Site 1173 is located in the outer margin of the trench fill of the Nankai Trough ~11 km seaward of the deformation front (Fig. F2). Cored to basement during Leg 190, Site 1173 provides a reference for the physical and chemical nature of the sediments at the subduction zone, before they are deformed either by incorporation into the accretionary prism or by being underthrust beneath the décollement zone. Such reference sites are essential in understanding processes at subduction zones because changes in the sediments entering the subduction zone occur rapidly.
Site 1173 penetrates ~80 m of the trench-wedge deposits and all of the subjacent sequence of the Shikoku Basin (Shipboard Scientific Party, 2001). At Sites 1174 and 808, this reference section is buried by more than 350 m of trench sediments (Moore et al., 2001) (Fig. F3). The "Leg 196 Summary" chapter and Moore et al. (2001) provide additional topical and regional background for Site 1173.
Legs 190 and 196 shared similar goals at Site 1173: to determine the physical and chemical state of the section incoming to the subduction zone as a reference in an overall program to define the interrelationship of the dynamics of deformation and fluid flow processes in an accretionary prism characterized by thick terrigenous sediments. Whereas Leg 190 approached these goals by coring, Leg 196 focused on the collection of LWD data and emplacement of a long-term ACORK hydrogeologic observatory at the site. Leg 196 sought to obtain a set of borehole logs and long-term hydrogeologic measurements that could be combined with the existing core and seismic reflection data to best define the initial state of materials incoming to the Nankai Trough along the Muroto Transect (Moore et al., 2001).
Specific Leg 196 objectives at Site 1173 included