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doi:10.2204/iodp.pr.314.2008

Introduction

The Nankai Trough Seismogenic Zone Experiment (NanTroSEIZE) is a multiyear, multistage, and multiplatform effort planned for the Integrated Ocean Drilling Program (IODP). Out of four operational stages, the first stage of NanTroSEIZE drilling operations comprises three individual expeditions (314, 315, and 316) being conducted by the new riser-capable drilling vessel, Chikyu, beginning September 2007. The primary objectives of Expedition 314 were to obtain a comprehensive suite of geophysical logs and other downhole measurements at six sites (Figs. F1, F2) using state-of-the-art logging-while-drilling (LWD) technology. These six sites are designed to accomplish the principal goals of the NanTroSEIZE Science Plan, including documenting the material inputs to the subduction conveyor (fluid, solids, and heat), the properties of major thrust faults and their wall rocks at depths shallower than ~1.4 km, and the geology of the accretionary prism and overlying slope basin sediments (Tobin and Kinoshita, 2006a; Kinoshita et al., 2006). Four of these six sites have been or are slated for continuous core sampling during the two subsequent Expeditions 315 and 316, immediately following Expedition 314.

During Expedition 314, we acquired borehole LWD data by drilling dedicated holes with a drill string with the logging instruments incorporated into drill collars just above the drilling bit. A number of previous Ocean Drilling Program (ODP) and IODP expeditions have employed LWD technology (Riedel et al., 2006; Flemings et al., 2006; Tréhu, Bohrmann, Rack, Torres, et al., 2003; Mikada, Becker, Moore, Klaus, et al., 2002; Kelemen, Kikawa, Miller, et al., 2004). We deployed Schlumberger LWD/measurement-while-drilling (MWD) tools including the following:

• geoVISION: records formation resistivity, including 360° borehole resistivity imaging and spectral natural gamma radiation

• adnVISION: mainly records neutron porosity and gamma ray density

• sonicVISION: provides sonic P-wave velocity

• seismicVISION: records check shot vertical seismic profile (VSP) arrivals during LWD drilling and therefore provides interval velocity.

Additional measurements primarily intended for monitoring drilling conditions (MWD), such as torque, downhole weight on bit, and so on, as well as borehole pore fluid pressure (annular pressure while drilling [APWD]) were recorded and also transmitted using MWD uphole telemetry.

Expedition 314 was entirely dedicated to the LWD effort, and only minor coring and no downhole measurement operations took place. Logging data were analyzed initially by the Shipboard Scientific Party and made available to the scientific parties of the other NanTroSEIZE Stage 1 expeditions.

Geological setting

The Nankai Trough is a subducting plate boundary, where the Philippine Sea plate underthrusts the southwestern Japan margin 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) down an interface dipping 3°–7° (Kodaira et al., 2000). The subducting lithosphere of the Shikoku Basin was formed by backarc spreading at 15–25 Ma (Okino et al., 1994). The Nankai subduction zone forms an "end-member" sediment-dominated accretionary prism. In the toe region off Muroto, a sedimentary section ~1 km thick is accreted to or underthrust below the margin (Moore et al., 2001).

The three major seismic stratigraphic sequences identified in the northern Shikoku Basin are the lower and upper Shikoku Basin sequences and the Quaternary turbidite sequences (Fig. F3). The upper Shikoku Basin facies off Kumano decreases modestly 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 et al., 2001). Seismic thickness decreases above larger 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.

The deformation front behavior off Kumano is fundamentally different than it is at previous targets of ODP drilling off Muroto or Ashizuri, several hundred kilometers to the southwest. 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 deeper Shikoku Basin strata (see "Proposal 603A-Full2" at www.iodp.org/nantroseize-downloads). One interpretation of the seismic profile is that the décollement steps up to the seafloor, thereby thrusting older accretionary prism strata (upper Shikoku Basin facies?) over the upper Quaternary trench-wedge facies (Fig. F3). 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 (MCS) 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, including the 1944 Tonankai M 8.2 earthquake and repeated previous earthquakes. 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; unpubl. data). The outer arc high coincides with the updip end of the splaying system of thrust faults that branch from a strong seismic reflector interpreted by Park et al. (2002) as a major out-of-sequence thrust, which we term the "megasplay" because it is a feature that traverses the entire wedge and has had a protracted history shown by the thick forearc basin trapped behind its leading edge (Moore et al., 2007). The megasplay is hypothesized to represent the mechanical boundary between the inner and outer accretionary wedge and between aseismic and seismogenic fault behavior (Wang and Hu, 2006). At depth this megasplay is a high-amplitude reflector (Fig. F2), and it branches into a family of thrust splays in the upper few kilometers below the seafloor, including thrust splay drilled on this expedition

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); however, the thrust sheets wedge into these deposits, causing tilt and slumping of even the deposits nearest to the surface.

Other direct evidence that the megasplay fault has been active in geological to recent times comes from Kumano forearc basin stratigraphy. The Kumano Basin is characterized by flat topography at ~2000 m depth and is filled with turbiditic sediments to a maximum thickness of ~2000 m. Little is known regarding the detailed stratigraphy of the Kumano Basin, but several remarkable features are recognized in the seismic profiles (Fig. F4). The overall sedimentary sequences filling the basin can be divided into four main units by unconformities based on seismic reflection stratigraphy. The sediments in the southern part of the basin are tilted northward, truncated by a flat erosional surface, and subsequently cut by normal faults (Park et al., 2002). The depositional center appears to have migrated northward after each successive unconformity.

The sequences above the unconformities are tilted less than those below the unconformities. All of the sediments pinch out toward the north. All of these features appear to be caused by uplift of the outer rise and potentially by postseismic relaxation after coseismic slip on the splay faults (Park et al., 2002).

Seismic studies/site survey data

The off-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 two-dimensional seismic reflection (e.g., Park et al., 2002), wide-angle refraction (Nakanishi et al., 2002; Nakanishi et al., submitted), passive seismicity (e.g., Obana et al., 2001), heat flow (Kinoshita 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, three-dimensional (3-D) seismic reflection survey was conducted by Japanese and U.S. scientists over a ~11 km x 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 inline 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. As the drilling proceeds, the 3-D seismic data will continue to be used to analyze physical properties of the subsurface through seismic attribute studies, to expand findings in the boreholes to wider areas, and to assess drilling safety.

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