Advances in crustal deformational studies in the Andes between 33° and 36°S

LAURA GIAMBIAGI 1, JOSÉ MESCUA1 AND FLORENCIA BECHIS 2

Workshop; First workshop IGCP 586Y project; 2010

The Andean of west-central Argentina (33°-36°S), above the normal subduction segment, presents important along-strike variations in topography, structural style and elevation, amount and rate of shortening, and crustal thickness. To analysis the controlling factors of these latitudinal changes we compare these parameters and the chronology of deformation along eleven balanced crustal cross-sections across the thrust front between 70° and 69°W. Our results indicate a synchronous onset of deformation along-strike at ca. 17-18 Ma, a smoothly southward reduction of total amount of shortening from 60 to 10 km, shortening rates from 3.3 to 0.6 mm/a and average topographic elevation from 3500 to 2200 m. Various controlling factors for upper-plate shortening have been proposed for the Southern Central Andes, although the mechanism of shortening drop remains debatable. Several discussed parameters, such as convergence and geometry of the Nazca plate, westward drift of the South American plate, the onset of deformation, and thickness of Pre-Andean sediments do not vary along the study area. On a regional scale, crustal thickness, topography and amount and rate of shortening show a progressive decrease that correlates with the decrease in age of the subducting slab and the presence of the Maipo orocline. Local variations cannot be fully explained by differential in the subducted slab age and fore-arc rotations. Instead, a close correlation is suggested between previous crustal and lithospheric anisotropies present in the upper plate and the style and amount of shortening. We explore the important effects of crustal and lithospheric heterogeneities related to Pre-Andean tectonic events, such as Paleozoic orogenies and Mesozoic extensional events. Our results indicate that plate geodynamic could control the overall pattern of deformation but local variations are more likely to be due to fault reactivation processes and lithospheric strength variations.