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doi:10.2204/iodp.proc.311.109.2006

Introduction

The northern Cascadia margin offshore Vancouver Island has been the focus of many marine geological and geophysical studies over the past two decades, including Ocean Drilling Program (ODP) Leg 146 (Westbrook, Carson, Musgrave, et al., 1994). Adjacent onshore studies of the LITHOPROBE program (e.g., Clowes et al., 1987a, 1987b), as well as large-scale seismic reflection profiling along the margin allowed the construction of continuous onshore–offshore structural cross sections (e.g., Hyndman, 1995). These earlier undertakings were continued in the forearc by the Seismic Hazard Investigations in Puget Sound (SHIPS) project in 1998 (Fisher et al., 1999), which resulted in new multichannel seismic (MCS) reflection lines and higher resolution structural maps of the forearc part of the subduction zone, including large-scale seismic tomography studies (e.g., Ramachandran et al., 2004, 2005).

More recent offshore detailed seismic reflection surveys focused on gas hydrate studies providing supporting data for the Cascadia Integrated Ocean Drilling Program (IODP) drilling objectives such as determining the origin of the bottom-simulating reflector (BSR), the role of sediment consolidation and fluid expulsion in regional gas hydrate formation, and the significance of newly found cold vents in the overall fluid budget of the accretionary prism.

This paper summarizes many of the previous findings, describes the structural background of the northern Cascadia margin, and introduces the area of detailed investigation along the transect of drill-sites established during IODP Expedition 311.

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