Erosion of the seafloor at the top of gas hydrate stability: Evidence from an uplifted ridge on the Hikurangi Margin, New Zealand.

Gas hydrates are thought to play a significant role in submarine slope failure, however, the exact mechanisms by which they affect seafloor stability are not yet understood. It is mostly assumed that overpressure caused by pore volume expansion during gas hydrate dissociation may lead to weakening and sliding along the base of gas hydrate stability (BGHS) in sediments. Sediments are predicted to be particularly vulnerable close to water depths where the BGHS pinches out (i.e., at the top of gas hydrate stability, TGHS, in the ocean) because of the lack of overburden sediments to compensate for overpressure at the BGHS. We have discovered a location on the Hikurangi margin, Southern Ritchie Ridge, where seafloor erosion appears to take place at the THGS. Late Cretaceous (?) strata are uplifted and appear to be eroded at about 600 m water depth. Pinchouts of bottom simulating reflections (BSRs) document that the ridge crest is close to the TGHS. We propose two possible mechanisms that may cause gas hydrate-related seafloor erosion. In a modification of the common model for slope failure from gas hydrate dissociation, we predict that during uplift, the BGHS moves upward with respect to the seafloor, leading to gas hydrate dissociation, overpressure, and ultimately slope failure along the BGHS. Subsequently, pressure-temperature conditions on the seafloor move back deeper into the gas hydrate stability field and the process can repeat itself during continued uplift. However, for this model, the seafloor would need to remain entirely within the stability field, which contradicts the presence of BSR pinchouts. We therefore favor a different mechanism for seafloor erosion, a process similar to frost heave. Temperature measurements at this water depth show mesoscale fluctuations by about ± 1^o C suggesting that the ridge repeatedly crosses the gas hydrate phase boundary. We propose that the repeated pore volume expansion and contraction during gas hydrate dissociation and formation leads to weakening of the seafloor. The weakened material does not support the relatively steep slopes of the uplifted ridge and is predicted to slide down the ridge flanks.