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doi:10.2204/iodp.proc.314315316.118.2009

Discussion and synthesis

Site C0006 is located at the toe of the entire accretionary prism near the trench floor, and drilling targeted the main frontal thrust at ~700 mbsf, as well as uplifted and complexly faulted sediments in the thrust sheet and less deformed trench fill in the footwall of the main frontal thrust. Hole C0006B was drilled with a full LWD tool string except for nuclear tools to TD (885.5 m LSF). Because the water depth was beyond the ROV limit of 3000 m, holes were drilled without ROV monitoring. Drilling was smooth; however, the loss of MWD communication effectively eliminated sonic data acquisition from 274 to 885.5 m LSF. Drilling at this site set the record for the deepest water depth (3871.5 m) drilled by the Chikyu.

Four logging units were defined based on differing trends and the character of LWD log responses. Logging Unit I (0–197.8 m LSF) is interpreted as sandy and muddy deposits. Logging Unit II (197.8–428.3 m LSF) is interpreted as mud with occasional thick (~5 m) sand layers. Logging Unit III (428.3–711.5 m LSF) is defined as alternating beds of mud and sand and is divided into two subunits. The base of logging Unit II is interpreted as a possible fault zone as well as a distinct lithologic boundary. Logging Unit III (711.5 m LSF to TD) is characterized by low gamma ray values and low resistivity and is interpreted as sandy deposits. Logging Unit IV has the lowest gamma ray values and is interpreted as underthrust, coarse, trench-fill sediments.

Faults within the thrust sheet

Within logging Unit II at Site C0006, sediment layers are cut by several northwest-dipping thrust faults that offset sedimentary reflections. The resistivity image and other properties at this site, however, show only moderate fracturing with no obvious major faults or fracture zones separating these layers.

Logging Unit II includes four highly conductive intervals, probably ~5 m thick sand beds. The sharp bases of these beds occur at 223, 244.5, 305, and 335 m LSF. The seismic depth section (Figs. F27, F30) shows faults between some of these distinctive beds, suggesting thrust displacement and potential repetition. The hypothesis that these beds are repeated by thrust faulting was tested by coring and associated dating during Expedition 316.

Main frontal thrust

The gamma ray baseline gradually increases with depth throughout logging Units I–III. Within logging Unit IV below 711 m LSF, the gamma ray value suddenly decreases from ~90 gAPI at the bottom of Unit III to 20–50 gAPI in Unit IV. Lower resistivity and larger borehole diameter accompany this low gamma ray trend, indicating that Unit IV is dominated by unconsolidated sand. From lithologic interpretation, we interpret that Unit IV corresponds to the underthrust trench-fill sediments and that the Unit III/IV boundary corresponds to the main frontal thrust.

Alternatively, from structural interpretation of resistivity images, a well-developed conductive fracture zone (~1 m thick) and a fold occur at 657 m LSF (Fig. F23) within logging Unit III, across which the resistivity abruptly decreases following a thick zone of high resistivity. Although no significant change in gamma ray response is present, this fracture zone and change in the resistivity log may correlate with the main frontal thrust below the zone of subsidiary thrusts, although we acknowledge that this feature does not appear as a major fault zone. The complex pattern of seismic reflections below the main frontal thrust may represent channels within the upper part of the trench sediment section that are being overridden by the thrust sheet.

Deformation and stress orientation near the toe of accretionary prism

Analysis of both bedding and fracture orientation at Site C0006 documents how the formation has been deformed under the local or regional stress field.

Within logging Unit I, bedding planes are west-dipping. In logging Unit II, bedding dip and orientation patterns show variability. Bedding is predominantly north-dipping in logging Units III and IV.

Fractures at Site C0006 are notable in that they are not as clustered into zones as those found at Sites C0001 or C0004, and no major deformed zones are identified, suggesting generally weaker deformation at Site C0006. Fractures mostly strike northwest–southeast in logging Units I and II (<428 m LSF), whereas fractures in Units III and IV (429–853 m LSF) strike northeast–southwest.

Overall, both fracture and bedding orientations at depths >429 m LSF are consistent with northwesterly directed shortening resulting from plate convergence. Fracture and bedding orientations at these depths are also consistent with breakout orientations. At shallower depths, fracture and bedding orientations deviate from this general orientation and probably result from combined regional tectonic and local effects (such as gravitational processes).

Borehole breakouts occur from 188 to 729 m LSF but are not readily discernible at greater depths. Taken as a whole, breakouts are more weakly developed at Site C0006 than at any of the upslope sites. Breakouts show mean S_Hmax orientation of 330° with little variation downhole, consistent with that observed at Sites C0001 and C0004 but slightly divergent from the S_Hmax expected from the convergence direction between the Philippine Sea plate and Japan. The lack of obvious breakouts below 729 m LSF occurs just below the transition to logging Unit IV, which is dominated by sand. Excessive washouts in this interval may have obscured the breakout signal.

In logging Unit III (429– 711 m LSF), fracture and bedding orientation is in agreement with the S_Hmax direction calculated from breakouts. Within logging Units I and II, however, fracture and bedding orientations are not what is predicted by S_Hmax orientation. The reason for this is unclear, but detailed postcruise research (integrating log, core, and seismic data) may help to resolve this issue.

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