Mei Xue, Richard M. Allen*
Department of Earth and Planetary Science, University of California Berkeley, USA
Received 22 March 2007; received in revised form 25 September 2007; accepted 29 September 2007
Editor: R.D. van der Hilst
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Abstract. Beneath the Pacific Northwest the Juan de Fuca plate, a remnant of the Farallon plate, continues subducting beneath the North
American continent. To the east of the Cascadia subduction zone lies the Yellowstone Hotspot Track. The origins of this track can be
traced back to the voluminous basaltic outpourings in theColumbia Plateau around 17Ma. If these basalts are the result of a large melting anomaly rising through themantle to the base of the NorthAmerican continent, such as amantle plume head, the anomalywould need to
punch through the subducting Juan de Fuca slab. Here, we use teleseismic body-wave travel-time tomography to investigate the fate of
the subducted slab and its possible interaction with a plume head. Our dataset is derived from the Oregon Array for Teleseismic Study
(OATS) deployment in Oregon and all other available seismic data in this region during the same period. In our JdF07models, we image
the subducted Juan de Fuca plate in the mantle east of theCascades beneath Oregon,where the slab has not been imaged before, to a depth
of 400 km but no deeper. The slab dips ∼50°E and has a thickness of ∼75 km. Immediately beneath the slab, we image a low velocity
layer with a similar geometry to the slab and extending down to at least ∼575 km depth in the Vs model. The total length of the high
velocity slab is∼660 km, about 180 kmlonger than the estimated length of slab subducted since 17 Ma.Assuming similar slab geometry
to today, this 180 km length of slab would reach ∼60 km depth, comparable to the thickness of continental lithosphere.We propose that
the absence of the slab below 400 km today is due to the arrival of the Yellowstone plume head ∼17 Ma, which destroyed the Juan de
Fuca slab at depths greater than the thickness of the continental lithosphere. Given this scenario, the low velocity anomaly beneath the
slab is likely the remnant plume head material which has been pulled down by traction with the subducting plate. The amplitude of the
observed low velocity anomaly is comparable with that expected for plume head material 100–300 °C hotter than the surrounding
asthenosphere. |
