Structural, stratigraphic, and geochronologic evidence for extension predating Palaeogene volcanism in the Sevier hinterland, east-central Nevada

Abstract

Alluvial and lacustrine deposits of the >1 km thick, uppermost Cretaceous to middle Eocene Sheep Pass Formation of east-central Nevada provide a unique opportunity to test models of the tectonic and palaeogeographic evolution of the Sevier hinterland. Within the south Egan Range, new 1:12,000 geologic mapping and stratigraphic observations reveal that latest Cretaceous initiation of the Sheep Pass basin was marked by megabreccia deposition, growth faults, and fanning dips that formed in response to down-to-the-northwest motion along the Ninemile fault, a presently low-angle normal fault displaying 4 km of stratigraphic throw. Continued Maastrichtian to Late Palaeocene motion along the Ninemile fault is suggested by widespread soft-sediment deformation within the Sheep Pass Formation, interpreted as seismites. Located 20 km to the south of the Sheep Pass Formation type section, the Shingle Pass fault similarly shows evidence for late Palaeocene motion. A subsequent episode of Eocene extension is recorded within the Sevier hinterland by a series of normal faults that repeat the Sheep Pass Formation type section, but are overlapped by the upper Eocene to Oligocene Garrett Ranch Group. These faults are interpreted as splays related to reactivation of the Ninemile fault system. Megabreccia deposits of middle to late Eocene age in the hanging wall of the Shingle Pass fault also record this younger event. New 40Ar/39Ar ages of Eocene volcanic strata in the Egan and Schell Creek ranges presented here indicate that, while this later period of extension overlapped ca. 38–35 Ma volcanism across a wide swath of east-central Nevada, renewed extension may have begun as early as the middle Eocene. Palaeocurrent data from uppermost Cretaceous to upper Eocene alluvial fanglomerates of the Egan and Schell Creek ranges record westward palaeoflow away from the foreland, and suggest that the area of the central Nevada/Utah borderlands formed a series of long-lived highlands bounded to the west by west-dipping normal faults. These data indicate that the Sevier hinterland of east-central Nevada was topographically more rugged than generally envisioned and experienced episodic extension throughout latest Cretaceous–Palaeogene time. Late Cretaceous to Palaeocene extensional basins overlapped temporally with previously documented mid-crustal extension within the Sevier hinterland, and with shortening within the Sevier foreland to the east. Orogen-top, synconvergent extensional basins are documented in both the modern Puna-Altiplano and Tibetan plateaus, and our new data strengthen their comparison with the Late Cretaceous to Palaeogene Sevier hinterland.