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doi:10.2204/iodp.proc.345.102.2014 Structural geologyConventions for structural studies established during previous “hard rock” drilling legs (e.g., ODP Legs 118, 131, 140, 147, 153, 176, 179, 206, and 209 and Expeditions 304/305, 309/312, and 335) were generally followed during Expedition 345 (Shipboard Scientific Party, 1989, 1991, 1992, 1993, 1995, 1999a, 1999b, 2003, 2004; Expedition 304/305 Scientists, 2006; Expedition 309/312 Scientists, 2006; Expedition 335 Scientists, 2009). Definitions of structural measurements and descriptive parameters were further refined from Expedition 335 to configure the DESClogik core description software application for hard rock descriptions. DESClogik is used to enter and upload the information into the LIMS database (see “Introduction”). Details specific to structural features are illustrated with comments and sketches in STRUCTUR in “Supplementary Material.” Sections of Leg 147 plutonic cores were redescribed using the techniques and methods outlined below to ensure consistency in descriptions between holes in plutonic crust at Hess Deep. Structural measurementsStructural features, categorized as magmatic, crystal-plastic, brittle, and veins were logged by interval in centimeters from the top of each section. Descriptions of all features were recorded using curated depths so that structural intervals are 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, where the feature intersected the center of the core piece. We measured structures on the archive half relative to the standard IODP core reference frame (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. F14A). The cut surface of the core, therefore, is a vertical plane (strike 090°–270°). Apparent dip angles of planar features were measured on the cut face of the archive half. To obtain a true dip value, a second apparent dip was measured, where possible, in a section perpendicular to the core face (second apparent orientation shown in Fig. F14B). 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 (Fig. F14C). 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. True dip and dip direction (azimuth) were calculated using a macro in Microsoft Excel written by Michael Cheadle for Leg 209, or by inputting data into the Stereonet 8 program developed by R. Allmendinger (www.geo.cornell.edu/geology/faculty/RWA/programs/stereonet-7-for-windows.html), and imported into DESClogik. Macroscopic core description and terminologyAll descriptions and structural measurements during Expedition 345 were made on the archive halves of the cores unless otherwise noted. Whole cores were oriented by a member of the structural geology group for cutting prior to curation. Cores were marked to maximize dip of planar structures so that the dominant structure dips toward 90° in the CRF (Fig. F2). Where no obvious structures were present, cores were marked to maximize contiguity with adjacent core pieces. During Expedition 345, the structural geologists worked together for the shift to minimize measurement inconsistencies, with each member of the team responsible for making a specific set of observations throughout the entire core (e.g., characterization of crystal-plastic fabric [CPF] intensity). For each section, detailed structural information was described and sketched on handwritten forms and subsequently entered into Excel spreadsheets. This information was then input into the DESClogik framework, which contains four worksheets for the following categories:
The DESClogik worksheets contain data on the interval, type of structure, intensity, orientation, any crosscutting relationships, and comments. The worksheets record up to six separate types of quantitative measurements using the deformation intensity scales summarized in Figure F15. Sketches of structural relations were made to illustrate the most representative structures and crosscutting relationships in a core section. All sketches were scanned and archived in STRUCTUR in “Supplementary material.” The most representative and/or prominent structural features in the cores recovered during Expedition 345 are plotted on the VCDs. These include intensity of magmatic and CPF alignment; density of brittle fractures; and precise locations of observed prominent structures, such as igneous contacts, magmatic banding and magmatic veins, alteration veins, vein networks, hydrothermal breccia, cataclastic zones, breccia, shear veins, faults, fractures, and folds, where recognizable. Short explanations for terms and abbreviations used in the respective categories are based on definitions given in Ramsay and Huber (1987), Twiss and Moores (1992), Passchier and Trouw (1996), and Davis et al. (2011) and are given below. Magmatic structuresIntrusive contacts, magmatic dikes, and magmatic veins (including plagioclase and pyroxene segregation veins) were measured and described in accordance with the igneous petrology workbook (see “Igneous petrology”). Descriptions include
Igneous layering/banding, where present, was measured and described in accordance with the igneous petrology workbook (see “Igneous petrology”). Descriptions include
Magmatic fabrics were defined by the presence and intensity of any shape-preferred orientation (SPO) of magmatic phases. Descriptions of magmatic fabric include
Crystal-plastic fabricsCPFs include planar or linear fabrics defined by grains exhibiting plastic strain. Descriptions for CPF include
Preferred orientation of serpentine networksSerpentinization confers a mesh texture in peridotite and/or olivine-rich intrusive rock that defines a macroscopic expression of serpentine growth at the expense of olivine (O’Hanley, 1996). The microscopic serpentine fibers may be randomly oriented and homogeneously distributed in the rock, forming an anastomosing isotropic network, or preferentially oriented and/or unevenly distributed in the rock leading to an anisotropic texture (foliation). Formation of a preferred orientation of serpentine meshes may include protolith anisotropy (either inheritance of a former shape preferred orientation or of an uneven distribution of the olivine and pyroxene grains/microlayering), serpentine growth in an anisotropic stress field, or transposition of the serpentine during and/or after growth. Descriptions of the preferred orientation of serpentine networks (Fig. F15) include a semiquantitative indication of the degree of preferred orientation from 0 (isotropic) to 3 (strong preferred orientation) and orientation (dip azimuth and dip angle of the planar fabric, where measurable. Brittle deformationBrittle fabrics described during Expedition 345 include breccias, faults (defined as fractures with shear displacement), and fractures (including open, drilling induced, and shear). Descriptions of brittle deformation include
Alteration veinsAlteration veins described during Expedition 345 include characterization of the vein margin, structure of the vein fill (undeformed, sheared, or crack-seal), chronology of vein intrusion, relations with other veins, and crosscutting relations between veins and other structures. The nature of the vein fill material was identified in cooperation with the metamorphic petrology group. For each mineralogical type, individual veins were numbered within each piece sequentially, starting with the first vein encountered (V1) and entered into the spreadsheet successively (V2, V3, etc.). Descriptions include
MicrostructuresBecause of the setting of the sites drilled during Expedition 345 (plate spreading), superposition of microstructures or deformation mechanisms is commonly attributable to different stages of extension and cooling. Thus, the physical state of the material during fabric development may span the transition from hypersolidus to subsolidus. Igneous fabrics defined entirely by minerals with no crystal-plastic deformation microstructures are termed magmatic. To 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, and the orientation is given relative to the CRF and was marked on each thin section. Marking two directions was necessary in rare cases 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, which are all scanned and included in STRUCTUR in “Supplementary material.” Microstructural notes were entered into the Structures tab in the thin section workbook of DESClogik. We followed the terminology used during Legs 153, 176, and 209 and Expedition 304/305 (Shipboard Scientific Party, 1995, 1999a, 2004; Expedition 304/305 Scientists, 2006). In the Structure tab of the thin section description workbook, we described the following microscopic features for each thin section:
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