Gas trapped below hydrate as a primer for submarine slope failures

•A shear zone is observed to sit immediately on a hydrate BSR.•The shear zone developed at the base of a partially developed failure.•A pre-failure gas accumulation has been interpreted.•The shear surface along the BSR can be critically stressed due to buoyancy.•A buoyancy-related model for submarine slope failure is proposed.

Interpretation of a three-dimensional (3-D) seismic dataset from offshore of Mauritania reveals a shear zone at the base of a partially developed slope failure. The shear zone is at a depth of ~ 220 m below the seabed, immediately above a hydrate bottom simulating reflector (BSR). We propose that a paleo-gas accumulation trapped below hydrate was the primer for what would have been a substantial submarine slope failure of ~ 220 m thickness, covering ~ 50 km2. This is based on the following observations: (a) the shear surface is, at the level of seismic resolution, coincident with some present gas accumulations located immediately below sediment that hosts hydrate; (b) there are remnants of a more extensive paleo gas accumulation that would have generated sufficient buoyancy pressure for the shear surface to be critically stressed and therefore primed the failure; (c) seismic pipes are a common seismic feature within the studied succession but absent in the area of the shear zone, which supports the hypothesis that a high gas column could have existed. This is a rare example of a shear zone that did not lead to the complete development of a slope failure. It provides the first seismic evidence that the buoyancy effect of gas below the hydrate rather than the hydrate dissociation is also a viable mechanism for large-scale slope failures.