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doi:10.2204/iodp.proc.334.103.2012

Structural geology

The primary structural geology objective during Expedition 334 was to describe and document style, geometry, and kinematics of structural features observed in the cores. At Site U1378, bedding dips are shallow (<20°, mostly <15°; Fig. F11). A faint northeast–southwest–trending alignment of poles to bedding planes is identified after paleomagnetic correction. Structures are dominated by normal faults. In addition, there are a large number of healed faults. There are four fault zones at 280 mbsf, 375–380 mbsf, 477 mbsf, and 498–520 mbsf. Healed faults and sediment veins occur at ~260 mbsf, ~20 m above the shallowest appearance of the fault zone. Paleomagnetic corrections were conducted for three normal faults. All are within 35° of an east–west strike and dip ~70° to the north or south.

Structures in slope sediments

Bedding

Bedding planes dip gently, commonly <15° throughout the borehole (Fig. F11). Below 300 mbsf, the number of steeper dipping bedding planes (15°–30°) increases slightly. Although bedding planes show shallow inclination, paleomagnetic correction identified a faint northeast–southwest–trending alignment of poles to the bedding plane (Fig. F12).

Brittle faults

Faults recognized in the Hole U1378B cores are characterized by striated and/or polished surfaces or by offset markers. The sense of slip is determined by offset markers such as lamination, bioturbation, and slickensteps on striated slip surfaces. The faults commonly show a normal sense of displacement, although some faults (n = 7) show a reverse sense of shear. In the interval between 204 mbsf and 510 mbsf, 33 normal faults were recognized (Fig. F11), with a mean dip >60°.

Only three of the 33 measured normal fault attitudes can be restored to the geographic frame because of the difficulties in paleomagnetic measurement due to the extensive biscuiting and magnetic overprinting. These three faults have similar orientations, striking within 35° of east–west and dipping ~70° to the north or south (Fig. F13).

Fractures and mineral veins

A total of 33 fractures and joints, closely associated with fault zones, were measured in Hole U1378B. The fractures are preferentially subhorizontal or steeply to subvertically dipping. Only one of the fractures could be restored using paleomagnetic data, resulting in an orientation of 82° dipping toward 56° (right-hand rule).

Mineral veins and en echelon veins were also identified. Most of the veins are filled with calcite and are steeply dipping ~70°. They are frequently located adjacent to faults and fractures.

Healed faults and sediment-filled veins

A total of 49 healed faults were concentrated in the interval between 249 and 502 mbsf within lithostratigraphic Unit II. The fillings are all less than a few millimeters thick and indicate fault cohesiveness. The cohesiveness of the fault plane may represent early stage, soft-sediment deformation. These healed faults have preferentially steep dip angles (45°–80°).

Sediment-filled veins aligned in arrays parallel or subparallel to the bedding planes were identified at 262–387 mbsf (Figs. F14, F15). Sediment-filled vein structures include a variety of structures, many of which have been identified during previous ocean drilling expeditions. This category of structures includes the sigmoidal-shaped suite of thin, mud-filled veins that are slightly darker than the surrounding materials and was first documented by Ogawa (1980) and Cowan (1982). Dark veins are spaced at regular intervals (0.5–2.0 mm) in the horizontal plane (Fig. F15). Each vein is preferentially oriented vertically to subvertically to the vein arrays. Apparent offsets across the vein arrays in the split cores indicate mostly normal displacements.

Fractured and brecciated zones

Fractured zones are defined as moderately sheared zones fractured into centimeter-size fragments. Brecciated zones represent intensively sheared zones composed mainly of angular millimeter to 1 cm sized fragments (Fig. F16). Each fragment has an angular shape with slickenlines showing a straight and parallel striation on all of its surfaces in a systematic direction. For the most part, these features are distinguishable from drilling-induced fractures and clasts (see “Structural geology” in the “Methods” chapter [Expedition 334 Scientists, 2012]).

All major fault zones shown in Figure F11 are characterized by alternating sequences of fractured and brecciated zones. The top and bottom boundaries of these zones are characterized by a weakly fractured zone and/or undisturbed host rock. Four fault zones were identified at 280, 375–380, 477, and 498–520 mbsf (Fig. F11). All fault zones are located within lithostratigraphic Unit II of Site U1378.

The shallowest fault zone occurs between 279.3 and 281.4 mbsf and consists of a >0.8 m thick brecciated zone in the center and accompanying fracture zones above and below.

The second shallowest fault zone ranges from 375.4 to 383.7 mbsf. This fault zone consists of two main brecciated zones, one ~2.0 m wide and the other 1.8 m wide. Because of the thickest brecciated intervals and the relatively smaller size of fragments than that in the other fault zones, the second fault zone appears to have experienced the most intense deformation. This depth interval corresponds to a low-density, high-porosity interval identified by LWD (see “Downhole logging”).

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