Upper mantle velocity structure beneath California and southern Nevada

Travel times collected by regional networks in California and southern Nevada are used to map the upper mantle structure beneath the plate boundary. First, Pn travel times are used to map the uppermost mantle velocity and anisotropy structure beneath California and southern Nevada. Then, a detailed three-dimensional study uses teleseismic travel times from the southern Great Basin to map the upper mantle structure beneath this region to a depth of 400 km.; Both the Pn velocity and the Pn anisotropy correlate with the geology and tectonics of the region and trend, in general, parallel to the plate boundary implying coherent crustal-mantle lid deformation. Pn velocities are lowest beneath mountain ranges like Sierra Nevada suggesting that low-density mantle is an essential component of isostasy (equilibrium of topography and crust and upper mantle density). Pn anisotropy directions compare well with shear wave splitting polarization only in some regions while in other regions they differ substantially. This suggests that lithospheric deformation correlates with the surface tectonics while teleseismic S-wave anisotropy is controlled more by mantle flow.; The detailed three-dimensional velocity distribution in the southern Great Basin shows small velocity variations in the crust, consistent with a flat crust-mantle boundary. In the lithosphere, the velocity distribution shows an asymmetric north-south-trending structure with the lowest velocities along the eastern side of the Great Basin and comparatively higher velocities to the west. These features extend through the upper mantle down to 150–200 km depth. Temperature variations and partial melt are most likely causing the velocity variations observed in the southern Great Basin. The broad low velocity region along the eastern margin of the basin suggests more lithospheric extension beneath the Lake Mead extension domain compared to the Death Valley extension domain situated to the west. Regions of lower velocities at the top of the asthenosphere indicate the location of upwelling hot mantle material, while an adjacent high velocity zone shows where cold mantle descends. Significantly lower amplitude velocity variations in the deeper region suggest the absence of a deeper mantle plume.