The Mantle Flow Field beneath Western North America

The Mantle Flow Field beneath Western North America Station-averaged shear wave splitting beneath the western United States. Fast polarization directions denoted by azimuth of bar; splitting times denoted by length of bar. Background is smoothed, contoured splitting time magnitude. Black symbols represent new measurements from this study; red symbols rep- resent splitting measurements from the SNEP experiment (courtesy Ian Bastow); white symboles represent measurements from Long et al. [2009]; gray symbols from other previous published studies. Clear, broad-scale regional similarities exist over hundreds of km, with significant complexity over shorter spatial scales in some regions. Large splitting times dominate most of region with the exception of very small splitting times beneath Great Basin.
The goal of this study is to image the mantle flow field beneath western North America. We utilize broadband data from regional and portable seismic arrays, with an emphasis on stations from EarthScope’s USArray Transportable Array, to image seismic anisotropy and constrain deformation in the lithosphere and asthenosphere across the region.
</p><p>Regional shear wave splitting parameters show clear variations with geologic terrane. In the Pacific NW, splitting times are large and fast directions are ~E-W with limited variability. Away from the Pacific-North American plate boundary, and sandwiched between broad regions of simple and strong splitting, stations within the Great Basin (GB) exhibit significant complexity. Fast directions show a clear rotation from E-W in the northern GB, to N-S in the eastern GB, to NE-SW in the southeastern GB. Splitting times reduce dramatically, approaching zero within the central GB.
</p><p>While lithospheric fabric likely contributes to the shear wave splitting signal at many of the stations in this study, the broad- scale trends in both fast directions and delay times argue for a substantial asthenospheric source to the anisotropy. The regional mantle flow field therefore appears to be strongly controlled by significant and recent changes in plate boundary geometry. Assuming a North American plate reference frame, mantle flow appears to be controlled by absolute plate motion away from tectonic North America; conversely, rapid westward slab rollback of the Juan de Fuca plate dominates the Pacific NW U.S. flow field. To fill this gap in asthenospheric material, mantle flows strongly eastward S of the Juan de Fuca plate. Beneath the Great Basin, the paucity of shear wave splitting, combined with tomographic images and a range of geochemical and geologic evidence, supports a model of downward mantle flow due to a lithospheric drip.
</p><p>Acknowledgements: This work would not have been possible without high quality seismic data provided through the hard work of the USArray Transportable Array team, the USArray Array Network Facility, and the IRIS Data Management Center. This research was supported by National Science Foundation awards EAR-0548288 (MJF EarthScope CAREER grant) and EAR-0507248 (MJF Continental Dynamics High Lava Plains grant).</p>


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