Shionomisaki Canyon, off Kii Peninsula, SW Japan, cuts through fi ve E-W-trending ridges developed in the Nankai accretionary prism. Using the submersible SHINKAI , 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 < . 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 fi nding to the precipitation of carbonate cement from CaCO -saturated fl uids 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 CaCO -saturated fl uids 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 b› ind the ridge front.

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A series of outcrops, m long and m high, were exposed during construction of a factory at Sajima, Yokosuka City, Kanagawa Prefecture, central Japan. Before these outcrops were covered with concrete, we described the stratigraphy and structures in detail. The upper part of the Misaki Formation (Aburatsubo Member) of the Miura Group (roughly Ma in age) dips gently northeastward and is m thick. The sequence consists mainly of marine pyroclastic sediments, including mafic scoria, lithic fragments, tuffaceous sandstones, and felsic pumice ash beds, which are centimeters to several meters thick. Several characteristic tephra layers are determined as key beds and numbered from Sj to Sj , in stratigraphically ascending order, among which the tephra key bed Sj is correlated with the previously known key bed So. Several thrust-duplex-like structures were recognized in parts of the outcrop, and we discuss here the formation processes of four representative duplex-like structures. One group of duplex structures developed along thrust faults that are orientated oblique to the bedding and are spatially associated with cataclasites. These duplex structures were tectonically induced and of late-stage origin. The other three duplexlike structures developed along the bedding surfaces and were associated with submarine landslides. These gravitationally induced structures are associated with smaller duplex-like structures and show slump-like folds. The directions of thrusting and submarine sliding are different from the modern attitude of depositional beds in the Aburatsubo Member, and suggest an original transport direction from NW to SE.