Insights into the Tectonic Evolution of the North American Cordilleran Hinterland from Detrital Zircon Double Dating of the Eocene Elko Formation

Abstract:

The North American Cordilleran hinterland, from eastern Nevada to western Utah, is the product of Mesozoic crustal thickening and eventual orogen collapse. In NE Nevada, the Eocene Elko Formation represents ~10 Myr of high-elevation (up to 3.5 km) lacustrine deposition within a Paleogene orogenic plateau interior, prior to Neogene extensional collapse. Detrital zircon U-Pb and (U-Th)/He (ZHe) double dating from the Elko Formation constrains the timing and magnitude of tectonic-scale processes as well as the evolution of hinterland paleohydrology and magmatism. Eocene maximum depositional ages from detrital zircon are largely consistent with new single crystal sanidine ⁴⁰Ar/³⁹Ar ages for Elko Formation tuff beds.

U-Pb-He double dating of Eocene sediments supports several phases of exhumation and surface uplift in the hinterland. ZHe ages, combined with U-Pb geochronology, clast assemblages, and paleocurrent analysis suggest a significant amount of detritus from the Roberts Mountain Allochthon (RMA) and back-arc plutons was transported to the Elko Basin during the middle Eocene. Detrital zircons sourced from the RMA record progressive unroofing and are characterized by Archean-Paleoproterozoic crystallization ages and Mesoproterozoic-Triassic cooling ages. The preponderance of Precambrian ZHe ages during the middle-late Eocene and the absence of reset ZHe ages throughout the basin imply that sediment burial did not exceed depths of >6 km during the Phanerozoic. Double dating of several grains from a sandstone below the base of the lacustrine section also confirms the presence of volcanic detritus derived from the Challis volcanic field of central Idaho 400 kilometers to the northeast.

Lag time analysis permits discrimination of syndepositional volcanic grains from grains derived from rapidly exhumed sources. Lag time analysis (excluding volcanic grains) indicates that several source areas west of the Elko Basin have undergone major exhumation during the late Eocene.