Quaternary intraplate deformation in the southeastern ... - Springer Link

Journal of Seismology 5: 399–409, 2001. © 2001 Kluwer Academic Publishers. Printed in the Netherlands.

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Quaternary intraplate deformation in the southeastern Sierras Pampeanas, Argentina Carlos H. Costa1 , M. Victoria Murillo2, Guillermo L. Sagripanti3 & Carlos E. Gardini1,4 1 Departamento

de Geolog´ıa, Universidad Nacional de San Luis, Chacabuco 917, 5700 San Luis, Argentina;

2 Departamento de Exploraci´ on y Producci´on, Shell, CAPSA, Av. R. Saenz Peña 788, 1383 Buenos Aires, Argentina; 3 Departamento

de Geolog´ıa, Universidad Nacional de R´ıo Cuarto, Ruta Nac. 36 Km 601, 5800 R´ıo Cuarto, Argentina; 4 CONICET Received 19 July 1999; accepted in revised form 15 August 2000

Key words: Argentina, neotectonics, Quaternary deformation, Sierras Pampeanas

Abstract Neogene strain from the subducting Nazca plate is widely distributed in the Andean foreland as a result of flatlying subduction beneath central western Argentina (28◦ –33◦S latitude). This fact is indicated by uplifted basement blocks bounded by reverse faults as far as 600 kms east of the Chilean trench axis. Some deformation in the southern Sierras de Córdoba (southeastern Sierras Pampeanas) indicates significant displacements during Quaternary and even late Holocene time. This region has low to moderate seismicity characterized by earthquake magnitudes ≤ 6.7 with no associated noticeable surface ruptures. This paper presents information recently gathered on the most conspicuous regional structures of the area (El Molino, Sierra Chica and Las Lagunas faults). The last movement along the El Molino fault thrust basement rocks over organic-rich (0.8–1.3 ka) sediment and fault relationships suggest previous Quaternary displacements. Along the Sierra Chica fault, Precambrian basement has been thrust a minimum of 13.5 m over Pleistocene conglomerates, and faulting also affects late Pleistocene-Holocene fluvial sediments. The Las Lagunas fault has been regarded as the source of the 1934 Ms 5.5 and 6.0 earthquakes, which heavily damaged the nearby village of Sampacho. The faulted surface is buried under Holocene loess, but its trace is expressed as a 24-km-long rectilinear scarp, despite continuous modification due to land use. Although we lack detailed information on probable rupture lengths during large Sierras Pampeanas thrust earthquakes, some preliminary considerations are made for the regional seismic hazard of these structures. The geologic evidence described here identifies these faults as possible sources of strong earthquakes in the future.

Introduction Between 28◦ S and 33◦ S, the subducting Nazca plate has a flat-lying orientation underneath the South American plate (Stauder, 1973; Barazanghi and Isacks, 1976; Smalley and Isacks, 1987, 1990; Smalley et al., 1993; Araujo and Suárez, 1994). Neogene deformation also appears to be distributed above the subhorizontal subducted slab, forming up-faulted basement blocks in the Andean foreland (Figure 1). These broken foreland blocks, together called Sierras Pampeanas (Pampean Ranges), are considered to be a modern analog of the Rocky Mountains Laramide

uplifts in North America (Jordan, 1995; Jordan and Allmendinger, 1986). The necessity of expanding the earthquake record back beyond the short time span of the historical and instrumental seismicity has raised interest in paleoseismic data along the Quaternary faults. Reconstructing the Quaternary deformation record would be a valuable tool for bridging the gap between historical/instrumental seismicity data and the long-term intervals that commonly characterize intraplate fault recurrence. Such a data base is critical in order to properly address the regional seismic potential. For the several large cities, water reservoirs, and a nuclear power plant located in the Sierras Pampeanas,

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Figure 1. Location of the Sierras Pampeanas province. Pampean blocks are displayed in grey stipple with master faults in solid black lines PC: Precordillera fold and thrust belt. The southeastern Sierras Pampeanas study area location (Figure 2) is indicated by a rectangle.

paleoseismic studies would also expand the knowledge of fault rupture characteristics and earthquake recurrence in compressive intraplate tectonic settings. This paper presents new data for three of the most significant Quaternary faults in the southeastern Sierras Pampeanas (El Molino, Las Lagunas and Sierra Chica faults), as first step to achieving the above-mentioned goal.

