LITHOSPHERIC GEOID ANOMALIES AND THE PALEOSTRUCTURE OF THE ALTIPLANO: IMPLICATIONS FOR PLATEAU UPLIFT MECHANISMS

The Altiplano is characterized both by its high elevation (average elevation nearly 4 km) as well as a very rapid uplift history with ~2.3-3.4 km of uplift since the late Miocene. While constraints on the timing of this uplift are emerging, the uplift mechanism is not well known and a better understanding of how the current topography is isostatically compensated can help constrain the uplift mechanisms as well as help interpret published uplift timing data. The buoyancy required for uplift can be provided by either crustal thickening, decreases in mantle density, or a combination of these two mechanisms. Each of these modes has very different tectonic implications, and result in quite different average depths of the compensating mass deficits, potentially detectable with geoid anomalies. Estimates of local isostatic equilibration and potential energy, in tandem with lithosphere-related geoid anomalies, can be used to estimate paleolithosphere thickness, providing a clearer understanding of how and why continental topography is developed. Here we exploit the information contained in the lithospheric geoid anomalies that record changes in elevation and potential energy experienced by continental lithosphere to help constrain the pre-deformation lithospheric structure beneath Colorado Plateau and the Altiplano. Our approach uses information about the short-wavelength geoid anomaly, present-day elevation and lithospheric structure, and uplift history to estimate the paleo-lithospheric structure, thereby providing estimates of topographic development and structural support over time. Application of this technique in the Western U.S. Cordillera suggests that the uplift of the Colorado Plateau is the result of processes in the crust and mantle lithosphere; and that the lithosphere of the pre-orogenic Southern Basin and Range was thinned relative to the Northern Basin and Range and Colorado Plateau. In contrast, we find the uplift of the Altiplano-Puna segment of the Andes is the result of mainly crustal thickening with only limited contribution from the mantle lithosphere, a conclusion contrary to that of other workers. This suggests the possibility that any dense mantle lithosphere had already been removed beneath the Andes before 25 Ma.