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doi:10.2204/iodp.proc.338.102.2014

Structural geology

During Expedition 338, two types of sample material were used for structural geology analyses: (1) cuttings (1–4 mm and >4 mm size fractions) sampled at 5–10 m intervals between 865.5 and 2004.5 mbsf during riser drilling of Hole C0002F and (2) cores recovered from 200.0 to 1112.84 mbsf in Holes C0002H, C0002J, C0002K, from 0 to 419.5 mbsf in Hole C0022B, and from 0 to 194.5 in Hole C0021B. Hole C0021B cores were not processed on board during Expedition 338 but were analyzed postexpedition during a shore-based sampling party at KCC. The methods we used to document the structural geology data of Expedition 338 cores and cuttings are largely based on those used by the Expedition 315 and 319 structural geologists (Expedition 315 Scientists, 2009a; Expedition 319 Scientists, 2010b). Depths reported for cores are in core depth below seafloor, Method A (CSF-A).

Description and data collection

Cuttings

Cuttings were investigated at 5 m depth intervals between 865.5 and 1065.5 mbsf. Below 1065.5 mbsf only every other sample (10 m interval) was routinely analyzed, but at distinct depths the frequency was increased to 5 m intervals (see CUTTINGS STRUCTURE.XLSX in STRUCTURE in “Supplementary material”). For the upper 510 m (865.5–1375.5 mbsf), structural descriptions were only carried out for the >4 mm size fraction. Between 1375.5 and 2004.5 mbsf, the 1–4 mm size fraction was also investigated.

After cuttings were sieved and vacuum-dried (see cuttings workflow, Fig. F5), they were studied with a binocular or digital microscope. Above 1415.5 mbsf, for the >4 mm size fraction, ~100 cuttings were randomly selected and investigated; below 1415.5 mbsf, we measured the volume of the observed cuttings and converted it to absolute numbers (conversion factor determined empirically: 12 cm³ = 100 cuttings). For the 1–4 mm size fraction, we always measured the volume of the analyzed cuttings and converted it to number of cuttings (conversion factor: 1.5 cm³ = 100 cuttings). The number of cuttings containing deformation structures is recorded in an Excel spreadsheet with descriptions of each structure (see “Structural geology” in the “Site C0002” chapter [Strasser et al., 2014b]). The percentage of cuttings with deformation structures per number of investigated cuttings is shown in CUTTINGS STRUCTURE.XLSX in STRUCTURE in “Supplementary material” and plotted in Figures F54 and F58, both in the “Site C0002” chapter (Strasser et al., 2014b). In addition to investigating deformation structures, we also estimated the ratio of sandstone to silty claystone in cuttings. These results were compared to the sandstone–silty claystone ratio derived by macroscopic lithologic observations on cuttings before they entered the cuttings workflow (see “Lithology”).

Deformation structures recognized in cuttings include vein structures, carbonate veins, slickenlined surfaces (or slickensides), and minor faults. Optical thin sections were made every 50–100 m in order to describe microstructures.

Cores

Structures preserved in cores were documented on split cores and on X-ray CT images of whole-round cores (see “X-ray computed tomography”). Observations on split cores were hand logged onto the structural geology observation sheet (Fig. F9) at the core table and then transferred to both a calculation sheet and the J-CORES database (see “Data processing”). Core observations and measurements followed procedures of previous ODP and IODP expeditions (e.g., ODP Legs 131, 170, and 190 and IODP Expeditions 315, 316, 319, 322, and 333).

We measured the orientations of all structures observed in cores using a modified plastic protractor (Fig. F10) and noted the measurements on the structural geology observation sheet along with descriptions and sketches of structures. The orientations of planar or linear features in cores were defined with respect to the core reference frame, where the core axis is directed vertically and the double line marked on the working half of the core liner is toward the north (0° or 360°) (Fig. F11). We followed the techniques developed during Leg 131 (Shipboard Scientific Party, 1991) and later refined during Expeditions 315, 316, 319, 322, and 333 (Expedition 315 Scientists, 2009a; Expedition 316 Scientists, 2009a; Expedition 319 Scientists, 2010b; Expedition 322 Scientists, 2010a; Expedition 333 Scientists, 2012a).

