Temporal and spatial evolution of a gas hydrate-bearing accretionary ridgeon the Oregon continental margin

A seismic-reflection survey on the Oregon continental margin conducted in 1 989 indicates the widespread presence of gas hydrate beneath the middle and lower slope of this accretionary margin. The seismic signature of gas hydr ate, a bottom simulating reflector (BSR) with negative polarity that locall y cuts across stratigraphic horizons, is especially well developed beneath Hydrate Ridge. This anomalously shadow accretionary ridge was drilled durin g Ocean Drilling Program Leg 146 to study fluid venting. In this paper we f ocus on the seismic data from the southern part of Hydrate Ridge, where lit tle evidence of active venting has previously been reported but where the s eismic data indicate a complicated subsurface plumbing system. Apparent dis ruptions of the BSR beneath the western ridge flank suggest dissociation of gas hydrate in response to slumping. A double BSR beneath the southern cre st suggests hydrate destabilization in response to tectonic uplift and fold ing. On the basis of these and other observations, we propose a qualitative model for the evolution of a hydrate-bearing ridge in an active accretiona ry complex in which gas hydrate initially stabilizes the sea floor, permitt ing construction of large ridges that are then eaten away by slumps along t heir margins. The north-to-south variation in sea-floor venting and subsurf ace seismic structure along Hydrate Ridge may reflect different stages in t he temporal evolution of one of these ridges.