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doi:10.2204/iodp.proc.319.103.2010

Structural geology

The structural geology of Hole C0009A included the study of cuttings from 1097.7 to 1512.7 m MSF (Samples 319-C0009A-87-SMW through 173-SMW) and cores from 1510.5 to 1593.9 m CSF (Cores 319-C0009A-1R through 9R). Analysis of FMI resistivity image data from 710 to 1580 m WMSF was also performed.

Types and distribution of structures in cuttings, 1097.7–1512.7 m MSF

Cuttings were studied with a binocular microscope. Approximately 20 grains were selected from the >4 mm size fraction at each 5 m depth interval; smaller grains were not large enough to systematically recognize structures. Structures recognized include slickensides, vein structures, and web structures. Slickensides are grooved, polished, and striated (or slickenlined) surfaces (Fig. F25). Vein structures are subparallel dark bands visible on the surfaces of the cuttings. There are no apparent offsets across vein structures at the millimeter scale, and in places they have anastamosing geometries and splays (Fig. F26A–F26E). Web structures are similar to vein structures in their appearance but form a wider array of crosscutting geometries and some accommodate small (<1 cm) displacement (e.g., Fig. F26F).

Thin sections of representative vein structures illustrate their microstructural characteristics (Fig. F27). The well-defined vein structures do not contain obviously fractured grains and did not fracture grains in the surrounding matrix. Cross-polarized views do not show a particular crystallographic difference between vein structures and the surrounding rock (Fig. F27A), but some do appear to be finer grained than the surrounding rock. Displacements across vein structures are not apparent though, locally, shear-strain indicators are present, such as inclined granular foliation (Fig. F27B, F27C). Lenses of opaque minerals are locally preserved adjacent to vein structures (Fig. F27D).

Discussion

The fact that grains were not fractured by the vein structures suggests that the sediments were incohesive at the time of deformation (see "Microstructures"). This interpretation is also consistent with previous studies. For example, similar structures have been observed on the Boso-Miura Peninsula in forearc basin sedimentary rocks where they are usually oriented perpendicular to bedding. Vein structures were also found in slope apron and slope basin deposits of the Middle America Trench off Guatemala (Cowan, 1982) and Peru (Lindsley-Griffin et al., 1990). Brothers et al. (1996) and Ohsumi and Ogawa (2008) have shown that vein structures have a relationship between height and spacing and are interpreted as forming by oscillatory deformation such as shaking of incohesive sediments (e.g., resulting from an earthquake or a debris flow).

The most important structural observation in the cuttings is that the vein structures are more numerous in an ~200 m depth interval between ~1300 and ~1500 m MSF (Unit IV) than in any other interval. Slickensided surfaces and web structures, although less common than the vein structures, are also concentrated in this interval. Vein structures are rare in the cuttings above 1332.7 m MSF (cuttings Sample 319-C0009A-136-SMW) and below 1482.7 m MSF (cuttings Sample 319-C0009A-167-SMW) (Fig. F28; see C0009_T2.XLS in STRUCGEOL in "Supplementary material"). Web structures were only identified in cuttings from Samples 319-C0009A-162-SMW and 165-SMW (1452.7 and 1467.7 m MSF, respectively). Slickensides have variable appearances (Fig. F25) and in most cases coincide with vein structures. Although some of the slickensides may have formed during drilling, their restricted depth range and the correlation of this depth to other primary structures suggests that they are original structures and have tectonic significance.

The depth interval of abundant vein structures in Hole C0009A is below Unconformity UC2 and in lithologic Unit IV, which is below the comparable depth range of vein structures recognized at Site C0002 (Expedition 315 Scientists, 2009). Based on previous research on vein structures, structures at Site C0009 may have formed during seismic shaking of then partially consolidated materials (Ogawa, 1980; Brothers et al., 1996).

