Site 808 penetrates the seaward edge of the imbricate thrust zone of the Nankai accretionary prism (Figs. F2, F3, F4). Previous drilling here during Leg 131 cored through the frontal thrust zone, the décollement zone, and ended in the top of the oceanic crust at a total depth (TD) of 1327 m (Shipboard Scientific Party, 1991). This site is the most complete documentation of the initial thrusting of a terrigenous clastic section in a subduction zone (Hill, Taira, Firth, et al., 1993). The décollement zone here is marked by a well-defined negative-polarity seismic reflection (Moore et al., 2001). The deeper part of the hole shows a large negative anomaly in pore water chloride content, spanning several hundred meters of formation surrounding the décollement zone. Ransom et al. (1995) interpreted this anomaly as a response to fluid flow from seismogenic depths, but in fact it may represent a combination of deeply sourced fluids and those derived from local diagenetic alteration (Moore et al., 2001). The "Leg 196 Summary" chapter and Moore et al. (2001) provide additional topical and regional background on this site.
Overall, the goal of Leg 196 at Site 808 was to determine the initial state of deformation and fluid flow in a subduction zone dominated by terrigenous sediment. Given that the site crosses both the frontal thrust and décollement zones and also penetrates into basement, it provides a near-ideal location for the combination of techniques used during Leg 196: (1) LWD to document variations of physical properties across structural features and within the sections away from such features, and (2) long-term monitoring of the hydrogeologic state and processes in these zones with an ACORK, extending the understanding derived from similar experiments at other convergent margins (e.g., Becker et al., 1997). We intend to combine the LWD results and results from long-term subseafloor monitoring with existing core data to determine the spatial distribution and temporal progression in deformation, and the hydrologic evolution of the frontal thrust and décollement zones.
Because no significant wireline log data were collected at Site 808 during Leg 131, penetration data acquisition by LWD was a prime Leg 196 objective to provide continuous profiles of in situ physical properties. The ACORK at Site 808 was originally planned with a screen and packer at top of basement and an array of packers and screens spanning the décollement zone, to emphasize long-term monitoring of the hydrogeologic environment in the basement and across the décollement zone. Ultimately, the combination of ACORK measurements and the information from the LWD data should allow estimation of pore pressures and permeabilities as well as direct sampling of fluid from the décollement zone.