Active Tectonics in the Argentine Precordille- Ra and Western Sierras Pampeanas

604 Revista de la Asociación Geológica Argentina 61 (4): 604-619 (2006)

ACTIVE TECTONICS IN THE ARGENTINE PRECORDILLE- RA AND WESTERN SIERRAS PAMPEANAS

Lionel L SIAME. 1*, Olivier BELLIER1 and Michel SEBRIER2

¹ Université Paul Cézanne (Aix-Marseille III) - CEREGE, Europôle de l'Arbois, B.P. 80, 13545 Aix-en-Provence Cedex 4, France. 2 Université Pierre et Marie Curie, Laboratoire de Tectonique, 4 Place Jussieu, 75005 Paris, France. * Corresponding author: [email protected]

ABSTRACT The Andean foreland of western Argentina (28°S-33°S) corresponds to retroarc deformations associated with the ongoing flat subduction of the Nazca plate beneath the South American lithosphere. This region is characterized by high levels of seismic activity and crustal active faulting. To improve earthquake source identification and characterization in the San Juan region, data from seismology, structural geology and quantitative geomorphology were integrated and combined to provide a seismotectonic model. In this seismotectonic model, the Andean back-arc of western Argentina can be regarded as an obliquely converging foreland where Plio-Quaternary deformations are partitioned between strike-slip and thrust motions that are localized on the E-verging, thin-skinned Argentine Precordillera, and the W-verging thick-skinned Sierras Pampeanas, respectively. In this seismotectonic model, the Sierra Pie de Palo appears to be a key structure playing a major role in the partitioning of the Plio-Quaternary deformations.

Keywords: Seismotectonics, cosmic ray exposure dating, active faulting.

RESUMEN: Tectónica activa en la Precordillera argentina y las Sierras Pampeanas occidentales. El antepais andino del centro-oeste de Argentina (28°S-33°S) está caracterizado por deformaciones asociadas a la subducción horizontal activa de la placa de Nazca debajo de la litósfera de la placa Sudamericana. En esta región se concentra una importante actividad sísmica y fallamiento cortical activo. Para mejorar la identificación y caracterización de las fuentes sismogénicas en la región de San Juan, fueron integrados y combinados datos de sismología, geología estructural y geomorfología cuantitativa para establecer un modelo sismotectónico. El mismo considera al retroarco del oeste argentino como un antearco con convergencia oblicua donde las deformaciones plio-cuaternarias son particionadas en movimientos compresivos y de transcurrencia dextral. Dichos movimientos están localizados respectivamente en la Precordillera, una faja de pliegues y escurrimientos de cobertura con vergencia hacia el este, y en las Sierras Pampeanas, una faja de fallas inversas de basamento con vergencia hacia el oeste. Según este modelo sismotectónico, la sierra de Pie de Palo corresponde a una estructura mayor que acomoda la partición de las deformaciones plio-cuaternarias.

Palabras clave: sismotectónica, edades de exposición por rayos cósmicos, fallas activas.

INTRODUCTION are related to reverse compressional faul- gime rely on inversion of fault plane data ting. Nevertheless, because earthquakes on (deviatoric stress) and slip rates derived The Andean foreland of western Argentina active reverse faults are not always accom- from geological data as well as cosmic ray (Fig. 1) of San Juan and Mendoza is among panied with surface ruptures, a paleoseis- exposure dating along selected active faults the most seismically active regions in the mological estimate of their associated seis- (e.g., Siame et al. 1997a,b, 2002). From the world. Indeed, many small to moderate mic hazard parameters is a difficult task. comparison of both Present-day and Neo- (M<6.4) and several large (M>6.4) earth- To improve earthquake source identifica- gene-Quaternary tectonic regimes, the An- quakes have occurred during the last cen- tion and characterization in the San Juan re- dean back-arc of western Argentina can be tury, most notably the destructive Ms = 7.4, gion, data from seismology, structural geo- regarded as an obliquely converging fore- 1944 San Juan earthquake (Instituto Nacio- logy and quantitative geomorphology were land where Plio-Quaternary deformations nal de Prevención Sísmica INPRÉS 1977, integrated and combined to provide a seis- are partitioned between strike-slip and Alvarado and Beck 2006) and Mw = 7.4, motectonic model (Siame et al. 2005). In thrust motions that are localized on the E- 1977, Caucete earthquake (Kadinsky-Cade this seismotectonic model, the Present-day verging thin-skinned Argentine Precordi- et al. 1985; Langer and Bollinger 1988) tectonic regime is based from inversion llera and the W-verging thick-skinned Sie- events. This seismic activity is produced by (deviatoric stress) and moment tensor sums rras Pampeanas, respectively (Siame et al. a number of crustal active faults and may (strain and seismic rate of deformation) of 2005). In this seismotectonic model, the result from the reactivation of ancient sutu- focal mechanism solutions of moderate to Sierra Pie de Palo appears to be a key struc- res between Paleozoic terranes (Smalley et large earthquakes (M = 5.0) localized along ture playing a major role in the Plio- al. 1993; Alvarado et al. 2005). In the San the Chilean margin and in the Andean fore- Quaternary deformations within the An- Juan region, most of the active structures land. The Neogene-Quaternary tectonic re- dean back-arc at about 31°S.

0004-4822/02 $00.00 + $00.50 C 2006 Revista de la Asociación Geológica Argentina Active tectonics in the Argentine Precordillera and Western Sierras Pampeanas 605

REGIONAL GEODYNAMIC for roughly 300 km at a depth of about 100 Ma ago (Kay and Abbruzzi 1996). Enhan- SETTING km (Cahilll and Isacks 1992; Smalley et al. ced plate coupling above a flat subduction 1993). Coinciding with this change of the has long been suggested to be responsible Along the Andes, variations in tectonic style subducted Nazca plate dip (Jordan et al. for the large-scale, thick-skinned Sierras coincide with changes in the geometry of 1983, Cahill and Isacks 1992), the Andean Pampeanas basement uplifts (Jordan et al. the subducted Nazca plate (e.g., Ramos foreland domain is characterized by two 1983, Smalley et al. 1993). On the other 1999 and references herein) (Fig. 1). A- opposite verging, compressional structural hand, thermal weakness of the crust asso- mong those variations, high levels of crus- provinces coexisting side-by-side: the E- ciated with eastward migration of arc mag- tal seismicity characterize the retroarc of verging, thin-skinned Argentine Precordi- matism as also been suggested to be res- the South American plate above flat-slab lera fold-and-thrust belt, and the W-ver- ponsible for thick-skinned basement uplift segments, seismic moment releases being ging, thick-skinned Sierras Pampeanas u- of the Sierras Pampeanas (Ramos et al. generally 3 or 5 times greater than above plifted basement blocks (Fig. 2). In this An- 2002). 30° dipping segments (Gutscher et al. 2000). dean region, the flat slab geometry is attri- Along Chilean margin (Fig. 1) and within The Andean foreland of western Argentina buted to the subduction of the Juan Fer- the foreland of San Juan (Fig. 2), the Pre- (28°S-33°S) corresponds to such back-arc nandez Ridge below the South American sent-day tectonic regime has been investiga- deformation associated with the ongoing margin (Pilger 1981), and expressed at the ted thanks to inversions (deviatoric stress) subduction of the Pampean flat-slab seg- magmatic arc by a change in the geoche- and moment tensor sums (strain and seis- ment which proceeds nearly horizontally mistry properties of the Neogene volca- mic rates of deformation) of focal mecha- beneath the South American lithosphere nism and a cessation of activity roughly 10 nism solutions (Siame et al. 2005). This

