Previous   |    Next

doi:10.2204/iodp.proc.314315316.113.2009

Structural geology and geomechanics

Structural interpretations are based on resistivity image data (see the “Expedition 314 methods” chapter). Our interpretations were primarily drawn from the shallow level of investigation, with a 34 cm estimated diameter of investigation, in Hole C0001D. The small variability of the images between shallow, medium, and deep levels of investigation with the resistivity imaging tool indicates good quality data, even in a section between 529 and 629 m LSF where drilling problems were encountered.

Identification of bedding planes needed special care and was achieved by comparing images and logging data at this site. The process of dynamic normalization sometimes creates artifacts that have features similar to bedding planes. In order to reduce such misinterpretation, we compared several different dynamic and static normalization images with bit resistivity, ring resistivity, and gamma ray logs.

Bedding

Bedding dips are mostly gentle (<10°) throughout the upper 400 m LSF of the hole with the exception of higher values within logging Subunit IB (190.5–198.9 m LSF) and immediately beneath it (Figs. F39, F40, F41). Below 400 m LSF, bedding dips gradually become steeper (averaging ~10°–20° at the base of the hole) and include values up to 60°. Bedding dips are more difficult to discern in the deeper parts of the hole (below 200 m LSF and particularly between 800 and 880 m LSF); here the higher and more uniform resistivity values and/or lack of coherent bedding planes do not provide the necessary image contrast to easily interpret bedding. Interpretation of the logging Unit I/II boundary is discussed in “Conclusions.”

Natural fractures

Natural fractures can be divided into the three following classes (Fig. F40):

1. Steep conductive fractures (~75°–90°) trending northwest–southeast and most commonly occurring from ~640 m LSF to the base of the hole,
2. Shallower dipping conductive and resistive fractures (~30°–75°) trending northwest–southeast and occurring throughout the hole (including below 640 m LSF), and
3. Conductive and resistive fractures of variable trend but predominantly northeast–southwest that are distributed throughout the hole.

These three fracture classes are distinct from drilling-induced tensile fractures which are described separately.

The shallower dipping fractures (Classes 2 and 3) generally can be fitted well with a single sinusoid, although some fractures are less distinct and consequently result in uncertainties in dip and azimuth. The conductivity of these fractures is often difficult to determine, particularly where fractures interact with conductive borehole breakouts or tensile fractures. The fracture aperture was too small to measure in almost all cases. This narrow aperture makes these fractures more difficult to recognize than the steeper conductive fractures that occur in the bottom half of the borehole.

The steep conductive fractures (Class 1) commonly have apertures of 50–60 mm, as measured in the images (Fig. F42). The fractures are steep and can extend along the length of the borehole for ~10–20 m. The fractures are undulating and typically have been compositely approximated by several sinusoids along their entire length. Although steep conductive fractures occur from 640 m LSF to the base of the hole, they are most intensely developed from 785 to 875 m LSF, which correlates with a zone of decreased resistivity and higher resistivity-derived porosity (see “Physical properties”). Because the steep fractures are locally offset by shallower dipping, less conductive fractures (Classes 2 and 3), we believe the former are natural fractures, formed before the borehole was drilled, and predate the more shallowly dipping fractures. In a few cases, offset of the steep conductive fractures can be determined (Fig. F42) and show normal fault offset.

Disrupted zone

At ~529 m LSF, breakouts stop and shallowly dipping bands of resistivity (interpreted as bedding) become partially to completely disrupted. For example, at 550–551 m LSF, circular bodies of high-resistivity material are separated horizontally and locally enveloped by high-conductivity material. At 584 m LSF high-conductivity material appears to intrude upward into the overlying high-resistivity layer (Fig. F43).

Borehole breakouts and drilling-induced tensile fractures

Breakouts comprise a strikingly obvious feature in the upper half of the borehole (Figs. F39, F44). To 110 m LSF the breakouts are associated with fractures that are perpendicular to them (Fig. F44). We interpret these fractures as drilling-induced tensile fractures (DITFs). The breakouts are obvious to 530 m LSF and patchy in the lower half of the hole, but the associated DITFs are generally less apparent. In general, the DITFs are more common in the lower 400 m of the hole, whereas breakouts are more common in the upper 530 m of the hole. An exception is the upper 70–100 m LSF, where strong breakouts and DITFs coexist. The azimuths of all breakouts and DITFs are nearly exactly perpendicular (Fig. F45).

The variable occurrence and character of breakouts suggest distinct zones and boundaries. We interpret the abrupt change in breakout occurrence at ~530 m LSF (see “Disrupted zone”) as resulting from an increase in borehole pressure during drilling rather than physical properties of the sediments (see “Analysis of breakouts and drilling-induced tensile fractures”). This is in contrast to the occurrence of breakouts at Site 808 (McNeill et al., 2004; Ienaga et al., 2006), which showed a relationship with lithology. Within the 70–530 m LSF zone of strong breakouts, their character (conductivity, width, and homogeneity) allows division into three zones: ~70–200, 200–300, and 300–530 m LSF (Fig. F39). Transitions between these zones coincide with log character change and mark logging unit boundaries at Subunits IB/IIA and IIA/IIB, respectively. On a meter scale, no clear correlation is observed between changing sediment properties from logs and breakout character. See “Log characterization and lithologic interpretation” for further details. No major change in breakout character is observed with depth of investigation (shallow or medium versus deep) except for increased conductivity of the widest breakouts in the shallow image. No significant or consistent relationship between breakout width and hole depth is observed.

