Seismotectonics of thin- and thick-skinned deformation in the Andean foreland from local network data: evidence for a seismogenic lower crust

Local network data from San Juan, Argentina, provides new information about crustal seismicity in the Andean foreland above a horizontal segment of the subducted Nazca Plate. Two areas of foreland seismicity are found, one associated with the Sierras Pampeanas basement uplifts, and the other beneath, but not within, the Precordillera foreland fold-thrust belt. The Precordillera seismicity provides direct evidence for basement deformation beneath the sediments of the thrust belt and supports the idea that its eastern part is significantly modified by underlying basement deformation. The depth distribution suggests a brittle-ductile transition near 30-35 km and that the upper plate may have a geotherm similar to stable cratonic regions. A spatial correlation of upper plate seismicity and structures with subducted plate seismicity raises the possibility that a major lithospheric structure cutting across the strike of the foreland near 31°S may be related to subduction of the Juan Fernandez Ridge. -from Author