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doi:10.2204/iodp.proc.314315316.123.2009

Structural geology

The main goal of the structural geologists during Expedition 315 was to describe and document the style, geometry, and kinematics of structural features observed in the cores. Drilling at Site C0001 retrieved >450 m of core from three holes (C0001E, C0001F, and C0001H), and drilling during Expedition 314 retrieved an additional 30 m of core at this site from Hole C0001B (0–32 m CSF) that were also described by Expedition 315 scientists. Although the total drilling depth at Site C0001 was shorter than the planned total depth of 1000 mbsf, >550 individual structural features, including 135 bedding measurements, were documented and described. Hole C0001H (the deepest hole cored at this site) also yielded the largest number and greatest variety of structures, although geologic structures and bedding measurements from Holes C0001B, C0001E, and C0001F provided additional information critical to understanding regional kinematics. Drilling-induced structures, identified primarily in Hole C0001H, were also documented. Many structural features were also reoriented to a geographic reference frame using shipboard paleomagnetic data. Overall, structural data show significant changes with depth, particularly above and below a zone of deformation that occurs 15–20 m below an unconformity that separates the slope apron from the underlying sediments thought to have originated on the ocean floor of the Philippine Sea. Kinematics derived from structures at Site C0001 confirm and expand upon the orientations of stresses obtained from LWD data during Expedition 314.

Types of structures

A range of planar deformation structures was observed in Expedition 315 cores. Most could be classified as belonging primarily to one of three types of structures: faults, shear zones (or deformation bands), or “vein structures.” This classification is not meant to imply that all structures were only one type—in fact, many structures showed features characteristic of all three types—but at this preliminary stage, and with limited thin section and other microscopic analyses, we chose to use a limited number of broader categories. Descriptive modifiers and sketches provide additional information on the similarities and differences of individual structures.

Faults

Faults are relatively planar, narrow zones of deformation characterized by a single band of concentrated deformation at the scale of the core and CT images. Faults also show clear evidence of displacement, including offset markers (e.g., bedding, burrows, or small pumice fragments), fault drag, and slickensided rough or polished surfaces (Fig. F11A, F11C, F11D). They are also typically slightly bright in CT images, suggesting a higher density than the adjacent wall rock (Fig. F11B).

Shear zones

Shear zones (or deformation bands) are generally wider (>2 mm) than faults and are often composed of multiple sets or bands of concentrated deformation (Fig. F12A). In many cases, the outer edges of a shear zone are sharp and more planar than the anastamosing surfaces of deformation within the zone. CT images also suggest a range of densities within individual zones and between different shear zones (Fig. F12B). On the cut surfaces of cores, shear zones commonly appear darker than wall rock (Fig. F12C), and in thin section, phyllosilicates display a range of crystallographic preferred orientations (CPOs), from well-developed bands to distributed clusters of phyllosilicates with different CPOs in different parts of the same thin section (Fig. F12D).

Vein structures

Vein structures include a variety of structures, many of which have been identified on previous ocean drilling expeditions and interpreted as dewatering structures. This category of structures includes the classic sigmoidal-shaped suite of thin, mud-filled veins that are slightly darker than the surrounding materials and first documented by Ogawa (1980) and Cowan (1982), as well as other veinlike structures that appear darker (Fig. F13) than the surrounding sediment and slightly brighter (or denser) in CT images (Fig. F13C). These other veinlike structures include single planar to subplanar features that are several millimeters thick but associated with only limited displacements and rare to poorly developed slickenlines. Dark veins are often spaced at regular intervals in the horizontal plane (Fig. F13A, F13B). In some cases, veinlike structures extend subparallel to the core axis for decimeters; in other cases, they occur at moderate to low angles to the core axis with limited extent. Apparent offsets across these structures in the split core indicate mostly normal displacements (Fig. F13B).

Steepened bedding

Steepened bedding is an important and relatively sensitive indicator of deformation. Seismic reflection studies and LWD data suggest that bedding dips were relatively shallow in the upper 500 m in the area of this site, although dips as steep as ~50° were documented at ~200 m LSF. These findings were largely corroborated during Expedition 315 through core and CT image logging efforts, although some important differences emerged, as described below.

Breccia

Breccia is marked by a relatively high concentration of small polished and slickenside, lens-shaped fragments of clayey silt or silty clay. Zones of breccia range in thickness from a few centimeters to 1 or 2 dm and are often associated with zones of poor recovery. Breccia was recognized at 160–170 and 218–220 m CSF.

Kink bands

Only two occurrences of kink bands were documented (Korig and Lundberg, 1990). More detailed analysis may show, however, that some of the structures classified as faults or shear zones evolved from kink or kinklike structures.

