Origin of deformation bands in argillaceous sediments at the toe of the Nankai accretionary prism, southwest Japan

Deformation bands in argillaceous sediments at the toe of the Nankai accretionary prism are roughly planar, ∼1–10 mm thick, bedding-oblique zones. The bands are typically developed as two oppositely dipping sets. A band of one set can be displaced by the slickenlined surface of the oppositely dipping band, showing a reverse slip with respect to a horizontal plane. Deformation bands commonly consist of thinner subbands that are oblique or parallel to the boundaries of the deformation bands. The subbands also display a reverse slip with respect to a horizontal plane, and particles within subbands are oriented perpendicular to the maximum principal stress (σ1), regardless of the dips of the subbands. Physical property analysis using an X-ray computed tomography scanner demonstrates that porosity reduction in the deformation bands relative to the surrounding undeformed material ranges from 1 to 6%. The deformation bands are compactive shear bands formed under subhorizontal σ1 associated with northwest-directed plate convergence, and subbands arise from a combination of reverse sense of shear and flattening perpendicular to σ1, with their geometries producing the S–C aspect observed in sheared clayey sediments or ductile metamorphic shear zones.