HR: 1330h
AN: S42A-0151    [PDF]
TI: Structure of the Juan de Fuca Plate and Forearc Mantle below Vancouver Island
AU: * Ramachandran, K
EM: kramacha@nrcan.gc.ca
AF: Pacific Geoscience Centre, Geological Survey of Canada 9860 West Saanich Road, Sidney, BC V8L 4B2 Canada
AU: Clowes, R M
EM: clowes@lithoprobe.ubc.ca
AF: Department of Earth and Ocean Sciences, The University of British Columbia Room 357 Geological Sciences 6339 Stores Road, Vancouver, BC V6T 1Z4 Canada
AB: LITHOPROBE multichannel seismic reflection data, recorded on Vancouver Island in 1984, provided the first images of the subducting Juan de Fuca (JdF) plate. At the western edge of the island, Line 84-01 identified the top of the JdF plate at ~10 s two-way time (TWT; ~30-33 km depth); in the southern part, Line 84-02 showed the plate at ~11.5-12.5 seconds TWT (~35-42 km depth). Data from these and two other short lines on the island plus Line 88-16, effectively a continuation of 84-01 on the mainland across Georgia Strait, have been reprocessed. Reflections from the top of the JdF crust and Moho are observed on NMO-corrected shot gathers. The new results extend mapping of the top of the subducting JdF crust and Moho to eastern Vancouver Island, where the top of the plate is identified at ~13 s TWT (~44-47 km depth). Our interpretation is consistent with locations of earthquake hypocenters. The results also show the position of the Cascadia forearc mantle wedge beneath southern Vancouver Island. At the end of Line 84-01 on eastern Vancouver Island, the forearc crust-mantle boundary is delineated at ~11 s TWT (~37 km depth). Across Georgia Strait, the forearc Moho also is identified at ~11 s TWT (~37 km depth) on the western end of Line 88-16. Previous seismic receiver function and refraction studies have estimated similar depths to continental Moho in this region. On the western end of Line 88-16, a band of NE-dipping upper mantle reflections is observed between 13-15 seconds TWT. This reflective unit seems to correlate with the prominent `E' reflections observed on Line 84-01. To strengthen this interpretation, further processing to improve the signal coherency of `E' reflections on the eastern end of Line 84-01 is in progress. Velocity models along the reflection lines on Vancouver Island are extracted from a 3-D P-wave tomographic study of onshore-offshore wide-angle data [1998 SHIPS (Seismic Hazards Investigation of Puget Sound) experiment] and earthquake data acquired north of the Strait of Juan de Fuca. Low P-wave velocities (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. Silent-slip earthquakes recently have been detected within this zone through continuous GPS studies.
DE: 7205 Continental crust (1242)
DE: 7218 Lithosphere and upper mantle
DE: 7220 Oceanic crust
SC: Seismology [S]
MN: 2003 Fall Meeting