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doi:10.2204/iodp.proc.348.102.2015

Structural geology

During Expedition 348, two types of sample material were used for structural geology analyses:

1. Cuttings (>4 mm size fraction) sampled at 5–10 m intervals between 870.5 and 2325.5 mbsf during riser drilling of Hole C0002N and between 1941.5 and 3058.5 mbsf during riser drilling of Hole C0002P.
2. Cores recovered from 475 to 507.64 mbsf in Hole C0002M and from 2163 to 2218.5 mbsf in Hole C0002P.

The methods we used to document the structural geology data of Expedition 348 cores and cuttings are largely based on those used by Expedition 315, 319, and 338 structural geologists (Expedition 315 Scientists, 2009a; Expedition 319 Scientists, 2010b; Strasser et al., 2014a). However, the method for cuttings description was modified to eliminate pillowed cuttings and DICA and to improve statistical counting of structural features. Depths reported for cores and cuttings are reported in CSF-A and MSF depths, respectively.

Description and data collection

Cuttings

Cuttings were investigated at 10 m depth intervals from 870.5 to 2325.5 mbsf in Hole C0002N and from 1941.5 to 3058.5 mbsf in Hole C0002P. Structural descriptions were conducted on sieved, washed, and vacuum-dried (see cuttings workflow in Fig. F1) cuttings collected from 870.5 to 2325.5 mbsf in Hole C0002N and just sieved and washed (wet) cuttings collected from 1941.5 to 3058.5 mbsf in Hole C0002P. Each cuttings bag was thoroughly washed again with seawater for several minutes on the core processing deck to reduce the percentage of DICA and pillowed cuttings. The samples were wet and left to stand for at least 5 min. This allowed the dried DICA and pillowed cuttings to soak up water and disaggregate into mush. Subsequent washing continued until the large majority of the pillowed cuttings were broken down and washed away. Sieving with a 4 and 1 mm mesh then allowed us to extract the >4 and 1–4 mm sized real lithologic cuttings (intact cuttings) for structural analysis. In some samples, 70%–95% of the total initial cuttings disaggregated entirely. The weak lithologies tended to be mostly disaggregated and mixed or destroyed by the drilling process. This potentially induces a bias toward more indurated lithologies (silty sandstone and siltstone). The assumption was that including the drilling-induced disturbances would incorrectly bias the statistical results for the different natural structural elements.

The washed and handpicked formation cuttings were studied with a binocular or digital microscope. For both the 1–4 and >4 mm size fractions, we recorded the total number of intact cuttings investigated and the number of cuttings containing deformation structures, together with the description of each structure, in an Excel spreadsheet.

Deformation structures recognized in the cuttings include bedding, carbonate or pyrite veins (sometimes brecciated), slickenlined surfaces (or slickensides), cataclastic bands, deformation bands (Maltman et al., 1993), web structures (Byrne, 1984), and scaly fabric. Thin sections were made and observed under optical microscope and scanning electron microscope (SEM) to describe representative or particularly interesting structural elements.

Cores

Structures preserved in the cores were documented on split cores and on XRCT images of unsplit cores (see “X-ray computed tomography”). Observations on split cores were hand logged onto the structural geology observation sheet (Fig. F7) 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 in Nankai and Costa Rica subduction zones (e.g., ODP Legs 131, 170, and 190 and IODP Expeditions 315, 316, 319, 322, 333, 334, 338, and 344). We measured the orientations of all structures observed in cores using a modified plastic protractor (Fig. F8) and then 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, for which the core axis is defined as “vertical” and the double line marked on the working half of the core liner is arbitrarily called “north,” 0° or 360° (Fig. F9; in unoriented core, this does not correspond to true north). We followed the techniques developed during Leg 131 (Shipboard Scientific Party, 1991) and later refined during Expeditions 315, 316, 319, 322, 333, and 338 (Expedition 315 Scientists, 2009a; Expedition 316 Scientists, 2009; Expedition 319 Scientists, 2010b; Expedition 322 Scientists, 2010; Expedition 333 Scientists, 2012; Strasser et al., 2014a). To determine the orientations of planes in this core reference frame (Fig. F9), the apparent dip angle of any planar feature was measured in two independent sections parallel to the core axis (Fig. F10). 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, bearing α₂ is either 0° or 180° (Fig. F10). Both β₁ and β₂ are between 0° and 90°. Similar measurements were made for planar features visible in XRCT 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. F11). We used the following convention in order to avoid confusion between two lines having the same rake but raking toward two opposite azimuths (e.g., a N45°E–60°SE fault bearing two striations, one raking 30°NE and the other raking 30°SW). If the linear feature rakes from an azimuth between N1°E and 179°E or between N181°E and N359°E, then 90° or 270°, respectively, will follow the value of the rake. In the example depicted in Figure F11, 270° would be added after the ϕ_a value. In the case of subvertical planes, +1° would 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 this information into account 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 348 cores (see “Structural geology” in the “Site C0002” chapter [Tobin et al., 2015]). These structures include bedding, fault planes, brittle fault zones, deformation bands, calcite-cemented breccia, fissility, and scaly foliation (or scaly fabric). These structures were observed by using the microscope, thin section, and SEM, as necessary.

Data processing

Orientation data calculation and true north correction

A spreadsheet developed during Expeditions 315, 316, 322, 319, 333, 334, 338, and 344 was used to calculate orientation data in the core reference frame (Fig. F12) (Expedition 315 Scientists, 2009a; Expedition 316 Scientists, 2009; Expedition 319 Scientists, 2010b; Expedition 322 Scientists, 2010; Expedition 333 Scientists, 2012; Expedition 334 Scientists, 2012; Strasser et al., 2014a; Harris et al., 2013). Based on the measured bearings (α₁ and α₂) and plunge angles (β₁ and β₂), this spreadsheet determines the strikes and dip angles of the 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 spreadsheet is further used to convert the core reference data in geographic coordinates. However, reorientation using paleomagnetic measurement was skipped for cores from Holes C0002M and C0002P because of the limited number of the structures and oversized diameter of SD-RCB cores for cryogenic magnetometer onboard.

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 the plots from the Composite Log Viewer. During Expedition 348, only the locations of structural features were entered in J-CORES, and orientation data management and analyses were performed with the spreadsheet as described above. For final publication, structural elements were converted to core-scale depictions using Strater software.

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