BUOYANCY AND DOWNWELLING: DISTRIBUTION OF ECLOGITE IN THE SIERRA NEVADA

Modern topography of the Sierra Nevada has been attributed to rapid uplift following a delamination event at <10 Ma; negatively buoyant eclogitic lithosphere is thought to have detached and foundered into the mantle, driving uplift of the overlying crust. The extent of this detachment and its development through time, however, remain poorly understood. To this end, we examine the subsurface density structure in the region using seismic data and mineralogical and isostatic considerations to infer the present distribution of negatively buoyant mafic lithosphere. We find that the western foothills of the Sierra lie more than 2 km lower in elevation than is predicted from seismic velocities and empirical velocity to density relations. Conversely, the southern High Sierra stands modestly (~1 km) higher than predicted by predicted subsurface density structure. Thus, garnet-rich material is likely still present in the western foothills of the Sierra, but its replacement by buoyant asthenosphere to the east now supports anomalously high topography. Further, we have examined the mass and volume constraints provided by observations of Quaternary flexural subsidence in the Central Valley and tomography, respectively. Two high velocity bodies, one extending NNW-SSE under the Sierran foothills and the other descending vertically from ~70 km to > 200 km depth, likely couple mechanically to load the crust of the San Joaquin Valley. The integrated volume of the vertical anomaly nearly matches the that expected of the original magmatic root to the Sierran batholith. Moreover, the calculated mass of the anomaly is comparable to that of the absent mafic lithosphere beneath the range. We thus argue that mafic material removed from the southern Sierra is foundering beneath the Central Valley, while it is still present beneath the western foothills.