STRUCTURAL INVESTIGATIONS OF GEOTHERMAL FIELDS: APPLICATIONS AND IMPLICATIONS

Structural studies are critical in assessing geothermal systems and selecting targets for geothermal wells, because faults are commonly the primary pathway for deeply circulating hydrothermal fluids. Important tools for structural analysis include detailed geologic mapping, kinematic analyses of faults, and determinations of stress orientations. These data can be integrated with 3-D stress-strain models to predict the location and orientation of permeable fractures at depth. Detailed characterization of the local structural setting is crucial for determining areas with the highest fracture density and faults with the greatest tendency for slip or dilation. Detailed structural assessments are particularly critical for geothermal fields in the Basin and Range and Aegean extensional provinces of western North America and western Turkey, respectively, where regional extension and transtension combine with high heat flow to generate abundant geothermal activity in regions with little recent volcanism.

In the western U.S., the northwestern Great Basin is one of the most geothermally active areas, probably in response to enhanced dilation on N- to NNE-striking normal faults induced by a transfer of NW-directed dextral shear from the Walker Lane to NW-directed extension. In Turkey, geothermal activity is focused in the Aegean extensional province where slab roll-back in the Hellenic arc and tectonic escape of the Anatolian block promote large-magnitude ~N-S extension. Our findings from these extensional provinces suggest that many geothermal systems occupy discrete steps in normal fault zones or lie in belts of intersecting, overlapping, and/or terminating faults. In addition, most fields are associated with steeply dipping faults and, in many cases, with Quaternary faults. The structural settings favoring geothermal activity generally involve subvertical conduits of highly fractured rock along fault zones oriented approximately perpendicular to the least principal stress. Features indicative of these settings that may be helpful in guiding exploration include: 1) major steps in normal faults, 2) interbasinal highs, 3) groups of relatively low, discontinuous ridges, indicative of en echelon sets of normal faults, and 4) the lateral, horse-tailing terminations of major range-front faults.