Hydromechanics of a high taper angle, low‐permeability prism: A case study from Peru

Abstract

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 −18 m 2 to a high of ∼10 −16 m 2 . Estimated compaction fluid sources exceed 5 × 10 −13 s −1 near the toe and remain above 10 −14 s −1 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 −16 m 2 and a décollement permeability range of 10 −15 –10 −17 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.