Submarine outcrop evidence for transpressional deformation within the Nankai Accretionary prism, Tenryu Canyon, Japan

Abstract

The Tenryu submarine canyon lies within the eastern edge of the Nankai Trough, Japan, and exposes the internal structure of the Nankai accretionary prism. SHINKAI 6500 submersible dives within Tenryu Canyon in 2008 collected important new data on the structural geology of well-bedded, Pleistocene turbidite sequences of fine-sand, silt, and mud. These data include estimates of the strike and dip of bedding, and of the joints and faults that cut these strata. Most strata are involved in broad, east-west trending first-order folds (trench parallel) that typify the accretionary prism structure. Aside from the trench-parallel fold hinges and faults, tight outcrop-scale folds and broader <1 km-scale folds have hinge-lines that plunge to the north (trench perpendicular). These trench-perpendicular folds are associated with outcrop-scale evidence for strike-slip faulting and appear to culminate in an oblique imbricate thrust zone near the trace of the Tokai thrust, an approximate equivalent to the ‘megasplay’ out-of-sequence thrust in the NanTroSEIZE region to the west. The preferred hypothesis links the transpressional deformation to a subducted seamount or paleo-Zenisu ridge, though it is alternatively possible that transpression accommodates the northward collision of the Izu-Bonin arc and Sagami trough, and oblique subduction of the Philippine Sea plate. The identification of trench-perpendicular fold hinges in Plio-Pleistocene strata illustrates a large component of previously unidentified elastic strain, and faulting that cuts this folded strata may have a strike-slip component. Therefore, these submersible observations are relevant to evaluating seismic hazards confronted by the Tokai region in Japan.