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doi:10.2204/iodp.proc.314315316.134.2009

Structural geology

Various deformation structures were observed in cores and X-ray CT images from Holes C0006E and C0006F. Figure F12 summarizes the structural observations. All structural data are given as supplementary material (see C0006_STRUCT_DATA.XLS in folder 316_STRUCTURE in “Supplementary material”).

Bedding and fissility surface attitude and evolution with depth

Among the 174 measurements of bedding surface attitudes obtained by visual inspection of cores, 126 were corrected for true magnetic north using paleomagnetic data. Thirty one measurements are relevant to lithologic Unit I (trench to slope sediments, between 0 and 27 m CSF). The remaining 95 measurements pertain to the accretionary prism sediments crossed between 27 and 600 m CSF. Of a total of 65 fissility measurements, 63 can be used with good reliability. All fissility measurements were obtained in the accretionary prism sediments at depths >100 m CSF (Fig. F12). Figure F12 shows a clear decrease of the dip angles of bedding surfaces and fissilities with increasing depth, from up to 75° to 30° or less. A similar change of bedding dip angle versus depth was also obtained from analysis of LWD data (see “Structural geology” in the “Expedition 314 Site C0006” chapter).

Most bedding surfaces in slope sediments strike north–south to northwest–southeast and dip westward (Fig. F13A). Strikes of bedding or fissility surfaces in accretionary prism sediments (below 27 m CSF) are variable and no preferred direction is apparent (Fig. F13B, F13C).

Deformation in lithologic Unit I

Trench to slope transition sediments of lithologic Unit I are poorly consolidated. Between 15 and 35 m CSF, they are affected by normal faults striking north–south to northwest–southeast and dipping steeply (Fig. F14) with offsets <10 cm. Because of poor consolidation, fault surfaces could not be separated in order to allow examination of possible striations. Between 28 and 31 m CSF, which is slightly below the normal fault interval, the sediments are cut by two reverse faults showing very small offsets (<1 cm).

Deformation structures within the fractured/​brecciated zone

Visual core description (VCD) and CT images allow us to distinguish a broad fractured/​brecciated zone extending from 230 to 545 m CSF in which fracture density and intensity is highly variable (Fig. F15). This zone differs from the sections above or below with the following characteristics:

1. Coherent pieces of cores are short, usually not exceeding one section (~1.4 m).

2. Cores are commonly strongly fractured.

3. Striated or polished planes are common.

4. The zone contains intervals of breccias that are unambiguously of tectonic origin.

By contrast, the sections above or below this fractured and brecciated interval are more coherent, less fractured, and generally do not include tectonic breccias. The transition between the fractured/​brecciated zone and the underlying section appears rather sharp.

Minor faults

Within the fractured/​brecciated zone defined above, minor faults of various types can be observed on cores, as well as on CT images.

CT images were used to investigate the fault geometry and offsets that existed prior to whole-round sampling and cutting. Figure F16 shows a CT image of a fault displacing worm tubes with a normal offset of ~1 cm. Visually, this fault displays parallel lineations, and the faces of the fault are mated. That the worm tubes are displaced when the fault surfaces are mated strongly suggests that the fault is natural. Paradoxically, this normal fault strikes approximately perpendicular to the maximum principal horizontal stress (σ_Hmax) direction inferred from borehole breakouts (see “Structural geology” in the “Expedition 314 Site C0006” chapter).

Within the fractured/​brecciated zone recognized above, dip angles of faults are scattered but clusters around 60° tend to dominate (Fig. F12). Fault plane strikes do not show any preferred direction (Fig. F17). Minor faults are characterized by striated and/or polished surfaces or by offset markers. The sense of displacement is given by the offset of markers or, most commonly, by steps borne by the surface. Of the 220 observed faults, normal faults are the most frequent (42%), followed by reverse faults (12%) and left-lateral (10%) or right-lateral (10%) faults. The remaining faults (26%) have undetermined or ambiguous senses of displacement. Preliminary kinematic analysis of populations of normal faults, reverse faults, and strike-slip faults has not yielded consistent results, implying that fault-slip data sets are not homogeneous (Fig. F17). Additional sorting is clearly needed in order to define homogeneous subsets.

