Chamberlain During orogenesis large amounts of tectonically driven rock uplift combined with erosional processes produces high topography. Knowledge of the timing of uplift is critical to many dynamic models of orogenesis, but paleotopographic history is difficult to determine. This project will attempt to develop a paleotopographic record for the Sierra Nevada by using oxygen isotopic data from smectite of known volcanic age and origin. The method assumes that the smectite developed from volcanic ash quickly, in situ, in equilibrium with the isotopic composition of rain water at the time of diagenesis, unaltered since, and that the measured isotopic ratios faithfully proxy the rain shadow effect of mountainous topography as weather patterns cross mountain ranges orthogonally thus providing a robust rain shadow effect proxy for the desired paleotopography. A successful outcome will provide critically needed data for cordilleran tectonic models, and will help provide and test this method for use in ancient orogens elsewhere.
