CONICET | Buscador de Institutos y Recursos Humanos

Twenty-five new long-period magnetotelluric sites near 31.5°S were collected in a west?east profile. Thisprofile and the previous one, aligned with and adjacent to the eastern end, have been merged to form a singleprofile of more than 700 km long, extending from the Precordillera to the Chaco-Pampean Plain.The geotectonic scenario is characterized by a modern flat subduction zone of the Nazca plate located at a depthof around 120 km and clearly defined by the distribution of earthquake hypocenters recorded by local and regionalnetworks. A ?bulge? shape at 68.5°W, with an anomalous dip to thewest, is observedwithin this segment.The smooth slab deformation might result from the restriction on eastward motion due to the presence of anelectrically resistive zone. The magnetotelluric model shows that this thick zone of increased resistivity isfound from shallow crustal levels to upper mantle depths. The bulge geometry allows hot fluids and volatilesto rise from the deeper asthenospheric wedge, and reach the lower crust reducing its viscosity and letting itflow. The zones of low resistivity in the lower crust show spatial correlation with the areas of foreland deformationfrom Precordillera to the Sierras Pampeanas and may also suggest a ductile regime.Shear zones reactivated by Cenozoic faulting must necessarily have their roots in the levels of the ductilelower crust associated to conductive channels. The zone where the lower crust is closer to the surface coincideswith the areas of greatest structural relief and erosion. The interface between the folded ductile lowercrust and the brittle upper crust might act as the main level of décollement of the bordering structures betweenthe Precordillera, Sierra de Pie de Palo and the Sierras Pampeanas. In addition, the geometry of the interfacemight be conditioning the vergence of those structures.