Structural Localization of Two Low Temperature Geothermal Systems Within the Gravity Defined Linkage Between Dixie Valley and Fairview Valley, Nevada, USA

摘要

This study offers an interpretation of the structural geometry of the Transition Zone (TZ) between the Dixie Valley Structural Basin DVSB and Fairview Valley Structural Basin FVSB based on new gravity data (300 stations) integrated with the rupture patterns of the 1954 Dixie Valley Fairview Peak earthquake sequence. The TZ is of potential interest for geothermal exploration as it hosts the Elevenmile Canyon and Pirouette Mountain low temperature geothermal occurrences. The TZ is also the locality of a 10 km left-step over and slip direction change from oblique to vertical in the rupture pattern of the 1954 earthquake sequence. The new Complete Bouguer Anomaly (CBA) gravity map reveals the basement structural relief in the TZ fairly effectively. Northwest gravity gradients were chosen as the approximate structural boundaries of the TZ which measures approx18 km north to south. An interpretation of basement depths assuming densities associated with Tertiary alluvium and volcanic basin fill over Mesozoic metasedimentary basement indicates the DVSB and FVSB have smaller structural basins joined to their main terminations. These form a roughly antisymmetric pair of north-south trending, right-stepping grabens (3 to 5 km wide and 500-700 m deep), which attenuate in relief toward a central overlap area. The grabens are consistent with the concept of a zone of maximum east-west extension cutting through the TZ, which aligns with the southward projection of the maximum east-west extension in western Dixie Valley reported by other studies. Additionally, the overlap area and significant rupture segments associated with the 1954 Fairview Peak - Dixie Valley earthquake sequence seem to be controlled by the interpreted boundaries of the gravity defined TZ. The two geothermal systems occur in asymmetric structural nodes of the TZ, where the inferred northwest TZ's boundaries intersect the north-south graben (range front) faults. More specifically, the Elevenmile Canyon geothermal occurrence is located near the intersection of the southern most extent of the 1954 Dixie Valley fault rupture and the TZ's southern NW trending boundary. And, the Pirouette Mountain (PM) occurrence has nearly the same geometric relation in an anti-symmetric sense; the trace of the Lauderback rupture segment (part of the Fairview Peak rupture group) projects toward the PM occurrence, but dies out 5 km away.