In May and June of 2001, scientists on Ocean Drilling Program (ODP) Leg 196 investigated the interrelationship between the fluid flow regime and deformation in the Nankai Trough accretionary prism (Fig. F1). During the first part of Leg 196, scientists examined two sites off Cape Muroto, Japan, by drilling with logging-while-drilling (LWD) tools for in situ physical property data (Mikada, Becker, Moore, Klaus, et al., 2002). Leg 196 is part of a larger effort to fully characterize the accretionary processes at the Nankai Trough that includes preceding ODP Legs 131 and 190 (Moore, Taira, Klaus, et al., 2001; Taira, Hill, Firth, et al., 1991), two-dimensional (2-D) and three-dimensional (3-D) seismic reflection and refraction experiments (Aoki et al., 1982, 1986; Bangs et al., 2004; Bangs and Gulick, this volume; Gulick et al., 2004; Kodaira et al., 2000; Leggett et al., 1985; Moore et al., 1991; Moore and Shipley, 1993; Moore et al., 1990, 2001; Nasu et al., 1982; Park et al., 1999, 2000; Tamano et al., 1984).
LWD data collected from Site 808 during Leg 196, combined with wireline and core data collected from Site 1174 during Leg 190 (Shipboard Scientific Party, 2001), provide details about fluid expulsion, deformation, diagenesis, and compaction at both the protothrust and frontal thrust zones of the Nankai accretionary prism (Fig. F2). These data are important for the study of accretionary processes on their own, but may prove to be invaluable in that they allow calibration of a 3-D seismic reflection volume that was collected off Cape Muroto in 1999 (Fig. F1) (Gulick et al., 2004; Bangs et al., 2004). Integration of 3-D seismic data with the logging data can greatly extend our understanding of the interplay among deformation, consolidation, and fluid and gas expulsion beyond the one-dimensional (1-D) view provided at individual drill sites.
Within the protothrust and imbricate thrust zones of the Nankai Trough accretionary prism, the existence, polarity, and strength of fault plane reflections vary substantially in 3-D (Fig. F3). In general, the protothrust faults are recognized from offsets in the Shikoku Basin strata and not as fault plane reflections, whereas the thrusts within the imbricate thrust zone display negative polarity fault plane reflections (e.g., Gulick et al., 2004) (Fig. F3). Leg 196 Hole 808I penetrated through the frontal thrust using LWD technology, whereas Leg 190 Hole 1174B cored through the protothrust. The Leg 190 cores were subsequently logged onboard the JOIDES Resolution. In this paper, we compare the velocity and density data from Sites 808 and 1174 to demonstrate that the presence of a negative polarity fault plane reflection at the frontal thrust, but not at the protothrust, is due to differences in the physical properties within the fault zones. We show evidence from the logging data for a distinct velocity and density drop at the frontal thrust at Site 808 and model the frontal thrust reflection in 1-D in Hole 808I. Our results suggest fluid-transported free gas may play an important role in generating the negative polarity fault plane reflection at Site 808. Using this information, we examine the pattern and discuss the implications of the negative polarity fault plane reflections present throughout the imbricate thrust zone.