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doi:10.2204/iodp.proc.304305.102.2006

Structural geology

Expeditions 304 and 305 generally followed the same conventions for structural studies in order to maintain compatibility of the data. The conventions adopted during Expedition 304 were based on previous “hard rock” drilling programs (e.g., ODP Legs 118, 131, 140, 147, 153, 176, 179, and 209). Several minor changes in nomenclature and procedure were adopted during Expedition 305. These changes are described below. Because no peridotite was found during Expedition 305, its note was mostly excluded. Where procedures followed directly from previous cruises, references to the appropriate ODP Initial Reports “Explanatory Notes” chapters are given.

Overview of macroscopic core descriptions

Whole cores were oriented for cutting prior to curation. Cores were marked to maximize dip on planar structures so that the dominant structure dips toward 270° in the core reference frame (i.e., toward the right looking down at the cut surface of the archive half of the core). Where no obvious structures were present, cores were marked to maximize contiguity with adjacent core pieces. The sign convention for the core follows the paleomagnetic conventions of a right-hand rule with +z vertical and down the core (Fig. F10).

All descriptions and structural measurements during Expeditions 304 and 305 were made on the archive half of the core. Following procedures described in the Leg 153 Initial Reports volume (Shipboard Scientific Party, 1995), data were entered into a VCD form (Figs. F2, F11) used in conjunction with two spreadsheet logs (see “Supplementary material”). 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 “Supplementary material”). During Expedition 304, paper copies of spreadsheet forms were used for recording specific structures and measurements during core description. Separate spreadsheets were used to record structural data on the following

• Alteration veins
• Brittle structures: breccias, faults, joints, drilling-induced fractures, serpentine foliation, cataclastic fabric intensity
• Crystal-plastic deformation: mylonitic foliation, sense of shear
• Igneous structures: magmatic foliation, compositional layering, igneous contacts

During Expedition 305, specific structures and measurements during core description were directly typed into spreadsheet forms. Separate spreadsheets were used to record structural data on the following:

• Brittle structures: breccias, faults, joints, drilling-induced fractures, serpentine foliation, cataclastic fabric intensity, alteration veins
• Magmatic and crystal-plastic structures: magmatic foliation, compositional layering, igneous contacts, crystal-plastic foliation, sense of shear

The description and orientation of all features were recorded using curated depth so that “structural intervals” could be correlated with lithologic core descriptions. The spreadsheets were organized to record six separate types of measurements using the deformation intensity scales summarized in Figure F12. During both Expeditions 304 and 305, the structural geologists worked together during the same shift to minimize measurement inconsistencies. During Expedition 304, 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). During Expedition 305, the structural geologists were divided into two teams: one was responsible for high-temperature structures including both magmatic and crystal-plastic deformation, and the other was responsible for low-temperature structures including veins, cataclasis, serpentine foliations, and so on. Each team was responsible for making a specific set of observations throughout the entire core (e.g., characterization of crystal-plastic fabric intensity).

Nomenclature

We used feature identifiers for structures similar to those outlined by the Leg 153 Shipboard Scientific Party (1995). Modifications to this scheme are shown in the comments checklist (Table T5). Where brittle fabrics overprint crystal-plastic fabrics or deformation was “semibrittle,” a note was made as “brittle-plastic” and documented in the magmatic and crystal-plastic structures spreadsheet (see “Supplementary material”).

The shorthand notation for structural units was as follows (more detail in Table T5):

• Brittle structures:
• Vein (V): hydrothermal (Vh), microfault with fibrous minerals (Vf), cataclasis accommodated (Vc)
• Breccia (B): hydrothermal breccia (Bh), cataclastic breccia (Bc)
• Fault (F): fault zone (FZ)
• Foliation (S): serpentine foliation (S or Ss)
• Magmatic and crystal-plastic structures:
• Vein (V): magmatic vein (Vm)
• Foliation (S): magmatic foliation (Sm), crystal-plastic foliation (Sp), brittle-plastic foliation (Bp)
• Alteration (A): alteration front (Af)
• Contacts (c): igneous contact (Ic)

Structural measurements

Structural features were recorded in centimeters from the top of each 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. F10) or, if the feature does not appear in the center of the core, the depth of the centroid of the feature projected to the center of the core. Where they occur, crosscutting relationships were described with core section depth. Apparent fault displacements of planar markers were recorded as they appeared on the cut face of the archive half of the core. Displacements observed on the vertical core cut face were treated as dip-slip components of movement and labeled in spreadsheets as either normal or reversed for faults inclined <90°; their displacement in millimeters was also recorded. Shear sense indicators were also marked on the spreadsheets. For vertical faults, displacements were recorded as up or down (e.g., west side up or east side down [+y is east]). Offset features visible on the upper and lower surfaces of core pieces (+z is upper side) were treated as strike-slip components of movement and marked either sinistral or dextral. Displacements were measured between offset markers displaced 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. The structures were oriented with respect to the core reference frame; the convention that was used for the core reference frame (Fig. F10) is shown at the top of the comments box in the structural data spreadsheets (see “Supplementary material”).

