The structural study of rocks sampled during Leg 180 was devoted mainly to understanding the mechanisms of extension in the footwall and hanging wall of a low-angle fault system just ahead of the tip zone of a propagating oceanic rift. Structural observations were made on unlithified sediments and sedimentary rocks, and on igneous and metamorphic rocks.
Conventions for structural studies established during previous drilling legs (e.g., Leg 131, Shipboard Scientific Party, 1991; Leg 134, Shipboard Scientific Party, 1992b; Leg 141, Shipboard Scientific Party, 1992a) were generally followed during Leg 180. In addition, approaches used recently for the study of structures in sediments and sedimentary rocks were also employed (e.g., Leg 160, Shipboard Scientific Party, l996a). As far as possible, systems for recording soft- and hard-rock structures were merged into a single unified system.
Given the specific aims of Leg 180, special attention was paid to the vertical variations of strain in the vicinity of the fault zone. The descriptions focused on structural features concerning the style of deformation (e.g., flattening and simple shear), the strain gradient, and the strain kinematics.
The following sections deal with the macroscopic and microscopic descriptions of sediments and hard-rock cores. The aim is to identify and describe the observed structural features in a systematic and, if possible, quantitative way and then to orient them in the core reference frame.
Descriptions and structural measurements were based on observations of the working half of the core (see "Structural Measurements"). We followed the procedures used for the description of hard rocks in the Initial Reports "Explanatory Notes" chapters for Leg 153 (Shipboard Scientific Party, 1995a) and Leg 176 (Shipboard Scientific Party, 1999b). For soft rocks we followed the system employed during Leg 160 (Shipboard Scientific Party, l996a). Gradation and overlap between different features were identified by adding modifiers, descriptive comments, and sketches. The structural identifiers, listed in Table T4 as a checklist, were entered into the structural logs that are included in the "Supplementary Materials" contents list.
A structural log spreadsheet (Table T5) was used to record all forms of structural data. The following categories of structural identifiers were recorded: (1) drilling-induced structure, bedding, and unconformity; (2) slump folds; (3) joints; (4) folds (with the hinge-zone angle noted when possible); (5) crenulation cleavage, disjunctive cleavage, and foliation; (6) fault, brittle shear zone, and ductile shear zone; (7) sedimentary breccia and tectonic breccia (clast/matrix ratio as well as maximum and average clast size); and (8) veins and dikes. The structural log spreadsheets are provided in Excel 5.0 format in "Supplementary Materials."
For each of the above structures, the intensity of deformation was noted in three categories. The cleavage/foliation category was further subdivided into more specific structures as appropriate. Similarly, different linear structures were also assigned specific symbols (e.g., slickenlines and mineral lineations). Sense of shear was recorded where possible. The description and orientation of structural features were recorded using curated depths so that structural intervals could easily be correlated with other lithologic intervals.
Natural structures are sometimes difficult to distinguish from those caused by drilling and coring disturbance (Kopf and Flecker, 1996). In general during Leg 180, planar structures having polished surfaces and/or linear grooves were regarded as tectonic induced rather than drilling induced. In zones of brecciation, features were attributed to drilling disturbance if their tectonic origin was in doubt. The recommendations of Lundberg and Moore (1986), as well as the approach adopted during Leg 160 (Shipboard Scientific Party, 1996a), were generally followed with regard to the recognition of drilling disturbance.
When dealing with sediments and weakly lithified sedimentary rocks, if the core length was relatively devoid of features we did not draw that interval, but merely marked the features on the data table (Table T5). The feature number was used to link data in the data table with drawings of the feature on the VCD form. In the data table, the feature is identified using the codes defined in the accompanying list of structural terms (Table T4).
Hard-rock structures were also recorded on the structural log spreadsheet as shown in Table T5.
All the 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 "Explanatory Notes" chapter of the Leg 153 Initial Reports volume (see fig. 15A, Shipboard Scientific Party, 1995a).
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 observed 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 taken with respect to the core reference frame (working half) (Fig. F10). The convention we used for the core reference frame is detailed in the "Explanatory Notes" chapter of the Leg 153 Initial Reports volume (Shipboard Scientific Party, 1995a) and shown at the top of the comments box in the structural data spreadsheets. 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), 153 (Shipboard Scientific Party, 1995a), and 160 (Shipboard Scientific Party, 1996a). 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 a two-digit number (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 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 and relative to the core reference frame. Fold hinge lines were identified in two mutually perpendicular sections and measured as discussed in the "Explanatory Notes" chapter of the Leg 153 Initial Reports volume (Shipboard Scientific Party, 1995a).
The two apparent dips and dip directions measured for each planar feature were used to calculate a true dip, following classical geometrical techniques.
A key for fabric intensity (Fig. F11) was used to refine identifier descriptions. Previous usage was modified to take account of the recovery during Leg 180. When feasible, (semi-)quantitative estimates of feature development were used to define a three-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 are only semiquantitative at best and could not be fully quantified during core description.
Intensity was recorded for the following categories:
Three textural classes were selected for macroscopic description. It must be stressed that these classes do not necessarily directly relate to any single physical parameter, such as stress or strain. The definitions below were designed primarily for hand-sample observations where thin sections were not necessarily available. "Gneissic" refers to composi-tional banding in a ductilely deformed rock in which 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. The terms "Mylonitic," "Ultra-mylonitic," and "Cataclastic" were used in accordance with the definitions presented by Twiss and Moores (1992; p. 53).
Thin sections of basement cores recovered during Leg 180 were examined to (1) confirm macroscopic descriptions of ductile and brittle structures; (2) determine the texture, the deformation at mineral scale, and the degree of recrystallization; (3) provide information regarding the kinematics of high-temperature ductile deformation and the time relationship with brittle deformation; and (4) document major structural zones and downhole textural variations. Where possible, the thin sections were oriented with respect to the core to preserve the original attitude of the core axis.
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, in the "Explanatory Notes" chapter of Shipboard Scientific Party, 1995a).