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doi:10.2204/iodp.proc.319.104.2010

Structural geology

We measured the attitudes of faults and bedding and borehole breakout orientations in Hole C0010A from LWD resistivity image data. We logged the borehole to 3034 m LRF (482 m LSF) in Run 1. After assessing data from Run 1 during a weather-related suspension of operations, we relogged between 2900 and 2970 m LRF (347–418 m LSF) to attempt to improve data quality and then continued logging while drilling from 3034 m LRF to TD at 3107 m LRF (482–555 m LSF). Data from both logging runs are presented here. The discussion below includes structural data, a discussion of data bias, borehole breakout analysis, and description of fault resistivity.

Structural data

The resistivity image data are horizontally "striped," reflecting artifacts related to tool heave because of ship motion. This artifact hindered, but did not preclude, our ability to interpret structural and sedimentary features. We used several criteria for interpreting bedding, including: (1) being able to fit a sine curve to the flattened image of the cylindrical borehole wall, (2) having resistivity values extending across the entire image, and (3) having consistent resistivity values above and below a feature (Fig. F9). Our criteria for interpreting faults are the same as those for bedding except that we interpreted faults only where inconsistent resistivity values were apparent above and below the fault (Fig. F10).

Most of the bedding dips eastward with significant scatter in dip values and orientation (Fig. F11; see C0010_T1.XLS in STRUCGEOL in "Supplementary material"). Bedding in slope deposits above the thrust wedge dips moderately to the east, at the top of the thrust wedge it dips moderately to the west, and bedding below the thrust wedge dips both east and west (Fig. F12; see C0010_T1.XLS and C0010_T2.XLS in STRUCGEOL in "Supplementary material"). Easterly dipping bedding below the thrust is not consistent with dips observed in the seismic reflection data.

Most of the faults occur within the base of the thrust wedge and exhibit a wide range of dips to the west and south (Fig. F13). The two logging runs through this section (2900–2970 m LRF yield different interpretations. Logging Run 1 data (during drilling) indicate more shallowly west-dipping faults. Run 2 data, recorded after reaming the open hole, indicate many more steeply dipping faults; the shallowly dipping faults from Run 1 data are not observed in Run 2. Figure F12 shows data from Run 1 between ~50 and 482 m LSF, except between 347 and 418 m LSF where Run 2 data are shown, plus data acquired while deepening the borehole from 482 m LSF (TD of Run 1) to TD at 555 m LSF.

We also documented whether the faults were resistive or conductive, an interpretation made difficult because of the generally poor imaging data quality. However, of the 27 faults observed, 4 were resistive, 8 conductive, and 15 undetermined (see C0010_T2.XLS in STRUCGEOL in "Supplementary material").

Discussion

There is little repeatability between the two logging efforts. Only three faults were interpreted with similar dips and dip directions in both runs (Fig. F14; see C0010_T2.XLS and C0010_T3.XLS in STRUCGEOL in "Supplementary material"). Logging Run 1 exhibits more shallowly dipping faults. The vertical resolution is probably worse than in Run 2 because of significant tool heave. A total of 18 new faults were observed in Run 2 after reaming the open hole; however, 9 faults from Run 1 were missed by Run 2 (Fig. F14). This could be partly due to errors in interpretation, but there are biases in both data sets: Run 1 shows more shallowly dipping faults but missed many others, whereas Run 2 shows more steeply dipping faults but missed the shallowly dipping faults.

Borehole breakouts

At Site C0010, LWD resistivity images show bands of low resistivity on opposite sides of the borehole. We interpret these as borehole breakouts; the enlarged borehole produces a low-resistivity signal because of the increased conductive water between the LWD resistivity tool and the borehole wall (Zoback, 2007). Criteria for mapping breakouts include observing low-resistivity areas on opposite sides of the borehole (180° apart) or a single area of low resistivity that directly extends as a vertical continuation from one member of an opposed pair. We mapped borehole breakouts from both logging Runs 1 and 2. In general, breakouts were most clearly imaged in Run 1 data and in Run 2 data where the borehole was deepened from 482 to 555 m LSF beyond Run 1. The borehole changed between the two logging runs (see also "Physical properties") where the logging runs overlap in the relogged section (348–418 m LSF); breakouts enlarged in size in the 3 days between the two runs (Fig. F15).

Breakouts are rare in the slope deposits, common in the thrust wedge, and abundant in the overridden slope deposits below the thrust wedge (Fig. F16; see C0010_T4.XLS in STRUCGEOL in "Supplementary material"). The poor image quality, especially in the slope deposits and thrust wedge, made picking the breakouts more difficult than in the overridden slope deposits. The mean azimuth of all breakouts is 55°–235° (Fig. F17). The orientations are consistent, with minimal scatter, in the overridden slope deposits below the thrust wedge (Fig. F16). Moving uphole, the orientations shift sharply to higher values (more east–west) in the thrust wedge (above 407 m LSF) and then gradually rotate within the thrust wedge and overlying slope deposits to a more northeast–southwest orientation upsection.

We interpret breakout orientations at 55°–235° as S_hmin, with S_Hmax 90° to this direction at 145°–325° (Zoback, 2007). This S_Hmax direction is parallel or subparallel to all other S_Hmax directions from the Kumano transect, except at IODP Site C0002 (Fig. F18) (Tobin et al., 2009a). The base of the thrust wedge at Site C0010 is marked by a transition in breakout orientation (407 m LSF). This sharp transition contrasts with the uniform trend of breakout orientation with depth at Site C0004, which extends from the overlying slope deposits down through the thrust wedge and into the overridden slope deposits (Expedition 314 Scientists, 2009b).

Summary

Mapped faults are concentrated mostly near the base of the thrust wedge and immediately below in the uppermost overridden slope deposits. These faults mostly dip to the south and west and are steep relative to reflectors in the thrust wedge on the seismic reflection data (Fig. F19). However, neither the faults nor the bedding orientations show clear trends, in contrast to Site C0004 where data quality was better and measurements were more numerous (Expedition 314 Scientists, 2009b). The breakouts show that S_Hmax trends to the northwest, similar to other sites on the outer slope along the NanTroSEIZE transect (IODP Sites C0001, C0004, and C0006) (Fig. F18). The sharp discontinuity in stress orientation across the base of the thrust wedge (Fig. F16) is consistent with a fault discontinuity at this depth (Barton and Zoback, 1994) but contrasts with the more uniform but constantly changing trend of breakouts across the base of the thrust wedge at Site C0004 (Expedition 314 Scientists, 2009b). The enlargement of breakouts during the interval between logging runs indicates that breakout width grows with time in this environment, in contrast to observations from more lithified rocks (Zoback, 2007).

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