Planar structures were oriented using the techniques outlined during Leg 176 (Shipboard Scientific Party, 1999). Apparent dips in the cut plane of the archive half were recorded as two-digit numbers (between 00° and 90°) with apparent-dip azimuth either as 090° or 270° (Fig. F10). 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 spreadsheet (see “Supplementary material”).

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.dstu.univ-montp2.fr/​TECTONOPHY/​index.html).

Fabric intensities

A semiquantitative scale of deformation and alteration fabric intensities was used by the shipboard structural geologists during core description (e.g., Cannat et al., 1991; Dick et al., 1991). These scales, shown in Figure F12, were modified from the deformation scales used during Legs 176 (Shipboard Scientific Party, 1999) and 209 (Shipboard Scientific Party, 2004). Where possible, we assigned specific values to intensity estimates (e.g., vein spacing or matrix percentage in a cataclastic zone). For some categories, however, classification is difficult (e.g., the intensity of any crystal-plastic fabric), in which case we used the previously adopted qualitative estimates of intensity based on hand-specimen and thin-section observations. We documented six distinct types of fabric intensity measurements (Fig. F12):

• Magmatic (Mf): presence and intensity of any shape-preferred orientation of magmatic phases. Four levels, from no shape-preferred orientation (0) to strong shape-preferred orientation (3), were used.
• Crystal-plastic (CPf): six levels of deformation intensity were used, ranging from a lack of any crystal-plastic fabric (0), through three stages of foliation and porphyroclast development (1–3), to mylonitic and ultramylonitic fabrics (4–5). The textural criterion used for gabbroic rocks, on which this was based, was modified slightly for peridotites. Occasionally, it proved difficult to differentiate between crystal-plastic and cataclastic deformation in relatively high strain shear zones based on hand-specimen observations only. In this case, we introduced a new classification: brittle-plastic foliations (Bp).
• Cataclastic (Cf): six levels of deformation intensity were used with fabrics categorized depending on the percentage of matrix present within each cataclastic zone. Thin section descriptions, wherever available, significantly aided this categorization.
• Joints (J: fractures without displacement): four levels of joint density were used, depending on the average frequency of joints across a 10 cm depth interval along the long axis of the core. Joints were distinguished from faults (cataclastic features) by the lack of any identifiable offset. Some sections of the core contained small subhorizontal microfractures commonly related to unloading accompanying drilling. The same scale as for joints (0–3) was used for the density of these drilling-induced features.
• Veins (V): six levels of vein density were used, depending on the average frequency of veins across a 10 cm depth interval along the long axis of the core. Alteration veins are those which contain metamorphic phases. In some cases, these may have originally been igneous, but no primary phases or their characteristic pseudomorphs were recognized. Magmatic veins are compositionally distinct mineral segregations that may be concordant or discordant.
• Serpentine foliation (Ss or S): formation of strong planar fabrics may occur during serpentinization by development of closely spaced subparallel veins termed ribbon texture (O’Hanley, 1996). The texture is characterized by anastomosing, commonly cross-fiber, replacement serpentine veins. In general, the veins wrap around relict or pseudomorphed pyroxene grains with little evidence of shear offset. This texture may parallel preexisting crystal-plastic foliation, and its intensity may reflect stress state or the presence of a preexisting fabric in the rock. The strength of serpentine foliation is rated on a scale from 0 (massive, no foliation) to 3 (strongly foliated). If more than one obvious serpentine foliation was present, the orientation and intensity of all fabrics were recorded.

Thin section descriptions

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, 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, in the core reference frame described in “Structural measurements.” A large variety of microstructures occur in gabbros (see “Igneous Petrology,” p. 12, and “Structural Geology,” p. 54, in Shipboard Scientific Party, 1999). For the purposes of entering data into spreadsheets, a number of textural types characterized by specific microstructural styles were used based on the Leg 153 and 176 descriptions (Table T6). There is a superposition of different microstructures or deformation mechanisms attributable to different stages of 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. We used four intensity categories:

• 0 = absent
• 1 = questionable
• 2 = weak
• 3 = strong

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. For crystal-plastic fabrics, we used six categories:

• 0 = absent
• 1 = only local nests of neoblasts
• 2 = ~10% neoblasts
• 3 = 40%–90% neoblasts
• 4 = mylonitic with rare porphyroclasts left or other independent evidence for high strain in rocks with no mylonitc appearance
• 5 = ultramylonitic

In addition, during Expedition 305, we estimated relative stress levels based on neoblast size:

• 0 = no neoblasts
• 1 = neoblast size >200 µm
• 2 = a typical range of neoblasts (50–250 µm)
• 3 = neoblast size <100 µm

A table with thin section intensities and short comments (30405STR.XLS) is available in “Supplementary material.” For a description of the methods for calculating averages from logged intervals, including MatLab calculations, see “Supplementary material.”

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