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doi:10.2204/iodp.proc.333.104.2012

Structural geology

Holes C0011C and C0011D provide sparse and subtle structures. Structural data measured on cores are given in C0011.XLS in STRUCTUR in “Supplementary material.” Where possible, planar structures were corrected to true geographic coordinates using shipboard paleomagnetic data (see “Structural geology” in the “Methods” chapter [Expedition 333 Scientists, 2012]). The distribution of planar structures with depth is shown in Figure F15. The main structural features encountered in Holes C0011C and C0011D are subhorizontal to moderately dipping beds, high- to moderately dipping faults, low-angle healed faults, shear zones, and vein structures in the shallow part.

Bedding

Bedding planes are recognized as boundaries between hemipelagic mud and sand or volcanic ash. In addition, a series of laminations in a turbidite sequence is measured at interval 333-C0011D-44X-6, 61–99 cm (338.68–339.06 mbsf). Dip angles of Hole C0011C and C0011D beds are mainly subhorizontal or gentle. In lithologic Subunit IA (0–251.54 mbsf), bedding strike is scattered and most dips range 0°–15° between 0 and ~100 mbsf (Fig. F16A). Below ~100 mbsf in lithologic Subunit IA to its base, bedding surfaces gently dip northwestward with dip angles of 10°–30° (Fig. F16B). Beds in lithologic Subunit IB and Unit II strike east–west and gently dip either northward or southward (Fig. F16C). These bedding dip measurements are roughly consistent with the dips deduced from LWD image analysis (Expedition 322 Scientists, 2010). Bedding below ~100 mbsf is steeper than that of present seafloor inclination. The change of bedding dip may suggest that a tilting event occurred at ~3 Ma, which is the approximate age of the sediments at 100 mbsf.

Faults

High-angle faults (mostly dipping 40°–subvertical) developed in lithologic Subunit IA (Fig. F15). They exhibit brittle deformation features without any gouge layer or X-ray computed tomography (CT) value variation (Fig. F17). The faults often contain striations inferred to be slickenlines on the fault surface that are indicative of dip-slip movement. Offsets observed on the split core surfaces range from 1 mm to 5 cm (in most cores <1 cm). Although the orientations of these faults are scattered, two conjugate sets of faults, one striking north-northeast–south-southwest and the other striking northwest–southeast, are dominant (Fig. F18). The orientation of north-northeast–south-southwest conjugate sets implies vertical loading and maximum horizontal principal stress (σ_Hmax) in a north-northeast–south-southwest direction, which coincides with ~N25° maximum horizontal stress direction inferred from borehole breakouts observed at 600–650 mbsf at Site C0011 (Expedition 322 Scientists, 2010). On the other hand, northwest–southeast conjugate sets suggest vertical loading and northwest–southeast σ_Hmax, which is consistent with the direction of present plate convergence.

Fault distribution is heterogeneous: in places, faults develop very densely at 10–20 cm intervals (e.g., Cores 333-C0011D-5H, 10H, 16H), but mainly one or two faults are observed in each section. Although these faults are commonly observed in HPCS cores, they are scarce in EPCS and ESCS cores. The difference in observed fault density may be caused by a difference in core quality. However, it is possible that some of the faults could be generated during coring. No high-angle faults are observed in lithologic Unit II.

Low-angle healed fault

Layer-parallel healed faults develop in lithologic Subunit IB and Unit II (Fig. F15). These faults have thicknesses of <5 mm and are characterized by higher bulk density in X-ray CT images (Fig. F19), which suggests shear-induced compaction. Severe biscuiting of ESCS cores only allowed paleomagnetic reorientation of three of these structures, which face eastward to southward (Fig. F20).

Low-angle healed faults formed in bioturbated hemipelagic mudstones at other Nankai Trough Seismogenic Zone Experiment (NanTroSEIZE) drilling sites such as IODP Sites C0006 and C0007 (Expedition 316 Scientists, 2009a, 2009b). The existence of low-angle healed faults at Site C0011 suggests that at least some of such low-angle healed faults at Site C0006 and Site C0007 were formed before sediments passed through the deformation front. In contrast, deformation bands with thrust-sense offsets are commonly found in nonbioturbated mudstone of Sites C0006 and C0007 (Expedition 316 Scientists, 2009a, 2009b) but are not observed in Holes C0011C and C0011D at all. This implies that deformation bands were formed at a later stage of deformation.

In Hole C0011D, several low-angle planar structures display no clear offset. In such cases, we described them as “dark zones.” There are several possibilities for the origin of dark zones: sedimentary structure, bioturbation, alteration-related reaction seam, low-angle healed fault, or wood fragments.

Shear zones

Shear zones are occasionally found at this site. They are characterized as continuous planar bands, thicker than several millimeters, and with high CT numbers (Fig. F21). Shear zones show high-angle dips and strike eastward to southward (Fig. F22). Displacement on these shear zones is larger than on other localized shear structures; the counterparts of offset markers are not observed within core.

Veins

Sediment-filled veins (vein structures) are recognized as parallel sets of sigmoidal or planar seams generally less than a few millimeters wide that tend to extend perpendicular to bedding (Fig. F23). Some of them have shear displacement. In X-ray CT images, sediment-filled veins are expressed as bands with slightly higher CT numbers, suggesting concentration of relatively denser minerals or enhancement of porosity reduction within veins. They are found in the shallower part of Site C0011 (above 102 mbsf).

Only one mineral vein occurs at Section 333-C0011D-41X-6 (317.69 mbsf) (Fig. F24). The mineral vein is composed of white-colored minerals that fill a crack within a dark zone.

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