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doi:10.2204/iodp.proc.319.102.2010

Structural geology

At Site C0009, two groups of sample materials were used for structural geology analysis: (1) cuttings (>4 mm fraction) sampled at 5 m intervals between 1097.7 and 1512.7 m MSF during drilling of Hole C0009A, and (2) core recovered between 1507.7 and 1593.9 m CSF from Hole C0009A. We did not study the structure preserved within cuttings from the cored interval. In addition to sample materials, we analyzed wireline FMI resistivity images and caliper log data for structure and geomechanics analysis from 703 to 1580 m WMSF at Site C0009. At Site C0010, only RAB images and associated log curves were used for structural interpretation.

Cuttings

Cuttings from Hole C0009A were studied with a binocular microscope. Approximately 20 cutting fragments were selected from the >4 mm size fraction at each 5 m depth interval; smaller fragments were not large enough to systematically recognize structures. Structures recognized include slickenlined surfaces (or slickensides), vein structures, and web structures (see "Structural geology" in the "Site C0009" chapter). The depth of each cuttings sample is recorded as the top of the 5 m interval from which it was collected.

Cores

Structures preserved in the core were documented through visual inspection of split cores and in optical thin sections. Detailed structural data were only collected on split cores. The data were hand logged onto a printed form at the core table and then transferred to both a spreadsheet and the J-CORES database. Core observations and measurements followed procedures of previous ODP expeditions (e.g., ODP Legs 131, 170, and 190). We used a modified plastic protractor to collect orientation data (Fig. F10) and noted results on a descriptive core log. Symbols for structures were entered on VCD sheets.

Following techniques developed on Leg 131 (Shipboard Scientific Party, 1991) and refined during Expedition 315 (Expedition 315 Scientists, 2009), orientations of planar and linear features in cored sediments were determined relative to the core axis, which represents vertical in the core reference frame, and the "double line" marked on the working half of the split core liner, which represents north (or 0° and 360°) in the core reference frame (Fig. F11). To determine the orientation of a plane, two apparent dips of the planar feature were measured in the core reference frame and used to define either a strike and dip or a dip and dip direction. This conversion was accomplished using a spreadsheet, as described in "Structural geology spreadsheet." One apparent dip of a planar feature was measured on the split face of the core with a dip direction and angle in the core reference frame. Apparent dip direction measurements on this core face had a trend of 90° or 270° and ranged in plunge from 0° to 90° (Fig. F11). The second apparent dip was usually measured on a cut or fractured surface at a high angle to the split face of the core (core face). In many cases this was a surface at 90° to the cut core face and either parallel or perpendicular to the core axis. In the former cases, the apparent dip lineation would trend 0° or 180° and plunge from 0° to 90°; in the latter cases, the trend would range from 0° to 360° and plunge 0°. Linear features observed in the cores were always associated with planar structures (typically faults or shear zones), and their orientations were determined by measuring either the trend and plunge in the core reference frame or the rake (or pitch) on the associated plane. All data were recorded on the log sheet with appropriate depths and descriptive information.

We observed a range of deformation structures in Expedition 319 cores (see "Structural geology" in the "Site C0009" chapter). Most can be classified as one of three types of structures: shear zones, faults, and slickenlined faults. We also logged bedding, a few sets of "vein structures" (Brothers et al., 1996; Cowan, 1982; Ogawa, 1980), and a range of other structures (e.g., fabrics defined by deformed trace fossils), though we did not measure the orientations of these other structures.

Borehole image data

Borehole resistivity imaging tools run during Expedition 319 also provide an in situ view of structural features. At Site C0009, the FMI wireline logging tool was run in the lower part of the hole (703.9–1580 m WMSF). This tool provides resistivity images with a spatial resolution up to 5 mm, but the four resistivity pads cover only part (~50%) of the interior of the borehole. At Site C0010, borehole images were acquired using the LWD geoVISION resistivity tool. This tool is based on RAB technology, which provides an image of the entire interior of the borehole while drilling (see "Logging" for information on image resolution). A 4 cm electrode is located 102 cm from the bottom of the tool and provides a focused lateral resistivity measurement (ring resistivity) with a vertical resolution of ~5–7.5 cm (Expedition 314 Scientists, 2009). Both the FMI and the RAB images are oriented with respect to north. All resistivity images were interpreted using GeoFrame and GMI Imager software to record the orientation of beds, faults, and various borehole failure features (see Expedition 314 Scientists, 2009). Caliper magnitude and orientation were used to determine borehole shape downhole to assess stress orientations.

Data analysis

Structural geology spreadsheet

A spreadsheet, developed and used during Expedition 315, was used to calculate orientation data in the core reference frame (Expedition 315 Scientists, 2009). This spreadsheet takes observations of apparent dips and determines the true dip of the feature in the core reference frame.

During rotary core barrel drilling, the core rotates with respect to the true geographic reference frame and individual core pieces undergo rotation during drilling, recovery, and preparation for visual descriptions. Thus, a correction routine is required to rotate structures from the core reference frame to the geographic reference frame. On previous expeditions (e.g., Expedition 315), paleomagnetic data were used to correct drilling-induced rotations of these coherent intervals. For Expedition 319, the cryogenic magnetometer was unavailable. However, paleomagnetic measurements were made during Expedition 322 on archive-half cores, and preliminary results are reported in "Paleomagnetism" in the "Site C0009" chapter. Reorientation using paleomagnetic constraints remains for postexpedition research.

Data recording

Structural data entered in the J-CORES database VCD program include a visual (macroscopic and/or microscopic) description of core structures at a given section index. Orientation data were entered into a spreadsheet separately for postacquisition processing.

Anisotropy of magnetic susceptibility

During Expedition 319, the physical properties group collected AMS from the "pmag" cubes (see "Physical properties"). The magnetic susceptibility (kij) is produced by the application of a magnetic field (Hj) to the natural magnetic remanence (Mi):

Mi = kij × Hj. (3)

By measuring the magnetic susceptibility in three directions (by rotating the sample within the Kappabridge), one can measure the three principal axes (kmax, kint, and kmin) of the AMS ellipsoid. The resulting AMS ellipsoid is then interpreted as resulting from preferred orientations (or in some instances distribution) of micrometer to submicrometer magnetite grains and/or other paramagnetic phases. Because of the fine-grained nature of materials in the cored sediment, the AMS ellipsoid is thought to be roughly proportional to finite strain in both direction and magnitude (Housen, 1997).

The AMS spreadsheet (see C0009_T1.XLS in STRUCGEOL in "Supplementary material") reports the mean magnetic susceptibility (kmean), kmax, kint, and kmin, and three statistics that describe the AMS ellipsoid (P, L, and F):

kmean = (kmax + kint + kmin)/3, (4)

P = kmax/kmin, (5)

L = kmax/kint, and (6)

F = kint/kmin. (7)

The spreadsheet also has columns in which to determine the geographic orientation of the AMS ellipsoid given paleomagnetic data.