GEOLOGIC MAPPING AT THE NORTHERN TERMINUS OF THE CENTRAL WALKER LANE, WESTERN NEVADA: KINEMATICS AND STRAIN TRANSFER MECHANISMS IN AN EVOLVING TRANSFORM PLATE BOUNDARY

Positioned between the Sierra Nevada microplate and Basin and Range in western North America, the Walker Lane (WL) accommodates ~20% of the dextral motion between the North American and Pacific plates on predominately NW-striking dextral and ENE to E-W-striking sinistral fault systems. The Terrill Mountains lie at the northern terminus of a domain of dextral faults accommodating translation of crustal-blocks in the central WL and at the southeast edge of sinistral faults accommodating oroclinal flexure and clockwise-rotation of blocks in the northern WL. 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 area make it an ideal site for studying the transfer of strain between regions undergoing differing styles of deformation and yet both accommodating dextral shear.

Detailed geologic mapping of the Terrill Mountains 7.5-minute quadrangle was completed to elucidate Neogene strain accommodation for this transitional region of the WL. Strain at the northernmost Terrill Mountains appears to be transferred from a system of NW-striking dextral faults to a system of ~E-W striking sinistral faults with associated clockwise flexure. The mapped Tertiary strata include at least 9 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, dextral Benton Springs fault that bounds the NE flank of the Terrill Mountains. Preliminary paleomagnetic results suggest vertical-axis rotation of crustal blocks south of the domain boundary in the system of NW-striking dextral faults, similar to some other domains of NW-striking dextral faults in the northern WL.

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