THE LITHOSPHERE-ASTHENOSPHERE BOUNDARY BENEATH THE WESTERN UNITED STATES

Abstract

S receiver functions from 67 broad-band seismic stations in the western United States clearly reveal the existence of a mantle discontinuity with velocity reduction downward, which we interpret as the lithosphere-asthenosphere boundary (LAB). The average depth of the LAB is ~70 km. The boundary is relatively sharp with an overall sharpness of less than 20 km. The boundary is more prominent south of the Mendocino Triple Junction, where the Farallon Plate has completely subducted. This may indicate partial melts at the base of the lithosphere caused by the upwelling of the asthenospheric flow through the slab window. A double low velocity zone is observed at base of the lithosphere beneath southern Sierra Nevada, implying a second melting zone at a depth of ~100 km, well correlated with previous studies of lithospheric delamination in the area.