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Geological setting

The Nankai Trough is formed by subduction of the Philippine Sea plate underneath southwestern Japan at a rate of ~4.1–6.5 cm/y along an azimuth of 300°–315°N (Seno et al., 1993; Miyazaki and Heki, 2001). In the outer forearc, the plate interface dips 3°–7° (Kodaira et al., 2000). The Nankai Trough subduction zone forms an end-member sediment-dominated accretionary prism similar to the Mediterranean Ridge (Kopf et al., 2003). In the toe region, a sedimentary section ~1–1.5 km thick is accreted to or underthrust below the margin (Moore, Taira, Klaus, et al., 2001; Moore et al., 2009).

The three major seismic stratigraphic sequences identified in the northern Shikoku Basin are the lower and upper Shikoku Basin sequences and the Quaternary turbidite sequence. The upper Shikoku Basin facies off Kumano decreases slightly in thickness toward the north, whereas the lower Shikoku Basin facies displays a much more complicated geometry as a result of the effects of basement topography (Le Pichon et al., 1987a, 1987b; Mazzotti et al., 2000; Moore, Taira, Klaus, et al., 2001; Ike et al., 2008a, 2008b). Its thickness decreases above basement highs, and a more transparent acoustic character indicates local absence of sand packages that characterize most other parts of the lower Shikoku Basin. The mechanical differences between subducting basement highs and subducting basement plains could be significant for fault zone dynamics and earthquake rupture behavior (Bilek et al., 2003).

The deformation front behavior off Kumano is fundamentally different than it is at previous targets of Ocean Drilling Program (ODP) drilling offshore of Capes Muroto and Ashizuri, ~200–300 km to the southwest (Taira, Hill, Firth, et al., 1991; Moore, Taira, Klaus, et al., 2001). Seismic reflection data off Kumano clearly delineate the frontal fault near the prism toe; however, there is little evidence for seaward propagation of the décollement within the deeper Shikoku Basin strata (see Proposal 603A-Full2 at​600/). One interpretation of the seismic profile is that the décollement steps up to the seafloor, thereby thrusting older accretionary prism strata over the upper Quaternary trench-wedge facies (Fig. F2). Manned submersible observations also indicate that semilithified strata of unknown age have been uplifted and exposed along a fault scarp at the prism toe (Ashi et al., 2002). Farther inboard, the fault ramps down into the lower Shikoku Basin facies (Park et al., 2002).

The lower forearc slope consists of a series of thrust faults that have shortened the accreted sedimentary units of the accretionary prism. A combination of swath bathymetric and multichannel seismic data show a pronounced continuous outer ridge (outer arc high) of topography extending >120 km along strike, which may be related to megasplay fault slip (e.g., Moore et al., 2007; Strasser et al., 2009). Remotely operated vehicle (ROV) and manned submersible diving surveys along this feature reveal a very steep slope on both sides of the ridge (Ashi et al., 2002; J. Ashi et al., unpubl. data). The outer arc high coincides with the seaward edge of a system of thrust faults that branch from the megasplay.

The megasplay is a major structural boundary within the accretionary wedge, traverses the entire wedge, and has had a protracted history of activity as shown by the thick forearc basin trapped behind its leading edge (Moore et al., 2007). The megasplay is also hypothesized to represent a discontinuity in rock physical properties and fault mechanical boundary between the inner and outer accretionary wedge and perhaps between aseismic and seismogenic fault behavior (Wang and Hu, 2006). At depth, the megasplay is imaged in seismic reflection data as a high-amplitude reflector (Bangs et al., 2009) (Fig. F2), and it branches into a family of smaller splays in the upper few kilometers below the seafloor, including the fault penetrated at IODP Sites C0004 and C0010. The most direct evidence for recent megasplay fault activity comes from stratigraphic relationships at the tips of the faults in young slope sediments. Direct fault intersections with the seafloor are not observed (Moore et al., 2007; Strasser et al., 2009); however, the thrust sheets wedge into these deposits, causing tilt and slumping of even the deposits nearest to the surface. Evidence for mass wasting complexes is consequently found at IODP Sites C0004, C0008, and C0010. The latter, as well as Site C0002 further landward, were the target areas of Expedition 332.