Types and distribution of structures in cores, 1510–1593 m CSF

We identified a number of structural features in the cores, including bedding and planar fabrics, shear zones, faults, slickenlined faults, and vein structures (see Fig. F29). These terms are only descriptive and do not imply a specific deformation mechanism.

Bedding

Bedding was measured systematically in the core. Bedding is best defined where sand or silt layers are in contact with adjacent finer layers or sedimentary structures are present. Bedding can also be recognized where trace fossils and zones of bioturbation are aligned parallel to bedding and are preserved in their undeformed state.

Planar fabrics

Planar fabrics are defined by discontinuous sand or silt layers, zones of bioturbation, or parting planes (folia) within muddy lithologies. Increasing fabric intensity toward shear zones and deformed trace fossils or sedimentary structures can be used to distinguish planar fabric from bedding.

Shear zones

Shear zones are planar bands that are dark relative to the surrounding rock and sediment (Fig. F29A). They range in width from <1 mm to ~1 cm. They are composed of fine sediment particles and transmit light in thin sections, even though they are dark or even black in the cores. They locally show evidence of shear, crosscut primary sedimentary features, and are crosscut by other structures; many do not show evidence of shear displacement (Fig. F29A). Locally, shear zones occur at the center or top of thicker (~10 cm) deformed intervals. Elsewhere, shear zones cut across apparently undeformed sediment and rock. Where offset markers or sense-of-shear indicators are present, the sense is clear reverse (thrust) displacement. On the cut face of the core, many shear zones are apparently oriented parallel to subparallel to bedding. However, 3-D examination generally indicates that these shear zones are oblique to bedding. Commonly, shear zones form networks of thinner anastomosing zones.

Faults

Faults are thin (<1 mm), planar, or listric structures that are dark relative to surrounding rock. Faults are generally discrete surfaces, whereas shear zones are thicker zones of deformation. Faults typically show clear evidence of displacement at core scale, including offset markers (e.g., bedding, burrows, or small pumice fragments) and fault drag. Faults are distinguishable from shear zones because they are thin and commonly show displacements. Locally, faults cut shear zones, although in many places they "sole" into shear zones or are part of fault networks adjacent to shear zones (Fig. F29B, F29D).

Slickenlined faults

Slickenlined faults have slickenlines and rough or polished surfaces. Slickenlined faults cut all other structures, and in most instances have striated surfaces. Some slickenlined faults clearly displace primary sedimentary or preexisting structural features. There are generally few recognizable markers, and shear sense has to be interpreted from the slickenlines' texture (Fig. F29C). Slickenlines are widely associated with steps or, less commonly, spoon-shaped indentations on the faulted or sheared surface, producing an asymmetry that reflects the slip sense during deformation (e.g., Petit, 1987; Angelier, 1994). Steps were interpreted as extension cracks (or "R" shears activated as extension cracks) that connect en echelon "P" shears within the fault or shear zone. In some cases, the combined set of P shears and extension cracks provides an indication of shear sense; in other cases, extension cracks or P shears alone were used as evidence for slip sense. Spoon-shaped indentations were interpreted as R shears within the fault or shear zone and indicated a sense-of-slip synthetic to the main fault or shear zone.

Vein structures

Vein structures are relatively rare. Vein structures in cores are dominantly of the sigmoidal thin mud-filled extensional crack or vein type (Ogawa, 1980; Cowan, 1982; Brothers et al., 1996). Although only six examples of vein structures were recognized in cores, numerous examples occur in cuttings from ~1325 to 1475 m MSF (see above). Figure F30 shows that they are filled with clay-sized particles.