Figure 1: Regional geodynamic setting. a) Map of the central Andes between 19°S and 36°S latitude (Globe shaded relief and bathymetry using Hastings and Dunbar (1998) and Smith and Sandwell (1997), respectively). Black cross represent the epicenter distribution of large to moderate earthquakes (U.S. Geological Survey, National Earthquake Information Center; Preliminary Determination Epicenter catalog, 1973-Present, depth < 70km, M >5.0), Open triangles correspond to Plio-Quaternary and active volcanoes. Black and white arrows represent the relative convergence of the Nazca and South-American plates following NUVEL-1 (DeMets et al. 1990, 1994) and continuous GPS observation (Kendrick et al. 1999; 2003). Numbers are given in mm/yr. b) Map showing the focal mechanism solutions available in this Andean region from Harvard catalog (1977-2003, depth<70km, M>5.0). Boxes locate the different zones where inversion and moment tensor sum have been applied along the Chilean trench (e.g., Siame et al. 2005). c) Lower-hemisphere stereoplots (Wulf) and synthetic focal solutions (Schmidt) showing inversion and moment tensor sum results, respectively. Histograms show distribution of deviation angles between the selected 'σ' and the predicted 't' slip-vector on each preferred seismic plane. Stress axes obtained by inversions are shown by diamonds (s1), triangles (s2) and squares (s3). Strain axes obtained by summations are shown by squares (P), and diamonds (T). d) AA' and BB' are cross-sections showing the depth distribution of relocated earthquakes (after Engdahl et al. 1998). Keys: PFS, Pampean flat-slab segment; JFR, Juan Fernandez Ridge. 606 L.L.SIAME, O. BELLIER AND M. SEBRIER

Figure 2: a) Map of San Juan region (shaded relief from SRTM90 digital topography; e.g., Rosen et al. 2000; Far and Kobrick, 2000) showing 1973- 2003 crustal seismicity from NEIC preliminary determination epicentres and locating cross-section. b) Cross-section (FF') showing shallow (solid circles) and intermediate-depth (open circles) earthquakes (modified after Smalley et al. 1993). c) Map of the San Juan foreland domain showing the location of the teleseismic (large grey beachballs; Harvard CMT catalogue) and local (small black beachballs; Régnier et al. 1992) focal mechanism solutions used in this study. Black numbers indicate Harvard CMT magnitudes. d) Synthetic focal solution of the moment tensor sum using the Harvard CMT solution (after Siame et al. 2005). e) Lower-hemisphere stereoplots (Wulf) showing inversion results for the teleseismic dataset (after Siame et al. 2005). F - Lower-hemisphere stereoplots (Wulf) showing inversion results for the local dataset (data after Régnier et al. 1992; inversion after Siame et al. 2005). Histograms show distribution of deviation angles between the selected 's' and the predicted 't' slip-vector on each preferred seismic plane. Stress axes obtained by inversions are shown by diamonds (s1), triangles (s2) and squares (s3). study raised the following points: (1) the derate to large earthquakes localized betwe- tains (Kendrick et al. 2003, Brook et al. slight convergence obliquity at the plate en 22 and 36°S shows that the amount of 2003), it seems that the clockwise rotation boundary is accommodated by the subduc- shortening is about the same (2-4 cm/yr) of the stress and strain axes respect to the tion zone itself, precluding any deformation between the fore-arcs of the 30° dipping plate convergence, evidenced from both partitioning at a lithospheric scale; (2), slab segments and the back-arc of the flat teleseismic and local focal solutions, may clockwise rotation of s1-axes and P-axes segment. Interestingly, the trench segment also be seen despite the elastic loading of within the Andean foreland of western corresponding to flat subduction is charac- the upper plate. Whether the strain partitio- Argentina suggest that deformation parti- terized by a much lower seismic moment ning suggested by both seismic data and tioning may occur in the upper plate at a release during the last 30 years. When com- GPS-derived velocity field is recorded at a crustal scale, (3) comparing seismic rates of pared to the GPS-derived velocity field much longer time scale than the last 30 shallow deformations determined from mo- (elastic strain field) for the Andean Moun- years has been investigated through the Active tectonics in the Argentine Precordillera and Western Sierras Pampeanas 607

Figure 3: Structural regional map of Argenti- ne Precordillera and western Sierras Pampeanas from Spot and Landsat image analysis (modified after Siame et al. 2005). It shows main structural sub-provinces and locates the major regional structures (dotted lines refer to either inferred or buried faults). (top) Schematic regional cross-section (CC') at about 31°30'S latitude showing thick-skinned deformation front: Western Precordillera: modified after Cristallini and Ramos (2000); Central Precor- dillera: modified after von Gosen (1992); Eastern Precordillera: Zapata and Allmendinger (1996); Siame et al. (2002); Structure at depth below Sierra Pie de Palo is modified according to Ramos et al. (2002) with seismicity after Régnier et al. (1992). Keys: WAP, Western Argentine Precordillera; CAP, Central Argentine Precordillera; EAP, Eastern Argentine Pre- cordillera; ETF, El Ti- gre Fault; FR, Francia ramp; MB, Matagusanos Basin; UB, Ullúm Basin; NPF: North Pie de Palo Fault; TVF, Tapias-Villicúm Fault; LF, La Laja Fault; RF, Rinconada Fault; CF, Las Chacras Fault; SBF, Salinas-Berros Fault; ANF, Ampacama-Niquizanga Fault (e.g., Siame 1998, Costa et al. 2000, Siame et al. 2002). Shaded relief from SRTM90 digital topography (e.g., Rosen et al. 2000; Far and Kobrick 2000). 608 L.L.SIAME, O. BELLIER AND M. SEBRIER

