Regional Geophysics and Structural Framework of the Vancouver Island Margin Accretionary Prism

Ocean Drilling Program (ODP) Leg 146 was directed at aspects of two related problems: (1) the role of sediment consolidation and fluid expulsion in the development of subduction zone accretionary prisms and (2) the nature and origin of bottom-simulating reflectors (BSR) interpreted to be generated by high-velocity methane hydrate and underlying low-velocity free gas. The portion of the leg assigned to drilling off Vancouver Island addressed diffuse fluid expulsion from the prism not associated with obvious faults or other structures, and the structure and composition of the hydrate. The two objectives are related through the possibility that fluid expulsion is required for the formation of the hydrate that forms BSRs. This article describes the regional geological framework and tectonics of the northern Cascadia Margin at Vancouver Island, presents regional survey data in the area of the ODP sites, and summarizes the scientific studies that led to the drilling objectives. An extensive reference list is given to assist the reader in obtaining more detailed information.

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