Microstructures

Thin sections from parts of the core were made to investigate three structures: vein structures, shear zones, and faults. Figure F30 shows that vein structures are filled with clay particles and microcrystalline residue or precipitate. Shear zones occur both parallel to and crosscutting sedimentary layering (Fig. F31). Locally, shear zones show demonstrable sense of shear. Shear zones are typically darker and finer grained than the adjacent bedding. A shear zone associated with bedding is shown in Figure F32. The shear zone has several finer grained discrete shear zones separated by more coarse grained regions (Fig. F32A, F32B). The orientation of possible Riedel shears (Fig. F32A, F32C) suggests a sinistral slip sense. Stratigraphic up is the top of the photograph, so overlying units verge in the updip direction. Faults are observed in thin sections as bedding truncations (Fig. F31D, F31E).

Mineralogy of shear zones

We used XRD analysis to document the mineralogy of a bedding-parallel shear zone and compare it to the sediments above and below. The results are shown in Figure F33 for material above the shear zone (Fig. F33A), within the shear zone (Fig. F33B), and below it (Fig. F33C). The bulk mineralogy of the three zones is nearly identical, showing basically quartz, feldspar, and minor illite, suggesting that the material within the shear zone formed from the same sediment surrounding it, with no mineralogical differentiation.

Structural orientations

Structures in cores show distinct statistical groupings in orientation. Bedding has modal dips of 30°–50°. Shear zones have modal dips of 30°–40°, reflecting that a number of the shear zones are parallel to bedding planes. Faults have two dip modes of 10°–30° and 50°–70°, possibly reflecting both thrust and normal faults. But because most faults do not have a clear sense of offset, the relationship of steepness of dips to slip sense is unclear. Slickenlined faults have modal dips of 20°–40° and 50°–60°, with the latter, steeper inclination more common. Further analysis of the kinematics and geographic orientations of the structures could further elucidate if they belong to separate geometric populations (Fig. F34).

The magnitude of bedding dips based on observations from the cores and FMI image data (see "FMI borehole image interpretation") show a change with depth from a range of ~10°–35° at 1510–1520 m CSF to ~40°–70° at 1570–1580 m CSF (Fig. F35).

Crosscutting relationships and kinematics

We interpret the relative age of structures as follows: the early shear zones are dominantly thrusts, intermediate generations of faults are mixed normal and thrust faults (potentially accommodating a spatially and temporally complex strain history), and the youngest faults accommodate dominantly oblique slip. Crosscutting relationships in support of this interpretation include the following:

• Slickenlined faults crosscut all other structures, and are therefore youngest;

• Faults are intermediate in age, in places crosscutting shear zones, but in other places soling into them or forming adjacent networks; and

• Shear zones appear to have formed when the sediments were relatively incohesive as they are locally associated with folds and fabrics that coincide with thinning and thickening of silt-sand horizons.

In detail, structures show diverse kinematic relationships. The shear zones are mostly thrusts (Fig. F31E), more rarely normal faults (Fig. F31D), or exhibit ambiguous kinematics associated with distributed strain at low angles to bedding (e.g., oblique strain fabrics when viewed in the strike plane). Faults exhibit either normal or reverse shear sense; however, markers clearly offset by strike-slip displacements are difficult to recognize in cores. Normal faults can be cut by thrust faults, which subsequently can be cut by normal faults. The normal faults, where documented, are minor in frequency and in displacement, and they appear to be the result of minor deformation that was concurrent with thrusting.

Markers offset by slickenlined faults and associated slickenlines are consistent with oblique or strike-slip deformation, though normal and reverse faults also occur. A number of these faults could be drilling induced.

Anisotropy of magnetic susceptibility

The measurement technique and theory of magnetic susceptibility (MS) is introduced in "Physical properties" in the "Methods" chapter and MS results are presented in "Physical properties." Anisotropy of magnetic susceptibility (AMS) terminology is described in "Structural geology" in the "Methods" chapter. Because AMS units are nondimensional and normalized to unit vectors, only the degree of anisotropy is included in AMS units (unlike MS, which is a scalar property). The AMS ellipsoid is a useful tool for quantifying deformation fabrics because it can approximate the finite strain ellipsoid (Borradaile, 1988).