analysis of the Neogene to Quaternary tec- the Cordillera Frontal has apparently beha- (Ortiz and Zambrano 1981, Baldis et al. tonic regimes (see section 4). ved as relatively rigid blocks, disrupted by 1982). Nevertheless, since it is thrusted wes- high-angle reverse faults. Allmendinger et al. twardly from the craton thanks to faults STRUCTURAL OVERVIEW (1990) suggested that, between 29° and that reach mid-crustal depths, it can be OF THE ANDEAN FORE- 31°S latitude, the Cordillera Frontal is uplif- attributed to the Sierras Pampeanas domain LAND OF SAN JUAN AND ted as a ramp-anticline over a mid-crustal (Zapata and Allmendinger 1996, Jordan et MENDOZA (29°S-33°S) décollement. al. 2001, Siame et al. 2002). The Central and The 30 km-wide Calingasta-Iglesia Valley is Eastern Precordillera thus form oppositely Between 29°S and 33°S latitude, the An- a piggy-back basin partly filled with Mio- verging thrust systems on the western and dean foreland of San Juan and Mendoza Pliocene sediments and andesites, and eastern side of the Ullúm-Matagusanos- Provinces is characterized by three major Quaternary deposits (Beer et al. 1990). The Huaco valleys (Fig. 3). Conversely to the N-trending mountainous ranges: The Cor- Quaternary sediments are mainly related Western and Central Precordillera, the dillera Principal, the Cordillera Frontal, the to Pleistocene to Holocene alluvial fans Eastern Precordillera exhibits a drastic a- Argentine Precordillera, and the Sierras (Siame et al. 1997a). In this basin, the conti- long-strike change of structural and geo- Pampeanas (Fig. 3). The Cordillera Prin- nental strata deposited from ~16 to at least morphic style at about 31°S latitude (Fig. 3). cipal was the first morphostructural unit 6 Ma (Beer et al. 1990), while crustal shorte- North of this latitude, the Eastern Precor- developed after the Farallones plate break- ning was occurring in the Cordillera Frontal dillera is characterized by thick-skinned, up and is closely related to the shifting of and the Precordillera. This basin has been blind thrusts and folds developed within the Andean orogenic front ever since. The interpreted to have been passively transpor- the Neogene strata of the Bermejo valley associated mountain building processes ted above a horizontal décollement that (Zapata and Allmendinger 1996), whereas it ended while the Andean shortening was links mid-crustal deformation beneath the is marked by thick-skinned emergent taking place in the Cordillera Frontal and Cordillera Frontal with the thin-skinned thrusts carrying mostly Paleozoic rocks to

Precordillera. The Precordillera mountain tectonics in the Precordillera (Beer et al. the south (Smalley et al. 1993, Siame et al . belt, which is nearly 400 km-long and 1990). In the Iglesia valley, the El Tigre 2002). The location of this change in the roughly 80 km-wide, is a thrust-and-fold Fault is a 120 km-long right-lateral strike- structural style of the Eastern Precordillera belt separated from the Cordillera Frontal slip fault along which geomorphic features coincides with the north Pie de Palo Fault by an N-S piggyback basin: the Calingasta- (fan and river offsets, sagponds) and surfa- (Fig. 3). In fact, it is most probably related Iglesia Valley (Fig. 3). To the east, the ce disruptions provide evidences for its to a major change in the basement depth Precordillera is flanked by the Bermejo Quaternary activity (Bastias 1985, Siame et from 8 to 17 km across the southern mar- foreland basin that separates the Precordi- al. 1997b, Costa et al. 2000 and references gin of the Bermejo basin (Jordan and llera thrust-belt from the Sierras Pampea- herein). Allmendinger 1986, Smalley et al. 1993). nas, a structural province characterized by Between 30° and 32°S, the Precordillera is South of 31°S latitude, the Eastern Precor- ranges uplifted above moderate to highly- classically divided into three structural dillera is bounded by one regional active dipping, mostly west-verging reverse faults provinces: Western, Central, and Eastern thrust, the Villicúm-Pedernal thrust (Costa that root deeply into the Precambrian base- (Ortiz and Zambrano 1981, Baldis et al. et al. 2000 and references herein; Siame et al. ment rocks (Ramos et al. 2002). The struc- 1982). Western and Central Precordillera 2002). This thrust system corresponds to a tural evolution of this region reflects the correspond to a mountainous topography 145 km-long, N20°E-trending fault that regional convergence between the Nazca predominantly characterized by a landscape runs on the western piedmont of the and South America plates, which led to the of linear ranges and basins (Fig. 3). Reverse Eastern Precordillera bounding Sierra de formation of the east-verging thin-skinned thrust faults bound Paleozoic ranges and Villicúm, Sierra Chica de Zonda and Sierra Precordillera and of the west-verging thick- narrow linear valleys filled with Neogene de Pedernal (Fig. 3). It is characterized by a skinned Sierras Pampeanas, forming two and Quaternary continental sediments (von surface fault trace that clearly displaces the opposite verging structural systems (Figs. 3 Gosen 1992, Jordan et al. 1993). Western Quaternary deposit surfaces (Siame et al. and 4) (Jordan et al. 1993, Zapata and and Central AP have been described as a 2002). Allmendinger 1996). typical thin-skinned thrust-and-fold belt The Bermejo basin is a Miocene to recent The Cordillera Frontal is the major Andean due to Neogene crustal shortening on west- foreland basin formed during the develop- topographic feature in the studied region, dipping, imbricated structures that root ment of the Andes and Sierras Pampeanas and reaches elevation higher than 5000 m. down to a 10-15 km deep main décollement mountains (Jordan et al. 2001). It developed This mountain belt is mainly composed of (Allmendinger et al. 1990, von Gosen 1992, firstly between 20 and approximately 7.3 a thick Carboniferous to Triassic volcanic Jordan et al. 1993, Cristallini and Ramos Ma as a simple foreland basin adjacent to rocks and batholiths (Llambias and Cami- 1995) (Fig. 2). Formerly, the Eastern Pre- the advancing thrusts of the Western and nos 1987), as well as significant Lower and cordillera was grouped in the Precordillera Central Precordillera (Jordan et al. 2001). Middle Miocene volcanic rocks (Ramos et because of its geographic proximity, and Then after a short period (<1 Myr) of sym- al. 1986). During the Andean deformation, mostly because Paleozoic characteristics metrical foreland, it developed during the Active tectonics in the Argentine Precordillera and Western Sierras Pampeanas 609

