Darcy's Law flow in a permeable medium, consisting of uniform parallel evenly spaced fractures, is used to elucidate how thermoelastic effects may modify the permeability and flow in fracture-controlled hydrothermal systems. Some simple permeability models are then used to investigate whether black smoker venting can result from focussing of low velocity porous flow into fractures at shallow depths (≂100 m.). The models indicate that: (a) thermoelastic processes may be important in controlling the temporal evolution of hydrothermal upflow zones; (b) permeability structure, not just the bulk value of the permeability, may be critical for the formation of black smokers; (c) a small zone extending to a depth of ≂100 m containing a few fractures a factor of 2 or more wider than average may be sufficient to focus upflow into discrete vents provided thermoelastic and chemical effects seal parts of the upper crust. ¿ American Geophysical Union 1990