Crust and mantle discontinuities, shear wave velocity structure, and azimuthal anisotropy beneath North America

This thesis contains four chapters which investigate seismic discontinuity structure and shear wave velocity structure in the crust and upper mantle beneath the eastern and western edges of the North American craton.; In chapters one and two, we imaged the Moho and discontinuities in the upper mantle and transition zone by stacking P to S converted phases on receiver functions recorded at the MOMA array in the eastern United States. The most interesting result is the relatively flat “410” discontinuity, which suggests that the North American lithospheric keel and any associated cold downwelling is largely confined in the upper mantle. A ∼20 km depression on the “660” discontinuity to southwest of the array may pinpoint the location of the subducted Farallon plate in the deep transition zone. Moho depth varies from 30 km to 49 km across the array, and a discontinuity observed at depths of 280 km to 320 km is most plausibly explained as the base of a low velocity zone.; In chapters three and four, we applied a surface wave inversion technique to the northeastern United States and to the Colorado Rocky Mountain region. Azimuthally anisotropic phase velocities were obtained and then inverted for shear wave velocity structure. The North American lithospheric keel is imaged with high velocity anomalies in both study areas. Keel morphology is complex at its eastern edge, possibly reflecting erosion by a mantle plume. Strong velocity variations in the crust and shallow upper mantle across the Rocky Mountain region indicate that the mountains are supported by both crustal and mantle buoyancy. The strength of anisotropy increases with period beneath the keel in the east and beneath the central Rockies in the west, suggesting a highly anisotropic asthenosphere and mechanical decoupling between the lithosphere and deeper mantle.