Figure 4: a) Frequency- grid derived from the PDE catalog (100x112, 4-km-large cells) together with a map of the principal tectonic structures (dashed lines). Quaternary structures are mapped after Siame et al. (1998) and Costa et al. (2000) (black hea- vy lines). Open circles show the epicenter distribution of relocated seismic event (Engdhal and Villaseñor 2002). b) 3D-view of the frequency-grid derived from the PDE catalog showing the high-peak of frequency centered on the Sierra Pie de Pa- lo, and two other peaks of moderate frequency centered on San Juan and Mendoza cities. c) Topographic cross-section (GG') together with a plot of the frequency-grid at about 31°S. Keys: BV, Bermejo valley; VF, Valle Fértil range; H, La Huerta range; TV, Tulum valley; MV, Matagusanos valley; PdP, Sierra Pie de Palo; FC, Cordillera Frontal; CI, Calingasta-Iglesia basin; WAP, Western Argentine Precordillera; CAP, Central Argentine Precordillera; EAP, Eastern Argentine Precordillera. last 6.5 Ma as an asymmetrical foreland The Sierras Pampeanas are approximately dipping back sides (Fig. 3). The topographi- associated with both thin-skinned thrusting N-trending basement block uplifts of Pre- cal back side corresponds to a Late Palaeo- in the Central Precordillera and thick-skin- cambrian metamorphic rocks with topogra- zoic erosional basement surface (Carignano ned tectonics in the Pampean basement phic characteristics that consist of steep et al. 1999). The basement blocks are sepa- (Jordan et al. 2001). sloped sides on the faulted front and gently rated by wide, shallow basins of unmeta - 610 L.L.SIAME, O. BELLIER AND M. SEBRIER

morphosed, relatively undeformed Carbo- km. From September 1987 to May 1988, basins. Some individual sites of measure- niferous and younger sediments (Salfity and Régnier et al. (1992) operated a digitally ments are also located on active faults Gorustovitch 1984). recording seismic network that provided a where the computed states of stress can be detailed view of the crustal seismicity in the regarded as significant for recent stress regi- PRESENT-DAY SEISMOTEC- area (with a maximum activity at about 25 mes. The results of these quantitative inver- TONIC REGIME km depth). Even if both seismicity patterns sions of fault slip-vectors have been pre- observed from teleseismic and local data sented in details in papers by Siame et al. Within the Andean foreland of western are quite similar, teleseismic data are cen- (2002, 2005). Argentina, earthquake activity is characteri- tred on the low-lying valley between Sierra Even if, at individual sites, some of the zed by two depth distributions (Fig. 2). Pie de Palo and Sierra de la Huerta, where- observed fault-slip data can be related to Earthquakes with hypocentral depths at as local data cluster below Sierra Pie de Palo earlier stress states (e.g., Triassic extension about 100 km show flat slab geometry of (Fig. 2). This slight difference in the epicen- in the Sierras Pampeanas), chronological re- the Nazca plate beneath South-America tre location is most probably due to syste- lationships allowed determining the most (Cahill and Isacks 1992, Smalley et al. 1993, matic differences in location obtained for recent set of fault slip-vectors to determine Engdahl et al. 1998), whereas earthquakes the two datasets. Best-fitting deviatoric the most recent states of stress which are with hypocentral depth ranging from 5 to stress tensors from inversions of both tele- discussed. The stress tensors determined 35 km correspond to crustal seismicity bet- seismic and local datasets are consistent using fault-slip measurements within Pa- ween the Argentine Precordillera and the with s1-axes striking N95 ± 2°E (Fig. 2). laeozoic and Mesozoic rocks from the Pre- Sierras Pampeanas (Smalley et al. 1993). In Moment tensor sum of the CMT solutions cordillera and Sierras Pampeanas are remar- this region, the crustal seismicity pattern provide consistent results with P-axes stri- kably consistent with the stress tensors derived from preliminary determination e- king N93°E (Fig. 2), parallel to the compu- deduced from inversion of fault-slip data picentre catalogue (USGS/NEIC) is domi- ted s1-axes. Both inversion and moment collected within the Neogene and Quater- nated by a cluster of events centred on the tensor sums indicates that s1-axes and P- nary sediments. Compared at a regional sca- Sierra Pie de Palo (Fig. 2). To efficiently axes are roughly orthogonal to the strike of le, the stress regimes determined at indivi- interpret this pattern, we applied a gridding the reverse faults that borders the Sierra Pie dual sites yield a regional stress pattern that algorithm to the seismic event frequency de Palo basement uplift, and consequently is in close agreement with the stress tensors distribution (Siame et al. 2005). The resul- slightly rotated clockwise with respect to determined using local and teleseismic focal ting map not only takes into account the the Nazca vs. South American plates con- mechanism solutions, as well as with the dense N-trending ellipsoid-shaped cluste- vergence trend (Fig. 2). When compared to elastic strain field deduced from the GPS- ring of events centred on the eastern side the GPS-derived velocity field for the derived velocity field (e.g., Brooks et al. of the Sierra Pie de Palo, but also clearly Andean Mountains (Kendrick et al. 1999, 2003). unveils two others areas of moderate fre- Kendrick et al. 2003, Brooks et al. 2003), it In the Precordillera, Paleozoic to Quater- quency located near San Juan and Mendoza seems that this clockwise rotation of the nary strata are involved in the deformation, cities (Fig. 4). Around San Juan city, this stress axes does not only result from local which is related to the Andean (<20 Myr) zone of moderate frequency corresponds boundary effects within the Sierras Pam- structuration of the region. Since the to the Tulum and Matagusanos valleys, peanas, as already suggested by Régnier et Precordillera developed eastwardly during where thick-skinned active faulting has al. (1992), but may have a more regional sig- the last 20 Ma, the fault-slip sites determi- been suspected to be responsible for the Ms nificance (Siame et al. 2005). ned for the Western Precordillera are most 7.4, 1944 San Juan earthquake (Smalley et al. probably older than those calculated for the 1993, Siame et al. 2002). The rest of the NEOGENE TO QUATER- Central, Eastern Argentine Precordilleras Precordillera is relatively aseismic, particu- NARY TECTONIC REGIME and Sierras Pampeanas. In the Western and larly north of the north Sierra Pie de Palo Central Precordillera, where the thrust STRESS REGIME FROM INVER- Fault, which is in agreement with epicentre mainly developed between 20 and 5 Ma distributions from local seismic networks SIONS OF FAULT SLIP-STRIAE (Jordan et al. 2001), the stress regime is (Régnier et al. 1992, Smalley et al. 1993), dominated by s1-axes striking parallel to relocated teleseismic events (Engdahl and In order to determine the most recent sta- that determined for the Chilean trench. In Villaseñor 2002), and with the location of tes of stress within the Precordillera and the westernmost Sierras Pampeanas and the regional active faults (Siame et al. 2005). westernmost Sierras Pampeanas, quantitati- Eastern Precordillera, where the thrust Within the Andean foreland of San Juan, ve inversions of fault slip-vectors (striae) initiated between 5 and 2.5 Myr, the stress earthquakes with Mw > 4.9 provide eviden- measured at individual in Paleozoic rocks regimes are dominated by N110°E-trending ce for predominantly reverse to strike-slip from the Precordillera, in Mesozoic rocks s1-axes south of 31°S, whereas it is striking faulting (Fig. 2), mostly indicate E-trending from the Sierras Pampeanas, and in Neo- parallel to that determined for the Chilean shortening on ~40°-dipping faults, and ha- gene to Quaternary continental deposits trench north of 31°S (Fig. 5). When com- ve their source depths ranging from 5 to 35 that fill the inter-mountainous and foreland pared to the stress regime pattern to the Active tectonics in the Argentine Precordillera and Western Sierras Pampeanas 611

Figure 5: Structural regional map of Argentine Precordillera and western Sierras Pampeanas showing the inversion results of fault kinematic data measured at individual sites (after Siame et al. 2005). Black arrows show compressional directions. Keys: Q, Quaternary; Pli, Pliocene; Mi, Miocene; Te, Tertiary; Tr, Trias; Pz, Paleozoic. White arrow (compressional direction) on Pie de Palo shows the inversion result for focal mechanism solutions. White arrow (compressional direction) on Sierra Morada show the compressional direction inferred from regional observations. Upper inset depicts the Plio-Quaternary strain partitioning model for the San Juan foreland domain, lower inset shows the GPS-derived velocity field (modified after Brook et al. 2003).

