Structural Controls of the MacFarlane Geothermal System, Humboldt County, Nevada: New Insights Based on Detailed Geologic Mapping, Shallow Temperature Surveys, and Magnetic Data

Detailed geologic mapping, structural analysis, magnetic and two-meter temperature data, integrated with previous datasets, constrain the structural controls of the MacFarlane geothermal system. MacFarlane hot springs and the travertine fissure ridges lie within a relay ramp. The relay ramp is formed between two overlapping, north-northeast-striking, west-dipping Holocene normal faults exposed in Lake Lahontan sediments. Other mapped faults near the hot spring include a north-striking, west-dipping Tertiary fault east of MacFarlane hot springs. The highest temperature gradient is found at the projected intersection between the Tertiary and north-northeast-striking Quaternary fault, ∼2.5 km northeast of the hot spring (Sibbett et al., 1982; Swanberg and Bowers, 1982). Our new data suggest other controls involving the relay ramp geometry of the Holocene faults.;The anomalous orientation of the travertine fissure ridge 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 east-trending travertine fissure ridge, which is ∼180 m long. The travertine fissure ridge is up to ∼2 m tall and ∼5 m wide, and has a central fissure along its long axis. The orientation of the travertine fissure ridge indicates local north-south extension, which is inconsistent with the regional west-northwest extension of the northwestern Basin and Range province. The anomalous travertine orientation is due to fractures that occurred during formation of a relay ramp between two overlapping fault segments.