HR: 0800h
AN: S51B-0167    [Abstracts]
TI: Structure of the Northern Cascadia Subduction Zone: A 3-D Tomographic P-wave Velocity Model
AU: * Ramachandran, K
EM: kramacha@nrcan.gc.ca
AF: Pacific Geoscience Centre, Geological Survey of Canada P.O. Box 6000, Sidney, BC V8L 4B2 Canada
AU: Hyndman, R D
EM: rhyndman@nrcan.gc.ca
AF: Pacific Geoscience Centre, Geological Survey of Canada P.O. Box 6000, Sidney, BC V8L 4B2 Canada
AU: Hyndman, R D
EM: rhyndman@nrcan.gc.ca
AF: School of Earth and Ocean Sciences, University of Victoria, Victoria, BC V8W 3Y2 Canada
AU: Brocher, T M
EM: brocher@usgs.gov
AF: U.S. Geological Survey, MS 977 345 Middlefield Road,, Menlo Park, CA 94025 United States
AB: A large-scale 3D P-wave velocity model to ~60 km depth has been constructed for SW British Columbia and NW Washington through tomographic inversion of first-arrival times from controlled source experiments together with local and regional earthquake travel-time data recorded at permanent stations. 150000 first-arrival times recorded at 225 temporary stations from the 1998 Seismic Hazards Investigation in Puget Sound (SHIPS) experiment, and 60000 first-arrival times from 3000 earthquakes recorded at 91 permanent recording stations are inverted for a minimum structure velocity model. The RMS residuals for the initial and final models are 764 and 132 ms, respectively, which represents a 97% variance reduction. Checkerboard resolution tests indicate a horizontal resolution of 30 km down to 20 km depth, and 50 km down to 60 km depth. The velocity model images the structure of the forearc crust/upper mantle, and the subducting Juan de Fuca plate geometry beneath the region. The sedimentary basins in the Straits of Georgia and Juan de Fuca and Puget Sound are well defined by the velocity model. The mafic Eocene Crescent Terrane (Metchosin Igneous Complex in southern Vancouver Island) is shown to dip beneath the margin to at least 20 km depth. This terrane is regionally extensive beneath the Strait of Juan de Fuca and the Puget lowland, with higher than average velocities of ~7 km/s at approximately 15 km depth. Beneath the Olympic Peninsula, the Core rocks (accretionary sedimentary prism) are under-thrust beneath the Crescent Terrane to a depth of at least 30 km. At this location most seismicity lies within the overlying Crescent Terrane; the under-thrusting Core rocks are aseismic. The strong Crescent terrane seismicity may be due to deformation induced by the underthrusting. Beneath southern Vancouver Island, the subduction thrust zone above the Juan de Fuca plate is characterized by low velocities of 6.4-6.6 km/s at a depth of 25-35 km. Such low velocities may be due to trapped fluids, sheared lower crustal rocks, and possibly underthrust accretionary sedimentary or metamorphic rocks. This low velocity region coincides with the high conductivity region mapped in previous magneto-telluric studies and with a dipping band of seismic reflectors; it is devoid of seismicity. It probably represents a zone of aseismic slip. Low velocities of 7.2-7.6 km/s are observed in the forearc upper mantle beneath the Strait of Georgia and Puget Sound. Such low upper mantle velocities are interpreted to be due to regional serpentinization of cool forearc mantle peridotite by fluids rising from the dehydrating underlying Juan de Fuca crust. The Tertiary sedimentary basins in the Strait of Georgia and Puget lowland lie directly above the zone of forearc upper mantle serpentinization. In contrast, the sedimentary basins in the Strait of Juan de Fuca lie in a synclinal depression in the Crescent volcanic Terrane.
DE: 7220 Oceanic crust
DE: 8105 Continental margins and sedimentary basins
DE: 7205 Continental crust (1242)
DE: 7218 Lithosphere and upper mantle
SC: Seismology [S]
MN: 2004 AGU Fall Meeting