Initial Tectonic Deformation of Hemipelagic Sediment at the Leading Edge of the Japan Convergent Margin

Abstract
Diatomaceous mudstones at depth under the deep sea terrace and\ud the trench inner slope off Japan have been variably affected by tectonic\ud stress. Veins, healed fractures, and microfaults occur at all sites\ud except the shallow Site 435 on the upper trench inner slope and Site\ud 436 on the Pacific Plate. Veins, fractures, and faults occur in cores\ud from below 620 meters (lower to middle Miocene) in the landward\ud sites (438 and 439) on the deep sea terrace, and are probably related\ud to normal faulting seen in seismic records. The depth to "consolidated"\ud sediment and to the first occurrence of veins and healed fractures\ud shallows progressively toward the trench. The intensity of deformation\ud also appears to increase seaward. However, no sediments\ud younger than upper Pliocene are deformed. Open fractures may exist\ud in situ at Sites 434 and 441 at levels between about 150 and 500 meters\ud sub-bottom.\ud The Japan Transect sediments—in contrast to deposits in the zone\ud of initial deformation at other convergent margins—though highly\ud deformed, are not highly overconsolidated. However, sediment at\ud depth in the trench inner slope sites is overconsolidated relative to\ud that at the same depth in the landward reference site. None of the deformed\ud Japan margin sediments recovered at Legs 56 and 57 sites\ud originated by accretion of oceanic plate material—also in contrast to\ud sediments at some of the margins previously studied. We suggest that\ud tectonic stress related to convergence has been communicated to the\ud slope sediments on the trench inner slope, either continuously or periodically,\ud causing rapid tectonic dewatering and inducing fracturing\ud and faulting. If episodic, the latest of these deformational periods\ud may have occurred during the late Pliocene. The faults and fractures\ud are either rehealed by continued overburden pressure (sediment loading)\ud or may remain open at shallower levels. Fracturing and dewatering\ud of semiconsolidated sediment beneath an unconsolidated but impermeable\ud mud veneer may cause overpressured zones at depths of\ud 200 to 500 meters. These overpressured zones possibly locally reduce\ud shear strength and cause downslope mass movement of sediment,\ud even on low-angle slopes on the trench inner slope

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