FAULT-HOSTED GEOTHERMAL RESOURCES IN THE GREAT BASIN REGION, USA –EVOLUTION OF STRUCTURAL-TECTONIC CHARACTERIZATION OVER THE PAST FOUR DECADES

The structural-tectonic characterization of fault-hosted geothermal systems in the Great Basin region has evolved through several stages over the past four decades. These stages include, (1) widespread recognition that most of these resources are amagmatic and are fault-hosted, (2) recognition that hydrothermal systems are associated with specific structural settings along faults, (3) awareness that geothermal favorability along faults is significantly influenced by strain rates, strain history, and type of structural setting, and (4) demonstration that geothermal favorability can be quantified with a statistical assessment of a set of benchmark geothermal resources and their associated structural-tectonic characteristics.

The first major wave of geothermal exploration in the Great Basin region was initiated in the 1970s by energy companies with primary experience associated with large volume, high-enthalpy resources. After drilling numerous holes, it was realized that Great Basin systems are smaller than many of the magmatic heated systems known at the time. Development of these initial resources in the 1980s defined relatively narrow permeable zones along high-angle normal faults. By the 1990s, normal faults were widely recognized as critically important for hosting amagmatic resources in the Great Basin region, and as a consequence, the delineation of faults became a key component of exploration.

In the late 1990s and early 2000s, it was recognized that geothermal fluid flow favors specific parts of normal fault systems (e.g., fault tips or step-overs). Throughout the 2000s, multiple studies demonstrated qualitatively and semi-quantitatively how geodetic strain rate, geologic strain rate, and recency of rupture of Quaternary faults could be used to rank the relative favorability of otherwise similar structural settings located in different portions of the Great Basin.

In the past few years, statistical analysis of structural-tectonic characteristics of benchmark geothermal systems has provided a quantitative framework for evaluating geothermal favorability in diverse structural settings. For example, the recency of faulting of Quaternary faults has a stronger correlation with power capable resources than the type of structural setting or the fault slip rate.