Evidence of abrupt lateral variation in fluid flux on the Blake Ridge

The Blake Ridge is a southeast trending sediment drift offshore South Carolina that has been identified by seismic reflection and borehole studies as a large methane hydrate province. New multi-channel reflection data reveal a 150-m-high step in the bottom simulating reflection (BSR). This step is in an area of uniform shallow dip of the seafloor, suggesting that the depth of the BSR is not controlled by the normal geothermal and pressure gradients in this area. Seismic observations suggest the presence of free gas to the southwest and sediments disturbed by fluid migration to the northeast. A weak crosscutting reflection connecting the BSR on the upper limb to BSR of the lower limb approximately 4.5 km down dip may represent a double or paleo-BSR. The strong termination of the upper limb of the step suggests that there is an abrupt lateral change in either the fluid or heat flux regime at the step. 2D pre-stack depth migrations and closely spaced 1D-waveform inversions reveal detailed velocity structure of the region. The shallow, weaker reflection may be the base of the gas hydrate stability zone and the deeper stronger reflection, which comprises the lower limb of the step, could be the top of the free gas zone. This system may be analogous to ODP site 994, where the top of the free gas zone does not coincide with the base of hydrate stability. The cause for this discrepancy may be a permeability seal at the deeper BSR that traps gas. This permeability barrier suggests that this surface was previously a long-lived base of hydrate. During this time, the upward migration of gas and fluid may have altered the lithology to create an impermeable seal that crosscuts strata. Gas may be prevented from migrating across the step due to fault control similar to that shown in the gas charged sediments of the upper limb.