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doi:10.2204/iodp.proc.343343T.103.2013 Structural geologySeveral diverse structures were recognized in the cores. This section leads off with a general description of the different structures. More specific details of those structures relevant to specific units are summarized below. Bedding was identified based on distinct changes (mostly centimeter scale) in lithology in the core or as coherent packages of layered bright and dark bands in X-ray CT images. Structures were classified as faults, normal faults, shear fractures, dark seams, dark bands, fissility, and sediment-filled veins. Faults are planar to curviplanar features that truncate and offset bedding or burrows. Dark seams are planar to curviplanar surfaces <1 mm thick marked by the presence of dark material, which is commonly bright in X-ray CT images relative to the host rock. Dark bands are tabular or curviplanar to irregular in thickness, are thicker than dark seams, and range up to 2–3 mm along their length. Drilling-induced damage is common throughout the core, manifesting as breccias, fractures, and biscuits. Unit 1 (slope sediments)
Structures in lithologic Unit 1 were identified in both halves of the core and X-ray CT images where possible, though many features apparent in X-ray CT images could not be identified in the core. Bedding orientations in Unit 1 were measured from two locations in the core and two additional locations using X-ray CT images. Average bedding dip is 30° ± 3° (Fig. F32). Appropriate paleomagnetic data to correct bedding strike to true azimuths were not obtained. Some faults are visible in both the core and X-ray CT images (Fig. F33), but more commonly they are only observed in X-ray CT images. In the core, faults are narrow (few millimeters thick) zones of deformed sediment with similar composition to the adjacent rock. In X-ray CT images, faults appear as tabular zones of bright material several millimeters thick. Recognized fault surfaces are polished and display slickenlines defined by aligned clay particles, which are correlated to bright surfaces in X-ray CT images. Of 10 mapped faults in this unit, all are dip-slip. Three are normal faults, and the shear sense of the others was not determined. Fault dip averages 69° ± 8°, ranging from 55° to 81°. Measured offsets are on the order of 1 cm, but several faults have offsets greater than the diameter of the core. Sediment-filled veins clustered in isolated intervals and were also identified from X-ray CT images. They form closely spaced, 0.5–1 mm thick bands that appear brighter in X-ray CT images than the surrounding siliceous mudstone. In this unit some of these fractures offset bedding and worm burrows by up to a few millimeters. Drilling-induced open fractures exploited existing faults, bedding planes, or other planes of material contrast within the core, and also occurred at random orientations. Biscuits were identified at places where the core was broken in a horizontal plane (perpendicular to core axis), minor breccia or soft (presumably remobilized) mud was observed within the break, and the bedding dip direction changed across the interval. Unit 2 (wedge sediments)
Cores 343-C0019E-2R and 3R are completely made up of drilling-induced breccia (Fig. F34); therefore, in Unit 2 we only have measured bedding and structural features on three small pieces of intact rock, representing two different lithologies. Structures in Unit 2 were identified in both halves of the core and in X-ray CT images where possible. The single probable bedding orientation measured (dip = 3°) is marked by a dark band that gradually darkens toward the top of the core. Additionally, five dip-slip faults were identified. Some were recognized in X-ray CT images as bright surfaces cutting burrows and were then located and measured in the core. Of these, faults cutting the grayish brown mudstone in Core 2R are coated by thin veins (<1 mm) of calcite. We identified a normal sense of shear on one fault on the basis of stepped striae. Two dark seams cut one of the calcite-veined faults with <1 mm displacement and truncate a burrow; they have been tentatively interpreted as possible pressure-solution seams, but might accommodate small shear displacement. Thin en echelon bands of dark material are interpreted as sediment-filled veins. Unit 3 (wedge sediments)
The upper surface of Unit 3 was not observed. Unit 3 terminates at its base against the highly sheared clays of Unit 4, although the contact was not present in the core. The contact surface must lie within the 1.5 m interval that was not recovered between Cores 343-C0019E-16R and 17R. Structures in Unit 3 were identified in the cores and X-ray CT images. The main structural features in Unit 3 are inclined bedding and fissility, dark seams, dark bands, fractures, brecciated zones, and faults/fault zones. Bedding and fissilityBedding is defined by parallel pale or dark laminations and compositional layering in the mudstone (Fig. F34A). Bedding may manifest as changes in brightness or may not be distinct in X-ray CT images (Fig. F34B). Bedding dips vary between 3° and 86° (Fig. F32). Locally, fissility is observed parallel to bedding, possibly opened because of drilling or unloading. Although bedding dip is highly variable, some patterns are found in particular intervals (Fig. F32). In the interval between ~690 and ~725 mbsf, dips average 37° ± 20° and are variable through the entire interval. From ~770 to 790 