Download Volume II Accomplisments (28 Mb pdf). - IRIS
Download Volume II Accomplisments (28 Mb pdf). - IRIS
Download Volume II Accomplisments (28 Mb pdf). - IRIS
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The Mantle Flow Field beneath Western North America<br />
Matthew J. Fouch (School of Earth and Space Exploration, Arizona State University), John D. West (School of Earth and<br />
Space Exploration, Arizona State University)<br />
The goal of this study is to image the mantle flow field beneath western North America. We utilize broadband data from<br />
regional and portable seismic arrays, with an emphasis on stations from EarthScope’s USArray Transportable Array, to image<br />
seismic anisotropy and constrain deformation in the lithosphere and asthenosphere across the region.<br />
Regional shear wave splitting parameters show clear variations with geologic terrane. In the Pacific NW, splitting times are<br />
large and fast directions are ~E-W with limited variability. Away from the Pacific-North American plate boundary, and sandwiched<br />
between broad regions of simple and strong splitting, stations within the Great Basin (GB) exhibit significant complexity.<br />
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<br />
GB. Splitting times reduce dramatically, approaching zero within the central GB.<br />
While lithospheric fabric likely contributes to the shear wave splitting signal at many of the stations in this study, the broadscale<br />
trends in both fast directions and delay times argue for a substantial asthenospheric source to the anisotropy. The regional<br />
mantle flow field therefore appears to be strongly controlled by significant and recent changes in plate boundary geometry.<br />
Assuming a North American plate reference frame, mantle flow appears to be controlled by absolute plate motion away from<br />
tectonic North America; conversely, rapid westward slab rollback of the Juan de Fuca plate dominates the Pacific NW U.S. flow<br />
field. To fill this gap in asthenospheric material, mantle flows strongly eastward S of the Juan de Fuca plate. Beneath the Great<br />
Basin, the paucity of shear wave splitting, combined with tomographic images and a range of geochemical and geologic evidence,<br />
supports a model of downward mantle flow due to a lithospheric drip.<br />
Acknowledgements: This work would not have been possible without high quality seismic data provided through the hard work of the USArray<br />
Transportable Array team, the USArray Array Network Facility, and the <strong>IRIS</strong> Data Management Center. This research was supported by<br />
National Science Foundation awards EAR-0548<strong>28</strong>8 (MJF EarthScope CAREER grant) and EAR-0507248 (MJF Continental Dynamics<br />
High Lava Plains grant).<br />
Station-averaged shear wave splitting beneath the western<br />
United States. Fast polarization directions denoted by azimuth<br />
of bar; splitting times denoted by length of bar. Background is<br />
smoothed, contoured splitting time magnitude. Black symbols<br />
represent new measurements from this study; red symbols represent<br />
splitting measurements from the SNEP experiment (courtesy<br />
Ian Bastow); white symboles represent measurements from<br />
Long et al. [2009]; gray symbols from other previous published<br />
studies. Clear, broad-scale regional similarities exist over hundreds<br />
of km, with significant complexity over shorter spatial<br />
scales in some regions. Large splitting times dominate most of<br />
region with the exception of very small splitting times beneath<br />
Great Basin.<br />
124 W 120 W 116 W 112 W 108 W 104 W<br />
48 N 48 N<br />
44 N 44 N<br />
40 N 40 N<br />
36 N 36 N<br />
1.0 s<br />
32 N 32 N<br />
2.0 s<br />
0.5 1.0 1.5 2.0 2.5<br />
Splitting Time (sec)<br />
<strong>II</strong>-2<strong>28</strong> | 2010 <strong>IRIS</strong> Core Programs Proposal | <strong>Volume</strong> <strong>II</strong> | Upper Mantle Structure and Dynamics