GPS-derived velocity field proposed by ved velocity vectors strike in a direction pa- clockwise rotation of s1-axes evidenced Brook et al. (2003), the stress regime pattern rallel to that of the plate convergence (Fig. from both focal mechanism (Present-day) derived from fault slip-vector data in the 5). Farther south, the comparison is not so and fault slip-vector (long-term) datasets Eastern Precordillera and westernmost conspicuous with GPS-derived velocity may also be seen in the GPS-derived velo- Sierras Pampeanas are in relative agree- vectors striking E-W (Fig. 5). Nevertheless, city field despite the upper plate elastic loa- ment. Indeed, the northern part of the San GPS-derived velocity vectors seem to be ding (Fig. 5). Juan foreland both s1-axes and GPS-deri- also clockwise rotated, suggesting that the 612 L.L.SIAME, O. BELLIER AND M. SEBRIER

GEOLOGICAL RATES OF DEFOR- methods have opened new prospects for Fault. On the basis of the fault trace geo- MATION long term surface processes analysis (e.g., metry, Siame et al. (1997b) proposed to divi- Previous study (e.g., Allmendinger et al. Gosse and Phillips 2001). During the past de the southern segment of the El Tigre 1990, Cristallini and Ramos 2000) have decade, the cosmic ray exposure dating Fault into central and southern segments, shown that Andean orogeny may have re- method provided relatively accurate estima- 26 and 48 km-long, respectively. The nor- sulted in more than 90 km of shortening tes of the Quaternary deformation rates thern segment is 46 km-long and extends as accommodated by the Precordillera, yiel- along active faults in the Andean Foreland far as Rodeo city (30.2ºS). Although the ding a mean shortening rate of 5 mm/yr if of San Juan (Siame et al. 1997a, 2002). N10°E fault strike is preserved, this seg- integrated over the last 20 myrs. Age of ment is composed of smaller 1 to 5 km- shortening in the east-verging AP is relati- HORIZONTAL AND VERTICAL spaced elementary fault strands, and the vely well-known at about 30°S (e.g., Jordan SLIP RATES ASSOCIATED TO THE surface deformation appears to be more et al. 1993, Zapata and Allmendinger 1996), EL TIGRE FAULT distributed. Considering these numerous whereas it is less constrained at ~31°S. spaced fault strands, we interpreted this Near 30°S, age and amount of shortening The El Tigre Fault is a 120 km-long right- segment as the northern El Tigre Fault have been estimated mostly through mag- lateral strike-slip fault along which geomor- "horse-tail"-like termination (Fig. 6). neto-stratigraphy and growth strata: the phic features (fan and river offsets, sag Along the southern segment of this active Western Precordillera experienced an early ponds) and surface disruptions demonstra- fault, alluvial fans skirting the western pied- phase of shortening roughly 20 Ma ago te its Quaternary activity (Bastias 1985, mont of the Precordillera are incised by a (rates <5 mm/yr) followed by a period of Siame et al. 1997b). The fault is located in drainage network of stream channels, gu- quiescence (Jordan et al. 1993). Thrusting in the Calingasta-Iglesia Valley which is a 30 llies, and ridges. Most of these geomorphic the Western and Central Precordillera then km-wide piggyback basin filled with Mio- markers provide evidence for right-lateral resumed roughly 15 Ma ago with a period Pliocene clastic sediments and andesites, displacements, with a maximum offset of of rapid rates (15-20 mm/yr) spanning at and Quaternary deposits (Fig. 3). The El 260±20 m cumulated during Late Quater- about 10-12 Ma, followed by declining Tigre Fault trace analyzed on SPOT images nary. Thanks a geomorphic analysis of the rates (Jordan et al. 1993a, Zapata and extends northward from the Río San Juan alluvial fan surfaces incised by the defor- Allmendinger 1996), even though some of to the Río Jáchal (Fig. 6), has a mean med drainage network, we estimated that the easternmost thrusts may have been acti- N10°E-striking, linear and discontinuous the channel incision took place between the ve throughout the Quaternary (Zapata and fault trace composed by 1 to 7 km-long abandonment of the A4 unit and the aban- Allmendinger 1996, Siame 1998). Those fault strands. At the surface, this fault zone donment of the A3 unit (Fig. 6). Cosmic ray previous studies have shown that the defor- affects chiefly Tertiary and Quaternary for- exposure dating of the abandoned fan sur- mation front of the AP has experienced a mations as well as some Paleozoic rocks in faces allowed constraining an horizontal total shortening of ~40 km during the last its southern part. The fault strands are slip-rate for the ETF of about 1 mm/yr 5 Myr, implying a mean geological shorte- separated by stepovers, bends or relay dis- (0.5-2 mm/yr). Along the central segment ning rate of ~8 mm/yr. Regarding the continuities (Fig. 6). of the El Tigre Fault, a beheaded series of Eastern Precordillera and the Bermejo The overall fault geometry may be descri- alluvial fans is uplifted by pressure ridges Valley, they accommodated ~17 km of bed considering two to three main seg- formed within local fault strands. Cosmic shortening during the last 3 Myr (Ramos et ments. The southern segment is 74 km- ray exposure dating of these alluvial surfa- al. 2002), yielding a mean deformation rate long, and exhibits a continuous trace, exten- ces yielded a local vertical slip rate on the of about 6.5 mm/yr. The amount of shor- ding from 31.2ºS, north of the San Juan order of 0.3 mm/yr. The vertical and hori- tening associated with the Sierra Pie de Palo River to ~30.9ºS (Fig. 6). It is the most pro- zontal slip-rates estimated at roughly 0.3 is on the order of 15 km and may have star- nounced fault segment on SPOT satellite and 1.0 mm/yr, respectively, yield a recons- ted between 5 and 3 myrs, yielding a mean images where it is clearly evidenced by off- tructed slip-rake of about 17° for the El shortening rate of 3-5 mm/yr (Ramos et al. set stream channels, 200-400 meters wide Tigre Fault, similar to the fault slip vector 2002). releasing and restraining stepovers, which observed in a trench on the fault plane are always marked by depressions and pres- (fault plane azimuth: N17ºE, dip: 78ºE, QUATERNARY RATES OF DEFOR- sure ridges, respectively (Fig. 6). From striae pitch: 14ºS). MATION FROM COSMIC RAY 30.9ºS to 30.6ºS, the fault trace is also cha- EXPOSURE DATING racterized by an east-facing steep slope, VERTICAL SLIP RATE ASSOCIATED commonly bounded by several sag ponds WITH THE LAS TAPIAS FAULT In active tectonic studies, quantification of on its eastern foot and by beheaded alluvial the tectonic forcing processes requires con- fans on its western side (Fig. 6). Southward, The Villicúm-Pedernal thrust is a 145 km- tinuous and well-controlled chronologies of the fault trace vanishes within the Precor- long, N20°E-trending structure that runs the landscape development over long-span- dilleran Paleozoic strata, and may be inter- on the western piedmont of the Eastern ned time scales. Cosmic ray exposure dating preted as the southern tip of the El Tigre Precordillera between 31°S and 32°20'S lati- Active tectonics in the Argentine Precordillera and Western Sierras Pampeanas 613

