The structural study of rocks sampled during Leg 149 was devoted to understanding the mechanisms of extension of the continental lithosphere during a rifting episode. Synrift-related deformation is recorded in the basement rocks and the synrift sedimentary sequences. However, structural features present in the basement rocks may have been inherited from older tectonic events, and later events must be invoked to explain structures in the post-rift sequences cored during Leg 149.
This section deals with the macroscopic and microscopic descriptions of sediment and basement rock cores. The intention of this record is to identify and describe in a systematic and, if possible, a quantitative way the structural features in the cores and their orientation. The method used here is derived from that used during previous legs (Leg 131, Taira, Hill, Firth, et al., 1991; Leg 134, Collot, Greene, Stokking, et al., 1992; Leg 140, Dick, Erzinger, Stokking, et al., 1992; Leg 141, Behrmann, Lewis, Musgrave, et al., 1992).
Structures were measured on the face of the archive half or the working half of the split core, depending on the availability of the cores at the time of description. Initial data were recorded graphically section-by-section on a scaled structural visual core description form (VCD). The location of a structure was recorded in centimeters from the top of the section, according to ODP convention. Where a structure extended over an interval, the top and bottom of its range were recorded. More detailed information, particularly the orientation of structures, was recorded on a working core description form adapted from those devised during Leg 131 (Taira, Hill, Firth, et al, 1991) and on a computer spreadsheet. A numerical identifier was used to correlate individual structures between the three forms.
The descriptive terminology for macroscopic features is listed in Figure 10. The terms may be modified by added descriptions and sketches. Natural structures often are difficult to distinguish from those caused by drilling and coring disturbance. Planar structures having polished surfaces and/or linear grooves were regarded as tectonic-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, p. 42-43) were followed for the sediments.
Several problems are inherent to this study. Commonly, only part of the core associated with any one core interval is actually recovered, leading to a sampling bias that for structural purposes is particularly acute. Friable material from fault zones and low temperature shear zones in particular may be missing in cases of incomplete recovery. When faulted or fractured rock from such zones is recovered, it is often highly disturbed and its original orientation altered.
Determining the orientation of observed structures (and intrusive dikes) is difficult. First, structures were oriented relative to core reference coordinates (see below). This arbitrary reference frame will be related, if possible, to true north and true vertical using paleomagnetic, and Formation MicroScanner (FMS), data when available.
Our measurements of the orientations of structures observed in the cores were facilitated by a simple tool described in the "Explanatory Notes" chapter of Leg 131 Initial Reports volume (Taira, Hill, Firth, et al., 1991). The dip of a structure exposed in the split core was recorded according to the convention illustrated in Figure 11. The plane normal to the axis of the borehole was referred to as the apparent horizontal plane. On this plane, a 360° net was used with a "pseudonorth" (000°) direction defined as the bottom of the semicircular archive-half core. Thus, the face of the split core (the core face) was the plane 090°/90° (strike, dip), and the plane at right angles to the core face and parallel to the core axis was the plane 000°/90° (strike, dip). The apparent dip on the core face, either dipping toward the "east" or toward the "west" (looking at the face of the archive half of the core with its top looking upward), was measured. A second apparent dip was measured in one of the two planes, depending on exposure. Usually, we measured the dip in the plane 000°/90° at right angles to the core face, with the apparent dip direction in this plane being toward "north" or "south." Sometimes, we measured the dip angle on plane 000°/00°, where the strike of the plane was recorded relative to the core reference frame. When possible, a direct measurement of the strike and dip was performed. Dips recorded at this stage were based on the assumption that the long axis of the core was vertical; that is, deviations of the hole from vertical were ignored.
The great circle of cylindrical best fit of the two apparent dips (regarded as lines) was calculated using the Stereonet plotting program of R.W. Allmendinger (version 3.5). The measurements on the core face or on plane 000°/90° were taken replacing N, E, S, and W with their azimuths, that is, 0°, 90°, 180°, and 270°, respectively. The orientation of this "best-fit great circle" provided the working azimuth and dip of the observed structure (i.e., the azimuth and dip within the core reference frame).
The orientations of linear structures were recorded as "working trends." These were measured in the direction of plunge and referred to the core reference frame in the same way as planar surfaces. In the case of some small pieces, the vertical axes could be identified from their cylindrical shape, but not the up-going direction, as they may have rolled in the core barrel. In these pieces, the true dip of planar features was recorded, but not the direction of dip, which was meaningless.
The sense of fault displacement was recorded and referred to as normal, reverse, or strike-slip with sinistral or dextral movement. The apparent magnitude of displacement was measured on the core face and/or on the top of broken pieces. Offset was normally measured on a plane normal to the displacement plane, as straight-line separation between displaced markers. Mineral and/or mylonitic foliations and lineations could be measured in much the same way as faults and veins, but as the cut surfaces were chosen to be parallel to lineations or perpendicular to foliations, they could often be measured as true orientations.
The structures were oriented in the core reference frame, but not with respect to geographical coordinates. Re-orientation will be attempted using only paleomagnetic data, as multishot and FMS data acquired during Leg 149 are missing or of poor quality, particularly in the basement.
Paleomagnetic measurements were obtained on the core using a pass-through cryogenic magnetometer and on discrete samples taken from the core. The declination and inclination of the natural remanent magnetism (NRM), when available, can be used for orienting the structures. This method is useful on unoriented cores obtained using the RCB system, which often disrupts the core by breaking it into pieces that rotate independently of each other within the core liner. These drilling-induced rotations sometimes can be estimated and removed on the basis of the magnetic declination of the archive half, measured using the pass-through cryogenic magnetometer. This method assumes that each segment having a constant value is a homogeneous drilling piece. Paleomagnetic convention employs a "pseudonorth," or 000° direction, in the working half of the core which is 180° different from our reference frame, based on the archive half. Orientation of the structural features using magnetic data thus requires a rotation of 180° minus the magnetic declination. In some cases, a component of viscous remanent magnetization (VRM) parallel to the present-day magnetic field can be used to orient cores.
Thin-sections of basement cores recovered during Leg 149 were examined (1) to confirm macroscopic descriptions of ductile and brittle structures; (2) to determine the texture, the deformation at mineral scale, and the degree of recrystallization; (3) to provide information regarding the kinematics of high-temperature ductile deformationand the time relationship with brittle deformation; and (4) to document major structural zones and downhole textural variations. Where possible, the thin sections were oriented with respect to the core (so that the original attitude of the core axis is preserved).