, 1990). Following the dominantly compressional setting,the regional tectonic regime underwent a major change towardsan extensional regime that was reflected by strong magmatic activ-ity, known as the Choiyoi Group (Caminos, 1965; Stipanicic et al.,1968; Rolleri and Criado Roque, 1969; Kay et al., 1989; Mpodozisand Kay, 1992; Llambías and Sato, 1995; Llambías et al., 2003),during the Late Permian to Middle Triassic (270–230 Ma, Martinez,2005). In the study area and southward, the Choiyoi rocks are ex-posed in a narrow NNW-trending zone (Fig. 1B) and have a strongspatial relationship with the San Rafael orogenic belt, suggestingthat the latter influenced the location of the Permo-Triassicextension.

2.1. Choiyoi Group

In southern South America, the Permo-Triassic magmatism ofthe Choiyoi Group extends along a NNW- to NW-trending beltfrom at least 28�S in the Cordillera Frontal to 41�S in the North Pat-agonian Massif. This magmatism is characterized by extrusiverocks 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 partcorresponds to acid pyroclastics flows. These rocks are subalkaline

Fig. 2. Geological map of the Potrerillos-Uspallata area where distinction between non-r

to alkaline and have geochemical characteristics attributed to amagmatic arc of post-orogenic character. This magmatism has

eactivated and reactivated Permo-Triassic faults is made. Location shown on Fig. 1.


been associated with an extensional regime, probably related tothe final stage of a subduction process (Llambías and Sato, 1995;Llambías, 1999). The heat source has been proposed to be injectionof extension-related basalt into the lower crust (Kay et al., 1989),creating a mafic underplating which would have contributed tothe high volumes of melt and generation of bimodal volcanism(Llambías et al., 2003).

In the study area (Fig. 2), Martinez (2005) grouped these rocksinto three sections (Fig. 3). The lower section is composed of con-tinental sandstones and conglomerates, as well as andesitic brecciaand lava flows with hornblende phenocrystals. It is separated fromthe Early to Late Paleozoic rocks by a high angular unconformity.The middle section groups rhyodacitic ignimbrite and lavas. Theupper section corresponds to lithic rhyodacitic ignimbrites as wellas granitic plutonic and subvolcanic rocks. The outcrops of theserocks are highly controlled by the presence of WNW to NNW-trending normal and sinistral oblique-slip faults (Fig. 2).

Trace-element characteristics of the Choiyoi rocks are useful inunderstanding their tectonic evolution. Particularly important todetermine an extensional environment are their flat HREE pattern,steep LREE levels and negative Eu and Sr anomalies (Martinez,2005). Rocks of the lower section have low SiO2 and K2O contentswith calc-alkaline signatures. Trace-element characteristics ofthese rocks are flat HREE patterns and variable LREE levels, withno Eu anomaly and a medium La/Yb ratio, suggesting an incipientthinner crust (Fig. 3). On the other hand, rocks of the middle sec-

Lithic ignimbrites

Andesitic lavas

Rhyodacitic ignimbrand lavas

Rhyoliticignimbrites

Granitic intrusivesand porphyries

Andesiticbreccia

San Rafael event

Conglomerates

Sandstones

noitcesreppU

noitceselddi

Mnoitcesre

woL

Lithic ignimbritesLithic ignimbrites

Fig. 3. Schematic columnar section of the Choiyoi Group rocks, not on scale, cropping outupper sections of the Choiyoi Group, normalizad to the Leedey chondrite of Masuda et

tion have high levels of K2O and SiO2 and are peraluminous witha calc-alkaline to alkaline imprinter. This trace-element patterncould indicate a low-pressure with a plagioclase-bearing source.These rocks present medium La/Yb ratios and can be related toan extensional environment. The granitic rocks of the upper sec-tion are mildly peraluminous and have high SiO2 with relativelyhigh K2O contents. These characteristics, together with high Ceand Ga values, determine them to be alkaline A-type granitoidsformed in a post-orogenic setting (Martinez, 2005). Their largenegative Eu and Sr anomalies indicate a shallow level of emplace-ment into a normal or thinned crust. Fig. 4 illustrates the La/Yb vs.Fe2O3 relationships for the three sections of the Choiyoi Group. Thelower section rocks show high Fe2O3 contents and a medium La/Ybratio, whereas the middle section rocks have less Fe2O3 and a sim-ilar La/Yb ratio, suggesting that both sections developed along anormal crust. The upper section rocks have a lower La/Yb ratio,indicating a thinner crust than the one present during the extru-sion of the lower and middle section rocks.

