Shear wave splitting and P, SKS, and S travel time residuals are calculated for teleseismic arrivals recorded on the Colorado Plateau-Great Basin Program for Array Seismic Studies of the Continental Lithosphere (PASSCAL) portable broadband seismic deployment and for permanent stations in the western United States. Little shear wave splitting is observed for broadband recordings in the northern Colorado Plateau, the Rocky Mountains, or the central Great Basin. The transition between the Colorado Plateau and the Great Basin is marked by moderate shear wave splitting (1.0 s) and unusually late teleseismic phase arrivals. This suggests material with a higher content of mantle melt or volatiles than regions to either side. Splitting in the transition between the Colorado Plateau and Great Basin is part of a pattern of fast polarizations that align in a semicircle, surrounding a central Great Basin region of null (no splitting) measurements. Away from the California plate boundary, splitting to the north and south of our study region aligns roughly parallel to the absolute plate motion of the North American plate. No simple spatial relation of splitting with geological and geophysical features such as mountain ranges, velocity anomalies, gravity, magnetics, or heat flow is evident in most of the western United States. However, splitting in the Great Basin is compatible with asthenospheric flow. The smallest shear wave splitting delay times coincide with the Eureka Low in heat flow, also having low S velocity at 300 km depth. We suggest that the circumferential pattern of fast polarization directions ringing a central region of nulls in the Great Basin is caused by mantle flow, by the interaction of mantle upwelling and the absolute motion of the North American plate. ¿ 2000 American Geophysical Union