Tectonic setting and regional seismicity The deformation processes associated with the Andean Orogeny in the Central Andes of Argentina starts after the break-up of the former Farallones plate into the Cocos and Nazca plates around 25 Ma (Pilger, 1984; Ramos, 1988). The resulting higher subduction rates caused mountain building with uplift of the Principal Cordillera (late Oligocene-early Miocene), the Frontal Cordillera (late Miocene-early Pliocene) and the Precordillera (late Pliocene-Pleistocene), as a progressive eastward shifting of the orogenic front occurred. The peak of Neogene deformation, both at the Andean orogen and the Sierras Pampeanas, was in

Figure 2. General sketch of the study area in the southeastern Sierras Pampeanas. Main Quaternary faults where tectonic activity has been described or suspected are indicated in solid black lines. (triangles in hanging wall of reverse faults). Stars refer to location, date and magnitude of major seismic events during the 20th century. 1. El Molino fault, 2. Comechingones fault; 3. Las Lagunas fault and 4. Sierra Chica fault. Arrows indicate location of fault descriptions shown in Figures 3 (El Molino); 6 (Las Lagunas) and 8 (Potrero de Garay-Sierra Chica).

Pliocene-Pleistocene time (Jordan et al., 1983; Ramos, 1988). According to Jordan et al. (1983), some surficial geological features above the flat-lying subduction plate (28◦S–33◦ S) are a consequence of the subducting plate’s geometry, such as the lack of recent volcanism and the development of foreland block uplifts. The presence of these Neogene uplifted blocks (Figure 1) indicates a wide distribution of Andean Neogene de-

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Figure 3. Stratigraphic and structural relationships of the El Molino fault as exposed at El Molino Creek. Numbers indicate stratigraphic position and letters indicate composition of deposits. B, crystalline basement rocks (migmatites) and shear zone C, brownish coarse colluvial scarp-derived and fluvial sediments, F, fine-grained fluvio-lacustrine and colluvial sediments, G, alluvial gravel and coarse sand, S, medium to coarse alluvial sand with gravel, T, gray Neogene subvertical sandstone. Stars show 14 C sample location. The zig-zag lines show lineations interpreted as probable continuation of the fault. Modified from Costa et al. (1994); Costa and Vita-Finzi (1996) and Murillo (1996).

formation in the South American intraplate, as these uplifts occur up to 600 kms east of the Chilean trench. The Sierras Pampeanas are widely distributed mountain blocks in west-central Argentina, separated by basins filled with Carboniferous and younger continental sediments. The ranges are mainly composed by metamorphic and igneous rocks of late Precambrian to early Paleozoic age and are bounded by Neogene reverse faults. In most cases, the Neogene uplift geometry results in an asymmetrical E-W profile, with a steep western slope and a gentle eastern slope that has remnants of a regional paleo-landsurface developed prior to the Andean deformation. The southeastern Sierras Pampeanas are comprised of the San Luis and Córdoba ranges (Figure 2). The most conspicuous evidence of neotectonics are on their western slopes, where west-verging (30◦–55◦E) master faults thrust crystalline basement over Late Paleozoic, Tertiary and Quaternary deposits (Costa, 1996). Models of the fault geometry based on the uplifted block’s topographic asymmetry generally favor listric faults according to both geometrical (González Bonorino, 1950; Jordan and Allmendinger, 1986; Costa, 1996) and geophysical (Introcaso et al., 1987)

