The sheep pass formation, a record of late Cretaceous and paleogene extension within the Sevier hinterland, East-Central Nevada

The Sevier hinterland of western North America is considered by many to be an ancient proxy for the modern Andean Puna-Altiplano or Tibetan Plateau. However, controversies exist as tectonic setting and overall paleogeography of the Sevier hinterland during the Late Cretaceous and Paleogene. The Sheep Pass Formation type section within the southern Egan Range of east-central Nevada comprises a > 1 km thick sedimentary succession spanning the latest Cretaceous to Eocene, and provides a rare opportunity to test prevailing tectonic and paleogeographic models for the Sevier hinterland. New 1:12,000 scale field mapping in the southern Egan Range indicates that up to three km of stratigraphic throw occurred along the Ninemile fault, a presently low-angle down-to-the-northwest normal fault, during deposition of the Sheep Pass Formation type section. Subsequent reactivation of the Ninemile fault produced an additional &sim1 km of stratigraphic throw during deposition of the Garrett Ranch Group, which unconformably overlies the Sheep Pass Formation type section. New U-Pb and (U-Th)/He detrital zircon dating and U-Pb carbonate age analyses from the Sheep Pass Formation type section indicate that the Ninemile fault system was active in latest Cretaceous time, and documents for the first time the presence of surface-breaking, synconvergent normal faults within the Sevier hinterland. New U-Pb detrital zircon and 40Ar/39Ar age analyses from the overlying Garrett Ranch Group document reactivation of the Ninemile fault in the middle to late Eocene, indicating that two discrete episodes of extension affected the Sevier hinterland. Movement along the Ninemile fault was coeval with Late Cretaceous and early Paleogene mid-crustal extension within the Sevier hinterland, and suggests a possible link. Middle to late Eocene extension was coeval with extension in the Sevier foreland of central Utah, and foundering of the Farallon slab. Evidence that extension significantly predated volcanism within the Sevier hinterland invalidates the theory that Paleogene volcanism drove coeval extension. Recognition of synconvergent extensional basins within the Sevier hinterland strengthens comparisons to the modern Puna-Altiplano and Tibetan plateau, where similar processes have been documented.