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doi:10.2204/iodp.proc.314315316.116.2009

Structural geology and geomechanics

Our interpretations of structure and in situ stress are based on resistivity images (see the “Expedition 314 methods” chapter). We used a variety of images of three different depths of investigation and both statically and dynamically normalized images, but final interpretation was primarily based on the shallow dynamic image. The quality of the image data is excellent, permitting clear interpretation of planar features; therefore, our interpretation results should have a high degree of accuracy (see the “Expedition 314 methods” chapter for interpretation errors in subhorizontal planes).

Site C0004 is located at the tip of a deformed thrust sheet above a thrust fault. The site penetrated shallow slope sediments, the deformed thrust sheet, and underthrust slope sediments. The structural characteristics identified in the borehole images define three structural domains (Fig. F25).

Since the lithology of this hole is predominantly homogeneous fine-grained sediments, we sometimes encountered difficulties distinguishing bedding planes and fractures. Most of the fractures are also bedding parallel. Therefore, some conductive planes have been interpreted both as bedding planes and conductive fractures.

Structural domains

Three structural domains were defined based on the pattern of fractures and borehole breakouts and sediment resistivity texture in the images (Fig. F25). Structural Domain 1 (0–95 m LSF) is characterized by a lack of fractures and weak breakouts. The background texture of the sediments in the images shows little variation within this domain. Structural Domain 2 (95–292 m LSF) is characterized by a series of heavily or moderately fractured conductive zones and intensive development of borehole breakouts. Structural Domain 3 (292–396 m LSF) includes a minor fractured zone, but most fractures are patchily developed. Breakouts in Domain 3 continue from Domain 2 but are generally reduced in width. Domain 3 has bedding planes that dip more shallowly than those in Domain 2. See the next section for discussion of the fractured zones.

Bedding

Bedding planes in structural Domain 1 (0–95 m LSF) are consistent and mostly strike east–west and dip 30°–40° to the south (Figs. F25, F26). The beds in structural Domain 2 are more scattered both in dip and azimuth but generally strike northeast–southwest and dip to the north. Beds dip at 20°–70°. Structural Domain 3 shows a less scattered distribution of poles to bedding planes. The dominant bedding plane strike is similar to Domain 2 (northeast–southwest) but the dips are generally gentler (average = ~20°; range = 5°–55°) and to the north.

Natural fractures and fractured zones

Fractures in borehole images have been analyzed according to their azimuth, dip, aperture, and conductivity and are classified into three types: conductive, resistive, and uncertain fractures (Fig. F27). Most fractures we identified are conductive, but this may be due to the better visibility of conductive sinusoids against the background sediments. The uncertain fractures were defined where breakouts show discontinuity, and these discontinuous features can be picked as a partial sinusoid. Such discontinuity in breakouts may also be caused by variability in lithology; therefore, there is a small possibility that these picked fractures are bedding planes or, alternatively, not real features. Tensile fractures were not observed at this site with the exception of some examples in the uppermost part of the image (56–61 m LSF).

Fractures in structural Domain 2 are mostly conductive but also include some resistive and uncertain fractures (Fig. F27). The fractures are scattered both in strike and dip but with a dominant trend of northeast–southwest and mostly steeply dipping (~30°–70°) to the north (Fig. F27A).

Structural Domain 3 includes fewer fractures than Domain 2. Most are conductive, but Domain 3 also exhibits some resistive and uncertain fractures (Fig. F27). The poles to the fracture planes form a dominant cluster corresponding to planes of northeast–southwest trend and gentler dip (10°–20°) to the north (Fig. F27A). There are also two minor clusters of northwest–southeast striking fractures with both eastward and westward dips.

