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doi:10.2204/iodp.proc.335.102.2012 Structural geologyThis section outlines the techniques used for macroscopic and microscopic description of structural features observed in Hole 1256D igneous rocks. During Expedition 335, scientists generally followed the same conventions for structural studies used during Expeditions 304/305 (Expedition 304/305 Scientists, 2006) and 309/312 (Expedition 309/312 Scientists, 2006), which were based on previous ODP hard rock drilling legs (118, 131, 135, 140, 141, 147, 148, 153, 176, 206, 209; Shipboard Scientific Party 1989, 1991, 1992a, 1992b, 1992c, 1993a, 1993b, 1995, 1999a, 1999b, 2003, 2004). Definitions of structural measurements and descriptive parameters, along with corresponding description dictionaries, were further refined as part of the process of configuring the new core description software application DESClogik for hard rock descriptions. DESClogik was used to enter and upload the information into the LIMS database (see “Information architecture”). Details specific to structural features were illustrated with comments and sketches in STRUCTURE_SKETCHES_335 in DESCRIPTIONS in “Supplementary material.” Expedition 312 plutonic cores were redescribed using the same techniques and methods to ensure consistency in descriptions of Hole 1256D plutonic rocks (see STRUCTURE_LOGS_312_335 in DESCRIPTIONS in “Supplementary material”). An attempt was made to restore orientations of important structures into the geographic reference frame using available paleomagnetic data (see “Structural restoration: geographic reorientation”), and obtained orientations were recorded to generate a coherent data set for the Hole 1256D plutonic section (see STRUCTURE_LOGS_312_335 in DESCRIPTIONS in “Supplementary material”). Structural measurementsStructural features, categorized as magmatic, crystal-plastic, and brittle, were logged in centimeters from the top of each section. Descriptions of all features were recorded using curated depths so that “structural intervals” could be correlated with lithologic core descriptions. Depth intervals of structures were recorded as the distance from the top of the section to the top and bottom of the feature. Depth to a midpoint of a structure was recorded for the structures with measurable width, such as veins or intervals with a magmatic foliation. Structures on the archive half were measured relative to the standard IODP CRF (see “Core reference frame for sample orientation” Fig. F2). 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 pseudosouth (180) pointing into the archive half and pseudonorth (000) pointing out of the archive half (Fig. F12). The cut surface of the core, therefore, is a vertical plane striking 090–270. 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, middle diagram, in Fig. F12). The two apparent dips and dip directions (or one apparent direction combined with the strike) measured for each planar feature were used to calculate the dip and dip direction in the core reference frame. If the feature intersected the upper or lower surface of the core piece, measurements of the strike were made in the core reference frame, recorded as the trend (strike) of a line with zero dip angle, and combined with an apparent dip measurement, measured on the cut surface of the core, to calculate the true dip value (right sketch in Fig. F12). If a feature was exposed on the surface of the core (i.e., a fracture defining the top or bottom of a piece), the true dip and dip azimuth were measured directly on the feature with no need for another measurement or a calculation (not illustrated). True dip and dip direction (azimuth) were calculated using the Stereonet plotting program developed by Allmendinger (www.geo.cornell.edu/geology/faculty/RWA/programs/) and then imported into DESClogik. Macroscopic core description and terminologyWe examined all material from both the working and archive halves. Sketches of structures and orientation measurements were made from the archive half, but observations on working-half pieces were also made for certain features that were better exposed on the working half. For each section, detailed structural information was described and sketched on handwritten forms and subsequently entered into Excel spreadsheet logs, as described above. This information was then input into the DESClogik framework. The sketches were scanned and are archived in STRUCTURE_SKETCHES_335 in DESCRIPTIONS in “Supplementary material.” The DESClogik workbooks contain data on location, types of structures, their intensity, and orientations. We separated the structural data into three categories:
Other structural features not included in the structure workbook include contacts and veins. Magmatic structures and textures, including igneous contacts, compositional banding, and textural banding were described and their orientations recorded in the Contacts and the Texture tab in the igneous petrology workbook (see “Igneous petrology”) using the method described above. Veins, defined here as fractures filled with secondary minerals, were described and their orientations recorded in the Veins and Halos tab in the metamorphic petrology workbook (see “Alteration and metamorphism”). The most representative and/or prominent structural features in the cores recovered during Expedition 335 are plotted on the VCDs. These are intensity of magmatic and crystal-plastic fabric alignment; density of brittle fractures; and precise locations of observed prominent structures, such as igneous contacts, magmatic banding and magmatic veins (see “Igneous petrology”), veins, vein networks, hydrothermal breccia (see “Alteration and metamorphism”), cataclastic zones, breccia, shear veins, faults, fractures (except for horizontal irregular fractures; see “Brittle deformation”), and folds, where recognizable. Short explanations for terms and abbreviations used in the respective categories are given below. We followed the terminology based on Ramsay and Huber (1983), Twiss and Moores (1992), Davis (1984), and Passchier and Trouw (1996). Magmatic fabricsMagmatic fabrics were defined macroscopically by shape-preferred orientation (SPO) of primary minerals with no evidence of crystal-plastic deformation. Descriptions of magmatic fabric include the following:
Crystal-plastic fabricsCrystal-plastic fabrics (CPFs) are lineations or foliations defined by grains exhibiting plastic strain. Descriptions for CPFs include the following:
