Structural studies performed on the cores from Leg 210 were designed to
The methods used here were designed following the "Explanatory Notes" chapters of the Initial Reports volumes for earlier ODP legs, mainly Legs 141 (Shipboard Scientific Party, 1992), 149 (Shipboard Scientific Party, 1994), 160 (Shipboard Scientific Party, 1996), and 173 (Shipboard Scientific Party, 1998). Modifications were made as appropriate in order to meet the specific needs and problems encountered during Leg 210.
An important consideration is that commonly only part of the cored interval is actually recovered. This leads to a sampling bias that for structural purposes can be acute. In particular, in cases of incomplete recovery, material from fault zones may be missing. When faulted or fractured rock from such zones is recovered, it is often highly disturbed and its original orientation has changed. Further limitations are created by drilling-induced deformation and rotation in cores. Features are considered to be drilling induced if their origin is doubtful.
The structures on the cut face of the archive half of the core were included on the VCD form where deformation structures were considered to be either important or common (see "Visual Core Descriptions"). Details that could not be included on the VCDs are available on the handwritten barrel sheets from the program data librarian. The descriptive terminology we used for deformation structures in hard and soft rocks follows Ramsay and Huber (1983, 1987), and terminology more particular to sediments follows Maltman (1994). For microstructures, we applied the terminology of Passchier and Trouw (1996).
Determining the orientation of observed structures (and intrusive dikes) in the geographical reference frame is difficult. Therefore, structures were oriented with respect to the core reference frame. The core reference frame was chosen according to the conventions used for ODP magnetic measurements to allow direct comparison with the paleomagnetic results (Fig. F8; also see Fig. F12). The plane perpendicular to the long axis of the core was taken as the horizontal. "North," "south," "east," and "west" were defined in this plane in the following manner. South (azimuth = 180°) is the direction perpendicular to the cut surface of the archive half, pointing toward the cylindrical outer surface of the archive half. Consequently, north (azimuth = 000°) is the opposite direction, pointing into the cut face of the working half. With the archive half oriented top up, west (270°) is directed toward the right side of the surface of the archive half and east (090°) is toward the left side. The orientations of planar and linear elements in the core were documented using dip azimuth and dip angle (Fig. F9). The orientation is always expressed by two numbers; the first indicates the dip azimuth, expressed by three digits, and the second indicates the dip angle, expressed by two digits (i.e., 090/45 means a plane or a line dipping or plunging east at an angle of 45°).
Measurements of orientations of planar structures observed in the cores were made using a protractor. For planar elements, the apparent dip on the core face was measured as either dipping toward the east or toward the west. The orientation of the planar element was then further constrained using an additional measurement either from the horizontal plane (top or bottom of core piece) or from the cylindrical outer surface of the core half, as shown in Figure F9. The two measurements yielded the orientations of two lines contained within the planar element. The orientation of the planar element itself was then determined from the orientation of the two lines using a stereonet. This procedure provided the working azimuth and dip of the observed structure (i.e., the azimuth and dip within the core reference frame). The dip is "true," as long as the core axis is approximately vertical. The orientations of linear structures were recorded as "working trends." These were measured using dip azimuth and dip angle and referred to the core reference frame in the same way as planar surfaces (Fig. F8). If cut surfaces were parallel to azimuth of lineations and perpendicular to azimuth of faults, dips could be measured directly. Mineral and/or mylonitic foliations and lineations were measured in much the same way as faults and veins.
The sense of fault displacement was recorded and referred to as normal, reverse, sinistral strike-slip, or dextral strike-slip. The apparent displacement was measured on the core face and/or on the top or bottom side of pieces. In cases where the direction of slip was visible, offset was measured in a plane normal to the fault and parallel to slip direction, as straight-line separation between displaced markers.
At this stage, structures were oriented in the core reference frame shown in Figure F8, but they were not oriented with respect to geographic ordinates. In some cases, viscous remanent magnetization (VRM), considered to be parallel to the present-day magnetic field, can be used to orient cores to the geographic frame. Unfortunately, the VRM was dominated by drilling effects during Leg 210 (see "Paleomagnetism"), so it was generally not possible to reorient structural features in cores.
No downhole logging was conducted during Leg 210 because of poor hole conditions, so no Formation MicroScanner data were available for core reorientation.
Thin sections of sedimentary, igneous, and metamorphic rocks recovered during Leg 210 were examined in order to
For the description of microstructures, we applied the terminology of Passchier and Trouw (1996). Shipboard thin sections were oriented, except when they were made from small pieces whose orientation with respect to the rest of the core was unknown. Orientation was in the core reference frame and was marked on each thin section by an arrow pointing upward and a short tick pointing toward west from the base of the arrow. Marking two directions is necessary in order to achieve complete orientation of thin sections, which are cut parallel to the split surface of the core.