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Abstract Eocene strata of the Elko Formation record lacustrine deposition within the Nevada hinterland of the North American Cordillera. We present a detailed geochemical stratigraphy enabled by high‐sampling‐resolution geochronology from lacus trine limestone and interbedded volcanic rocks of the Elko Formation. Two intervals of lacustrine deposition, an early Eocene “Lake Adobe” of limited aerial extent and a laterally extensive middle Eocene “Lake Elko,” are separated by ∼5 m.y. of apparent unconformity. Sediments deposited in the apparently short‐lived (49.5–48.5 Ma) early Eocene Lake Adobe exhibit high‐amplitude covariation of δ¹⁸O, δ¹³C and⁸⁷Sr/⁸⁶Sr, which suggests a dynamically changing catchment and precipitation regime. Lake Elko formed during the middle Eocene, and its strata record three geochemically distinct phases, indicating it was a single interconnected water body that became increasingly evaporative over time. The lower Elko Formation (44.0–42.5 Ma) was deposited in a freshwater lake. Middle Elko Formation (42.5–41.2 Ma) lithofacies and geochemistry suggest that an increasingly saline and alkaline Lake Elko experienced salinity stratification‐induced hypolimnion disoxia and burial of¹²C‐rich organic matter. The upper Elko Formation (41.2–40.5 Ma) records a shallow final phase of Lake Elko that experienced short residence times and a breakdown in stratification. A sharp decline of⁸⁷Sr/⁸⁶Sr in the upper Elko Formation reflects an increasing aerial extent of low‐⁸⁷Sr/⁸⁶Sr volcanic deposits from nearby calderas. Middle Eocene strata record ponding of paleodrainage, increasing hydrologic isolation and volcanism, consistent with progressive north to south removal of the Farallon flat slab and/or delamination of the lower lithospheric mantle of the North American plate. 

Sedimentary basins record crustal-scale tectonic processes related to the construction and demise of orogenic belts, making them an invaluable archive for the reconstruction of the evolution of the North American Cordillera. In southwest Montana, USA, the Renova Formation, considered to locally represent the earliest accumulation following Mesozoic−Cenozoic compressional deformation, is widespread but remains poorly dated, and its origin is debated. Herein, we employed detrital zircon U-Pb and (U-Th)/He double dating and sanidine 40Ar/39Ar geochronology in the context of decimeter-scale measured stratigraphic sections in the Renova Formation of the Muddy Creek Basin to determine basin evolution and sediment provenance and place the basin-scale record within a regional context to illuminate the lithospheric processes driving extension and subsidence. The Muddy Creek Basin is an extensional half graben in southwest Montana that is ∼22 km long and ∼7 km wide, with a >800-m-thick sedimentary package. Basin deposition began ca. 49 Ma, as marked by multiple ignimbrites sourced from the Challis volcanic field, which are overlain by a tuffaceous fluvial section. Fluvial strata are capped by a 46.8 Ma Challis ignimbrite constrained by sanidine 40Ar/39Ar dating. An overlying fossiliferous limestone records the first instance of basinal ponding, which was coeval with the cessation of delivery of Challis volcanics−derived sediment into the Green River Basin. We attribute initial ponding to regional drainage reorganization and damning of the paleo−Idaho River due to uplift and doming of the southern Absaroka volcanic province, resulting in its diversion away from the Green River Basin and backfilling of the Lemhi Pass paleovalley. Detrital zircon maximum depositional ages and sanidine 40Ar/39Ar ages show alternating fluvial sandstone and lacustrine mudstone deposition from 46 Ma to 40 Ma in the Muddy Creek Basin. Sediment provenance was dominated by regionally sourced, Challis volcanics−aged and Idaho Batholith−aged grains, while detrital zircon (U-Th)/He (ZHe) data are dominated by Eocene cooling ages. Basin deposition became fully lacustrine by ca. 40 Ma, based on an increasing frequency of organic-rich mudstone with rare interbedded sandstone. Coarse-grained lithofacies became prominent again starting ca. 37 Ma, coeval with a major shift in sediment provenance due to extension and local footwall unroofing. Detrital zircon U-Pb and corresponding ZHe ages from the upper part of the section are predominantly Paleozoic in age, sourced from the Paleozoic sedimentary strata exposed in the eastern footwall of the Muddy Creek detachment fault. Paleocurrents shift from south- to west-directed trends, supporting the shift to local sources, consistent with initiation of the Muddy Creek detachment fault. Detrital zircon maximum depositional ages from the youngest strata in the basin suggest deposition continuing until at least 36 Ma. These data show that extension in the Muddy Creek Basin, which we attribute to continued lithospheric thermal weakening, initiated ∼10 m.y. later than in the Anaconda and Bitterroot metamorphic core complexes. This points to potentially different drivers of extension in western Montana and fits previously proposed models of a regional southward sweep of extension related to Farallon slab removal. 

High elevation regions cover a relatively small portion of Earth’s surface, but have an out-sized influence on regional hydrology, continental sediment transport, and global climate. What factors control the lifespan of mountains, and what are the mechanics of the transition to orogenic collapse? The Cenozoic morphology and evolution of the Cordilleran orogen is widely studied and debated in both North and South America, particularly the timing and rates of uplift, the links to possible mantle delamination, and the controls on extensional collapse. At least part of this discrepancy is due to the inherent complexity of atmospheric circulation and moisture sourcing in continental interiors, such as in the modern Rocky Mountains and Altiplano, where the distribution of hydrogen and oxygen isotope ratios (δD/δ18O) of precipitation does not correlate with temperature. Here we reconstruct past elevations across the northern and southern Basin and Range of the western U.S. and the Central Andes of southern Peru, using hydrogen isotope ratios (δD) of paleo-precipitation preserved in volcanic glass from widespread air-fall ashes and ignimbrites. Data span from the paleo-Pacific shoreline across the Cordilleran orogen and are paired with new sanidine 40Ar/39Ar geochronology and detailed chemo- and litho-stratigraphy, providing a novel approach to interpreting changes in δD values over space and time. Paleogene data in the western U.S. show that Pacific-sourced moisture crossed a high elevation Cordilleran hinterland prior to reaching what are now the Rocky Mountains and Great Plains. Mixing with Gulf-sourced moisture likely took place over the central and southern Rocky Mountains, making applications of the modern lapse rate or a Rayleigh modeled lapse rate inaccurate as both use a single moisture source. Data from west of the Sevier foreland, dominated by Pacific-sourced moisture, however, reveal the locations of peak Paleogene elevations. Across the southern Peruvian Altiplano, we observe a similar pattern of paleo-air-mass mixing from relatively higher and lower elevation moisture sources.