Quantitative shape-preferred orientation analysisQuantitative SPO analysis of plagioclase-rich leucocratic patches was applied to Expedition 312 Gabbro 1, where grains were recognizable in close-up photographs. SPO analysis involves three successive processes: image acquisition, image processing, and image analysis:
The main results of SPO analysis are the modal percentage of the analyzed phase, the shape ratio of the SPO ellipse, and the angle between the ellipse long axis and core axis, recorded in a clockwise sense from the core face. The current SPO methodology provides consistent results when SPO shape ratio > 1.01. Specimens with shape ratio < 1.01 were thus considered methodologically isotropic, and those with shape ratio > 1.01 were considered methodologically anisotropic. Practically, magmatic and crystal-plastic fabrics are macroscopically visible only when shape ratio > 1.05. When compared to the intensity of macroscopic fabric alignment (magmatic or crystal plastic), specimens are ranked as follows:
Brittle deformationIn the Brittle deformation tab, we described and measured features including faults, defined as fractures with shear displacement, and joints, defined as fractures with no shear displacement. We used fracture as a general term, indicating brittle failure with or without displacement. Microfault was used when describing faults with <1 mm of related width of deformation or faults with displacement measurable at core scale. Shear veins, defined here as fractures with secondary mineralization and a recognizable shear sense (such as slickenlines and fibrous mineral growth), were logged independently in the Brittle deformation tab, with orientation of slickenlines and sense of shear, where measurable. Orientations of open fractures that occur along the broken surface of veins were also recorded in the Brittle deformation tab rather than in the Vein tab. Descriptions for the Brittle deformation tab include the following:
Subhorizontal irregular fractures are identified as irregular or concave, horizontal to subhorizontal (<20°) fractures, without secondary mineralization, at the piece ends and/or confined in a piece, and may be related to drilling-induced fracture or preexisting anisotropy of the rocks. MicrostructuresTo better characterize different types of deformation, we studied the microstructural features of interesting and/or prominent mesoscopic structures. Thin sections of recovered material were examined in order to
For descriptions of microstructures, the terminology of Passchier and Trouw (1996) was used. Shipboard thin sections were oriented; the orientation is given relative to the core reference frame and was marked on each thin section (Fig. F2). Marking two directions is necessary to achieve unambiguous orientation of thin sections cut parallel to the cut surface of the core. Orientation of structures measured during the macroscopic core description was confirmed, and macroscopic observations were refined by the microscopic description. Digital photomicrographs and sketches were taken and annotated to document features described in thin sections (see STRUCTURE_SKETCHES_335 in DESCRIPTIONS in “Supplementary material.” Microstructural notes were entered into the Structures tab in the thin section workbook of DESClogik. Structural domains were introduced into the Structural tab in the thin section workbook. Several terms were introduced in the macroscopic descriptions, such as “bands” and “patches,” that might have different deformation mechanisms in different parts of a thin section. These terms are nongenetic and are meant to describe inhomogeneous distributions of phases and aggregates of phases. Such inhomogeneous distributions of phases were separated into structural domains for the documentation of microstructures and described separately. We generally followed terminology used during Expedition 312 (Expedition 309/312 Scientists, 2006). Additional classifications and terminology were incorporated from Legs 153, 176, and 209 and Expedition 304/305 (Expedition 304/305 Scientists, 2006). In the Structure tab of the thin section description workbook, we described the following microscopic features for each structural domain:
Additional work on Hole 1256D plutonic sectionStructural restoration: geographic reorientationWe attempted to azimuthally reorient structures into the geographic reference frame using a compilation of paleomagnetic data consisting of Expedition 335 shipboard data and unpublished Expedition 312 postcruise data of R. Anma and D.S. Wilson (See “Paleomagnetism” in the “Site 1256” chapter [Expedition 335 Scientists, 2012b]). Stable endpoint remanence directions determined using principal component analysis (PCA) may be used to perform a first-order reorientation of structures in individual core pieces by making the following assumptions: (1) the sampled section records a normal polarity remanence, (2) the stable remanence for each core piece approximates the time-averaged geomagnetic field direction at the site at the time of accretion, and (3) no significant tilting of the section has occurred. Structures may then be reoriented by applying a simple rotation to core pieces around the axis of the hole to align piece declinations with north. The first assumption is supported by General Purpose Inclinometry Tool (GPIT) data (see Teagle, Alt, Umino, Miyashita, Banerjee, Wilson, and the Expedition 309/312 Scientists, 2006) that indicate normal polarity of the plutonic section and by U-Pb ages of Gabbro 1 (15.04 Ma ± 0.18 m.y. and 15.06 Ma ± 0.37 m.y.; R. Anma, unpubl. data) that coincide with normal polarity Chron C5Bn.2n (15.034–15.155 Ma; Cande and Kent, 1995). However, Gabbro 2 yielded a U-Pb age of 15.20 Ma ± 0.17 m.y. (R. Anma, unpubl. data), falling within reversed polarity Chron C5Br (15.155–16.014 Ma; Cande and Kent, 1995). We note that assuming a reversed polarity (i.e., original south declination) in the restoration process would reverse the dip direction of restored structures but would not affect their restored strike. Magnetic susceptibility measurements on split coreTo support the delineation of magmatic fabric units, a series of magnetic susceptibility measurements were made on split cores. These measurements were performed using the Bartington MS2 magnetic susceptibility meter and the surface probe MS2F, which has a measuring volume equivalent to a half sphere of 40 mm diameter (see “Physical properties”). Measurements were performed on the center of fresh pieces, chosen to be as homogeneous, representative, and free of veins as possible. Measurements were taken at an average spacing of 50 cm depending on core recovery. The values are given in 10–5 (SI). |
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