We image the upper mantle velocity structure of central California and southern Nevada by inverting teleseismic P-wave residuals. Beneath the southern Great Valley and adjacent Sierran foothills 4% high velocities extended to a depth of 200--240 km. The magnitude and the depth extent of this anomaly suggest that it is sinking lithosphere. Possible sources of lithospheric material include fragments of slab from Laramide or post-Laramide time or perhaps North American mantle lithosphere removed from neighboring regions. A west to east contrast from high to low velocities in the shallow mantle beneath the Sierra Nevada correlates with crustal constrasts from low to high heat flow and low to high seismicity, suggesting that the high velocities beneath the western foothills correspond to mantle lithosphere relatively chilled and strengthened by sub-horizontal Cenozoic subduction. In southern Nevada, low velocity mantle is imaged along the SW extension of the St. George Volcanic lineament, and high velocity mantle is found to the NW beneath most of the Miocene SW Nevada Volcanic Field. Variations in partial melt content most simply explain upper mantle velocity contrasts observed there. ¿ American Geophysical Union 1992