Faults in a variety of tectonic settings can act as both conduits for and barriers to fluid flow, sometimes simultaneously. Documenting the interaction between hydrologic and tectonic processes in active faults in situ is the key to understanding their mechanical behavior and large-scale fluid transport properties. We present observations of the plate boundary décollement zone at the Middle America Trench off Costa Rica, showing that it is structurally divisible into an upper brittle-fracture–dominated domain overlying a lower, ductile domain. Pore-water geochemical evidence shows that along-fault flow is occurring specifically in the upper brittle domain, but is hydrologically isolated from fluids in the underlying footwall sediments. We propose a model for the mechanics of these contrasting domains in which differing stress paths coexist in the upper and lower parts of the décollement zone. The data suggest a mechanically controlled permeability anisotropy at a scale of several meters to ∼10 m across the décollement zone. This documentation of separate yet simultaneously active mechanical and hydrologic subregimes within a décollement provides a relatively simple explanation for enhanced along-fault permeability coexisting with reduced cross-fault permeability, without requiring matrix-scale permeability anisotropy.