OGAWA, Y., University of Tsukuba, Tsukuba, Japan; S. HIRANO, and K. KOBAYASHI, JAMSTEC, Yokosuka, Japan; and K. KAWAMURA, University of Tsukuba, Tsukuba, Japan
Abstract: Patterns of Orientation of Chemosynthetic Bio-Communities in the Cold Seepage Areas of the Japanese Plate Convergent Boundaries
Tectonically significant pattenrs of chemosynthetic biocommunities (mostly Calyptogenas and vestimentiferan) in the cold seepage areas in convergent margins have been known on sharp lines just on fault traces or on top of benches above a largescale detachment faults as from the toe of the eastern Nankai trough by KAIKONankai and -Tokai projects. In addition the JAMSTEC submersibles Shinkai 6500 and 2000, and ROVs Dolphin 3K and Kaiko found many more examples in the Japan trench, and Sagami Bay. The zone of communities do not only align along fault traces but also show dominant patterns of orientation of each community, most particularly in en echelon or two or three different straight patterns, commonly forming lozenge shape of conjugate fractures. Bacterial mats sometimes delineate the similar patterns but they are not always form a zone but rather form diffused patterns.
The dominant directions of zones of communities are always very straight along fault scarps at oblique transpressional mechanical boundary between the Pacific and Okhotsk plates in the Japan trench, and along active faults of N-S and NW-SE two directions in the Sagami Bay. However, the dominant community directions are reflecting local shear patterns such as Riedel shears or extension fractures. These patterns were interpreteded as good indicators of shearing in the covering sediments on the faults which are the conduits of methane-bearing fluid along a detachment thrust (decollement) or along a strike-slip fault, and the fluid finally seeps along local, shallow fractures at the seabottom, and supports the chemosynthtic bio-communities.
AAPG Search and Discovery Article #90920©1999 AAPG Pacific Section Meeting, Monterey, California