| We use seismological methods to investigate mantle structure in two highly deformed regions of the western U.S., the Yellowstone Swell and the Cheyenne Belt. Using shear wave splitting to measure anisotropy, and thus infer mantle flow, we find a single layer of simply sheared mantle under the Yellowstone Swell. Outside the region affected by the passage of the hotspot, we can measure no anisotropy. This result is unusual, and we interpret it to mean that the upper mantle in the western U.S. is not being strained by plate motion, perhaps indicating that plate motion is being accommodated at a much greater depth than previously thought, perhaps in the transition zone.;We also measure seismic velocities in a transect across the Yellowstone Swell. We find significant VS and V P/VS structure. This is interpreted as being mostly to the presence of melt beneath the Snake River Plain, probably between .5%--1.5%.;This dissertation includes both previously published and unpublished co-authored materials. |
