[1] The Peru frontal prism represents a contrasting hydrologic setting to the better characterized systems of Nankai and Barbados. The high taper angle and convergence rate result in rapid burial of low-permeability sediments. Critical taper stability calculations were combined with a flow model to understand how a steep taper angle could be stable under these conditions. Constant rate flow tests of the slope and prism sediments gave permeability values ranging from a low of $10 À18 m 2 to a high of $10 À16 m 2 . Estimated compaction fluid sources exceed 5 Â 10 À13 s À1 near the toe and remain above 10 À14 s À1 for 20 km arcward in the underthrust sediments. These data were used to constrain a two-dimensional, steady state model of pore pressure along a cross section at 9°S. The model domain is composed of a 15-km-wide frontal prism with a crystalline buttress at the back and 500 m of permeable basement at the base. The combined mechanical stability and flow model analysis yielded a feasible basement permeability of $10 À16 m 2 and a décollement permeability range of 10 À15 -10 À17 m 2 . The model results demonstrate that basement flow and use of different pressure ratio values for the wedge and the base in stability calculations are important for systems with steep taper angles.
A cased and sealed borehole in the Northern Barbados accretionary complex was the site of the first attempts to measure permeability in situ along a plate boundary dé collement. Three separate efforts at Hole 949C yielded permeability estimates for the dé collement spanning four orders of magnitude. An analysis of problems encountered during installation of the casing and seals provides insights into how the borehole conditions may have led to the wide range of results. During the installation, sediments from the surrounding formation repeatedly intruded into the borehole and casing. Stress analysis shows that the weak sediments were deforming plastically and the radial and tangential stresses around the borehole were significantly lower than lithostatic. This perturbed stress state may explain why the test pressure records showed indications of hydrofracture at pressures below lithostatic, and permeabilities rose rapidly as the estimated effective stress dropped below 0.8 MPa. Even after the borehole was sealed, the plastic deformation of the formation and relatively large gap of the wire wrapped screen allowed sediment to flow into the casing. Force equilibrium calculations predict sediment would have filled the borehole to 10 cm above the top of the screen by the time slug tests were conducted 1.5 years after the borehole was sealed. Reanalysis of the slug test results with these conditions yields several orders of magnitude higher permeability estimates than the original analysis which assumed an open casing. Overall the results based on only the tests with no sign of hydrofracture yield a permeability range of 10 )14 -10 )15 m 2 and a rate of increase in permeability with decreasing effective stress consistent with laboratory tests on samples from the dé collement zone.
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