Seismic Velocity Structure of the Pacific Northwestern United States From Tomographic Inversion of Teleseismic P-wave Travel Time Residuals

The goal of this study is to examine the structure and dynamics of the Cascadia subduction system as they relate to the evolution of tectonomagmatism across the backarc, including the Columbia River basalts, the High Lava Plains, and the time-progressive Newberry and Yellowstone hotspot tracks. We evaluate relative delay times from several broadband seismic networks, including the USArray Transportable Array (TA), the linear Cascadia array operated by Oregon State University in 1993-1994, and the currently operating High Lava Plains (HLP) broadband seismic array operated by Arizona State University and the Carnegie Institution of Washington. To date, we have made 8852 high-quality relative delay time measurements from 186 teleseismic events recorded at 359 stations spanning across Oregon and Washington, as well as parts of western Idaho, northern California and northwest Nevada. We applied a linear inversion using the VanDecar [1990] method to obtain a relative P- wave velocity perturbation model from the delay time residual data. The primary features of our model highlight the complex nature of this region. First, we find clear evidence for large lateral variations in the subducting Juan de Fuca slab as illuminated by higher than average seismic wavespeeds. The slab extends to a depth of perhaps greater than 400km beneath Washington, and appears to extend laterally to the east. Conversely, the slab is noticeably absent below a depth of approximately 250km beneath much of central and southern Oregon. Adjacent to this "slab window" is a region of lower than average seismic velocities located east of the slab in the back-arc that does not correlate directly with either the Columbia River basalt trend or the High Lava Plains. We also find preliminary evidence for the western extent of the Precambrian continent, generally coincident with the 87Sr/86Sr=0.706 line, a demarcation of the geochemical edge of the cratonic North American lithosphere. The strongly reduced velocities are likely the result of high asthenospheric temperatures in the mantle wedge, and perhaps provide evidence for anomalously elevated temperatures produced by a localized mantle upwelling in the region. In addition, the absence of a strong slab signature in this area suggests that elevated temperatures have significantly modified the slab in this region and is perhaps the cause for the dearth of subduction-related seismicity in the area.