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doi:10.2204/iodp.sp.315.2007

Scientific objectives

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


Proposed Site NT2-03B will target the main splay at an ultimate depth of ~3500 mbsf during Stage 2. Stage 1 drilling is a pilot hole with a total depth of 1000 mbsf for riser drilling planned in Stage 2. The expected lithologies are young slope sediments (hemipelagites and slump deposits) in the upper section and old accreted trench sediments in the underlying section. We will not penetrate the underlying thrust during Stage 1, but this hole will be deepened during the riser drilling stage to the megasplay fault penetrated at proposed Site NT2-01. Acquisition of core and logging data (by logging while drilling [LWD] during Expedition 314) and their integration in this pilot hole is not only geotechnically but also scientifically important. In future riser drilling, whole-core recovery is not necessarily guaranteed for two reasons. First, some less scientifically important intervals might be skipped in the interest of saving time. Second, even if we try to recover the whole core, recovery may be poor in some sections, especially in highly fractured lithologies. To compensate for incomplete core information, wireline logging and mud logging should be utilized as much as possible to understand lithologic, stratigraphic, geophysical, and geochemical properties of the downhole formations. Therefore, it is of primary importance to establish a complete correlation between the core, logging, and seismic data in this pilot hole, as was done during the German Continental Deep Drilling Program (KTB), which produced highly successful results (Emmermann and Lauterjung, 1997).

Temperature measurement also provides important information on estimating the geothermal gradient to 3500 mbsf, which is essential for designing the long-term monitoring observatory.

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


Defining the upper limit of a splay thrust

As stated above, 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) was actually composed of two main fault systems referred to as the upper splay fault (USF) and the lower splay fault (LSF), locally associated with prominent reflectors (Fig. F4). Proposed Site NT2-03B for drilling the lower splay fault system at intermediate depth (3500 mbsf) during Stage 2 was chosen in a location ~2 km seaward of the outer ridge crest. This site has been chosen primarily to simultaneously meet two requirements that are difficult to find together: (1) a small slope angle to the seabed and (2) significant amplitude in the deep splay fault reflector. These requirements have been found in a small (1 km wide) bench on the seaward slope of the crest corresponding to a ~200 m thick series of layered reflectors, which may partly correspond to a slope basin (Fig. F5). The USF system does not seem to outcrop here, and we can question whether it is presently inactive or simply a blind thrust. To understand the development of this complex wedge-shaped basin is key to answering this question. The boundary between the basin and the underlying and locally transparent unit (probably deformed turbiditic sequences) appears in this part of the 3-D box as a high-amplitude reflector with normal polarity at 210 mbsf. The meaning of this reflector is one of the most interesting problems we have to solve during Expedition 315. Is it (1) a simple sedimentary unconformity between the slope basin and the underlying deformed accretionary sediments in front of a blind splay thrust or (2) the updip extension of the upper splay fault? In the first case, the propagation of the deformation related to the USF stops landward of proposed Site NT2-03B (Fig. F6A), whereas in the latter case, the deformation propagates on a detachment surface along, or more probably close, to the basal unconformity (Fig. F6B). At the location of proposed Site NT2-03B, a faint landward-dipping reflector cutting the basin reflectors can be interpreted as a thrust, although we cannot completely rule it out as a sedimentary feature. Such a thrust could be the updip extension of the USF covered with only a very thin gliding unit (Fig. F6C). Defining the updip extension of the USF and thus choosing between these different working hypotheses is one of the major structural goals of drilling at proposed Site NT2-03B during Stage 1.

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

The 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 (Fig. F7) and is probably due to the development of a pull-apart basin (Fig. F8). Although the strike-slip component seems to be concentrated mostly on the landward side of the outer ridge, we shall carefully check the core-scale structures at proposed Site NT2-03B for indications of such a strike-slip deformation.

Extensional versus compressive deformation

The Kumano Basin's seaward edge is cut by normal faults whose spacing rapidly decreases toward the outer ridge depression. On the other hand, some minor normal faults can be observed locally just seaward of the LSF (Fig. F9). 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 Stage 1 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.

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 proposed Site NT2-03B during submersible dives (Fig. F3) (Ashi et al., 2002). Pore fluids of the surface sediments are characterized by low chlorinity, low D, and low ¹⁸O (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 (Fig. F1B). Seafloor observations with ROV and deep-towed video cameras near proposed Site NT2-03B have revealed thick hemipelagic cover and no evidence for cold seepage (Fig. F3). 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. Determining the migrating path of fluids, therefore, is of primary importance. We will penetrate the probable USF at a shallow depth, and we expect to obtain pristine fluids by drilling both from the fault zone and the over/underlying sediments with much less contamination from seawater than sampling at the seafloor. Their chemistries will provide valuable information regarding their origins and migrating paths. Integration with the fluid chemistries around the LSF and its hanging wall, to be drilled at proposed Site NT2-01B during IODP Expedition 316 (Fig. F4), 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). We are also interested in understanding the relationships between formation and migration of methane and sediment deformations. Formation and dissociation of gas hydrate probably affect methane circulation, although a bottom-simulating reflector (BSR) has not been identified near proposed Site NT2-03B.

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


Surface strata at proposed Site NT2-03 are expected to consist of reworked sediments and hemipelagite. A piston core sample taken from the base of the fault scarp 30 km southwest of proposed Site NT2-03B included more than 10 debris layers intercalated with hemipelagic sediments, suggesting repeated falling or sliding at the time of fault displacement and/or nearby earthquake. The recurrence time of probable event depositions is estimated to be ~1000 y between 15 and 27 ka based on carbon isotope ages of foraminifer fossils (Ikehara, unpubl. data, 2006). A similar sedimentary sequence is expected in the shallow part of proposed Site NT2-03B (Fig. F4) that provides longer records of recent activities of the megasplay faults.

Old accreted sedimentary sequences are expected beneath the 210 mbsf boundary. This unit is characterized by transparent acoustic features in 3-D seismic profiles that suggest the presence of highly dipping bedding reflectors and/or highly disrupted sequences (Fig. F4). 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. Proposed Site NT2-01B, 2 km south of proposed Site NT2-03B, is planned to penetrate the LSF. Evolution of the megasplay fault zone associated with prism growth will be revealed by integration and comparison of data from both sites.

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