CONICET | Buscador de Institutos y Recursos Humanos

The Western Sierras Pampeanas region is characterized by basement cored uplifts with approximately N-S trending ranges of average 2500-m heights, thick-skinned deformation and a high frequency occurrence of seismic activity. They correlate with the flat slab segment of the Nazca plate subducting at about 100-km depth beneath South America. Lying immediately to the east of the Argentine thin-skinned fold-and-thrust belt Precordillera, mafic and ultramafic lithologies seem to have a record of Paleozoic accretion of terranes as well as extensionalprocesses. This might have caused fracturation and a fault system that remains active.In this study we analyze the Sierra de Valle Fértil using seismic stations from INPRES (Argentina) and the CHARGE and INPRES broadband experiments. This range of approximately 180 km length separates the Cuyania terrane in the west from the Pampia terrane in the east. We observe a high seismic activity with focal depths < 25 km, mainly reverse focal mechanisms and other solutions including a strike-slip component, a crustal model of high P-wave /S-wave velocity ratio in the first 3 km depth, which is different from the lower crust between 36 and 47-km showing a low P-wave/S-wave velocity ratio. These results are consistent with a more fractured basement and/or the presence of more sediments in the upper crustal level on top of a partially eclogitized lower crust. To the east of the Sierra de Valle Fertil the crust is thinner exhibiting about 40 km-km thickness. In addition, the results are consistent whit 2D reflection cross section Analyses at 30° of latitude east of the Western Sierras Pampeanas. Our seismic results agree also with Apatite (U-Th)/He results (AHe) and neotectonic observations across the northern part of the Sierra de Valle Fértil which suggest recent exhumation likely related to uplift since Pleistocene.