The Nankai Trough accretionary prism represents an "end-member" prism, accreting a thick terrigenous sediment section in a setting with structural simplicity and unparalleled resolution by seismic and other geophysical techniques. It therefore represents a superb setting for addressing the Ocean Drilling Program's (ODP's) Long Range Plan objectives for accretionary prism coring, in situ monitoring, and refinement of mechanical and hydrological models.
Legs 190 and 196 represent a two-part drilling program designed to measure the in situ physical properties and provide long-term monitoring of the physical and chemical states of the initial deformation along the subduction megathrust between the Philippine Sea and Eurasian plates in the central Nankai Trough (Fig. F1). Previously, Deep Sea Drilling Project (DSDP) Legs 31 and 87 and ODP Leg 131 drilling in this area, along with regional seismic reflection data, documented frontal offscraping and sediment dewatering at the toe of the accretionary prism. Although penetration was excellent and core recovery was very good, logging and installing borehole instrumentation were largely unsuccessful. A return to the Nankai Trough with advanced technology was thus enthusiastically endorsed by the ODP planning structure.
Leg 190 drilling included coring two reference sites, one in the Muroto Transect (Site 1173) and one in the Ashizuri Transect (Site 1177); one site in the protothrust zone off Muroto (Site 1174); and three slope sites off Muroto (Sites 1175, 1176, and 1178). Wireline logging was accomplished at Site 1173.
Leg 196 employed logging-while-drilling (LWD) technology, revisiting Sites 808 (originally drilled during Leg 131) and 1173. Advanced Circulation Obviation Retrofit Kits (ACORKs) were installed at both sites for long-term monitoring of in situ fluid pressures at multiple depths.
The Nankai Trough is the subducting plate boundary between the Shikoku Basin and the southwest Japan arc (Eurasian plate) (Fig. F1). The Shikoku Basin is part of the Philippine Sea plate, which is subducting to the northwest under southwest Japan at a rate of 2–4 cm/yr, oriented ~310°–315° (Karig and Angevine, 1986; Seno et al., 1993) down an interface dipping 3°–7° (Kodaira et al., 2000). Repeated great earthquakes (magnitude > 8) with an average recurrence interval of ~180 yr (Ando, 1975) have occurred along the Nankai Trough throughout history. A sedimentary section ~1 km thick (Fig. F2) is accreted to or underthrust beneath the margin in the style of a fold and thrust belt at the Nankai Trough, creating a large accretionary prism (Moore et al., 2001b).
Currently, the margin is locked, with convergence between Japan and the Philippine Sea plate stored locally as elastic stress (Mazzotti et al., 2000). The convergent margin of southwest Japan has a geologic record of accretion of deep-sea deposits extending to at least the Cretaceous (Taira et al., 1988). However, rocks cored during Leg 190 (Figs. F1, F2), even those subducted to seismogenic depths, entered the subduction zone no earlier than the Pliocene (Moore et al., 2001b).
In the area of Legs 190 and 196 drilling along the Muroto and Ashizuri Transects (Fig. F1), the sedimentary section at the basin-to-margin transition can be divided into the undeformed Shikoku Basin and overlying trench fill of the Nankai Trough, the protothrust zone, the imbricate thrust zone, and the out-of-sequence thrust (OOST) or "splay fault" zone (Fig. F2). A condensed summary of these tectonic provinces from Moore et al. (2001a) follows.
The Philippine Sea plate entering the Nankai Trough along the Muroto Transect is near the axis of an extinct spreading center marked by the Kinan Seamounts (Okino et al., 1994). As documented at Site 1173, the 16-Ma oceanic crust of the Shikoku Basin is overlain by, successively, volcaniclastic facies, middle Miocene to mid-Pliocene massive hemipelagic mudstones of the lower Shikoku Basin facies, upper Pliocene to lower Pleistocene hemipelagic mudstones with tephra layers (upper Shikoku Basin facies), a Pleistocene turbidite to hemipelagic facies transition sequence, and a Pleistocene to Holocene trench turbidite unit.
Subduction of the Kinan Seamounts has significantly modified the accretionary prism. The large embayment east of the Muroto Transect (Fig. F1) was caused by subduction of a particularly large seamount (Yamazaki and Okamura, 1989).
Entering the protothrust zone, the ~1-km-thick sedimentary section initially deforms above a protodécollement zone or incipient detachment surface developed in the uppermost Miocene massive hemipelagic mudstone layer. The lower part of this massive hemipelagic section is underthrust beneath the accretionary prism along with the underlying volcaniclastic sequence and oceanic crust. Coring at Site 1174 and seismic studies demonstrate that initial deformation in the protothrust zone (PTZ) consists of small thrust faults associated with subtle folding on scales well resolved in seismic profiles (Moore et al., 1990) and development of minor faults at core scales (Moore et al., 2001b; Morgan and Karig, 1995a).
Major thrust faulting and growth of the accretionary prism initiate at the frontal thrust and continue upslope (Fig. F2). Immediately landward of the frontal thrust, which was penetrated at Site 808, the imbricate thrust zone consists of a series of well-developed seaward-vergent imbricate packets spaced several kilometers apart. OOSTs overprint the imbricate thrusts, starting at the frontal OOST (Fig. F2). These OOSTs were probably initially imbricated from thick turbidite sand sequences of the Shikoku Basin (Moore et al., 2001b). The region upslope from the OOST zone is less well imaged than more seaward areas of the prism. However, the landward-dipping reflectors probably represent thrust boundaries and, in some cases, tilted sedimentary layering (Ujiie et al., this volume). The 1946 Nankai Trough earthquake ruptured offshore of Kii Peninsula and Shikoku. Sites 1175, 1176, and 1178 are located close to the seaward limit of this rupture zone (Baba et al., 2002).
The Legs 190 and 196 drilling area shows high heat flow (Yamano et al., 2003) because the Muroto Transect is located near the extension of a ridge on the Philippine Sea plate (represented by the Kinan Seamounts) that ceased spreading at only 15 Ma. The young crust is blanketed by a thick sequence of sediment that restricts its convective cooling. Conductive heat flow values range from 180 mW/m2 at Sites 1173 and 1174 (Shipboard Scientific Party, 2001a, 2001b) to 130 mW/m2 at Site 808 (Shipboard Scientific Party, 1991). Heat flow values decrease rapidly upslope from the Nankai Trough, verifying the anomalously warm nature of the trench area (Yamano et al., 2003). The simple extrapolation of conductive heat flow with depth may overestimate temperature because of the potential of heat advection associated with fluid expulsion from the deforming sedimentary sequence. The temperatures at the top of oceanic crust at Sites 1173 and 808 were estimated at 110° and 120°C, respectively, by a conductive extrapolation of shallower measurements (Shipboard Scientific Party, 1991, 2001a). A less reliable extrapolation of only two very shallow data points at Site 1174 suggests a comparable basement temperature there (Shipboard Scientific Party, 2001b). Thus, the evidence indicates that basement temperatures exceed 100°C at Sites 1173, 1174, and 808. These high temperatures drive diagenetic reactions that significantly influence logging response.