1.Evaluation of overall prism consolidation and velocity-porosity relationships: LWD data obtained at five sites during Leg 171A and two sites during Leg 156 provide an overview of prism consolidation. Two reference sections at Site 1044 and 1045 establish a baseline for evaluating consolidation of both the offscraped and underthrust sections up to 6 km east of the deformation front. Comparisons of the density curves from the reference sites enable calculation of underthrust sequence consolidation because of loading by the overlying accretionary prism at the LWD sites west of the deformation front. Velocity-porosity relationships could not be evaluated because of the absence of an LWD velocity tool that could measure sediments with such low velocities and the lack of time to conduct wireline velocity measurements.

2.Correlation of physical properties of faults with displacement and fluid flow: We found no correlation between consolidation and amount of underthrusting.

3.Evaluation of the consolidation state of sediments in and around faults: The localization of the décollement zone beneath the northern Barbados accretionary prism is controlled by the physical properties of the incoming sedimentary section, providing a classic example of the principle of sedimentary inheritance. A siliceous mudstone interval at reference Sites 1044 and 1048 (mean depth range = 178 to 198 mbsf) is characterized by a wet bulk density (mean = 1.47 g/cm3) that is lower than the mudstones immediately above and below and comparable to the surface sediment. This lower Miocene low-density layer biostratigraphically correlates to the décollement zone at five ODP sites that penetrate through the accretionary prism.

The low density of the radiolarian mudstone layer indicates underconsolidation and high fluid pressure, which facilitates décollement formation. Based on LWD data from boreholes penetrating the décollement beneath the accretionary prism, the low-density radiolarian mudstone layer consolidates, probably because of dewatering associated with shearing. Consolidation locally thins and more sharply defines the top and bottom of the low-density layer. Here negative and positive seismic reflections respectively from the top and bottom of the layer interfere or "tune," producing a strong negative polarity seismic reflection. Producing the strong negative polarity reflections by selective consolidation contrasts with previous interpretations that invoke dilation and hydrofracture to generate the reflections. Both interpretations are viable, but the data from 171A may be more simply interpreted as a consolidation response. Where consolidation has eliminated the low density signature of the décollement zone, its seismic reflection becomes positive because of a preserved positive shift in density and impedance at the base of the radiolarian mudstone interval. Thus, the areas of negative polarity reflections of the décollement zone are intervals of arrested partial consolidation. This pattern of arrested consolidation bears no simple relationship to the prism geometry, distance of underthrusting, or underthrust sediment form.

4.Determination of the origin of the negative polarity reflections in fault zones: LWD data were obtained through a negative polarity reflection, thereby documenting the in situ physical properties that produce these distinctive reflections.

In summary, Leg 171A partially accomplished Objective 1 by determining prism consolidation. However, we did not establish velocity-porosity relationships of these sediments. We tested the prediction of Objective 2 and found no correlation between consolidation and the amount of underthrusting; but we were successful in determining the consolidation state of sediments around the décollement (Objective 3). We recorded logs through a negative polarity reflection, established that it is probably a residual fluid accumulation, and now have documented the physical property variations that produce these distinctive reflections (Objective 4).

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