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doi:10.2204/iodp.proc.348.205.2017

Results

Macroscopic observations

The available photographs of the whole Section 5R-4 can be seen in Figure F2. These images correspond to the archive half photographed on board the ship and the two surfaces of the slab made for thin section preparation at Kochi Core Center. As the slab is composed of three pieces, its total length is slightly longer than that of the core. Moreover, the photographs are not continuous.

The whole Section 5R-4 can be considered as an incohesive fault breccia in the sense of Sibson (1977) with more than 30% large clasts (>2 mm) in a clay-rich matrix. The clasts are formed of either silty claystones and/or sandstones and their size is heterogeneous, occupying the whole core diameter (6.4 cm) to <0.1 cm. From 57 to 91 cm of the archive half, the clasts are smaller than those in the remainder of the fault zone (Fig. F2A). This is considered the most damaged part of the fault breccia in Section 5R-4. This is also the only part of the core where calcite veins appear (whitish tones, clearly observable around 59, 79, and 87 cm offset in the archive half) (see Fig. F3).

The anastomosing cataclastic foliation, whose mean orientation is indicated on the right side of Figure F2, is defined by the preferred orientation of the centimeter- to submillimeter-spaced, parallel fracture systems, which affect the silty claystone and sandstone, surrounding elongated lentil-shaped clasts. Note that the three pieces of Section 5R-4 rotated differentially; accordingly, the present-day orientation of the cataclastic foliation in the section varies from one piece to the other.

The anastomosing character of this foliation appears broadly as two families of clear or dark lines (Fig. F3A or F3B, respectively) cutting the silty claystones at an angle of around 50˚–60˚. This foliation corresponds to microfaults. These microfaults systematically produce lengthening of the markers. For example, a domino-like structure can be seen in Figure F3B. In Figure F3A, an asymmetric clast of sandstone, 4 cm long, appears in the most damaged zone of the fault breccia. Its shape is consistent with the microfault kinematics, which are indicated in Figures F2 and F3.

Deformation structures observed in thin sections

The thin sections correspond to the surface of the slab shown in Figure F2C. They are located according to IODP conventional numeration, based on the measuring tape situated below Figure F3C, and are named as such. Nevertheless, the thin sections do not cover the entire specified length, as shown in Figure F3, where the exact area of the thin sections has been drawn on the selected slab surface. The thin sections are vertical, and the top of the core is indicated in each of the photographs.

The characteristic sedimentary texture of the undeformed rocks of the fault zone are shown in Figure F4. The rocks consist of silty claystones and sandstones. The boundary between sandstone and fine-grained silty claystone, which can be used as a marker of the deformation, is generally sharp and consists of microfaults. Sometimes, the bedding is preserved and a transitional boundary between both lithologies can be observed. Also, some elongated patches of claystones within the sandstones and/or finer grained claystones are present (mud ball, Fig. F4A–F4B). The sand grains within the sandstones are angular and embedded in sparse matrix/cement (Fig. F4C). Clasts of quartz are dominant (monocrystalline grains) and those of feldspar are abundant, whereas lithic fragments are scarce. Few fossils are preserved (Fig. F4D). A complete description of the lithology of the undeformed rocks can be found in the “Site C0002” chapter (Tobin et al., 2015).

Microfaults are the most common deformation structure observed in the fault zone thin sections. At core scale (Figs. F2, F3), the geometry of the microfaults forms lenticular structures surrounding clasts of sandstones or claystones (Fig. F5). The microfaults and the lenticular structures define the anastomosing character of the cataclastic foliation. The shear sense along this cataclastic foliation is revealed by the deflection and/or displacement of the markers, as shown in Figure F5A. In this example, with respect to the top of the core, the shear sense component is that of a normal fault (present-day position).

Within the microfault planes, the clayey minerals show a preferred orientation, highlighted under crossed nicols in Figure F6. These faults are located generally along the boundary between competent sandstones and less competent claystones, if both lithologies are present in the thin section (Fig. F7). Sometimes, deformational structures are concentrated along the vicinity of this boundary, as shown in Figure F7A–F7B, where an array of microfaults developed in the claystones. The boundary between both lithologies can also be cut by the microfaults that comprise the cataclastic foliation (Fig. F7C). In this case, the quartz grains are crushed and reduced along the microfault plane (Fig. F7D). Opaque material also concentrates along the same plane.

In the vicinity of these microfaults, drag folds can be present (Fig. F8). The drag is generally marked by the clayey material included in a previous microfault but can also be drawn by the fragments of broken fossils. In this case, observe how the drag fold, shown by the progressive alignment of the fossil fragments more marked toward the microfault, is coherent with the movement of the brownish claystone marker of Figure F8B.

Concerning the kinematics of the microfaults, it is striking to note that they systematically produce lengthening of the markers (original bedding, layer bounded by previous microfaults, layer of opaques, etc.). For example, this is very clear in Figure F9A, where an array of microfaults cut a layer of claystones intercalated between sandstones. With respect to the present-day position of the core top, as indicated in each photograph, most of the microfaults show a component of normal shear sense along their apparent dips (indicated in Figs. F5, F7, F8, F9). Finally, the frequent staircase geometry of the band of opaque minerals is coherent with both the observed microfault shear sense and the marker lengthening (Fig. F9), which in turn is seen at all scales, from core scale (Fig. F3) to microscopic scale (Figs. F5, F7, F8, F9).

