DEEP REFLECTORS AND THE POSSIBLE EMPLACEMENT OF CRUSTAL ROCKS INTO THE FOREARC MANTLE OF THE CASCADIA SUBDUCTION ZONE

Deep seismic reflection profiles, which were acquired across the Cascadia forearc in 1998 as part of the SHIPS (Seismic Hazards Investigation in Puget Sound) program, show that reflectors exist at depths as great as 50 km. These reflectors occur beneath parts of the Strait of Georgia, the Strait of Juan de Fuca, and beneath parts of Puget Sound. Under Vancouver Island, a 10-15 km thick band of reflections, previously interpreted as a shear zone developed in the continental crust above the subducting Juan de Fuca plate, extends downward to at least 50 km depth. These deep reflectors are interpreted to be close to the top of the subducting Juan de Fuca plate, and they occur along the underside of a region with P wave velocities of 6600-7000 m/s that exists in the corner of the mantle wedge. Elsewhere beneath the forearc, the upper mantle, which is likely partially serpentinized, exhibits velocities of approximately 7500 m/s. The anomalously low velocity rocks could result from a higher degree of serpentinization in this region of the mantle. Alternatively, these low velocity rocks may be of crustal origin. Splitting of the deep reflections into two reflection bands at 30-33 km depth indicates two shear zones that bound the low velocity unit, and were associated with its tectonic emplacement at mantle depths. Shallower reflectors within the forearc mantle indicate either an earlier stage of subduction or deformation linked to the present oblique subduction regime.