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

Diatomaceous mudstones at depth under the deep sea terrace and\udthe trench inner slope off Japan have been variably affected by tectonic\udstress. Veins, healed fractures, and microfaults occur at all sites\udexcept the shallow Site 435 on the upper trench inner slope and Site\ud436 on the Pacific Plate. Veins, fractures, and faults occur in cores\udfrom below 620 meters (lower to middle Miocene) in the landward\udsites (438 and 439) on the deep sea terrace, and are probably related\udto normal faulting seen in seismic records. The depth to "consolidated"\udsediment and to the first occurrence of veins and healed fractures\udshallows progressively toward the trench. The intensity of deformation\udalso appears to increase seaward. However, no sediments\udyounger than upper Pliocene are deformed. Open fractures may exist\udin situ at Sites 434 and 441 at levels between about 150 and 500 meters\udsub-bottom.\udThe Japan Transect sediments—in contrast to deposits in the zone\udof initial deformation at other convergent margins—though highly\uddeformed, are not highly overconsolidated. However, sediment at\uddepth in the trench inner slope sites is overconsolidated relative to\udthat at the same depth in the landward reference site. None of the deformed\udJapan margin sediments recovered at Legs 56 and 57 sites\udoriginated by accretion of oceanic plate material—also in contrast to\udsediments at some of the margins previously studied. We suggest that\udtectonic stress related to convergence has been communicated to the\udslope sediments on the trench inner slope, either continuously or periodically,\udcausing rapid tectonic dewatering and inducing fracturing\udand faulting. If episodic, the latest of these deformational periods\udmay have occurred during the late Pliocene. The faults and fractures\udare either rehealed by continued overburden pressure (sediment loading)\udor may remain open at shallower levels. Fracturing and dewatering\udof semiconsolidated sediment beneath an unconsolidated but impermeable\udmud veneer may cause overpressured zones at depths of\ud200 to 500 meters. These overpressured zones possibly locally reduce\udshear strength and cause downslope mass movement of sediment,\udeven on low-angle slopes on the trench inner slope