Site C0010, located 3.5 km southeast of previously completed Site C0004, was first drilled during Expedition 319 (Saffer, McNeill, Byrne, Araki, Toczko, Eguchi, Takahashi, and the Expedition 319 Scientists, 2010). Operations during that expedition included drilling through the megasplay fault zone and into its footwall using LWD, setting casing with screens across the fault zone, and installing a simple and temporary borehole observatory (a SmartPlug) to monitor fluid pressure and temperature in the shallow megasplay. Major lithologic boundaries as well as the location of the megasplay fault at ~407 meters below seafloor (mbsf) were identified in LWD data and were used to select a depth interval spanning the fault for placement of the two screened casing joints. Three distinct lithologic packages were observed at Site C0010: slope deposits (Unit I, 0–182.5 mbsf), thrust wedge (Unit II, 182.5–407 mbsf), and overridden slope deposits (Unit III, 407 mbsf to total depth [TD]) (Fig. F3). Unit I is similar to Unit I described previously at Site C0004 and was interpreted as hemipelagic slope sediments composed primarily of mud with minor distal turbidite interbeds. The thrust wedge (Unit II) is clayey and characterized by high and variable gamma ray and resistivity values compared with the units above and below. Unit III is composed of hemipelagic muds with minor turbidite interbeds and rare volcanic ash layers.

Site C0002 is located near the southeastern edge of the Kumano Basin (Fig. F2). The Kumano Basin has a generally flat bathymetry, with a water depth of ~2000 m. The sediments in the southern part of the Kumano Basin are tilted northward, truncated by a flat erosional surface, and subsequently cut by normal faults (Park et al., 2002; Ashi et al., 2008; Moore et al., 2009). Lithostratigraphy at Site C0002 (Fig. F3) is characterized by turbiditic sediments to ~830 mbsf, underlain by older rocks of the accretionary prism and/or early slope basin sediments deposited prior to the development of the megasplay fault, which were drilled and partly cored during Expeditions 314 and 315. Expedition 332 revisited Site C0002, drilling with a limited suite of LWD/measurement-while-drilling (MWD) tools, for reconnaissance and to identify the most suitable depth intervals to place sensors of the long-term borehole monitoring system (LTBMS) (see “Site C0002 riserless observatory”).

Seismic studies/Site survey data

The Kii and Kumano Basin region is among the best-studied subduction zone forearcs in the world. A significant volume of site survey data has been collected in the drilling area over many years, including multiple generations of 2-D seismic reflection (e.g., Park et al., 2002), wide-angle refraction (Nakanishi et al., 2002, 2008), passive seismicity (e.g., Obana et al., 2001), heat flow (Yamano et al., 2003), side-scan sonar, and swath bathymetry as well as submersible and ROV dive studies (Ashi et al., 2002). In 2006, a joint, 3-D seismic reflection survey was conducted by Japanese and US scientists over a ~11 km × 55 km area, acquired under contract by Petroleum GeoServices, an industry service company (Fig. F1) (Moore et al., 2007). The poststack trace spacing is 12.5 m in the in-line direction and 18.75 m in the cross-line direction. This 3-D volume—the first deep-penetration, fully 3-D marine survey ever acquired for basic research purposes—has been used to refine the selection of drill sites and targets in the complex megasplay fault region and define the regional structure and seismic stratigraphy (Moore et al., 2009). As NanTroSEIZE drilling proceeds, the 3-D seismic data will continue to be used to refine drilling and operational strategies, to analyze physical properties of the subsurface through seismic attribute studies, to extend findings in the boreholes to wider areas, and to assess drilling safety.