STRUCTURE OF THE CASCADIA SUBDUCTION ZONE BELOW VANCOUVER ISLAND: JUAN DE FUCA PLATE AND FOREARC MANTLE

The top of the subducting Juan de Fuca (JdF) plate and the position of the Cascadia forearc mantle wedge beneath southern Vancouver Island are mapped from a joint analysis of reprocessed LITHOPROBE seismic multichannel reflection data and recent tomographic velocity models. Reprocessed data from Line 84-01 across Vancouver Island and Line 88-16, effectively a continuation of 84-01 on the mainland across Georgia Strait, plus the original data from three short lines (84-02, -03 AND -04) on Vancouver Island provide the reflection dataset. The velocity models derive from a 3-D tomographic study of onshore-offshore wide-angle data [1998 SHIPS (Seismic Hazards in Puget Sound) experiment] and earthquake data acquired north of the Strait of Juan de Fuca.

Reflections observed on NMO-corrected shot gathers from the reprocessed reflection data on Vancouver Island are identified with the top of the crust and the oceanic Moho of the JdF plate, the position of which also is constrained by earthquake hypocenters. Thus, the reprocessed reflection data define the plate underneath most of Vancouver Island, in contrast to earlier interpretations that only showed it below the western part of the island. The forearc crust-mantle boundary also is delineated at ~37 km depth at the eastern end of Line 84-01, the first evidence for a continental Moho beneath eastern Vancouver Island. Across Georgia Strait, the Moho also is identified at ~37 km on the western end of Line 88-16. In this region, previous seismic receiver function and refraction studies have estimated similar depths to continental Moho. The P-wave velocity structure along the reflection lines was extracted from the 3-D velocity model. Low values of 7.2-7.4 km/s characterize the mantle wedge at depths of 40-50 km beneath eastern Vancouver Island. We attribute these low velocities to metamorphic dehydration reactions in the crust of the JdF plate and subsequent release of aqueous fluids, resulting in hydration and serpentinization of the forearc mantle. This is the zone within which silent-slip earthquakes have recently been detected through continuous GPS studies.