TECTONICS, TOPOGRAPHY AND CLIMATE OF THE SOUTHERN CENTRAL ANDES

The southern central Andes (15-27°S lat) are an integral part of a continental-scale topographic barrier to atmospheric circulation in South America, separating sharply contrasting environmental conditions and geomorphic process regimes in the humid Chaco foreland, the Subandes and the Cordillera Oriental from the arid Altiplano-Puna plateau in the orogen interior and the western flanks. Being situated in a subtropical region with atmospheric subsidence and cold upwelling along the W coast of the continent, the Andes are inherently arid in this corridor. This aridity is especially pronounced in the Atacama Desert, which receives about 20 mm/yr rainfall and constitutes a region with one of the lowest erosion rates on Earth. In contrast, the E flank of the orogen is humid and receives up to 3000 mm/yr rainfall in NW Argentina and S Bolivia. The primary source of precipitation is associated with Atlantic moisture recycled via the Amazon and moisture related to the South Atlantic Convergence Zone. The latitudinal aridity of the W flank and the orogen interior has been exacerbated by the construction of the plateau and range uplifts along its E margin and the broken foreland, thus preventing westward moisture transport and successive moisture starvation of the orogen interior. This provided the conditions for slow erosion in the interior and thick sedimentary fills and coalesced basins in the course of shortening. This tectonic style may have ultimately aided the outward propagation of basement uplifts into the foreland owing to an increase in lithostatic stress. The barriers along the plateau margin focus precipitation and runoff, promoting headward erosion and fluvial connectivity with the foreland, which prevents these areas from becoming incorporated into the plateau realm. Humid environments at the E plateau margin seem paradoxical at this latitude, but can be interpreted to result from the establishment of the South American Monsoon related to the uplift of the Andean orogen. Stable C and O isotope data from pedogenic carbonates and stable compound-specific H-isotope ratios of bulk leaf-wax samples suggest that these humid conditions must have existed by 10 to 8 Ma, while the subsequent aridification of the eastern Andean flanks took place in several stages, related to eastward-migrating range uplifts.