Pacific Northwest Crust and Lithosphere Structure Imaged with Ambient Noise Tomography

Pacific Northwest Crust and Lithosphere Structure Imaged with Ambient Noise Tomography Tomography maps show the inverted isotropic shear-wave velocity structures (perturbations of ± 0.5 km/s relative to the average velocity; blue is fast). Triangles show the Quaternary volcanoes of the Cascade arc, and P.B. is the Pasco Basin.
<p>
Rayleigh-wave ambient noise tomography from periods 6-40 seconds is used to study the Pacific Northwest crust and uppermost mantle structures with the methods of Yao et al. [2006]. We include a total of about 300 broadband stations recording from 2006-2009, including EarthScope US Transportable Array, the Wallowa flex-array, a portion of the High Lava Plains flex array, and seven permanent stations. The western U.S. model of Yang et al. [2008] is used for shear-wave velocity reference. We focus on three areas:
</p><p>1) Cascades. In the Washington Cascades, where magmatic production diminishes northward to low values, the upper crust near magmatic centers usually is fast and the lower crust is slow. In Oregon, magmatic production rate is high and the crust and upper mantle are slow, whereas the old western Cascades upper crust is quite fast. These observations are consistent with the idea that magmatic intrusions make crust fast, but that high temperature can be dominant.
</p><p>2) Pasco Basin. The upper crust is very slow, suggesting that this deep sedimentary basin (which is covered by Columbia River flood basalt flows) is larger than previously mapped.
</p><p>3) Siletzia. The fast lower crust and upper mantle of eastern Washington and north-central Oregon is attributed to Siletzia, a fragment of ocean lithosphere that accreted ~ 50 Ma. The SE boundary of Siletzia (the Klamath-Blue Mountains gravity lineament; dashed line) is thought to represent a trans- form suture, and is well defined by the tomography. The NE suture is thought to be a subduction thrust system that trends NW to Vancouver Island (dotted line). We suggest that eastern Washington’s low-lying (see dash-dot line), tectonically rigid and seismically faster lower crust and upper mantle is under- thrust Siletzia lithosphere.
</p><p>References
</p><p>Yang, Y., Ritzwoller, M.H., Lin, F.-C., Moschetti, M.P., and Shapiro, N.M., Structure of the crust and uppermost mantle beneath the western United States revealed by ambient noise and earthquake tomography, J. Geophys. Res., 113, doi:10.1029/2008JB005833, 2008.
</p><p>Yao, H., van der Hilst, R.D., and de Hoop, M.V., Surface-Wave array tomography in SE Tibet from ambient seismic noise and two-station analysis – I. Phase velocity maps, Geophys. J. Int., 166(2), 732-744, doi:10.1111/j.1365-246X.2006.03028.x., 2006.
</p><p>Gao, H., E. D. Humphreys , H. Yao, and R. D. van der Hilst, Crustal and lithosphere structure of the Pacific Northwest with ambient noise tomography, in prep.
</p><p>Acknowledgements: This research is supported by NSF award EAR-051000.</p>

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