PERMEABILITY GENERATION AND MAINTENANCE AT STRUCTURAL DISCONTINUITIES: A PERSPECTIVE FROM GEOTHERMAL FIELDS (Invited Presentation)

Structural discontinuities, i.e. fault terminations, fault step-overs, intersections, bends, and other fault interaction areas, host fluid flow in many different types of geologic systems, including geothermal fields. We examine stress transfer associated with modeled slip of the Holocene-to-historically active Rainbow Mountain fault zone in order to evaluate permeability development and maintenance at the Salt Wells geothermal field in western Nevada, USA. Results suggest that the discrete locations of maximum stress perturbation that result from slip occur within discontinuities along this fault system. Well field data, surface geothermal manifestations, and subsurface temperature data, each a proxy for modern circulation in the geothermal field, indicate that geothermal fluid upwelling is focused in these same areas where stresses are most highly perturbed as a result of slip. These results suggest that sub meter- to meter-scale slip on fault systems generates stress perturbations that are sufficiently large to promote slip on an array of secondary structures spanning the footprint of the modern geothermal activity, implying an important role in generation of fluid flow pathways. Still, mineralization is expected to seal permeability along faults and fractures over time-scales that are generally shorter than either earthquake recurrence intervals or the estimated life-span of geothermal fields. This suggests that though stress perturbations resulting from fault-slip are broadly important for defining the location and areal extent of enhanced permeability at structural discontinuities, continual generation and maintenance flow conduits throughout these areas is probably dependent on the deformation mechanism(s) affecting each involved structure.