Quaternary intraplate deformation in the southeastern Sierras Pampeanas, Argentina

Neogene strain from the subducting Nazca plate is widely distributed in the Andean foreland as a result of flat-lying subduction beneath central weste rn Argentina (28 degrees -33 degreesS latitude). This fact is indicated by uplifted basement blocks bounded by reverse faults as far as 600 kms east o f the Chilean trench axis. Some deformation in the southern Sierras de Cord oba (southeastern Sierras Pampeanas) indicates significant displacements du ring Quaternary and even late Holocene time. This region has low to moderat e seismicity characterized by earthquake magnitudes less than or equal to 6 .7 with no associated noticeable surface ruptures. This paper presents info rmation recently gathered on the most conspicuous regional structures of th e 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 displaceme nts. Along the Sierra Chica fault, Precambrian basement has been thrust a m inimum 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 heavi ly damaged the nearby village of Sampacho. The faulted surface is buried un der Holocene loess, but its trace is expressed as a 24-km-long rectilinear scarp, despite continuous modification due to land use. Although we lack de tailed information on probable rupture lengths during large Sierras Pampean as thrust earthquakes, some preliminary considerations are made for the reg ional seismic hazard of these structures. The geologic evidence described h ere identifies these faults as possible sources of strong earthquakes in th e future.