Seismic anisotropy and mantle flow from the Great Basin to the Great Plains, western United States

Shear wave splitting and P, SKS, and S travel time residuals are calculated for teleseismic arrivals recorded on the Colorado Plateau-Great Basin Prog ram far Array Seismic Studies of the Continental Lithosphere (PASSCAL) port able broadband seismic deployment and for permanent stations in the western United States. Little shear wave splitting is observed for broadband recor dings in the northern Colorado Plateau, the Rocky Mountains, or the central Great Basin. The transition between the Colorado Plateau and the Great Bas in is marked by moderate shear wave splitting (1.0 s) and unusually late te leseismic phase arrivals. This suggests material with a higher content of m antle melt or volatiles than regions to either side. Splitting in the trans ition 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 fe atures such as mountain ranges, velocity anomalies, gravity, magnetics, or heat flow is evident in most of the western United States. However, splitti ng 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 circumfere ntial pattern of fast polarization directions ringing a central region of n ulls in the Great Basin is caused by mantle flow, by the interaction of man tle upwelling and the absolute motion of the North American plate.