Relating permeability to the structural setting of a fault‐controlled hydrothermal system in southeast Oregon, USA

Abstract

It is generally accepted that high‐permeability pathways are required to bring hydrothermal fluids to the surface from depth at elevated temperatures, and these pathways are commonly associated with faulting. The orientation and mode of fractures that develop as a result of fault slip are dependent on the state of stress and the geometry of the fault system, which in turn control the near‐fault permeability structure. We hypothesize that temperature data collected in fault‐controlled hydrothermal systems may be used to delineate the extent of the near‐surface breakdown region of the controlling fault, and may provide insight regarding the fault geometry and stress field. Here we present a geostatistical analysis of 1550 ground and spring temperature measurements in an area of active hydrothermal discharge located at Mickey Hot Springs in southeast Oregon. Indicator kriging was used to treat heterogeneity across the site prior to in‐category simulation of temperature as a continuous variable. The analysis indicates that zones of high, medium, and low temperature at the site arise from different physical mechanisms, which we propose are related to the underlying modes of fracturing and the dominant mechanisms of heat and mass transport (i.e., by advection or diffusion). The findings of this study suggest that thermal‐hydrologic data of the type presented here may be useful for understanding fault zone characteristics, and for developing conceptual models of fault‐controlled fluid flow.