DEVELOPMENT OF TRENCH-SLOPE BREAK NEAR SPLAY FAULT ZONE IN THE NANKAI TROUGH: RESULTS OF SUBMERSIBLE STUDIES

Aiming to understand distribution of outcrop-scale structures and strain in the accretionary complex of the Nanaki trough, field mappings were conducted using submersible "SHINKAI 6500" along the Shionomisaki submarine canyon, off Kii peninsula, Southwest Japan. Outcrops were investigated in the seaward-most part (frontal thrust zone), the landward-most part (splay fault zone) and an intermediate part (imbricate thrust zone) of the complex. Turbiditic sediments in the frontal thrust zone were deformed to form a ridge-scale gentle anticline. Minor normal faults striking parallel to the trough axis developed in the crest of the anticline. As sediments transferred into the imbricate thrust zone, both ridge-scale open anticline and outcrop-scale open to tight folds were developed in the turbiditic sequence. Brittle thrust faults that accompany tight folds and development of shear fabrics were also observed.

An out-of-sequence thrust (splay fault) was then developed in frontal part of the landward-most ridge comprised of thick Pliocene to Recent (< 4.3 Ma) turbidite sequences. Sandstones just above the splay fault were cemented by carbonates. Behind the cemented zone, bifurcations of the splay fault were distributed in the sandstone-rich strata dipping steeply to the south. Folds with wavelength of ~ 200 m were developed in mudstone-rich turbidites behind the bifurcating fault zone. The uniaxial compressional strength and degree of fabric development increase, and the porosity decreases southward toward the splay fault. The cementation strengthened the frontal part of the splay fault, which in turn, acted as an indenter for the inner part of the accretionary wedge, and as a backstop for the outer wedge. Sediments behind and in front of the hardened ridge were thickened as trench-slope break developed in such manner. The model accounts for the over all strain distribution, porosity increasing and strength decreasing northward behind the ridge front.