Seismic anisotropy and mantle deformation in the western United States and southwestern Canada

Strain aligns highly anisotropic olivine crystals in the mantle, and thus measurements of anisotropy can be used to examine deformation in the mantle. Western North America is one of the best-studied regions for anisotropy. Comprised of present subduction, strike-slip transform faulting, extension regions, and a hotspot, it includes most types of tectonic settings, and should serve as a model for the rest of the world. Yet the interpretation of the anisotropy in the region is enigmatic, with numerous competing hypotheses, some of which use the same datasets. Theoretieal considerations and results from much of the world suggest that strike-slip faulting should provide the clearest manifestation of anisotropy, with the fast direction parallel to the plate boundary. This theory has been invoked to explain the maxim that anisotropy in mountain ranges usually has the fast direction parallel to the ranges. Yet shear-wave polarizations near the San Andreas fault are mostly E-W, at 45degrees from the boundary. The same orientation is found for stations within the Sierra Nevada mountain range. Only a few stations directly over the fault exhibit the expected fast direction, and then only when a two-layer model of anisotropy is invoked, with the lower-layer anisotropy yielding an E-W fast direction. This layer has recently been modeled as caused by deep mantle flow at the bottom of the asthenosphere in response to the passage of the subducted portion of the former Farallon plate. Spreading regions were initially expected to yield anisotropy with fast directions parallel to the spreading direction, whereas results from the Basin and Range suggest that such directions may only be present in a thin upper layer, and the Rio Grande Rift has fast directions in between rift-parallel and perpendicular. These results have been explained in some studies as resulting from asthenospheric flow rather than lithospheric deformation. Other papers suggest that the asthenosphere is deep or does not produce measurable anisotropy. Finally, results above the Juan de Fuca slab provide one of the few regions in which the fast direction of anisotropy follows the expected flow of the mantle in response to subduction.