Descripción: Modeling of broadband seismic waveforms recorded by global and Chilean networks for two moderate crustal earthquakes of the northwestern Sierras Pampeanas shows their focal mechanisms, depths and seismotectonic features. The magnitude Mw 5.8 earthquake on 28 May 2002, located in the eastern flank of the sierra de Velasco and the Mw 6.2 earthquake on 7 September 2004, with epicenter in the southwestern part of the sierra de Ambato, have shallow focal depths of 10 and 8 km, respectively. These results combined with the historical seismicity of the region allow us to estimate the deep structure of the Sierras Pampeanas in the study region. The seismic analyses together with interpretations of the surface structure and previous neotectonic studies ruled out extensional or strike slip deformation as the main responsible mechanism of the Present structure of this sector of the sierras de Ambato and Velasco in the northwestern Sierras Pampeanas. The comparison between the Nazca-South America plate convergence orientation as well as GPS velocities in the upper plate with the summation of the seismic moment tensor for the largest seismic energy released by crustal earthquakes of this region in the last 30 years, shows a clockwise rotation of 50° of the average P-axis orientation from the convergence orientation to the northeast suggesting important strain partition. This partition is controlled by the Eopaleozoic basement fabric, which has guided the orientation and vergence of the Andean faults.

Descripción: The Central Precordillera of San Juan is a thin-skinned fold and thrust belt bounded to the east by the thick-skinned structures of the Eastern Precordillera. The objective of this study is to describe and interpret the structural features found at the southern end of this fold and thrust belt. The lomas de Andapaico's folds are of asymmetric, open chevron type, and are genetically related to fault propagation. Toward the north, the folding becomes tighter and broken by faults. The regional structure consists of an imbricated fan of an array of separated to overlapping fault propagation folds, and out-of-sequence structures related to the Eastern Precordillera uplift. The major décollement thrust is located near the top of the San Juan Formation limestone. Deformation occurred in two main stages. First, an imbricate array of fault-propagation folds was developed as the thrust front migrated toward the foreland. Then, the uplift of the eastern Precordillera impeded the migration of the thrust front and may have caused the development of out-of-sequence structures in the Central Precordillera. The minimum horizontal shortening calculated is 4.8 km, and the whole structure shows a southward decreasing shortening that is shown by structures of different orders and a variety of styles of deformation.