Paper No. 4
Presentation Time: 2:20 PM


HOKE, Gregory D.1, WALCEK, Alina A.1 and GRABER, Nathan R.2, (1)Department of Earth Sciences, Syracuse University, 204 Heroy Geology Laboratory, Syracuse, NY 13244, (2)Earth Sciences, Syracuse University, 204 HGL, Syracuse, NY 13244,

The linkages between tectonic and geodynamic processes in generating the topography of the Earth’s surface provide fundamental constraints for refining our construct of how orogens evolve in time and space. The eastern flanks of the southern central Andes present a unique laboratory for exploring how the topography of a mountain range has developed over the Neogene, with dramatic gradients in total crustal shortening and the shape of the subducting Nazca slab. We combine data from paleoaltimetry, thermochronology and tectonic geomorphology to examine the amount of pre-existing topography, when high topography developed, and explore what factors drive topographic change.

At 33°S, the eastern flank of the Andes is divided in the Principal Cordillera, Frontal Cordillera and Precordillera ranges, which give way to the foreland. South of 33ºS the Precordillera ends, however the Neogene strata south of the Precordillera are deformed. The traditional tectonic model describing the development of the Andes advocates east-stepping propagation of deformation, from what is today the core of the range, towards the foreland. Neogene strata preserved between each range have been interpreted in previous studies to reflect a continuous, unbroken foreland, whose western edge migrated eastward with the propagation of deformation. Our data from river profiles and cosmogenic nuclide erosion rates demonstrate uplift of the range commenced by ~ 10 Ma and that substantial Neogene topography existed in the southernmost 100 km of the Precordillera. Similarly, thermochronology data indicate that the Frontal Cordillera was never buried by a thick foreland foredeep sequence, which suggests that the planation surfaces which today lie at 4 - 5 km elevation were at the surface for much of the Cenozoic. Finally, stable isotope data from strata exposed in the western edge of the foreland and intermontane basins suggest both were at similar elevations until ~ 10 Ma. Considering the modern elevation of the intermontane strata, 1.5 to 2.5 km of surface uplift occurred in the Late Miocene to Pliocene, consistent with geomorphic studies in Chile. Our results indicate the Andes rose significantly during the last major pulse of deformation and that the area was not a simple foreland, but rather one with a complex paleogeography.