3. Regional structures

The prolonged history of convergence of oceanic crust againstthe Pacific edge of Gondwana resulted in several episodes of defor-mation during Paleozoic to Cenozoic times (Ramos, 1988; vonGosen, 1995; Cortés et al., 1997; Davis et al., 2000). Overprintingrelationships between different structures in the Precordillera

ites

etirdnohc/kcoR

1

1

1

Upper Section

Middle Section

Lower Section

etirdnohc/kcoR

etirdnohc/kcoR

1000

1000

1000

100

10

100

10

100

10

in the Potrerillos area and trace-element diagrams for rocks of the lower, middle andal. (1973).

02468

101214161820

0 2 4 6 8 10Fe O2 3

bY/aL

Andesitic lavas

Rhyodacitic lavas

hyodacitic ignimbritesGranitic intrusiveshyolitic ignimbrites

basic dykes

cid dykesr

r

a

Uppersection

Middlesection

Lowersection

LS

MS

US

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-tional events that occurred since the Early Paleozoic: (1) a Middleto Late Devonian collisional event; (2) the Late Paleozoic San Rafaelorogeny; (3) the Permo-Triassic extension; and (4) the NeogeneAndean orogeny. In the western part of the Precordillera, the lowerPaleozoic metamorphic rocks are affected by N–S to NE–SW trend-ing folds verging toward the west and northwest (von Gosen,1995) and high-angle NNE-trending west-vergent faults (Fig. 2;Cortés et al., 1997; Folguera et al., 2001; Folguera and Giambiagi,2002). These structures developed during Middle to Late Devoniantime (von Gosen, 1995; Davis et al., 2000) and are thought to be re-lated to the collision between the Cuyania and Chilenia terranes(Ramos et al., 1986; Davis et al., 2000).

The Late Paleozoic orogenic belt has a NNW-trend, and its loca-tion coincides, in the study area and southwards, with the inferredEarly Paleozoic suture between the Cuyania and Chilenia terranes.During this time a thin-skinned thrust belt developed in the fore-land, located immediately towards the east of the study area, inthe eastern part of the Precordillera, where sheets composed ofSilurian to Carboniferous strata were thrust eastward by low-angleNE- to NNE-trending faults (Folguera et al., 2001; Folguera andGiambiagi, 2002). In the internal zone of the orogen, in the easternflank of the Cordillera Frontal, a series of N- to NNE-trending re-verse high-angle faults were involved in what is known as the LaCarrera fault system (Caminos, 1965, 1979; Polanski, 1972;Fig. 2). Locally, the inconsistency between NNW-trending regionaland NNE- to NE-trending local Late Paleozoic structures can be ex-plained by clockwise block rotations inferred to have taken placebetween 280 and 265 Ma, before the extrusion of the Choiyoi vol-canics (Rapalini and Vilas, 1991). These crustal block rotationshave been found in the Uspallata-Calingasta valley and have beenattributed by Rapalini and Vilas (1991) to dextral strike-slip move-ment parallel or subparallel to the continental margin. Dextralmovements along NNW-trending San Rafael structures have alsobeen found in the San Rafael block area by Japas and Kleiman(2004). In accordance with Rapalini and Vilas (1991), Japas andKleiman (2004) proposed the existence of a NE–SW oriented trans-pressional regimen during the Early Permian, suggesting the reac-tivation of a pre-existing crustal fabric.

Following the dominantly compressional setting that led to theSan Rafael orogeny, the regional tectonic regime underwent, dur-ing the Late Permian to Middle Triassic, a major change towardsan extensional regime that was reflected by the magmatic activityand structural development that will be analysed and discussed inthis paper. The extensional regime continued during the Triassicand led to the formation of a series of rift systems, with overall

NNW trend, formed along the western margin of Gondwana(Charrier, 1979; Uliana et al., 1989). One of these basins, the Cuyobasin, corresponds to a NNW-trending narrow basin parallel to theinferred suture zone between the Chilenia and Cuyania terranes(Fig. 1B).

The present morphology of the Precordillera and CordilleraFrontal is the result of Miocene to present Andean shorteningand uplift. The tectonic style of the western part of the Precordill-era and the eastern flank of the Cordillera Frontal is strongly influ-enced by pre-Andean paleogeographic features and structures,indicating that the resulting structural styles were predeterminedby conditions established in earlier tectonic events as was previ-ously pointed out by von Gosen (1995).

