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Drilling at Site 808 (ODP Leg 131) provided an extensive record of the discrete brittle structures (small faults and shear bands) at the toe of the Nankai accretionary complex. Brittle failure is occurring throughout the hole, although most of the deformation has been observed between the frontal thrust and the décollement (from 365 to 963 mbsf). Brittle failure occurs in the turbiditic trench fill (0 to 600 mbsf) but also in the ash-bearing hemipelagites from the upper Shikoku Basin (600 to 800 mbsf) and the ash-free hemipelagites of the lower Shikoku Basin sequences, down to the décollement (963 mbsf). The geometry of the tectonic features has been recorded in a local frame related to the core liner and then corrected to an absolute frame by the use of paleomagnetic measurements. We used the resulting geometry of homogeneous populations of slickenlined faults to estimate the reduced stress tensor. The main result is to show three consistent stress patterns. Most of the fault clusters from the frontal thrust down to the décollement are in good agreement with a compressional regime with a northwest-trending σj (azimuth N305° to N315 °). The compression direction is thus roughly parallel to the local direction of the relative convergence. It is also perpendicular to the trend of the anticlinal ridges. Discrete faults, locally appearing as clusters (e.g., 30 m above the décollement) agree with a quite different compression direction (west-southwest-east-northeast) and are restricted to the hemipelagic sequences above the décollement. They could be related to the heterogeneous internal deformation of this layer. Although isolated normal faults exist, their occurrence as clusters is restricted to the Shikoku Basin hemipelagic sequences and indicate a east-west to northwest-southeast extension axis (σ3). Finally, the analysis of Mohr diagrams related to the best-fitting tensors shows some similarities and differences between the fault populations of the turbidite and those of the hemipelagite sequences. The angle of friction corresponding to 95% of the data is 30° ± 5°, whereas including the remaining 5% in turbidites and hemipelagites would result in a lower friction angle (18° ± 8°). The major difference concerns the angle between conjugate sets which are about 60° in the turbidites and lower (35°40°) in a 200 m thick zone above the décollement and can be explained by higher fluid pressure in the hemipelagites or by odd mechanical properties of this hemipelagic material.

Stress Tensors at the Toe of the Nankai Accretionary Prism: An Application of Inverse Methods to Slickenlined Faults