台湾南西沖，海底マウンド分布域の流体循環 : ―大陸棚斜面上のリッジにおける高精度地震探査について―

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A high-resolution seismic reflection survey was conducted over submarine mounds on continental slope ridges in passive margin offshore southwestern Taiwan during the NT07-05 cruise in March 2007. The continental slope ridges were formed by an erosion system in which submarine canyons incise the continental slope and control ridge formations. The seismic survey revealed the geologic structure of continental slope ridges where there are target locations, site F and site G, of submarine mounds. Site F corresponds to the southern peak of the Formosa Ridge, where sedimentary strata are generally flat lying. On the Formosa Ridge, methane hydrate BSR (bottom simulating reflector) was widely observed, but it is chopped off just below site F. There is also a vertical narrow reflection blanking between the cut of the BSR and site F on the seafloor. SeaBat multibeam echo sounder detected a gas plume rising from the mound of site F. ROV Hyper-Dolphin discovered very large and dense chemosynthetic communities at the top of the mound. Site G is also a small submarine mound in another continental slope ridge. The mound is thought to be a mud volcano. There is no clear BSR, but there are high-amplitude beds below the mound. The high-amplitude beds peak just below site G, and there is also a vertical, high amplitude zone between the peak and the submarine mound of site G. Thus, the geologic structures in sites F and G have morphologically similar characteristics. It can be considered that the BSR in the Formosa Ridge should act as a good cap for trapping gassy fluid, and the narrow blanking just below site F is a fluid conduit where strata have been disturbed. On the other hand, in site G, the high-amplitude beds acts in the same way as BSR in site F, and the vertical high-amplitude zone just below site G is considered also to be the mark of a conduit from the peak of the high-amplitude beds to the submarine mound, where remnant gas components in the conduit may have emphasized partial acoustic impedance, although the Hyper-Dolphin could not find any present activity on the mound. We found the cold seep site at the top of site F in the Formosa Ridge. By integrating bathymetric characters and seismic reflection data, we can have a better understanding of the fluid circulation system in the cold seep site. In site F, the BSR that forms a trap as a gas reservoir is shallower than the canyon floors on either side of the ridge. We suggest that this reservoir configuration may enable a thermal pump system in the ridge and the surrounding cold seawater to enter the fluid system, and form a notable fluid circulation and unusual chemosynthetic communities.

台湾南西沖，海底マウンド分布域の流体循環 : ―大陸棚斜面上のリッジにおける高精度地震探査について―

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