3.1. Analysis of Permo-Triassic faults

Field analysis consisted of detailed geological mapping andstructural analysis of the Potrerillos-Uspallata area (Fig. 2A). Wechose this area because it has preserved a great amount of Per-mo-Triassic structures, and it has only been affected by largehigh-angle Cenozoic faults, the movement of which can be easilyreconstructed. In this area, the volcanic rocks of the Choiyoi Groupand older rocks are affected by normal and oblique-slip normalfaults with WNW to NW trends. The great thickness and facies vari-ations of these volcanic rocks near these structures, and the rela-tionship between structures and the intrusion of plutonic rocksallow us to consider them as synmagmatic Permo-Triassic struc-tures. The two most prominent Permo-Triassic faults identified cor-respond to the Polcura and La Manga faults (Figs. 2A and 4). ThePolcura fault extends for more than 30 km, has a NW trend and avertical displacement of at least 500 m. Slickensides on exposed slipsurfaces indicate two sets of striae (Fig. 2B). The older set shows asinistral oblique-slip normal movement and the younger a sinistralstrike-slip one. The fault does not show constant strike trends, butvaries between 305� and 345� in a stair mode. The La Manga fault isparallel to the Polcura fault and merges toward this structure by aseries of en echelon strike-slip faults. A detailed analysis of itssoutheastern termination shows that the fault was reactivated dur-ing the Andean shortening. The La Polcura and La Manga faults,trending parallel but dipping one against the other, delineate a gra-ben structure that apparently controlled the localization of theandesitic breccias and the lithic ignimbrites of the middle andupper sections of the Choiyoi Group, respectively (Figs. 2 and 5).

We measured fault orientation and sense of displacement of thePermo-Triassic structures, the criteria of which include: offset of localhorizons, growth direction of mineral fibres (quartz-slickenfibres),

0

1

2

3

4

5

4km0

FaultPolcuraLa Manga

Fault

SW NEKm

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 normalfaults which controlled the deposition of breccias and ignimbrites of the Choiyoi Group.


secondary striated and tensile fractures, and associated steps,grooves on the fault surface, and drag folds. Our data set consistsof 72 measures on faults with sense of displacement indicators

n: 72

n: 19

n: 22

n: 31

A

B

C

D

Mean strike vector:

Mean strike vector:

Total population

Fig. 6. Equal-area stereographic projections of fault-slip data of the meso-scale area. n itrend and plunge of striae and relative movement of the hanging wall. We used the orienaxes using Richard Allmendinger’s FaultKinWin program. (A) Heterogeneous populatioHomogeneous data subset integrating faults with normal offsets. (C) Homogeneouscompressional faults.

(Fig. 6). Slips on individual faults are integrated to determine theprincipal directions of brittle strain within the area. The heteroge-neous population of fault-slip data (Fig. 6A) was divided into three

1

2

3

2

1

3

3

2

1

LINKED EINGHAM AXESEigenvalue Eigenvector (T&P)1. 0.4173 18.3º 3.5º2. 0.0031 288.5º 1.7º3. -.4204 173.0º 86.1º

LINKED EINGHAM AXESEigenvalue Eigenvector (T&P)1. 0.4597º 205.7º 2.8º2. -.0856º 295.9º 4.5º3. -.3741º 83.9º 84.8º

LINKED EINGHAM AXESEigenvalue Eigenvector (T&P)1. 0.2574º 182.6º 25.7º2. 0.1417º 352.2º 63.9º3. -.3992º 90.6º 4.1º

295º

316º

s the number of data. Great circle represents fault plane, ball and arrow representstation of P (black dots) and T axes (white squares) to estimate principal strain rate

n of all fault slip data obtained. The whole set was divided into three subsets. (B)data subset of faults with sinistral oblique-normal offset. and (D) Oblique-slip


homogeneous subsets on the basis of orientation and sense of dis-placement. The first population strikes WNW (�295�) and displaysnormal offsets with minor or no component of strike-slip (Fig. 6B).The second population corresponds to NW-trending oblique-slipnormal faults (�316�), with sinistral offset (Fig. 6C). Because An-dean deformation reactivated some of these structures in astrike-slip mode, we identified superposition of striations on faultplanes whenever possible. The third population corresponds tostrike-slip faults with few compressive dip–slip components andsinistral offsets (Fig. 6D). Some of these faults are cutting the An-dean dip–slip compressional structures, indicating that they areAndean in age.

