Cenozoic tectonic evolution of the Sierra de Valle Fértil, Sierras Pampeanas, Argentina: a thermochronology and seismological approach

ORTIZ, G.; ALVARADO, P.; VENERDINI, A.; FOSDICK, J.; CARRAPA, B.

SANTIAGO

Simposio; SIMPOSIO DE TECTONICA SUDAMERICANA; 2016

The Sierras Pampeanas (SP) (27°S-33°S) basement-cored uplifts in the Andean retroarc of the western-central region of Argentina, correlate with the region were flattening processes of the subducting Nazca slab beneath South America have been observed. Large crustal earthquakes showing reactivation of preexisting weaknesses zones and an absence of active arc volcanism are observed even six hundred kilometers eastward from the trench. Deformation of this thick-skinned region and their entire exhumation has been associated to the passage of the aseismic Juan Fernández Ridge (JFR) beneath the continental plate increasing also lithospheric coupling. Further details of this relation between plate coupling and exhumation in the SP region however, have not been fully studied. They can help to analyze the tectonic evolution of the basement-cored uplifts in the mountain building process.The Sierra de Valle Fértil (SVF) in the western SP extends for about 180 km length in a NW-SE orientation. It is located to the east of the Argentine Precordillera fold-and-thrust belt.Deformation is thus concentrated in the narrow zone between these two SP and Precordilleran structural provinces. We used low-temperature thermochronology consisting of apatite (U-Th)/He estimations (AHe)to put quantitative constrains on the timing and magnitude of rock exhumation. In addition, we analyzed crustal seismicity of the SVFand integrated results using 2DMove structural models..We found Mio-Pliocene AHe ages from the SVF consistent with recent cooling and inferred erosional denudation concentrated along the western border of this mountain range where the SVF fault is located. In addition Paleogene AHe data characterize the central part of the SFV. These results together support the idea that a positive, or near surface relief already existed before Neogene exhumation. Modern seismicity in the same region shows focal depths between 10 and 38 km depth and mainly reverse focal mechanisms. We concluded that the exhumation of the SVFstarted from its central part and migrated toward its north and south edges, in good agreement with the time of flattening of the Nazca slab and the NE-SW elongated geometry of the JFR. Presently regional earthquake stress field exhibits a nearly east-west horizontal compressive axis and an almost vertical tensional axis.