Site 1175, drilled into an actively tilting lower slope basin, exhibits little evidence for tectonic deformation at the core scale; however, there is copious soft-sediment slumping in lithostratigraphic Unit I, the upper slope-basin facies (Fig. F9), and minor faulting in Units II and III. Structural data are presented in Table T7. Overall, the upper 205 m is characterized by extensive soft-sediment slump folding and contortion, whereas the lower interval, from 205 to 445 mbsf, is dominated by subhorizontal bedding and minor faults, mostly steeply dipping and of normal sense. Near the base of the hole, core recovery was extremely poor, but a few pieces of recovered sandstone exhibiting web structure and small faults, as well as a short interval of steeper dips up to 21°, may be consistent with penetration into accreted trench sediments.
In the upper 204 mbsf, APC coring permitted geographic reorientation of well-preserved soft sediment structure with data from the Tensor orientation tool (see "Structural Geology" in the "Explanatory Notes" chapter). Below this depth, extensive XCB biscuiting of the partially lithified sediments mostly precluded reorientation, although a few structures were reoriented with paleomagnetic declination data.
Starting only a few meters below seafloor in the first recovered core, slump features are well developed and common down to 204 mbsf. This slump-dominated interval coincides with lithostratigraphic Unit I. Large variations in bedding attitude, from horizontal to vertical, but only rarely clearly overturned, along with core-scale recumbent to isoclinal folds, chaotically mixed bedding, and rare boudins are the defining features of this interval. Fold amplitudes vary from a few centimeters, visible in the split cores, to several meters, inferred from downcore changes in bedding attitudes. True slump folding is distinguished from possible artificial core flow by the recovery of tight fold hinges with sharp and apparently undistorted intersections with the core margin (e.g., Fig. F10). Intervals from 0 to 20 mbsf, 54 to 83 mbsf, and 103 to 204 mbsf all exhibit disaggregated bedding and small folds marking slumped deposits. Interlayered with these strongly slumped zones are intervals from 20 to 54 and 83 to 103 mbsf that exhibit horizontal to gently inclined bedding.
Bedding dips from 0 to 204 mbsf are markedly variable with depth (Fig. F9), but zones of horizontal beds unaffected by slumping are also apparent. Attitudes (reoriented to true azimuth with Tensor tool data) of bedding exhibit a great deal of scatter but very broadly define a north-south girdle of dip directions (Fig. F11A). Where possible, we measured fold axial planes and axes of small folds observed in the cores. These few axial planes strike northwest-southeast and dip to the south-southwest (Fig. F11B), and the fold axes have westward trends and shallow to moderate inclinations.
We interpret this zone of slumping as having formed in response to syndepositional tectonic tilting of the slope basin as its southeastern margin was uplifted by slip along the OOSTs imaged in three-dimensional (3-D) seismic reflection data (see "Seismic Stratigraphy"). The seismic data indicate that the fanning reflectors of the basin dip generally northward in this location. The axial-plane data suggest northward vergence and, along with bedding, are consistent with downslope slumping toward the north.
Below 220 mbsf, bedding is much more consistently subhorizontal to gently inclined, with a few localized zones of chaotic bedding between 350 and 388 mbsf. It is possible that more extensive slumped sediments were also penetrated below 205 mbsf but were not recognized in the poorly recovered and badly biscuited XCB cores. Significant dips, up to 21°, were observed only in Core 190-1175A-43X at ~400 mbsf; however, core recovery from 400 mbsf to the total depth of 445 mbsf was only 4%, and little is known about this interval.
Beginning at ~298 mbsf, we observed a zone of small, steep faults of mostly normal and occasionally reverse sense, typically showing displacements of <1 mm to a few millimeters. These minor structures are present from 298 to 302 mbsf and sporadically from 340 to 435 mbsf and are similar to those observed at Sites 1173 and 1174. As at those sites, these faults do not exhibit strong preferred orientation (Fig. F12) and hence are interpreted as compaction-related features produced during sediment burial and dewatering, although they could also be extensional response to tilting and uplift of the basin.
In Core 190-1175A-44X at 406.9 mbsf, a several-centimeters-long interval in poorly lithified sand exhibits web structure (Fig. F13). At 425.8 mbsf, Core 190-1175A-46X recovered only one 2-cm-long piece of sandstone, which contains several low-angle small faults. Unfortunately, core recovery was too poor from 400 to 445 mbsf for us to meaningfully interpret the deformational environment in which these structures formed; the deformation observed is consistent with either deformed slope basin or frontally accreted trench sediments.
Determinations made with the gas permeameter at Site 1175, again subject to the provisos outlined in "Structural Geology" in the "Explanatory Notes" chapter, illustrate the huge influence of lithology on gas permeability. The data are summarized in Figure F14. As at the other sites, the overall values center on the hemipelagic clays that dominate the section. Results range chiefly between 10-16 and 10-17 m2, although the lowest part of the hole becomes siltier and yields slightly higher values.
A coarse, friable black ash at 23 mbsf gave an exceptionally high measurement (1.2 × 10-11 m2), and turbiditic sands between 60 and 90 mbsf also yielded large values (>8 × 10-12 m2). Thin bands of gray-white ash also gave relatively high determinations (~10-12 m2), in line with shallow, unaltered equivalents at the other sites.