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doi:10.2204/iodp.proc.314315316.116.2009

Discussion and synthesis

The primary goals of drilling and logging at Site C0004 were, in order of priority, the characterization of

1. The major splay fault,
2. The overlying thrust sheet,
3. The underthrust sediments originally deposited in a now-overridden slope basin setting, and
4. The young slope deposits in the uppermost unit.

Log data are consistent with the various units being clastic sediments ranging from hemipelagic to turbiditic origin. The logging units divide the drilled formations into young slope deposits, with logging Unit I representing well-stratified deposits (and logging Subunit IIA being likely slump deposits incorporated into the thrust wedge), the thrust sheet represented by logging Unit II, and the underthrust section, also stratified clastic deposits, marked by logging Unit III. Physical properties and structural interpretations support division into three structural domains that overlap but are distinct from the logging units. Logging Unit I and Subunit IIA lack distinct fractures or well-developed breakouts and are assigned to a structural domain (1) that has seen little or no apparent tectonic deformation. Structural Domain 2 is the thrust sheet, exhibiting numerous zones of fracturing and intense breakouts. The interpretation of logging Subunits IIB–IID as finer grained hemipelagic sediments is consistent with the structural interpretation of a distinct domain if it is an uplifted thrust package, perhaps of older Shikoku Basin sediments. Within structural Domain 2, a number of intervals of concentrated fractures likely correlate with subsidiary thrust faults within the thrust sheet (Fig. F33).

The interval between the two major conductive fractured zones at 247–269 m LSF and 284–292 m LSF (see “Structural geology and geomechanics”) may represent either a zone of anastomosing thrusts forming a ~50 m thick broad fault zone or a sliver of relatively intact sedimentary strata bounded by two distinct thrusts. Discriminating between these alternatives will require analysis of Expedition 316 cores.

One perhaps surprising discovery is that the section immediately underthust beneath the splay fault below 292 m LSF is a zone of higher P-wave velocity and resistivity, when the opposite might be expected, based on the assumption that older, more deeply buried and denser rocks were thrust over younger slope sediments. It is possible that intense fracture development through tectonic deformation of the ~50 m thick thrust zone has lowered its resistivity and velocity locally. The resistivity below the fault zone is only elevated for ~20–30 m, then declines to values lower than those above 247 m LSF. This may represent the actual contrast between the bulk properties of the thrust sheet and underthrust section, unaltered by local fracturing. Again, core analysis may shed light on this log observation.

The observation that breakouts are well developed in both the thrust sheet (structural Domain 2) and the underthrust section (structural Domain 3) suggests that this fault is not a locus of a strong stress decoupling; that is, both domains are still subject to the overall present-day tectonic stress. The breakout orientation is similar in general to that at Site C0001 but differs by ~16° in a statistically significant way. This deviation is likely a product of relatively small 3-D variations of the stress field caused by local structural variability, perhaps as a result of surface slope and gravitational stresses.

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