Small‐scale deformation structures and physical properties related to convergence in Japan Trench slope sediments

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Abstract
Diatomaceous mudstones, cored on DSDP legs 56/57 off Japan, contain numerous postdepositional veins, healed fractures, and microfaults. Beneath the forearc basin, these secondary structures occur only at depths greater than 620 m (Miocene) in a zone of normal faulting. Beneath the landward slope of the trench, however, they occur at depths as shallow as 250 m (Pliocene) in hemipelagic sediments which are inferred to blanket tectonically accreted deposits. The intensity of deformation in the trench slope deposits increases in a seaward direction. Bulk densities indicate that the trench slope deposits have not dewatered normally but are, above 400 m, more consolidated than sediment deposited on the deep sea terrace. A decline in sediment bulk density with increasing depth suggests that open fractures may exist in situ, perhaps induced and/or maintained by excess pore pressure. Below about 550 m, a normal pattern of increasing density with increasing depth may reflect fracture closure and consolidation under lithostatic load. We suggest that tectonic stress related to plate convergence has been communicated to the sediment on the landward Japan Trench slope, causing tectonic dewatering within the subduction complex and inducing pervasive but small‐scale veining, fracturing, and faulting in near‐surface (≤700 m) hemipelagic deposits. This deformation occurs in sediment which has not been subject to large tectonic movement and does not eradicate larger (50–100 m thick) structural/depositional units, which are apparent on seismic reflection records. The brecciation induced by deformation may reflect natural hydrofracturing under abnormally high pore pressures or opening of fractures formed earlier (at shallower depths) during the dewatering process. The overpressures may, in turn, locally reduce the sediment shear strength at shallow levels in the sediment column and induce downslope mass sediment movement.

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