Rocks from Site 1183 exhibit relatively few structural features. The sedimentary rocks that overlie basaltic basement are characterized by horizontal bedding planes and appear to have experienced little tilting or faulting (see "Lithostratigraphy"). Veins are common in the basaltic basement, which has been subdivided into eight units on the basis of sedimentary interbeds, basalt texture, and phenocryst abundance and size (see "Igneous Petrology," Fig. F36). No ductile or cataclastic fault zones, or tectonic breccias were encountered in the cores.
Location, vein width, and mineral filling were documented for 849 veins at this site. A total of 303 calculated and measured vein orientations are included in this data set. The graphic presentation of true dip, fracture density, and vein abundance in Figure F87 provides some information about the relation between vein characteristics and basement depth. This figure graphically depicts the measured and calculated dip inclinations vs. depth. Fracture density has been calculated per meter of core section (see "Structural Geology" in the "Explanatory Notes" chapter). Vein abundance has been calculated by determining the total thickness of the veins in millimeters per meter. For veins defined as hairline, we assigned a thickness of 0.25 mm, assumed to be a representative average for these types of veins.
The distribution of true vein dip shows no systematic variation with depth in basement (Fig. F87). Although the graph is affected by a pronounced data gap in Core 192-1183A-60R, which was followed by a change of the drill bit, the fracture density appears to be high at 1145-1165 and ~1195-1210 mbsf. These zones of high fracture density are also distinct in the graph of vein abundance, which shows maxima in approximately the same intervals.
The absence of downhole logging information and structural marker planes (e.g., sedimentary bedding and basaltic layering) precludes reorientation of the measured veins and the calculation of dip azimuths. Paleomagnetic results may enable the reorientation of some of the veins on the larger pieces during shore-based analysis of the data (see "Paleomagnetism") and allow calculation of true dip azimuths.