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The hinterland of the Sevier orogenic belt of western North America is widely interpreted as a Cretaceous to Paleogene orogenic plateau. Although evidence for mid-crustal extension of Late Cretaceous age within the Sevier hinterland is widespread, coeval surface-breaking normal fault systems have not been documented. New 1:12,000-scale mapping within the type section of the latest Cretaceous to Eocene Sheep Pass Formation of east-central Nevada suggests that deposition occurred in response to normal fault movement recording up to 4 km of Late Cretaceous and Paleogene stratigraphic throw. Intrabasinal normal faulting caused lateral thickness variations within the basal Sheep Pass Formation, although upper members are largely unaffected. An extensional basin setting best explains the fanning of bedding dips, the deposition of megabreccia, and the presence of syndepositional normal faults within the Sheep Pass Formation. Deposition of the basal member of the Sheep Pass Formation is bracketed between ca. 81.3 ± 3.7 Ma and 66.1 ± 5.4 Ma, based on the (U-Th)/ He cooling ages of detrital zircons, and on a U-Pb carbonate age derived from the overlying lacustrine limestone member. These new data provide the strongest evidence to date for the existence of Late Cretaceous, surface-breaking normal faults in the Sevier hinterland. Normal faulting was coeval with mid-crustal hinterland extension and with continued contraction within the Sevier foreland to the east. on June 8, 2015 geology.gsapubs.org Downloaded from

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The Late Cretaceous to Paleogene Sevier hinterland of east-central Nevada is widely regarded as an orogenic plateau that has since undergone topographic collapse. New U-Pb detrital zircon age data consisting of 1296 analyses from the Lower Cretaceous Newark Canyon Formation and the Upper Cretaceous to Eocene Sheep Pass Formation indicate that Precambrian detrital zircon populations recycled from local Paleozoic strata are dominant. Subordinate Mesozoic zircon populations are derived mainly from local backarc volcanic centers of Late Jurassic and Early Cretaceous age, while ca. 38-36 Ma detrital zircon age peaks record the local onset of Eocene volcanism. Sevier hinter land deposits of east-central Nevada lack signifi cant Triassic, Early Jurassic, and Late Cretaceous populations common in terranes of western Nevada and the Sierra Nevada magmatic arc. These data suggest that long-term evolution of the Sevier Plateau involved geographic isolation through a combination of high relief and rugged topography related to Early Cretaceous shortening, and continued isolation through development of latest Cretaceous to Eocene internally drained, extensional basins. The (U-Th)/He zircon ages obtained from the Sheep Pass Formation record late Paleozoic, Early Cretaceous, and Late Cretaceous cooling through 180 °C. Preservation of late Paleozoic (ca. 265 Ma) cooling ages indicates that much of the Upper Paleozoic section within east-central Nevada that contributed detritus to the Sheep Pass basin was un affected by deep thrust burial, or by burial beneath thick Mesozoic sedimentary cover. Early Cretaceous (ca. 135 Ma) cooling ages are potentially coeval with shortening along the central Nevada fold-and-thrust belt, although ca. 80 Ma cooling ages within the Sheep Pass Formation are coeval with hinter land midcrustal extension. Together, these new data provide support for previous interpretations that the Sevier hinterland represents an ancient high-elevation orogenic plateau, and that the latest Cretaceous locally marks a transition from contraction to extension.

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