Previous   |   Next

doi:10.2204/iodp.proc.322.103.2010

Structural geology

Structures observed at Site C0011 are sparse and subtle (as expected for subduction input sediments) in common with those observed at Sites 1173 and 1177 (reference sites for the Nankai accretionary prism off Cape Muroto and Cape Ashizuri, respectively). Although bedding is generally recognizable throughout Hole C0011B, highly bioturbated hemipelagic sediments often prevented actual surface measurement. Structural data measured on cores are given in C0011_STRUCT_DATA.XLS in STRUCTUR in "Supplementary material." Where possible, planar structures were corrected to true geographic coordinates using shipboard paleomagnetic data. The distribution of planar structures and lithologic divisions with depth are shown in Figure F20. The main structural features encountered in Hole C0011B are subhorizontal bedding attitudes and fault distribution such that layer-parallel faults are found at shallow depths and high-angle faults are found at greater depths.

Bedding

Bedding planes dip almost horizontal or 10°–20°, and faults that are parallel to bedding planes characterize the structure in Hole C0011B. Bedding in lithologic Unit III is scattered between 0° and 30°, whereas bedding dips in Units II, IV, and V are concentrated below 20°. High-angle bedding planes are distributed only in the interval corresponding to the chaotic deposits in Core 322-C0011B-8R and Section 28R-2 (Fig. F20). Planar structures were reoriented to the geographic coordinate system using paleomagnetic data listed in Table T7. Poles of bedding and faults cluster around the subhorizontal inclination (Fig. F21). Cylindrical best fit of the bedding pole indicates a weak distribution along the north-northwest–south-southeast, perpendicular to the trench axis. These structural data of bedding planes correspond reasonably well with the data deduced from LWD image analysis (Fig. F22). These data indicate the bedding inclination was controlled by trenchward-dipping bathymetric slopes on the seaward side of the trench.

Sandstone layers display a lighter color under X-ray computed tomography (CT) exposure (Fig. F23A) because of their higher bulk density than the mudstone layers. X-ray CT images, therefore, are useful for distinguishing lithology even if the lithologic boundary is not clear using conventional observation methods. However, it tends to be difficult to detect bioturbation in the greenish clay-rich layers that contain strong bioturbation, probably because of the small difference in bulk density or chemical composition both within and outside the burrows. Boundaries between different lithologies, such as silty claystone, clayey siltstone, and sandstone, are distinct in X-ray CT images (Fig. F23), which indicates a difference in their bulk densities. In some cases, however, sandstone and silty claystone or clayey siltstone do not show significant differences in X-ray CT images (e.g., Fig. F24). These exceptions should always be considered for whole-round sampling or lithologic evaluation before splitting.

Deformation structures

Layer-parallel faults

Although small-scale healed faults with high-angle dips are present, most faults in lithologic Units II and III are characterized by healed faults with layer-parallel attitudes (Figs. F20, F25). These faults are characterized by higher bulk density under X-ray CT image observation (Fig. F26), which is indicative of shear-induced compaction. No such faults were observed in Units IV and V.

Although layer-parallel faults were not reported at Sites 1173 and 1177, healed faults with high-angle dips were reported (Moore, Taira, Klaus, et al., 2001). Layer-parallel faults are well identified in the late Miocene–Pliocene accretionary complex and Pliocene–Pleistocene cover sediments in the Miura Peninsula, central Japan (e.g., Yamamoto et al., 2005). Similar structures are also reported from the toe of the Nankai accretionary complex, off Muroto, at Sites 808 and 1174 (Maltman et al., 1993; Ujiie et al., 2004). These analogs from ancient and modern accretionary complexes and cover sediments formed soon after sedimentation (the primary stage of deformation history).

High-angle faults and fractures

High-angle faults and fractures dipping 45°–70° developed in lithologic Units III, IV, and V (Fig. F20). They exhibit brittle deformation features without gouge or CT value variation (Fig. F27). The faults contain striations inferred to be slickenlines on the fault surface, which is indicative of dip-slip movement. These faults apparently correspond with a normal fault system because the dip angles are too high to make thrust faults in such a stable sedimentary basin. Also, burrows or layers cut by these faults indicate normal displacement (Fig. F27). Although healed high-angle faults were reported at Sites 1173 and 1177, faults in Hole C0011B have not been healed, apparently because of the difference in sediment physical properties during deformation.

Although variations in dip angle correspond well to LWD image data, the distribution pattern is different. Core observation revealed that the high-angle faults/fractures were distributed deeper than 580 m CSF to at least 881 m CSF (TD), whereas LWD image analysis shows the distribution between 150 and 700 m LSF. This difference might be caused by poor core recovery in the shallower interval and lower quality borehole images in the deeper section.

