UPLIFT OF THE SIERRA NEVADA: HOW MUCH IS MANTLE AND HOW MUCH IS CRUSTAL?

New mantle-tomography data and mantle-dynamics modeling have indicated that mantle dynamics are responsible for many of the topographic anomalies of the interior western United States. However, in many cases mantle-driven uplift results in positive crustal feedbacks (e.g., faulting and erosion). How much of the geodetically observed uplift in the western U.S. results from mantle forcing and how much arises from feedbacks?

To address this question we have focused on the southern Sierra Nevada where current uplift has been well defined using geodetic methods (Hammond et al., 2012, Geology 40 667). In the same area, Saleeby et al. (2013, Geosphere 9 394) have modeled the vertical velocities imparted by asthenospheric upwelling in response to delamination of mantle lithosphere. We have attempted to quantify the shallow contributions to uplift by characterizing fault geometries, displacement rates, and amounts of erosion and basin-fill deposition in a transect stretching from the White Mountains west to the San Andreas fault. We have used these to drive a simple flexural isostatic-deflection model of the consequent surface compensations. Our preliminary results indicate that roughly half of the total vertical uplift is directly driven by mantle forcing. Approximately half of the residual uplift can be attributed to isostatic compensation for erosion from mountain ranges and deposition in the basins and the other half to tectonic denudation as a result of extensional faulting. Our results thus indicate that in the most tectonically active portion of the interior West, mantle forcing and crustal feedbacks result in approximately equal contributions to the observed uplift.