University of California at Santa Cruz, United States

Leg 196 is the second of a two-leg program of drilling, logging, and installing long-term observatories in the Nankai Trough, the type example of a convergent margin accreting a thick section of clastic sediments. The two-leg program was designed to define the interrelationship of the dynamics of deformation and fluid-flow processes in an accretionary prism characterized by thick terrigenous sediments. In 2000, Leg 190 investigation focused on coring at sites in undeformed to highly deformed zones along two transects across the Nankai Trough to investigate variability in deformational and hydrologic character. In 2001, Leg 196 research will first conduct logging-while-drilling (LWD) activities through the sediment section at three sites previously cored on the Muroto Transect during Legs 131 and 190: Sites 808 and 1174 near the deformation front at the toe of the prism and Site 1173, a reference site about 12 km seaward. During Leg 196, two holes at Sites 808 and 1173 will be sealed with multipacker advanced circulation obviation retrofit kits (ACORKs) for long-term monitoring of fluid-flow and tectonic processes. Contingent upon funding and JOIDES advisory structure approval, the ACORK at Site 1173 will incorporate a broadband seismometer cemented into the basement section, approximately 50 m of which will be cored. In concert with measurements made during Leg 190, the Leg 196 LWD data will document how physical properties change during accretion and provide critical information on stress, pore pressures, and permeability. Instrumenting the two holes with ACORKs will begin a long-term program of observation of seismicity, fluid-flow parameters, and fluid geochemistry at the Nankai Trough, a program that will involve future revisits by manned and unmanned submersibles.