Abstract

On the northern Cascadia accretionary margin off Vancouver Island, Canada, there are numerous sedimentary slide features near the base of the slope, and significant amounts of gas hydrate found beneath the ridge suggest a possible connection. A 2-km-wide collapse structure with a 300-m-high headwall has been studied by multibeam bathymetry and a single-channel seismic reflection grid. Migrated seismic reflection lines on the frontal ridge image multiple 15–75-m-high seafloor scarps perpendicular to the margin that are the seafloor expressions of normal faults that cut deeply into the sediment section. Two of the largest faults are aligned with the sidewalls of the slide, indicating that the lateral extent of the collapse is fault controlled. The presence of marine gas hydrate beneath the ridge is based on a widespread hydrate bottom-simulating reflector (BSR), high velocities determined by ocean bottom seismograph data, and sediment core samples and downhole logs collected by the Integrated Ocean Drilling Program at Site U1326. The depth of the BSR at 255 (±15) m coincides closely with the estimated depth of the glide plane beneath the slide. This suggests that the base of the slope failure is related to the contrast between strong hydrate-cemented sediments above the BSR and underlying weak sediments containing free gas. Strong earthquake shaking on this convergent margin likely provides the trigger for the slide.

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