interpretations, although a planar fault geometry has also been proposed (Martino et al., 1995). The current position of the Andean orogenic front at 31◦ –33◦S is between the eastern Precordillera foothills and the western Sierras Pampeanas (Figure 1) (Smalley and Isacks, 1987; 1990; Regnier et al., 1992; Smalley et al., 1993). The orogenic front is characterized by a concentration of ongoing tectonic processes highlighted by active faulting, growing structures and strong seismicity. More than 90 percent of the total continental seismic-moment release in the Andean back-arc from Colombia to Patagonia has occurred in the area between 30◦ –34◦S (Chinn and Isacks, 1983). The cratonward active seismicity and deformation are responsable for Argentina’s two most destructive earthquakes during the 20t h century (1944 M 7.4 San Juan and 1977 M 7.4 Caucete). Within the southeastern Sierras Pampeanas, five earthquakes with estimated magnitudes up to 6.7 have occurred during the past 100 years (Figure 2) with depths ranging from 10 to 35 km. Historical records indicate that a similar number of events damaged cities and villages in the nineteenth century. However, the spatial distribution of instrumental seismicity does not clearly image the master faults where evidence

402 the western range fronts and adjacent areas, as we illustrate in the following examples. The southern Sierras de Córdoba are divided into two N-S elongate ranges, the Sierra de Comechingones and the Sierra Chica (Figure 2). Within the Sierra de Comechingones, recent tectonic activity is observed along a secondary N-S trending fault west of the main range-bounding fault. This nearly 50km-long fault, the El Molino, has clear geomorphic expression of en echelon scarps, strongly suggesting Quaternary movement. The Las Lagunas fault is 20 kms south of the Sierra de Córdoba and is the expression of faulted shallow basement.

El Molino fault

Figure 4. Schematic diagram showing interpreted structural evolution of El Molino fault during Quaternary time, based on relations shown in Figure 3.

of Quaternary deformation is observed. The Las Lagunas fault is one exception, its location is outlined by sustained local seismicity.

Quaternary faulting in the southeastern Sierras Pampeanas Even if traditionally regarded as a stable continental region, several examples of Quaternary faulting have been reported recently for the Sierras Pampeanas region, particularly to the southeast in the Sierra de San Luis and Sierra de Córdoba (Massabie, 1976, 1987; Massabie and Szlafstein, 1991; Costa et al., 1992; Costa, 1996; Costa et al., 1994; Costa and Vita-Finzi, 1996). Quaternary faulting has been described along

A clear exposure of this structure can be found at El Molino Creek (Figure 3) where Costa et al. (1992; 1994); Costa and Vita-Finzi (1996), and Murillo (1996) described highly sheared migmatites of the crystalline basement that are thrust over fluvial deposits that range from Late Tertiary(?) to late Holocene in age. The example is located in a N-S trending section where the average dip is 45◦ to the east. Quaternary alluvial surfaces east of the exposure are tilted eastward and indicate the same faulting geometry as the main range block. At this site, basement rocks override a suite of Quaternary colluvial, fluvial and loessal sediments that contain interbedded blocks of basement rock. 14 C dating of the organic-rich loess yielded ages of 1170–800 BP and 1300–1150 BP (Costa and Vita-Finzi, 1996) (Figure 3). The absence of Quaternary units in the hanging wall of the exposure hampers proper reconstruction of the fault’s displacement history, but by restoring the predicted basement rocks slip, we calculate 2.10 m of dip-slip since 1300 BP and a minimum Quaternary displacement of 3.10 m, according to the age of the dated units (correspond to m-m’ and m-m” respectively, in Figure 3). Evolution of the presently buried scarp is interpreted to have developed progressively as suggested by the stages in Figure 4. We believe that the present El Molino fault scarp at the study location, resulted from two main slip stages (A and D), assuming that the fault evolution has been driven by coseismic displacements rather than aseismic creep. Fault slip related to stages A and D seems to be too large for a single event (see for instance empirical relationships provided by Slemmons, 1982 and Wells and Coppersmith, 1994) and it is suspected that the cumulative

