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Allen CV
Seismo Lab
Earth & Planetary
UC Berkeley


Convective interactions in the mantle beneath the Pacific Northwest: The fate of the Juan de Fuca plate

Richard M. Allen and Mei Xue
University of California Berkeley

Earthscope meeting, Monterey, March 2007



Figure Vertical cross-section through the 3D S-velocity model beneath Oregon showing the high-velocity (blue) Juan de Fuca slab diving down into the mantle with a dip of 50°.

Beneath the Pacific Northwest remanents of the Farallon plate continue to subduct beneath the North American continent. While the subducting slab has been imaged to the transition zone beneath British Columbia, previous studies suggest that the maximum imaged depth decreases to the south, reaching ~400 km beneath northern Washington, ~300km beneath southern Washington, and perhaps only ~150km beneath Oregon. To the east of the Cascadia subduction system lies the Yellowstone hotspot track. The origins of this track can be traced back to the voluminous basaltic outpourings of the Columbia River Basalts around 17 Ma. If the Columbia River Basalts are the result of a large melting anomaly rising through the mantle to the base of the North America continent, the anomaly would need to punch through the subducting Farallon slab.

We image the subducting Juan de Fuca slab beneath Oregon using teleseismic body-wave travel-time tomography and the stations of the OATS deployment across Oregon combined with data from regional seismic networks and the Earthscope Transportable Array. The 3D compressional and shear-velocity models show the high velocity slab extending to a depth of ~400km with a dip of ~50°, slightly shallower than the observed dip to the north which ranges from 60 to 65°. Resolution tests show that the dataset used would resolve the slab to greater depths if it was present in the mantle, suggesting that the slab ends abruptly at 400 km depth.

Global plate motion models constrain the convergence rate of the Juan de Fuca plate with respect to North America allowing us to trace the current location of the bottom edge of the imaged slab back to its location at 17 Ma when the source of the Columbia River Basalts reached the surface. We estimate that the bottom of the imaged slab would have been at the trench at 17 Ma. One explanation for the observed lower end of the slab is therefore that an upwelling responsible for the voluminous basaltic outpouring caused the Farallon slab to disintegrate around 17 Ma leaving no remanent of the slab deeper than the 400 km maximum depth extent that we observe today.

© Richard M Allen