Structural Geology of the Nankai Accretionary Prism; Tenryu Submarine Canyon, Japan

The walls of Tenryu Canyon, which cut nearly perpendicular to the Nankai trench, provide a unique cross-sectional view of structures within an active accretionary prism. These exposures were studied in a series of dives in the manned deep submergence vehicle, SHINKAI 6500. The Nankai prism comprises thick turbidite sequences of weak to moderately consolidated mudstone, siltstone, and fine-grained sandstone. The strike and dip of bedding, joints, and minor faults observed during the dives were estimated using data on the submarine heading and the yaw and pitch of remote controlled video cameras. These structural data were combined with time and position data to produce schematic geologic maps and cross sections of each dive site.

Our survey of the Nankai prism reveals three distinct structural fabrics in the vicinity of Tenryu Canyon. First, bedding within the prism (elsewhere dated at 0.12-0.46 Ma) strikes southwest to northwest and dips predominantly to the north towards the arc, which suggests strata are involved in a series of east-west trending, kilometer-scale folds in the hanging-walls of north dipping thrust faults. Second, strata are also involved in an unusual series of north-northeast trending folds that have 20 to 50 m wavelengths and deform preexisting structures. Finally, prominent northeast trending scarps truncate all other structures. Some of these scarps were formed by thrusts, but others may be the trace of trench-parallel strike-slip faults. While kilometer-scale thrust related folds are typical in accretionary prisms, the origins of the possible strike-slip faults and north-northeast trending folds are unclear. However, it is likely that these structures are essentially active and associated with out-of-sequence thrusting, oroclinal bending, and gravitational collapse associated with the frequent subduction of seamounts in the Zenisu Ridge Complex.