Samples from cores exhibit degrees of anisotropy (in terms of the ratio of maximum to minimum principal axes of the AMS ellipsoid (P) between 1.014 and 1.059 (Fig. F36A). These values are similar to those found in the Barbados accretionary prism (Housen et al., 1996). There is no obvious statistical pattern of distribution in anisotropy values (Fig. F36A), nor does the anisotropy change over the limited range of core sample depths.

The shapes of the AMS ellipsoids are given by the comparison of the ratio of the maximum to the intermediate principal axes of the AMS ellipsoid (L) and the ratio of the intermediate to minimum principal axes (F) (Fig. F36B). Similar to a Flinn diagram for finite strain, L versus F values that plot below a line with a slope of unity are oblate, whereas those that plot above the line are prolate. Nearly all data plot in the oblate field, though a few are near or above the L = F line. In at least one case, we attribute the latter to analytical error (for a sample with an AMS L value of >1.04) and in the other cases to essentially isotropic samples (with respect to the AMS ellipsoid).

Orientation of the principal axes of the AMS ellipsoid in geographic space is not known without paleomagnetic reorientation of the cores. From ~1530 to ~1590 m CSF, the inclination of the minimum axis (also equal to the oblate ellipsoid inclination because the intermediate and maximum axes are similar) decreases from ~80° to ~10° (Fig. F36C), with a few outlier data points that may be due to analytical error. The change in the AMS ellipsoid orientation with depth is similar to the change in bedding orientation within the cored interval, consistent with the minimum axis of the AMS data being approximately perpendicular to bedding.

Using caliper logs to document breakouts

Failure and enlargement of a borehole on opposite walls forms a breakout that is commonly interpreted as a compressive failure. Breakouts provide specific information about stress orientation and, in some cases, stress magnitude (Zoback, 2007) and are commonly detected with borehole imaging tools, including resistivity tools (Zoback, 2007). Calipers can also measure borehole enlargement associated with breakouts. The four arms of the FMI tool act as a caliper (Fig. F7 in the "Methods" chapter) with pairs of opposing arms (Caliper 1 and Caliper 2).

The FMI caliper shows an in-gauge borehole (~12 inch diameter) at depths shallower than 1285 m WMSF. Below this depth to the base of the borehole, the hole is significantly enlarged, with one caliper commonly indicating ≥16–18 inches diameter and the other typically close to 12 inches diameter. The greater enlargement is recorded by Caliper 1 and 2 alternately because of tool rotation as the tool is drawn up the hole. These shifts in largest caliper measurement lead to division of the borehole into Interval A (1300–1455 m WMSF), Interval B (1465–1530 m WMSF), and Interval C (1545–1578 m WMSF) (Fig. F37). These intervals are not contiguous but represent the depths of orientation stability between the shorter sections where the caliper drifts.

Caliper 1 orientation is defined magnetically and recorded as Pad 1 azimuth (P1AZ) in logging data files (Fig. F37). Caliper 2 is located 90° from Caliper 1. Caliper 1 orientation above 1285 m WMSF rotated smoothly and spiraled clockwise (viewed downhole) as the tool was drawn uphole (Fig. F37). Below this depth, caliper orientation was stable over Intervals A, B, and C but jumped clockwise at discrete depths as the tool was pulled uphole. We believe two of the opposing arms of the caliper were confined to the bilaterally enlarged portion of the borehole within each of these intervals.

The orientation of the enlarged portion of the borehole is determined from the largest caliper value and from the orientation of Caliper 1 (Table T7). The orientation of the largest caliper (i.e., borehole enlargement) is remarkably stable even as the entire tool rotated in discrete clockwise increments. The mean value of that orientation, weighted for the depth of borehole sampled in each interval, defines an alignment 46°–226° (northeast–southwest). We interpret this orientation as a series of breakouts. Accordingly, this would represent the direction of minimum stress in the horizontal plane (S_hmin) with the maximum stress in the horizontal plane (S_Hmax) at 136°–316° (southeast–northwest), 90° to S_hmin. This direction of S_Hmax, plotted with the previous results of Expedition 314 (Fig. F38), is similar to S_Hmax at sites drilled on the prism slope seaward of the Kumano Basin and is nearly perpendicular to S_Hmax at Site C0002 near the seaward edge of the Kumano Basin (Tobin et al., 2009a).

