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doi:10.2204/iodp.proc.348.103.2015

Structural geology

Structural analyses at Site C0002 included description of cuttings in Holes C0002N (875.5–2330 mbsf) and C0002P (1939.5–3058.5 mbsf) and analyses of cores in Holes C0002M (475–512.5 mbsf) and C0002P (2163.0–2218 mbsf).

Cuttings description

Sampling and quality control of intact cuttings

In Holes C0002N and C0002P, deformation structures in cuttings from both the 1–4 and >4 mm size fractions were investigated from 875.5 to 2330 and 1939.5 to 3058.5 mbsf. It should be noted that in the shallow portion of the hole, intact cuttings that are apparently internally undisturbed by drilling processes represent only a small fraction of the initial amount of the sample prior to hard washing (see the “Methods” chapter [Tobin et al., 2015]).

For the coarser fraction (>4 mm), the total number of cuttings per sample was generally <100 in the upper section, and the maximum number of intact cuttings counted was 100 (Fig. F28). In some samples, all of the initial cuttings disaggregated entirely. The ~200 cm³ initial amount present prior to hard washing was taken throughout the section from 1200 to 3058.5 mbsf in Holes C0002N and C0002P. In the section above 1200 mbsf, the volume of the initial sample was small (frequently ~20 cm³), and few intact cuttings were retained after hard washing.

WOW or drilling operations that add additional time between the initial drill bit penetration and sampling can have some implications for the retrieval of cuttings. Indeed, during this downtime, some cuttings and cavings can become detached from the borehole walls and fall along the mud column. The denser intact cuttings should fall more rapidly than the drilling-induced aggregates (see “Physical properties”). As a consequence, the amount of intact cuttings (and cavings) retrieved after these incidents often increases dramatically. In Hole C0002N, these incidents occurred at 1219, 1677.5, 2008.5, and 2036 mbsf (Fig. F28) and are associated with a corresponding increase of retrieved intact cuttings, although the increase of the cuttings observed at ~1200 mbsf is less pronounced than the others, probably due to the relatively short down time. In Hole C0002P, similar occurrences took place at 2067.7, 2163.0, 2218.5, and 2601.5 mbsf. The increases in volume at 2163.0 and 2218.5 mbsf are likely due to coring operations, but no influence is observed due to the WOW incident that took place at 2067.7 mbsf. At 2601.5 mbsf, the intact cutting recovery is high, and the influence of the wiper trip cannot be observed.

As a whole, the amount of examined intact cuttings increases from 1045.5 to 3058.5 mbsf (Fig. F28). Shallower than 1045.5 mbsf it was not possible to retain any material at all after washing.

Distribution of deformation structures in intact cuttings samples

Observed deformation structures besides drilling-induced disturbance in intact cuttings (from here onward called “cuttings”) from the 1–4 and >4 mm size fractions include slickenlined surfaces, scaly fabric, deformation bands, opaque bands, minor faults, and mineral veins.

Figure F29 shows the percentage of deformed cuttings obtained by dividing the number of cuttings that show deformation structures by the total number of retrieved cuttings as a function of depth for both Holes C0002N and C0002P. Because of the highly variable number of retrieved intact cuttings, three categories are represented: >40 cuttings, >20 cuttings, and at least 1 cutting.

For the >4 mm size cuttings, the percentages based on 21–40 or >41 cuttings are similar, but the percentages based on very few cuttings are more scattered, in particular above 1500 mbsf in Hole C0002N. Accordingly, we consider the percentages based on >20 cuttings as representative of the actual distribution of the deformation. In that case, in Hole C0002N, the percentage of deformed cuttings varies mainly between 5% and 25%, and the mean percentage reaches 16%. In Hole C0002P, the rate of deformed cuttings identified is more frequent than in Hole C0002N, and the percentage of deformed cuttings varies mainly between 10% and 30%. The mean is ~21% for Hole C0002P.

