Documenting the interaction between hydrologic and tectonic processes in active faults in situ is key to understanding both the mechanical behavior of faults and their large-scale fluid transport properties. Drilling into accretionary prism decollements has shown that they are among the most hydrologically and tectonically active faults on Earth. The fluid flow systems have generally been conceptualized as fracture flow through scaly-fabric dominated faults tens of meters thick. However, porewater geochemistry and other evidence suggests that along-fault flow and cross-fault barriers to flow co-exist, which has been difficult to explain without invoking permeability anisotropy at the matrix or fracture scale. Recent results from ODP drilling at the North Barbados Ridge, Costa Rica, and the Nankai Trough provide much more detail on fault zone structure, the locus of flow, and pore pressure regimes. The Costa Rica and Barbados decollements exhibit mechanical partitioning into subdomains of brittle fracture and of macroscopically ductile deformation, generating a strongly asymmetric structure across the fault zone. Evidence for fluid flow in both of these systems suggests that it occurs only in the brittle subdomain, and that the underlying ductile portion of the faults are barriers to cross-fault flow. In contrast, the Nankai decollement exhibits only a brittle fracture domain, and has no evidence for along-fault fluid transport. I present a model for the mechanics governing formation of these contrasting structural domains, in which differing stress paths co-exist in the upper and lower parts of the decollement zone, producing a mechanically-controlled permeability anisotropy at a tens of meters scale. This model can qualitatively explain localized along-fault flow without calling on matrix-scale permeability anisotropy. Results from all these margins will be synthesized into a global model for the tectonic hydrogeology of prism decollements in which permeability structure and fault mechanics are seen as completely interdependent.