Scientific objectives

Site C0002 background

The primary drilling plan for Expedition 338 is to extend Hole C0002F to ~3600 mbsf through riser drilling with the Chikyu. The hole will be suspended after casing is installed and cemented at the 13⅜ inch casing set point (Fig. F2). During Expedition 326 in 2010, the wellhead was installed and a 20 inch casing string was cemented in place to 860 mbsf.

The uppermost 1400 mbsf section at Site C0002 was previously logged with a comprehensive LWD program during Expedition 314 (Expedition 314 Scientists, 2009). The intervals 0–204 and 475–1057 mbsf were cored during Expedition 315 (Expedition 315 Scientists, 2009b). The Kumano forearc basin sedimentary package composes the interval from 0 to 940 mbsf, and it is underlain by the “inner wedge” deformed accretionary wedge package. The seismic reflection character of the entire zone from ~940 mbsf to the megasplay reflector at ~5200 mbsf exhibits virtually no coherent reflections that would indicate intact stratal packages, which is in contrast to the outer accretionary wedge seaward of the megasplay fault system (Figs. F2, F3) (also see Moore et al., 2009). This seismic character is thought to indicate complex deformation within the inner wedge, perhaps best characterized as a subduction mélange or protomélange. The anticipated lithology to be encountered during Expedition 338 is Miocene age hemipelagic mudstone and sand/silt turbidites with sparse volcanic ash, judging from the core recoveries and logs recovered during Expeditions 314 and 315. Whether the deeper accreted strata represent trench-wedge deposits, Shikoku Basin deposits, or both remains to be determined.

Site C0002 objectives

Accordingly, the main research objectives for this interval are to (1) sample the interior of the accretionary complex in the midslope region beneath the Kumano forearc basin with both cores and drill cuttings and (2) collect an extensive suite of LWD to characterize the formation. Sampling this previously unsampled interval will allow the (1) determination of the composition, age, stratigraphy, and internal style of deformation of the Miocene accretionary complex; (2) reconstruction of its thermal, diagenetic, and metamorphic history and comparison with present pressure-temperature (P-T) conditions; (3) determination of minimal horizontal stress within the deep interior of the inner wedge; (4) investigation of the mechanical state and behavior of the formation; and (5) characterization of the overall structural evolution of the Nankai accretionary prism and the current state of the upper plate above seismogenic plate boundary thrust.

The interval from 856 mbsf to target depth (proposed to be 3600 mbsf) will be drilled with continuous LWD resistivity, gamma radiation, and annulus fluid pressure data. During this riser drilling, mud return will allow for comprehensive analysis of drill cuttings and mud gas, as was performed at Site C0009 and described in the Expedition 319 Preliminary Report (Saffer et al., 2009). Coring (100 m total) is also planned to sample the inner wedge but is restricted to one interval from 2300 to 2400 mbsf.

Site C0002 drilling will therefore access the interior of the landward region of an active accretionary prism for the first time by scientific ocean drilling, testing hypotheses for the transition from aseismic prism growth to a strong hanging wall regime defining the outer edge of the geodetically locked or partially locked seismogenic plate boundary. Additionally, it will shed light on the nature of prism formation and evolution. The data collected will also define the physical properties of the sediments that create the discontinuous seismic signature. At the end of Expedition 338, the borehole will be suspended for reentry and further deepening to the planned plate boundary target during the 2013–2014 IODP riser drilling season.

Specific questions to be addressed by drilling into the deep interior of the inner wedge include

  • What is the thermal, diagenetic, and metamorphic history of the sedimentary rock below the Kumano Basin?

  • What is the budget for hydrous minerals (e.g., smectite group clays) and the extent of dehydration reaction progress as a function of depth?

  • What is the mechanical and structural evolution of the inner wedge?

  • Are there indicators of low effective stress, high pore pressure zones related to deformation?

  • How do the properties of the inner wedge sediments compare with the Shikoku Basin sediments that are input to the wedge?

  • What is the orientation of minimal horizontal stress within the deep interior of the inner wedge? How does the stress orientation relate to the current state of the earthquake cycle?

  • What is the mechanical state and behavior of the formation and how does it relate to the current state of the upper plate above seismogenic plate boundary thrust?

  • What are faulting processes and mechanisms and how do they vary with depth in the inner accretionary wedge?

Answering these questions will allow for inferences on the structural style (subduction mélange or protomélange versus deformed former outer wedge), connections between sediment dewatering and fluid pressure, and thus long-term evolution of the Nankai accretionary prism. The answers also provide a robust characterization of the inner wedge, which will ultimately be related to the deep section near the megasplay and plate boundary faults.

Performing experiments using cuttings at presumably in situ conditions, we can constrain mechanical and hydrological properties of the inner wedge materials. Analyses of continuous series of cuttings, even with poor resolution from mixing, would also provide information on the lithologic constituents and their variation with depth in the inner accretionary wedge and also compare them with those properties estimated by LWD.