High-Resolution Imaging of the Mantle Flow Field Beneath Western North America

The goal of this study is to provide an improved understanding the nature of deformation in the crust and lithospheric mantle and its relationship to the mantle flow field beneath western North America. We utilize broadband data from regional and portable seismic arrays, including EarthScope's USArray Transportable Array and the ~120 stations of the High Lava Plains seismic array to image seismic anisotropy in the crust and mantle to constrain deformation in the crust, mantle lithosphere, and asthenosphere across the region. Regional shear wave splitting parameters show clear variations with geologic terrane. In the Pacific Northwest, splitting times are large (2.25+ sec) and fast directions are ~E-W with limited variability. Beneath the southern Basin and Range/Colorado Plateau region, splitting times are also large (~1.75+ sec) and fast directions are oriented ~NE-SW (similar to absolute plate motion). Stations near the San Andreas fault exhibit more variability between measurements at individual stations, but regionally exhibit a general rotation toward NW-SE for stations closer to the fault. Analyses from a dense array across the fault near Parkfield exhibit fast direction variations of ~30 degrees over ~15 km, indicating that uppermost crustal structure plays a significant role in some regions. Away from the Pacific-North American plate boundary, and sandwiched between broad regions of simple (i.e., regionally similar fast directions) and strong (i.e., large splitting times) azimuthal anisotropy, stations within the Great Basin exhibit significant complexity. Fast directions show a clear rotation from E-W in the northern Great Basin, to N-S in the eastern Great Basin, to NE-SW in the southeastern Great Basin. Splitting times reduce dramatically, approaching zero within the central Great Basin. At many stations within the Great Basin, particularly those that have been in operation for many years, we observe backazimuthal variations in splitting parameters that suggest a crustal/lithospheric mantle signature. The broad-scale trends in both fast directions and delay times, however, clearly argue for a sublithospheric source. Studies are currently underway to assess the degree of crustal anisotropy at many of these stations via receiver function analysis. Based on our current dataset, the overall regional mantle flow field appears to be strongly controlled by significant and recent changes in plate boundary geometry. For instance, beneath the Pacific Northwest, flow is strongly controlled by rapid westward slab rollback, while beneath the Great Basin, mantle downwelling dominates the local flow field. We will evaluate further our results of seismic anisotropy in the context of both the regional geologic history and new body and surface wave seismic tomography images.