Deformation and in situ stress in the Nankai Accretionary Prism from resistivity-at-bit images, ODP Leg 196

Borehole resistivity images from ODP Leg 196 allow rapid and complete qualitative assessment of deformation within the toe of the Nankai prism, Japan. Borehole breakouts were common within the prism but prominent in the trench-wedge unit around the frontal thrust, suggesting reduced sediment strength. Breakouts indicate consistent ?2 orientations (?050°), compatible with northwesterly convergence. Deformation is dominated by discrete zones, including the frontal thrust and décollement zone. Prism fractures trend ?NE–SW, consistent with convergence. The décollement shows minimal deformation and the dominant structural trend is ?N–S. Prism deformation zones are characterized by high resistivity (compaction), whereas the décollement is apparently dilated, both with conductive fractures. Distribution of fracture orientations varies between log units confirming lithologic and rheologic influence. Pore pressure is elevated within the décollement and the misalignment of conductive fractures may reduce permeability