Eight fractured “zones” in Hole C0004B (Fig. F28) were defined by intense development of fractures (mostly conductive) and wide breakouts and were classified as “major” or “minor” based on their intensity of deformation and conductivity. It is difficult to identify individual fractures and to determine their dip and azimuth within the very conductive parts of these fractured zones. The characteristics of each fractured zone are shown in Figure F28. In structural Domain 2, three major fractured zones and four minor fractured zones are identified (Zones 1–7). Structural Domain 3 includes a minor fractured zone (Zone 8). The uppermost part of fractured Zones 3 and 6 is characterized by merged breakouts of extremely broad width producing uniform high conductivity. The depth ranges of each fractured zone are

• Fractured Zone 1 (minor): 96–112 m LSF,
• Fractured Zone 2 (minor): 133–141 m LSF,
• Fractured Zone 3 (major): 170–184 m LSF (broad breakout conductive zone: 170–175 m LSF) (Fig. F29),
• Fractured Zone 4 (minor): 208–213 m LSF,
• Fractured Zone 5 (minor): 230–235 m LSF,
• Fractured Zone 6 (major): 247–269 m LSF (broad breakout conductive zone: 247–251 m LSF),
• Fractured Zone 7 (major): 284–292 m LSF (Fig. F30), and
• Fractured Zone 8 (minor): 308–324 m LSF.

Borehole breakouts

Breakouts were well developed in structural Domains 2 and 3, and the breakout orientation is approximately the same in these two domains (Figs. F25, F29, F30). Breakout widths range from 10° to >180° but with a modal range of ~50°–80° and an average of 70° (Fig. F31). Structural Domain 1 has patchy narrow breakouts.

Breakout azimuth is generally constant but width varies with depth (Fig. F22). Figure F31 illustrates that the azimuth scatters around a mean of 050° throughout the borehole. The main exception is the azimuth within structural Domain 1, which is ~040°. Breakout width at the borehole wall (Fig. F22) shows a significant increase from ~30° to ~70° at ~100 m LSF at the boundary between structural Domains 1 and 2 and a minor drop at ~290 m LSF at the boundary between structural Domains 2 and 3. These agree with changes in physical properties of the sediments at these boundaries (see “Physical properties,” “Log characterization and lithologic interpretation,” and discussion in the next section).

The mean azimuth of breakouts is 050° (northeast–southwest) with a range of 020°–080°, indicating that S_Hmax is oriented northwest–southeast (~140° or 320°). Our statistical analysis on 1289 breakout measurements throughout the borehole shows that the standard deviation is 15.82 and the 95% confidence interval is <1° of azimuth (Table T6). This orientation of S_Hmax is therefore similar to, but statistically distinct from, the orientation at Site C0001 (mean breakout azimuth = 066°; standard deviation = 14°; 95% confidence interval = 0.82°; mean S_Hmax orientation = 156° or 336°).

Stress magnitude analysis from breakout widths

Attempts to constrain in situ stress magnitudes were made using empirically estimated rock strengths based on physical properties (see the “Expedition 314 methods” chapter). Two depths were chosen for stress magnitude analyses within structural Domains 2 (200 m LSF) and 3 (325 m LSF). Average widths of borehole breakouts at these depths are 75° and 60°, respectively. Figure F32 shows stress polygons for the two depths. Formation pressures are assumed to be hydrostatic. The two stress polygons show that the unconfined compressive strengths of rocks at respective depths should be >0.9 MPa (200 m LSF) and >1.6 MPa (325 m LSF).

Physical properties that are useful for strength estimation in these clay-rich rock formations are P-wave velocity and porosity. Velocities at 200 and 325 m LSF were ~1890 and ~1990 m/s, respectively; only a 5% difference. Since no measurement of porosity was conducted in this hole, indirect porosities (derived from resistivity) were used (see “Physical properties”). Estimated porosities at the two depths are ~49.5% and ~50%, respectively; therefore, the rocks at the two given depths appear to possess very similar physical properties, which in turn suggest similar strengths.

