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doi:10.2204/iodp.proc.314315316.113.2009

Discussion and synthesis

Site C0001 sampled deep into the thrust sheet of what has previously been interpreted as a shallow branch of the megasplay fault in the Kumano transect region (Moore et al., 2007). A 976 m interval was drilled and logged, made up of apparently strongly deformed mudstones with some silty to sandy sediments. Prior to drilling and based on seismic interpretation, this interval was hypothesized to represent relatively old and well-lithified mudstone of the interior of the Nankai accretionary prism overlain by ~200 m of hemipelagic slope deposits. Log and vertical seismic data largely support this overall model and provide the first in situ information on physical properties, lithology, structure, and state of stress in this key element of the Nankai subduction system. Cores collected during subsequent Expedition 315 tested these interpretations of log data and provided age and textural information not available from the logs.

Importantly, the riserless drilling at Site C0001 fulfilled a key project goal of piloting the planned deep-riser drilling at this location. For the riser hole planned in NanTroSEIZE Stage 2, it was imperative that we obtain information on the rock properties and potential qualities for drilling and casing of the uppermost kilometer as preparation for installation of riser casing. Drilling parameters and physical property logging have accomplished this goal, and the well planning for the riser operation will proceed with this knowledge. In particular, establishing the uppermost few hundred meters of a riser borehole actually requires riserless drilling, because the near-surface sediments are generally not strong enough to withstand the hydrostatic pressure in a riser column of weighted mud extending to the sea surface more than 2000 m above the seabed. Hence, the strength and in situ pore pressure in the sediments are key parameters in determining how deep the drilling must proceed in open hole before casing can be set, the blowout preventer connected, and the riser set up for mud-based drilling.

Make-up of the thrust sheet

As described in the sections of this chapter, we found that the interior of the thrust sheet is remarkably homogeneous in apparent composition, with gamma ray and resistivity values indicating that mudstones dominate. In addition, the high seismic/sonic velocity (reaching >2700 m/s at 1000 m LSF) and relatively low apparent porosity suggest that the rocks in the lower part of the drilled section are overconsolidated relative to their present depth below the surface and have therefore been uplifted with erosion of a significant overlying section or alternatively have been diagenetically altered by cementation at their present depth. The available information favors the former interpretation.

The “Disrupted zone” extending from 529 to 629 m LSF may be a zone of major tectonic deformation, as described in the “Log-seismic correlation” and “Structural geology and geomechanics” sections of this chapter. This disrupted zone corresponds exactly to the discontinuous patch of anomalously bright, moderately seaward-dipping reflectivity in the seismic profiles (see Fig. F2). If so, it is possible that it represents a zone of dilated fractures. The seismic imaging suggests that the zone is underlain by a steeply dipping fault (Figs. F3, F46) with shallowly dipping bedding abutting the fault, consistent with interpretation of the zone as one of the following:

1. A thrust fault that has been rotated to a steep dip by subsequent thrusting on a deeper structure;
2. A normal fault on the backlimb of a deeper thrust structure, accommodating the hanging wall folding; or
3. A thrust reactivated as a normal fault.

The interpretation will require analyses of core samples, such as age determination, to assess which is most likely.

State of stress and pore pressure

The borehole breakout and drilling-induced tensile fracture information presented in this chapter place bounds on the orientation and permissible magnitude of the ambient principal stresses at this location. Two different zones of breakouts are divided by the disrupted zone (~529–629 m LSF), with strong breakouts but few DITFs above, and less-prominent breakouts but well-developed DITFs below. The orientation of S_Hmax at ~335° azimuth is perpendicular to the trend of the major prism structures and significantly oblique to the far-field plate tectonic relative motion vector of 300°–315° (Miyazaki and Heki, 2001; Seno et al., 1993; Heki, 2007). This implies significant partitioning between convergent and strike-slip motion within the outer accretionary prism, consistent with the apparent stress state favoring strike-slip faulting implied by analysis of the tensile fractures and breakouts. To produce tighter constraints on stress magnitude will require information on sediment strength and on ambient pore fluid pressure. The former can be provided by core studies; the latter is more difficult based on the available data. While installing casing during later expeditions, it may be possible to perform leak-off tests that will provide information on S_hmin and ambient pore pressure.

No direct pore fluid pressure information was obtained during Expedition 314; however, the MWD-APWD data provide information on borehole annular pressure in the circulating fluid (seawater plus occasional mud pills) while drilling. Normally during drilling in open hole, this value should remain near the hydrostatic pressure except when heavy mud is being pumped into the hole or when the formation is “packing off” around the drill string. Therefore, it typically shows transient higher pressures at intervals while drilling. However, in both Holes C0001A and C0001D, the APWD and related ECD measurements became elevated and remained elevated from the ~500 m LSF level to the bottom of the hole. This is in marked contrast to other sites drilled during this expedition and may only represent charging of formation fractures with drilling fluids during pumping and sweep operations. Alternatively, it may be a qualitative indicator of elevated in situ ambient pressure in the boreholes.

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