Bioturbated dark deformation bands

Although layer-parallel faults commonly accompany very thin deformation bands (~1–2 mm), some of them have thick (>1 cm), dark deformation bands (Fig. F28). These deformation bands are filled with well-sorted, fine, dark material and have some round clasts of silty claystone. These apparent clasts are burrows, which connect to the upper adjacent layer in X-ray CT images. These bioturbated claystones were also reported at Site 1173 during ODP Leg 190 (Moore, Taira, Klaus, et al., 2001). The band that occurs at interval 322-C0011B-31R-3, 100–117 cm (Fig. F28C), accompanies a much darker, thin layer at its bottom, which indicates shear flow. The dark matrix has a slightly less dense CT signature than the host rock. These deformation structures indicate submarine soft-sediment sliding or creep in a shallow enough subsurface to be bioturbated. These deformation bands disappear in lithologic Units IV and V.

Veins

Mineral-filled veins are observed only below 762 m CSF and are composed mostly of calcite (Fig. F29). Although at least five mineral-filled veins were identified in Hole C0011B, they are quite rare at Sites 1173 and 1177 (only one mineral-filled vein was identified from Site 1173). The calcite veins form dogteeth texture, though some are partly fragmented. On the basis of the striation that was recognized on the outside veins and this fragmented occurrence, the vein minerals are thought to have crystallized along faults, and some faults appear to show repeated movement, as indicated by different phases of crystal growth.

Interpretation

In general, the deformation structures observed in Hole C0011B correspond with lateral extension and vertical compaction. Layer-parallel faults and the bioturbated dark deformation bands exposed only in lithologic Units II and III likely formed by soft-sediment creep soon after sedimentation. The disappearance of these structures in the lower part of the hole (Units IV and V) implies that there was no cause of further deformation (e.g., seafloor tilting) after or during the deposition of Units IV and V. High-angle faults/fractures exposed in Units III, IV, and V developed at a later stage because they exhibit brittle deformation features. Although attitudes of faults/fractures were reoriented to the geographic coordinate system using paleomagnetic data, the restored data are limited and scattered. Additional reliable postcruise paleomagnetic data are required to evaluate the stress conditions.

Documentation of the structures in Hole C0011B provides a further important structural datum against which we can compare more highly deformed sites within the prism. The scientific results obtained from this subduction input site will help scientists recognize structures at other sites that are the result of deformation within the accretionary prism.

Drilling-induced deformation

RCB coring induced syndrilling fractures. In addition to visual observation, X-ray CT images were used extensively for the evaluation of drilling-induced deformation.

Syndrilling fractures are commonly observed in X-ray CT images as noncompleted cracks. Nevertheless, the cracks fully propagate after splitting (Fig. F30). A jigsaw-puzzle fracture in which pieces are reconstructible with similar uneven plane indicates in situ deformation without displacement (Fig. F31). The fracture that has a linear plane but shows bending at the rim of the core also has the same nature as other jigsaw-puzzle fractures.

Drilling-induced conjugate faults typically occur in a convex shape (Fig. F30). A possible explanation is that during drilling, a fracture is generated at the contact of the drill bit in the bottom of the borehole and propagates upward into the core with a certain angle that is constrained by the angle of internal friction.

Cuttings (the scrapings of sediments or rocks during drilling) sometimes strayed into the core liner in significant amounts, even though they are usually washed out through the outside drill pipe. Core 322-C0011B-43R, which records 224% of recovery, was filled with cuttings in the bottom 4.38 m (from Section 322-C0011B-43R-5, 64 cm, through 43R-CC, 17.5 cm; Fig. F32). Subtracting the cuttings, Core 322-C0011B-43R recovery becomes 114.5%. Well-sorted coarse angular fragments can superficially be mistaken for a sandy layer. In Sections 322-C0011B-48R-5 and 48R-6, a grading of poorly sorted round cuttings was observed (Fig. F33). Those cuttings are uniformly composed of adjacent silty claystone. They appear similar or less dense than the claystone in X-ray CT images, which may be a result of higher porosity. Using X-ray CT images, cuttings are distinguished from sandstone, which usually has higher bulk density than clayey siltstone (see Fig. F23). Round vesicles are also a unique feature of cuttings-filled cores (Fig. F32). The size of cuttings varies from millimeters to centimeters. Fragments are either angular (in Core 322-C0011B-43R, Fig. F32) or round (in Core 48R, Fig. F33).

Top of page   |   Previous   |   Next