Depth constraints on azimuthal anisotropy in the Great Basin from Rayleigh-wave phase velocity maps

We present fundamental-mode Rayleigh-wave azimuthally anisotropic phase velocity maps obtained for the Great Basin region at periods between 16 s and 102 s. These maps offer the first depth constraints on the origin of the semi-circular shear-wave splitting pattern observed in central Nevada, around a weak azimuthal anisotropy zone. A variety of explanations have been proposed to explain this signal, including an upwelling, toroidal mantle flow around a slab, lithospheric drip, and a megadetachment, but no consensus has been reached. Our phase velocity study helps constrain the three-dimensional anisotropic structure of the upper mantle in this region and contributes to a better understanding of the deformation mechanisms taking place beneath the western United States. The dispersion measurements were made using data from the USArray Transportable Array. At periods of 16 s and 18 s, which mostly sample the crust, we find a region of low anisotropy in central Nevada coinciding with locally reduced phase velocities, and surrounded by a semicircular pattern of fast seismic directions. Away from central Nevada the fast directions are similar to N-S in the eastern Great Basin, NW-SE in the Walker Lane region, and they transition from E-W to N-S in the northwestern Great Basin. Our short-period phase velocity maps, combined with recent crustal receiver function results, are consistent with the presence of a semi-circular anisotropy signal in the lithosphere in the vicinity of a locally thick crust. At longer periods (28-102 s), which sample the uppermost mantle, isotropic phase velocities are significantly reduced across the study region, and fast directions are more uniform with an similar to E-W fast axis. The transition in phase velocities and anisotropy can be attributed to the lithosphere-asthenosphere boundary at depths of similar to 60 km. We interpret the fast seismic directions observed at longer periods in terms of present-day asthenospheric flow-driven deformation, possibly related to a combination of Juan de Fuca slab rollback and eastward-driven mantle flow from the Pacific asthenosphere. Our results also provide context to regional SKS splitting observations. We find that our short-period phase velocity anisotropy can only explain similar to 30% of the SKS splitting times, despite similar patterns in fast directions. This implies that the origin of the regional shear-wave splitting signal is complex and must also have a significant sublithospheric component. (C) 2009 Elsevier B.V. All rights reserved.