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Figure 5. Low-sun angle photograph of the Las Lagunas fault scarp looking southwest. Note the linear trace of the scarp with heights ranging from 0.5 to 2 m, modified by land use. The fault trace is emphasized by dashed lines at both extremes of the photo. A secondary scarp with the same southwest trend can be seen in the central part of the photo, north of the main scarp.

slip preserved the older sediments is the result of a larger and unknown number of faulting events. The basement blocks (3b) showed in Figure 3 are interpreted as gravity blocks derived from the uplifted bedrock, after stage A (Figure 4). The nearvertical bedrock-colluvium contact was probably a surface-rupture scarp. After the subsequent burial of the basement blocks by units 3G and 4C, additional faulting event(s) took place, disturbing units 4C, 4F and probably unit 5F (Figure 4).

Las Lagunas fault This fault is revealed by a 24-km-long NE-SW linear scarp on the loessoial plain south of the Sierras de Córdoba (Figure 2). The NW-facing scarp ranges from 1 to 11 m in height, has maximum slope angles of 26◦ (Sagripanti et al., 1999), and controls the location of several lagoons. Las Lagunas fault has been regarded as the source of the M 5.5 and M 6.0 June 11, 1934 Sampacho earthquakes (Mingorance, 1991). Based on fresh scarps in the loess, Mingorance claimed that 8 kms of the fault ruptured to the surface along a southwestern section of the fault scarp, as a consequence of the 1934 events. Low-sun-angle reconnaissance allowed the identification of second-

Figure 6. General sketch of Las Lagunas fault scarp (in solid line), whith height in meters indicated by circled numbers. See Figure 2 for location of this area. Pre-Quaternary rocks are shown in grey. Fluvio-lacustrine sediments in dotted pattern, loessoial sediments in white, and lagoons in stippled pattern. Arrow shows location of inset at the northeastern end of Las Lagunas fault. Note the structureless body of mixed and disrupted layers with dark lacustrine clay (in black), fluvial sand (in grey) and a loessoial (silty sand) sediments (dotted pattern). Fine bedding within the disrupted layers and fluvial sand indicates the original bedding attitude.

404 ary scarps related to the Las Lagunas fault that are commonly less than 1 m high (Figure 5). These geomorphic features are clearly expressed with linear traces in the unconsolidated loess and loessial materials (Pampeano Formation, late Pleistocene age), despite intense agricultural activity in the area. Trench studies of the fault zone revealed shallow crystalline basement rock in the hanging wall, buried by fluvial, eolian, and fluvio-lacustrine sediments. It is suspected that the Las Lagunas fault is a high-angle reverse structure because of the linearity of its trace. Although no concrete evidence of fault propagation into Quaternary materials has been found yet, a topographic map of unknown source (dated 1916) indicates that this location was a lagoon at that time. The enlarged area of an exploratory trench displayed in Figure 6, shows a fragmented layer composed of fine sands, dark brown clays and loessial sediments whose internal bedding has been completely distorted. Some marker horizons, such as thin pelitic layers, can be found in different fine sandy clay units indicating that their present orientation is not flat-lying but disrupted into a chaotic disposition (‘flame’ structures, M. Meghraoui, personal communication, 1999). Clastic dikes of fine sand (1–3 cm wide) vertically intrude thin loessial and clay beds (2–7 cm thick) at the extreme northwest side of the trench, suggesting lateral spreading or liquefaction. The origin of such features has yet not been satisfactory explained and non-tectonic causes might also be advocated. However, the liquefactionprone character of the sediments and a high water level until recent years, suggest association of these structureless horizons with paleoliquefaction induced by ground shaking. A distant large earthquake/s could have caused of the distorted structures, but the fact that historic seismicity occurs along the Las Lagunas fault strongly suggests that the liquefaction resulted from recent activity of this fault.

