Seafloor Manifestations of Fluid Venting and Shallow Gas-hydrate BSRs at the Outer Ridge of the Eastern Nankai Trough

A multichannel seismic profile across the outer ridge of the eastern Nankai Trough shows abnormal shallow bottom simulating reflectors (BSRs) of gas hydrate in proportion to water depth and regional heat flow. Such shallow BSRs suggest fluid venting along the active fault estimated from swath bathymetry and subbottom profile survey. We conducted detail seafloor observation and sampling by JAMSTEC deep-tow video system, ROV "Dolphin 3K" and submersible "Shinkai 2000" in 2002. Dense clam colonies were observed around the summit of the ridge at a water depth of 1050m. Each colony mainly consists of Calyptogena sp., tubeworms and white bacterial mats, and sometimes includes small colony of Bathymodiolus sp. Existence of fossil cemented shell within active colony indicates repetition of methane seep at the same locality. Chimney-like or Pinnacle-like structures, composed of carbonate cemented sandstone, are observed at the northern slope of the ridge. These chimneys are regarded as ancient fluid conduits. Their exposure up to 2 meters above the seafloor indicates very active erosion after cold seepage. The seafloor near the summit is partly covered by thin sandy sediments and mostly exhibits carbonate pavement with a large number of NW-SE trending fractures. Shapes of some clam colonies are also elongated toward NW-SE suggesting effect of fracture patterns developed in the hard-ground below thin cover sediment. These fractures are probably caused by right lateral displacement of active faults across the ridge. It is inferred that up-dip fluid/gas migration through this fault causes pervasive fluid expulsion as well as shallow BSRs.