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doi:10.2204/iodp.proc.348.205.2017

Discussion

Figure F18 summarizes the deformation structures observed at both macroscopic and microscopic scale in the fault zone drilled in Hole C0002P (Section 5R-4) combined with the main characteristics of the veins, in terms of size and geometry of calcite grains together with their orientation (apparent dip) with respect to the cataclastic foliation.

Deformation regime and kinematics

The deformation structures are characteristic of a brittle regime, which crushes the silty claystones with few intercalations of sandstones affected by faulting. The rock fragments are surrounded by an anastomosing cataclastic foliation formed of arrays of microfaults in which the clay minerals show a preferred orientation.

The microfaults systematically produce lengthening of the markers. Moreover, the shear sense along with the apparent dip of the cataclastic foliation with respect to the top of the core in both meso- (Figs. F2, F3) and microscale (see shear sense and core orientation in the thin sections; Figs. F5, F7, F8, F9, F11, F12) shows either a normal component or a neutral one (horizontal apparent dip of the microfault planes). Both criteria are coherent with an extensional regime of the fault that generated the breccia, that is, an apparent normal fault sense with respect to its present-day position (see the “Site C0002” chapter [Tobin et al., 2015]), reaching the same conclusion made during core description on the ship.

Brecciation and timing of the veining

Veins are present only in the most damaged part of the fault zone. They can be accompanied by scarce carbonate cementation, indicative of the role of this interval as a fluid path. The veins are filled by the growth of calcite, a mineral that is not the main constituent of the wall rock, neither of the lithologic unit affected by the described fault nor of any of the other units drilled along Site C0002 (Expedition 314 Scientists, 2009; Expedition 315 Scientists, 2009; Strasser et al., 2014; see the “Site C0002” chapter [Tobin et al., 2015]). Hence, two working hypotheses can be formulated for the origin of the carbon:

  1. The carbonate was transported toward the fault zone from outside the drilled part of the accretionary prism, and/or
  2. Organic material in the matrix of the sediments or small traces of soluble calcite (e.g., the foraminifers of Figs. F4D, F8C) supplied local calcite.

Crespo-Blanc et al. (2016) shows that the fault veins of Section 5R-4 have generally lower δ18O values than carbonate cements in the sedimentary matrix (up to –8.7‰ for δ18O Vienna Peedee belemnite). The depleted δ18O values are consistent with veins forming at higher temperatures than those present during formation of the matrix carbonate, favoring the first hypothesis for the origin of the carbonate.

In veins, the elongated shape of the grains broadly defines their mean opening direction. In the studied case, the calcite grain elongation shows that this opening direction is mostly subperpendicular to the veinlets, independent of their orientation. Therefore, the displacement vector of the fracture is perpendicular to the fracture plane and there was almost no shear component during opening. This is characteristic of extensional fractures, which form perpendicular to the minimum principal stress (Passchier and Trouw, 2005; Bons et al., 2012). With respect to vertical of Section 5R-4, the mean orientation of the elongated shape of the calcite grains within the six observed veins are shown in Figure F18. The orientation of the vein planes are also shown, although it must be taken into account that these orientations are apparent. With the few available data, it is not possible to determine a mean opening direction, if there was such.

The frequent double palisade geometry of the elongated grains associated with the lack of optical continuity of the grains from one part to the other of the wall rock indicate that the growth direction of the elongated grains is most likely syntaxial (Durney and Ramsay, 1973). This type of growth is expected in the case of veins filled by minerals scarcely present in the wall rock (Passchier and Trouw, 2005). Growth takes place on a single plane with a consistent position within the vein, not only somewhere in the vein middle, but also nearer to one of the vein sides, as shown by the symmetry or asymmetry of the palisades, respectively. It is generally assumed that this plane is a fracture that is sealed by inward growth on both surfaces of that fracture (Bons and Montenari, 2005; Bons et al., 2012). Single grains bounding both wall rock sides indicate that growth can also be accomplished fully on one side.

A crack-seal mechanism during growth of the veins can be inferred from the presence of frequent inclusion bands of wall rock (Ramsay, 1980; Bons et al., 2012). Moreover, in the studied case, more than one inclusion band can appear on the sides of the veinlets, sometimes with irregular geometry. Accordingly, the successive crack plane can cut not only within the vein but also through the wall rock.

In terms of the timing relationship with the cataclastic foliation, the geometry of the calcite veins clearly shows that there are least two generations of these veins. The veins formed of aggregates of very small calcite grains are interpreted as early veins with initially larger grains, crushed during subsequent brittle deformation and associated with size reduction. This late deformation is evidenced by the lentil-type structure drawn by the aggregates of fine-grained calcite or the normal faults that affect this type of vein. The other four veins, formed of relatively coarse grained calcite, can be considered broadly contemporaneous and mainly postdate the cataclastic foliation. This timing relationship is clear for the veinlets that cut and seal this foliation, but the relationships of the foliation with the veins that show a rhombic distribution of the veinlets that form them cannot be directly established. Nevertheless, as in this case, the calcite grains are elongated subperpendicular to the wall rock/vein boundary, independent of the veinlet orientation, indicating that these are extensional veins. Moreover, it is not possible to observe any difference of habitus between the elongated calcite grains of veinlets that cut the cataclastic foliation and those of veins that follow it. As a consequence, it is suggested that their opening occurred along the already formed cataclastic foliation planes, which acted as weakness planes. In this case, the veins would mimic the cataclastic foliation distribution. Finally, some of these coarse-grained calcite veins deformed by extensional microfaults possibly formed domino-like structures.

These observations lead to proposing the following multistage evolution of the fault breccia and associated veins. The presence of the veins restricted to the fault zone shows that the fault zone acted as a fluid path. Accordingly, it is likely that opening of the extensional fracture took place due to the effect of elevated fluid pressure (hydrofracture). The carbon-rich fluids precipitated in the fracture, taking advantage of the cataclasitic foliation plane to develop. Then, as deformation proceeded, it affected the veins formed in the first stages, and new veins developed which in turn are deformed. This describes a reiterative history of brittle deformation and veining during an (apparent) extensional regime.

Concluding remarks

  1. The shear zone from Hole C0002P was formed during a brittle regime, with the development of an anastomosed cataclastic foliation in silty claystone and scarce levels of sandstones.
  2. The kinematic indicators observed in thin section are coherent with an apparent normal fault sense with respect to its present-day position.
  3. The veins present in the most damaged part of the fault zone are extensional veins filled with calcite, probably transported from outside of the drilled part of the accretionary prism.
  4. The veins are likely of syntaxial type and opened through a crack-seal mechanism.
  5. Two generations of veins were observed: a first one strongly affected by brittle deformation and a second one sealing it, although this second generation is scarcely affected by extensional microfaults.

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