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doi:10.2204/iodp.proc.314315316.135.2009

Structural geology

Site C0007 yielded a variety of deformation structures, as expected for penetration through the toe of the prism and the frontal thrust. All structural data are given as a supplementary material (see C0007_STRUCT_DATA.XLS in folder 316_STRUCTURE in “Supplementary material”). Where possible, we corrected the measurements of planar and linear structures to true geographic coordinates using paleomagnetic data (see “Structural geology” in the “Expedition 316 methods” chapter). The distribution of planar structures and lithologic divisions with depth is shown in Figure F16. Slump-related structures are observed in lithologic Unit I (0.00–33.94 m CSF). The chief structural features in the prism are subhorizontal bedding and fissility, deformation bands in less sandy portions of lithologic Unit II, healed faults and sediment-filled veins in lithologic Unit III, and three fault zones developed in the hanging wall of the frontal thrust.

Slump-related structures in Unit I

Bedding dips between 20° and 70° in Unit I, in contrast to gently dipping (0°–30°) beds in the prism below (Fig. F16). Bedding dips southeastward after paleomagnetic orientation (Fig. F17A). In places, faults offset bedding with normal or reverse sense of shear and displacement of less than a few centimeters (Fig. F18). Faults show scattered orientations after paleomagnetic correction (Fig. F17B). Sandy layers are locally disturbed and form inclusions in the muddy matrix (Fig. F18). In the extreme case, sandy layers are chaotically mixed with muddy layers. Faults tend to be present near disturbed or chaotically mixed bedding. These features and large variations in bedding attitude most likely record slumping of sediments above the prism.

Deformation structures in the prism

Bedding and fissility (foliation parallel to bedding) dip gently (generally ~30°) throughout the prism (Fig. F16). The orientations of bedding and fissility after paleomagnetic correction are shown in Figure F19. Deformation features are not present in Unit II where sediments are dominated by coarser-grained material such as sands and gravels. Deformation structures are concentrated in mudstone-dominated Units II and III.

Deformation bands

Deformation bands are developed in portions of Unit II where fissility is well developed in nonbioturbated mudstone (Fig. F16B; C0007_STRUCT_DATA.XLS in folder 316_STRUCTURE in “Supplementary material”). In the cores, bands are defined by bedding oblique dark bands that change in thickness from ~1 to 5 mm along their length. Typically, deformation bands occur as two sets that dip in opposite directions, and one of the bisectors of the dihedral angle (~90°) is oriented parallel to the bedding-parallel fissility (Fig. F20A, F20B). Where there is offset between these two sets of deformation bands, shear sense is always reverse and displacement is less than a few millimeters. These features clearly indicate that deformation bands were formed by layer-parallel contraction. The orientations of deformation bands after paleomagnetic correction predominantly strike northeast (Fig. F21).

On X-ray CT images, deformation bands are displayed as bright bands or seams with relatively high CT numbers (Fig. F20C). This suggests that the bands are denser than the surrounding material. Overall, structural features seen in cores and CT images are consistent with those observed at Site C0006 (see “Structural geology” in the “Expedition 316 Site C0006” chapter) and deformation bands in the Muroto transect in the Nankai accretionary prism (e.g., Ujiie et al., 2004), which were formed by northwest-directed plate convergence–related tectonic deformation in the accretionary prism.

Healed faults and sediment-filled veins

Healed faults and sediment-filled veins are observed in Unit III where bioturbation is well developed in hemipelagic mudstone. Two types of healed faults are recognized in the cores. One type of healed fault is defined by dark seams <1 mm thick that dip between 30° and 90°. The faults are braided and curviplanar to irregular. Where offset markers occur, the sense of shear is always normal (Fig. F22A). The orientations of these healed faults, after paleomagnetic correction, exhibit random strike (Fig. F22B). This type of healed fault is similar to that reported in the hemipelagic mudstone at the base of Site C0006 (see “Structural geology” in the “Expedition 316 Site C0006” chapter) and in the Muroto transect in the Nankai accretionary prism (Moore, Taira, Klaus, et al., 2001) and is thought to result from vertical compaction of sediments during burial.

The other type of healed fault is defined by dark bands generally <5 mm thick, which are parallel or subparallel to bedding (dips <30°). The orientations of these healed faults after paleomagnetic correction predominantly dip southward (Fig. F22C). The bedding-parallel or subparallel occurrence of the healed faults is similar to layer-parallel faults observed in mudstone in the accretionary prism on land (Hanamura and Ogawa, 1993). The origin of this type of healed fault is controversial; it either developed as slip surface associated with slumping or resulted from thrust faulting during frontal accretion.

Sediment-filled veins are recognized as parallel sets of sigmoidal or planar seams generally less than a few millimeters wide (Fig. F23) that tend to extend perpendicular to bedding with length ranging from 1 to 28 cm. Some sediment-filled veins offset bedding or greenish layers and show normal slip with displacement less than a few millimeters.

Fault zones

Three fault zones were identified in the prism of Hole C0007D (Fig. F16B) based on the following criteria:

• Rocks are broken along polished and striated fracture surfaces with spacing of fractures <10 cm.
• Preservation of fault breccia and fault gouge. The size of brecciated fragments is ~1–10 mm.
• Changes in lithology, deformation style, and kinematics of structures with depth.

Outside fault zones, rocks are not brecciated except for drilling-induced brecciation (see definition of drilling-induced breccia in “Structural geology” in the “Expedition 316 methods” chapter) and the spacing of natural fractures is >10 cm.

