Title: Preliminary geologic map of the Terrill Mountains quadrangle, Churchill and Mineral counties, Nevada

Author: Chad W. Carlson
Year: 2014
Series: Open-File Report 2014-04
Version: superseded by Map 187
Format: 33 x 27 inches, color
Scale: 1:24,000

The Terrill Mountains 7.5-minute quadrangle is located about 45 km south of Fallon and incorporates the bulk of the Terrill Mountains, southern Calico Hills, northwest Red Ridge, and part of the Rawhide Flats valley just east of U.S. Highway 95. The Terrill Mountains quadrangle has significant relevance to understanding the evolving tectonic framework of the region, as it straddles a major domain boundary in the Walker Lane. Positioned between the Sierra Nevada microplate and Basin and Range, the Walker Lane accommodates ~20% of the right-lateral transform motion between the North American and Pacific plates. This motion is accommodated on NW-striking right-lateral and ENE to E-W-striking left-lateral fault systems. The Terrill Mountains lie at the northern terminus of right-lateral fault zones translating crustal-blocks of the central Walker Lane and at the southeastern edge of left-lateral faults accommodating clockwise-rotation of blocks in the northern Walker Lane. As the mechanisms of strain transfer between these disparate fault systems are poorly understood, the thick Oligocene to Pliocene volcanic strata of the Terrill Mountains make it an ideal site for studying the transfer of strain between regions undergoing differing styles of deformation and yet both accommodating right-lateral motions. Further, it contains several major Quaternary faults capable of producing large earthquakes and the Camp Terrill mining district.

Detailed geologic mapping of the Terrill Mountains quadrangle was completed to help elucidate the Neogene styles of, and changes in, strain accommodation for this region of the Walker Lane. The mapped Tertiary strata include at least nine late Oligocene to early Miocene ash-flow tuffs. Several tuffs, not previously identified in the Terrill Mountains, are tentatively correlated to regionally extensive units in the western Great Basin, including the 25.3 Ma Nine Hill Tuff. A distinct ~23 Ma paleosol is locally preserved below the tuff of Toiyabe and provides an important marker bed. This paleosol is offset ~6 km across a strand of the NW-striking, right-lateral Benton Springs fault that bounds the NE flank of the Terrill Mountains. Strain at the northernmost Terrill Mountains appears to be transferred from a system of NW-striking right-lateral faults to a system of ~E-W striking left-lateral faults with associated clockwise flexure. The northern Terrill Mountains may represent a localized region of strain transfer analogous to the greater transition between the central and northern Walker Lane.

The detailed mapping of the Terrill Mountains quadrangle, completed through the EDMAP component of the National Cooperative Geologic Mapping Program in cooperation with the U.S. Geological Survey, has provided a robust foundation for ongoing and future structural, paleomagnetic, and geochronologic investigations in the region.