To determine the orientations of planes in the core reference frame, the apparent dip angle of any planar feature was measured in two independent sections parallel to the core axis (Fig. F12). The orientation was then calculated using a calculation sheet (see “Data processing”). In practice, one section is typically the split surface of the core, on which the trace of the plane has a bearing (α₁) and a plunge angle (β₁) in the core reference frame. α₁ is either 90° or 270°. The other section is, in most cases, a cut or fractured surface at a right or high angle to the split core surface, on which the bearing (α₂) and plunge angle (β₂) of the trace of the plane are measured. In the case where the second measurement surface is perpendicular to the core split surface, α₂ is either 0° or 180°. Both β₁ and β₂ are between 0° and 90°. Similar measurements were made for planar features visible in X-ray CT images.

Linear features (e.g., slickenlines) were commonly observed on planar structures (typically fault or shear zone surfaces). Their orientations were determined in the core reference frame by measuring either their bearing and plunge or their rakes (or pitches) (ϕ_a) on the planes (Fig. F13). When using rakes, in order to avoid confusion between two lines having the same rake but raking toward two opposite azimuths (e.g., a N45°–60°SE fault bearing two striations, one raking 30°NE and the other raking 30°SW), we used the following convention, which applies for all planes except for subvertical planes: if the linear feature rakes from an azimuth between N1°E and N179°E or between N181°E and N359°E, then “90” or “270” will follow the value of the rake. In the example depicted in Figure F13, “270” will be added after the ϕ_a value. In the case of subvertical planes, “+1” will follow the rake value to indicate rakes from the top of the core and “–1” to indicate rakes from the bottom of the core. The calculation sheet takes account of this information for data processing.

All the above-mentioned data as well as any necessary descriptive information were recorded on the structural geology observation sheet. We observed a variety of deformation structures in Expedition 338 cores (see “Structural geology” sections in the relevant site chapters). These included beddings, faults, shear zones, deformation bands, calcite-cemented breccias, fissility surfaces, scaly foliations, vein structures (Brothers et al., 1996; Cowan, 1982; Ogawa, 1980; Ohsumi and Ogawa, 2008), and so forth.

Data processing

Orientation data calculation and true north correction

An Excel spreadsheet developed during Expeditions 315, 316, 319, 322, and 333 was used to calculate orientation data in the core reference frame (Fig. F14; see STRUCTURES_NEW.XLSX in STRUCTURE in “Supplementary material”) (Expedition 315 Scientists, 2009a; Expedition 316 Scientists, 2009a; Expedition 319 Scientists, 2010b; Expedition 322 Scientists, 2010a; Expedition 333 Scientists, 2012a). Based on the measured bearings (α₁ and α₂) and plunge angles (β₁ and β₂), this spreadsheet determines the strikes and dip angles of planar features in the core reference frame. Because of drilling-induced core fragmentation (e.g., biscuiting) and ensuing core recovery and core preparation operations, the orientation of the core with respect to the present-day magnetic north is lost. A correction routine is therefore required to rotate orientations measured in the core reference frame back to the magnetic reference frame. Paleomagnetic data taken by the long-core cryogenic magnetometer on the Chikyu (see “Paleomagnetism”) were used to correct drilling-induced rotations of cored sediment whenever there was a paleomagnetic datum point within the same coherent interval. If paleomagnetic data are available, the Excel spreadsheet further converts the core reference data in geographic coordinates.

J-CORES structural database

The J-CORES database has a VCD program to store visual (macroscopic and/or microscopic) descriptions of core structures at a given section index and a record of planar structures in the core coordinate system. The orientations of such features are saved as commentary notes but do not appear on plots from the Composite Log Viewer. During Expedition 338, only the locations of structural features were entered in the J-CORES database, whereas orientation data management and analyses were performed with an Excel spreadsheet as described above. For final publication, structural elements were converted to core-scale depictions using Strater software (Golden Software, Inc.).

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