Fault Zone 1 (237.5–259.3 m CSF)

The structural elements in fault Zone 1 are shown in Figure F24. Rock in fault Zone 1 is predominately fractured. Fault breccias are 17–19 cm thick and are characterized by angular to subangular fragments 1–10 mm in size (Fig. F25). Brecciated fragments are commonly polished and slickenlined and lineations indicate multiple slip directions. The 21.8 m thick fault Zone 1 is developed in the muddy portion of Unit II and is located just above the sand- and gravel-dominated portion of Unit II (Fig. F16). Deformation bands, which are developed in Unit II between 230 and 248 m CSF, are absent below fault Zone 1, consistent with a change in lithology across fault Zone 1. These features suggest that fault Zone 1 is related to the thrust faulting that places mudstone onto sands and gravels.

The prominent feature in fault Zone 1 is preservation of light grayish ash layers. On CT images, three such ash layers are similar in character (see also “Lithology”). They are recognized as dark layers bounded above and below by bright layers (Fig. F24). The thicknesses of the ash layers are similar: 15, 10, and 12 cm from lowest to highest in depth. Occurrence of these similar ash layers may simply indicate the deposition of volcanic ash at three different times. Alternatively, the three ash layers within fault Zone 1 may reflect a repetition of the same layer by thrust faulting.

Fault Zone 2 (341.5–362.3 m CSF)

Fault Zone 2 is located where lithology changes from mudstone in Unit II to bioturbated hemipelagic mudstone in Unit III (Fig. F16B). The location of this zone is comparable to the interval where a subhorizontal strong reflector is recognized in a nearby seismic line (Fig. F2). The structural elements in fault Zone 2 are shown in Figure F26 and consist mostly of fractured rocks characterized by fragmentation along sets of polished and striated surfaces (Fig. F27A). Concentration of deformation at the bottom of fault Zone 2 is indicated by occurrence of breccia and fault gouge (Fig. F27B). Fault breccia includes subangular to rounded fragments ~1 mm–2 cm in size and a random texture. Fault breccia is replaced by fault gouge at 362.1 m CSF. This change is marked by a decrease in size and volume fraction of fragments to <2 mm and 30%, respectively. Fault gouge is weakly foliated and the angle between a foliation defined by alignment of fragments and the horizontal plane is 38°.

It is possible that fault breccia and fault gouge have been affected by drilling-related deformation because they are often recovered within the core catcher. However, in this hole, the core below is characterized by relatively unbroken hemipelagic mudstone, showing an asymmetric distribution of fractured rocks with respect to fault breccia/​fault gouge zone. As in the case of fault Zone 1, deformation bands are concentrated above fault Zone 2 and band kinematics show reverse slip associated with northwest-directed contraction (Figs. F16B, F20, F21). Deformation bands are absent below fault Zone 2, but healed faults are well developed and show normal slip consistent with vertical compaction of sediments during burial (Fig. F22). The changes in deformation style and kinematics of structures across fault Zone 2, as well as the asymmetric distribution of fractured rock, suggest that the major slip zone is located at the bottom of fault Zone 2, possibly represented by the fault breccia/​fault gouge zone. Although distribution of deformation bands could be controlled by lithology, it is also possible that deformation bands record stress concentration in the hanging wall of the fault.

Fault Zone 3 (398.5–446.0 m CSF)

Fault Zone 3 is located at the basal part of the prism above the frontal thrust that juxtaposes the hemipelagic mudstone (Unit III) above and sand-dominated sediments (Unit IV) below. The structural elements of fault Zone 3 are shown in Figure F28. This zone is marked by a heterogeneous distribution of fractured and brecciated hemipelagic mudstone (Fig. F29). Despite intense fracturing, bedding has not been rotated significantly along the fractures. Fractures commonly have polished and slickenlined surfaces, and steps on the surfaces were used to determine shear direction and shear sense. The orientations of fracture surfaces after paleomagnetic correction are scattered with complex and heterogeneous slip directions and shear senses (Fig. F30).

Two zones of concentrated deformation are recognized within fault Zone 3: one contains a foliated fault gouge at 418.83–418.94 m CSF, and the other contains a 2 mm thick dark layer at 438.57 m CSF. The fault gouge at 418.83–418.94 m CSF was recovered in the core catcher and thus could be affected by drilling-related deformation. Despite this fact, the core sample preserves a foliation defined by an alignment of <1 mm size fragments in a clayey matrix and color banding (Fig. F31). The dip direction of the foliation changes across a drilling-induced biscuit boundary, suggesting that the foliation was formed prior to drilling and is a product of natural shear deformation. The foliation is cut by slip surfaces, imparting a composite planar fabric (Chester and Logan, 1987). The shear sense of fault gouge indicated by this composite planar fabric is consistent with thrust faulting. In places, fragments are surrounded by anastomosing slip surfaces.

The lowermost part of fault Zone 3 at 438.28–438.57 m CSF is intensely brecciated into fragments ~1–10 mm in size (Fig. F32A, F32B). This 29 cm thick breccia shows a foliated aspect from an anastomosing network of polished and striated surfaces. At the base of this zone, the 2 mm thick dark layer sharply separates intensely brecciated hemipelagic mudstone above from unbroken hemipelagic mudstone and ash below (Fig. F32C). There is a biostratigraphic age reversal across the lowermost part of fault Zone 3; an intensely brecciated interval above is older than the coherent sediments below (see “Biostratigraphy”). These features indicate that the thin dark layer most likely represents extreme localization of slip associated with thrust faulting.

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