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doi:10.2204/iodp.proc.322.104.2010

Structural geology

Most of the recovered cores at Site C0012 are relatively good quality for the observation of structures; however, poor recovery and highly bioturbated hemipelagic sediments limited the amount of data. Structural data measured on cores are given in C0012_STRUCT_DATA.XLS in STRUCTUR in "Supplementary material." The correction of planar structure orientation using shipboard paleomagnetic data was performed the same as at Site C0011 for as long as possible. The distribution of planar structures and lithologic units with depth are shown in Figure F24.

Bedding

From Cores 322-C0012A-1R through 7R, bedding planes could not be defined because of heavy bioturbation and drilling disturbance. Bedding planes generally dip <25° in lithologic Units I, II, IV, and V. On the other hand, from Cores 322-C0012A-22R through 24R in the upper part of Unit III, bedding shows anomalously steep inclination with a maximum dip of 45°. Although no bedding could be measured in Cores 322-C0012A-25R through 29R because of bioturbation, core surface lineaments, such as Zoophycos burrows, are apparently steep. Thus, Unit III is apparently characterized throughout by steep-dipping bedding planes. The bottom of the steep-bedding interval is expected to be between Sections 322-C0012A-29R-4 and 31R-1 (which is not clear in Section 29R-CC and there was no recovery for Core 30R). A similar bedding-angle variation is also identified in Unit III at Site C0011, though the variation is faint. Small-scale chaotic deposits are observed in Sections 322-C0012A-12R-CC and 49R-1 (Fig. F24).

Figure F25 shows the reoriented plot of the planar structures to the geographic coordinate system using paleomagnetic data listed in Table T6. Dominant bedding planes concentrate around the subhorizontal inclination dipping northward. Data from a steeply inclined interval in Unit III show girdle distribution trending north-northwest–south-southeast (Fig. F25). From these data we infer that although bedding planes at this site are tilted to the north, bedding planes in Unit III show large block rotation possibly because of northward sliding.

Deformation structures

Faults

Three types of faults were described at Site C0011 (layer-parallel faults, high-angle faults, and bioturbated dark deformation bands), and they are all observed at Site C0012. Layer-parallel faults are distributed from Units III to V (Fig. F26). High-angle faults/fractures that strike north-northwest–south-southeast are common throughout Hole C0012A from Units I to V (Fig. F27), whereas they develop only in Unit III and deeper at Site C0011. The high-angle faults commonly accompany slickenlines on the fault surface, indicating dip-slip movement. Displacement markers are occasionally identified, and all indicate normal fault sense (e.g., Fig. F27).

Dark deformation bands with bioturbation are also observed at Site C0012 (Fig. F28). Similar to those at Site C0011, finer material fills layer-subparallel bands. The bands occur from Units II to V, whereas we only observed them in Unit III at Site C0011.

Veins

A mineral-filled vein ~5 mm thick is observed in Unit IV (Section 322-C0012A-33R-4), and we observed similar veins in Unit IV at Site C0011. This type of vein is apparently composed of calcite (Fig. F29A). Another type of vein that occurs in Unit V is a layer-parallel vein (Fig. F29B, F29C). Layer-parallel veins appear to develop along former parallel laminae in a thin turbidite package. In interval 322-C0012A-46R-3, 10–12 cm, a layer with thin veins accompanies kinklike folding (Fig. F29B). This may result from coarser grain size with some fluid flow and associated shear concentration.

Sheath folds

Small-scale flow folds with interlimb angles <20° are well developed in laminated sandstone in interval 322-C0012A-45R-1, 80–96 cm (Fig. F30). Although the fold axes are oriented horizontally, the trends of axes curve and exhibit sheath folds. Muddy material injected into the sandy part and the saw-shaped boundary indicate that these structures were formed soon after sedimentation. The sheath fold is a common structure in mass transport deposits (e.g., sediment slides); however, no such structures were reported from Sites 1173 and 1177 (reference sites off Cape Muroto and Cape Ashizuri in the Nankai Trough, respectively).

Interpretation

In general, the deformation structures observed in Hole C0012A correspond with lateral extension and vertical compaction. High-angle bedding that is characteristic in Unit III likely formed because of large block sliding on the northward-tilted seafloor. The upper boundary of this inclined block was not recognized in the cores; however, it is expected to be between Section 322-C0012A-21R-CC and 22R-1. Biostratigraphy data show a possible hiatus in interval 322-C0012A-19R-1, 32–34 cm, through 21R-3, 7 cm (see Table T7), which does not overlap with the slide-induced unconformity. This slight incompatibility in the possible unconformity can be explained by two distinct events. However, we propose a model to interpret this discrepancy with one event because the evidence lies very close together in depth. During block rotation by submarine sliding, the surface sediments may have been soft enough to be suspended and redeposited, followed by further fine-grained sedimentation to attempt to smooth any residual seafloor topography (Fig. F31). In this case, the hiatus apparently shifts upward. In addition, Unit III consists almost entirely of hemipelagic claystone, which makes structural features hard to detect from the seismic profile. The lower boundary or sliding surface is expected to be between Sections 322-C0012A-29R-4 and 31R-1, which is above the top of Unit IV (Section 31R-4, 74 cm).

High-angle faults/fractures strike north-northwest–south-southeast, and the poles show girdle distribution trending east-northeast–west-southwest. Because most of the slickenlines on the fault surfaces exhibit dip-slip movement, the intermediate principal stress (σ₂) apparently trends north-northwest–south-southeast, perpendicular to the trench, which is similar to the stress trend at Site C0011. Documentation of the structures in Hole C0012A provides a further important structural datum against which we can compare more highly deformed sites within the nearby accretionary prism.

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