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doi:10.2204/iodp.pr.315.2008

Objectives

Scientific objectives

1. Acquisition of geotechnical data and establishment of core-log-seismic integration for deep riser drilling.

Site C0001 (proposed Site NT2-03B) will target the main splay at an ultimate depth of ~3500 meters below seafloor (mbsf) during Nankai Trough Seismogenic Zone Experiment (NanTroSEIZE) Stage 2. Site C0002 (proposed Site NT3-01A) will target the main splay and plate boundary faults at ~3000–4000 and ~6000 mbsf, respectively, during NanTroSEIZE Stage 3. During NanTroSEIZE Stage 1A, a pilot hole was drilled with a total depth (TD) of ~1000 mbsf for riser drilling planned in Stages 2 and 3. Drilling will penetrate the young slope/forearc sediments (hemipelagites and turbidites) in the upper section and obtain the underlying old accretionary prism materials. Acquisition of core and logging data and their integration were geotechnically of primary importance for riser well planning for Stages 2 and 3 and for designing the long-term borehole monitoring system for NanTroSEIZE Stage 4. These pilot holes are important not only geotechnically but also scientifically. In future riser drilling, whole-core recovery is not necessarily guaranteed for two reasons. First, some less scientifically important intervals could be skipped in the interest of saving time. Second, even when attempting to recover whole cores, recovery may be expected to be poor in some sections, especially in highly fractured lithologies. To compensate for incomplete core information, wireline logging and mud logging would be utilized as much as possible to understand lithologic, stratigraphic, geophysical, and geochemical properties of the downhole formations. Therefore, it was of primary importance to establish a complete correlation between the core, log, and seismic data in this pilot hole such as was done in the German Continental Deep Drilling Program KTB, which produced highly successful results (Emmermann and Lauterjung, 1997).

2. Structural investigations of strain partitioning between the prism and the forearc basin in oblique subduction.

Defining the upper limit of a splay thrust (Site C0001)

The recent 3-D seismic survey showed that the splay fault system, originally defined on 2-D seismic lines (e.g., Park et al., 2002), is actually composed of two main fault systems referred to as the upper splay fault and the lower splay fault, locally associated with prominent reflectors. The boundary between the slope basin and the underlying locally transparent unit (probably deformed turbiditic sequences) appears in this part of the 3-D box as a high-amplitude reflector with a normal polarity at 200 mbsf. We had working hypotheses for this boundary: a simple sedimentary unconformity between the slope basin and the underlying accretionary sediments, or the updip extension of the upper splay fault. Another interesting structure was a faint landward dipping reflector cutting the slope basin. This reflector possibly corresponded to the updip extension of the upper splay, although no deformation was recognized in the reflectors of the surface basin sediment. Defining the updip extension of the upper splay fault was one of the major structural goals of drilling at Site C0001 during NanTroSEIZE Stage 1A.

Strain partitioning between the active accretionary wedge and the forearc basin domain

Seafloor morphology of the outer ridge domain in the Kumano transect shows numerous linear features. This strongly suggests that it is a strike-slip component of the deformation, which can be interpreted as a strain partitioning of the oblique convergent motion with right-lateral motion mostly expressed along the wedge/forearc boundary. Similar partitioning has been described within the forearc basin of the neighboring Tokai domain (Huchon et al., 1998). Although the main linear depression located landward of the outer ridge might be partly due to erosional processes, it seems to be structurally controlled as shown by a sharp linear scarp seen on the Wadatsumi side-scan sonar images and is probably due to the development of a pull-apart basin. Although the strike-slip component seems to be concentrated mostly on the landward side of the outer ridge, we should carefully check the core-scale structures at Sites C0001 and C0002 for indications of such a strike-slip deformation.

Extensional versus compressive deformation

The Kumano Basin's seaward edge is cut by normal faults with rapidly decreasing spacing toward the outer ridge depression. On the other hand, some minor normal faults can be observed locally just seaward of the lower splay fault. In a working hypothesis that the splay fault moves rapidly during the coseismic period, these features could be related to gravity relaxation of the outer ridge domain during the interseismic period. Observation of such extensional features in a core scale during NanTroSEIZE Stage 1A drilling, as well as their mutual relationships with the compressional structures, would be of primary interest in understanding the deformation cycle of a splay fault system. Structural analyses during IODP Expedition 314, based mainly upon resistivity borehole image analysis, provided northwest–southeast direction compressional and north-northwest–south-southeast direction tensional stress regimes for Sites C0001 and C0002, respectively. We attempted to confirm these stress regimes in core analyses, as well as revealing paleostress regimes recorded in core structures.

3. Geochemical investigations of migrating fluids through splay faults.

Geochemistry of pore fluids reflects conditions of the deep prism where structural deformations, diagenesis, and metamorphism occur. Active cold seeps, with bacterial mats and clam colonies, have been observed at the scarp base of the megasplay 30 km southwest of Site C0001 by submersible dives (Ashi et al., 2002). Surface sediment pore fluids are characterized by low chlorinity, low δD, and low δ¹⁸O (Toki et al., 2004). These geochemical and isotopic features prefer land-derived groundwater for their origin; however, it is hard to explain the hydrological connection between land and the seep site 70 km offshore. Seafloor observations with ROV and deep-towed video cameras near Site C0001 have revealed thick hemipelagic cover and no evidence for cold seepage. In shallow cover sediments, diffusion is the dominant mode of upward migration of fluids; however, in the deeper part, channel flow through fault zones and/or fractures may be the dominant mode of fluid migration rather than diffusion with decreasing permeability. Therefore, determining the migrating path of fluids is of primary importance. We penetrated the boundary between the slope basin and the underlying accretionary complex at ~210 m core depth below seafloor (CSF; IODP method A: core expansion lengths overlap [are not scaled]) and obtained pristine fluids from the boundary and the over- and underlying sediments with much less contamination from seawater than sampling at the seafloor. Onshore fluid chemistry analyses will provide valuable information regarding their origins and migrating paths. Integration with fluid chemistries around the lower splay fault and its hanging wall, to be drilled at proposed Site NT2-01B in future NanTroSEIZE expeditions, will help further our understanding of fluid migration.

