Faults are often assumed to be either barriers or conduits for subsurface fluid flow, although they may act as both, depending on the hydraulic architecture of the fault and the direction of flow with respect to the fault plane. Here we use high-resolution (5 ¿ 5 m spacing) ground temperature measurements to track geothermal discharge in the step-over region of an active ¿chelon normal fault in southeast Oregon. Our analysis demonstrates that the fault acts as a combination conduit-barrier system and reveals complex, 3-dimensional circulation patterns in the area of the fault step-over. Although complex flow circulation patterns are likely to be present in most fault-controlled flow systems, they are generally neglected in conceptual and numerical models. Improved understanding of this aspect of subsurface fluid flow is essential for developing better models of fault hydrology.