With the assumption that the stress field was homogeneousduring deformation affecting the study area, and that faults didnot interact mechanically, we used the orientation of the shorten-ing and extension axes (P and T axes) to estimate principal direc-tions of brittle strain (Marrett and Allmendinger, 1990) whichprovide an approximate orientation of the principal strain rate axes(Twiss and Unruh, 1998). The kinematic axes of the two popula-tions of faults indicate that the Permo-Triassic deformational phasewas characterized by NNE–SSW oriented, subhorizontal extension

AB

B

C

a

e

A

A

B

c

Fig. 7. Normal and oblique-slip faulting contemporaneous with the extrusion of the Chdeposition of sedimentary rocks (A) and andesitic breccias (B). (b) Angular unconformityChoiyoi Group with the rhyodacitic lavas (C) of the middle part of the Choiyoi Group. NotNW-trending oblique-slip normal faults with thickness variations across them, indicatinChoiyoi Group but occurred before the extrusion of the rhyodacitic lavas. (c) Several WNandesitic lavas. (d) Normal and sinistral strike-slip normal faults contemporaneous withChoiyoi Group. Several small normal faults are constrained during the time of the effusionof the rhyodacitic ignimbrites (A) and lower part of the rhyodacitic lavas (B). The upperfaults developed during the extrusion of the lithic ignimbrites of the upper part of the C

direction and almost vertical shortening (Fig. 6B and C). The varia-tion in the orientation of the extension axis between the two do-mains is small (7�) and is not significant in terms of kinematicanalysis. The kinematic analysis of the third population showsgreat diversity. The reactivation of Permo-Triassic structures of dif-ferent orientations is interpreted to be the cause of this diversity.Its solution indicates strike-slip mode with a small component ofeast–west shortening. We combined the first and second popula-tions which are interpreted to reflect the same deformation eventand obtained the direction of 22.9� and 0.2� for the extension axis,suggesting that the NW- and WNW-trending structures are the re-sult of an oblique extensional regime.

4. Relative timing of the extension

In previous sections, we used kinematic and geometric studiesof faults developed synchronously with the Choiyoi rocks to provethe presence of an oblique extensional setting during Permo-Trias-sic times. In this section, we try to assign a relative time for theduration of this deformational event by: (i) relationships betweendifferent extensional structures; and (ii) relationships between the

A

B

Cb

d

f

A

A

BB

f

oiyoi volcanics. (a) Normal and oblique-slip NW-trending faults active during thebetween the sedimentary rocks (A) and andesitic lavas (B) of the lower part of the

e that the sedimentary rocks are affected both by WNW-trending normal faults andg that faulting was synchronic with the sedimentation and extrusion of the lowerW-trending normal faults with different relative times of movement related to thethe rhyodacitic ignimbrites (A) and rhyodacitic lavas (B) of the middle part of theof the ignimbrites. (e) WNW-trending normal fault active during the sedimentation

part of the rhyodacitic lavas (C) unconformably covers the fault. (f). Several normalhoiyoi Group.


Choiyoi stratigraphical units and extensional structures. As waspointed out above, the kinematic structural analysis suggests thatsinistral oblique-slip and normal faults evolved contemporane-ously under a NNE–SSW oriented maximum extension previousto the Andean horizontal shortening. No clear cross-cutting rela-tionships can be observed for faults of the first and second popula-tions, but the compartmentalization of smaller displacementWNW-trending faults between larger displacement NW-trendingfaults is consistent with a similar age.

Further evidence for the relative time of the oblique extensionaldeformation is derived from detailed stratigraphical studies of theChoiyoi rocks. These rocks show regional-scale lateral changes inthickness and facies which have a tectonic origin and sometimesproduce the total omission of the units, indicating syntectonicmagmatism. The complex relationship between faults and the dif-ferent units of the Choiyoi Group can be seen in Fig. 7. The oblique-slip and normal faults mark some of the contacts between sedi-mentary and volcanic rocks and are interpreted as being synerup-tive with: (i) sedimentary rocks and andesitic lavas and breccia ofthe lower section of the Choiyoi Group (Fig. 7a–c); (ii) rhyodaciticlavas and associated ignimbrites of the middle section (Fig. 7d ande); and (iii) rhyolitic ignimbrites of the upper section (Fig. 7f). Sim-ilar field observations have been carried out by von Gosen (1995)in the Uspallata area. This author studied several N to NW-trendingfaults associated with slump folds, breccia, and breccia horizonswithin the Permo-Triassic deposits and interpreted them as synse-dimentary normal faults. This allows us to postulate that the extru-sives and shallow plutonic rocks of the lower, middle and uppersections of the Choiyoi Group were generated under a constantNNE–SSW stretching direction.

