Conventions for structural studies established during previous hard-rock drilling legs (e.g., Leg 118, Shipboard Scientific Party, 1989a; Leg 131, Shipboard Scientific Party, 1991; Leg 135, Shipboard Scientific Party, 1992a; Leg 140, Shipboard Scientific Party, 1992b; Leg 141, Shipboard Scientific Party, 1992c; Leg 147, Shipboard Scientific Party, 1993a; Leg 148, Shipboard Scientific Party, 1993b; Leg 153, Shipboard Scientific Party, 1995; Leg 176, Shipboard Scientific Party, 1999) were generally followed during Leg 179. 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 "Explanatory Notes" chapters are given. Leg 179 was originally designed as an engineering leg, but because of contingencies that developed before and during the cruise that required core recovery, the leg was significantly understaffed, based on the core recovered. Description efforts were concentrated, therefore, on completing structural information for the VCDs (Fig. F4). VCD information was entered as basic log sketches with positions of measurements on the core. In addition, preliminary descriptions of each core section were made and recorded in structural notebooks, and the information was later transcribed into a word-processing program and summarized for the VCD form. The positions of as many major structural features as possible were also logged, and many of these features were oriented on the core face. These data were entered into the structural log spreadsheet (Fig. F8). More detailed structural measurements and descriptions of the core and thin sections will await shore-based studies.
Following procedures described in the Leg 153 Initial Reports volume (Shipboard Scientific Party, 1995), data were entered onto a VCD form (Fig. F4). The structural sketches drawn onto the VCD were designed to present the most representative structures and crosscutting relationships present on a core, in conjunction with the brief general descriptions printed on the same form. Where no structures or fabrics were present, the structural column was left blank. Six spreadsheet logs used during previous legs (see Cannat, Karson, Miller, et al., 1995; Dick, Natland, Miller et al., 1998) were combined into a single structure log spreadsheet (Fig. F8) for identifying the positions and describing major structural features, fabric zones, or structural unit boundaries.
Paper copies of the spreadsheet form were used for recording specific structures and measurements during core description. The structural log spreadsheet was used to record data on joints, veins, foliations, folds, fault and magmatic breccias, faults and fault zones, cataclastic, mylonitic and gneissic shear zones, crystal-plastic fabrics, sense of shear indicators, magmatic fabrics, mineral lineations, slickenlines, compositional layering, igneous plutonic contacts, and crosscutting relationships. Each feature and structural zone has been logged into the structure log with its position. Limited orientation data were gathered because of the lack of sufficient time given the small number of scientific personnel assigned to the leg. The description and orientation of structural features were recorded using curated depth so that "structural intervals" can easily be correlated with other lithologic intervals. Descriptions and structural measurements were based on observations on the working half of the core (see "Structural Measurements").
To maintain consistency of core descriptions of plutonic rocks during previous legs, we used feature "identifiers" for structures similar to those outlined during Leg 153 (Shipboard Scientific Party, 1995) and Leg 176, (Shipboard Scientific Party, 1999) although some modifications should be noted in Figure F9. The characteristics of each structure in the identifier list that were investigated are listed in Figure F10. For major structural features, a planar and/or linear orientation, usually limited to a single apparent dip (or plunge) on the core face, was recorded, and the comments section in the structural log (Fig. F8) was used for additional explanatory remarks.
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 in the Leg 153 Initial Reports "Explanatory Notes" chapter (see fig. 15A in Shipboard Scientific Party, 1995). 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. 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 between apparent dip-slip, oblique-slip, and strike-slip displacements.
The measurement of the orientations of observed structures was oriented with respect to the core reference frame (working half). The convention we used for the core reference frame is the same as detailed in Shipboard Scientific Party (1995) and shown at the top of the comments box in the structural data spreadsheets (see the "Appendix" contents list). All spreadsheet orientations are given in the core reference frame.
Planar structures were oriented using the techniques outlined during Legs 131 (Shipboard Scientific Party, 1991) and 153 (Shipboard Scientific Party, 1995). 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 will be obtained where possible during postcruise investigations in a section perpendicular to the core face (second apparent orientation). Apparent dips in the cut plane of the working core were recorded as a two-digit number (between 00º and 90º) with a dip direction to 090º or 270º. In the second plane, apparent dip directions will be recorded as either 000º or 180º. The dip and the dip direction for 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. All structural measurements for each feature were entered into the structural log spreadsheet (Fig. F8).
The second stage of core orientation, involving a combination of two apparent dips to calculate a true dip and rotating the azimuthal data to fit with the paleomagnetic data, was not completed on board because no paleomagnetist sailed during Leg 179. In this step, adding or subtracting, as appropriate, the difference between the 000º reference direction and magnetic north, the core measurements can be rotated into a geographically correct orientation. This procedure is outlined in steps in figure 6 of Shipboard Scientific Party (1991). Detailed measurements will be completed onshore.
A key for fabric intensity (Fig. F11) and a list of textural abbreviations (Fig. F12) were used to refine identifier descriptions. When feasible, quantitative and semi-quantitative estimates of feature development were used to define a five-category relative intensity scale. It is important to recognize that this scale is based on different characteristics for different types of structures and that not all of the identifiers could be appropriately assigned an intensity value. We also emphasize that the intensity estimates were only semiquantitative at best and could not be fully quantified during core description. Frequent cross-checks were used between shipboard structural geologists to ensure consistency during core description. The intensity scales are represented in Figure F11 and summarized below:
Nine textural classes were selected for the purpose of macroscopic description (Fig. F12). It must be stressed that these classes do not necessarily directly relate to any single physical parameter such as stress or strain. Moreover, some of these terms apply only to mafic rocks, others to ultramafic rocks, and some to both of them. The primary reason for selecting these classes was that they are relatively unambiguous textural terms. The definitions below were designed primarily for hand-sample observations where thin sections were not necessarily available. Coarse-grained equigranular refers to a rock that has been deformed but shows no marked grain-size reduction. Porphyroclastic refers to a polymodal grain-size distribution; medium-grained (1-5 mm) elongated porphyroclasts are embedded in a relatively fine-grained (typically <<1 mm) matrix. Gneissic refers to compositional banding in a ductilely deformed rock, where porphyroclasts are commonly elongated parallel to the banding. Schistose refers to the visibility of platy or prismatic metamorphic minerals that define a preferred dimensional orientation and typically a parallel fissility. Magmatic was used for those rocks displaying a well-developed shape-preferred orientation, but where the magmatic character of the individual crystals has not been destroyed by solid-state deformation. The terms mylonitic, ultra-mylonitic, and cataclastic were used in accordance with the definitions presented by Twiss and Moores (1992).
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 postkinematic alteration, and the relative timing of microstructures.
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 of Shipboard Scientific Party, 1995). We adopted and modified the thin-section description form used by the structural geologists during Leg 140 (Shipboard Scientific Party, 1992b) and 153 (Shipboard Scientific Party, 1995), and microstructural information is reported in the thin-section report.
The terms used to describe microstructures generally follow those used during Leg 153 and Leg 176. 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 associated with melt-enhanced diffusion-related mechanisms including (1) melt-enhanced diffusion creep (Hirth and Kohlstedt, 1995), (2) submagmatic microfracturing (Bouchez et al., 1992), and/or (3) contact melting or pressure solution (Means and Park, 1994; Nicolas and Ildefonse, 1996), we term the physical state crystal-plastic ± melt. Where fabric development is accommodated entirely by dislocation climb/creep, we use the term crystal-plastic to define the physical state of the rock. The other groups refer to rocks with magmatic and/or crystal-plastic texture overprinted by brittle deformation.