We operated a digitally recording seismic network from September 1987 to May 1988 in the San Juan Province of northwestern Argentina. The network was located in the thin-skinned Precordillera and thick-skinned Sierras Pampeanas tectonic provinces of the Andean foreland. Our data provide a detailed view of the crustal seismicity of Sierra Pie de Palo, one of the most seismically active mountain blocks of the Sierra Pampeanas and the site of the destructive MS 7.3 Caucete earthquake in 1977. Most of the well-located shallow earthquakes we recorded occurred directly beneath the range between 9 and 30 km depth, with a maximum activity around 25 km depth. The morphology of the range, seismicity pattern, and focal mechanisms strongly suggest Sierra Pie de Palo is cut into two blocks by a northeast-southwest striking fracture. In the northern block, seismicity occurs along a well-defined west-dipping seismic zone, while the southern block is characterized by east-dipping seismic layers. Both dipping zones connect to a subhorizontal mid-crustal detachment that we interpret as the lower boundary of the brittle failure domain. The complex fault geometry and large variety of focal mechanism orientations we observe suggest that the ongoing seismic activity is a continuation of the aftershock sequence of the 1977 Caucete earthquake. We therefore interpret the two principal dipping zones to be buried thrusts activated during the complex rupture of the 1977 earthquake. Inversion of the focal mechanism data for the orientation of the regional deviatoric stress tensor shows the maximum compressive stress, σ1, oriented at an azimuth of N107°E. This direction is approximately orthogonal to the trend of the Sierra but rotated 27° clockwise with respect to the current Nazca-South America convergence direction. This result suggests that local boundary effects within the Sierras Pampeanas Province partially control the shortening direction in the crust. It may also indicate that a component of sinistral strike-slip motion exits within the Precordillera to the west of the Sierras Pampeanas.

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