Mantle structure and flow beneath the central-western US: Constraints from anisotropic tomography

To investigate lateral and depth variations of seismic anisotropy beneath the central-western United States, we determined a detailed 3-D model of P-wave anisotropic tomography by inverting a large number of arrival-time data of local and teleseismic events. Our results reveal significant azimuthal anisotropies in the crust and lithosphere, which are associated with ancient orogenic collisional and magmatic activities. As depth increases, the fast-velocity direction (FVD) pattern becomes gradually trended and small features fade away. There is a boundary in the FVD distribution, which separates the tectonically active region in the west from the stable cratonic region in the east. Frozen-in anisotropy with a NW-SE FVD is preserved in the thick Wyoming cratonic lithosphere that exhibits as a high-velocity (high-V) anomaly to a depth of ~250 km. In the asthenosphere beneath the western thin lithosphere, FVDs are generally parallel with the absolute motion direction of the North American plate due to shearing between the plate and the asthenosphere. In the deeper areas, the subducted and fragmented slab exhibiting as high-V anomalies leads to slab-related mantle flows. These results indicate that seismic anisotropies exist in both the lithosphere and asthenosphere with different geodynamic mechanisms and it is feasible to link the P-wave azimuthal anisotropy to lithospheric deformations, fossil anisotropy in the lithosphere, and flows in the asthenosphere.