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STT-07 Three-dimensional aspects of subduction zone processes: Insight from dynamic modelling, tectonic reconstructions and obse
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Physical properties, textural variation and fluid flow near the Nankai Mega-Splay Fault zone exposed in the Shionomisaki canyon, off Kii Peninsula, Southwest Japan
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Ryo Anma, University of Tsukuba (Japan)
Yujiro Ogawa, University of Tsukuba (Japan)
Kiichiro Kawamura, Fukada Geological Institute (Japan)
Yoko Michiguchi, University of Tsukuba (Japan)
YK05-08 Shipboard Scientific Party, JAMSTEC (Japan)
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The Shionomisaki canyon, off Kii Peninsula in Southwest Japan, cuts five EW-trending ridges developed in the Nankai accretionary prism. We traversed outcrops exposed along the eastern canyon wall using submersible ?gSHINKAI 6500?h, along the landwardmost ridge where the extension of a mega-splay fault detected by previous seismic studies is exposed. Four dives were conducted to obtain lateral variations of physical properties and texture of sediments along the mega splay fault zone. Southward (seaward) dipping strata are predominant in gently folded, often steeply inclined turbidites; the structures are consistent with the younging direction confirmed by radiolarian biostratigraphy. Pliocene to Recent sediments (< 4.3 Ma) comprise the strata of this region. The ridge itself is disrupted by numerous EW-trending gullies. Detailed observations on specimens revealed soft sediment deformation structures such as web structure, vein structure and deformation bands. The position of the mega-splay fault and its bifurcations was deduced from topography and distribution of the deformed rocks and chemosynthetic biocommunities (vesicomyid bivalves and tube worms) that mark the active cold seepage zone. Porosity decreases southward toward the position where the mega-splay fault was deduced. The porosity decrease has an almost negative correlation with the age of the sedimentation: older sediments have higher porosity. This implies progressive tectonic compaction toward the fault. Uniaxial compressional strength calculated from a needle penetration test indicates that the strength of sandstones increases sharply just above the active cold seepage zone due to carbonate cementation. We attribute this cementation to precipitation from CaCO3-saturated fluids migrating along the cold seepage zone from deeper parts of the accretionary prism. The progressive porosity decrease toward the south due to lateral tectonic push, folding and development of prolate fabrics in mudstones, and the carbonate cementation took place in this order. Sandstones with high pore-connectivity and permeability above faults acted as a channel through which CaCO3-saturated fluids migrated.
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