Seismic anisotropy, lithospheric deformation, and mantle flow in subduction zones, continental keels, and the core-mantle boundary

This thesis consists of four chapters that evaluate the location, orientation, and strength of seismic anisotropy in the lithosphere and mantle in several tectonic settings, including two western Pacific subduction zones, the tectonically stable region of eastern North America, and two regions of the core-mantle boundary beneath the Pacific Ocean. The analyses in these chapters not only utilize existing methods, but also develop new, innovative techniques to determine and investigate patterns of shear wave splitting. Beneath northwest Pacific subduction zones, we found evidence for seismic anisotropy to depths as great as 410 km in some regions. In addition, we determined that fast directions are roughly parallel to the direction of absolute Pacific plate motion beneath Izu-Bonin, roughly parallel to the strike of the trench near Japan, and roughly parallel to the direction of transpressional shear in the southern Kurils near Sakhalin Island. In the Marianas subduction zone, we found strong evidence of frequency dependence in fast directions from phases that sample the northwestern portion of the subducting slab, but did not find evidence for frequency dependence in splitting times. All of the data can be explained by models containing anisotropy in the subducting slab and mantle wedge, and possibly anisotropy in the overriding Philippine Sea plate. Beneath eastern North America, we performed measurements of shear wave splitting and combined them with the results of a simple finite-difference model to examine mantle flow around a realistic continental keel. Using this model, we calculated predicted shear wave splitting produced in the mantle resulting from flow around and beneath the keel. We found that splitting produced by modified mantle flow can explain most, but not all, of the shear wave splitting observations in this region. Beneath the central and northern Pacific Oceans, we found evidence for seismic anisotropy within the lowermost mantle (D″ ). Beneath the northern Pacific, this anisotropy may be due to lateral flow along the core-mantle boundary induced by lateral spreading of downwelling paleoslab material. Beneath the central Pacific, anisotropy may be due to lateral flow of lower mantle material toward the Hawai'ian plume source.