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doi:10.2204/iodp.proc.338.106.2014

Structural geology

Structures observed in cores retrieved from Hole C0021B are mainly of three types: bedding, faults, and shear zones. Fissility was rarely observed and was likely induced by drilling because it is systematically observed at the very top of the first section of the cores.

Bedding

A total of 98 bedding measurements were made on Hole C0021B cores (Fig. F15). In undisturbed slope basin sediment, the recognition of bedding is straightforward. It is, however, harder to recognize in MTD intervals. In such cases, X-ray CT images were also used for bedding identification and measurement. Bedding dips mostly (74 of 98 measurements) between 0° and 20° but occasionally between 20° and 50°. In terms of bedding dip angles versus depth, three intervals are clearly visible. The upper interval from 80 to ~100 mbsf is characterized by gentle dip angles (<20°). The intermediate interval from ~100 to 180 mbsf is characterized by dip angles between 0° and 50°, likely reflecting sedimentary disturbance during MTD emplacement. The lower interval (correlative to lithologic Subunit IB), with one exception likely linked to a local disturbance (e.g., slumping), is characterized by gentle dip angles between 0° and 16°.

Figure F16 shows the poles to bedding for Subunits IA and IB reoriented based on paleomagnetic data. No strong preferred orientation can be recognized, but east–west strikes seem to predominate in Subunit IA.

Minor faults

A total of 19 minor faults were observed in Hole C0021B cores. These faults are characterized by sharp fault surfaces, along which cores can be split away. Fault surfaces are mostly planar and sometimes bear fine striations (Fig. F17A). Rakes of striations are close to 90°, suggesting predominantly dip slip. A normal sense of slip could be determined for two minor faults. Offset beds indicate separation of 6 and 37 mm for these two faults. A reverse sense of slip along one minor fault (interval 338-C0021B-7H-2, 84–90 cm) is suggested by intense striations on the restraining bends arrangement of possible P surfaces, but the lack of offset markers on either side of the fault precludes further confirmation.

Most faults (78%) cluster in two intervals: 94–96 and 129–134 mbsf (Fig. F18). Most faults dip between 44° and 66°, but three faults dip between 74° and 88°. Some faults observed in MTDs clearly offset mud clasts, indicating that faulting postdates MTD emplacement (Fig. F19).

Figure F20A shows the projections of faults from intervals for which corrections based on paleomagnetic data were available (the section between MTDs A and B). Faults preferentially strike north–south to northwest–southeast and dip consistently to the west or southwest. The scarcity of striations borne by the fault surfaces precludes any interpretation in terms of paleostress.

Shear zones

A total of 45 shear zones were measured in cores from Hole C0021B. Shear zones typically appear on cores as faint, 1 or 2 mm thick, rectilinear zones that are slightly darker than the surrounding host sediment (Fig. F17B). On X-ray CT images, shear zones clearly appear as bright zones with higher density than the host sediment. Although most shear zones are ~1 or 2 mm thick, several centimeter-thick shear zones were also observed (e.g., Fig. F17C). Particularly, the 1 cm thick shear zone observed at interval 338-C0021B-13T-1, 60–66 cm (Fig. F21), could represent the basal shear zone of MTD B (see “Lithology”). Sense of displacement along shear zones could not be determined. However, in one case (interval 10H-7, 1–21 cm), the en echelon arrangement of three shear zone segments suggests a reverse component of slip, but the lack of independent indicators, such as offset markers, prevents further confirmation.

Shear zones are mainly observed in two intervals (Fig. F18): 100–130 and 147–176.16 mbsf. In either interval, shear zones dip gently (~20°) to almost vertical but mostly dip between 30° and 70°. Unlike faults, shear zone attitudes are scattered and no preferred orientation can be observed (Fig. F20B).

Summary

Structural observations of core from Hole C0021B confirm the conclusions obtained in nearby Hole C0018A (Expedition 333 Scientists, 2012b). Slope sediments are characterized by flat or gently dipping bedding and a lack of shear zones. In contrast, MTDs are characterized by a wider range of bedding dip angles (0°–50°) and by the occurrence of millimeter to centimeter thick shear zones, reflecting disruption during emplacement. The post-MTD emplacement deformation style is characterized mostly by normal-sense faulting.

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