UCLA

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 semi-circular pattern of fast seismic directions. Away from central Nevada the fast directions are ∼ 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 ∼ E–W fast axis. The transition in phase velocities and anisotropy can be attributed to the lithosphere–asthenosphere boundary at depths of ∼ 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 ∼ 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.