Drilling-related structures

Drilling, particularly with the RCB in Hole C0001H, induced a range of styles of fractures and slickensided surfaces that in some cases appeared to be very similar to small faults and in other cases overprinted preexisting faults or shear zones. Fractures that appeared to be clearly related to drilling ranged from planar surfaces perpendicular to the core axis and displaying concentric or circular slickenlines to conspicuous cones or “witch hats” that point vertically up in the core reference frame. Planar features, in addition to circular slickenlines, display unusually flat and polished surfaces. Cone-shaped fractures, in contrast, often expose broken and only slightly polished surfaces, and slickenlines, if present, spiral up the sides of the cone. In cases where drilling-induced slip occurs on preexisting faults or shear zones, faulted surfaces appear unusually smooth and polished, similar to the polished surfaces with circular slickenlines. Reactivation of preexisting faults was also interpreted based on kinematic analysis, as described below, and a selection of these kinematically identified faults showed some of the features associated with clearly drilling-induced faults (e.g., unusually smooth and polished surfaces).

Distribution of structures

Figures F14 and F15 show the distribution and dip of the dominant deformation structures at Site C0001 and highlight both the heterogeneity of deformation throughout the site and the dominance of structures at or below a zone of breccia and concentrated deformation at ~220 m CSF.

Bedding generally dips gently with only three anomalous zones: ~80–100, 140, and ~200–205 m CSF (Fig. F14). Each of these zones of increased bedding dip also appears to generally correlate with structural features identified at Site C0001. For example, the zone between 80 and 100 m CSF occurs just above an interval of slumping documented in the lithology, whereas the cluster of increased bedding dips at ~140 m CSF correlates with a possible normal fault suggested by an apparent offset of seismic reflectors (Fig. F15) and is likely related to a breccia zone at 160–170 m CSF, as well as a cluster of normal faults at 150–160 m CSF (Fig. F14). Bedding-dip directions in the interval 140–205 m CSF are also significantly more organized than elsewhere, ranging from ~270° to 360° (Fig. F14). Finally, the well-defined zone of steepened bedding at 200–205 m CSF occurs just above the Unit I/II boundary and the zone of concentrated deformation suggested by clusters of shear zones and normal faults (Figs. F14, F15).

Shear zones are present almost exclusively below the Unit I/II boundary and form three groups with different dip magnitudes: very gently dipping, moderately dipping, and steeply dipping (Fig. F14). Moderately dipping shear zones dip ~50°, are present just below the Unit I/II boundary and correlate with a cluster of moderately deepening normal faults at the same depth interval. Very gently dipping and steeply dipping shear zones have mean dips of 15° and 80°, respectively, and occur below ~240 m CSF (Fig. F14).

Faults are generally present at all depths at Site C0001, but only normal faults and a few relatively minor thrust faults occur above ~220 m CSF. Clusters of normal faults occur from 150 to 160 and 220 to 240 m CSF (Fig. F14), and both clusters are associated with breccia zones. Normal faults above ~240 m CSF have steep to moderate dips similar to the cluster of shear zones at the same depth (Fig. F14). Below ~240 m CSF, the range of dip magnitudes increases, although many of the normal faults have relatively steep dips (e.g., 80°), which is also similar to the dip of the shear zones below this depth (Fig. F14). Both thrust and strike-slip faults have a range of dip magnitudes (Fig. F14).

Geometry and crosscutting relationships

In the most deformed sections of Hole C0001H, structures have been found in association with one another, allowing sense of movement and crosscutting relationships to be established. As a general rule, the oldest structures observed are shallow-dipping (<30°) black shear zones. In most cases, sense of shear could not be determined for these shear zones, although based on rare shear sense indicators, it seems that some of them might have a thrust offset. These shallow-dipping shear zones are commonly cut by steeper (>60°) dark-colored shear zones. Steeper shear zones are often observed to merge by coalescence of several thin black “dewatering” veins. At the hand specimen scale, steep shear zones display both reverse and normal offsets, but normal offsets are more frequently observed and are consistent with the microscale normal movement of dewatering veins (see thin bed at middle of Fig. F13B).

Shear zones and dewatering structures are systematically cut by faults, indicating that the faults are younger. The fact that faults are not filled with black material indicates either that these structures are too recent for chemical alteration to have occurred or that interstitial fluid chemistry has changed since the black deformation structures formed. Alternatively, these differences may reflect fundamentally different grain scale micromechanical processes (e.g., dilational versus compactive processes). Normal and thrust faults display mutually crosscutting relations, and both are crosscut by shallow-dipping strike-slip faults.

Kinematics

Using shipboard paleomagnetic data, ~30% of the planar structures were reoriented to true north. The remaining data could not be reoriented because of extensive whole-round sampling, relatively strong drilling-related magnetic overprint, and drilling-related disturbances. Reoriented data provide key insights into the state of stress in sediments cored at Site C0001.

