Structural Controls of the MacFarlane Geothermal Sysytem, Humboldt County, Nevada

Sabina M. Kraushaar and Patricia H. Cashman

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 mapped faults near the hot spring include a Nstriking, 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 and step-over formation, or related to a single fault termination and/or unknown buried faults. Many of these faults are covered by Quaternary sediments, so a magnetic survey and detailed Quaternary mapping are in progress to resolve the specific structural setting.

The anomalous orientation of the travertine mound 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.

AAPG Search and Discovery Article #90181©2013 AAPG/SEG Rocky Mountain Rendezvous, University of Wyoming, Laramie, Wyoming, September 27-30, 2013