Figure 6: The El Tigre Fault. a) Map of the 120 km-long Quaternary fault trace of the ETF overlain on Landsat TM (after Siame et al. 1997b). b) This figure presents partly the ETF southern and central segments (after Siame et al. 1997b), locates inset c and field photographs (d, e). c) The southern segment. It is composed of a unique trace which cross-cuts Quaternary alluvial fans where displaced geomorphic features can be observed. The color map presents the Quaternary alluvial fan deposits from field observations and panchromatic (10 m/pixel) SPOT image interpretation. The numbers refer to the minimum surface exposure ages calculated from 10Be concentrations measured in surface boulders (Siame et al. 1997a). Spot image extract shows the maximum horizontal cumulative displacements recorded by the drainage network. d) field photograph presenting the ETF trace (black arrows) in the landscape along the southern segment. e) Field photograph presenting the ETF trace (black arrows) in the landscape along the central segment tude (Fig. 3), bounding Sierra de Villicúm, cúm and Las Tapias segments are characte- northern Sierra Chica de Zonda fault bran- Sierra Chica de Zonda and Sierra de Peder- rized by a fault trace that clearly affects the ches. In the low-lying region localized bet- nal (Fig. 7). Structural segments separated Quaternary deposits whereas, the fault trace ween the Sierra de Villicúm and the Sierra by oblique N40°E-trending fault branches of Zonda-Pedernal segment is less conspi- Chica de Zonda, the Las Tapias Fault a- define the Villicúm-Pedernal thrust. These cuous (Siame et al. 2002). ffects Quaternary alluvial deposits that segments are the Villicúm, the Las Tapias, The Las Tapias Fault is 18 km-long and ex- overly unconformably the Neogene fore- and the Zonda-Pedernal segments, from tends with the same N22°E trend between land strata and forms discontinuous and north to south, respectively. Both the Villi- the southern Sierra de Villicúm and the degraded west-facing scarps (Fig. 7). Minor 614 L.L.SIAME, O. BELLIER AND M. SEBRIER

Figure 7: The Las Tapias Fault. a) Structural map of the 65 km-long Quaternary segment of the Villicúm-Pedernal thrust overlain on Landsat TM (after Siame et al. 2002). b) Structural map of the Las Tapias Fault shown on an aerial photograph centered on the low-lying region between Sierra de Villicúm and Sierra Chica de Zonda (after Siame et al. 2002). c) Panoramic view of the Las Tapias Fault escarpment showing the three different alluvial levels. d) Map of the alluvial deposits affected by the Las Tapias Fault based on both stratigraphic relationships and surficial morphology observed in the field and on aerial photograph. The numbers refer to the minimum surface exposure ages calculated from 10Be concentrations measured in surface boulders (Siame et al. 2002). e) N110°E-striking topographic profiles constructed perpendicularly to the Las Tapias scarps (circled numbers refer to locations on d). Active tectonics in the Argentine Precordillera and Western Sierras Pampeanas 615

