Structural data were determined from RAB images using Schlumberger's Geoframe software (see "Interpreting Structure from RAB Images" in the "Explanatory Notes" chapter). All structural interpretations were made using RAB images of medium-focused button resistivity, which images at a penetration depth of 3 in (7.6 cm). Data were taken from Hole 1173B for the overlapping region of Holes 1173B and 1173C.
Deformation is sparse at Site 1173, as was observed in the cores and wireline data of Hole 1173A (Shipboard Scientific Party, 2001). Apparent bedding dips are low throughout the hole and range from ~0° to 35°, with the majority <~5° (Fig. F17). Based on calculations of maximum resolution of RAB data and hole width, minimum resolvable dip is ~4°-7°. Bedding dips exceeding ~5° occur at 50-200, 280-560, and 600-720 mbsf (above basaltic basement). Bedding dip is subhorizontal between these regions (200-280 and 560-600 mbsf). An increase in average bedding dips is observed below ~370 mbsf, but subhorizontal bedding is still common below this depth (Fig. F17). An abrupt increase in bedding dip was observed at the same depth in cores from Leg 190 Hole 1173A (Shipboard Scientific Party, 2001). Direct comparison of Leg 190 and 196 data (Fig. F17) suggests greater bedding dips for the former. However, if dips <5° (approximate minimum resolvable dip for both Leg 190 core data and Leg 196 RAB image data) are removed and the remaining data are replotted, moving averages are very similar. The apparent discrepancy is therefore thought to be a result of the sampling and recording methods of these two types of subhorizontal data. The nonsystematic variability of bedding dips with depth at Holes 1173B and 1173C may reflect a slump rather than tectonic origin.
Fractures (including faults) are rare but fairly evenly distributed throughout Holes 1173B and 1173C (Fig. F18). Fracturing was identified between 80 and 200 mbsf, 320 and 570 mbsf, and 660 and 700 mbsf, with the majority of deformation concentrated between 380 and 520 mbsf. This zone correlates with increases in bedding dip (Fig. F17). Specific concentrations of fractures within this interval occur at 378-384 and 410-417 mbsf. Fracture dip is high (40°-80°), with the majority dipping 50°-70°. This is inconsistent with low-angle thrust faulting as observed within the accretionary wedge (not expected at the reference site seaward of the deformation front), but consistent with normal or reverse faulting. Where offset of stratigraphic markers was observed in the RAB images, displacement was normal (e.g., Fig. F19) and of the order of 10-20 cm. Conjugate faults are observed locally. Fracture strike shows random orientation (Figs. F18, F20). A second example of a high-angle fault cutting a high-resistivity ash layer is shown in Figure F19B. Fractures are both resistive and conductive with nonsystematic variation with depth in the hole. Resistive fractures dominate and many fractures show a change from high to low resistivity along the fault plane (across the RAB image). High conductivity would suggest open fractures with high porosity whereas high resistivity might reflect nonconductive clay gouge, mineralization, or porosity collapse due to compaction. However, mineralization and veins were rare in cores recovered from Hole 1173A (Shipboard Scientific Party, 2001) with a few exceptions in lower cores (~600 mbsf). High-angle fractures (68°-73°; south trending) are also observed within the basaltic basement at ~730 mbsf. These fractures cut a highly conductive near-vertical feature, which is interpreted as a borehole breakout induced by drilling (Fig. F21).
At 500 mbsf, bands of increased and heterogeneous ("mottled") resistivity are observed, with poorly defined bedding. More fractures are observed within these zones but identifying individual planar fractures is hindered by the heterogeneity of resistivity within the RAB image. These zones may indicate brecciation, intensive fracturing filled with high-resistivity sediments, or partial porosity collapse due to compaction. Sediment-filled fractures were noted at this depth in Hole 1173A (Shipboard Scientific Party, 2001).
In general, structural data from RAB images at Holes 1173B and 1173C show good correspondence with seismic data (i.e., where visible in data, subhorizontal bedding from ~0 to 300 mbsf followed by increased stratigraphic dip below this level). Deformation at Site 1173 (including high-angle faulting, conjugate faults, random orientation, and normal displacement) is consistent with a model of extensional faulting related to compaction and burial. Extensional faulting is also observed in seismic sections crossing Site 1173 (Figs. F3, F4). Extensional deformation may also be related to basement structures (ridge related) or flexure of the incoming subducting plate.