ANISOTROPY FROM JOINT ANALYSIS OF SPLIT DIRECT-S AND SKS IMAGES LITHOSPHERIC FOUNDERING UNDER THE SIERRA NEVADA, CA

The Sierra Nevada, California, with a ~3km mean elevation, lacks the thick crust generally thought to support high topography. Studies have suggested that a dense, mafic root has been removed from the Sierra Nevada since the Miocene, triggering ~1 km of uplift of the overlying surface. To understand the removal processes, we investigate seismic anisotropy in the southern Sierra Nevada, supplementing previous SKS analysis with 260 split direct S-waves. We constrain the depth of an assumed single anisotropic layer by back projecting direct S-waves to a range of depths and comparing these projections with SKS. The best-fitting depth is 150-200 km, consistent with an absolute plate motion origin for regional anisotropy. Furthermore, our results reveal a ~100 km wavelength disruption in this pattern that trends from the south-central Sierra to the high wavespeed Isabella Anomaly. This suggests that the Isabella Anomaly is dense lower crust removed from beneath the southern Sierra Nevada. This eddy also coincides with the most felsic part of the batholith and the N-S transition from low to high velocity material at 170-220 km depth, further substantiating the hypothesis that replacement of mafic material by asthenosphere is observed in patterns of asthenospheric anisotropy.