Conventions for structural studies established during previous "hard rock" drilling legs (e.g., Legs 118, 131, 140, 147, 153, 176, and 179) were generally followed during Leg 209. 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.
All descriptions and structural measurements during Leg 209 were made on the archive halves of the cores. Following procedures described in the Leg 153 Initial Reports volume (Shipboard Scientific Party, 1995), data were entered into a VCD form (Fig. F2) used in conjunction with four spreadsheet logs. The structural sketches are intended to illustrate the most representative structures and crosscutting relationships in a core section; in addition, a brief general description of the structures is printed on the VCD form (see the "Supplementary Materials" contents list). Paper copies of spreadsheet forms were used for recording specific structures and measurements during core description. Separate spreadsheets were used to record data on
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 record seven separate types of measurements using the deformation intensity scales summarized in Figure F6. 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 crystal-plastic fabric intensity).
To maintain consistency of core descriptions, we used feature identifiers for structures similar to those outlined in Shipboard Scientific Party (1995). Modifications to this scheme are shown in the comments checklist (Table T4) and include designation of breccia type (hydrothermal [Bh], cataclastic [Bc], or magmatic [Bm]). Where brittle fabrics overprint crystal-plastic fabrics or deformation was "semibrittle," a note was made in the comments section and documented in the cataclastic fabrics (brittle structures) spreadsheet (see the "Supplementary Materials" contents list).
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 archive half of the core (Fig. F7) or, if the feature does not appear in the center of the core, the depth of the centroid of the feature in the archive half of the core. Crosscutting relationships were described in intervals delimited by top and bottom depth. Apparent fault separations of planar markers along faults were recorded as they appeared on the cut face of the archive half of the core and the end of broken pieces. Separations seen on the vertical core cut face were treated as dip-slip components of movement and labeled either normal or reverse for faults inclined <90°; their displacement in millimeters was also recorded. Shear sense indicators were also marked on the core barrel sheets. For vertical fractures, vertical separations were recorded as up or down (e.g., up to the west or down to the east). Separations of features visible on the upper and lower surfaces of core pieces (horizontal planes) were treated as strike-slip components of movement and either marked sinistral or dextral. Separations were measured between displaced planar markers, parallel to the trace of the fault. Slickenside and/or slickenfiber orientation trend and plunge measurements or the trend and plunge direction of the slip line between offset linear markers were incorporated wherever possible to determine dip-slip, oblique-slip, or strike-slip components. Spinel lineations were determined on acid-leached oriented cubes sawn from representative core sections in the working half. The structures were oriented with respect to the core reference frame; the convention that was used for the core reference frame is explained in Shipboard Scientific Party (1995) (Fig. F7) and shown at the top of the comments box in the structural data spreadsheets (see "Supplementary Materials" contents list).
Planar structures were oriented using the techniques outlined during Leg 176 (Shipboard Scientific Party, 1999a). Apparent dips in the cut plane of the archive core were recorded as two-digit numbers (between 00° and 90°) with apparent-dip azimuth either as 090° or 270° (Fig. F7). A second apparent dip was recorded in a different orientation with different apparent-dip azimuths (usually either 000° or 180°). The two apparent dips and their azimuths were used to calculate the true dip and strike direction with respect to the core reference frame. These calculations were performed using a macro routine within each Excel spreadsheet.
The true dip and strike directions of the samples were reoriented using available paleomagnetic declinations to rotate the measurements to a common alignment. The orientation of this common alignment was chosen to have an azimuth of zero, but this alignment does not necessarily correspond to true north because of the effects of tectonic rotation on the orientation of the magnetic declination direction. The data were plotted on lower-hemisphere stereographic projections using the shareware of R. Allmendinger (www.geo.cornell.edu/geology/faculty/RWA/RWA.html) and the careware of D. Mainprice (www. isteem.univ-montp2.fr/TECTONOPHY/petrophysics/software/petrophysics_software.html).
A semiquantitative scale of deformation and magmatic and alteration fabric intensities was used by the shipboard structural geologists during core description. This scale, shown in Figure F6, was modified from the deformation scales used during Leg 147 (Shipboard Scientific Party, 1993, table 2) and the fabric intensity scales used during Legs 153 and 176 (Shipboard Scientific Party, 1995, fig. 13; 1999a, table T2) and conforms to the deformation texture scales used by Cannat et al. (1991) and Dick et al. (1991). Wherever possible we have assigned specific values to intensity estimates (e.g., vein spacing or matrix percentage in a cataclastic zone) to maintain consistency throughout the core. However, for some categories this proved difficult (e.g., the intensity of any crystal-plastic fabric), in which cases we used qualitative estimates of intensity based on hand-specimen and thin section observations. We documented six distinct types of fabric intensity measurements (Fig. F6):
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 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. Some sections of the core contained small subhorizontal microfractures commonly related to unloading accompanying drilling. To avoid unnecessary measurements of these drilling-induced features, a column was added to the cataclastic fabric spreadsheet to indicate the density of these features using the same scale as for joints.
Abyssal peridotites represent a special problem for grading crystal-plastic deformation, as they have usually been emplaced from the Earth's deep interior by high-temperature crystal-plastic creep processes and, hence, lack a primary igneous texture. They generally have either protogranular or porphyroclastic textures when unmodified by relatively shallow deformation processes associated with unroofing and exposure to the seafloor. Protogranular textures are generally the earliest fabric and are characterized by smoothly curved grain boundaries with complex cusps and lobes (Fig. F6). However, alteration has generally entirely obscured olivine grain size and shape in hand specimen in abyssal peridotites, so the visual core description is almost entirely based on pyroxene and spinel textures. Rocks with a purely protogranular texture are graded 0, as this is the earliest formed texture and may or may not have a preferred crystallographic mineral fabric or a shape fabric. Porphyroclastic textures are generally superimposed on protogranular, and frequently elements of both are present. In this case we used the name of the predominant texture, but the intensity grade 0.5 indicates that elements of both textures are prominent. Rocks with porphyroclastic texture are graded 1 if there is only a weak or no pyroxene shape fabric developed. When pyroxenes exhibit a significant shape fabric, then the sample is graded a 2 under the crystal-plastic field, noting that at this grade only a few protogranular textural elements are present. These samples are still referred to as porphyroclastic. When the shape fabric starts to become strong and a foliation develops, accompanied by significant grain size reduction (protogranular abyssal peridotites generally have a pyroxene grain size of around 3–6 mm), the texture is referred to as protomylonite and graded 3 under crystal-plastic deformation. At this grade there are generally no protogranular textural elements left. When the peridotite has significant grain size reduction and consists of a fine-grained mass of olivine with embedded pyroxene porphyroclasts and a prominent foliation, the rock is graded 4 and listed as a mylonite. If there is no visible foliation due to extreme grain-size reduction, the rock is listed as an ultramylonite and graded 5. This deformation scale closely parallels that used for gabbro here and during previous legs and represents very similar or the same intensities of deformation at each grade.
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 (where obvious) 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. 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. 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 (see the "Supplementary Materials" contents list).
A large variety of microstructures occur in gabbros (see "Igneous Petrology" and "Structural Geology" in Shipboard Scientific Party, 1999b). For the purposes of entering data into spreadsheets here, a number of textural types characterized by specific microstructural styles were used based on the Leg 176 and 153 systems, modified slightly to accommodate description of peridotites. These are summarized in Table T5. It is possible that superposition of different microstructures or deformation mechanisms may occur during 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. Where local crystal-plastic fabrics are 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.