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doi:10.2204/iodp.proc.314315316.133.2009 Structural geologySite C0004 penetrated the shallow portion of a splay fault crossing the prism. As can be expected from such a location, various deformation structures were observed in cores. All structural data are given as a supplementary material (see C0004_STRUCT_DATA.XLS in folder 316_STRUCTURE in “Supplementary material”). Where possible, we corrected the measurements of planar and linear structures to true geographic coordinates using paleomagnetic data (see “Structural geology” in the “Expedition 316 methods” chapter). The distribution of planar structures and lithologic divisions with depth are shown in Figure F14. The chief structural features at this site are southeast-dipping beds and normal faults in lithologic Unit I (upper slope sediments) (0–78 m CSF), reverse faults and shear zones in Unit II (prism) (78–256 m CSF), a fractured/brecciated zone associated with Unit III (fault-bounded unit) (256–315 m CSF), and subhorizontal beds and fissility in Unit IV (underthrust slope sediments) (315–400 m CSF). Structures in Unit I (upper slope sediments)Sediments deposited on the slope are only weakly deformed. After paleomagnetic correction, bedding surfaces dip gently to moderately southeastward (Fig. F15), which is consistent with the regional strike of the Nankai accretionary margin and southeast-dipping beds displayed in the seismic reflection profile passing through Site C0004 (see Fig. F3 in the “Expedition 316 summary” chapter). The high-angle faults are concentrated between 50 and 80 m CSF (Fig. F14). Most faults are planar, but some faults are bifurcated and curviplanar to irregular. Where displacement can be determined, all faults show a normal offset with displacements ranging from 2 to 10 mm (Fig. F16). After paleomagnetic correction, the fault orientations show a scattered distribution about the vertical (Fig. F17), possibly reflecting vertical compaction of the sediments. Structures in Unit II (prism)There is no obvious deformation in the 78–100 m CSF interval of Hole C0004C. Drilling disturbance (i.e., biscuiting, fracturing, and brecciation) occurs throughout the cores recovered from prism sediments in the 100–256 m CSF interval of Hole C0004D. Particularly, drilling-induced brecciation often complicates identification of deformation structures and precludes measurements of structures. Intense drilling-induced brecciation coupled with poor core recovery cause scarcity of measurements in the prism sediments at 100–256 m CSF (Fig. F14). Despite these facts, faults and shear zones can locally be observed within the fragments of drilling-induced breccias. In addition, rare sediment-filled veins (vein structures) are observed within the fragments. In order to maintain consistent terminology between NanTroSEIZE Expeditions 315 and 316, the definitions of shear zones and sediment-filled veins follow those used during Expedition 315 (see the “Expedition 315 Site C0001” chapter). In the cores, faults and shear zones are darker than the surrounding material and are spatially close to each other. In most cases, faults cut and offset shear zones and have a reverse sense of shear with displacement less than a few millimeters (Fig. F18A). Faults are planar and <1 mm thick. In contrast, shear zones change in thickness from 1 to 5 mm along their length and tend to bifurcate into twin strands that coalesce along zones (Fig. F18B). The margins of shear zones are polished and slickenlined. On X-ray CT images, shear zones are expressed as bright bands or seams with CT numbers ~150–200 higher than those of the surrounding material (Fig. F19). CT images suggest that shear zones are denser than the surrounding material. This may result from shear-induced consolidation. Sediment-filled veins are characterized by parallel sets of sigmoidal or planar seams <1 mm wide. One fragment of drilling-induced breccia shows sediment-filled veins being displaced 2 mm along a shear zone with a reverse sense of shear (Fig. F18C). Structures in Unit III (fault-bounded unit)Below 256 m CSF, drilling-induced brecciation still dominates but core recovery improved. We define a fractured/brecciated zone between 256 and 315 m CSF based on the following evidence:
Therefore, the fractured/brecciated zone is ~60 m thick. Drilling-induced breccia between 256 and 315 m CSF is most likely derived from naturally fractured and brecciated intervals that have been enhanced by the drilling process. The seismic reflection profile passing through Site C0004 (see Fig. F3 in the “Expedition 316 summary” chapter) suggests that the projected depth of the splay fault is ~290 m CSF. There are two inversions of biostratigraphic age at ~259 and ~308 m CSF (see “Biostratigraphy”), which can be correlated with reverse faulting along the splay fault zone. Based on the correlation between these signals and our observations, we conclude that the ~60 m thick fractured/brecciated zone represents the splay fault zone. Structural elements in the splay fault zoneThe structural elements in the splay fault zone are fractured rocks, fault breccia, and microbreccia. The distribution of each element is shown in Figure F20. Our observation indicates no obvious evidence of fluid-rock interaction (e.g., mineralized veins and alteration) in the fault zone. Fractured rocksFractured rocks are defined by a breakage of rocks into trapezoidal fragments along sets of fractures (Fig. F21). Some fragments are aligned along the inclined fractures. The trapezoidal fragments are commonly centimeter size with lengths <10 cm, but their sizes locally decrease to millimeter size and are locally replaced by brecciated material. Fragments do not exhibit visible internal deformation structures. Most fracture surfaces are polished and striated. Some fracture measurements can be reoriented to true geographic