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doi:10.2204/iodp.proc.324.102.2010

Structural geology

Structural geology was an important facet of visual core description during Expedition 324. The priorities set by the structural geologists were

  1. To document all structures in the core and record evidence for the relative timing of the various structures and other processes such as magmatic cooling, tectonic processes, and hydrothermal alteration;

  2. To record the orientation of all structures on the core face frame and, where possible, to orient these in three dimensions in the core reference frame;

  3. To obtain evidence from the style, geometry, and microstructure of individual structures that might bear upon the processes and conditions of deformation and the finite strain that can be inferred; and

  4. To construct plausible models of the tectonic environment from all of these data.

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 Expedition 309/312 (Teagle et al., 2006) and ODP hard rock drilling (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; Leg 206, Shipboard Scientific Party, 2003b) were generally followed during Expedition 324. However, several minor changes in nomenclature and procedure were adopted.

Graphic symbols and terminology

All material from both working and archive halves was examined, although the records of the structures and orientation measurements were made on the archive half. The most representative structural features in the cores recovered during Expedition 324 are summarized on the VCD form (see "Core descriptions"). For some important sections, more detailed structural information is described and sketched on a separate Structural Geology Description form (Fig. F15). All structural data were entered in structural spreadsheet logs using DESClogik, with reference to the structural geology and symbols checklist (Fig. F8; Table T4).

We used a set of structural feature "identifiers" in order to maintain consistency of core descriptions. 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 (Teagle et al., 2006) and symbols commonly used in the structural description are sketched in Figures F8 and F16:

  1. Igneous contacts (demonstrably extrusive or dike contact),

  2. Lineation (preferred orientational minerals in the shear zone, oriented minerals in magmatic foliation, or slickenlines in the fault surface),

  3. Magmatic fabric (lineations and foliations, referred to as magmatic foliation when recognized, defined by shape-preferred orientation of primary minerals with no evidence of crystal-plastic deformation),

  4. Magmatic foliation (shape-preferred orientation of primary minerals with no evidence of crystal-plastic deformation),

  5. Shear veins (obliquely opening veins with minor shear displacement filled with slickenfibers or overlapping fibers),

  6. Veins (extensional open fractures filled with secondary minerals),

  7. Fractures (brittle failure with or without displacement),

  8. Joints (fractures where the two sides show no differential displacement [relative to the naked eye or 10x pocket lens] and have no filling material), and

  9. Faults (fractures with kinematic evidence for shear displacement across the discontinuity or with an associated cataclasite; we adopted the term "microfault" when the scale of the offset is millimetric).

This division of structures does not imply that all features fall into distinct and exclusive categories. We prefer to use the term "veins" for all healed fractures, avoiding the usual division based on fracture width (e.g., Ramsay and Huber [1987] defined veins as having >1 mm filling material), mainly to be consistent with the DESClogik alteration vein log template. Rigid boundaries between the adopted structural categories do not exist. Where necessary, details specific of structural features are illustrated with comments and sketches.

Brecciated core intervals are described in detail in a separate structural template in DESClogik, in which the compositional and textural features of the breccias are recorded. Where common types of breccia can be unambiguously recognized, such as fault-related, hyaloclastite, or hydraulic 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 and metamorphic petrology").

Geometric reference frame

Structures are measured on the archive half relative to the core face frame and core reference frame based on previous ODP/IODP procedures (Leg 141, Shipboard Scientific Party, 1992a; Leg 206, Shipboard Scientific Party, 2003b; Expedition 309/312 Scientists, 2006). 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 pseudosouth (180°) pointing out of the archive half and a pseudonorth (0°) pointing into the archive half and perpendicular to the cut surface of the core (Fig. F17). The cut surface of the core, also called core face or cut face, 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.

However, for the long term we should recover real orientation of the structures. Therefore, following the recording of orientation data on the descriptive sheets as outlined above, it would be desirable to convert these local orientations to geographical coordinates. The multishot and Formation MicroScanner (FMS) logging techniques allow cores to be oriented with respect to magnetic north and allow the arbitrary local reference frame to be positioned. A paleomagnetic approach can be also employed. It is especially useful for cores obtained by RCB drilling because these techniques often cause the core to break into several pieces that rotate independently of each other within the core liner. To remove these drilling-induced rotations, FMS data could be gathered from sections of core that were considered to be structurally continuous.

Thin section description

Thin sections of basement rocks recovered during Expedition 324 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 and the relatively temporal sequence between different veins and joints; and (5) document downhole variations of structured zones.

The microstructural notes were entered into the DESClogik Thin Section Description template (see "Igneous petrology" for details of the template). For the description of microstructures we primarily applied 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. Digital photomicrographs were taken during the cruise to document features described in the thin sections.

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