mbsf, steeper dips dominate, including some potentially overturned intervals with very steep dips (average = 65° ± 15°). From ~800 mbsf to the base of the unit at ~820 mbsf, moderate dips dominate (average = 40° ± 12°). Dark seams and dark bandsSome dark seams in this unit are anastomosing (Fig. F35). When arranged in sufficient density, these networks may form dark bands. On X-ray CT images, many dark seams and dark bands are marked by bright seams and bright bands, respectively, but a few dark bands do not display contrast in X-ray CT images. Dark seams and dark bands show offsets commonly less than a few millimeters and might truncate burrows, mottled textural features, sedimentary layering in the mudstone, and other dark seams and bands. Drilling-induced fractures forming striated surfaces exploit many of the dark bands and seams. In some X-ray CT images, the brightness of the host rock changes across bright seams or bright bands, suggesting a lithologic contrast caused by displacement along seams or bands. In the middle part of this unit (Cores 343-C0019E-9R and 10R), dark bands locally contain visible (>100 µm) ellipsoidal to elongate pyrite aggregates, whereas toward the base of the unit, pyrite occurs within burrow-fill (see “Unit 3 (gray mudstone)” for more detail). Truncation/offset relationships and X-ray CT brightness properties of dark seams in this unit suggest they are likely solution surfaces and/or very thin shear surfaces. Dark bands are typically shear surfaces, but may also rarely be bedding or bedding-parallel shear surfaces. In most cases, shear-related surfaces can be distinguished from bedding laminae because sheared bedding or dark bands have sharper, more planar boundaries, offset burrows observed in X-ray CT images, and are not parallel to bedding features. Bright band near the H2 concentration maximumAt 697.2 mbsf, the X-ray CT image displays an ~2–7 mm thick bright band that dips 10° with respect to a horizontal plane (Fig. F36). The CT numbers of the bright band range from 1406 to 1875, whereas those of surrounding material range between ~1200 and ~1300. These high X-ray CT numbers indicate relatively high bulk density (i.e., more compacted material) or relatively high concentration of elements of high atomic number in the bright band (e.g., concentration of Fe-rich minerals). The section above the bright band shows an inclined fissility in homogeneous sediment, whereas fissility is absent in the section below, which is distinctively mottled. During Expedition 343, the bright band was only described using the X-ray CT image because the interval including this band was taken as a structure whole-round sample for postcruise research. The interval including the bright band is close to the location of an H2 anomaly at 697.9 mbsf that may have been generated by recent faulting (see “Geochemistry”). Although it is possible that the bright band is similar to dark bands commonly seen in Unit 3, apparent changes in lithology and orientation across the bright band lead us to the other interpretation that the bright band is a localized slip zone. Fault zoneOver a 27 cm depth interval centered around 719.85 mbsf, beds are truncated and crosscut by a 15 cm wide, 60° dipping zone of deformed rock. Narrow (<15 mm), anastomosing strands are located at the upper and lower edges of the zone. The rock in between these strands is darker in X-ray CT images, contains multiple narrow shear surfaces, and lacks coherent bedding, indicating the entire interval is deformed. From 719 to 725 mbsf, the mudstone is broken into angular fragments, ranging from 1 to 10 cm in diameter, along sets of inclined fractures that may be parallel to, or exploit, dark seams (Fig. F37). The fractures are commonly polished and slickenlined and sharply cut burrows, mottled texture, and compositional layering in the mudstone. Downdip plunging, stepped slickensides on slip surfaces indicate a reverse sense of motion with respect to the horizontal plane. Bedding dips abruptly increase in the fault zone. This pattern of dip changes, if attributed to drag along the fault, also is consistent with a reverse shear sense. From 721.5 to 725 mbsf, brecciated zones with millimeter-scale clasts are common in the fractured mudstone. The intervals dominated by brecciated material are 10–30 cm thick and show heterogeneous distribution. On the basis of the above features, we identify the fractured and brecciated interval between 719 and 725 mbsf as a fault zone. The location of this fault zone is well correlated with the interval of low resistivity identified from LWD (see “Logging while drilling”). No H2 anomaly was reported from gas chemistry analysis in this interval (“Geochemistry”). As cores were not recovered below 725 mbsf, the base of this fault zone cannot be identified. Sediment-filled veinsCore 343-C0019R-13R contains arrays of submillimeter-thick sediment-filled veins with lengths of 1–2 cm, in en echelon arrangement. These arrays are generally perpendicular to sedimentary layering (Fig. F38). Younging indicatorsEvidence of younging could not be identified within most of the sedimentary sequence composing Unit 3. But Cores 343-C0019E-13R and 14R have sedimentary contacts between fine- and coarse-grained, <5 cm thick beds that are either sharp or gradational. If the beds are normally graded, these beds young downcore in Core 13R, whereas they young upcore in Core 14R. In this case, younging reversals suggest the presence of an overturned syncline or rotated