5. Discussion: geodynamic model for the Permo-Triassicextension

In continental rift zones, much of the deformation from rifting iscontrolled by pre-existing 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 relatedto a reactivation of the upper crust discrete or pervasive fabric ora reworking of the lithospheric-scale volume of rocks which corre-sponds to the repeated focusing of metamorphism, deformationand magmatism at an orogenic scale (Holdsworth et al., 2001). Riftsystems tend to follow pre-existing orogenic belts and suturezones containing long-lived zones of mechanical weakness, whichcan be preserved in the lithospheric crust but also in the litho-spheric mantle, that undergo repeated deformation and failure(Tommasi and Vauchez, 2001). Mantle anisotropies, such as rheo-logical heterogeneities and mechanical anisotropies, developedduring orogenic events and subsequently frozen into the litho-spheric mantle, can survive for a long period of time and play a rolein preferential rift localization several hundred million years afterorogenesis (Vauchez et al., 1997, 1998). These large-scale mechan-ical anisotropies of the lithospheric mantle are due to the forma-tion of a pervasive crystallographic/tectonic fabric produced bythe preferred alignment of olivine crystals during major tectonicepisodes (Tommasi and Vauchez, 2001). The tectonic inheritancepermitted the systematic reactivation of ancient tectonic zonesby strain localization even if the extension direction is not normalto the structural trend of the belt (Vauchez et al., 1997). Moreover,major shear zones have been considered to represent importantcontrolling features in both magma ascent and pluton emplace-ment processes (Vigneresse, 1995; Rosenberg, 2004).

In the study area a pervasive fabric, related to schistosity oflower Paleozoic rocks, and a discrete fabrics, marked by upperPaleozoic faults and fault-related folds, have been identified. These

structural grains, developed during Early and Late Paleozoic defor-mational events respectively, strike N–S to NE–SW, suggesting thatthe development of the WNW- to NW-trending Permo-Triassicstructures was not influenced by the upper crust fabric. Instead,the parallelism between the NNW-trending Devonian deforma-tional zone, related to the inferred suture zone between the Chile-nia and Cuyania terranes, the Late Paleozoic San Rafael orogenicbelt and the Permo-Triassic Choiyoi Group outcrops suggests thepresence of a lithospheric weakness zone, which induces strainlocalization and guided lithospheric reworking during subsequentdeformational events.

In the tectonic model presented here we propose that the NNW-trending inferred suture zone and the Late Paleozoic orogenic beltacted as a lithospheric anisotropy which accounts for the genera-tion of an oblique extensional regime during Permo-Triassic timesand the presence of permeable structures which favored magmaascent. The reactivation of this pre-existing weakness has resultedin the generation of a new complex fault system under obliqueextensional conditions. The remarkable consistency between theNW- and WNW-trending orientations and their associated stressfield permits us to consider that these extensional structures werenewly created faults, the orientations of which were determined bythe regional stress field. This suggests that the fault array devel-oped synchronously as one largely heterogeneous system. Theseconclusions match those found for the Choiyoi Group in the SanRafael block (Fig. 1B) by Japas and Kleiman (2004), who proposeda sinistral oblique-slip regime during the evolution of the upperpart of the Choiyoi Group as a result of a NNE–SSW directedstretching direction.

6. Conclusions

Structural, stratigraphical and geochemical studies allow us topropose a consistent tectonic scheme for the development of anoblique extensional deformational event during Late Permian toEarly Triassic times in southwestern South America. During thistime, a NNW-trending deformational zone developed along apre-existing lithospheric anisotropy located along an inferred su-ture zone between the Chilenia and Cuyania terranes. This zonecorresponds to a dense network of normal and oblique-slip normalfaults along which the Permo-Triassic intraplate sinistral exten-sional movements, related to a NNE–SSW stretching (N23�E), wereaccommodated. It comprises NW-trending sinistral oblique-slipnormal faults and WNW-trending normal faults. Geometric, tem-poral and kinematic relationships indicate that these faults aregenetically linked to the formation of the Choiyoi volcanic rocks.

