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High-permeability faults, acting as preferential pathways for fluid migration, are important geological structures for fluid, energy, and solute transport. This paper examines the interaction of thermally driven convective circulation in a steeply dipping fault zone and groundwater flow through the surrounding country rock that is driven by a regional topographic gradient. We consider a geometry where a fault zone with a homogeneous, isotropic permeability is located beneath a narrow valley in a region with substantial topographic relief. System behavior is best characterized in terms of the largescale permeabilities of the country rock and the fault zone. Using three-dimensional numerical simulations, we map in permeability space four fluid flow and heat transfer regimes within a fault zone: conductive, advective, steady convective, and unsteady convective. The patterns of fluid flow and/or heat transfer are substantially different in each of these regimes. Maximum discharge temperatures can also be plotted in permeability space; the maximum discharge temperature in the advective regime is in general lower than that in the steady convective regime. A higher basal heat flux expands the convective regime in permeability space, as does a greater fault depth. Higher topographic relief on the regional water table compresses the convective regime, with the advective regime suppressing convective circulation at lower country rock permeabilities. If convective cells with aspect ratios close to 1 cannot form, the steady convective regime is smaller in permeability space, and the boundary between steady and unsteady convection occurs at lower values of fault zone permeability. At low country rock permeabilities a water table gradient along the surface trace of the fault of approximately 0.3% suppresses convective cells; at higher country rock permeabilities, convection can be suppressed by smaller gradients on the water table. 1489

Catastrophic volcanic collapse, without precursory magmatic activity, is characteristic of many volcanic disasters. The extent and locations of hydrothermal discharges at Nevado del Ruiz volcano, Colombia, suggest that at many volcanoes collapse may result from the interactions between hydrothermal fluids and the volcanic edifice. Rock dissolution and hydrothermal mineral alteration, combined with physical triggers such as earth-quakes, can produce volcanic collapse. Hot spring water compositions, residence times, and flow paths through faults were used to model potential collapse at Ruiz. Caldera dimensions, deposits, and alteration mineral volumes are consistent with parameters observed at other volcanoes.

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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.

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