Breakouts and drilling-induced tensile fractures from FMI images

We also examined FMI data for breakouts and drilling-induced tensile fractures (DITFs), both of which can provide information on stress orientations. Breakout geometry is obvious from the deviations of caliper measurements (see "Using caliper logs to document breakouts"). A breakout should appear as an area of low resistivity, representing the enlargement formed by failure and spallation of the borehole wall. Although a lowered resistivity value may occur in a breakout interval, this low-resistivity signal is not typically bilateral at Site C0009. For example, in Figure F39, low resistivity values are recorded on all four pads in the breakout area, making picking the opposing areas of low resistivity problematic. The inability to recognize breakouts in FMI data may result from the extension of the four tool arms into the local borehole diameter. If all of the arms remain in contact with the borehole wall, any lowered resistivity signal of the elongated borehole axis will be minimized. The ~50% coverage of the borehole by the FMI also may have limited our ability to clearly identify breakouts. Overall, investigation of the breakout interval below 1285 m WMSF showed inconsistent definition of breakouts using resistivity image data.

We studied FMI data for evidence of fractures, specifically DITFs (Fig. F39). DITFs should be vertical and normal to the borehole wall and occur in matched pairs on opposite sides of the borehole. We did observe some fractures from ~800 to 1000 m WMSF. Of these fractures, we only picked those that were vertical and apparently normal to the borehole. Only one example shows another paired fracture on the opposite side of the borehole. Because of the ~50% coverage of the borehole by the FMI, the matching pairs may have been missed. The individual fractures appear to have a dominant orientation trending west-northwest–east-southeast (Fig. F40; see C0009_T4.XLS in STRUCGEOL in "Supplementary material"). This orientation deviates somewhat westerly from the orientation of S_Hmax from caliper data.

FMI borehole image interpretation

Bedding dips and faults were gathered from FMI data. Bedding dips (N = 189) were shallow to the north-northwest Fig. F41; see C0009_T5.XLS in STRUCGEOL in "Supplementary material"). Faults (N = 39) dip moderately to shallowly to the northwest, including one documented example of a normal fault (Fig. F42; see C0009_T6.XLS in STRUCGEOL in "Supplementary material"). The mean inclination of the faults is ~45° with a modal dip between 60° and 70°. The faults are all resistive in character with the exception of one of undetermined nature. We examined the changes in bedding attitude of the different units. Unit I and upper Unit II were not sampled by FMI data. Subunit IIIA had mostly shallow bedding dips to the north-northwest–northeast (Fig. F43). The contact between Subunits IIIA and IIIB appears to be a 3 m thick deformed zone of three faults dipping 3°–10° to the northeast. In between these three faults, bedding dips steeply (up to 78°) (see Fig. F5 in the "Expedition 319 summary" chapter). This deformed zone is interpreted as a thrust fault system. Subunit IIIB has shallow bedding dips mostly to the northwest (Fig. F44). Unit IV has shallow to moderate dips consistently to 340°–350° (Fig. F45).

Summary

Most faults identified in FMI images dip to the northwest and strike northeast–southwest. S_Hmax, however, is also oriented to the northwest. Thus, if the northwest-dipping faults are active, it precludes S_Hmax from being σ₂ because σ₂ should be parallel to the strike of the associated faults. In this case, σ₁ must be horizontal and oriented northwest–southeast, suggesting that the northwest-dipping faults are thrust faults and σ₃ is vertical.

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