THE EVOLUTION OF DEFORMATION AND TOPOGRAPHY OF HIGH ELEVATED PLATEAUS.2. APPLICATION TO THE CENTRAL ANDES

The central Andes form a wide elevated plateau flanked in the west by a steep slope toward the deep Chilean Trench and in the east by a gent le slope that subsides gradually toward the Brazilian Shield. The low elevated trench topography is dynamically supported, whereas the high Andean mountain topography is mostly isostatically supported by a thic k crust. The last mountain building phase, which thickened the crust a nd formed the present-day Andes, began 26 m.y. ago, in the late Oligoc ene, with the increase of the convergence rate between the Nazca and t he South American plates. We investigate the time evolution of the And ean deformation and topography by applying a temperature dependent vis coplastic flow model of continental lithosphere to the South American plate. The model predicts the observed present-day topographic profile across the central Andes, from the trench across the high Altiplano p lateau to the Brazilian Shield. Our numerical results, combined with o bservations of the spatial and temporal distribution of igneous activi ty in the central Andes, lead us to conclude that the Altiplano develo ped and extended to its present width of 400 km as a result of thermal weakening of the lithosphere since late Oligocene until present. The model also predicts the observed eastward migration of the locus of th e Andean crustal deformation with time. At early stages, both the crus tal and mantle loci of deformation lie in the thermally weak region, w hich results in crustal thickening in this finite region. At later sta ges, as the crust thickens, it induces increased buoyancy forces, whic h resist crustal thickening beyond 65 km. As a result, the locus of cr ustal deformation migrates eastward. The detachment of the crustal loc us of deformation from that of the mantle can explain the observed cha nge in deformation pattern from thick-skinned tectonism during early s tages of the deformation to thin-skinned tectonism during the more rec ent stages.