Fault orientations are strongly contrasted above and below the highly deformed zone at ~220 m CSF. Above this zone, ~90% of the faults described are sets of normal faults dipping at 60° either northeast or southwest (Fig. F16A). These normal faults are consistent with a vertical maximum principal stress (σ₁), a minimum principal stress (σ₃) oriented northeast, and an intermediate principal stress (σ₂) oriented southeast (Fig. F16B). Deformation in the slope apron is therefore dominated by extension subparallel to the trench. Stress state is consistent with both seismic reflection profiles that show steep, northwest-striking normal faults (see Fig. F2 in the “Expedition 314 Site C0001” chapter) and borehole breakout data that show a north-northwest–trending maximum horizontal stress (see the “Expedition 314 Site C0001” chapter). A few moderately dipping thrust faults were also encountered above 220 m CSF. Fault kinematics (corrected for drilling-induced rotations) indicate northwest–southeast shortening subparallel to the direction of plate convergence (Fig. F17).

The geometry and kinematics of planar structures display greater variation below ~222 m CSF than above it (Figs. F18, F19, F20, F21). Nonetheless, kinematic analyses computed from normal (Fig. F18B) and thrust faults (Fig. F19) are respectively consistent with northeast–southwest extension and northwest–southeast shortening, as observed in the slope apron.

Strike-slip faults only occur below ~220 m CSF and display heterogeneous kinematics, including an unusually large number of low-dipping fault planes. Stereographic projections of fault planes and associated slickenlines, along with tangent lineation diagrams where the pole to the fault and the movement of the footwall relative to the hanging wall are shown, however, suggest two populations of faults (Figs. F20, F21). One population contains faults dipping <50° and records a clockwise sense of motion for the hanging wall relative to the footwall (Fig. F20B), which is the same sense of motion as the drilling pipe during drilling on the Chikyu. These faults also generally strike north–south (Fig. F20A). The second set contains more steeply dipping faults (generally >50°) and shows northwest–southeast shortening. Fault surfaces of the low-dipping strike-slip faults also appear to have more irregular steps than true tectonic faults. Consequently, we interpret the set of low-dipping strike-slip faults to be preexisting faults reactivated as strike-slip faults during drilling. The fact that shallow-dipping strike-slip faults crosscut all other structures supports this interpretation. After removal of shallow-dipping faults, the remaining steep strike-slip faults yield a kinematic solution consistent with the northwest–southeast shortening and northeast–southwest extension (Fig. F21) inferred from the other faults (Figs. F18B, F19B).

Discussion

The superposition of normal, thrust, and strike-slip faults as observed in the deeper sections of Site C0001 may have a number of causes. One possibility is that as the sediments moved through the accretionary prism and away from the trench the overburden-induced (i.e., vertical) stress became proportionally more important. This may have involved reorientation of σ₃ from vertical to horizontal and northeast trending (Fig. F22) and a reorientation of σ₁ from northwest trending to vertical. This interpretation would explain, in particular, why older sediments of the accretionary prism are affected by faults with a range of kinematics, whereas the slope apron is mostly deformed by extension. Alternatively, principal stress permutations could be caused by increases and decreases of the northwest–southeast plate convergence stress related to the Nankai seismic cycle. This would explain the absence of clear overprinting relations between fault types. A third possibility is that differential stresses are small and minor local perturbations of the stress field are the cause of stress permutations and resulting diversity of fault types.

Structural data collected from Site C0001 cores provide a critical third dimension to the stress data obtained from LWD during Expedition 314, especially for stress orientations in the sediments of the slope apron. For example, borehole breakouts observed in LWD logs consistently show that σ_Hmax trends northwest–southeast at least in the uppermost 500 m of the hole where the breakouts were observed. This trend for σ_Hmax is approximately perpendicular to the plate margin, suggesting that it represents σ₁. Site C0001 core-based structural data show, however, that the uppermost 200 m (i.e., the sediments of the slope apron) are dominated by normal faults that trend north-northwest. These data suggest that σ₁ is vertical rather than horizontal and that σ₂ is equivalent to σ_Hmax, which trends perpendicular to the margin. Thus, relative magnitudes of the horizontal stresses interpreted from the LWD borehole breakout data are consistent with the fault data, but actual principal stresses can only be deduced from fault kinematics.

The extension direction interpreted from normal faults in the slope apron (i.e., margin parallel) also contrasts with the extension direction interpreted from Site C0002 borehole breakout data, which show margin-perpendicular extension. The regional significance of this change across the margin from shelf to accretionary prism will be part of our postcruise research program, but these preliminary data suggest Site C0001 may represent the transition from horizontal compression near the trench to horizontal extension on the shelf.

The consistent trend of σ_h determined from borehole breakout data at Site C0001 suggests continuity of stresses throughout the hole, although the uppermost 200 m may be dominated by gravitationally driven slope stresses, whereas the lower few hundred meters are dominated by tectonic stresses. The vertical transition from gravitationally driven to tectonically driven stresses may therefore reflect the change in magnitudes of these stresses with depth.

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