Previous   |    Next

doi:10.2204/iodp.proc.314315316.115.2009

Discussion and synthesis

Because our data from Site C0003 is limited to the real-time data transmitted uphole during drilling from 0 to 525 m LSF, we have relatively little information on which to draw conclusions at this site. However, we can draw some inferences on the composition of the thrust sheet in the drilled interval. Consistent with the other midslope sites, the lithology is of apparently relatively coarse silt-sand and hemipelagic mud in the shallowest portion, interpreted as slope sediments based on seismic interpretation (logging Units I and possibly II). These coarser sediments are underlain by more clay rich, generally homogeneous muddy deposits of logging Unit III, interrupted by prominent zones of washouts interpreted as brecciated intervals, indicating likely faults. The seismic reflection imaging shows that this site was drilled into a series of at least three individual thrust sheets within the hanging wall of the mega-splay fault system (Fig. F28). The seismic character with numerous landward-dipping reflectors indicates that the hanging wall block (or thrust sheet) contains numerous thrusts below the resolution of seismic reflection data. In fact, PSDM versions of the data that became available just as we went to sea made this fabric clear, though it was not visible in the time-processed seismic volume. Therefore, the major washout zones at ~240 and 420–450 m LSF are likely to be strongly brecciated damage zones from subsidiary thrusts.

The seismic interpretation is consistent with substantial uplift of the thrust sheet bringing older accretionary prism rocks to within 500 m of the seafloor. Log density data show relatively constant and high values for this depth, suggesting that anomalously well-indurated rocks make up the thrust sheet. When the broken section of drill pipe was recovered to the rig floor after loss of the LWD string, it was plugged with cuttings and numerous large blocks (as large as 5–8 cm in diameter) of cavings that had come from an unknown position in the hole. This material had a nannofossil age of late Miocene (5.5–7.2 Ma), bulk density of 2.1 g/cm³ (implying a porosity of ~35%), and P-wave velocity of ~2.1 km/s. It is remarkably well lithified for material from <530 mbsf, which is also consistent with the thrust sheet rocks having been uplifted from substantially greater depth within the accretionary complex.

In summary, the portion of the thrust sheet penetrated at Site C0003 was largely homogeneous material, most likely sandy to clayey hemipelagic muds, and clearly heavily deformed by multiple faults and associated brittle fractures. Some evidence suggests that this thrust sheet has been transported a substantial distance vertically by thrusting, bringing older, denser rocks to shallow depths.

Top of page   |    Previous   |    Next