PALEO-ELEVATION OF THE SOUTHERN SIERRA NEVADA FROM LOW-TEMPERATURE THERMOCHRONOLOGY AND GEOMORPHOLOGY

The historic to recent debate over the paleoelevation of the Sierra Nevada, CA remains important to our understanding of the Cenozoic geodynamic evolution of the western plate boundary of North America. Geomorphic studies of the southern Sierra Nevada define high-elevation, low-relief “erosion surfaces” which cap the south-dipping landscape within the Kern River drainage basin. These surfaces are spatially continuous with west-dipping surfaces in the Kings and San Joaquin drainage basins and sediment-mantled surfaces in the north-central Sierra. New apatite (U-Th)/He thermochronometry ages from the southern Sierra (Kern River area) range from 75-20 Ma and are positively correlated with depth beneath the erosion surface with apparent exhumation rates of 0.03-0.06 mm/yr. These exhumation rates are consistent with long-term preservation of a low-relief landscape developed in a low-energy environment, probably at low elevations (< 2 km). Moreover, preservation of Late Cretaceous (U-Th)/He ages and correlation with sediment mantled sub-Eocene surfaces further north suggest limited erosional exhumation of the southern Sierra (<2 km) during Cenozoic time. However, approximately 1-2 km of deep river canyons have incised into the higher elevation surfaces which indicate a more recent [~20-3 Ma] increase in erosional power of the landscape that has not yet propagated entirely throughout the drainage network. Incised rivers and principal drainage patterns observed in the southern Sierra are consistent with a model of increased mean elevation due to changes in mantle buoyancy, rather than slow reduction of thickened continental crust. We are working to reconcile these data with previously published (U-Th)/He ages of House et al., 1997; 2001 who inferred late Cretaceous high elevation in the Sierra that has decreased through Cenozoic time.