In situ bulk permeability was measured in a borehole that intersected the decollement zone (a low-angle detachment fault) between the North American and Caribbean plates. Permeability measurements were made at a variety of fluid-pressure conditions, defining a quantitative relation between bulk permeability and effective stress for this plate-boundary fault zone. The bulk permeability in this zone changed by several orders of magnitude as a consistent function of fluid pressure. This relation may help to explain the dynamics of fluid-fault interactions and the transient nature of hydrologic processes during deformation at convergent margins.