A series of empirical relations between physical properties and strengths give a strength range between 1.0 and 5.8 MPa, with an average of 4.1 ± 2.0 MPa. Because of a relatively large uncertainty in rock strength and the relatively small size of polygons, it is not clear which stress regime should fit the state of stress. If only average values of strengths are used, the stress states at both depths will be either in the strike-slip or thrust fault stress regime. For a given strength value, the stress state at 200 m LSF lies in a region more favorable for strike-slip or thrust fault regimes than that at 325 m LSF.

Discussion and conclusions

Correlation of structural domains and logging units

The characteristics of the structural domains correspond to physical properties of the sediments as well as structural character and can therefore be generally correlated with the logging units. Structural Domain 1 (0–95 m LSF) includes logging Unit I and Subunit IIA; we see no structural evidence for division at the Unit I/IIA boundary. The minimal change in structural character in the resistivity images between these logging units suggests small changes in physical properties (and therefore rock mechanics parameters) within structural Domain 1. This is supported by the characteristics of the sonic log, which shows that both logging Unit I and Subunit IIA have extremely slow P-wave velocities (see “Log characterization and lithologic interpretation”). The structural features and log characteristics of structural Domain 1 could be interpreted as two sedimentation stages of slope deposits; the upper slope sediments correspond to Unit I and the lower slope sediments correspond to Subunit IIA. The strong reflectivity at the Unit I/​Subunit IIA boundary suggests that the reflection coefficient is larger than at the boundary between Subunits IIA and IIB. Subunit IIA may therefore be composed of reworked or slumped sediments, mostly from the deformed thrust sheet.

The boundary between structural Domains 2 and 3 at 292 m LSF lies within logging Subunit IID, a proposed transition zone between the thrust sheet deposits and overridden sediments (see “Log characterization and lithologic interpretation”). Deformation characteristics from resistivity images at the boundary of structural Domains 2 and 3 suggest a thin (a few meters) transition zone (Fig. F30) between the thrust sheet and underthrust section around fractured Zone 7 (284–292 m LSF), contrasting with the relatively thick logging Subunit IID (236.4–323.8 m LSF).

Structural comparisons between borehole and seismic reflection data

The structural features identified in the borehole resistivity images are well correlated with the structural style in the seismic reflection profiles. Bedding planes in structural Domain 1 dip to the south, consistent with the reflection horizons. The scattered seismic pattern in the thrust sheet can be explained by steeply dipping beds in Domain 2. The beds in Domain 3 dip gently north and agree with the structural style of the underthrust section in the seismic profile, although the borehole beds tend to show higher dips.

The fractured zones also match the seismic profile well (Fig. F33). The major fractured Zone 3 (170–184 m LSF) is exactly at the crosscutting point of a possible landward-dipping thrust fault at this site. The major fractured Zones 6 (247–269 m LSF) and 7 (284–292 m LSF) match with distinct positive reflection surfaces both inclined to the north. The boundary between the thrust sheet and underthrust sediments is at fractured Zone 7 and correlates with a major increase in velocity (impedance).

Breakouts and convergence directions

The breakout directions show mean S_Hmax shortening at 320° (Figs. F22, F31), which is between the mean S_Hmax direction at Site C0001 (336°) and the convergence direction (300°–315°) of the Philippine Sea plate and southwest Japan (Miyazaki and Heki, 2001; Seno et al., 1993; Heki, 2007). The difference between the plate convergence direction and the S_Hmax direction at Site C0004 may be due to partitioning of oblique shortening within the forearc. The 16° difference in the S_Hmax direction between Sites C0001 and C0004 may be caused by minor strike-slip faulting (partitioning of strain) or seafloor topography. Such a topographic effect on the local stress field is observed at the northern margin of the Kumano forearc basin (Yamamoto, 2006) and also in numerical models of accretionary prisms (Yamada et al., 2006). However, Site C0004 is located where the seafloor slopes gently toward the trench; therefore, topographic effects should be minimal.

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