Conventions for structural studies established during previous "hard-rock" drilling legs (e.g., Legs 118, 131, 140, 147, and 153) were generally followed during Leg 176. However, several minor changes in nomenclature and procedure were adopted. These changes are described below. Where procedures followed directly from previous legs, references to the appropriate Initial Reports "Explanatory Notes" chapter are given.
Because of the high recovery rate during Leg 176, archive halves of the cores were stored immediately, and all structural measurements had to be made on the working halves. Following procedures described in the Leg 153 Initial Reports volume (Shipboard Scientific Party, 1995a), data were entered into a VCD form (Fig. F4), used in conjunction with five spreadsheet logs (see "Appendix," in the "Leg 176 Summary" chapter). The structural sketches drawn on the VCD form are intended to illustrate the most representative structures and crosscutting relationships present on a core section; in addition, a brief general description of the structures is printed on the same form. Where no magmatic fabric, or only a weak one, was present (predating any other structure), the structural column was left blank.
Paper copies of spreadsheet forms were used for recording specific structures and measurements during core description. Separate spreadsheets were used to record data on (1) joints, veins, cleavage, and folds; (2) breccias, faults, and cataclastic fabrics; (3) crystal-plastic fabrics and sense of shear indicators; (4) magmatic fabrics, compositional layering, and igneous contacts; and (5) crosscutting relationships. The description and orientation of all features were recorded using curated depth so that "structural intervals" could be correlated with other lithologic intervals. The spreadsheets were organized to identify five separate types of structural intervals using deformation intensity scales summarized in Table T2. The structural geologists worked together during the same shift to minimize measurement inconsistencies. Each member of the team was responsible for making a specific set of observations throughout the entire core (e.g., characterization of magmatic fabric intensity). Descriptions and structural measurements were based on observations on the working half of the core (see orientation of structures below).
To maintain consistency of core descriptions, we used feature "identifiers" for structures similar to those outlined in Shipboard Scientific Party (1995a). Modifications to this scheme are shown in the comments checklist (Table T3) and include designation of breccia type (hydro-thermal - Bh, cataclastic - Bc, or magmatic - Bm). In core sections where crystal-plastic fabrics clearly overprint brittle hydrothermal or magmatic breccia, a note was made in the comment section of the crystal-plastic fabric spreadsheet that deformation occurred in the retrograde metamorphic assemblage. Where brittle fabrics overprint crystal-plastic fabrics, or deformation was "semi-brittle," a note was made in the comments section and documented in the crosscutting relations spreadsheet.
Structural features were recorded relative to core section depths in centimeters from the top of the core section. Depth was defined as the point where the structure intersects the center of the cut face of the working half of the core, as detailed during Leg 153 (see fig. 15A in Shipboard Scientific Party, 1995a). Crosscutting relationships were described in intervals delimited by top and bottom depth.
Apparent fault displacements were recorded as they appeared on the cut face of the archive half of the core and the end of broken pieces. Displacements seen on the core face were treated as components of dip-slip movement, either normal or reverse. Strike-slip displacements of vertical features were termed either sinistral or dextral. Displacements of features visible on the upper and lower surfaces of core pieces were treated as components of strike slip and termed sinistral or dextral. Displacements were measured between displaced planar markers, parallel to the trace of the fault. Additional cuts and slickenside orientations were incorporated wherever possible to differentiate among apparent dip slip, oblique slip, and strike slip.
The structures were oriented with respect to the core reference frame; the convention we used for the core reference frame is explained in Shipboard Scientific Party (1995a) and shown at the top of the comments box in the structural data spreadsheets (see "Appendix," in the "Leg 176 Summary" chapter).
Planar structures were oriented using the techniques outlined during Legs 131 and 153 (Shipboard Scientific Party, 1991; 1995a). Apparent-dip angles of planar features were measured on the cut face of the working 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). Apparent dips in the cut plane of the working core were recorded as two-digit numbers (between 00º and 90º) with a dip direction to 090º or 270º. In the "second" plane, apparent-dip directions were recorded as either 000º or 180º. The dip and the dip direction with respect to the working half of the core were recorded on the spreadsheet together with second plane measurements. If the feature intersected the upper or lower surface of the core piece, measurements were made directly of the strike and dip in the core reference frame. Where broken surfaces exposed lineations or striations, the trend and plunge were measured directly, relative to the core reference frame.
