Fluid circulation, heat transfer, and the development of thermal springs are examined for vertical fault zones with anisotropic permeability and internal heterogeneity. Interactions between thermally driven convective circulation in the fault zone and topographically driven groundwater flow through the surrounding country rock are mapped in permeability space (permeability of the country rock versus fault zone permeability) and compared to earlier results for homogeneous, isotropic fault zones. Simulations with a fault zone 4--10 times more permeable in the strike than in the dip direction show that the field of steady convection expands in permeability space, promoting stable convection at both higher and lower flux rates. Higher groundwater discharge temperatures (by 12--18 ¿C) are predicted relative to an isotropic fault because this anisotropy favors the formation of a smaller number of convection cells, creating a flow pattern that is more efficient in capturing heat from the country rock and transmitting it to a reduced number of discharge sites. Simulations with a fault zone 4--10 times less permeable in the strike than the dip direction indicate the regional flow from the country rock overrides buoyancy-driven convective circulation in the fault zone at lower values for the country-rock permeability. Heterogeneity internal to the fault creates complex patterns of flow and variations in the geothermal gradient that reflect the connections of higher-permeability regions interior to and along the surface trace of the fault. Channeling of the flow leads to minor differences in the maximum discharge temperature relative to the homogeneous case but creates significant enhancement in the local heat flux owing to the higher groundwater discharge rates. ¿ American Geophysical Union 1996