STRUCTURES, TEXTURES, PHYSICAL PROPERTIES OF ACCRETIONARY PRISM SEDIMENTS AND FLUID FLOW NEAR THE SPLAY FAULT ZONE IN THE NANKAI TROUGH, SW JAPAN

Submarine canyons provide excellent opportunities to map structures along an accretionary prism. In this paper, we submit a new deformation history of the Nankai Trough Splay Fault zone nearby the NanTroSIEZE observatory. The Shionomisaki canyon, off Kii peninsula, SW Japan, cuts five EW-trending ridges developed in the Nankai accretionary prism. Using submersible "SHINKAI 6500", we investigated outcrops exposed along the eastern canyon wall in the landward-most ridge. Four dives were conducted to obtain lateral variations of structure, physical properties and texture of sediments along the Splay Fault zone. The ridge itself is disrupted by numerous EW-trending gullies that continue to bifurcations of the Splay Fault. Southward (seaward) dipping strata are predominant in gently folded, often steeply inclined sediments of gravity flow origin. The south upward sequence is consistent with radiolarian biostratigraphy. Pliocene to Recent sediments (< 4.3 Ma) comprise the strata of this region. Detailed observations on specimens revealed soft sediment deformation structures such as web structure, vein structures and deformation bands. The distribution of the structures and chemosynthetic biocommunities (vesicomyid bivalves and tube worms) marks the position of faults that accompany active cold seepages. Porosity decreases southward toward the Splay Fault. 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 Splay Fault. Uniaxial compressional strength calculated from a needle penetration test indicates that the strength of sandstones increases sharply just above the deduced fault zones. We attribute this to carbonate cementation precipitated from CaCO3-saturated fluids migrated along the Splay Fault and its bifurcations. Sandstones with high pore-connectivity and permeability above the faults acted as a channel through which CaCO3-saturated fluids migrated. Such cementation strengthened the frontal part of the ridge that eventually influenced the southward porosity decrease behind it.