Acknowledgements

This research was supported by Grants from the University ofBuenos Aires (UBACYT TW87) and CONICET (PICT 07-10942). Wegratefully acknowledge the use of Richard Allmendinger’s Fault-KinWin program and Ernesto Cristallinís Estereografica GR pro-gram. Special thanks are due to Natalia Marchese, GuillermoPeralta, Cristian Guerra, Diego Márquez, Diego Di Carlo and Ezequ-iel García Morabito for their help in the field.

References

Aceñolaza, F.G., Miller, H., Toselli, A.J., 2002. Proterozoic-early Paleozoic evolution inwestern South America – a discussion. Tectonophysics 354, 121–137.

Azcuy, C.L., Caminos, R., 1987. Diastrofismo. In: Archangelsky, S. (Ed.), El sistemaCarbonífero en la República Argentina. Academia Nacional de Ciencias, Córdoba,pp. 239–251.

Butler, R.W.H., Holdsworth, R.E., Lloyd, G.E., 1997. The role of basement reactivationin continental deformation. Geological Society of London, Journal 154, 69–71.

Caminos, R., 1965. Geología de la vertiente oriental del Cordón del Plata, CordilleraFrontal de Mendoza. Asociación Geológica Argentina, Revista 20, 351–392.


Caminos, R., 1979. Cordillera Frontal. In: Turner, J.C.M. (Ed.), Segundo Simposio deGeología Regional Argentina, vol. 1. Academia Nacional de Ciencias, Córdoba,pp. 397–453.

Charrier, R., 1979. El Triásico de Chile y regiones adyacentes de Argentina: unareconstrucción paleogeográfica y paleoclimática. Comunicaciones 26, 1–37.

Chernicoff, C.J., Zappettini, E., 2003. Delimitación de los terrenostectonoestratigráficos de la región centro-austral argentina: evidenciasaeromagnéticas. Revista Geológica de Chile 30, 299–316.

Cingolani, C.A., Manassero, M., Abre, P., 2003. Composition, provenance and tectonicsetting of Ordovician siliciclastic rocks in the San Rafael block: southernextension of the Precordillera Crustal fragment, Argentina. Journal of SouthAmerican Earth Sciences , Special Issue Pacific Gondwana Margin 16, 91–106.

Coira, B., Nullo, F., Proserpio, P., Ramos, V., 1975. Tectónica de basamento de laregión occidental del Macizo Nordpatagónico. Asociación Geológica Argentina,Revista 30, 361–383.

Cortés, J.M., González Bonorino, G., Koukharsky, M., Pereyra, F., Brodtkorb, A., 1997.Hoja 3369-09, Uspallata. Dirección Nacional del Servicio Geológico,Subsecretaría de Minería de la Nación, Buenos Aires, p. 204.

Davis, J.S., Roeske, S.M., McClelland, W.C., Kay, S.M., 2000. Mafic and ultramaficcrustal fragments of the southwestern Precordillera terrane and their bearingon tectonic models of the early Paleozoic in western Argentina. Geology 28,171–174.

Dunbar, J.A., Sawyer, D.S., 1989. How preexisting weaknesses control the style ofcontinental breakup. Journal of Geophysical Research 94, 7278–7292.

Folguera, A., Giambiagi, L.B., 2002. Estructura de la Precordillera mendocina y sectororiental del Cordón del Plata, entre Potrerillos y Uspallata, 32�200 a 33�000 S. XVCongreso Geológico Argentino, Actas 3, Buenos Aires, pp. 176–180.

Folguera, A., Etcheverria, M., Pazos, P., Giambiagi, L.B., Cortés, J.M., Fauqué, L., Fusari,C., Rodriguez, M.F., 2001. Descripción de la Hoja Geológica Potrerillos(1:100.000). Subsecretaría de Minería de la Nación. Dirección Nacional delServicio Geológico, 262.

Franzese, J.R., Spalletti, L.A., 2001. Late Triassic-early Jurassic continental extensionin southwestern Gondwana: tectonic segmentation and pre-break-up rifting.Journal of South American Earth Sciences 14, 257–270.

Gosen, W. von, 1995. Polyphase structural evolution of the southwestern ArgentinePrecordillera. Journal of South American Earth Sciences 8, 377–404.

