GEOMAR, Research Center for Marine Geosciences, Federal Republic of Germany

A seismic-reflection survey on the Oregon continental margin conducted in 1989 indicates the widespread presence of gas hydrate beneath the middle and lower slope of this accretionary margin. The seismic signature of gas hydrate, a bottom simulating reflector (BSR) with negative polarity that locally cuts across stratigraphic horizons, is especially well developed beneath Hydrate Ridge. This anomalously shallow accretionary ridge was drilled during Ocean Drilling Program Leg 146 to study fluid venting. In this paper we focus on the seismic data from the southern part of Hydrate Ridge, where little evidence of active venting has previously been reported but where the seismic data indicate a complicated subsurface plumbing system. Apparent disruptions of the BSR beneath the western ridge flank suggest dissociation of gas hydrate in response to slumping. A double BSR beneath the southern crest suggests hydrate destabilization in response to tectonic uplift and folding. On the basis of these and other observations, we propose a qualitative model for the evolution of a hydrate-bearing ridge in an active accretionary complex in which gas hydrate initially stabilizes the sea floor, permitting construction of large ridges that are then eaten away by slumps along their margins. The north-to-south variation in sea-floor venting and subsurface seismic structure along Hydrate Ridge may reflect different stages in the temporal evolution of one of these ridges.