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doi:10.2204/iodp.sp.332.2010

Background

Geological setting

The Nankai Trough is a convergent plate boundary where the Philippine Sea plate underthrusts the southwestern Japan margin at rates of 4–6.5 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 by 15–25 Ma (Okino et al., 1994). The Nankai subduction zone forms an example of a sediment-dominated accretionary prism end-member. In the toe region off the Muroto transect, a sediment section ~1 km thick is accreted to or underthrusts below the margin (Moore, Taira, Klaus, 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 (Kinoshita, Tobin, Ashi, Kimura, Lallemant, Screaton, Curewitz, Masago, Moe, and the Expedition 314/315/316 Scientists, 2009). The upper Shikoku Basin facies off the Kumano Basin pinches out toward the north, whereas the lower succession has a more complicated geometry resulting from basement topographic influence (Le Pichon et al., 1987a, 1987b; Mazzotti et al., 2000; Moore, Taira, Klaus, et al., 2001): seismic thickness decreases above large basement highs, and acoustically transparent units indicate local absence of the sand packages that characterize most other parts of the lower Shikoku Basin. The mechanical differences between subducting basement highs and plains could be significant for fault zone dynamics and earthquake rupture behavior.

The deformation front behavior off the Kumano Basin is fundamentally different than that at previous targets of Ocean Drilling Program (ODP) drilling off the Muroto and Ashizuri transects, several hundred kilometers to the southwest. Seismic reflection data off the Kumano Basin 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 www.iodp.org/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 in board, 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 prism. A combination of swath bathymetric and multichannel seismic data show a pronounced and continuous outer ridge (outer arc high) of topography extending >120 km along strike, which may be related to the megasplay fault slip, including the 1944 Tonankai M 8.2 earthquake and repeated previous earthquakes. Remotely operated vehicle (ROV) and manned submersible surveys along this feature reveal a very steep slope on both sides of the ridge (Ashi et al., 2002). The other 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 OOST. This 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 sedimentary successions 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). It branches into a family of thrust splays in the upper few kilometers below the seafloor, including the thrust splay drilled during Expedition 314.

The most direct evidence for activity in the megasplay during geologically recent times comes from Kumano forearc basin stratigraphy. The Kumano Basin is characterized by flat topography at ~2000 m and is filled with turbiditic sediments to a maximum thickness of ~2 km. Little is known regarding the detailed stratigraphy of the Kumano Basin, but several remarkable features are recognized in the seismic profiles (Fig. F3): the overall sedimentary sequences filling the basin can be divided into four main lithologic units (I–IV) by unconformities based on the seismic stratigraphy as well as LWD during Expedition 314. 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 them; all of the sediments pinch out toward the north. All 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).

Previous drilling achievements

Site C0002

Site C0002 was previously drilled during Expedition 314, as Hole C0002A, in which 1401 m of Kumano forearc basin and accretionary prism sediments were successfully drilled and logged with a full suite of LWD and measurement-while-drilling (MWD) tools. Despite strong Kuroshio Current conditions, the expedition retrieved an excellent series of logs and seismic VSP data. They drilled and logged four units, separated by unconformities:

• Logging Unit I: slope basin deposits;

• Logging Unit II: basin fill comprised of repeating turbidite deposits (contains two potentially gas-bearing sandy intervals);

• Logging Unit III: basin fill consisting of homogeneous clay-rich mudstone; an angular unconformity cuts Unit III, but with no discernible lithologic changes across the boundary; and

• Logging Unit IV: accretionary prism sediment with very variable responses in the downhole logs.

Expedition 315 cored the 475–1057 mbsf interval (middle of logging Unit I to top of logging Unit IV), confirming the boundaries of the logging units and adding lithologic detail through core description as well as preliminary nannofossil-based biostratigraphy. Unit II is of Pleistocene age, Unit III is from the Pliocene, and Unit IV is of late Miocene age. Paleomagnetic measurements support this age model by placing the Bruhnes/Matayuama border at 850 m core depth below seafloor. Unit IV has a strongly varied dip and azimuth, confirming the presence of highly deformed strata as suggested by the reflection profiles.

Site C0010

Operations at Site C0010 during Expedition 319 in 2009 included drilling with LWD/MWD across the megasplay fault to a TD of 555 m LWD depth below seafloor, casing the borehole (with casing screens at the fault), conducting an observatory dummy run to test strainmeter and seismometer deployment procedures, and installation of a simple pore pressure and temperature monitoring system (SmartPlug).

Although the SmartPlug is relatively simple, it marks the first observatory placement in NanTroSEIZE. All of the planned science objectives for Site C0010 were achieved, although casing operations were adjusted to fit hole conditions after drilling to TD (560 m drilling depth below seafloor [DSF]) with casing to 500 m DSF instead of the planned 525 mbsf outlined in the Expedition 319 Scientific Prospectus (Saffer et al., 2009). LWD/MWD data (gamma ray and resistivity, including resistivity-at-the-bit images, see Fig. F4) were collected allowing (1) definition of major lithologic unit boundaries and of the shallow megasplay fault zone and (2) determination of the preferred placement of the screened joints interval within the fracture zone interval for the temporary observatory. Through comparison with previously drilled Site C0004 (Kinoshita, Tobin, Ashi, Kimura, Lallemant, Screaton, Curewitz, Masago, Moe, and the Expedition 314/315/316 Scientists, 2009) these data also provide insights into along-strike differences in the architecture of the megasplay fault and hanging wall.

After drilling the hole and in preparation for a future permanent observatory installation, a sensor dummy run test was carried out to evaluate reentry operations during instrument deployment. After casing was completed and the borehole was cemented, two dummy run tests were conducted, including adjustments for the effects of the Kuroshio Current, which reached speeds of 4.5 kt during the experiment.

After the dummy run reentry simulations were completed, the SmartPlug instrument package was installed for short-term (1–2 y) data collection and storage to monitor temperature and pore pressure within the megasplay fault zone. It was installed just beneath a mechanically set retrievable casing packer. The retrievable packer was set inside casing above two screened casing joints; the SmartPlug and screen placement in the casing were configured to continuously monitor pore pressure and temperature in an isolated interval of formation including the splay fault and to also monitor hydrostatic pressure as a reference. The SmartPlug contains two high-precision pressure transducers with period counters and four temperature sensors (one as part of each pressure gauge for compensation, one platinum chip thermistor, and one stand-alone miniature temperature logger) in a shockproof housing. The self-contained instrument has a recording lifetime of ~7 y.

The SmartPlug entered the hole and the packer was set at 364 m DSF. Retrieval of the bridge plug and instrument package is anticipated as a contingency plan during this expedition, where a more sophisticated temporary monitoring system will replace the SmartPlug.

Site survey data

The supporting site survey data for Expedition 332 are archived at the IODP Site Survey Data Bank.

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