Analysis of breakouts and drilling-induced tensile fractures

Breakouts form perpendicular to the orientation of maximum horizontal principal stress (S_Hmax) and parallel to the minimum principal horizontal stress (S_hmin) (Zoback et al., 2003). Accordingly, in Hole C0001D S_Hmax is oriented 336° or north-northwest (Fig. F44). Because the breakouts and DITFs are oriented parallel to the borehole, which is vertical, one of the principal stresses is also vertical.

The occurrence of DITFs provides constraints on the magnitudes of stresses at this site. Figure F45 shows stress polygons at four different depths in Hole C0001D (80, 720, 820, and 910 m LSF) where we observed clear tensile fractures (Fig. F44) (see the “Expedition 314 methods” chapter for details on determination of stress polygons). The red line in each plot is the trace of S_Hmax and S_hmin, along which the hoop stress becomes zero. This calculation assumes hydrostatic pore pressure and uses the APWD to constrain the borehole pressure (see Zoback et al., 2003, for details of calculation). The occurrence of DITFs indicates that the state of stress should lie somewhere above the red line.

Although the actual coefficient of friction value is not available for these weak sediments, Figure F46 implies that it should be at least 0.6 or higher in order to allow a possible stress range that can induce tensile hoop stresses at the borehole wall. If the coefficient of friction was <0.6, the stress polygon would shrink and lie under the red line and the prism would be in an impossible state of stress (assuming that the breakouts conform to the Mohr-Coulomb failure model). Since weak sediments tend to have low resistance to shear stress, we assume a frictional coefficient of 0.6 for faults in this site.

At 80 m LSF, almost all possible magnitudes of S_Hmax and S_hmin are within the strike-slip fault stress regime (S_Hmax > S_v > S_hmin) and lie close to the state of frictional equilibrium; that is, the upper left boundary of the stress polygon (Fig. F46A). At 720, 820, and 910 m LSF, the possible state of stress should lie in any region above the red lines, in which case the faulting stress regime could be any type. At other depths, where no tensile fractures were observed, the magnitudes of S_Hmax and S_hmin should be below the tensile stress criterion (red line). This means that the state of stress should lie close to the red lines in Figure F45 unless there are drastic changes in stress field with depth.

During drilling, the borehole pressure gradient (indicated by equivalent circulating density calculated from annulus pressure) was kept nearly constant at hydrostatic state from the seafloor to ~500 m LSF, and thereafter it was elevated significantly over the hydrostatic pressure. This probably suppressed breakouts in the lower part of the hole.

Conclusions

Logging Unit I/II boundary: an unconformity

The seismic reflector corresponding to the logging Unit I/II boundary at 198.9 m LSF had previously been inferred to be either a fault or unconformity. The image data do not show significant disruption of bedding here except for within logging Subunit IB. Subunit IB shows a systematic increase in bedding dip from 16° to 46° at 191–192 m LSF then a gradual decrease to 22° at its base (Fig. F41). The dip of beds within logging Unit II is generally gentle (<7°), except for the uppermost beds that show steeper dips (14°–17°). The moderately dipping beds of Subunit IB could be explained by oblique stratification of sediments or slumping along a stratigraphic contact rather than tectonic fracturing or faulting.

Disrupted zone: fault or mass transport deposit?

The block in matrix and possible layer injection textures of the disrupted layer (Fig. F43) are similar to stratal disruption observed in mass transport deposits or in tectonically disrupted units from exhumed accretionary prisms (Cowan, 1985). Accordingly, we interpret the interval between 529 and 629 m LSF as a mass transport deposit or fault zone. The combination of environment, thickness of the zone, physical properties (including increased porosity), and comparison with seismic reflection data suggest to us that this is a fault zone. The resistivity-derived porosity decreases below the 529–629 m LSF interval (see “Physical properties”). Therefore, the most likely fault type would be normal, moving less consolidated material down to overlie more consolidated material. However, this assumes faulting of an undisrupted stratigraphy with porosity decreasing gradually with depth as a result of compaction. In an accretionary prism with a long history of deformation, this pattern may not occur, thus preventing determination of fault type; therefore, thrust faulting may also be a likely scenario.

Breakouts and drilling-induced tensile fractures: north-northwest shortening and east-northeast extension and predictions of fault types

The breakouts unequivocally show S_Hmax shortening at ~335° and extension at ~065° (Fig. F44). This indicates that the accretionary prism at Site C0001 is shortening perpendicular to the trend of the plate boundary and major prism structures and not parallel to the Global Positioning System (GPS)-constrained displacement of the Kii Peninsula to the northwest nor the plate tectonic convergence direction of the Philippine Sea plate and southwest Japan (300°–315°) (Miyazaki and Heki, 2001; Heki, 2007; Seno et al., 1993). Thus, the difference between the shortening direction of the prism at Site C0001 and the plate convergence direction must be accounted for elsewhere in either convergence oblique to the trend of the prism and/or in strike-slip faulting.

Top of page   |    Previous   |    Next