Dewatering structure and soft-sediment deformation controlled by slope instability: examples from the late Miocene to Pliocene Miura–Boso accretionary prism and trench-slope basin, central Japan

Systematic arrays of dewatering structure and soft-sediment deformation (e.g. convolute laminations and flame structures possibly associated with dewatering) were examined in the late Miocene to Pliocene Miura–Boso accretionary prism and Plio-Pleistocene trench-slope basin, central Japan, with respect to paleoslope inclination. Numerous preferred orientations of these structures strike perpendicular to the slumping direction. The observed features, including the measured orientations and field observations that dewatering-induced foliations terminated by the slip plane of a slope failure deposit, indicate that the structures formed under the influence of slope instability. liations from both the upper part of the accretionary prism and the slope basin correspond to plausible “pseudo-hydraulic fracture” planes, parallel to σ1 and perpendicular to σ3, and are associated with slope instability. Pore fluids or liquidized silt escaped along the foliations until pore fluid pressures along the uppermost and lowermost levels of the foliated zone were balanced. Consequently, pore fluid pressure increased just below the relatively impermeable shear planes. This pore fluid migration and increase in pore fluid pressure would have been critical for slope instability and could have triggered failures, especially within sediments close to the critical state. The findings indicate that pore-fluid behavior associated with shear stress controls the failure patterns and their locations.