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doi:10.2204/iodp.proc.338.107.2014

Structural geology

Structures observed in cores retrieved from Hole C0022B fall into two categories: structures that can be observed along the entire cored section and structures observed at a specific interval within which the tip of the splay fault, initially identified on seismic profiles and subsequently drilled through at Site C0004 during Expedition 314, may be located (Expedition 314 Scientists, 2009b; Expedition 316 Scientists, 2009a; Moore et al., 2009). We first describe structures distributed along the entire cored section. Structures relevant to the inferred splay fault will be described in a separate section.

Bedding

A total of 190 bedding measurements were made on cores from Hole C0022B. Most bedding dips are between 0° and 20°, whereas bedding dips steeper than 20° (up to ~50°) are exclusively observed between 73.49 and 143.82 mbsf (Fig. F21). In this bedding disturbed zone (shaded section in figure), bedding dips fluctuate but, in total, progressively increase toward the bottom. From 145 mbsf downward, bedding dips remain <15°.

Bedding strike does not show any noticeable change with depth, whether above or below the bedding disturbed zone (Figs. F21, F22B, F22D). Bedding orientations within the bedding disturbed zone show scattered distributions (Fig. F22C).

Minor faults and deformation bands

Minor faults and deformation bands are common in Hole C0022B cores (Figs. F23, F24). Of 26 faults measured in Hole C0022B cores, 25 are clustered in two intervals: between 50 and 83 mbsf (fault Cluster 1), and between 386 and 405 mbsf (fault Cluster 2). With one exception dipping at 32°, all faults dip between 50° and 85°. Figure F24 does not show any consistent variation of fault strike with depth. Hole C0022B faults show two preferred orientations (Fig. F25): north–south to N20°E trending faults and N100°E to N145°E trending faults. Separately plotting faults from fault Cluster 1 and those from fault Cluster 2 shows that the former strike around northwest–southeast (Fig. F25B), whereas the latter preferentially strike north–south to northeast–southwest (Fig. F25C). Such a difference in fault orientation suggests different stress conditions between the two fault clusters.

Among the 26 faults observed on core split surfaces, 15 show a normal component of displacement. Displacement along the remaining 11 faults cannot be established. Striations observed on 4 fault surfaces (all from the footwall) have rakes around 45°, indicating oblique slips along these faults.

Deformation bands (Ujiie et al., 2004; Milliken and Reed, 2011) appear dark and homogeneous, with thickness ranging between <0.5 and 2 mm. Boundaries with the host sediment are commonly sharp but in some instances are diffuse. Because they are never crossed by burrows, even in heavily bioturbated intervals, deformation bands likely postdate sediment bioturbation. In most cases, deformation bands occur sporadically but anastomosed networks are also observed.

A total of 61 deformation bands were observed and measured in Hole C0022B core, all in the splay fault footwall and most below 275 mbsf. Figure F24 does not show any trend in the strike of deformation bands with depth. In contrast, sets of deformation bands can be distinguished on a stereographic projection (Fig. F26): N60°E to N70°E trending high-angle deformation bands, N140°E to N190°E trending high-angle deformation bands, and low-angle (<10°) deformation bands without preferred strike. In some cases, high-angle deformation bands show a component of normal displacement. Conversely, low-angle deformation bands in rare instances show reverse offsets of markers.

Megasplay fault

Three-dimensional seismic reflection studies (Moore et al., 2007, 2009) suggest that Hole C0022B may intersect the extension of the megasplay fault. In Hole C0022B, the bedding disturbed zone (73.49–143.82 mbsf) could correspond to the megasplay fault zone or flexure zone. Top and bottom depth of the bedding disturbed zone is consistent with two biostratigraphic age reversals: 76.48–84.48 and 132.38–148.46 mbsf (see “Biostratigraphy”).

In Hole C0022B between 93.36 and 95.36 mbsf, silty claystone exhibiting weakly marked planar fabrics was observed in intervals 338-C0022B-10T-5, 49–51 cm, 10T-CC, 19–20 cm, and 11H-1, 18–19 cm (Fig. F27). The planar fabrics, which could correspond to R- or P-type shear surfaces, bear faint striations. The rakes of the striations are close to 90°, suggesting dip-slip motion (pure normal slip or pure reverse slip). The above-mentioned intervals could correspond to secondary branches of a first-order fault that may not have been sampled during drilling. Indeed, although Core 10T recovery is 100%, Core 11T recovery is only 17.2% (0.86 m). This implies that 82.8% (4.14 m) of the interval drilled for Core 11T remains unsampled and hence unexplored. In summary, the interval 94.272–99.5 mbsf in Hole C0022B corresponds to a possible shear zone related to the splay fault. This estimate agrees well with the LWD data obtained in Hole C0022A (see “Logging while drilling”), which show a low-resistivity interval at 100–101 mbsf, located in a fractured zone extending from 86.6 to 105.2 mbsf.

Summary

The recovery of the bedding disturbed zone as well as foliated claystone intervals suggest that the megasplay fault zone intersected at Site C0004 (Expedition 316 Scientists, 2009a) may extend southeastward to Site C0022. However, incomplete recovery at this site precludes a definitive conclusion. Moreover, smearing during core splitting renders observation of core surfaces difficult. Additional shore-based studies, including resin impregnation of the foliated claystone intervals and thin section making, are required to better characterize the planar fabrics observed on core surfaces.