ONSET OF CENOZOIC VOLCANISM AND REGIONAL EXTENSION IN NORTHEAST NEVADA

From 60 to 35 Ma, the western U.S. Cordillera experienced a time-transgressive transition from large-scale crustal shortening to regional extension. Proposed mechanisms for this transition include evolution of plate boundary conditions and buoyancy forces due to Mesozoic crustal thickening and/or replacement of mantle lithosphere by hotter, more buoyant asthenosphere. Widespread volcanism across the Cordillera is broadly synchronous with the onset of extension and has been linked to asthenospheric buoyancy models, but few studies have specifically documented this linkage. This study documents the co-evolution of extension and volcanism at Copper Basin in northeast Nevada (Elko County). Copper Basin is flanked on the west by the Copper Creek detachment fault and consists of >1.5 km of late Eocene to Oligocene strata grading conformably upward from the volcaniclastic Dead Horse Formation into the conglomeratic Meadow Fork Formation. Petrographic evaluation and clast counts of the conglomerates from this sequence indicate an inverted stratigraphy recording the unroofing of footwall lithologies. The appearance of footwall clasts of metamorphic and granitic rock in the Meadow Fork Formation defines a younger age bracket for the onset of faulting. However, the chronology of basin evolution is presently constrained only by dates of 41 Ma from the lower Dead Horse Formation, 37.4 Ma from the uppermost Dead Horse Formation and 29.4 Ma from the upper Meadow Fork Formation. Field work, including measured sections, reveal the existence of at least five major ash fall deposits within the upper Dead Horse Formation and sporadic ash fall horizons throughout the Meadow Fork Formation. Consequently, planned dating of tuffs from the Tertiary sequence offers an opportunity to clock the pace of unroofing, volcanism and syntectonic basin development. In addition, future U-Th/He dating of samples of 50 granitoid cobbles from four different horizons within the Meadow Fork Formation promises to constrain the lower temperature cooling histories of the eroded footwall rocks and thus their exhumation along the fault.