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doi:10.2204/iodp.proc.314315316.102.2009

Introduction

The Nankai Trough, as the type example of a subduction zone accreting a thick terrigenous sediment section, has been a site of intense study, including Deep Sea Drilling Project (DSDP) and Ocean Drilling Program (ODP) drilling (Karig, Ingle, et al., 1975; Taira, Hill, Firth, et al., 1991; Moore et al., 2001), numerous two-dimensional (2-D) and three-dimensional (3-D) seismic reflection surveys and ocean bottom seismometer wide-angle seismic surveys (e.g., Aoki et al., 1982; Moore et al., 1990; Park et al., 2002a; 2002b; Taira et al., 2005; Kodaira et al., 2006; Nakanishi et al., 2008), and heat flow and submersible surveys (e.g., Ashi et al., 2002). As part of a continuing multidisciplinary scientific investigation of seismogenic processes in the Nankai Trough, the Integrated Ocean Drilling Program (IODP) Nankai Trough Seismogenic Zone Experiment (NanTroSEIZE) is focused on a transect south of Kii Peninsula, Honshu, Japan (Fig. F1). Several 2-D seismic reflection data sets across the Kumano Basin region of the Nankai Trough (Fig. F2) were used to define the regional tectonic setting and locate the NanTroSEIZE drilling transect, but these data sets lacked both adequate imaging to interpret the complex 3-D structure and the detail needed to prepare for the planned deep riser drilling. In order to better image this region for proper structural interpretation, choose final site locations, and characterize these locations for riser drilling, we carried out a commercial 3-D seismic survey, funded by Japanese (Ministry of Education, Sports, Culture, and Technology) and U.S. (National Science Foundation) sources. In this chapter we describe the acquisition and processing of this 3-D data set and present preliminary interpretations of the structure and seismic stratigraphy of the NanTroSEIZE drill sites based on all of the available regional bathymetric and 2-D and 3-D seismic data sets.

Regional setting of the NanTroSEIZE transect

The Philippine Sea plate (PSP) is being subducted beneath southwest Japan at the Nankai Trough subduction zone at a rate of ~4–6.5 cm/y at an azimuth of ~300°–315° (Seno et al., 1993; Miyazaki and Heki, 2001; Zang et al., 2002). The northern margin of the PSP (Fig. F1), the Shikoku Basin, was formed by backarc spreading behind the Izu-Bonin arc system ~25–15 Ma (Okino et al., 1994). Late-stage reorientations of spreading in Shikoku Basin caused large variations in basement relief with associated variations in thickness and sediment type in overlying sedimentary sections (Ike et al., 2008b). For instance, Miocene turbidites filled deep basement grabens, but these turbidites only onlap adjacent basement highs; the basement highs are draped by thin hemipelagic sediment (Ike et al., 2008a). The Miocene and younger strata were designated the lower and upper Shikoku Basin facies at ODP Site 808 (Taira, Hill, Firth, et al., 1991). A trench sediment wedge ~1.3 km thick overlies ~1.1 km of Shikoku Basin deposits in the NanTroSEIZE transect area, where all of the trench deposits and at least half of the Shikoku Basin deposits are accreted to form a wide accretionary prism (Park et al., 2002a).

The Nankai Trough region has a long record of historic great earthquakes that have generated large tsunamis (Ando, 1975). The Kumano transect is within the region that ruptured during the 1944 and 1946 earthquakes (Baba and Cummins, 2005), and this portion of the seismogenic zone is the target of future NanTroSEIZE drilling (Tobin and Kinoshita, 2006).

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