Penetrating the sedimentary sequence seaward of the subduction zone, reference Sites 1044 and 1048 show that the proto-décollement occurs in a lower Miocene radiolarian mudstone unit of unusually low density. Beneath the accretionary prism, this radiolarian mudstone unit is synonymous with the décollement zone at every cored site, indicating that special properties of the mudstone foster detachment along this surface. Logs at Sites 1045, 1046, and 1047 that penetrated the accretionary prism show various stages of consolidation in the décollement zone. The initial low density of the radiolarian mudstone indicates high fluid pressure and low strength that trigger and localize décollement formation.
As underthrusting proceeds, burial and shear strain lead to consolidation of the décollement zone, erasing the original low-density signature. At Site 1045, however, a local area of arrested consolidation of the décollement zone shows a strong negative polarity seismic reflection. Logs there indicate that the negative polarity reflection is a sharply defined low-density interval in the consolidating low-density radiolarian mudstone. The thickness of the underconsolidated interval causes constructive interference or "tuning" of the incident seismic waveform producing the strong negative polarity reflection.
The degree of consolidation along the décollement zone does not vary systematically with distance of underthrusting; perhaps local features such as faults in the overlying prism may be important in controlling fluid escape from, and therefore, consolidation of the décollement zone. At greater depths than we drilled, the décollement zone displays extensive areas of negative polarity reflections, indicating either large regions of arrested consolidation or other processes that dilate the fault zone.
Comparison of the density profiles between sites penetrating the incoming sedimentary sequence and equivalent strata thrust beneath the accretionary prism quantifies the magnitude of consolidation due to loading by the prism. In the underthrust strata, a turbidite sequence of higher porosity and permeability than adjacent deposits apparently transports some of this fluid seaward and produces the anomalously high thermal gradient at Site 672 (Leg 171A, LWD Site 1044).
The high quality of the LWD data coupled with information from the boreholes and seismic data calibrate the logs for recognition of geologic features. Thrust faults are evident as repeated log signatures, often as density inversions. Fluid conduits documented by the pore-water chemistry in the cores can be recognized in the logs as low-density intervals. Known sediment compositions allow inversion of log signatures to specify sediment types in uncored areas.
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