coordinates using paleomagnetic data. The reoriented fracture surfaces do not show any preferred orientation (Fig. F22A), and slip directions are complex and heterogeneous (Fig. F22B). Fault brecciaFault breccia is characterized by a predominance of angular to subangular fragments of centimeter to millimeter size. Brecciated fragments are commonly slickensided with variable slip directions. Fault breccia is locally characterized by the alignment of fragments that are inclined at moderate angle with respect to a horizontal plane, showing a foliated aspect. Despite intense fracturing and brecciation, the original layering of ash and mudstone has been preserved (Fig. F23). This layering suggests distributed fracturing and shearing with very small displacements along individual slip surfaces. On CT images, brecciated intervals are expressed as bright zones in which fragments are included in the matrix (Fig. F24). The contrast between the matrix and fragments is low, and the matrix CT numbers are commonly >1100 (Fig. F25A, F25B). These features contrast with those of drilling-induced breccias, which exhibit spherical voids and bright fragments in a much darker matrix characterized by low CT numbers (700–900) (Fig. F25C, F25D). In general, high CT numbers correspond to bright colors in CT images, reflecting relatively high bulk density (low porosity), and there is a positive linear relationship between CT number and bulk density (see “X-ray computed tomography” in the “Expedition 316 methods” chapter). Apparently, fault breccia matrix is much denser than drilling-induced breccia matrix. MicrobrecciaMicrobreccia is marked by a zone of comminution that is mainly composed of angular to subangular fragments of millimeter to <1 mm size (Fig. F26). The CT character of microbreccia is similar to that of fault breccia: low contrast between the matrix and fragments and matrix CT numbers >1100. Internal characteristics of the fault zoneTwo biostratigraphic age reversals correspond to intervals of poor core recovery within the fault zone (Fig. F20). Based on recovered cores, fractured rocks and drilling-induced breccias are widely distributed in the fault zone, whereas fault breccia and microbreccia have relatively limited distribution. The upper and lower boundaries of fault breccia zones and microbreccia zones are commonly obscured by drilling-induced brecciation and nonrecovery of cores. At 291 m CSF, however, a 6 cm thick microbreccia is bounded above and below by fault breccia ~50 cm thick, which in turn is bounded above and below by fractured rocks (Figs. F20, F26). The localized comminution in microbreccia zones contrasts with distributed deformation in fractured rocks and fault breccia. Structures in Unit IV (underthrust slope sediments)Overall, the underthrust slope sediments exhibit horizontal to gently dipping bedding and fissility (Figs. F14, F27), which is consistent with the bedding dips acquired by the borehole images at this site (see the “Expedition 314 Site C0004” chapter) and the seismic reflection profile (see Fig. F3 in the “Expedition 316 summary” chapter). Fissility was reported elsewhere in the Nankai accretionary margin and is thought to result from burial compaction (Taira, Hill, Firth, et al., 1991; Moore, Taira, Klaus, et al., 2001). Steeply dipping faults are sporadically distributed in the underthrust sediments. After paleomagnetic correction, the orientations of these faults show highly scattered distribution with variable shear sense, suggesting a lack of tectonic influence associated with plate convergence (Fig. F28). Bedding-oblique foliation is recognized in mudstone at 355–360 m CSF. This foliation lacks polished and striated surfaces. The crosscutting relationship between bedding-oblique foliation and fissility is invisible at the core scale. Bedding-oblique foliation was documented at ODP Site 1178 in the upper part of the Nankai accretionary prism and is considered to correspond to the flattening plane associated with shear deformation in a fault zone (Ujiie et al., 2003). Another possible origin of bedding-oblique foliation includes an axial planar cleavage associated with folding. In the underthrust slope sediments of Site C0004, however, neither distinct fault zones nor folds are recognized in the cores or in the seismic reflection profile (see Fig. F3 in the “Expedition 316 summary” chapter); the origin of bedding-oblique foliation at Site C0004 remains unclear. DiscussionThe shallow portion of the splay fault zone is marked by brittle deformation. The aspects of fracturing and brecciation in the fault zone are similar to those of the décollement zone in the Nankai accretionary prism off Muroto (ODP Sites 1174 and 808) (Taira, Hill, Firth, et al., 1991; Moore, Taira, Klaus, et al., 2001) and the fault zones in the accretionary prism at Site C0002 (see the “Expedition 315 Site C0002” chapter). However, the splay fault zone contains microbreccia zones, including fragments that are thinner and finer than those previously reported in fault zones in shallow parts (<1 km in depth) of the Nankai accretionary prism. Microbreccia possibly represents a zone of concentrated shear within the splay fault zone. However, the location of microbrecciation is not correlated with two inversions of biostratigraphic age (Figs. F14, F20). Two intervals at which biostratigraphic age reversals occur are also candidates for concentrated shear in the fault zone. Borehole image analysis of the structurally defined fault zone (we considered depth shift between Holes C0004B and C0004D) shows that conductive fractures are relatively concentrated in these intervals (see the “Expedition 314 Site C0004” chapter). The presence of conductive fractures and/or highly deformed material associated with the concentrated shear may have caused poor recovery at these intervals. |
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