EARLY CENOZOIC EXTENSION AND ACCOMMODATION IN THE BASIN AND RANGE: IMPLICATIONS FROM APATITE AND ZIRCON DOUBLE DATING

Paleogene sedimentary basins in the North American Cordilleran hinterland record the paleogeography of a high-elevation landscape at the onset of orogenic collapse. Detrital apatite and zircon U-Pb and (U-Th)/He double dating of Paleogene sedimentary rocks in northeastern Nevada facilitates evaluation of magmatism and the initiation of extension. We evaluate sedimentary lag times in the Cordilleran hinterland within a chronostratigraphic framework developed from new single crystal sanidine ⁴⁰Ar/³⁹Ar ages for Elko Formation tuff beds. This study demonstrates the utility of combined apatite and zircon double dating for assessing basin forming mechanisms and relative rates of source area exhumation. This method is applicable to a wide array of terranes, especially those that have well-constrained chronologies and well-established sedimentary provenance. Double dating of clastic sediments deposited across the width of the Cordilleran hinterland indicate slow rates of erosional exhumation of multi-cycle Paleozoic miogeocline sediments preceded drainage ponding during the early Eocene. Lag time analysis of non-volcanic grains reveals a ~150 Myr decrease in lag time and an inferred >1 km/Myr increase in source exhumation rate between the northern Elko Basin and Copper Basin, 50 km to the north, coincident with the onset of volcanism in northeastern Nevada. Subsidence of the northern Elko Basin occurred amidst slow exhumation, and was spatially isolated from magmatism. During the onset of proximal volcanism, Paleogene sediments deposited in Copper Basin show a middle Eocene transition from muted accumulation of primarily volcaniclastic detritus to rapid exhumation and basin-filling with footwall-derived sediment. Initial subsidence of the northern Elko Basin in the absence of volcanism and extension supports the hypothesis that early Eocene basins formed instead as a product of broad wavelength ponding of east-flowing paleorivers due to uplift driven by rollback of the shallow Farallon slab. A late Eocene transition to rapid exhumation and voluminous magmatism is consistent with early-onset extension across the Basin and Range being triggered locally by lithospheric weakening during southwest-directed slab rollback heating and magmatism.