Structural settings of hydrothermal outflow: Fracture permeability maintained by fault propagation and interaction

Hydrothermal outflow occurs most commonly at the terminations of individual faults and where multiple faults interact. These areas of fault propagation and interaction are sites of elevated stress termed breakdown regions. Here, stress concentrations cause active fracturing and continual re-opening of fluid-flow conduits, permitting long-lived hydrothermal flow despite potential clogging of fractures due to mineral precipitation. As fault systems evolve, propagation, interaction, and linkage of fault segments result in the migration and eventual localization of breakdown regions in kinematically favorable sites such as fault bends or fault intersections. Concurrent migration of hydrothermal outflow sites along with these areas of elevated permeability leads to predictable patterns of hydrothermal deposition along fault zones. Thus, the distribution of active outflow sites and preserved deposits along fault zones can potentially provide a tool for studying fault-zone evolution.