Deformation structures at Site 1173 are sparse as expected for a site designed to be a reference site oceanward of the prism. Most of them were observed in Unit III, the lower Shikoku Basin sequence. Bedding dips <5° characterize the upper part of the hole; dips up to 25° were observed between ~375 and 550 mbsf and sporadically below this interval (Table T7; Fig. F11). It was often difficult, however, to detect actual bedding surfaces in the highly bioturbated hemipelagic sediments in the deeper units. The lack of systematic trends in bedding dips from core to core suggests that these dips are not due to hole deviation from vertical. Using paleomagnetic declination data (see "Paleomagnetism"), we were able to correct 42 bedding measurements to true geographic coordinates. The resulting distribution of poles to bedding shows scatter about vertical, but with a general north-south strike (Fig. F12).
Drilling disturbances, particularly biscuiting, fracturing, and brecciation, occurred throughout the cores, complicating identification of primary deformation structures. Moreover, many of these apparently artificial fractures are slickensided and lineated. However, several zones of breccia and planar, high-angle microfaults were observed entirely within coherent drill biscuits and hence were taken to be natural. The microfaults are concentrated between 250 and 275 mbsf (Fig. F11). Where displacement could be determined, the microfaults typically show a normal sense of movement on the order of 2-10 mm. At 270 mbsf, a healed fault showing apparent normal displacement is underlain by a zone of antithetic faults and chaotic breccia (e.g., interval 190-1173A-33X-4, 111-122 cm; Fig. F13). At ~440 mbsf, a 30-cm zone of foliated breccia, intensely broken in places and characterized by low-angle planar contacts between variably colored, elongate fragments of silty claystone, suggests a narrow interval of greater deformation extending from 190-1173A-47X-2, 140 cm, to 47X-3, 0-24 cm (Fig. F14). High-angle fractures are also present, one of which contains a white mineral, probably microcrystalline silica.
A few thin veins were observed below 500 mbsf. Sediment-filled veins, in one case associated with a possible fluid-escape structure (interval 190-1173A-53X-5, 90-97 cm; Fig. F15), crosscut and distort stratigraphic and diagenetic boundaries. A set of subvertical, white, mineralized extensional veins are present in a rare carbonate horizon at 615 mbsf (interval 190-1173A-65X-CC, 30-36 cm; Fig. F16). The white veins, possibly composed of calcite, extend across the carbonate layer and die out in the overlying silty claystone within a zone of fine mineralized pores. The veins crosscut a darker set of en echelon veinlike features (not visible in Fig. F16, being apparent only on the working half) with an apparent orientation perpendicular to the 41° dipping bedding plane.
In general, the deformation structures observed at Site 1173 are consistent with lateral extension and with vertical compaction of the sediments during burial. Deviations of bedding from horizontal may be associated with the bending of the Philippine Sea plate at the outer trench high of the Nankai Trough or related to the normal faults apparent on the seismic sections oceanward of the site. The core-scale faults and sediment-filled veins are likely to have formed in relatively soft sediments during burial and compaction. The greater abundance of deformation features within the lower Shikoku Basin sequence relative to the trench-wedge deposits also implies an early deformation history. The microfaults and extensional veins are reminiscent of similar features observed at Deep Sea Drilling Project (DSDP) Site 582, the reference site along the western Nankai transect (Shipboard Scientific Party, 1986a).
Documentation of the structures at Site 1173 provides a further important structural datum against which the more highly deformed sites within the prism can be compared and helps us to recognize structures at other sites that are a result of deformation within the prism.
Reconnaissance uncalibrated gas-permeability data were collected using the gas-probe permeameter (see "Structural Geology" in the "Explanatory Notes" chapter) and, encouragingly, showed good correlation with the kinds of values expected from the lithologic variations (Fig. F17). Measurements on cores from above 100 mbsf are unreliable because of the softness of the sediments and because the measurements were made while we refined the technique. In lithostratigraphic Unit II, the hemipelagites consistently showed poor uncalibrated permeabilities, whereas ash bands showed markedly higher values, possibly by at least four orders of magnitude. Figure F18 shows detailed variations within two of the ash bands and illustrates well the high resolution allowed by the probe permeameter. Measurements in Unit III are at the lower limit of the instrument's capabilities but appear to be uniformly low. Although thin ash bands are present in this unit, in contrast with Unit II, they are no more permeable than the hemipelagites, perhaps because of their greater degree of alteration.