Depth Dependent Azimuthal Anisotropy in the Western US Upper Mantle

Depth Dependent Azimuthal Anisotropy in the Western US Upper Mantle, Figure 1 Figure 1.
Azimuthal anisotropy variations with depth. Black bars indicate the fast axis direction and the bar length is proportional to the anisotropy strength. Blue, green and red arrows show the absolute plate motion (APM) directions of the North American, JdF, and the Pacific plates respectively, computed at each location using the HS3-NUVEL 1A model.
We present the results of a joint inversion [Yuan and Romanowicz, 2010] of long period seismic waveforms and SKS splitting measurements for 3D lateral variations of anisotropy in the upper mantle beneath the western US, incorporating recent data- sets generated by the USArray deployment as well as other temporary stations in the region. We find that shallow azimuthal anisotropy (Figure 1) closely reflects plate motion generated shear in the asthenosphere in the shallow upper mantle (70-150 km depth), whereas at depths greater than 150 km, it is dominated by northward and upward flow associated with the extension of the East-Pacific Rise under the continent, constrained to the east by the western edge of the north-American craton, and to the north, by the presence of the East-West trending subduction zone. (Figure 1 here) The strong lateral and vertical variations throughout the western US revealed by our azimuthal anisotropy model reflect complex past and present tectonic processes. In particular, the depth integrated effects of this anisotropy (Figure 2) explain the apparent circular pattern of SKS splitting measurements observed in Nevada without the need to invoke any local anomalous structures (e.g. ascending plumes or sinking lithospheric instabilities [Savage and Sheehan, 2000; West et al., 2009]): the circular pattern results from the depth-integrated effects of the lithosphere-asthenosphere coupling to the NA, Pacific and JdF plates at shallow depths, and in the depth range 200- 400 km, northward flow from the EPR channeled along the craton edge and deflected by the JdF slab, and more generally slab related anisotropy. With the accumulating high quality TA data, surface wave azimuthal anisotropy makes it possible to resolve complex depth dependent anisotropic domains in the North American upper mantle. (Figure 2 here)
</p><p>Savage, M. K., and A. F. Sheehan (2000), Seismic anisotropy and mantle flow from the Great Basin to the Great Plains, western United States, J. Geophys. Res., 105(6), 13,715-713,734.
</p><p>West, J. D., M. J. Fouch, J. B. Roth, and L. T. Elkins-Tanton (2009), Vertical mantle flow associated with a lithospheric drip beneath the Great Basin, Nature Geosci., 2(6), 439-444.
</p><p>Yuan, H., and B. Romanowicz (2010), Depth Dependent Azimuthal anisotropy in the western US upper mantle, Earth Planet. Sci. Lett., in revision.
</p><p>Acknowledgements: We thank the IRIS DMC and the Geological Survey of Canada for providing the waveforms used in this study and K. Liu, M. Fouch, R. Allen, A. Frederiksen and A. Courtier for sharing their SKS splitting measurements with us. This study was supported by NSF grant EAR-0643060. This is BSL contribution #10-05.</p>


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