This section outlines the techniques used for macroscopic and microscopic description of structural features observed in hard rock basement cores. Conventions for structural studies established during previous hard rock drilling legs (Leg 118, Shipboard Scientific Party, 1989; Leg 131, Shipboard Scientific Party, 1991; Leg 135, Shipboard Scientific Party, 1992c; Leg 140, Shipboard Scientific Party, 1992b; Leg 141, Shipboard Scientific Party, 1992a; Leg 147, Shipboard Scientific Party, 1993b; Leg 148, Shipboard Scientific Party, 1993a; Leg 153, Shipboard Scientific Party, 1995; Leg 176, Shipboard Scientific Party, 1999) were generally followed during Leg 206. However, several minor changes in nomenclature and procedure have been adopted.
All material from both working and archive halves was examined, although the sketches of the structures and orientation measurements were made on the archive half. The most representative structural features in the cores recovered during Leg 206 are summarized on the VCD form (see the "Core Descriptions" content list). For each section, more detailed structural information is described and sketched on a separate "Structural Geology Description" form (Fig. F9). Structural data were tabulated in two spreadsheet logs, "Structural Log" and "Breccia Log" (Tables T7, T8), with reference to the structural geology checklist (Table T9).
To maintain consistency of core descriptions we used a set of structural feature "identifiers." Brittle deformation identifiers include joint, vein, shear vein, fault, and breccia. Identification of these features is based on the presence of fractures, filling phases, and evidence of shear displacement. The terminology adopted generally follows that of Ramsay and Huber (1987), Twiss and Moores (1992), and Passchier and Trouw (1996) and is consistent with the terminology used during Leg 153 for brittle deformation (Shipboard Scientific Party, 1995). Some of the terms commonly used in the structural description are sketched in Figure F10:
This subdivision of the structures does not imply that all features fall into distinct and exclusive categories. We prefer to use the term veins for all the healed fractures, avoiding the usual subdivision based on fracture width (e.g., Ramsay and Huber [1987] defined veins as having >1 mm filling material), mainly to be consistent with the vein log (see "Alteration" in "Igneous Rocks"). There are not rigid boundaries between the adopted structural categories; where necessary, details specific of structural features are illustrated with comments and sketches.
In the VCD and Structural Log, the term "late magmatic vein" is used for texturally and/or compositionally distinct mineral products of late magmatic fluids that cut the primary features of the basalts. Ductile and brittle-ductile structures comprise folds, tension gashes, and shear veins, and most of them are related to magmatic and late magmatic events.
Brecciated core intervals are described in detail in a separate Breccia Log (Table T8), which records the compositional and textural features of the breccias. Where common types of breccia can be unambiguously recognized, such as hyaloclastite or jigsaw-puzzle breccia, a note was added in the Comments column. Data on the composition and alteration of both veins and breccias were integrated with the Alteration and Vein Logs (see "Alteration" in "Igneous Rocks").
Structures are measured on the archive half relative to the core reference frame used by ODP. The plane normal to the axis of the borehole is referred to as the horizontal plane. On this plane, a 360° net is used with a pseudo-south (180°) pointing into the archive half and a pseudo-north (0°) pointing out of the archive half and perpendicular to the cut surface of the core (Fig. F11). The cut surface of the core, therefore, is a vertical plane striking 90°-270°.
Apparent dip angles of planar features were measured on the cut face of the archive half of the core. To obtain a true dip value, a second apparent dip reading was obtained where possible in a section perpendicular to the core face (second apparent orientation). The dip and the dip direction with respect to the archive half of the core are recorded on the spreadsheet together with second plane measurements. If the feature intersected the upper or lower surface of the core piece, measurements of the strike were made directly in the core reference frame and combined with the apparent dip measurements to calculate the true dip values. The two apparent dips and dip directions (or one apparent direction combined with the strike) measured for each planar feature are used to calculate the true orientation using the "LinesToPlane" Macintosh program by S.D. Hurst.
Thin sections of basement rocks recovered during Leg 206 were examined in order to (1) confirm macroscopic descriptions of brittle structures; (2) characterize the microstructure of the rocks; (3) provide information on the kinematics of brittle and brittle-ductile deformation; (4) identify time relationships between deformation, magmatic, and alteration processes; (5) assess the role of fluid in contributing to deformation; and (6) document major structural zones and downhole variations. The microstructural notes were entered into the "Thin Section Description Form" spreadsheet database (see Fig. F8). For the description of microstructures we applied mostly the terminology of Passchier and Trouw (1996). Shipboard thin sections were generally oriented; the orientation is given relative to 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 cut parallel to the cut surface of the core. Digital photomicrographs were taken during the cruise to document features described in the thin sections. A list of available images, any of which can be obtained from the data librarian at ODP is given in Table T30 in the "Site 1256" chapter.