Scientific objectives

Site C0002 background

The primary drilling plan for Expedition 348 is to extend Hole C0002F to ~3600 mbsf (or deeper) through riser drilling. The hole will be suspended after casing is installed and cemented at the 11¾ inch casing set point (Fig. F3). During Expedition 326 in 2010, the wellhead was installed and a 20 inch casing string was cemented in place to 860 mbsf. During Expedition 338, the riser hole was extended to 2005 mbsf; however, the hole was not cased because of operational difficulties.

The uppermost 1400 mbsf 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). During Expedition 338, riser operations extended the hole from 842 to 2005.5 mbsf, collecting a full suite of LWD and measurement-while-drilling (MWD), mud gas, and cuttings data (Moore et al., 2013). Additional cores were collected at adjacent holes during Expedition 338 with riserless drilling to 1120 mbsf (Moore et al., 2013). The Kumano forearc basin sedimentary package comprises the interval from 0 to 940 mbsf, and it is underlain by the inner accretionary wedge. The entire interval from ~940 mbsf to the megasplay reflector at ~5200 mbsf exhibits few coherent seismic reflections that would indicate intact stratal packages, which is in contrast to the outer accretionary wedge seaward of the Kumano Basin region (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 348 is Miocene age hemipelagic mudstone and sand/silt turbidites with sparse volcanic ash, based on cores and logs obtained during Expeditions 314, 315, 319, and 338. Whether the deeper accreted strata represent trench-wedge deposits, Shikoku Basin deposits, or both remains to be determined.

Site C0002 objectives

The main research objectives for Expedition 348 are to (1) sample the interior of the accretionary complex in the midslope region beneath the Kumano forearc basin by collection of both cores and drill cuttings and (2) collect an extensive suite of LWD logs to characterize the formation. Sampling this previously unsampled interval will allow (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 conditions; (3) determination of horizontal stress orientations and potentially magnitudes within the deep interior of the inner accretionary wedge; (4) investigation of the mechanical and hydrological and behavior of the wedge; and (5) characterization of the overall structural evolution of the Nankai accretionary prism and the current state of the upper plate above the presumed locked seismogenic plate boundary thrust.

The interval from 860 mbsf to target depth (3600–4400 mbsf, depending on drilling conditions) will be drilled with continuous LWD resistivity, azimuthal resistivity imaging, sonic transit time, gamma radiation, and annular pressure data. During this riser drilling, mud return will allow for a comprehensive analysis of drill cuttings and mud gas, as was performed at Site C0009 and Hole C0002F and described by Expedition 319 Scientists (2010) and Moore et al. (2013), respectively. Coring (100 m total) is also planned to sample the inner wedge but is restricted to one interval from 2300 to 2400 mbsf, just below the 13⅜ inch casing shoe. The overall target depth planned for the expedition depends on operational time, hole stability, and weather conditions. If these are all favorable and drilling proceeds smoothly with sufficient time remaining, the goal will be to drill to 4400 mbsf (instead of 3600 mbsf) and to case the hole to that depth with a 9⅝ inch liner (instead of the 11¾ inch casing). The decision between these options will be made as the expedition progresses.

Site C0002 drilling will access the interior of the landward region of an active accretionary wedge for the first time by scientific ocean drilling, testing hypotheses for the transition from aseismic wedge 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 wedge formation and evolution. The data collected will also define the physical properties of the sediments that create the observed discontinuous seismic signature across the megasplay (e.g., Park et al., 2002; Moore et al., 2009; Kamei et al., 2013). At the end of Expedition 348, the borehole will be suspended for reentry and further deepening to the planned plate boundary target during 2015.

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 are the horizontal stress orientations and magnitude 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 the 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, deformed former outer wedge, or some unanticipated lithology and fabric), connections between sediment dewatering and fluid pressure, and thus the long-term evolution of the Nankai accretionary complex. The answers also provide a robust characterization of the inner wedge, which will ultimately be related to the deep section near the plate boundary fault.

Performing experiments using cuttings at presumed 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.