The Lithosphere-Asthenosphere Boundary Beneath the Western United States

Abundant studies have focused on the tectonic activity in the western US. Direct constraints on lithosphere thickness are, however, rare. Receiver function technique is powerful to observe seismic discontinuities in the crust and upper mantle. In the western US, the P receiver functions reveal complicated crust structures. A strong intra-crust interface at a depth of ~ 20 km effectively masks the converted signals from the Moho and lithosphere interfaces at many sites. The S receiver function method overcomes this problem. S receiver functions from 67 broadband stations in the western US clearly reveal the existence of a mantle discontinuity with velocity reduction downward, which we interpreted as the lithosphere-asthenosphere boundary (LAB). Waveforms of the S receiver functions provide constrains on nature of the boundary. The average depth of the LAB is ~ 70 km, which is consistent with other geophysical observations. The boundary is relatively sharp with an overall sharpness of less than 20 km. It is much sharper and stronger 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 indicated at base of the lithosphere beneath southern Sierra Nevada, implying a second melting zone at a depth of ~ 100 km, well corroborated with previous studies of lithospheric delamination in the area. Although the S receiver functions show a crust interface besides the Moho at some stations, it may interfere with the Moho due to long wavelength, and result in mis-interpretation of the crust structure by S receiver function alone.