Links and Feedbacks between Salt Diapirs, Hydrates, and Submarine Landslides: Example from Cape Fear, offshore North Carolina, U.S.A.

Abstract:

New 2-dimensional multi-channel seismic data acquired offshore east coast U.S.A in autumn 2014 provide high-resolution insight into the post-rift evolution of the margin by dynamic, interrelated processes of sediment transport, slope failure, salt diapirism, and gas hydrate formation and dissociation. This area contains some of the largest slope failure complexes in the North Atlantic and on-going salt tectonics and large-scale growth faulting continue to shape the margin. In addition, there is strong evidence for the existence of gas hydrate via bottom-simulating reflectors. The best example of where salt diapirism, hydrates, and landslides are affecting near-surface sediments is the Cape Fear Slide Complex in which two salt diapirs are surrounded by the 120-meter tall amphitheater-shaped lower headwall of the Cape Fear landslide, which occurred approximately 24-42 kya. One of the diapirs currently stands above the present-day seafloor. Previous interpretations propose that the Cape Fear landslide was triggered by the rising salt diaper. We test this by integrating our geophysical observations with cores from Ocean Drilling Program Leg 172 and an analytical model that solves for the upward velocity of salt diapirs based on regional basin sediment thickness and diapir diameter. We find that the rate of salt rise is 4-15 m/My. This indicates less than 1 meter post-landslide rise has occurred and thus that the present-day morphology as imaged in seismic data represents the geometry at the time of the Cape Fear landslide. Current sediment angles on the flanks of the diapir are a maximum of 7°, which are statically stable at hydrostatic pore pressure. This suggests that simple oversteepening is not enough to explain the landslide. The hydraulic conductivity of sediments is estimated from nearby ODP sites to be an order of magnitude greater than the upward salt velocity, which suggests that overpressure in the roof sediments was unlikely. We tentatively propose that hydrate dissociation during salt rise may have been an important factor in triggering the landslide. Salt, has a higher thermal conductivity than sediment. Therefore it is expected that while the salt was rising, warming of the surrounding hydrate-bearing sediment may have resulted in overpressure that allowed landsliding at low angles.­