In the >4 mm size cuttings in Hole C0002N, four intervals deviate from the mean value of 16%. A clear peak appears at 1235.5 mbsf, but it is defined by relatively few samples (maximum of 52% deformed cuttings) recovered from a position a few tens of meters after a hole cleaning (1219 mbsf). Accordingly, although the observation is correct in terms of percentage, the exact depth of the cuttings is perhaps not precise, and some may be cavings. A second deviation from the mean can be observed from 1565.5 to 1695.5 mbsf, with a maximum of 27% deformed cuttings. A third peak is present between 1870.5 and 1905.5 mbsf (maximum of 38% deformed cuttings). These two increases are not influenced by waiting on weather, hole cleaning, or mud loss treatment incidents. Finally, a fourth increase can be observed between 2015.5 and 2145.5 mbsf (maximum of 27% deformed cuttings). In this case, the upper part of this broad peak could have been influenced by the waiting on weather and mud loss treatment just before retrieval of these cuttings, at least in terms of exact position of the sample.

In Hole C0002P, the percentages of deformed cuttings of the >4 mm size fraction show a variable trend with peaks of >35%. No localized deformation zone can be clearly shown based on high percentage distribution of the deformation around a particular depth.

From both holes, the relationships between the deformation percentages in the >4 and 1–4 mm fractions are not clear. For the 1–4 mm size cuttings in Hole C0002N, the ratio of deformation increases with depth shallower than 1235.5 mbsf. The percentage reached 30%, although it is generally ~8%. By contrast, in sections where a high percentage of deformation was observed in the >4 mm size cuttings, the 1–4 mm size fraction has a high percentage of undeformed cuttings. Indeed, in the 1–4 mm fraction, the size of the cuttings is perhaps too small to exhibit any type of distributed deformation structures. In Hole C0002P, the deformation seems to be penetrative, as it can also be observed even in the 1–4 mm size cuttings. In the intervals where observation was possible, a similar variable trend of the percentage of deformation is observed, but no correspondence between size fractions can be established.

Type of deformation structures as a function of depth

Figure F29 shows depth distribution of cuttings with deformed structures: slickenlined surfaces, scaly fabric, deformation bands, opaque bands, minor faults, and mineral veins. Cuttings with slickenlines were observed throughout the entire section below 1045.5 mbsf. In Hole C0002P below 2430.5 mbsf, the slickenlined surfaces are so abundant in the cuttings that a scaly fabric develops in almost all the samples. In Hole C0002N, in the >4 mm size fraction, a cluster of deformation band observations is noted around a peak of deformed cuttings at 1235.5 mbsf. The abundance of such structures is scattered but they continue to appear downhole to 2215.5 mbsf. In Hole C0002P, these structures appear below 2380.5 mbsf, although they are not frequently observed. The first appearance of veins is at 1245.5 mbsf (Hole C0002N). They appear irregularly below this depth to 2225.5 mbsf, but below that depth they are present regularly in Hole C0002N and in all of Hole C0002P. Rarely, arrays of minor faults were identified below 2410.5 mbsf.

Description of deformation structures

All observed deformation structures except for clearly drilling-induced ones are summarized in CUTTINGS STRUCTURE.XLSX in STRUCTUR in “Supplementary material.” In this file, we differentiated the lithology in which the different types of deformation structures were observed. We also counted the number of the cuttings of different lithologies in each sample, including nondeformed cuttings. Both 1–4 and >4 mm size cuttings include slickenlined surfaces, scaly fabric, deformation bands, opaque bands, minor faults, and mineral veins.

Slickenlined surfaces

Cuttings with slickenlined surfaces were observed throughout the entire section below 1045.5 mbsf. A slickenlined surface is the polished surface of a cutting that shows striations. At shallow depths, the cuttings of silty claystone with slickenlines generally show a shiny planar surface with very fine striations (in Fig. F30A at 1235.5 mbsf). Striations on surfaces in similar lithology can be easily observed with increasing depth (in Fig. F30B, F30C, F30D at 1665.5, 2095.5, and 2560.5 mbsf, respectively). Lens-shaped cuttings are commonly observed, surrounded by slickenlined surfaces. Under the optical microscope, clay mineral–rich zones are observed along these surfaces, reaching 200 µm thick (Fig. F30E). It is has been suggested that the thickness of such zones can increase with depth, together with the preferred alignment of clay minerals (Strasser et al., 2014b). This is confirmed by scanning electron microscope (SEM) images (see “SEM description”). Slickenlines are commonly associated with steps (Fig. F30F) from which the sense of shear can be inferred (e.g., Petit, 1987). The appearance of the slickenlined surfaces also depends on the lithology of the affected rock. In quartz-rich silty sandstone, the slickenlines are not coated by clay minerals (Fig. F31A, F31B). Stepped slickenlines can also be observed, but they are very tiny (<10 µm between steps; Fig. F31C). Under the optical microscope, deformation is very localized near the slickenlined surface. In one example, only a band of ~120 µm is affected by brittle shear that displaced opaque bands, which acquired an en echelon geometry (Fig. F31D). Figure F31E shows a detail of this band, where a plagioclase grain shows undulose extension probably due to the shearing along the slickenlined surface. A broken and displaced quartz grain indicates the sense of shear, dextral in the present position of the thin section (Fig. F31F).

