THERMOCHRONOLOGICAL EVIDENCE FOR EOCENE EXHUMATION IN THE RUBY MOUNTAINS - EAST HUMBOLDT RANGE - WOOD HILLS METAMORPHIC CORE COMPLEX, NE NEVADA

Cordilleran Metamorphic Core Complexes expose thick sections of the crust providing long and detailed records of deformation, metamorphism, and exhumation. The Ruby Mountains - East Humboldt Range - Wood Hills (REHW) Metamorphic Core Complex in NE Nevada forms the farthest west and most deeply exhumed of the northern Great Basin’s three classic core complexes and thus preserves a crucial record of Cordilleran lithospheric evolution. Exposures in the REHW contain evidence of both Mesozoic prograde metamorphism and later extensional exhumation and related cooling.

Although the west flank of the REHW records Early to Middle Miocene cooling and exhumation, significant Late Eocene cooling and exhumation of the southeastern parts of the core complex began at ~43 Ma as recorded by biotite, muscovite, and K-feldspar ⁴⁰Ar/³⁹Ar and zircon (U-Th)/He data, in some cases from the same sample locations. Importantly, this cooling event from >~400°C to <~200°C is recorded at multiple structural levels by multiple thermochronologic systems, thus disfavoring the possibility that is is the product of residence in a partial retention zone. Thermal modeling of key samples supports these results and helps to better resolve the rate and magnitude of exhumation. Key structural relationships lend additional support for the extensional mechanism of exhumation. For example, samples from the extensional mylonitic zone in the NW Wood Hills record cooling through mylonitic temperatures by the late Oligocene. In the southern East Humboldt Range, cooling to <180 °C occurred in kyanite-bearing rocks directly underlying an extensional detachment fault that carried sedimentary rocks as young as Triassic in its hanging wall.

This Eocene cooling event likely continued into the Oligocene and is separate and distinct from later Miocene exhumation also documented in the REHW and throughout the Basin and Range. Thus the REHW is clearly the product of a composite Cenozoic extensional history. Widely cited mechanisms for initiating extension in the Basin and Range, such as the arrival of the Yellowstone hotspot, cannot viably explain Eocene exhumation. Instead, we suggest that crustal weakening and gravitational collapse accompanying the Late Eocene ignimbrite flare-up was the most likely tectonic mechanism for Late Eocene extension.