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Structural geology

This section outlines the techniques used for macroscopic and microscopic description of structural features observed in Expedition 330 cores. Conventions for these structural studies generally follow those established during recent IODP hard rock drilling expeditions (Expedition 309/312 Scientists, 2006; Expedition 324 Scientists, 2010).

Detailed observations of structures in the recovered igneous basement of the drilled seamounts allowed us to (1) determine if there has been any posteruption orientation change of the lava flows, either by tectonic activity close to the Kermadec subduction zone or by seamount flank collapse; (2) document structures in the core and record evidence for the relative timing of the formation of joints, veins, and other structural features caused by magmatic cooling, tectonic processes, and hydrothermal alteration; (3) where possible, record the three-dimensional orientation of all structures (Fig. F9); and (4) obtain orientations of sedimentary features (bedding, erosional surfaces, and geopetals) that may provide information for sedimentologic studies of these flat-topped seamounts. All observations were entered into the LIMS database using the DESClogik application.

Several features were logged in conjunction with other core describing laboratory groups. The orientation and distribution of veins were first logged by the structural geology group, and subsequently mineral infills and alteration halos were described by the alteration petrology group (see “Alteration petrology”). Hyaloclastites, volcanic breccia, and chilled margins were first identified and described by the igneous petrology and sedimentology groups (see “Igneous petrology and volcanology” and “Sedimentology”), after which the orientations of these structures were entered into DESClogik by the structural geology group. When aligned vesicles occurred because of magma flow, vesicle orientations were recorded in DESClogik after the igneous petrologists had logged the vesicles. Sedimentary bedding was described together with the sedimentologists to ensure consistency in core descriptions.

Graphic symbols and terminology

Material from both the working and archive halves of the core were examined, but structures and orientations were measured exclusively on the archive halves.

A predefined set of structural features was used to maintain consistency in the core descriptions (Fig. F7). Brittle deformation identifiers include fractures, veins, faults, and breccia. Identification of these features was 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). The terms and symbols used in the structural descriptions are presented in Figures F7 and F8. The features identified include the following:

  1. Igneous contacts (demonstrably extrusive or intrusive contacts).

  2. Fractures (brittle failure with unknown displacement and no secondary infill minerals). Joints were identified as fractures when the two sides showed no differential displacement relative to the naked eye or a 10× hand lens. Faults are fractures with kinematic evidence for shear displacement across the discontinuity. Breaks clearly resulting from drilling were not logged as fractures.

  3. Veins (extensional open fractures filled with secondary minerals).

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

  5. Lineations (oriented minerals in shear zones or slickenlines on a fault surface).

  6. Magmatic fabrics (magmatic foliations defined by shape-preferred orientations of primary minerals, with no evidence of crystal-plastic deformation).

  7. Aligned vesicles (filled or unfilled primary igneous gas bubbles resulting from the outgassing of the lava).

  8. Sedimentary bedding.

  9. Geopetals (vesicles or voids filled with sedimentary or hydrothermal minerals originally deposited or precipitated horizontally).

This division of structures does not imply that all features fall into distinct and exclusive categories. The term “vein” is preferably used for all healed fractures, avoiding the usual division based on fracture width (e.g., Ramsay and Huber, 1987).

Structural features in the cores recovered during Expedition 330 are summarized in the VCDs (Figs. F7, F10; see “Core descriptions”). For some important intervals, more detailed structural information is described and photographed in figures within the respective site reports.

Geometric reference frame

To determine the orientation of a structure in the recovered core, a three-step process was followed. In step one, the attitude of a feature on the archive-half core face frame was recorded with a goniometer (Fig. F11) according to the conventions illustrated in Figure F9. The orientation of planar (Fig. F9A) or linear (Fig. F9B) structures in the core was obtained with the goniometer by rotating and holding its moveable (i.e., hinged) plane parallel to the feature to be measured. In the core face frame, the plane along which the core was split is referred to as the apparent horizontal plane, on which 000° points downcore on the archive half and 180° points upcore (Fig. F9A). When looking onto the face of the archive half in this 360° coordinate system 270° is on the right and 90° is on the left (Figs. F9, F10).

In step two, the orientations of structures were rotated 90° clockwise about the horizontal 090°–270° axis to transform the measurements from the core face frame into the core reference frame (Fig. F9C). These rotation calculations were performed using a Perl script written by Expedition 330 scientist L. Kalnins. In the core reference frame, the cut surface of the core is a vertical plane striking 90°–270°, with pseudonorth (000°) pointing out of the cut surface of the archive half and pseudosouth (180°) pointing into the cut surface of the archive half (Fig. F9C). The core reference frame used herein is consistent with that used by the paleomagnetism group (see “Paleomagnetism”). All dip and dip direction values listed in the VCDs (Fig. F10) are reported in the core reference frame. It is important to note that the dips in the core reference frame represent the actual dips of the features within the seamount, provided that the drill hole is vertical. Dip directions, however, will have been affected by rotary drilling because the core pieces are free to rotate independently around the vertical axis while inside the core barrel. Regardless of the transformation of the measurements into the core reference frame, dip directions are still referenced relative to the plane upon which the core was split (Fig. F9).

The full in situ orientation of the core (i.e., both dip and dip direction) can only be obtained when independent constraints are available, specifically from Formation MicroScanner downhole data (see “Downhole logging”). Core reorientation is possible by matching structural features recorded on the core to structures on the borehole wall that are imaged by the Formation MicroScanner tool and measured in the true geographic reference frame. This analysis, which forms step three of the structural orientation process, is a target of postexpedition research. For sites without downhole logging, paleomagnetic measurements may potentially be useful for core reorientation, but because of the high paleolatitude of the seamounts, the uncertainty will probably be large (several tens of degrees).

Thin section description

Thin sections made during Expedition 330 were examined in order to (1) confirm macroscopic descriptions of structures, (2) characterize the microstructure of the rocks, (3) identify timing relationships between magmatic and alteration processes and the relative temporal sequence between different groups of veins and joints, and (4) document downhole variations within structured zones.

The microstructural notes were entered into the DESClogik thin section description template (see “Igneous petrology and volcanology” for details about template). For the description of microstructures we primarily applied the terminology of Passchier and Trouw (1996). Shipboard thin sections were oriented relative to the core face frame, which was marked on each thin section by an arrow pointing upward. Digital photomicrographs were taken during the expedition to document features described in the thin sections. These photomicrographs were also uploaded into the LIMS database.