POLYPHASE (EOCENE AND MIOCENE) EXTENSIONAL FAULTING OF THE CENTRAL SCHELL CREEK RANGE, WHITE PINE COUNTY, NEVADA

The central Schell Creek Range (SCR) in eastern Nevada records some of the oldest extensional faulting in the eastern Great Basin. The geometries and kinematics of these early normal faults are poorly documented in this portion of the range, yet they have major implications for regional scale extensional structures and ore deposits in the Great Basin. New detailed 1:12,000 scale geologic field mapping and structural analysis has documented six separate tilt domains along with numerous high- and low-angle, east-and west-dipping-normal faults. Two of the largest and oldest faults are the low-angle, west-directed, Majors Place Detachment (MPD), and the low-angle, east-directed, Schell Peaks Detachment (SPD). These faults along with their hanging wall splays have thinned the exposed rocks underlying the range from 7 km to 2-3 km. The stratigraphy in the footwall of the SPD and MPD is sub-parallel to faults and is largely unmetamorphosed. The SPD has top-to-the-east displacement and shows a switch in the tilt polarity in the hanging wall from west-tilted in the south to east-tilted in the north. The MPD is a west-directed normal fault, and ⁴⁰Ar/³⁹Ar analysis of footwall rocks and syntectonic ignimbrites in the hanging wall of the MPD demonstrates the fault was active between 40-35 Ma. The MPD cuts hanging wall structures of the SPD, implying the SPD is older than the MPD. The SPD and MPD are cut by a second generation of high-angle normal faults associated with the modern range bounding fault system.

The data presented here suggests a three stage model for the structural evolution of the central SCR. 1) Cretaceous shortening created ~NNE trending folds across the eastern Great Basin. 2) Extension, beginning in the Eocene, forming closely spaced normal faults – the SPD and MPD. The SPD and MPD possibly exploited the limbs of a pre-existing antiform transecting the SCR. The east-directed SPD exploited the east-dipping limb and the west-directed MPD exploited the west-dipping limb. 3) Miocene and younger range-bounding-normal fault systems form at a high angle to bedding and cut the low-angle faults within the range.