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Structural geology

Site C0018 is located within the slope basin. Here, drilling was performed to 313.655 mbsf to penetrate and recover MTDs and surrounding cores. Core structural data are given in C0018.XLS in STRUCTUR in “Supplementary material.” Where possible, we corrected planar structures to true geographic coordinates using shipboard paleomagnetic data (see “Structural geology” in the “Methods” chapter [Expedition 333 Scientists, 2012]). The distribution of planar structures is shown in Figure F12. At this site, six intervals with evidence for MTDs were carefully identified based on lithologic and structural visual core descriptions and observation of X-ray CT images (see detailed X-ray CT description notes in Table T4).

Structural features of this site mainly record sedimentation and mass-movement processes and gravitational tectonics within a slope basin. The main structural features at this site are

  • Southeast-dipping beds, normal faults, shear zones, and a slump fold above MTD 6 (thickest MTD) of lithologic Subunit IA (0–127.545 mbsf);

  • Scattered beds, shear zones, and flow structures within MTD 6 (127.545–188.573 mbsf); and

  • Subhorizontal beds and fissility within lithologic Subunit IB (below MTD 6) (188.573–313.655 mbsf).

Structural elements above MTD 6 (within lithologic Subunit IA)

Gently to moderately dipping beds, normal faults, and a slump fold are observed above MTD 6 in lithologic Subunit IA (Fig. F12). After paleomagnetic correction, bedding surfaces gently dip southeastward except for several scattered points (Fig. F13). This trend is consistent with local seafloor topography and southeast-dipping beds displayed in seismic reflection profiles (Fig. F1). Steeply dipping beds (dip angle >40°) are scarce and distributed in limited depths: 2.78, 3.20, 44.76, 45.92, 64.27, 65.80, and 92.22 mbsf. These depths range within or nearby the lithologically identified MTDs 1–5 (gray zones in Fig. F12), suggesting that beds were tilted in response to the deposition of MTDs.

Minor faults develop below Section 333-C0018A-6H-5 (49.00 mbsf). Faults are recognized as dark or greenish thin planes with thickness <1 mm. Their offsets in split core surfaces range up to 28 mm. Most of the faults show normal offsets of beds and burrows (Fig. F14A), except for one example with reverse offset (Fig. F14B). Faults are also expressed as bright planes with high CT numbers on X-ray CT images (Fig. F14), suggesting that fault materials are denser than the surrounding host sediments. This might reflect shear-induced consolidation (porosity reduction) or mineral precipitation within the fault plane. Dip angles of faults are moderate to high (generally 50°–80°). After paleomagnetic correction, orientations of faults show scattered distribution along the vertical axis (Fig. F15), possibly reflecting vertical maximum principal stress within the sediments. These observations suggest that high-angle faults above MTD 6 were mainly formed by gravitational collapse in response to the sedimentation and burial processes of the slope basin.

A cylindrical fold occurs in Section 333-C0018A-5H-6 (41.62 mbsf) within MTD 2 (Fig. F16). Lack of systematic change in orientation of the bedding plane within lithologic Subunit IA suggests that the fold was formed as a synsedimentary slump fold. Two ash beds are involved in this fold, and orientations of folded ash layers are carefully measured in X-ray CT images. Poles to the folded layers lie on a great circle, which is perpendicular to the fold axis. After paleomagnetic reorientation, the fold axis strikes east–west and dips nearly horizontal (Fig. F17), whereas the axial plane also dips subhorizontal. The orientation of the fold axis implies that the fold was formed in response to gravity flow of north–south direction that locally occurred in the slope.

Structural elements within MTD 6 (127.545–188.573 mbsf)

Scattered beds, shear zones, flow structures, minor faults, and web structures are observed within the thickest MTD (i.e., MTD 6, 127.545–188.573 mbsf). Reflecting remobilization of sediments, paleomagnetic poles of this depth range are scattered evenly in each core (see “Paleomagnetism”). As a result, paleomagnetic correction of structural elements could not have been performed within MTD 6. Moreover, HPCS coring-induced disturbance (flow-in) especially occurs in the lower part of MTD 6 (Cores 333-C0018A-18H, 19H, 20H, 21H, and 22H).

Despite the difficulty of structural analyses caused by coring-induced disturbance and the lack of reoriented data, structural features of this interval explore the internal deformation style of the MTD. Dip angles of bedding range from subhorizontal to nearly vertical (Fig. F12). In contrast, subhorizontal beds, which are commonly distributed both above and below MTD 6, are not observed. Scattered dip angles suggest incoherent folding within MTD 6 formed during movement of a large submarine landslide.

Shear zones and healed, dark-colored planar fabrics dividing lithologies are recognized in Core 333-C0018A-17H (145.18–152.51 mbsf) (Fig. F18). At Section 333-C0018A-17H-6 (150.39–150.53 mbsf), two shear zones truncate dark grayish silty clays including fragments of volcanic ash. Greenish and brownish light-colored clay occurs between the two shear zones. On the split core surface and X-ray CT image, planar fabric is recognized in this light-colored part (Fig. F18). Shear zones also occur in Section 333-C0018A-22H-8.

Flow structures occur in Sections 333-C0018A-22H-4 through ~22H-9 (178.45–184.01 mbsf) (Fig. F19). At Section 333-C0018A-22H-6, greenish and brownish light-colored clay-rich sediments, including fragments of volcanic ash and burrows, consist of wavy-shaped flow structures (Fig. F19).

Both shear zones and flow structures exhibit typical occurrences of unconsolidated soft-sediment deformation, which is carried by independent particulate flow.

Faults are not common within MTD 6; however, faults with slickenlines and steps locally occur in Core 333-C0018A-16H (141.09–143.09 mbsf) (Fig. F20). In contrast to the faults above MTD 6, planes of these faults are not healed by relatively dense materials. Slickenlines and steps indicate that those faults have normal dip-slip offsets. Web structures, anastomosing dark-colored seams (each with thickness <2 mm), occur in Sections 333-C0018A-17H-7 to 17H-8 (Fig. F21). Web structures truncate beds and burrows. These nonhealed faults and web structures appear to occur posterior to MTD formation.

The bottom of MTD 6 is bounded by a structureless, thick sand layer.

Structural elements below MTD 6 (lithologic Subunit IB)

Subunit IB below MTD 6 is characterized by gently dipping beds of turbidites (<30°) (Fig. F22). Deformation is generally weak, and no fault is recognized within this unit. Fissility, cleavagelike planar fabric within the muddy part, weakly develops below Core 333-C0018A-31X (257.15 mbsf) (Fig. F23).

Coring-induced disturbances are commonly recognized, especially in cores sampled by ESCS (Core 333-C0018A-31X and below); however, some contain a boundary between sand and mud, providing evidence that they possess original stratigraphic relations.