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Westbrook, G.K., Carson, B., Musgrave, R.J., et al., 1994Proceedings of the Ocean Drilling Program, Initial Reports, Vol. 146 (Part 1)10. REGIONAL GEOPHYSICS AND STRUCTURAL FRAMEWORK OF THE VANCOUVERISLAND MARGIN ACCRETIONARY PRISM1R.D. Hyndman,2'3 G.D. Spence,3 T. Yuan,3 and E.E. Davis2ABSTRACTOcean Drilling Program (ODP) Leg 146 was directed at aspects of two related problems: (1) the role of sediment consolidationand fluid expulsion in the development of subduction zone accretionary prisms and (2) the nature and origin of bottom-simulatingreflectors (BSR) interpreted to be generated by high-velocity methane hydrate and underlying low-velocity free gas. The portionof the leg assigned to drilling off Vancouver Island addressed diffuse fluid expulsion from the prism not associated with obviousfaults or other structures, and the structure and composition of the hydrate. The two objectives are related through the possibilitythat fluid expulsion is required for the formation of the hydrate that forms BSRs. This article describes the regional geologicalframework and tectonics of the northern Cascadia Margin at Vancouver Island, presents regional survey data in the area of theODP sites, and summarizes the scientific studies that led to the drilling objectives. An extensive reference list is given to assistthe reader in obtaining more detailed information.INTRODUCTIONThe southern Vancouver Island margin in the area of Sites 888,889, and 890 has been the focus of comprehensive marine geologicaland geophysical surveys (see Davis and Hyndman, 1989; Hyndmanet al., 1990) complemented by adjacent onshore studies through theLITHOPROBE program (e.g., Clowes, Brandon, et al., 1987; Yorath,in press), allowing the construction of a continuous onshore-offshorestructural cross-section. The more recent detailed surveys were di-rected at providing supporting data for the primary Cascadia ODPdrilling objectives, including (1) elucidation of the role of sedimentconsolidation and fluid expulsion in the development of the accre-tionary wedge and (2) the origin of bottom-simulating reflectors(BSRs) interpreted to be generated by high-velocity methane hydrate"ice" and underlying concentrations of free gas. A supplementaryobjective was to obtain data that would help constrain the location ofthe "locked" portion of the décollement on which great subductionthrust earthquakes may occur.Present Plate Tectonic RegimeMost of the continental margin of western North America liesalong the right-lateral transform boundary between the Pacific andNorth America plates. The San Andreas fault system extends from theGulf of California to Cape Mendocino in northern California, and theQueen Charlotte fault system extends from just north of VancouverIsland to the Aleutian Trench of Alaska (Fig. 1). Along the interveningmargin of northern California, Oregon, Washington, and southernBritish Columbia, subduction of the Juan de Fuca Plate takes place inthe Cascadia subduction zone. The Juan de Fuca Plate is the remnantof the large Farallon Plate that has fragmented and diminished in sizeas a result of the convergence of the East Pacific Rise spreading centerand the North American continent, and the northward movement ofthe southern (Mendocino) triple junction (Atwater, 1970). The north-ern triple junction appears to have remained stable off northern Van-couver Island since the Eocene (e.g., Riddihough, 1984; Stock andMolnar, 1988; Engebretson et al., 1984, 1985; Atwater, 1989; Hynd-1 Westbrook, G.K., Carson, B., Musgrave, R.J., et al, 1994. Proc. ODP, Init. Repts.,146 (Pt. 1): College Station, TX (Ocean Drilling Program).Pacific Geoscience Centre, Geological Survey of Canada, P.O. Box 6000, Sidney,British Columbia V8L 4B2, Canada.3 School of Earth and Ocean Sciences, University of Victoria, Victoria, BritishColumbia V8W 3P6, Canada.130°W126C122CFigure 1. Large-scale plate regime of the Cascadia Margin. The box includesthe northern drilling area and marks the area of Figures 4 and 8. The dashedline indicates the location of the seismicity, thermal, seismic reflection, andstructure profiles of Figures 2, 3, 6, 7, and 18.man and Hamilton, 1993). The present plate configuration had itsorigin in a major reorganization of the northeast Pacific Plate regimein the Eocene at about 43 Ma.The Juan de Fuca Plate system is complex, but the present plateregime has been delineated through analyses of detailed magneticanomaly surveys and other studies. Several subplates (Explorer,Winona Block) exist to the north of the Nootka transform fault thatmeets the coast off north central Vancouver Island (Carbotte et al.,1989; Hyndman et al., 1979); off southern Oregon and northern Cali-fornia the southern portion of the Juan de Fuca Plate is deforming and399R.D. HYNDMAN, G.D. SPENCE, T. YUAN, E.E. DAVISfracturing (Wilson, 1989). In the intervening region, however, themain portion of the Juan de Fuca Plate has been converging towardthe northeast and underthrusting the continent since the Eocene. Thepresent rate of convergence increases slightly northward and is about45 mm/yr off southern Vancouver Island; the direction is approxi-mately orthogonal to the margin off southern Vancouver Island andabout 20° from orthogonal off Washington and Oregon (Riddihough,1984; Nishimura et al., 1984; DeMets et al., 1990). The North Amer-ica Plate has been advancing westward over the asthenosphere andoverriding the Pacific Plate at a rate of about 20 mm/yr for at least thepast 10 m.y. and probably at a similar rate throughout the Cenozoic(e.g., Riddihough, 1984; Pollitz, 1988), resulting in a relatively shal-low angle of subduction.The modern Juan de Fuca Plate convergence and underthrustingis evident in most of the common characteristics of subduction zones.These include (1) a series of active volcanoes extending from northernCalifornia to southern British Columbia (e.g., Wells et al., 1984) (Fig.1); (2) extensive seismicity (although there have been no historicalsubduction thrust events) (e.g., Crosson and Owens, 1987; Rogers,1983) (Fig. 2); (3) the characteristic pattern of low heat flow in theforearc inland from the margin, with an abrupt increase just seawardof the volcanic arc (Lewis et al., 1988, 1991; Blackwell, 1991) (Fig.3); (4) the characteristic low-high gravity couple represented by lowvalues over the outer margin (trench) and high values over the edgeof the continent (e.g., Riddihough, 1979) (Fig. 4); and (5) an extensiveaccretionary sedimentary prism


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