CHARACTERISTICS OF GEOTHERMAL RESOURCES IN EXTENSIONAL SETTINGS IN THE WESTERN UNITED STATES

Data collected through ongoing US Geological Survey geothermal resource assessment work provide the opportunity to examine characteristics of hydrothermal systems of varied types in diverse geologic and tectonic settings. Although hydrothermal systems are distributed throughout the western United States, including Alaska and Hawaii, the greatest concentrations of higher temperature systems (T>90C) are found in regions characterized by extensional, or extensional and strike-slip, faulting. Observed flow rates and temperatures of thermal springs, chemical geothermometry, and thermal models for the upflow of water along near-vertical faults are generally consistent with relatively rapid movement of water from depths in the range from 3 to 8km at rates in excess of 1km/100years. However, available age data on geothermal waters produced from these systems generally fall in the range of 10,000 to 50,000 years. Reconciliation of these observations suggests flow is highly variable over time, with episodes of faulting and deformation that enhance fault permeability occurring within longer time scales of declining permeability due to mineralization and increasing effective normal stress. Quantifying the magnitudes and rates of permeability changes in these systems over the depth of water circulation will better identify those characteristics that determine the locations of known active hydrothermal systems and provide the basis for understanding the potential for undiscovered, “blind” hydrothermal systems.