Fractures without displacement

The distinction between natural fractures and drilling-induced fractures is not easy at Site C0006. A fracture that can be followed across several “parted” (“biscuited”) core segments and that spans a distance greater than the borehole diameter is likely a natural fracture. Indeed, propagation of a fracture along the core is likely to stop at parting surfaces. The presence of plumose structures on fracture surfaces also supports natural propagation. A convincing example of such a tectonic fracture is found in interval 316-C0006F-18R-1, 70–125 cm. The surface of this fracture shows well-preserved plumose structures (Fig. F18) that clearly indicate a pure mode-I opening that initiated at some distance from the core and propagated approximately horizontally. Moreover, this fracture strikes parallel to the direction of σ_Hmax inferred on the basis of borehole breakouts in Expedition 314 (see “Structural geology” in the “Expedition 314 Site C0006” chapter); hence it is not drilling induced. However, when fractures are of small size (less than three times the core diameter) and lack plumose structures, there remains much ambiguity regarding the origin of the measured planes.

Deformation bands

Visually, deformation bands appear on cores as undulating bands that are darker than the host rock. Shear (tangential displacement) along deformation bands is established in several instances by offset markers. Deformation bands are visually observed in cores between 300 and 460 m CSF with a clear predominance between 300 and 405 m CSF (Fig. F12), which is within the fractured/​brecciated zone mentioned above. Most deformation bands dip more steeply than 30°, and the sense of shear is reverse (Fig. F19). Figure F20 shows the equal-area projection of deformation bands for which paleomagnetic correction is reliable. These bands predominantly strike about northeast–southwest and dip either northwestward or southeastward, forming two sets with an apparent conjugate geometry. Each set was fitted by a mean plane, and the obtuse dihedral angle between the two mean planes is ~97°. The bisecting plane of this angle strikes ~48° and is almost vertical. Given the reverse sense of slip observed along these deformation bands, this geometry suggests a shortening axis oriented horizontal along 138°. A similar direction of shortening was previously deduced from deformation bands in the Muroto transect in the Nankai accretionary prism (Ujiie et al., 2004).

Deformation bands appear as bright bands in CT images. The CT number for deformation bands is often ~100 greater than that of the surrounding rock and likely reflects densification within the bands. A systematic search of CT images over the depth interval from 160 to 400 m CSF (Fig. F21) confirms the heterogeneous density distribution of bands determined by visual inspection.

Combined VCD and CT image analyses indicate that deformation bands are concentrated between 240 and 400 m CSF. The lower density of deformation bands below 400 m CSF may also be explained by a lithologic change because deformation bands tend to be developed in mudstones which lack bioturbation. Below 400 m CSF, the sedimentary pile is dominated by bioturbated mudstone. Bioturbation may prevent deformation bands from forming, possibly because of a random reorganization of initially well aligned clay particles.

Sediment-filled veins

Sediment-filled veins (vein structures) are uncommon at Site C0006. The clearest examples were observed at six locations between 395 and 510 m CSF. Sediment-filled veins are planar or sigmoidal. One occurrence shows sediment-filled veins displaced in a reverse sense by deformation bands (Fig. F19C).

Sand injection

Sand injection cut by biscuiting boundaries associated with ESCS coring can be observed at 370.18 m CSF. Injection is either vertical (sand dike, Fig. F22) or horizontal (sand sill). Boundaries with the host rock are sharp and planar or irregular. The injected sand is medium to coarse grained, and is well sorted. It is homogeneous except for elongated wood fragments that can reach 1 cm in length. No internal structure can be recognized in the filling.

