STRUCTURAL CONTROLS OF THE MACFARLANE GEOTHERMAL SYSTEM, HUMBOLDT COUNTY, NEVADA: A PROGRESS REPORT

Detailed geologic mapping, structural analysis and two-meter temperature data, integrated with previous datasets, constrain the structural controls of the MacFarlane geothermal system. MacFarlane hot spring and travertine mound lie between two NNE-striking, west-dipping Quaternary faults in a dextral step-over or relay ramp zone. Other faults near the hot spring include a N-striking, west-dipping Tertiary fault east of MacFarlane hot spring, and a ENE-striking Quaternary fault mapped ~500 m north of the hot spring by Sibbett et al. (1982). The highest temperature gradient is found at the projected intersection between the Tertiary and NNE-striking Quaternary fault, ~2.5 km NE of the hot spring (Sibbett et al, 1982; Swanberg et al, 1982). Our new data suggest other controls involving the geometry of the Quaternary faults. The anomalously oriented travertine fissure ridge might be due to perturbations in the stress field during relay ramp formation, or related to a single fault termination, the dextral step-over between two N-striking faults, or a buried E-striking fault. Many of these faults are covered by Quaternary sediments, so a geophysical survey and more detailed Quaternary mapping are in progress to resolve the specific structural setting.

The anomalous character of MacFarlane hot spring motivated this study of the structural controls of the geothermal system. MacFarlane hot spring is located on the eastern margin of the Black Rock Desert, ~85 km west of Winnemucca, in Humboldt County, Nevada. The active hot spring emerges from the west end of an E-trending travertine mound, which is ~180 m long. The travertine mound is up to ~2 m tall and ~5 m wide, and has a central fissure ridge along its long axis. The orientation of the travertine fissure ridge indicates local N-S extension, which is inconsistent with the regional ESE-WNW extension of the Basin and Range Province.