Abstract: Structural Geology and Tectonic Implications of a Part of the Northern Stillwater Range, Nevada

Gabriel L. Plank

The east flank of the Stillwater Range adjacent the Dixie Valley geothermal area near Fallon, Nevada, hosts one of the best exposures of Mesozoic thrust faults in the Basin and Range province. The rangefront comprises four imbricate lithologic packages. The Triassic Star Peak Group sits structurally lowest beneath Triassic phyllite of the Fencemaker-B allochthon. Bedded quartz arenite of the Jurassic Boyer Ranch Formation lies above the phyllite along the Boyer thrust. Rocks of the Humboldt Igneous Complex sit structurally highest in brittle fault contact with both the arenite and phyllite. The Fencemaker thrust is a major Jurassic structure in west-central Nevada which places Triassic basinal strata northeastward over shelf carbonates of the Star Peak Group, which deposi ionally overlie the Golconda allochthon. Locally, the Fencemaker thrust lies within a high strain zone characterized by mylonitic marble, phlogopite-bearing calcareous argillite schist, boudinaged siliciclastics, and phyllonite. Consistently southeast-dipping penetrative foliations and down-dip stretching lineations in these Triassic metasedimentary rocks are, however, inconsistent with northeast directed thrusting. This suggests that northwest vergent thrusting also occurred here, possibly along the Willow Creek thrust. In contrast to the Fencemaker thrust, the Boyer thrust is characterized by close folds in the hanging wall, a narrow zone of fault gouge, and crenulation of footwall foliations, indicating a less ductile regime. East dipping Tertiary extensional faults expose these thrus s along the Stillwater rangefront and displace the thrust sheets downdip beneath Dixie Valley. The geometry of these thrust sheets in the subsurface is critical to the production of geothermal wells in the area.

AAPG Search and Discovery Article #90952©1996 AAPG Rocky Mountain Section Meeting, Billings, Montana