Scaly fabric

Scaly fabric was observed in Hole C0002P from 2430.5 mbsf downhole. It broadly corresponds to the relative enrichment in clayey materials of the sediment (see “Lithology”). At the cuttings scale, the scaly fabric appears related to slickenlined surfaces (Fig. F32). It also corresponds to reorientation of clay particles into preferred alignment (see “SEM description”).

Deformation bands

Deformation bands (Maltman et al., 1993) are not frequent, but they appear from the top to the bottom of the studied section of Holes C0002N and C0002P (see CUTTINGS STRUCTURE.XLSX in STRUCTUR in “Supplementary material”). These structures were observed in silty claystones at 1235.5 mbsf and in silty sandstones at 2380.5 mbsf. Their distribution along Holes C0002N and C0002P is shown in Figure F32.

In the cuttings, the deformation bands are characterized by thin planar dark gray bands (Fig. F33A–F33D), which in some cases show slickenlined surfaces (Fig. F33A, F33C) or stepovers (Fig. F33B). These bands generally do not appear as single planes but as a set of bands of variable orientation, which define a web structure (Byrne, 1984) (Fig. F33A, F33C, F33D). In thin section, the deformation bands have a thickness of >100 µm and are composed of dark brown clay minerals and angular quartz or feldspar grains in which it is not possible to recognize any preferred orientation (Fig. F33E, F33F). In Figure F33F, a quartz grain adjacent to the fault does not show any deformation.

Opaque bands

When thin sections were made to study the cuttings with a microscope, a type of structure not visible with binocular magnifying glass was revealed in a sample of silty claystone from 2015.5 mbsf (Sample 348-C0002N-259-SMW). Two systems of apparent veins of opaque material go through the whole sample (Fig. F34A–F34C), although primarily in the more clayey lithology of the thin section. The two systems define an apparent angle of 40° between them. They are very thin (no more than 5 µm thick), and no material other than opaque minerals can be seen within them. Geometries reminiscent of stepovers can be observed (Fig. F33D). This suggests that they could be microfaults, separating lenses, or rhombs of rocks within which no deformation is observed (Fig. F34A–F34C). In that case, their appearance is very different from the deformation bands described in the previous paragraph.

Minor faults

Arrays of minor faults were observed in a few samples below 2410.5 mbsf. The faults exhibit spacing of 0.5 mm or less and displacement of the same order of magnitude (Fig. F35A). Figure F35B shows that each minor fault is in turn a fault zone, constituted by various coalescing microfaults.

Mineral veins

Veins appear in all the lithologies observed in Holes C0002N and C0002P (Fig. F36A, F36B). They were also observed rarely as individual cuttings, as they are probably more resistant than other materials to the drilling and washing processes. The distribution of these veins in Holes C0002N and C0002P is shown in Figure F39.

The veins are generally less than a few millimeters wide and most commonly consist of carbonate minerals (Fig. F36A), although veins composed of pyrite were sometimes observed (Fig. F36B). Occasionally, open veins appear and are lined by carbonate minerals (Fig. F36C). In some cases, these veins are formed by fibers, occasionally stepped (Fig. F36D), which suggests shear deformation during vein formation. This is consistent with the orientation of some veins perpendicular to the slickenlines. Locally, wall rocks were brecciated during vein formation, and both the fibers of the vein and the slickensides of the wall rocks indicate the same direction, which suggests a common sense of shear (Fig. F36E). A few samples show veining and repeated brecciation, reminiscent of hydraulic fracturing processes (Fig. F36F).