beds associated with faulting or slumping. Tectonic fractures reactivated during drillingThe entire suite of cores making up this unit is variably dissected by open fractures that divide the rock into angular fragments. The open surfaces are commonly decorated by millimeter-scale striations. It is common for these striations to trend approximately downdip. In rare cases, evidence indicates that these fractures have opened along tectonic discontinuities. For example, in interval 343-C0019E-13R-1, 26–30 cm, an open fracture offsets bedding by 2 mm in a reverse sense. The fracture surface appears to have both 0.5 cm wavelength and amplitude ridges plunging downdip and millimeter-scale striations. It is inferred the former are tectonic, whereas the latter were developed during drilling. Open fractures that display millimeter-scale offsets commonly gently undulate, with wavelength >5 cm and amplitude ~0.5 mm. Unit 4 (sheared clay)Unit 4 is equivalent to Core 343-C0019E-17R (Fig. F39). Structures in Unit 4 were identified by observation of the whole-round and X-ray CT images. Most of the core is composed of clay with a variably intense scaly fabric. The scaly fabric is defined by polished lustrous surfaces, commonly striated, enclosing narrow, variably shaped and sized lenses of less fissile material, termed phacoids. There is also an interval of cohesive mudstone with sheared boundaries, between 22 and 35 cm below the top of the section. Several 0.5–4 cm thick bands of red-brown and dark brown to black clay alternate at the top of the core. Downhole from the mudstone interval, dark brown and black clay are dominant (Fig. F39). The red-brown material includes patches of the dark brown material, and vice versa. Within each of these materials, patches have sharp edges, sometimes corresponding with the shear surfaces defining phacoids. The long axes of the phacoids define the dominant pervasive foliation. In any observation section (e.g., axial, coronal, or sagittal) the phacoids appear to be bounded by two predominant orientations of surfaces, but locally in the coronal section one orientation predominates. In the uppermost 22 cm of the core, phacoids viewed in the sagittal section locally have asymmetric geometry, suggesting reverse shear sense (Fig. F40). Phacoid boundaries cannot be discerned in the X-ray CT image because the polished surfaces are too thin compared to the resolution of the X-ray CT image (~0.188 mm). The orientation of phacoid long axes is consistent at all scales, but smaller phacoids have a lower aspect ratio. In general, the fabric gradually becomes less intense downcore and phacoids become larger, with one relatively abrupt increase in phacoid size. However, more intense fabric is present in some discrete intervals also at the bottom of the core. The scaly fabric is generally slightly more intense in the red-brown clay than in the dark brown and black clay. The most intense scaly fabric is observed between 0 and 20 cm inside the red-brown clay where aligned platy (short axis <1 mm) phacoids define the foliation. Within this interval, a sharp curviplanar contact separates the predominantly red-brown from predominantly dark brown to black intervals. This contact is slightly wavy at the centimeter scale (amplitude <1 mm) and has no thickness; the foliation on either side is continuous up to the contact and is truncated against the contact, without deflecting. Just above the contact, the foliation forms ~2–3 cm asymmetric folds that are cut by the contact. On either side of the mudstone interval (20–22 and 35–40 cm), intermediate grayish brown material is observed close to the mudstone. Above and below the mudstone, the foliation orientation in the intermediate material is different to that in the adjacent rock and changes orientation across a nonplanar contact. Twisting of the clay fabric is observed along the boundaries of the relatively competent mudstone. This is likely related to reworking of the fabric during drilling. The mudstone interval includes three major sets of intersecting dark seams, some of which offset each other by a few millimeters. The major dark seams are subparallel to the bounding surfaces of lozenges in the mudstone clasts (most obvious in split working half, as illustrated in the structure tracing; Fig. F40). Below 49 cm, the scaly fabric intensity decreases markedly across a transition zone of ~1 cm. Above this contact, the long axis of the phacoids averages ~8 mm long. Below the contact, less deformed lenses (5–30 mm long) of dark brown clay containing a weak foliation are present. More intensely deformed bands of black clay containing smaller phacoids surround these lenses. InterpretationUnlike the other units in Hole C0019E, in Unit 4 the primary bedding has been completely destroyed by the shear deformation that formed the scaly fabric. Black and red-brown clays are complexly intermixed and displaced along sets of shear surfaces. Structures formed during at least two stages of deformation have been observed, with the scaly fabric (Stage 1) being locally crenulated and cut by shear surfaces (Stage 2). Less intense scaly fabric has also been observed in discrete intervals at the top of Unit 5 (maximum thickness = 10 cm). Therefore, Unit 4 has been interpreted as the most deformed within the cored interval. The section has not been oriented, but dominance of one orientation of phacoid-bounding surfaces in the coronal section (defining a less asymmetric fabric than that observed in other