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doi:10.2204/iodp.proc.314315316.118.2009

Structural geology and geomechanics

Very good quality borehole resistivity images provide information on orientation of bedding, fractures, and breakouts at Site C0006 (Fig. F21). In addition, the overall stratigraphic sequence and seismic reflection images allow interpretation of the structural geology.

Observations

Bedding

Bedding dips are shallow to moderate with most dips <45°. Bedding orientation is partitioned downhole with mostly westward dips above 198 m LSF (logging Unit I) (Figs. F21, F22). Dips below 428 m LSF (logging Units III and IV) are northwestward. Dips between 198 and 428 m LSF (logging Unit II) are diverse and do not meaningfully cluster.

Natural fractures and fractured zones

Fracture analysis from borehole images is primarily based on orientation, dip, and resistive character relative to surrounding sediments. Most fractures are conductive (Figs. F21, F23) with more resistive fractures identified within logging Units II and III. Fracture dips range from ~30° to 80° with no clear pattern in dip magnitude variation between logging units.

Overall orientation of fractures within the borehole is scattered. However, when fractures are divided into the four logging units (Fig. F24), distinct trends can be identified. Logging Units I and II are characterized by fractures predominantly striking northwest–southeast. In contrast, logging Units III and IV are characterized by fractures striking northeast–southwest.

In logging Unit I a second set of fractures strike northeast–southwest and dip southeastward. For the major fracture trend (northwest–southeast), fractures evenly dip toward the northeast and southwest. In logging Unit II (predominant northwest–southeast trend) many fractures dip toward the northeast. Fractures in logging Unit III are moderately scattered but with the largest subset dipping toward the northwest (trending northeast–southwest). Finally, fractures are more difficult to identify in logging Unit IV because of the high background conductivity of the borehole. For those fractures that have been identified, the predominant trend is closer to west-southwest–east-northeast and the majority dip toward the north-northwest.

Borehole breakouts

Borehole breakouts occur from 188 to 729 m LSF but are not readily discernible at greater depths. The mean azimuth of the breakouts is 060° (Fig. F25A) with some variation but no clear trend with depth (Fig. F25B). The overall mean width of breakouts is 44° (Fig. F25C). The weighted average of breakout widths decreases downhole, although there is considerable scatter locally to larger widths (Fig. F25D).

Interpretations

Bedding

Overall the bedding dip orientations below 428 m LSF are consistent with north-northwestward directed shortening, which could be caused by the plate convergence. The generally westward dips above 198 m LSF may reflect a rotation by gravitational slumping caused by a locally steep southwestward component of the topographic slope (Fig. F26). The westward dips above 198 m LSF could therefore be explained by a northwestward tilt caused by plate convergence with a superimposed southwestward tilt caused by gravitational slumping.

Correlations of conductive fractures with seismically inferred faults

Although the borehole images do not show clear-cut fracture zones, some conductive fractures may correlate with seismically inferred faults (Fig. F27). For example, in the zone of subsidiary thrust faulting between 200 and 600 m LSF conductive fractures occur at 360 and 381 m LSF that are close to the seismically inferred thrust faults. Moreover, a well-developed conductive fracture zone and a fold occurs at 657 m LSF (Fig. F23) that may correlate with the main frontal thrust below the zone of subsidiary thrusts. Fractures recognized in the borehole images are generally moderately dipping. There may be others of shallow dip that are indistinguishable from bedding and not identified in our analysis. Notably, the fractures discussed here are not well-developed major deformation zones and their correlations with seismic data are tentative and will be tested by coring.

Logging Unit II includes four highly conductive intervals, probably sand beds. The sharp bases of the beds occur at 223, 244.5, 305, and 335 m LSF (Fig. F1). The seismic depth section (Fig. F27) shows thrust faults between some of these distinctive beds, suggesting displacement and potential repetition. The borehole images do not show obvious major faults or fracture zones separating these beds. However, as mentioned above, our detection of faults and fractures is biased toward those that are more steeply dipping. The hypothesis that these beds are repeated by thrust faulting can be tested by coring and associated dating.

Fractures

Although fracture orientations are not strongly clustered, a significant shift occurs below 428 m LSF. Northwest–southeast striking fractures in logging Units I and II (above 428 m LSF) may be related to the southwest component of slope. That is, the southwest and northeast dips could represent a conjugate fracture system related to gravitational failure along the southwest facing slope. Overall fracture orientation below 428 m LSF is consistent with northwestward directed shortening and breakout orientations. Both fractures and bedding show orientations at shallow depths that may reflect gravitational processes, whereas deeper fractures and bedding can be better explained by tectonic processes.

In comparison to IODP Sites C0001 and C0004 upslope, there are no distinct highly deformed zones or concentrations of fractures at Site C0006. Deformation at this frontal thrust location appears to be in a less evolved state than sites farther landward in the accretionary prism. Although Site C0006 shows some discrete conductive fractures that arguably correlate with seismically inferred faults, localization and intensity of structural features are significantly less than those found at Site 808 (a similar frontal thrust location) along the Muroto transect of the Nankai Trough (Ienaga et al., 2006; McNeill et al., 2004). This apparent greater intensity of strain at Site 808 relative to Site C0006 may reflect the thinner incoming sedimentary sequence at Site 808 (about half as thick as those formed at Site C0006) with approximately equivalent convergence rate or may potentially reflect different sediment properties and consequently differing style of deformation.

Borehole breakouts

The decrease in mean breakout width with depth may indicate increasing rock strength with depth, although considerable scatter toward higher values suggests weak intervals persist. The lack of obvious breakouts below 729 m LSF occurs just below the transition to logging Unit IV, which is interpreted as dominated by sand. Excessive washouts in this interval may obscure the breakouts, but some sandy units uphole also show large caliper readings and do produce distinguishable breakouts. The breakouts indicate an S_Hmax orientation of 330°, which is generally consistent with that observed at Sites C0001 and C0004 but slightly divergent from the S_Hmax expected from the convergence direction of the Philippine Sea plate and Japan (Fig. F26).

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