NEW CONSTRAINTS ON THE TIMING, RATE, AND STYLE OF EXHUMATION OF THE WOOD HILLS AND PEQUOP MOUNTAINS, ELKO COUNTY, NEVADA

Zircon (U-Th)/He analysis constrains the timing, rate, and style of exhumation of the Wood Hills and Pequop Mountains in northeast Nevada. The Ruby Mountains-East Humboldt Range-Wood Hills (REHW) metamorphic core complex (MCC) is a premier natural laboratory for studying the dynamics of MCCs because it exposes middle to lower crustal rocks (up to >35 km deep) in the hinterland of the Late Cretaceous Sevier orogenic belt. In addition, the kinematics of exhumation are well known from an extensional mylonitic shear zone on the western flank of the complex. Ongoing debate focuses on the timing of onset and the progressive evolution of exhumation along the Ruby Mountains Detachment Fault. Some argue that it began during Miocene or younger times contemporaneous with the dominant phase of extension throughout the Basin and Range, whereas others infer a pre–Miocene age (e.g., in the Oligocene or Eocene). New zircon (U-Th)/He age results from a transect across the Wood Hills in the direction of tectonic transport yield ages ranging from ~39 Ma in the SE to ~27 Ma in the NW, indicating that the entire Wood Hills were cooled through (U-Th)/He closure temperatures (~180 °C) by the late Oligocene. In addition, the ~27 Ma age from the NW Wood Hills comes from a sample of extensional quartzite mylonite that must have been deformed at temperatures >300°C; thus mylonitization must have predated ~27 Ma. Taken together, the results from the Wood Hills indicate that extensional unroofing of the up-dip parts of the REHW must have begun by at least Oligocene, or more probably Late Eocene time. In the Pequop Mountains farther to the SE, zircon (U-Th)/He dating yields an age of ~43 Ma from the structurally deepest levels near the western base of the range and a date of ~61 Ma from quartzite immediately underlying the Pequop normal fault near the crest of the range. However, whether the ages from the Pequop Mountains represent forced cooling due to extensional exhumation, conductive cooling, or partial retention ages cannot be evaluated without additional data.