SUMMARY OF LWD RESULTS PERTINENT TO ACCRETIONARY PRISM EVOLUTION

  1. The décollement zone develops in a low-density interval in the sedimentary section incoming to the subduction zone. Initiation of the décollement zone in this interval is apparently caused by its low strength, in part because of its low density.
  2. The low density of the proto-décollement zone makes it the weak sole fault for listric normal faults in the incoming sedimentary section.
  3. The transition from extension (normal faulting) to compression is virtually at the toe of the accretionary prism, indicating that compressional stresses are not propagated far seaward of the accretionary prism.
  4. High heat flow seaward of the accretionary prism is explained by fluid flow in the subjacent sand layers, not in the proto-décollement zone.
  5. With underthrusting beneath the accretionary prism, the décollement zone densifies in a patchy manner, primarily because of the collapse of clay mineral fabric. The one area penetrated by LWD with retarded densification corresponds to negative-polarity seismic reflections in the décollement zone.
  6. The density data from the LWD holes allow a confident inversion of the seismic data for density along the proto-décollement zone and décollement zone.
  7. The strong negative-polarity reflections in the décollement zone indicate areas of arrested consolidation, not hydrofracture. These areas of arrested consolidation represent fluid compartments in the décollement zone and are locally bounded by high-angle faults. The areas of arrested consolidation may represent channels of focused fluid flow from depth, which tends to retard consolidation.
  8. The drilling results and, arguably, the seismic reflection data indicate the décollement zone remains localized in an interval of radiolarian claystone. Weakening of this smectite-rich lithology during shear explains the tendency of the décollement zone to remain localized in its initial position.
  9. During LWD operations, an inadvertent two-hole hydrologic test indicated significantly higher permeability of a scale of 45 m more than was previously inferred from single borehole tests.

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