In this section we describe the brittle structures, focusing on those with lengths that are greater than core diameter. Numerous smaller-scale structures are also visible, particularly when the core surface has dried, but detailed description of these is beyond the scope of the shipboard analyses. The occurrence of brittle deformational structures is restricted to the cored section below 300 mbsf, with the number of structures becoming much more abundant below 400 mbsf. The depth of apparent faulting begins at the top of lithologic Unit II, which is also the depth at which the sediment lithification changes from soft to firm. Faults are rare, however, down to the top of lithologic Unit III, which is where we commenced counting faults (Table T20, also available in ASCII format).
For the large-scale structures, we have assessed the abundance and type of structures that occur. For abundances, we have divided fractures into two types: open and healed. Most of the brittle structures were healed and filled with fine-grained dark gray clayey gauge. The width of the infill varies from 2 mm to a few tenths of a millimeter. No secondary mineralization or recrystallization (fibrous crystal growth) were observed. Open structures, though not as common as healed structures, were also abundant. These were probably drilling induced, although they were often formed along pre-existing weakness planes. These fractures were not considered in the structural interpretation unless they were partially filled.
Within the healed fracture category, we have further assessed the abundance of faults and joints. The distinction between faults and joints is based on the visible apparent displacement. Because sedimentary layering was rarely observed, the displacement was estimated primarily from the offset of burrows, carbonate nodule-like accumulations, and dissolution and precipitation rims around manganese or iron. As a result, the number of faults per meter is probably underestimated and the number of joints is overestimated. The fault planes, if accessible by open fracturing, usually show slickenside striation and structures indicating the direction of relative movement on the fault plane.
In addition, we measured the apparent dip of the large-scale structures with a conventional contact goniometer. The apparent dip is less than or equal to the real dip of any plane; therefore, dip angles that are reported here are minimum values. For curved fault traces (e.g., listric faults) the predominant apparent dip was measured. True dip measurements of faults are presented in "Downhole Measurements". Finally, we reconstructed the orientation of healed and open fractures and bedding planes relative to geographical north using paleomagnetic directions.
The types of observed faults are similar to those described for Site 1150 (See "Structural Geology" in the "Site 1150" chapter). Also similar to Site 1150, displacements recorded for the different types of faults range from some few millimeters to tens of centimeters. However, large displacements are visible only for normal faults and were more common than at Site 1150. The displacements along strike-slip faults is difficult to estimate, but the intense slickenside formation with deep grooves on polished surfaces and long mineral fibers indicate displacements of considerable size (several centimeters to decimeters).
An example of a steep normal fault is shown in Figure F42. The tip of this fault begins below a coarse-grained dark intercalation of a discontinuous sand/silt layer, and the vertical displacement amount increases downward along the fault. A specific feature of the hard sedimentary rocks in Hole 1151A is the sets of small pinnate fractures, parallel and S-shaped, commonly occurring as precursors of larger shear fractures. These tension gashes are so abundant that counting them was not feasible (Fig. F43). Some fractures consist of broad zones of anastomosing and branching joints, which resemble dewatering zones (Fig. F44).
Black and brown precipitation rims adjacent to fault planes and joint planes are very common, possibly indicating that past fluid movement occurred along the faults and joints. Usually, these rims extend in a cloud-like shape above the fracture planes, possibly indicating that the main flow direction was mainly parallel to the fractures, but with an upward component.
The different kinds of structures were counted, and the total number was divided by recovered length in meters (Fig F45; Table T20). The open fractures are most common. Significant numbers of joints are present below 500 mbsf but are rare above 500 mbsf. The values essentially show peaks from 650 to 720 mbsf, at 800 mbsf, and from 920 to 980 mbsf, with the amplitude of the peaks decreasing with depth. The number of normal faults is low, with one sharp peak at 680 mbsf and some small but broad peaks at 700-800 mbsf, 840-930 mbsf, and 1000-1110 mbsf. Strike-slip and reverse faults are very rare, with the strike-slip faults restricted to depths below 750 mbsf. Unidentified faults (i.e., those where the sense of motion could not be determined) occur predominantly in lithologic Unit V. This is probably caused by the changing lithology with completely different bioturbation that obscures features of fault motion. The pattern found at Hole 1150B, where the number of faults showed an overall increase with depth, does not occur at Site 1151.
We measured the orientations of healed fractures from recovered cores and then reoriented these into geographic coordinates using paleomagnetic declinations (Table T21, also available in ASCII format). Histograms of azimuths and dip angles are shown in Figure F46. The clusters of reoriented dip azimuths are weakly concentrated in 40°-160° and 260°-340° directions, with small peaks at 0°-20° and 200°-220°. The distribution of dip angles shows a unimodal pattern. Most angles range between 40° and 80°, which is the same distribution observed for Hole 1150B.
From the downhole variation of dip azimuth, we subdivided the data into domains (Fig. F47):
The poles to the fracture planes typically have bimodal distributions for all three domains (Fig. F48). In the upper domain these poles are concentrated in the southeast and northwest, whereas the poles for the other two domains are concentrated to the east and west.
Two dominant fracture directions, northwest-southeast and east-west, are also recognized in Hole 1150B. In Hole 1150B, however, the lower domain (1050-1180 mbsf) has the northwest-southeast orientation rather than the upper domain as in Hole 1151A. The middle domains are similar at both sites.
Variations of structural orientations with age as well as with depth will need to be investigated postcruise, in addition to the statistical significance of these variations. At both sites the overall combined poles give a west-northwest-east-southeast dominant direction (see "Accomplishments and Interesting Observations" in the "Leg 186 Summary" chapter).
Open fractures were reoriented in the same manner as healed fractures (Table T22, also available in ASCII format). The poles to the open fractures have been divided into the same domains as discussed above (Fig. F49). The orientations of the open fractures are more scattered than those for the healed fractures, although dip angles are concentrated at about 20°.
The tilt of bedding planes at Site 1151 differs from that of those observed below ~800 mbsf at Site 1150. Above this depth the bedding is nearly horizontal. Below 900 mbsf most bedding planes dip more than 10°, with the dip increasing gradually downhole (Fig. F50B; Table T23, also available in ASCII format). The reoriented dip azimuths of the bedding plane point dominantly to the east (Fig. F50A). Above 900 mbsf no dominant dip direction is evident, but below 900 mbsf the strata dip to the east.
We recognized clear striations that indicate the direction of fault movement on a few fracture planes. The number of planes with striations represents only a small fraction of all the fracture planes, and the planes with striations are more common below Core 186-1151A-94R, where the recovery rate of core decreased. In most cases the striations indicate that the motion along the fault is in the reverse sense.