INSTITUTO ARGENTINO DE NIVOLOGIA, GLACIOLOGIA Y CIENCIAS AMBIENTALES
Unidad Ejecutora - UE
congresos y reuniones científicas
Structural and geophysical estimates of shortening of the Andean foreland thrust belt at 35ºS: implications for Andean structure and pre-Andean history.
MESCUA, J. F.; GIAMBIAGI, L. B.; TASSARA, A.; RAMOS, V. A.
Congreso; Geological Society of America 125th Anniversary Annual Meeting and Exposition; 2013
Geological Society of America
The Andean foreland thrust belt at 35ºS is characterized by a thick-skinned structure in which basement reverse faults transfer displacement to a Mesozoic sedimentary cover. Deformation commenced in the Early Miocene, and advanced from west to east consistently with an evolution governed by critical wedge theory. Different structural models have been proposed for this thrust-belt, depending on the interpretation of the main structures as inverted Mesozoic normal faults, newly-formed Andean thrusts or both. We compare the shortening estimates from balanced cross-sections built using these different structural models with an estimate derived from a crustal model based on an extensive compilation of geophysical data. Structural modeling constrains shortening between 23 km, for an inversion model with subordinate thrusts, and 35 km for a model based on Andean thrusting. On the other hand, the shortening calculated with the geophysical model varies greatly depending on the initial (pre-deformation) crustal thickness (T0) used. For T0 < 36 km, the calculated shortening is higher than 95 km, which is clearly too large with respect to structural estimates. Values of shortening in the range of the structural estimates are obtained with T0= 39-40 km. Taking into account the pre-Andean history of the region, this implies a complex crustal geometry before Andean deformation started, with thickened regions (probably a relict of the Late Paleozoic San Rafael orogeny) in the west, and a normal/thin crust in the east formed during Mesozoic extension related to the Neuquén basin development. Finally, we compare the regional trends of maximum crustal thickness and structurally derived shortening between 33º30?S and 36ºS. Our results indicate that the pre-existing normal faults have played an important role in the Andean deformation at 35ºS, either through fault inversion or controlling the dip of Andean thrusts.