Structure, texture, and physical properties of accretionary prism sediments and fluid flow near the splay fault zone in the Nankai Trough, off Kii Peninsula

Abstract

Shionomisaki Canyon, off Kii Peninsula, SW Japan, cuts through five E–W-trending ridges developed in the Nankai accretionary prism. Using the submersible SHINKAI 6500, we investigated outcrops along the eastern canyon wall in the landward-most ridge. Four dives yielded information on lateral variations in the structure, texture, and physical properties of sediments along the splay fault zone in the accretionary prism. The ridge itself is disrupted by E–W-trending gullies that continue to bifurcations in the splay fault. Southward (seaward) dipping strata are predominant in gently folded, commonly steeply inclined sediments of gravity flow origin. The south-younging sequence is consistent with the radiolarian biostratigraphy. The sediments in this region are Pliocene to Recent in age (<4.3 Ma). Detailed observations reveal soft-sediment deformation structures such as web structure, vein structure, and deformation bands. The distribution of 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, showing a negative correlation with the age of the sedimentation (i.e., older sediments have higher porosity). This finding indicates progressive tectonic compaction toward the splay fault. The uniaxial compressive strength of sandstone, as calculated from a needle penetration test, shows a marked increase above the deduced fault zones. We attribute this finding to the precipitation of carbonate cement from CaCO₃-saturated fluids that migrated along the splay fault and its bifurcations. Sandstones with high pore-connectivity and permeability, located above the faults, acted as a channel through which CaCO₃-saturated fluids migrated. The related cementation acted to strengthen the frontal part of the ridge, which may act as an indenter for the inner wedge and as a backstop for the outer wedge. The proposed model accounts for the southward decrease in porosity behind the ridge front.