N40°E-trending topographic features, mar- mont surface, and best proxies for the important seismotectonic activity (Fig. 4). ked by roughly 1 m-high, and 1 to 5 km- actual abandonment ages of 6.9 ± 1.0 and In this area, evidences of Quaternary active long scarps, also affect the Quaternary de- 18.7 ± 2.3 10Be-kyr for the uplifted surfaces, deformation are found along the eastern posits in the Las Tapias Fault area. These respectively (Fig. 7). and northern borders of the Sierra Pie de topographic features strike obliquely to the The displaced alluvial deposits geometry Palo as well as along the western borders of major N22°E-trending Las Tapias Fault with respect to the fault scarps has been the Sierra de Valle Fértil and Sierra de la and can be regarded as the continuation, studied to estimate the uplift rate for the Huerta (Fig. 8) (Costa et al. 2000, Siame et al. within the Quaternary deposits, of the hanging wall of the Las Tapias Fault. In- 2002, Siame et al. 2005). With a dissymme- N40°E-trending oblique faults that affect deed, the uplifted terraces result from epi- trical arch-liked shape (Fig. 4), its topo- the southern end of Sierra de Villicúm. sodic fault incision into the up-thrown graphy is characterized by steep-sloped si- Both sets of faults at the southern Sierra de block of the Las Tapias Fault. The vertical des on the eastern and northern fronts and Villicúm and the northern Sierra Chica de separation between the projections of these western and southern gently dipping back Zonda are bedding-parallel, and interpreted terraces, measured at the inferred fault sides (Fig. 4). The surface envelope of Sie- as flexural-slip faults. plane may thus provide a first approxima- rra Pie de Palo corresponds to an inherited, Along strike, the Las Tapias Fault forms tion of the vertical component of fault dis- Late Paleozoic erosional surface (Carignano west facing, ~10 m-high discontinuous and placement. To constrain the vertical com- et al. 1999) affecting in Precambrian meta - degraded scarps which affect Quaternary ponent of fault displacement, three topo- morphic rocks. Since, geologic evidence alluvial sediments (Fig. 7). The scarps are graphic profiles have been surveyed per- indicates that the range was covered by Late west facing, which leads to pounding on the pendicular to the fault trace (Fig. 7). The Pliocene distal synorogenic deposits deri- footwall of clastic deposits transported scarps bounding A2 and A3 terraces have ved from the Precordillera (Ramos and from Loma de Ullúm. Streambeds are drai- been constructed through time by repeated Vujovich 2000), the deformation associated ning across the scarps from east to west faulting on the Las Tapias Fault. Since to- with the growth of the Sierra Pie de Palo such that the thrust scarps grown against pographic profiles provide constraints on may have started between 5 and 3 Myr main drainage. Nevertheless, the interaction minimum vertical displacement between (Ramos et al. 2002). The eastern and wes- of alluvial cutting and recurrent surface the geomorphic surfaces and 10Be cosmic tern flanks of the Sierra Pie de Palo are cha- faulting has generated a strath terrace se- ray exposure ages provide lower limits for racterized by east-dipping and west-dipping quence that records the faulting history on time elapsed since surface abandonment, Pliocene strata, respectively, indicating that the hanging wall. Stream gradients are rela- these data allow estimating lower limits for the surface structure of Pie de Palo is cha- tively high (<8°), and recurrent faulting on the long-term uplift rate. All together, these racterized by a basement fold (Ramos et al. the Las Tapias Fault sufficiently raised the estimates yield to a mean uplift and shorte- 2002). This basement structure passively hanging wall to produce a significant verti- ning rate of 0.7 ± 0.3 mm/yr and 0.8 ± 0.5 folded the Pliocene strata and the uncon- cal down cutting. This stream incision into mm/yr, respectively (Siame et al. 2002, formity that separates them from the base- the up-thrown block has isolated a couple 2005). This shortening rate is consistent ment, in a similar way than other Sierras of alluvial terraces that end abruptly at the with the ~1 mm/yr shortening rate deter- Pampeanas (García and George 2004). Ba- fault scarp. These terraces can be easily dis- mined further north over the past 3 Myr by sed on that, Ramos et al. (2002) proposed tinguished by their differential elevations, as Zapata and Allmendinger (1996) across that the structure is controlled by a base- well as by the difference of both incision Eastern Precordillera and Bermejo basin ment wedge detached at about 15-20 km pattern and incision degree at their surfaces individual structures. depth (Fig. 3), according to the crustal seis- (Fig. 7). The clastic source for alluvial mate- micity provided by a local network (Régnier rial is the relief of Loma de Ullúm, and the THE ROLE OF THE SIERRA PIE DE et al. 1992). In this context, the role of the streams flow direction is roughly SW-NE PALO Niquizanga Fault (ANF) is still unclear. (Fig. 7). The piedmont is still active as sug- Indeed, at depth this fault may correspond gested by both a well-developed network of Within the structural context of the to a west-dipping, thick-skinned reverse braided streambeds incised ~0.5 m into it, Andean back-arc at about 31°S, the Sierra structure that most probably ruptured and a well-preserved surface morphology Pie de Palo appears to play a key role in the during the 1977 Caucete earthquake with bars and swales formed by slightly var- partitioning of the Plio-Quaternary defor- (Kadinsky-Cade et al. 1985, Langer and nished abraded boulders and sub angular mations. Located in the westernmost Sie- Bollinger 1988). However, it produced very cobbles. In an attempt to establish absolute rras Pampeanas, the Sierra Pie de Palo is little surface ruptures, suggesting that this ages for this stratigraphically-based chrono- roughly NNE-striking. It forms an 80 km- fault is mostly buried. Indeed, along the logy, terrace surfaces have been sampled for long and 35-40 km-wide, ellipsoid shape eastern flank of Sierra Pie de Palo, the Plio- cosmic ray exposure dating. Based on the that reaches elevation as high as 3162 m cene beds are dipping to the east, and near cosmic ray exposure ages published by (Fig. 3 and Fig. 5). This mountain range is Niquizanga, those strata are slightly folded Siame et al. (2002), we propose a minimum an actively growing basement fold (e.g., and unaffected by the fault (V. Ramos, per- CRE age of 1.9 ± 0.8 10Be-kyr for the pied- Ramos et al. 2002) associated with an sonn. comm.). To explain these observa - 616 L.L.SIAME, O. BELLIER AND M. SEBRIER

Figure 8: Active tectonics associated with the Sierra Pie de Palo. This map presents the surface fault traces associated to the active structures of the Eastern Precordillera and westernmost Sierras Pampeanas (after Siame 1998, Costa et al. 2000, Siame et al. 2002, 2005). Shaded area and black solid isolines present the gridded regional seismicity frequency (e.g., Fig. 4). The drainage basins that contribute to the dissection of the surface envelope of the Sierra Pie de Palo are mapped with grey levels according to their hypsometric integral values.

tions, Ramos et al. (2002) have proposed fairly constant throughout the whole area, lues of hypsometric integral are preferen- that east-dipping faults along the eastern any active tectonic perturbation should thus tially located along the northern and eastern border of Sierra Pie de Palo might be rela - be discriminated using the geomorphic in- borders of the Sierra Pie de Palo. Moreover, ted to W-verging, thick-skinned fault propa- dices associated to the drainage network along the eastern border, the drainage ba- gation associated to the Sierra de la Huerta that dissects the Sierra Pie de Palo. Among sins show rectilinear shapes that correlate and Sierra Valle Fértil. However, such a W- those geomorphic indices, the hypsometric the seismicity frequency (see above), and verging thick-skinned propagation would integral value, related to the degree of dis- the steep slope that characterizes this side necessary have produced a significant relief section of a given landscape, is commonly of the Sierra Pie de Palo. Considering these instead of the subsiding Bermejo valley. To used to discriminate between tectonically combined observations, and the November our opinion, even if buried along most of active and inactive regions (Strahler 1952). 23, 1977, Caucete, earthquake sequence its length, an E-verging, Niquizanga Fault Indeed, high values of the hypsometric in- (Kadinsky-Cade et al. 1985, Langer and system should be regarded as the structure tegral (i.e., values close to 1) indicate youth- Bollinger 1988), it appears that the eastern responsible for the dissymmetrical, long- ful topographies whereas intermediate to and northern flanks of the Sierra Pie de wave length shape of the surface envelope low values (i.e., values close to 0) are related Palo are probably among the most active of the Sierra Pie de Palo (Fig. 3). to more matured and dissected landscapes. zones in the area. To evaluate the degree of tectonic activity In the Sierra Pie de Palo, one can consider around the Sierra Pie de Palo, some geo- that the hypsometric integral value of a DISCUSSION OF A SEISMOTECTO- morphic lines of evidence can be drawn given drainage basin is directly related to its NIC MODEL FOR THE ANDEAN from the study of the drainage network. In- degree of tectonic activity. The figure 8 pre- FORELAND OF SAN JUAN AND deed, even if the lithological and climatic sents a map of the drainage basins together MENDOZA conditions (semi-arid climate) contributed with the hypsometric integral values calcu- to a good preservation of its surface enve - lated using the SRTM90 digital topography The Andean back-arc of western Argentina lope, the Sierra Pie de Palo is dissected by (e.g., Rosen et al. 2000, Far and Kobrick, can be regarded as an obliquely converging fluvial incision. Moreover, taking into 2000). This morphometric approach un- foreland where Plio-Quaternary deforma- account that the lithological conditions are veils that the drainage basins with high va- tions are partitioned between strike-slip and Active tectonics in the Argentine Precordillera and Western Sierras Pampeanas 617

