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doi:10.2204/iodp.proc.334.105.2012

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. Site U1380 was cored in the interval 397–477 mbsf and potentially corresponds to the lower succession cored in Hole U1378B. Only one lithostratigraphic unit has been defined at this site (see “Lithostratigraphy and petrology”). Bedding dips are shallow to moderate, with a mean of ~40°. Two fault zones have been identified at 407–419 and 454–477 mbsf that consist of relatively intense faulting and fractured and brecciated zones. These zones show both normal and reverse sense of shear. Brecciated zones are 10–20 cm thick and are associated with fracture zones. No paleomagnetic data were available to restore structural data to true geographic coordinates.

Structures in slope sediments

Bedding

Bedding planes were recognized as boundary surfaces in thin layers of sand and ash within a clayey silt and silty clay sequence. Bedding was also recognized as thin laminations in sediments. Bedding dips are shallow to moderate, ranging from 27° to 49° (Fig. F5).

Brittle faults

Faults recognized in the cores from Site U1380 are characterized by striated and/or polished surfaces or by offset markers. The slip sense is determined by offset markers such as lamination, bioturbation, and slickensteps on striated slip surfaces. Incohesive faults show both normal and reverse displacement sense (Fig. F5). Unfortunately, no paleomagnetic data were available to perform rotations of structures to true geographic coordinates. However, we plan to conduct paleomagnetic corrections postexpedition.

Healed faults and sediment-filled veins

Two healed faults occur between 397.3 and 398.8 mbsf (Fig. F5). The cohesiveness of the healed fault surfaces may indicate early stage soft-sediment deformation. These healed faults have moderate to relatively steep dip angles (43° and 51°). Two sets of sediment-filled veins were identified in a 2.2 m interval from 417.2 to 419.4 mbsf. The sediment-filled veins are aligned in arrays parallel or subparallel to the bedding planes, as observed in the cores from Holes U1378B and U1379C (see “Structural geology” in the “Site U1378” chapter [Expedition 334 Scientists, 2012b] and “Structural geology” in the “Site U1379” chapter [Expedition 334 Scientists, 2012c]). Sediment-filled veins form 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 that were first documented by Ogawa (1980) and Cowan (1982). Dark veins are spaced at regular intervals (0.5–2.0 mm) in the horizontal plane.

Fractured and brecciated zones

Fractured zones are defined as moderately sheared zones fractured into few centimeter-sized fragments. Brecciated zones are intensively sheared zones composed of few millimeters to 1 cm sized angular fragments (Fig. F6). Each fragment has an angular shape with straight and parallel striation on its surfaces. For the most part, these features are distinguishable from drilling-induced fractures and fragments (see “Structural geology” in the “Methods” chapter [Expedition 334 Scientists, 2012a]).

Fault zones shown in Figure F5 are characterized by alternating sequences of fractured and brecciated zones (Fig. F6), which are always terminating through a weakly fractured zone in the undisturbed host rock below. Two fault zones have been identified at 407.3–419.4 and 453.8–477.7 mbsf (Fig. F5). The upper fault zone consists of a fractured zone and 0.1–0.3 m thick brecciated zones close to the uppermost and lowermost boundaries of the fault zone. The lower fault zone consists of fractured zones and three ~0.4 m thick brecciated zones.

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