Mantle flow under the western United States from shear wave splitting

We show that SKS splitting in the westernmost United States (polarization of the fastest shear waves and splitting times, including their back-azimuthal dependence) can be explained by a geodynamic model that includes a continuum-mechanics description of plate motions and underlying asthenospheric circulation. Models that include a counterflow at depths of ∼ 300 km are preferred, which may indicate a far-field effect of the Farallon slab anomaly sinking underneath the central continental United States. This finding is broadly consistent with earlier suggestions, and we demonstrate that a mechanically coupled system, though with a strong viscosity contrast with depth, is consistent with the data. We explore the depth dependence of predicted anisotropy by means of computing seismogram synthetics and comparing synthetic splits with observations. Some patterns in the data, including null observations, are matched well. Linked models of geodynamic flow and mineral alignment in the mantle provide a means to test the relationship between strain and the saturation of texturing. Lower fabric saturation strains than for global models are preferred by the data, which may reflect the relatively active tectonic setting and thin asthenosphere of the study region. In general, our results show that seismic anisotropy, when interpreted jointly with mineral physics theories, may be used to quantitatively constrain the spatial character of flow, and the degree of force coupling, at depth.