Gosen, W. von, Loske, W., 2004. Tectonic history of the Calcatapul Formation,Chubut province, Argentina, and the ‘‘Gastre fault system”. Journal of SouthAmerican Earth Sciences 18, 73–88.

Haller, M.J., Ramos, V.A., 1984. Las ofiolitas famatinianas (Eopaleozoico) de lasProvincias de San Juan y Mendoza. IX Congreso Geológico Argentino, Actas 2,San Luis, pp. 66–83..

Holdsworth, R.E., Butler, C.A., Roberts, A.M., 1997. The recognition of reactivationduring continental deformation. Journal of the Geological Society 154, 73–78.

Holdsworth, R.E., Hand, M., Miller, J.A., Buick, I.S. 2001. Continental reactivation andreworking: an introduction. In: Miller, J.A., Holdsworth, R.E., Buick, I.S., Hand, M.(Eds.), Continental Reactivation and Reworking, vol. 184. Geological Society ofLondon, Special Pubication 1–12.

Jacques, J.M., 2003. A tectonostratigraphic sintesis of the Sub-Andean basins:inferences on the position of South American intraplate accommodation zonesand their control on South Atlantic opening. Journal of the Geological Society160, 703–717.

Japas, M.S., Kleiman, L.E., 2004. El ciclo Choiyoi en el bloque de San Rafael(Mendoza): de la orogénesis tardía a la relajación mecánica. AsociaciónGeológica Argentina. Serie D: Publicación Especial 7, 89–100.

Kay, S.M., Ramos, V.A., Mpodozis, C., Sruoga, P., 1989. Late Paleozoic to Jurassic silicomagmatism at the Gondwanaland margin: analogy to the Middle Proterozoic inNorth America? Geology 17, 324–328.

Llambías, E., 1999. El magmatismo gondwánico durante el Paleozoico superior –Triásico. In: Caminos, R., Panza, J. (Eds.), Geología Argentina, vol. 29. ServicioGeológico Minero Argentino, Buenos Aires, Anales, pp. 373–376.

Llambías, E., Sato, A., 1995. El batolito de Colongüil: transición entre orogénesis yanorogénesis. Asociación Geológica Argentina. Revista 50, 111–131.

Llambías, E., Quenardelle, S., Montenegro, T., 2003. The Choiyoi Group from centralArgentina: a subalkaline transitional to alkaline association in the cratonadjacent to the active margin of the Gondwana continent. Journal of SouthAmerican Earth Sciences 16, 243–257.

Marrett, R., Allmendinger, R.W., 1990. Kinematic análisis of fault-slip data. Journalof Structural Geology 12, 973–986.

Martinez, A.N., 2005. Secuencias volcánicas Permo-Triásicas de los cordones delPortillo y del Plata, Cordillera Frontal, Mendoza: su interpretación tectónica.Unpublished PhD Thesis, Universidad de Buenos Aires, Buenos Aires, p. 275.

Masuda, A., Nakamura, N., Tanaka, T., 1973. Fine structures of mutually normalizedrare-earth patterns of chondrites. Geochemical and Cosmochemical Acta 37,239–248.

Mpodozis, C., Kay, S.M., 1990. Provincias magmáticas ácidas y evolución tectónicade Gondwana: andes Chilenos (28�S–31�S). Revista Geológica de Chile 17, 153–180.

Mpodozis, C., Kay, S.M., 1992. Late Paleozoic to Triassic evolution of the Gondwanamargin: evidence from Chielan Frontal Cordilleran batholiths (28�S to 31�S).Geological Society of America Bulletin 104, 999–1014.

Polanski, J., 1972. Descripción geológica de la Hoja 24 a-b – Cerro Tupungato,provincia de Mendoza. Dirección Nacional de Geología y Minería, Buenos Aires,Boletín 128, pp. 1–110.

Ramos, V.A., 1988. The tectonics of the Central Andes: 30� to 33�S latitude. In: Clark,S., Burchfiel, D. (Ed.), Processes in Continental Lithospheric Deformation.Geological Society of America, Special Paper 18, pp. 31–54.

Ramos, V.A., 2004. Cuyania, an exotic block to Gondwana: review of a historicalsuccess and the present problems. Gondwana Research 7, 1009–1026.

Ramos, V., Jordan, T., Allmendinger, R., Mpodozis, C., Kay, R., Cortés, J., Palma, M.,1986. Paleozoic terranes of the Central Argentine–Chilean Andes. Tectonics 5,855–880.

