HR: 16:30h
AN: S42I-03    [PDF]
TI: Deep Seismic Reflection Evidence for Duplex Formation Within the Northern Cascadia Inter-Plate Boundary Zone
AU: * Calvert, A J
EM: acalvert@sfu.ca
AF: Simon Fraser University, Department of Earth Sciences, 8888 University Drive, Burnaby, BC V5A 1S6 Canada
AB: Deep seismic reflection surveys shot over the last twenty years show that two distinct regionally extensive seismic reflectors, denoted E and F, exist beneath southern Vancouver Island and the Strait of Juan de Fuca. The E reflection package is a prominent 5-15 km thick zone of high amplitude reflectors that dips between $4^\circ$ and $15^\circ$ to the northeast, and increases in depth from approximately 20 km to 40 km across Vancouver Island. The F reflection is less than 2 km thick, and is intermittently observed beneath Vancouver Island approximately 1.8 s after the deepest E reflection. But near the west coast of Vancouver Island, the F reflector shallows and approaches the base of the overlying E reflectors. In both the deep ocean basin and beneath the continental shelf, the Moho of the incoming oceanic plate is observed 1.8-2.2 s after the top of the igneous crust, implying that the F reflector cannot originate at the oceanic Moho. The F reflector is best interpreted as the top of the subducting Juan de Fuca plate for four reasons: first, beneath central Vancouver Island receiver function analyses place the oceanic Moho 6 km deeper then the F reflector; second, near the west coast of Vancouver Island, relocated inslab seismicity does not occur above the F reflector; third, large lateral velocity variations immediately above the F reflector imply that the relatively homogeneous oceanic crust must lie below it; fourth, the F reflector can be correlated with the top of the subducting plate beneath the continental shelf. The F reflector also approaches the deepest of the E reflectors east of Vancouver Island where their dip increases to $15^\circ$. The top of the subducting Juan de Fuca plate thus exhibits a slightly tilted ramp-flat-ramp geometry, and defines the floor thrust of a $>$100 km-wide duplex structure whose roof thrust is formed by the deeper E reflectors. Deep seismicity occurs primarily beneath the F reflector where the dip of the subducting plate suddenly increases, but a small number of earthquakes occur close to the E reflectors near the eastern edge of Vancouver Island. These earthquakes may be associated with imbrication above the subducting plate, and could imply that the E reflectors are an active roof thrust extending north from the Strait of Juan de Fuca to central Vancouver Island. In this region, the inter-plate boundary may be best viewed as a $>$6 km-thick zone of active deformation, involving two major thrusts above the subducting Juan de Fuca plate.
DE: 7205 Continental crust (1242)
DE: 7223 Seismic hazard assessment and prediction
DE: 8015 Local crustal structure
DE: 8105 Continental margins and sedimentary basins
SC: Seismology [S]
MN: 2003 Fall Meeting