Frontal Ridge Slope Failure at the Northern Cascadia Margin: Fault Control and Possible Relation with Gas Hydrate Dissociation

A collapse structure is observed in newly acquired multi-beam bathymetry data and in seismic reflection data at the frontal ridge at the base of the slope off Vancouver Island, just landward of the deformation front. The head wall of the Frontal Ridge slide is ~250 m high and the slump has eroded a ~2.5 km long section into the ridge. The region is characterized by a wide distribution of gas hydrates based on the observation of the `Bottom Simulating Reflector' (BSR), that is assumed to coincide with the base of the methane hydrate stability zone. Hydrate was also observed in drilling by Integrated Ocean Drilling Program (IODP) Expedition 311. Since hydrates prevent sediment compaction, their dissociation in sediment pores is thought to decrease seafloor strength, potentially facilitating submarine landslides on continental slopes. Migrated seismic reflection data image a set of normal faults that clearly outcrop at the seafloor and can be traced from the surface through the sedimentary section to depths below the BSR in some locations. Seafloors scarps show that seafloor displacements of ~25 m to 75 m are generated. The faults strike in the NE-SW direction, perpendicular to the margin and parallel to the direction of convergence. Extensional motion on the faults is oriented NW-SE, perpendicular to the direction of compression on the margin. Disturbed sediments are confined to the vicinity of the normal faults. The two faults with the largest seafloor scarps clearly bound the region of slope failure on the frontal ridge, dramatically indicating that the lateral extent of slumping is fault-controlled. No BSR is observed within the slide region, but it is present in the surrounding region. The causal relationship between slope failure and gas hydrate dissociation is difficult to document, but a number of observations support a potential connection.