ALBERT

Description: This paper presents results of an interdisciplinary investigation of the relation between fluids, fluid flow, and deformation in the toe region of the Nankai accretionary prism. The techniques include thin-section petrography, SEM, TEM and microprobe analyses, and X-ray computed tomography as well as laboratory experiments. Together, the data suggest three structural/hydrologic regimes within the prism. These are: (1) the accreting sediments above the décollement zone, (2) the décollement zone, and (3) the underthrust sediments. The regime above the décollement is characterized by sediments that are progressively dewatered through both a penetrative fabric and a pervasive, but apparently poorly interconnected, set of core-scale deformation structures. The décollement is characterized by a relatively high density of structures/meter and is considered to be a regime of low stress but frequent failure. Hydrologically the décollement retards the vertical flow of fluids and enhances the potential for overpressuring in the footwall. Finally, the footwall regime contains very few tectonic structures and is structurally isolated from the stresses related to plate convergence. This regime provides an important component to the tectonics of the Nankai prism, however, because it supplies the overpressured fluids that cause the d?collement to fail at relatively low shear stresses.

Description: 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 sigma 1 (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 (?3). 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.

Description: Understanding the role of fluids in active accretionary prisms requires quantitative knowledge of parameters such as permeability. We report here the results of permeability tests on four samples from Ocean Drilling Program Leg 190 at the Nankai Trough accretionary prism-two from Site 1173 and two from Site 1174. Volcanic ash is present in one of the samples; otherwise, the material is hemipelagic mud. A constant-rate-of-flow technique was used at various effective pressures and rates of flow. The permeability of the four samples ranges between 10**-15 and 10**-18 m**2, with the ash-bearing sample showing the highest values.