Volatiles that are transported by subducting lithospheric plates to depths greater than 100 km are thought to induce partial melting in the overlying mantle wedge, resulting in arc magmatism and the addition of significant quantities of material to the overlying lithosphere. Asthenospheric flow and upwelling within the wedge produce increased lithospheric temperatures in this back-arc region, but the forearc mantle (in the corner of the wedge) is thought to be significantly cooler. Here we explore the structure of the mantle wedge in the southern Cascadia subduction zone using scattered teleseismic waves recorded on a dense portable array of broadband seismometers. We find very low shear-wave velocities in the cold forearc mantle indicated by the exceptional occurrence of an 'inverted' continental Moho, which reverts to normal polarity seaward of the Cascade arc. This observation provides compelling evidence for a highly hydrated and serpentinized forearc region, consistent with thermal and petrological models of the forearc mantle wedge. This serpentinized material is thought to have low strength and may therefore control the down-dip rupture limit of great thrust earthquakes, as well as the nature of large-scale flow in the mantle wedge.

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[1] S-to-P (Sp) scattered energy independently confirms the existence of a seismic velocity discontinuity at the lithosphere-asthenosphere boundary that was previously imaged using P-to-S (Ps) scattered energy in eastern North America. Exploration of the different sensitivities of Ps and Sp scattered energy suggests that the phases contain independent yet complementary high-resolution information regarding velocity contrasts. Combined inversions of Ps and Sp energy have the potential to tightly constrain associated velocity gradients. In eastern North America, inversions of Sp and Ps data require a strong, 5-10% velocity contrast that is also sharp, occurring over less than 11 km at 87-105 km depth. Thermal gradients alone are insufficient to create such a sharp boundary, and therefore another mechanism is required. A boundary in composition, hydration, or a change in anisotropic signature could easily produce a sufficiently localized velocity gradient. Taken separately, the magnitudes of the effects of these mechanisms are too small to match our observed velocity gradients. However, our observations may be explained by a boundary in hydration coupled with a boundary in depletion and/or anisotropy. Alternatively, a small amount of melt in the asthenosphere could explain the velocity gradient. The tight constraints on velocity gradients achieved by combined modeling of Ps and Sp energy offer promise for defining the character of the lithosphere-asthenosphere boundary globally.Citation: Rychert, C. A., S. Rondenay, and K. M. Fischer (2007), P-to-S and S-to-P imaging of a sharp lithosphere-asthenosphere boundary beneath eastern North America,