Site 1178 yielded a broad range of deformation structures consistent with penetration through slumped slope deposits into the more highly deformed rocks of the accretionary prism below (Table T6). We have divided the sequence into four structural domains (Fig. F9). Domain I (0 to 200 mbsf) comprises the slope sediments. Intervals of variably dipping beds, reaching vertical in places, are present in the silty clays, commonly bounded by subhorizontal contacts or sand layers. We interpret this arrangement to reflect a series of discrete slump packages. Paleomagnetic and Tensor tool reorientations indicate a general east-west strike for the bedding (Fig. F10A), presumably reflecting the local bathymetry of the time.
Domain II (200 to 400 mbsf) consists of accreted sediments that appear to show only small-scale deformation features, although poor core quality precluded good observations. Structures include minor high-angle normal faults, sparse deformation bands, and a few thin, black fault seams. Few of these could be reoriented because of the small size of the drilling biscuits. Bedding dips are gentle to moderate (Fig. F9) and show no clear pattern after reorientation (Fig. F10B).
Domain III (400 to 506 mbsf) is characterized by marked deformation, although the intensity varies and is greatest at the base of the domain. The domain extends downward from 400 mbsf, where it is seen both in the lowermost cores from Hole 1178A and the uppermost cores from Hole 1178B (to 506 mbsf). The deformation is manifested in Hole 1178A in two ways, depending on lithology. In silty clays, it is recorded by an anastomosing, low-angle, penetrative foliation and an incipient breccia with irregular fine black seams (Fig. F11). In sands, it is represented by crisscrossing ribs, <1 mm across (Fig. F12), in a weblike arrangement.
Deformation in Hole 1178B has produced four chief effects in Domain III. First, bedding dips are nowhere horizontal and range up to 55° (Figs. F9, F13A). Second, a more or less penetrative foliation has developed in some silts and sands and is clearly oblique to bedding (Figs. F13B, F14). The sediment tends to break along these planes, and hence the fabric could reasonably be termed a cleavage, a structure that is little documented in the literature on poorly lithified and shallowly buried sediments. Third, many of the sediments both in this domain and in underlying undeformed intervals show a tendency to break into planes with an appearance similar to the foliation mentioned above but with an orientation parallel to bedding. Because of this bedding-parallel nature and its presence in undeformed sediments, we term this structure fissility, although its relation to the similar-looking bedding-oblique foliation in deformed intervals remains unclear. Fourth, sets of fractures are common, especially in more clayey materials, varying in spacing and orientation and causing the sediment to brecciate into slickensided fragments that are roughly trapezoidal in shape and a few millimeters to a centimeter or more in size (Figs. F13C, F15). Several of the fracture sets crosscut and clearly postdate the foliation/fissility. They may denote faulting superimposed upon the more ductile pervasive strains in the prism recorded by the foliation.
At the base of Domain III (506 mbsf), the fracture sets are spaced as little as a few millimeters and slickenlines on the fracture surfaces are uniformly approximately downdip. In places, the fractures have an anastomosing form, with the resulting lenticles having distinctly polished surfaces. Consequently, the sediment has the appearance of scaly clay. Below 506 mbsf, the spacing of fractures rapidly increases such that by 509 mbsf, deformation of the cores is negligible. Bedding dips, too, show an abrupt decrease (Fig. F9). The intensity of brittle deformation above 506 mbsf and contrast in fracture density and bedding dips across this interval suggest that this zone represents a major thrust fault in the prism.
Domain IV, therefore, is characterized by much less deformation than Domain III. It extends from 506 mbsf to the base of the hole at 673 mbsf, although significant deformation effects are seen at 550 mbsf and from 622 mbsf to the bottom of Hole 1178B. The orientation data are summarized in Figure F16. From the shallow bedding dips at ~509 mbsf, values steadily steepen to 550 mbsf, where core-scale structures also peak, and reach a maximum at 600 mbsf.
Below 550 mbsf, most of the cores show little deformation, although bedding dips are mostly in the 25°-45° range. Fracture sets are sporadically distributed, and some sand intervals preserve a near-vertical anastomosing grain fabric, in some cases crosscut by subhorizontal, sediment-filled veinlets. Between 622 and 673 mbsf, however, such veinlets are mutually crosscutting in a weblike arrangement (Fig. F17), fracture intensity increases, and the clay-rich sediments are criss-crossed by irregular black seams similar to those observed near the base of Hole 1178A. The sediment has the appearance of an incipient breccia, in places scaly and foliated (Fig. F18). The sets of fractures locally developed in this interval (622-673 mbsf) appear similar to those in Domain III. We therefore interpret these structures to represent zones of shearing, presumably additional thrust faults in the prism. The thrusts in Domain IV may contribute to the apparent thickening of the C4A Subchron in the magnetostratigraphy data (Fig. F19) and possible repetitions in the biostratigraphy. Thus Domain IV appears to contain several thrust slices, each internally deformed less than the sheet overlying the major thrust at the base of Domain III.
In summary, Site 1178 consists structurally of slope deposits containing packets of slumped sediment overlying an accretionary sequence that comprises several thrust sheets, one of which occupies a zone of shearing >100 m thick.
Uncalibrated gas-permeameter measurements show a range of values similar to other Leg 190 sites, and the variation is again correlated with differences in lithology (Fig. F20). Fractured and brecciated intervals representing thrust faults were impossible to measure because of the probe design of the instrument. The background hemipelagites gave measurements ranging from 10-17 m2 to just over 10-16 m2 and showed no tendency to decrease with depth. As at other sites, there is no obvious correlation between measured uncalibrated permeability and average porosity measured shipboard. At the top of the section, some of the higher values were given by volcanic ashes, but the other high determinations were all due to silt and sand. Even at depths of ~600 mbsf, the turbiditic sands are sufficiently uncemented to give values as high as almost 10-12 m2.