thrust motions that are localized on the E- mm/yr) is in close agreement with the geo- to say since the Sierra Pie de Palo is actively verging thin-skinned Argentine Precordi- logical rates estimated from crustal balan- growing. llera and the W-verging thick-skinned Sie- cing studies (e.g., Allmendinger et al. 1990, rras Pampeanas, respectively. In this parti- Cristallini and Ramos 2000). Since this ran- ACKNOWLEDGEMENT tioning model the El Tigre Fault, Eastern ge of shortening rates has to be distributed Precordillera and western Sierras Pampea- over several active thrusts, it is also consis- We thank M. Araujo at the INPRES for nas faults play important roles. The El Ti- tent with a minimum shortening rate of 0.8 fieldwork facilities and fruitful discussions, gre Fault geometry and segmentation is ± 0.5 mm/yr determined for the Las Tapias M. Perez and G. Racciopi for helpful assis- tightly associated with the geometry of the Fault of the Eastern Precordillera south of tance during fieldwork. We thank IFEA for Precordillera ranges. Indeed, slight varia- 31°S over the Holocene period (Siame et al. travel grants. SPOT images were provided tions of strike in the Precordillera range 2002). Even if one argued as to the degree by the Tectoscope-Andes and ISIS pro- appear to parallel the El Tigre Fault geo- to which the GPS data are accurate, these grams (CNES/INSU/CNRS) Funding for metry and discontinuities (Siame et al. data suggest that, south of 31°S, 7-9 the CRE analyses was partly provided by 1997b). For example, the southern termina- mm/yr of E-trending shortening have to the PROSE programme (CNRS-INSU). tion of the El Tigre Fault coincides with be accommodated between the Calingasta The CNRS, CEA and IN2P3 support the the Precordillera ranges bending from Valley and Sierra Pie de Palo, with about 4 Tandétron (French ASM facility, Gif-sur- N160°E to N10°E close to Río San Juan. mm/yr between Eastern Precordillera and Yvette, France) operation. The authors also Along its northern segment, the El Tigre Sierra Pie de Palo (Brook et al. 2003). Those thank V. Ramos for fruitful discussions and Fault steps westwardly, splaying as a horse- GPS-derived shortening estimates are thus constructive comments about Sierra Pie de tail termination, coinciding to another o- slightly higher than the long-term geologi- Palo, and J. Cortés and D. Ragona for revie- rientation change of the Precordilleran ran- cal and Quaternary rates. Moreover, north wing their manuscript. ges (Fig. 5). Among others, these observa - of 31°S, GPS data suggest that only 2-3 tions lead Siame et al. (1997b) to propose mm/yr are accommodated between the WORKS CITED IN THE TEXT that the El Tigre Fault may correspond to a western Precordillera and the Sierra de Va- crustal-scale strike-slip fault, closely related lle Fértil, with less than 1 mm/yr within the Allmendinger, R. W., Figueroa, D., Snyder, D., to the Precordilleran fold-and-thrust belt. Bermejo valley. GPS-data thus seem to con- Beer, J., Mpodozis, C. and Isacks, B. L. 1990. In this Plio-Quaternary transpressive sys- firm that most of the Present-day tectonic Foreland shortening and crustal balancing in tem, the El Tigre Fault should accommoda- activity is concentrated south of 31°S, in the Andes at 30°S Latitude. Tectonics 9: 789- te the dextral strike-slip component while the Sierra Pie de Palo area. In this context, 809. the Eastern Precordillera-Pampean thrusts the shortening rate estimated from seismic Alvarado, P. and Beck, S. 2006. Source characte- should accommodate N110°E-trending moment tensor sum appears to be one or- rization of the San Juan (Argentina) crustal shortening (Fig. 5). Indeed, if one assumes der of magnitude greater (3-4 cm/yr) than earthquake of 15 January 1944 (Mw 7.0) and that the convergence obliquity is regionally those estimated above from the GPS-deri- 11 June 1952 (Mw 6.8). Earth and Planetary accommodated by the El Tigre Fault, Qua- ved velocity field (Siame et al. 2002, 2005). Science Letters (doi: 10.1016/j.epsl.2006. ternary slip-rates estimated thanks to cos- This high rate of co-seismic shortening 01.015). mic ray exposure dating (~1 mm/yr) imply strongly suggests that the November 23, Alvarado, P., Beck, S., Zandt G., Araujo, M. and that 3-8 mm/yr of shortening have to be 1977, Caucete, earthquake sequence may Triep, E. 2005. Crustal deformation in the accommodated throughout the Eastern Pre - have released the elastic deformation accu- south-central Andes back-arc terranes as vie- cordillera-Pampean system in a N110°E- mulated in this region, the high discrepancy wed from regional broad-band seismic wave- trending direction (Fig. 5). Interestingly, this with GPS-derived and geological shorte- form modelling. Geophysical Journal direction corresponds to the orientation of ning rates being much probably due to the International (doi: 10.111/j.1365-246X.2005. the s1-axes computed from both focal me- too short-spanned time window. South of 02759.x): 1-19. chanism (Present-day) and fault kinematic 31°S, the regional compressional stress regi- Baldis, B.A., Beresi, M.S., Bordonaro, O. and (long-term) datasets within the Eastern me is dominated by N110 ± 10°E-trending Vaca A. 1982. Síntesis evolutiva de la Pre-cor- Precordillera and westernmost Sierras Pam- s1-axis perpendicular to the trend of the dillera de Argentina. 5° Congreso Latinoa- peanas. This direction also coincides with Sierra Pie de Palo, and parallel to the trend mericano de Geología (Buenos Aires), Actas: the trend of the North Pie de Palo Fault of the north Pie de Palo Fault (Fig. 5). In 31-42. that marks the northern limit of the gro- this context, the process of basement uplift Bastias, H. 1985. Fallamiento Cuaternario en la wing Sierra Pie de Palo (Fig. 3), and corres- that exhumed the Sierra Pie de Palo has region sismotectonica de precordillera: San ponds to a regional structure playing a ma- most probably triggered the regional clock- Juan, Argentina. Tesis Doctoral, Facultad de jor role regarding the seismicity distribution wise reorientation of the compressional Ciencias Exactas, Fisicas y Naturales, Uni- pattern (Fig. 2 and Fig. 4). axes, observed at the long-term (fault striae) versidad Nacional de San Juan, inédita, 160 p. In our partitioning model, the estimated and short-term (seismicity) time windows. Beer, J.A., Allmendinger, R.W., Figueroa, D.A. range of Quaternary shortening rates (3-8 This may have started 5-3 Myr ago, that is and Jordan T.E. 1990. Seismic stratigraphy of 618 L.L.SIAME, O. BELLIER AND M. SEBRIER

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