Rapalini, A.E., Vilas, J.F., 1991. Tectonic rotations in the Late Palaeozoic continentalmargin of southern South America determined and dated by palaeomegnetism.Geophysical Journal International 107, 333–351.

Rapela, C.W., Pankhurst, R.J., 1992. The granites of northern Patagonia and theGastre Fault System in relation to the break-up of Gondwana. In: Storey, B.C.,Alabaster, T., Pankhurst, R.J., (Eds.), Magmatism and the Causes of ContinentalBreak-Up. Geological Society of London, Special Publication 68, pp. 209–220.

Rapela, C.W., Días, G.F., Franzese, J.R., Alonso, G., Benvenuto, A.R., 1991. El batolitode la Patagonia central: evidencias de un magmatismo triásico-jurásicoasociado a fallas transcurrentes. Revista Geológica de Chile 18, 121–138.

Rolleri, E.O., Criado Roque, P., 1969. Geología de la provincia de Mendoza. 4�Jornadas Geológicas Argentinas 2, 1–60.

Rosenberg, C.L., 2004. Shear zones and magma ascent: a model based on a review ofthe Tertiary magmatism in the Alps. Tectonics 23, TC2003. doi:10.1029/2003TC001526.

Sato, A.M., Llambías, E.J., Shaw, S.E., Castro, C.E., 1990. El batolito de Colangüil:modelo del magmatismo neopaleozoico de la provincia de San Juan. XICongreso Geológico Argentino, San Juan, Actas, Relatorio, pp. 100–122.

Stipanicic, P.N., Rodrigo, F., Baulies, O.L., Matinez, C.G., 1968. Las formacionespresenonianas en el denominado Macizo Nordpatagónico y regionesadyacentes. Revista de la Asociación Geológica Argentina 23, 76–98.

Tickyj, H., Dimieri, L.V., Llambías, E.J., Sato, A.M., 1997. Cerro de Los Viejos (38�280S–64�260O): cizallamiento dúctil en el sudeste de La Pampa. Asociación GeológicaArgentina. Revista 52, 311–321.

Tommasi, A., Vauchez, A., 2001. Continental rifting parallel to ancient collisionalbelts: an effect of the mechanical anisotropy of the lithospheric mantle. Earthand Planetary Science Letters 185, 199–210.

Twiss, R.J., Unruh, J.R., 1998. Analysis of fault slip inversions: do they constrainstress or strain rate? Journal of Geophysical Research 103, 12,205–12,222.

Uliana, M.A., Biddle, K., Cerdán, J., 1989. Mesozoic extension and the formation ofArgentine sedimentary basins. In: Margins, Tankard, A., Balkwill, H.R. (Eds.),Extensional Tectonics and Stratigraphy of the North Atlantic, vol. 46. AmericanAssociation of Petroleum Geologists, Memoir, pp. 599–614.

Urien, C.M., Zambrano, J.J., Irigoyen, M.R., 1995. Petroleum basins of southern SouthAmerica: an overview. In: Tankard, A.J., Suárez, R., Welsink, H.J. (Eds.),Petroleum basins of South America, vol. 62. American Association ofPetroleum Geologists, Memoir, pp. 63–77.

Vauchez, A., Barruol, G., Tommasi, A., 1997. Why do continents break-up parallel toancient orogenic belts? Terra Nova 9, 62–66.

Vauchez, A., Tommasi, A., Barruol, G., 1998. Rheological heterogeneity, mechanicalanisotropy and deformation of the continental lithosphere. Tectonophysics 296,61–86.

Vigneresse, J.L., 1995. Control of granite emplacement by regional deformation.Tectonophysics 249, 173–186.

Visser, J.N.J., Praekelt, H.E., 1998. Late Paleozoic crustal block rotations within theGondwana sector of Pangea. Tectonophysics 287, 201–212.

http://dx.doi.org/10.1029/2003TC001526http://dx.doi.org/10.1029/2003TC001526

Permo-Triassic oblique extension in the Potrerillos-Uspallata area, western ArgentinaIntroductionGeological settingChoiyoi GroupGroup

Regional structuresAnalysis of Permo-triassic Permo-Triassic faults

Relative timing of the extensionDiscussion: geodynamic model for the Permo-Triassic extensionConclusionsAcknowledgementsReferences

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