Definition of zones of concentrated deformation within the fractured/​brecciated zone

The distribution of structural elements in the fractured/​brecciated zone is shown in Figure F23. Core and thin section observation and CT image analysis allow us to define several intervals within the fractured/​brecciated zone that appear significantly more deformed than the background (and that could be interpreted as faults). Because the whole zone is affected by tectonic fracturing as well as by drilling-induced disturbance, the definition of deformed intervals was based on all possible observations and will be further tested by onshore studies. Key observations that, taken together, allow us to define these intervals are

1. The presence of fault rocks such as breccia or gouge;

2. The presence of strongly fractured intervals (by strongly fractured, we mean that the intact core segments, if any, have lengths smaller than the core diameter because of natural fracturing [biscuiting excluded]);

3. The presence of discontinuities of any type, such as abrupt changes in lithology, fracture, or deformation band intensity; and

4. A progressive change that leads to locally high density and/or preferred orientation of fabric elements, such as deformation bands and fractures, commonly associated with faults.

Based on these criteria, the following intervals appear as concentrated zones of deformation:

• 235–243 m CSF (Cores 316-C0006E-31X through 32X),
• 277–297 m CSF (Core 316-C0006E-36X through the top of 38X),
• 367.5–369.5 m CSF (Core 316-C0006E-45X),
• 433.75–440.00 m CSF (Cores 316-C0006F-5R through 6R), and
• 526–545 m CSF (Cores 316-C0006F-15R through 17R).

The 433.75–440.00 m CSF interval coincides with a biostratigraphic hiatus whose origin is still undetermined (see “Biostratigraphy”).

235–243 m CSF (Cores 316-C0006E-31X and 32X)

On the basis of CT image measurements, linear density of deformation bands increases with depth from 210 m CSF to a maximum of 21 bands/m in Section 316-C0006E-32X-1 (Fig. F21). Linear density of deformation bands decreases abruptly in the middle of Section 316-C0006E-32X-2 and then remains low downhole through Section 316-C0006E-32X-5. Fracturing also progressively increases in intensity with depth through Core 316-C0006E-31X, displays maximum intensity in intervals 316-C0006E-32X-1 and 32X-2, and decreases through Section 316-C0006E-32X-5. Locally in Sections 316-C0006E-31X-5 through 32X-2, finely fractured rocks are evident where fracture spacing is <1 cm and the fractures display strong preferred orientations. On the basis of VCDs, these intervals are classified as highly fractured rocks.

A prominent fracture surface in Section 316-C0006E-31X-5 (235.30 m CSF) juxtaposes two fractured units with contrasting CT numbers (Fig. F24). With a correction based on paleomagnetic data, the surface is oriented 043°, 29° southeast, which is approximately parallel to one of the preferred orientations of deformation bands with reverse sense of shear and thus is compatible with regional northwest–southeast contraction.

277–297 m CSF (Core 316-C0006E-36X)

Core 316-C0006E-35X is characterized by unbroken mudstone in which subhorizontal bedding and fissility are well preserved. At 277.39 m CSF (Section 316-C0006E-36X-1, 6 cm), a network of anastomosing fractures cuts the subhorizontal layering (Fig. F25A). Below this depth, fractured and brecciated mudstones are observed throughout Core 316-C0006E-36X (Fig. F25B). The fracture sets are spaced as little as a few millimeters apart and have an anastomosing geometry, suggesting incipient brecciation (protobreccia). Brecciated fragments are centimeter to millimeter size with distinctly polished and slickenlined surfaces. Mudstone tends to be brecciated along the sets of inclined and subhorizontal surfaces, showing a foliated aspect. The abrupt occurrence of millimeter-spaced anastomosing fracturing and brecciation across the top of Core 316-C0006E-36X constitutes a significant discontinuity that could correlate to a fault surface.

