COMPLEX SUBSURFACE PLUMBING BENEATH THE SOUTHERN HYDRATE RIDGE, OREGON CONTINENTAL MARGIN, FROM HIGH-RESOLUTION 3D SEISMIC REFLECTION AND OBS DATA

In June-July, 2000, we conducted a high-resolution 3-D seismic survey of a 4 x 11 km2 region around the summit of southern Hydrate Ridge, where seafloor seep communities have been observed and from which massive hydrates have been recovered. The survey also includes a 30-m-high, actively-venting carbonate tower 400 m southwest of the summit and a slope basin east of the ridge and provides new information on the regional subsurface plumbing system. Two distinct sets of stratigraphic reflections exhibit anomalously high amplitudes. A bright, negative-polarity reflection on the western flank of the ridge can be traced over 4 km2 as it shallows toward the summit, where it is a subhorizontal "bright spot" underlying the BSR. We speculate that methane-rich fluids migrate toward the summit along this surface, which may be an unconformity. We further speculate that variations in permeability initially favored flow along this horizon, resulting in diagenetic reactions that further enhancing flow. These speculations will be tested by ODP drilling during Leg 204. Overlying the "bright spot" is a zone of chaotic bright reflectivity that extends from the seafloor to ~ 30 m depth. We interpret this to indicate the subsurface extent of shallow massive hydrate. The mechanism whereby methane migrates from the subBSR "bright spot" to the seafloor remains enigmatic. On the eastern flank of the ridge, of a pair of bright reflections covering > 10 km2 cuts across the BSR. These bright reflections originate at the unconformity between "accretionary complex" material containing no coherent reflections and stratified sediments interpreted to be an uplifted and deformed slope basin. They are cut by small-offset, eastward-dipping normal faults and are associated with a subtle shallowing of the BSR that suggests an increase in heat flow where they approach the seafloor. The absence of seafloor "bright spots" or of observed seep fauna overlying these reflections indicates the absence of focused venting on the seafloor. We speculate that the many small offsets in the bright reflections lead to high fracture permeability and increased but diffuse fluid flux. These structures will also be a target of drilling during leg 204. Modeling of large aperture data recorded on ocean bottom seismometers deployed during the 3D survey provides additional constraints on this complex system.