Core description

Hole C0002M

Observation of the Hole C0002M cores and XRCT images revealed few structures in the lower part of the Kumano Basin deposits (lithologic Unit II). The orientation of seven bedding planes and one vein were measured on core from Hole C0002M. Because of the limited number of structures and oversized diameter of SD-RCB cores for the onboard cryogenic magnetometer, reorientation using paleomagnetic measurement was skipped for cores from Hole C0002M.

Bedding

Several bedding planes and laminations were identified in Hole C0002M (lower Kumano Basin sediment; lithologic Unit II; see “Lithology”). The bedding planes were identified at the bottoms of sandstone layers corresponding to the base of muddy distal turbidites. Their attitudes are almost horizontal; all bedding planes dip less than 5° (Fig. F37).

Faults and veins

In the cored interval in the Kumano Basin lithologic Unit II (Hole C0002M), faults were not observed by visual core description or XRCT observation. One vein composed of precipitated pyrite was observed in interval 348-C0002M-1R-2, 11–16 cm (476.52–476.57 mbsf; Fig. F38). The XRCT image shows that this vein has a planar shape as a whole, but in detail it shows a wavy, ptygmatic form. The origin of this vein is unknown. The ptygmatic shape suggests that the vein was deformed during compaction (lithification). In that case, it developed during an early stage. A possible genesis through bioturbation could be invoked, although the planar shape of the vein suggests it formed as a vein along a crack in shallow sediment.

Hole C0002P

Observations of Hole C0002P cores and associated XRCT images revealed bedding and other structures. A total of 41 bedding planes, 27 minor faults, and 4 carbonate veins were measured from the cored interval (2163.0–2217.5 mbsf). A wide fault zone between 2204.9 and 2205.8 mbsf was also observed.

Bedding

In most of the cored interval, bedding dips steeply (>75°) (Fig. F39). In some places, a group of layers was traceable in adjacent biscuits of the core. In that case, the attitude of the bedding was measured using only one segment to avoid duplicate measurements. Some boundaries between sandstone and silty claystone show flame structures and load cast indicative of younging direction. Sand layer grading was also used to determine the polarity of the bedding, indicating that most of layers are upright. Postcruise paleomagnetic research will hopefully permit restoration of the attitudes of the bedding planes in true azimuthal coordinates.

Faults

A total of 27 faults were observed in the core (Fig. F39). They are discrete planes that bear slickenlines (Fig. F40A, F40B). Sense of movement was deduced from slickensteps, the offsets of bedding, and asymmetric scratching and grooving along fault surfaces. The designation of reverse or normal faulting is based on the structure’s present-day position, but because the current bedding dips are very steep, the original attitude at the time of deformation is generally unknown.

There are 12 faults with an apparent reverse sense of faulting and 15 faults with apparent normal sense of faulting in their current orientation. The apparent normal faults appear scattered in the hanging wall of the fault zone, whereas the apparent reverse faults are distributed around the main fault zone, ~10 m above and below it. Interestingly, the main fault zone at ~2205 mbsf has a normal sense of motion (Fig. F39). One apparent reverse fault far from that fault zone lies at 2178 mbsf. The normal and reverse faults dip 23°–78° and 31°–70°, respectively. In one case, a cross-cutting relationship between both types of faults was observed: a normal fault cuts two conjugated reverse faults (2177.14–2177.27 mbsf; Fig. F40C, F40D). The mixing of apparent normal and reverse faults suggests at least two phases of deformation have occurred with possible bed rotation to steep angles occurring between them. This makes a simple kinematic interpretation of this region difficult.

Brittle fault zone

A brittle fault zone characterized by fault breccia appears between Sections 348-C0002P-5R-4, 30 cm, and 5R-5, 30 cm (2204.9–2205.8 mbsf; Fig. F41). The light brown matrix (~50%) involves angular brecciated clasts, generally <2 mm. Scaly fabric characterizes most of the clasts.

A few slickensteps observed along discrete minor faults in the fault zone were measured. With respect to the present core coordinates, the slickenlines show a high rake (~60°). Moreover, the 3-D XRCT images show that the observed surface of the working/archive half is oriented at a high angle (40°–60°) to the fault plane, which is adequate for determining the movement along the fault zone.

