SEISMICITY AND TECTONICS IN JUJUY PROVINCE, NORTHWESTERN ARGENTINA

The Portable Array for Numerical Digital Analysis (PANDA) network, a digitally recorded seismic array, operated for nine months in Jujuy province of northwestern Argentina. The network was deployed along the eastern edge of the Altiplano-Puna plateau in a major N-S thrust belt that is transitional in style between the thin-skinned deformation of the Bolivian foreland to the north and basement-involved deformation of the Pampean region to the south. Teleseismic locations of crustal earthquakes in the region indicate that seismicity is associated with compressional structures found near the eastern deformation front. No crustal seismicity was detected beneath the Puna plateau to the west. Peak seismicity levels beneath the foreland occurred between 20 and 25 km depth, a sharp decrease in seismicity was observed below 25 km. A estimate of 42 lan for the thickness of the Jujuy foreland crust was inferred from wide-angle Moho reflections observed on the digital seismograms. The highest concentration of crustal seismicity was located beneath Sierra de Zapla, a broad anticline immediately east of San Salvador de Jujuy. Many of the earthquakes in the 20-25 km depth range have a shallow, west dipping nodal plane as does the focal mechanism solution for a moderately large 1973 earthquake. Inversion of focal mechanism data for the orientation of principal stresses shows that maximum compression is oriented at azimuth 74-degrees, closely paralleling both the current Nazca-South America convergence direction and the shortening direction derived from regional Quaternary fault slip data. We interpret the earthquakes as occurring on planes of weakness first produced during Cretaceous rifting and later reactivated by Neogene compressive stresses. Crustal seismicity patterns and fault plane solutions suggest the presence of a midcrustal detachment, along which significant late Cenozoic E-W shortening has occurred.