Potrero de Garay fault The Potrero de Garay fault is a branch of the Sierra Chica fault, which is the Neogene uplift front of the Sierra Chica de Córdoba (Figure 2). This 160km-long fault is composed of several sections with submeridional trends, that are concave both westward and eastward. Quaternary deformation has been documented along its trace where east-dipping basement rocks are thrust over Tertiary and Quaternary alluvial sediments (Gross, 1948; Schlagintweit, 1954; Len-

cinas and Timonieri, 1968; Massabie, 1976, 1987; Kraemer et al., 1988; Wagner Manslau, 1988; Massabie and Szlafstein, 1991; Szlafstein, 1991). The fault has 13.5 m of exposed slip in coarse-grained Pleistocene fluvial deposits (Figure 7), 35 km south of the section described in this paper. The fault sketched in Figure 8 is at the northernmost part of Los Molinos reservoir, 200 m to the east of the San Pedro River, and was first described by Schlagintweit (1954) and Szlafstein (1991). The site shows no clear geomorphic evidence of Quaternary faulting, although Precambrian gneiss is thrust over unconsolidated alluvial sand interbedded with loessoial sediment. The fault trace is buried by about 50 cm of colluvium. The original fabric of the gneiss has been remarkably transposed by ductile and brittle deformation concentrated in a 10-to 50-cm-wide shear zone. The probable Quaternary units are composed of grey-brown unconsolidated fluvial deposits that include coarse sand and gravel, poorly sorted gravel, and impure gravel with a silty (loess) matrix. Discontinuous beds of loessoial sediment, composed dominantly of silt are interbedded with sand and micaceous laminae. Fragments of redeposited bones of an unidentified mammal (Scelidoterium?, Prado personal communication, 1999) were found in the sand layers but were not suitable for 14 C analysis. Castellanos (cited in Schlagintweit, 1954) also reported isolated Quaternary mammal bones in the same sediments nearby. The fault has an average dip of 38◦ E, where exposed, and a sinuous trace. It is slightly concave downward in the upper part (29◦ E average dip) and concave upward in the lower part (47◦E average dip). At the base of the excavation, there is an exposed net dip-slip of 2.15 m. As a result of faulting a 5-to 20cm-wide zone of sheared sediments has developed, in addition to the main fault plane. Dips in the loessoial beds range from flat-lying to subvertical, where they are deformed by fault slip or drag. The fault plane flattens at the base of unit 4C, where hill creep is occurring and it is not clear if unit 4C is affected by faulting. Our interpretion of the recent evolution of this fault is shown in Figure 9. A significant break in the stratigraphic record could explain the resulting disrupted geometry, although stratigraphic discontinuities in this vertical section are not clearly visible. During Stage A, a subhorizontal fluvial-eolian sequence was deposited on basement topography. The upper part of the alluvial sequence was probably eroded. The trench

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Figure 7. Exposure of the Sierra Chica fault (Santa Rosa section) along Route N◦ 5 at Santa Rosa de Calamuchita village, where Precambrian gneiss is thrust over horizontal Pleistocene conglomerates. The fault dip decreases upward and this structure shows no diagnostic geomorphology despite its 13.5 m of minimum slip during the Quaternary. View is looking south.

relationships do not conclusively prove that a faultrelated scarp was present in the bedrock, but it is quite probable, considering the proximity of the underlying basement. Later, a faulting event or events dragged loessoial material and unconsolidated sand along the fault plane, resulting in a counter clock-wise rotation of beds (Stage B). Erosion probably modified the fault scarp soon after the coseismic uplift. Another fluvio-eolian sequence was deposited (Stage C), and was in turn, deformed by an unknown number of faulting events (present stage at Figure 8). However, the stratigraphic markers were not inverted by counter clock-wise dragging, but tilted with the same dip as the fault plane. Additional studies on other fault exposures are necessary in order to improve the understanding of the Quaternary deformational history of this fault.