The shape and geometric distribution of clasts is highly asymmetric (Fig. F41), indicative of the apparent normal faulting sense of the fault in its current orientation. The few slickensteps observed along discrete minor faults in the fault zone are also coherent with the apparent normal faulting sense.

Some calcite veins were identified in the fault zone and appear at the top and bottom of the fault zone and near a 2 cm deformed sandstone clast (Fig. F42). The XRCT image shows that the geometry of these veins is not planar; rather they are generally curviplanar. Also, the veins are imbricated (Fig. F42). Given these occurrences and their limited distribution developed only inside the fault zone, the veins apparently developed during the faulting and have been partially disrupted by later movement.

SEM description

Hole C0002N: shallow section (850–2330 mbsf)

SEM images were made of broken surfaces of intact cuttings. We tried to image “exposures” as close as possible to the striated surfaces. However, cuttings often broke in a variety of orientations, and not all internal surfaces had striation orientations that were parallel to those visible on the outer surfaces. Compaction fabrics with grain alignment are weakly developed at 850–2100 mbsf in undeformed cuttings away from obvious shear zones (Fig. F43). Local collapse and alignment of the clay fabric initiated at ~1500–2000 mbsf but does so in an irregular box-framework pattern enclosing uncollapsed and poorly oriented subregions. Locally, siliceous microfossils that remained intact during burial and deformation are present at least to 1225.5 mbsf. Little evidence exists for opal diagenesis, suggesting in situ temperature is low (<80°C at 1225.5 mbsf), which is consistent with the estimated thermal gradient of Spinelli and Harris (2011). The lack of strong compaction foliation may occur because the beds, or parts of them, rotated during accretion and are now dipping at a high angle, as measured in Hole C0002P (mostly >60°) and in the LWD resistivity imaging data from both C0002F and C0002P. This would cause a superposition of compaction strains that limits ultimate grain alignment.

Surface images of striated microfaults in cuttings reveal that striations occur at all scales down to the micrometer level. Examples are shown in Figure F44A and F44B from a variety of depths. Striations occur in all lithologies from silty clay to sands. Striated surfaces of sand-rich inclusions often show polished glassy surfaces that may contain poorly resolved broken grains/cements. Shear surfaces have a microlayered internal texture, but individual shear surfaces are generally thin (<1–5 µm; Fig. F44A–F44D). In silty clays, the striations on shear surfaces can occur in extremely localized shear zones ≤1 µm thick, in which grain alignment is intense to the point of it being difficult to resolve individual clay grains at the highest resolution of the shipboard SEM (0.1–1 µm generally, depending on sea state) (Fig. F44B–F44F, F44G–I, F44K).

Typically, shears in silty clays form distributed anastomosing incipient scaly fabric networks enclosing interiors of lenticular phacoids composed of poorly oriented clays and silt grains (Fig. F44G–F44I). This lack of a strong consistent internal clay alignment away from the “nano-thin” shears appears typical of many of the deformed cuttings from the shallow to intermediate depths of the part of the wedge transected by Hole C0002N and may signify distributed low-intensity bulk strain. In other, rarer, cases, more penetrative foliation of clays along thicker slickensided microshear zones is apparent. An example from 2205.5 mbsf (Fig. F44E), the same depth as the cored fault zone (2204.9–2205.8 mbsf), has an intense scaly fabric forming a broad shear zone a few millimeters thick. These zones comprise many individual thin shear zones (<<1 µm thick) and represent full scaly fabric development. The location of the scaly clay cuttings sample shown in Figure F44E at the depth of the fault zone in the cores is probably not a coincidence, but such scaly fabrics also become generally more common with depth below 2000 mbsf (see below).

Hole C0002P: cores and deeper section (1939–3058 mbsf)

Examples of the deformation fabrics revealed in specimens from the cored section (2163.0–2217.5 mbsf in Hole C0002P) are shown in Figure F45.

