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

[1] 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 sigma(2) orientations (similar to050degrees), compatible with northwesterly convergence. Deformation is dominated by discrete zones, including the frontal thrust and decollement zone. Prism fractures trend similar toNE-SW, consistent with convergence. The decollement shows minimal deformation and the dominant structural trend is similar toN-S. Prism deformation zones are characterized by high resistivity (compaction), whereas the decollement 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 decollement and the misalignment of conductive fractures may reduce permeability.