The two apparent dips and dip directions measured for each planar feature were used to calculate a true dip. These calculations were performed using either (1) LinesToPlane by S.D. Hurst or (2) App2truedip by B. Celerier (see "Appendix," in the "Leg 176 Summary" chapter). Calculated data can be read directly into the shareware stereonet plotting program Stereoplot 3.05 of Neil Mancktelow.
A semiquantitative scale of deformation intensities was used by the shipboard structural geologists during core description. This scale, shown in Table T2, has been modified from the deformation scale used during Leg 147 (table 2 in Shipboard Scientific Party, 1993) and the fabric intensity scales used during Leg 153 (fig. 13 in Shipboard Scientific Party, 1995a) and conforms to the scales for deformation textures used by Cannat et al. (1991) and Dick et al. (1991b) in the Leg 118 Scientific Results volume. Wherever possible we have assigned specific values to intensity estimates (e.g., the spacing of veins; the percentage of matrix in a cataclastic zone) to maintain consistency throughout the core. How-ever, for some categories this proved difficult (e.g., the intensity of any crystal-plastic fabric) in which cases we have used qualitative estimates of intensity based on hand-specimen and thin-section observations. We chose five distinct types of deformation for intensity measurements (Table T2):
Occasionally it proved difficult to differentiate between crystal-plastic and cataclastic deformation in relatively high-strain shear zones based on hand-specimen observations only. For this reason, as during Leg 147, we have tried to classify both of these deformation styles using parallel scales where a certain intensity of cataclasite is a direct equivalent to the same intensity of crystal-plastic deformation.
Initial inspections of the core indicated a prevalence of small, sub-horizontal microfractures throughout the core; to avoid unnecessary measurements of these regular features, a column was introduced into the structural spreadsheet to indicate the density of these features using the same density scale as for joints. The microfractures, which were likely induced by drilling, have been termed "subhorizontal microfractures" (SHM).
Thin sections were examined to characterize the microstructural aspects of important mesoscopic structures in the core. Classes of information that were obtained include deformation mechanisms on a mineral-by-mineral basis, kinematic indicators, crystallographic and shape fabrics, qualitative estimates of the degree of crystallographic preferred orientation along local, principal finite strain axes, syn- and post-kinematic alteration, and the relative timing of microstructures.
The orientation of thin sections relative to the deformation fabrics and core axes is noted in the comments section of the spreadsheet. Thin sections were oriented, where possible, with respect to the core axis and in the core reference frame described in "Structural Measurements". Selected samples were cut perpendicular to the foliation and parallel to any lineation to examine kinematic indicators and the shape-preferred orientation of minerals (see fig. 15, Shipboard Scientific Party, 1995a). We adopted and modified the thin-section description form used by the structural geologists during Leg 140 (Shipboard Scientific Party, 1992) to enter the microstructural information into the spreadsheet database.
The terms used to describe microstructures generally follow those used during Leg 153. Microstructures of gabbroic rocks recovered at Site 735 are discussed in detail in "Igneous Petrology" and "Structural Geology" in the "Site 735" chapter. Although a large spectrum of microstructures occurs, for the purposes of entering data into spreadsheets a number of textural types characterized by specific microstructural styles were defined and are keyed to the spreadsheet. These are summarized for gabbroic rock in Table T4.
It is possible that superposition of different microstructures or deformation mechanisms may occur during solidification and subsolidus cooling. Thus the physical state of the material during fabric development may span the transition from magmatic to solid state. Fabrics defined entirely by igneous minerals with no crystal-plastic deformation microstructures we term "magmatic." Where local crystal-plastic fabrics are possibly produced in the presence of melt (e.g., Hirth and Kohlstedt, 1995; Bouchez et al., 1992; Means and Park, 1994; Nicolas and Ildefonse, 1996), we term the physical state "crystal-plastic ± melt." Where fabric development is produced entirely by dislocation creep, we use the term "crystal-plastic" to define the physical state of the rock. The other groups refer to rocks whose magmatic and/or crystal-plastic texture is overprinted by brittle deformation.
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