367.5–369.5 m CSF (Core 316-C0006E-45X)

Core 316-C0006E-44X and Sections 316-C0006E-45X-1 through 45X-5, 35 cm, are dominated by fractured mudstones. Interval 316-C0006E-45X-5, 35 cm, through 45X-6, 71 cm, is characterized by breccia, including centimeter- to millimeter-sized fragments that are ubiquitously polished and striated (Fig. F26A). At 369.18 m CSF (Section 316-C0006E-45X-6, 71 cm), a subhorizontal surface parallel to bedding/​fissility separates the brecciated interval above from the coherent interval below (Fig. F26B). Below this depth, original layering of sands and mudstones are well preserved and sands locally intrude into mudstone (see above, sand injection), which in turn are cut by biscuiting boundaries. The change in deformation style from fractured mudstone to breccia with depth and the prominent boundary between breccia and unbroken sands and mudstone layers at 369.18 m CSF suggest the presence of a fault.

433.75–440.00 m CSF (Cores 316-C0006F-5R and 6R)

Following Cores 316-C0006F-3R and 4R, which consist of ~4 m of highly fractured rock of unknown origin (tectonic, drilling-induced, or splitting-induced), Core 316-C0006F-5R includes coherent pieces of mudstones (interval 316-C0006F-5R-1, 0–73 cm) underlain by highly fractured mudstones nearly 2 m thick. Fragments have sizes <1 cm and commonly show polished/​striated surfaces. The presence in interval 316-C0006F-5R-CC, 4–23 cm, of a piece of tectonic breccia (Fig. F27) is consistent with the presence of a major fault nearby. Core 316-C0006F-6R is similar to the above-lying Core 316-C0006F-5R. It consists of 2 m of highly fractured mudstones with ~20 cm of tectonic breccia in the core catcher, confirming deductions from Core 316-C0006F-5R.

526–545 m CSF (Cores 316-C0006F-15R through 17R)

Section 316-C0006F-15R-3 shows a downhole increase in brecciation intensity. At 526.51 m CSF (Section 316-C0006F-15R-3, 48 cm), breccia is replaced by microbreccia (most brecciated fragments are <1 mm in size), which in turn changes to fault gouge at 526.65 m CSF (Fig. F28A). The thickness of the gouge zone is 8 cm. Careful observation of fault gouge both in core and thin section reveals that fault gouge is locally foliated, preserving composite planar fabric associated with shear (Fig. F28B). Below the gouge zone, breccia occurs again in Cores 316-C0006F-16R (Fig. F28C) and 316-C0006F-17R. Core 316-C0006F-18R is characterized by jointed mudstone with little or no evidence of brecciation.

Deformation below the fractured/​brecciated zone

Sediments found in the 545–603 m CSF interval, which consist of bioturbated hemipelagic mudstones (lithologic Unit III), are less fractured or brecciated than above 545 m CSF. Dip angles of fault planes are scattered between 20° and 88°, but steep angles predominate (Fig. F12). No preferred orientation of fault plane directions can be recognized. Among these faults, normal faults predominate (Fig. F29).

Discussion

The five intervals described above appear more deformed than the surrounding cores. In particular, they all include tectonically brecciated intervals recognized either visually or by relying on CT images. These five intervals are interpreted as faults within the fractured/​brecciated zone. Such interpretations remain hypothetical, partly because of the generally poor recovery near the considered intervals. Keeping in mind these uncertainties, it is then possible to propose correlations between these fault intervals and the reverse faults recognized on seismic lines passing nearby Holes C0006E and C0006F (Fig. F30; see Moore et al.).

Conclusion

Despite poor recovery and drilling- or splittinginduced disturbance, CT scan image analysis and visual examination of cores from Holes C0006E and C0006F allow us to define a broad fractured/​brecciated zone that includes at least five intervals of concentrated deformation. The presence of these five intervals shows that deformation is not evenly distributed along the whole fractured/​brecciated zone but is localized along narrow zones corresponding to faults, as suggested by seismic reflection profiles. Because of poor recovery, it is impossible to propose a typical thickness of fault core and damaged zone for any of the five recognized faults.

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