| Abstract |
The Great Basin region (GBR) in the western U.S. hosts vast amounts of undiscovered conventional geothermal resources. However, as much as 75% of the geothermal resources in the GBR may be blind or hidden. Nearly all geothermal systems in the GBR reside in six types of favorable structural settings, including normal fault terminations, step-overs (i.e., relay ramps) in normal faults, fault intersections, accommodation zones, displacement transfer zones whereby strike-slip faults end in arrays of normal faults, and pull-aparts in strike-slip faults.  Although the affinity between geothermal systems and these structural settings is widely known, little research has been conducted to distinguish which geometries of a particular structural setting are more conducive for geothermal activity.  Distinguishing the most favorable geometries is crucial to improving exploration strategies for hidden systems, selecting optimal drilling targets, and increasing the efficiency of existing power plants. Step-overs in Quaternary normal fault zones are the most common favorable structural setting for known geothermal activity in the GBR. Complex fault geometries in the step-overs, including multiple minor faults connecting the major overlapping fault strands, enhance permeability and generate efficient pathways for hydrothermal fluid flow. Step-overs are common in normal fault zones, and more than 450 were identified across the region in this study, with only a fraction associated with known geothermal activity. It is difficult to distinguish which step-overs might host a hidden geothermal system.  However, step-overs come in a variety of geometries depending on relative overlap, underlap, and spacing between major fault strands.  In addition, the geometry of minor faults that breach the step-over can vary from oblique-slip faults that connect the major fault strands to en échelon faults that parallel the major fault strands. Step-overs were therefore further classified based on orientation, sense of stepping (right vs. left), amount of overlap and spacing between main fault strands, and linkage style (hard vs. soft). Of identified step-overs in the GBR, ~54% are right stepping, ~51% hard linked, and ~50% underlapping. Relay ramp widths ranged from 0.1-14.6 km, with an average of 2.8 km. Step-overs associated with higher-temperature ( greater than 120°C) geothermal systems are ~56% right stepping, ~70% hard-linked, and ~56% overlapping; about ~74% lie between fault strands oriented north to north-northeast. Average relay ramp width for the higher-temperature step-overs is 3.3 km. Producing systems (i.e., containing operating power plants) in step-overs preferentially step left (~64%), are hard-linked (~73%), overlap (~55%), and have an average relay ramp width of 3.4 km. These data suggest that higher-temperature systems favor overlapping, hard-linked geometries, which have relatively high densities of fractures, faults, and fault intersections, all of which enhance permeability. Additional structural complexity provides subvertical conduits of enhanced permeability that facilitates transport of hot fluids from greater depths. These attributes combined with faults optimally oriented to accommodate dilation in the current stress field provide long-lived permeable pathways that can host higher-temperature geothermal systems. This information may facilitate more efficient exploration of hidden geothermal systems across the GBR. |