LATE CENOZOIC GLACIAL EROSION AND EXHUMATION: IMPLICATIONS FOR THE PALEOELEVATION OF THE CENTRAL ANDES AND LHASA

We have developed a three-dimensional finite-difference model for the distribution of glaciers, glacial erosion, and flexural-isostatic uplift in alpine terrain. Our glacial model is based on an ideal plastic rheology and predicts the distribution and thickness of alpine glaciers in any landscape given the yield stress of ice and the ELA. Combined with a model for glacial erosion and isostatic rebound, a powerful tool for alpine landscape evolution has been developed. We apply our model to the history of glacial erosion and exhumation of an antiform following a lowering of ELA. Even a small lowering of ELA can result in large changes in the extent and rates glacial erosion and exhumation through a positive feedback. Localized erosion (determined by the mechanics of glaciers) followed by rock uplift over a broader region (related to the flexural wavelength) leads to a nonlinear increase in the volume and area covered by glaciers which in turn drives more erosion and uplift. The geology and topography predicted by the model include the exhumation of a broad (~100 km wide) region capped by a narrow (~10 km wide) peak or massif which can be several km in elevation above the surrounding topography. Narrow peaks form by erosion localized at the base of the peak; higher elevations are not eroded due to the relative absense of ice in high, steep topography (as predicted by the ideal plastic model). Thermochronologic data from these exhumed rocks (granitoids) may have a very different cooling history than nearby rocks whose uplift histories reflect older tectonic shortening with little recent exhumation. These ideas are applied to the geology and thermochronology of the high peaks of the central Andes and Lhasa using available data sets. Our results suggest that the Miocene exhumation of these regions may reflect global cooling rather than surface uplift, indicating that the rocks were already near their present elevations at or before the late Miocene.