sections) suggests this section may be perpendicular to the dominant shear direction in the rock. The asymmetry of the fabric is consistent with reverse shear sense. The fabric of three sets of intersecting dark bands in the mudstone interval is similar to that observed within Unit 3 in Core 343-C0019E-15R, whereas the fracture sets appear similar to that opened during drilling in the top of Unit 5 (Cores 18R and 19R). It is not clear which of these two units is a better candidate for correlation. The structures within the mudstone interval may have developed during shearing of the whole unit. Alternatively, since similar structures are observed in Units 3 and 5, they could predate the incorporation of the clast inside the shear zone as a tectonic lens. Bedding dips change abruptly across the interval of scaly clay in Unit 4; the sediments above show moderate dips whereas those below are subhorizontal (Fig. F32), consistent with the abrupt change in bedding dips identified by LWD borehole image analysis (see “Logging while drilling”). We interpret that the sheared clay interval at 821.5–822.5 mbsf represents the plate boundary décollement zone. Recovery of Core 17R was not sufficient to identify the upper and lower boundaries of the décollement zone. If intervals not recovered during drilling from Cores 16R through 18R comprised sheared clay, the maximum thickness of the décollement zone could be 4.86 m. The décollement zone beneath the toe of the frontal prism at Site C0019 is localized in pelagic clay and is remarkably thinner than the few tens of meters thick décollement zones in other subduction zones such as Nankai and Barbados (Maltman et al., 1997; Moore et al., 2001). Unit 5 (underthrust sediments)
The rock type of this unit is brown silty clayey mudstone (see “Lithology”). Bedding is poorly defined, except by rare patches of discontinuous, dark mudstone. The unit is extensively bioturbated on the millimeter to centimeter scale. In places, discrete burrows can be recognized, but elsewhere there has been complete disruption of bedding. Overall, the unit is mostly homogeneous, although it has been traversed by shear fractures and, locally, dark bands are present. These brittle deformation features are observed in the core face as well as in X-ray CT images. BeddingCompositional layering in brown silty clayey mudstone with dark elongated patches of dark particles (tentatively identified as Mn oxide nodules) identifies the bedding. However, the orientation is not clear in the core face. Evidence of burrows in the X-ray CT image suggests extensive bioturbation may be responsible for destruction of bedding. However, the sedimentary contact at the base of this unit dips 7°, similar to the few observed stratigraphic contacts within the unit (bottom of Fig. F41). Locally, burrows observed on the cut core face are apparently sheared (parallel elongation and increase of aspect ratio; see “Shear surfaces” below). Weak scaly fabricWithin the unit we observed discrete zones with particularly high density of fractures. Weak, poorly defined scaly fabrics are suggested by intersecting fracture networks toward the top of this unit. The maximum thickness of any individual zone containing these incipient scaly fabrics is <10 cm. Dark seamsDark seams in this unit are planar and appear bright on the X-ray CT images. In some locations, dark seams cut burrows in the mudstone at a low angle. X-ray CT images show subtle changes in brightness across the bright bands, indicating lithologic contrasts across the bands. Therefore, we tentatively identify the dark seams as localized shear zones, deformation bands, or healed faults. Shear surfacesShear surfaces are planar discontinuities oriented oblique to the bedding plane. They are not uniformly distributed. Visual observations on the core face indicate that shear surfaces are concentrated in distinct zones from 827.2 to 827.5 mbsf. Such zones are more dominant toward the lower part of this unit, where the bands form anastomosing networks around elliptical pods, which may be primary sedimentary structures or burrows (top of Fig. F41). These dip between 31° and 62°, averaging ~45°. Units 6 and 7 (underthrust sediments)Unit 6 consists of variably colored beds of soft clay. Unit 7 is chert, represented only by a few broken clasts stuck in the clay at the end of Section 343-C0019E-20R-2 and in Sections 21R-1 and 21R-CC. This relationship is attributed to drilling damage. The primary structural measurements in these units were of bedding surfaces in the clays (average dip = 6° ± 2°). Paleomagnetic data are not available for these intervals, so bedding strikes have not been corrected to geographic orientations. Only three tectonic faults were identified (dipping 14°, 55°, and 78°). These structures offset beds but not core margins and were not associated with regions of incoherent clay that we interpret to have developed during the drilling process (Fig. F42). It is significant that drilling-induced damage included fluidization of specific clay beds, resulting in extrusion of the clay and faulting in the adjacent layers to accommodate the extrusion. Other apparent faults probably result from drilling-induced damage because they are bounded by significant amounts of soft, structureless, intruded clay. Overall, intact lamination in Unit 6 indicates that very little structural deformation and no bioturbation affected these units of the underthrust sediments. |
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