In contrast to the unidentified source of low chlorinity fluids mentioned above, hydrocarbon analyses indicate a light carbon isotopic composition from dissolved methane and a low C₂H₆/CH₄ ratio, suggesting a biological origin for the methane (Toki et al., 2004). At Site C0002, a clear bottom simulating reflector was located on seismic profile at ~400 mbsf. We are also interested in understanding the relationships between formation and migration of methane and sediment deformation. Formation and dissociation of methane hydrates probably affect methane circulation.

4. Reconstruction of the prism and the forearc basin evolution based on stratigraphic records.

Surface strata at Site C0001 were expected to consist of reworked sediments and hemipelagite. A piston core sample taken from the base of the fault scarp 30 km southwest of Site C0001 included >10 debris layers intercalated with hemipelagic sediments, suggesting repeated falling or sliding at the time of fault displacement and/or a nearby earthquake. The recurrence time of probable event depositions was estimated to be ~1000 y between 15 and 27 k.y. ago based on carbon isotope ages of foraminifer fossils (Ikehara, unpubl. data). A similar sedimentary sequence was expected in the shallow part of Site C0001, which provides longer records of recent activities of the megasplay faults. On the other hand, the upper 920 m strata at Site C0002 was expected to consist of alternation of hemipelagites and turbidites. Drilling at Site C0002 covered most sequences of the Kumano Basin, although its deepest parts were missing. High-resolution bio- and magnetostratigraphies provided age constraints to several unconformities, which may reveal that the history of the basin evolution correlated with the megasplay fault system activity.

Old accreted sedimentary sequences were expected beneath the boundary at 200 mbsf for Site C0001 and 920 mbsf for Site C0002. These accreted sequences are characterized by transparent acoustic features in 3-D seismic profiles at Site C0001. At Site C0002, these sequences are characterized by a stack of strong but discontinuous reflectors. These features are interpreted as highly dipping bedding reflectors and/or highly disrupted sequences. Microfossil age determinations, thermal histories from clay diagenesis, and fluid inclusion thermobarometries in quartz and calcite veins could provide information about the growth and exhumation of the accretionary prisms.

Strategy

Diagenetic processes, microstructures, chemical compositions, bio- and magnetostratigraphies, and potential sealing/healing processes were examined in the accretionary prism and the overlying slope/forearc basin sediments. Physical property data, including porosity, electric resistivity, and seismic velocity, were key parameters in assessing mechanical and hydrologic behaviors and for core-log-seismic integration. Whole-round and discrete core sampling are necessary for postexpedition studies, such as permeability and consolidation experiments and mechanical (rock friction and triaxial) tests. Pore fluid chemistry data will be important in constraining the hydrologic behavior of faults and unconformities and the source of any chemically and/or isotopically distinct deeply sourced fluids. Expected lithologies from seismic and logging data were hemipelagite and slump deposits for the upper ~200 m section and well-consolidated and highly deformed sediments in the underlying accretionary prism section. We were also expecting to encounter fault rocks that were cataclastically deformed to various extents.

During Expedition 314, LWD holes had already been drilled at Sites C0001 and C0002. In addition, a pilot hole and a short geotechnical hole (~30 m) were also drilled at Site C0001. We planned two more holes for coring at each site during this expedition. At Site C0001, the first hole was to be cored with the hydraulic piston coring system (HPCS)/extended shoe coring system (ESCS) to refusal depth. Refusal depth was estimated as ~600 mbsf in the original plan described in the Expedition 315 Scientific Prospectus (Ashi et al., 2007); however, we had to modify the depth to ~200 mbsf based upon our experience during Expedition 314. After refusal, we planned to drill a new hole for rotary coring. During Expedition 314 we encountered a zone of drilling difficulty at ~450–600 m CSF. Unstable borehole conditions were expected in this so-called "sticky zone." Therefore, we planned to stop rotary core barrel (RCB) coring just above this zone and drill through it with a normal drilling assembly and then restart coring from 600 mbsf. We planned to core the skipped intervals with a 4 inch petroleum-type coring system after reaching TD (1000 mbsf) (Fig. F4A). We prepared a sufficient amount of bentonite Hi-Vis mud and weighted kill-mud for potential overpressure.

Site C0002 was one of the high-priority global contingency sites. Because the riser top-hole casing operation during Expedition 315 was cancelled, we had plenty of time to drill/core another site. Based on the opinion of the expedition's riser pilot, our primary targets were the accretionary prism and its upper boundary to the overlying forearc basin. For this reason, we started coring from 475 mbsf, aiming for TD (1400 mbsf), and allocated the remaining days for shallow coring with the HPCS (Fig. F4B).

Estimating the geothermal gradient was one of the key issues for future deep riser drilling. The advanced piston corer temperature tool (APCT3) was used to measure in situ formation temperatures. The APCT3 is generally used every 30 m for HPCS coring intervals.

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