RECEIVER FUNCTION IMAGES OF THE LITHOSPHERE IN THE WESTERN U.S

We have begun making P- and S-wave receiver functions from the data available from USArray Transportable Array stations to examine lithospheric and upper mantle structure beneath the western US. At present the Transportable Array extends as far west as ~108^o to 113^oW, and thus crosses the Snake River Plain and Columbia flood basalts, most of the Basin and Range, part of the Rocky Mountains, and about half of the Colorado Plateau. Our preliminary results are for PdS receiver functions: We constructed common conversion point (CCP) stacked volumes for the western U.S. using a standard reference velocity model for ray tracing, with both linear and 4th root stacking methods, the latter to enhance lateral correlation. The stacks contains ~12,000 receiver functions from 175 earthquakes recorded at 331 stations.

The preliminary stack shows a number of interesting lithospheric features, particularly the lithosphere-asthenosphere boundary (LAB). We identify the LAB as a negative polarity event at various depths beneath the Moho, above the zone of contamination from crustal multiples in PdS receiver functions. In northwestern Nevada near (120^o-117^oW, 40^o-42^oN), the LAB appears to shallow almost to Moho depths (~30 km) in the region of modern Basin and Range extension, but is otherwise at about 60km depth beneath the rest of the Basin and Range. These results are in good agreement with SdP receiver functions by Li et al., 2007. To the north beneath the Snake River plain and Columbia flood basalts, the LAB is deeper, broader, and centered at about ~75 km depth. The southernmost Sierra Nevada, Mojave block, and southern Basin and Range have an eastward thickening lithosphere, with the LAB increasing in depth from ~50km to 70km under the southwestern edge of the Colorado Plateau. The Sierra Nevada has lower crust and upper mantle complications previously identified by Zandt et al., 2004. Further work will include incorporating a laterally variable crustal model derived from active source studies to recalculate the CCP stacks. Working with other datasets we have observed that an accurate crustal velocity model can reposition Moho depth by several times the formal depth error obtained from a general reference model. This is also presumably true for the depth to the LAB.