FATE OF WATER IN THE CASCADIA FOREARC UNVEILED BY TELESEISMIC IMAGING

The past decade has witnessed the deployment of large numbers of broadband seismometers along the Cascadia forearc. A high density of sampling has permitted the mapping of subduction zone structure along and across strike using scattered teleseismic body waves. Forearc structural signatures across profiles sampling central Oregon, Puget Sound, and Vancouver Island are remarkably similar. A ~5 km thick, dipping low-velocity zone (LVZ) extending to depths of >50 km is the dominant feature on these profiles and is inferred here to represent subducting oceanic crust of the Juan de Fuca plate. Estimates of Poisson's ratio within the shallow LVZ are extremely high, between 0.35 and 0.4, and cannot be attributed to lithology. Rather, they are interpreted to manifest near-lithostatic, pore-fluid pressures generated through metamorphic dehydration reactions within the oceanic crust. Accordingly, the plate interface must represent an impermeable boundary at these depths. Farther down dip, the signature of the LVZ fades as its upper followed by lower boundaries become seismically indistinguishable from ambient mantle. Thermo-petrological models predict that metabasalt undergo a final transformation to nominally anhydrous eclogite at these depths, consistent with the seismic observation. Immediately above the LVZ, within the mantle wedge, there is a significant velocity reduction that is sufficient to erase the seismic contrast that typically marks the transition from mantle wedge to continental crust. This feature is inferred to be due to the combined presence of antigorite and free water (at elevated pore pressure) resulting from eclogitization, consistent with thermo-petrological models. A number of other geophysical (seismic reflection, gravity, magnetics) observations lend support to the serpentinization interpretation and demonstrate its continuity along strike. Both serpentinization and eclogitization involve large (10%-20%) changes in volume that are likely accommodated through fracturing, thereby promoting a dramatic increase in permeability across the plate interface near the mantle wedge corner. This location coincides with hypocentral locations of non-volcanic tremor and, given the oft-inferred association of tremor with fluids, we speculate that changes in plate interface permeability may play a key role in tremor generation.