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Summary and implications

Together with coring during Expeditions 315 and 316, the objectives of Expedition 314 were to drill a transect of riserless sites that would characterize the lithologies, physical and mechanical properties, structures, and stress conditions in the sediments at relatively shallow depths below seafloor on the Nankai margin and establish the riserless “top-hole” conditions and properties at the two sites slated for deep riser drilling in later stages of NanTroSEIZE drilling. Our specific goal on Expedition 314 was to obtain the maximum possible in situ data by logging in dedicated LWD holes, with Expeditions 315 and 316 slated to core twinned holes at each of the sites we logged. Our targets were the accretionary prism and faults therein and the major forearc basin overlying the hypothesized seismogenic zone. To this end, we drilled and logged (Fig. F8; Table T1) from 0 to 885.5 mbsf at the frontal thrust site at the toe of the entire accretionary prism (Site C0006), 0 to 400 mbsf across the out-of-sequence megasplay fault system (Site C0004), 0 to 1000 mbsf into the thrust sheet above the megasplay in a pilot hole for future riser drilling (Site C0001), and 0 to 532 mbsf at a second site in that thrust sheet (Site C0003), and 0 to 1401 mbsf at a site crossing 976 m of the Kumano forearc basin and 425 m of the underlying older accretionary prism rocks (Site C0002), completing the pilot hole for a second future riser drilling effort.

At these sites, we conducted an extensive suite of LWD logging, including gamma ray values, resistivity imaging, sonic velocity, neutron porosity, lithodensity, and check shot seismic. This body of data is collectively the most advanced LWD data recorded in scientific ocean drilling to date and is also the largest number of meters logged with LWD during a single expedition in IODP or ODP. Hole C0002B is the deepest penetration into an accretionary prism and the deepest LWD hole ever in scientific ocean drilling.

Not all sites have all logs available. In some cases equipment malfunctions caused loss of some logging data (e.g., sonic log at Site C0006), and after the loss of the LWD BHA at Site C0003, we were unable to record porosity or density data at the subsequent sites because of the lack of a radioactive source.

Key results

Lithology and structure of faults and thrust sheets

As expected, we found that logs are consistent in showing sediments dominated by hemipelagic muds/​mudstones and turbidites, with relatively little sand except at the trench site (C0006) and in the slope basin deposits (Fig. F21). In the absence of cores, our main tool for structural analysis of deformed zones, fractures and faults, bedding attitude, and borehole stress was resistivity imaging. We documented bedding attitudes consistent with the seismic interpretation of structural domains: relatively shallow bedding in basinal settings with well-imaged, continuous reflectors and more variable and steeper dips in areas imaged as strongly deformed thrust sheets, accretionary mélange, and apparent fault zones. The thrust sheet drilled in the hanging wall of the megasplay fault at Sites C0001 and C0004 is a strongly deformed zone and has bedding attitudes clearly differentiated from the slope basin sediments that overlie and also the strata that underthrust the fault. Similarly, the Kumano forearc basin section at Site C0002 exhibits gentle dip, in close agreement with the seismic reflection interpretations, whereas the underlying “basement,” interpreted as older accreted sedimentary rocks, shows highly variable bedding orientation. Rocks of that underlying prism and of the thrust sheet below 500 m LSF at Site C0001 are anomalously dense with high sonic/​seismic velocity for their burial depth, indicating more advanced lithification. In the case of Sites C0001 and C0004, this suggests significant uplift and exhumation along the splay thrust, though that hypothesis must be confirmed with core data. At Site C0006, the frontal thrust region, the bedding is less variable in general, in agreement with the incipient nature of the deformation at this site, and dips are generally shallow. Physical properties are consistent with shallowly buried sediments that have been only weakly lithified at this site.

Borehole breakouts and stress

Borehole breakouts were observed at all four sites for which we have imaging data and show very systematic orientations (see below) (Fig. F22). In a vertical borehole, the orientation of breakouts is a well-established indicator of the orientation of the horizontal maximum principal stress in the present-day stress field (Zoback et al., 2003). Less frequently, drilling-induced tensile fractures were observed, primarily at Site C0001 in the splay fault thrust sheet. Breakout orientations at Sites C0001, C0004, and C0006 all indicate northwest–southeast azimuths of the maximum horizontal principal stress (SHmax) (Fig. F23). In the thrust-dominated tectonic environment, this is consistent with trench-normal shortening, although strike-slip and/or normal faulting stress states are also permissible. Variations of the SHmax direction among these three sites of as much as 10°–20° are statistically significant and may be due to local structure influencing local stress orientation. SHmax orientations deviate from the far-field plate motion vectors based on Global Positioning System results (Heki, 2007), implying strain partitioning between convergence and strike-slip motions.

At Site C0002 in the Kumano Basin, by contrast, the orientation of SHmax is northeast–southwest, or near-perpendicular to that in the more trenchward sites. This is consistent with a normal faulting stress state which extends through the basin section (which exhibits normal faults) and also in the upper 400 m, at least, of the underlying prism. This contrast suggests that the tectonic stress magnitudes differ greatly in the upper part of the prism at sites just a few kilometers apart along our transect and that gravitationally driven extension may be important above the megasplay where the outer arc high has been uplifted.

Gas hydrates and bottom-simulating reflector

At Site C0002, a well-developed BSR is imaged in the seismic data, and LWD logs recorded comprehensive in situ information about the nature of this BSR (see “Site C0002”). Resistivity logs showed a pattern of elevated resistivity background and spikes for ~200 m above the BSR depth at ~400 m LSF (Fig. F24). The gamma response indicated that the spikes are in especially sandy intervals as thick as 1–2 m, interpreted as the coarse basal beds in turbidite deposits. Sonic and resistivity responses are consistent with pore space in these sands being partially filled and cemented with gas hydrate. In contrast, similar sands below the BSR depth show no elevated resistivity response. In a zone 80 m deeper and ~70 m thick, a low resistivity response in the sandy beds suggests a potential gas-charged interval beneath the gas hydrate stability field. The logging data indicate that the BSR reflectivity is a response to both a small velocity high from hydrate cement in the hydrate stability zone and a more significant velocity low caused by the presence of uncemented sediments and/or free gas below the stability field. Ongoing further analysis will quantify the amount of pore space charged with hydrate in the zone above the BSR and, integrated with 3-D seismic analysis, the total amount of gas in this region of the Kumano Basin section.

Taken as a whole, the results of Expedition 314 LWD show that both the tectonic history and present-day stress conditions in the forearc basin region differ greatly from those in the outer accretionary prism thrust sheets and faults. This was manifested in both the logging and image data and in the difficult drilling conditions in the splay fault thrust sheet, where borehole caving and collapse was a near-constant risk. We tentatively conclude that this is an indication that the basic concept of the NanTroSEIZE transect—to span the lateral boundary between the aseismic outer accretionary prism and the seismogenic inner accretionary prism and forearc basin—is validated even by the LWD-based observations in the upper 400 to 1400 m LSF. In particular, one hypothesis to explain the apparent boundary in the state of stress between Sites C0001 and C0002 (only 10 km apart) is that these sites span the updip locked boundary zone. We hope that future deep riser drilling will test these inferences as we drill from the forearc basin region down to the actual plate interface fault system.