QUANTIFYING EOCENE AND MIOCENE EXTENSION IN THE SEVIER HINTERLAND WITH IMPLICATIONS FOR MINERAL AND ENERGY RESOURCES IN NORTHEASTERN NEVADA

Rocks exposed in the Ruby-East Humboldt metamorphic core-complex, NE Nevada, provide a guide for reconstructing the Eocene crustal structure in the hinterland of the Sevier Orogen. These rocks occupy the footwall of a major west-dipping normal-sense shear system that may extend ~50km further west under part of the Carlin trend. Previous thermochronologic studies show that mineral cooling ages from the Ruby Mountains core complex generally grow younger to the WNW in the direction of fault slip. Biotite K-Ar ages young in the westward direction from ca. 33 Ma to ca. 20 Ma at an inverse rate of 0.5 to 1 km/m.y. The age progression of younger ages to the west can be interpreted to suggest the rate of westward unroofing and cooling of the core complex, or an east-tilted oblique section through a zone of partial argon retention. New 40Ar/39Ar biotite data show apparent ages between ca. 31 and 21 Ma are, therefore, consistent with the previous K- Ar data in showing a decrease in cooling age from east to west. These data also indicate an episode of rapid exhumation at about 20 Ma. Apatite fission track apparent ages from a traverse through the central Ruby Mountains range in age from about 20.4 to 14.1 Ma, and generally decrease in age to the WNW at an inverse rate of about 5 km/m.y. These samples all show mean track lengths of 14.1 to 14.9 microns, which indicates rapid cooling between ca. 20 and 14 Ma. New 40Ar/39Ar data from three additional transects will also be presented.