Calcite veins and their relationships with deformation structures observed in the fault zone

Calcite veins appear only in the most damaged part of the cored fault zone, from 57 to 91 cm (archive half), which is within the 34 cm interval where the cataclastic foliation is particularly well developed (Fig. F3). At macroscopic scale, these veins appear as whitish ribbons (Fig. F2A, particularly around 59, 79, and 87 cm offset in the composite section).

Six of these ribbons are present in thin sections TS 57–64, TS 64–71, TS 71–78, and TS 78–85. The locations of these ribbons are shown in Figure F3C, and the large side of the rectangle coincides with their mean direction. The ribbons show different habits: two veins are formed of aggregates of very small calcite grains, and four of these veins appear as a swarm of veinlets composed of relatively coarse grained calcite.

Veins formed of aggregates of very small calcite grains

The aggregates of very small calcite grains are concentrated along bands subparallel to the cataclastic foliation (Fig. F10). The boundaries between wall rock and veins are generally well defined (Fig. F11A, upper part of Fig. F11B) but they can also be fuzzy, as calcite cements impregnate the wall rock in the vicinity of the vein boundary (lower part of Fig. F11B, F11D). In these veins, the calcite grain size varies from approximately 0.05 to 0.005 mm (e.g., Figs. F11, F12). This variation can be distributed within the whole aggregate or concentrated along bands subparallel to the cataclastic foliation (e.g., in the upper part of the vein shown in Fig. F12B, F12D).

Deformation affected the geometry of these aggregates of grains and they can show a lenticular shape, similar to the clasts composed by claystones situated in their vicinity (Figs. F11A, F11C, F12A, F12B). At the same time, these calcite aggregates can surround the claystone lentil-shaped clasts. They can be distributed along two families of bands that draw rhombs underlining the microfault arrays of the cataclastic foliation (Fig. F12B, F12C). The shear sense deduced from the microfaults that affected the aggregates of calcite grain shows a normal component (except Fig. F12B, in which the component of movement is neutral). This is coherent with the conjugated normal microfaults marked by the very fine grained bands in Figure F11B, F11D.

Finally, it is interesting to note that in Figure F11A another type of vein can be observed over the band of aggregates (left of the photograph, also see detail in Fig. F15A). The figure shows coarser calcite grains, which clearly indicate the presence of at least two generations of veins.

Veins formed of relatively coarse grained calcite

A swarm of veinlets make up a second type of veins (Figs. F13A, F13B, F14A, F14B). In the vicinity of these veinlets, isolated grains of calcite can also impregnate the wall rock (Fig. F13C, F13D). Within the veinlets, the calcite grains are generally coarse and elongated. The elongation is subperpendicular to the vein/wall rock boundaries and the crystals show little or no increase in width along their growth direction. The vein/wall rock boundaries are mostly sharp, although not perfectly straight, as there are some irregularities at the scale of the grain width (Figs. F14, F15, F16).

The geometry and distribution of these veinlets can vary. Very frequently, these veinlets draw a rhombic network with the same orientation as the microfault planes that surround the lentil-type claystone clasts, although a continuum can be observed from the rhombic network that mimics the cataclastic foliation (Fig. F14) up to the veinlets that cut and seal this foliation (Fig. F15). The calcite grains that form the veinlets are undeformed, as shown by their optical continuity, without undulose extinction (Fig. F14C). It must be stressed that independently of the veinlet orientation, the direction of the calcite grain elongation is subperpendicular to the vein boundaries (Fig. F14A, F14B).

Locally, some late deformation of this type of vein is shown by the domino-like structure drawn by the calcite grains (Fig. F15B) or by a microfault cutting the vein (Fig. F16A, F16B). It is worthwhile to note that the example in Figure F15 is in the vicinity of the band with very small calcite grains (lower part of Fig. F15A).

Within the veinlets are inclusion bands formed of fragments of the wall rock (Figs. F13C, F13D, F15A, F15D, F16C, F16D, F17A–F17C). These wall rock inclusion bands mostly show irregular geometry, although broadly subparallel to the veinlet boundaries. At microscopic scale, these wall rock inclusion bands are relatively wide, but in a few cases, very thin lines parallel with the vein/wall rock boundary can also be interpreted as inclusion bands (Fig. F17D, F17E).

The calcite grains are generally straight, with optical continuity (Figs. F14C, F15D, F16B, F17C, F17E). The veinlets are filled either by two palisades of grains that converge within the veins with an irregular boundary (radiator shaped according to Bons et al., 2012; Figs. F14, F15, F16, F17) or by single grains that join both wall rocks (Figs. F15C, F16C, F16D). In the case of two grain palisades, these latter single grains can be either symmetrical or asymmetrical. If symmetrical, the crystals are not in optical continuity across the vein. Consequently, they probably do not grow from a median suture line toward the walls. If they are asymmetrical, it is worth noting that this asymmetry is very high and that the shortest grains of calcite generally coincide with the presence of wall rock inclusion bands (Fig. F17A–F17C).

Rarely, the calcite grains can show a slightly curved geometry. In this case, no undulose extinction is observed (Figs. F16C, F16D, F17D, F17E). Accordingly, this geometry is due to a variation of the veinlet opening direction and not late deformation superimposed on the calcite crystallization. Finally, broadening of the calcite grains along the same vein likely indicates propagation of the vein tip (to the left in the case of the lowest vein in Fig. F16C, F16D).

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