Seismic-hazard considerations Fault segmentation based on paleoseismological criteria (i.e., Machette et al., 1991) has not been applied here because the present knowledge of seismicity

and Quaternary deformation on these faults is insufficient. However, we make some preliminary estimates about the seismogenic potential of the faults, based on maximum potential of rupture length and the poorly constrained average slip per event. A minimum cumulative slip of 2.10 m occurred on the El Molino fault during the last 1300 yrs (mm’ in Figure 3) (Costa and Vita-Finzi, 1996), based on the age and displacement of Unit 4F, and at least 3.10 m of slip (m-m” in Figure 3) took place during an unknown amount of time in the Quaternary (measured slip considering Unit 3G). Because no relevant historical earthquakes have been reported after the settlement of the nearby village of Merlo in 1797, it is believed that the 2.10 m of recorded slip took place between 200 and 1300 yrs ago. This results in a maximum time interval of 1100 yrs. Considering the tectonic setting, such a slip might be the result of several earthquakes, unless we assume a coseismic slip of 2.10 m in a single faulting event. This possibility would give rise to an M 6.6 event. According to the El Molino fault’s step-like pattern in plan view (Costa and Vita-Finzi, 1996), we

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Figure 8. Quaternary deformation of the Sierra Chica fault as expressed by the Potrero de Garay fault at the northern end of Los Molinos Reservoir. Numbers indicate stratigraphic position and letters indicate lithology. C, colluvium and modern soils, R, gravel with sandy matrix, M, brownish fine to medium silty sand with disperse mica, S, grey-brown sand, poorly sorted, with very loose fabric, L, brownish loessoial sediment, moderately consolidated, with fine layers of sands, F, sand and gravel with a cataclastic fabric, G, poorly sorted loosely consolidated, grey-light brown sand with gravel, B, tectonized Precambrian gneiss with a pervasive bounding shear zone. Fragments of mammal bones are shown in black.

interpret the maximum single rupture length for one segment to be as much as 1.6 km. Such a short length is below the confidence interval of empirical scaling laws for reverse faulting (Wells and Coppersmith, 1994). An extrapolation of the corresponding curve (magnitude vs. surface rupture length) indicates a magnitude of about M 5.2, which would not result in surface faulting. Considering subtle lineaments associated with the El Molino scarp, we could interpret a potential rupture length of up to 4 km, which would yield about M 5.8 events. According to Wells and Coppersmith’s (1994) rupture parameters, this is at the lower magnitude limit for producing surface rupture. In fact, historical earthquakes in this region with assigned magnitudes up to 6.2 did not produce noticeable ruptures. Since we show that this fault ruptured the surface generating enough relief as to induce block fall (minimum coseismic vertical displacement of 0.70 m, see Figure 3), the resulting earthquake must have been at least M 6.5. Alternate interpretations for the length vs. magnitude relation are 1) to consider longer single surface ruptures than suggested by fault scarp geometry, like multiple ‘steps’ of the fault trace rupturing at once

or 2) to consider that part of the rupture is blind and coseismic surface faulting could occur without clear surface expression and discrete lengths but significant magnitudes (≥ 6.5). On the other hand, a 4-km-long rupture corresponds with 0.6 m of coseismic slip in the maximum displacement versus surface rupture length regression curve (Wells and Coppersmith, 1994). If this is a typical event, the 2.10 m of slip (Figure 3) might have resulted from of 3–4 surface-rupturing earthquakes in 1100 yrs (between 200 and 1300 yrs BP), suggesting a recurrence interval of 275–366 yrs. Despite the short historical record, this is likely to be too short of a recurrence interval for typical intraplate faults. Two possible explanations for the problem are: 1) the measured slip results from fewer events but with larger coseismic slip. In the case that the 2.10 m of slip were the result of a single faulting event, this might have been the result of a M 6.6 earthquake, following Wells and Coppersmith scaling laws), or 2) earthquakes rupturing the surface are not randomly distributed through time but clustered during the last millenium. Mingorance (1991) reported that the Ms 5.5 and 6.0 1934 events produced a surface rupture 8 km long