The cores show a variably deformed hanging wall sequence overlying the main fault zone identified in Section 348-C0002P-5R-4. Bedding is steeply oriented (generally >75°), as are the measured faults (mostly between 40° and 70°) within the cores (Fig. F39). SEM images were taken from a portion of Section 2R-3 (2176.28–2177.70 mbsf) in the hanging wall from the interstitial water whole round excavated during retrieval of material for pore water sampling (see “Geochemistry”). The strata in this section are dismembered to form lenticular sandstone blocks in a clay-rich matrix with scaly fabrics (Fig. F45). Observations on SEM images of the edges of lenticular sand-rich blocks suggest that they are dismembered by the generation of a deformed outer rind that is progressively sheared off into the surrounding matrix. For example, Figure F46 shows a series of increasingly higher magnification images of a deformed rind of a muddy sandstone block in which many individually thin (<0.1–1 µm), spaced, clay-dominated shears develop, with intervening less deformed ~40 µm thick panels containing the sand grains and more weakly aligned clays. The progressive localization of the shear zones into the clays suggests mechanical control. The ~40 µm spacing of the shear zones is quite regular and may have been dictated by the average size of sand grains, resembling the spaced cleavage seen in the early stages of cleavage development in low-grade metamorphic rocks. However, in this case a simple shear component dominates, given the common development of striations on the shear planes in the scaly fabrics and asymmetric fabrics in some locations (Fig. 46F).

Toward the base of the hole below 2625 mbsf, more clay-rich sediment dominates the lithology. The percentages of deformed, striated, and scaly (Figs. F29, F47) cuttings are consistently elevated, suggesting distributed deformation is pervasive in this unit. SEM images indicate that apparently unsheared or undeformed cuttings at binocular microscope scale also exhibit a progressively developing penetrative clay alignment fabric at SEM scale. SEM views roughly perpendicular to the pervasive clay alignment fabric in a sample at 2980 mbsf illustrate this (Fig. F48). The flattening fabric can presumably be attributed to compaction. Porosities (uncorrected for any clay-bound water) fall below 20% by the base of the hole, and such compaction fabrics are to be expected in deeply buried sediments. XRD studies will reveal if the progressive development of pervasive clay alignment fabrics is associated with recrystallization of smectite to illite.

Together with the compaction fabrics, clay alignment in shear zones might be expected to also intensify with depth (e.g., Fig. F47) through the development of dense low-porosity shear fabrics, although such shear fabrics may also just represent regions with elevated shear strains. However, there are numerous clear examples of single <0.1–1 µm shear zones that cut directly across quite intense preexisting clay-compaction fabrics with almost no wall-damage zone, other than bending the ends of clays that intersect minor shears at a high angle (Fig. F48). This implies extreme strain localization.

Preliminary interpretations

The key observations made on cuttings in Holes C0002N and C0002P, along with structural analyses of cores retrieved in Hole C0002P are as follows:

1. The main structures observed in intact cuttings include slickenlined surfaces, scaly fabric, deformation bands, minor faults, and mineral veins. Slickenlines are observed throughout the whole interval, but scaly fabric is increasingly observed below ~2400 mbsf. The other types of structures are scattered throughout the whole section.
2. SEM images in the upper part of Hole C0002N show little evidence for opal diagenesis, implying that in situ temperatures are <60°–80°C at 1225.5 mbsf. In Hole C0002N, the fabric lacks a strongly preferred orientation in clay-rich materials, except along striated microfaults formed by clays. These zones are extremely localized and a few micrometers or less thick. Only in the deeper part of Hole C0002N do these microfaults reach a few millimeters thick, and even then they are composed of many individual ≤1 µm thick shear zones. In Hole C0002P below 2200 mbsf, SEM images show the development of regularly spaced fabric in sandstones, constituted by thin (<0.1–1 µm), clay-dominated shear planes. Toward the base of the hole below 2625 mbsf, compaction fabrics in clay-rich materials can be observed. This fabric is commonly cut by very thin shear zones with almost no wall-damage zone.
3. A 90 cm thick fault zone with 2 mm angular clasts is present in one of the cores (2204.9–2205.8 mbsf). In its present position, the brittle fault zone is associated with a normal faulting sense. It is unclear if this represents an early prerotation thrust or a late normal fault.

Structural features observed in Holes C0002N and C0002P are characteristic of relatively shallow deformation processes, consistent with deformational features identified from modern accretionary wedges and other shallowly buried accretionary prisms (Taira et al., 1992; Maltman et al., 1993; Yamamoto et al., 2005). The overall character of the deformation (independent particulate flow with limited evidence for cataclastic deformation) is suggestive of a relatively shallow environment (~0–4 km in burial depth).

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