CENOZOIC TECTONIC EVOLUTION OF THE SIERRA DE VALLE FÉRTIL, SIERRAS PAMPEANAS, ARGENTINA: A THERMOCRONOLOGY AND SEISMOLOGICAL APPROACH

AGOSTINA VENERDINI; BARBARA CARRAPA; GUSTAVO ORTIZ; ALVARADO PATRICIA

Santiago de Chile

Simposio; Primer Simposio de Tectónica Sudamericana; 2016

Asociación de Tectónica Sudamericana

The Sierras Pampeanas (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 Nazca slab subducting beneath South America have been observed. Large crustal earthquakes and absence of active arc volcanism are observed even six hundred miles eastward from the trench showing reactivation ofpreexisting weaknesses zones. 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 causing also lithospheric coupling. Further details of this relation between plate coupling and exhumationin the Sierras Pampeanas (SP) region however, have not been fully determined. Their estimation can help to analyze a model for the tectonic evolution of the basement-cored uplifts. The Sierra de Valle Fértil (SVF) in the western SP region extends for about 180 km length in a NW-SE orientation. Its location eastof the Argentine Precordillera fold-and-thrust belt may suggest that deformation is concentrated in the narrow zone between these two structural provinces. We used low-temperature thermochronology due to the fact that this technique may place quantitative constrains on the timing and magnitude of rock exhumation. In this work we present new apatite (U-Th)/He results (AHe) and seismic analysis for the SVF. Furthermore, structural models using 2DMove have been also developed for constraining these results. We found Mio-Pliocene AHe ages from the SVF indicating that recent cooling and inferred erosional denudation were concentrated along the western border of this mountain range; Paleogene AHedata instead characterize the central part of the SFV. This supports 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 with mainly reverse focal mechanisms. We concluded that theexhumation of the SVF started from its central part 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 stress field exhibits a nearly east-west horizontal compressive axis and an almost vertical tensional axis.