407 The present geometry of the strata could be a result of two main faulting stages, and an unknown number of earthquakes. The lack of a clear geomorphic signature related to the Potrero de Garay fault and the complex fracture pattern of the crystalline basement precludes the identification of probable fault rupture lengths. However, maximum coseismic slips might be 1 m (see Stage B at Figure 4), which corresponds to M 6.5 events according to Wells and Coppersmith (1994).

Concluding remarks

Figure 9. Schematic diagram showing probable evolution of the Potrero de Garay fault interpreted from the relations sketched at Figure 8. Units are the same as in Figure 8.

with 0.25 m of average displacement along the Las Lagunas fault. He estimated a maximum potential earthquake of Ms 7.0, considering a rupture along the entire fault scarp (24 km) using the relations proposed by Slemmons (1982). The Wells and Coppersmith (1994) empirical relationships allows for a M 6.6 maximum event. According to these scaling laws, the Ms 6.0 1934 earthquake would have produced a surface rupture approximately 7 km long, which agrees with the data reported by Mingorance (1991). The evidence for coseismic surface faulting during the 1934 earthquake has not been fully corroborated during our field studies. However the data reported here, suggest significant ground shaking occurred during the late Pleistocene-Holocene. Careful examination of new trench exposures of the fault zone may help to better understand the Las Lagunas fault’s deformation record. Quaternary surface faulting has not been clearly shown to be associated with the Sierra Chica fault at Potrero de Garay, although faulting affects thin fluvial sequences deposited over the basement surface.

Quaternary faulting in the Sierras Pampeanas is a consequence of modern mountain building driven by foreland thrusting resulting from the subhorizontal subduction of the Nazca plate. Even though the study area in the southeastern Sierras Pampeanas does not have extensive modern seismicity, the examples of recent faulting presented here suggest that significant paleoearthquakes are more likely to occur than once thought. These preliminary data underline the need to collect more extensive and complete paleoseismological information to compliment the short historical and instrumental database in Argentina. Paleoseismological studies in this area has been limited by the preservation of faulted sequences in the hanging wall, which prevents a complete reconstruction of the Quaternary faulting (i.e. number of events associated to the overall fault slip and individual amounts of coseismic slip). We have presented evidence showing that these faults have experienced late Quaternary movements (i.e. Potrero de Garay fault) even though they lack distinctive geomorphic signatures. The results of this study suggests that there might be other potential seismic sources in the Sierras Pampeanas, which are unrecognized and could be easily ignored or missed. Both the El Molino and Potrero de Garay faults are secondary branches of the main range uplift front. Hence, further studies to recognize recent faulting on secondary and/or subtle geomorphic features rather than along major range-bounding fault scarps are needed to fully understand the seismic hazard in the region. Although the data discussed here do not constitute a robust paleoseismic record of the studied faults, they suggest that 1) coseismic surface ruptures have occurred during the Holocene and are probably associated with M 6.5 or larger events and 2) at least two

408 of such events took place along El Molino fault during the last 1300 yrs.

Acknowledgements Helpful and careful reviews from an anonymous reviewer, M. Machette, H. Stenner, T. Crone and M. Meghraoui helped considerably to improve the final version of the manuscript. We also are grateful for the help of H. Schiavo, A. Dolso, A. Giaccardi and D. Aguilera and to enthusiastic field assistance of UNRC students at the Las Lagunas trench. This research is supported by the Universidad Nacional de San Luis through the Grupo de Investigaciones Geológicas Aplicadas and the Universidad Nacional de Río Cuarto.

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