Early Oligocene Surface Uplift in Southwestern Montana During the North American Cordilleran Extension

Abstract

AbstractThe topographic history of the North American Cordillera holds the key to understanding the tectonic and geodynamic processes of orogenesis and post‐orogenic collapse. In southwestern Montana and its adjacent regions, previous studies have suggested that the early middle Eocene rollback of the Farallon flat‐slab caused the latest topographic growth, after which regional mean elevations were progressively lowered due to extensional collapse. In this study, we report 60 new Eocene‐Miocene stable hydrogen isotopic values (δD) of hydrated volcanic glass from southwestern Montana to examine regional topographic history during the extension. The δD values show a two‐stage negative shift of ∼35‰ during the late Paleogene, the timing of which is further refined by published ages and new zircon U‐Pb ages. About 19‰ of the negative shift occurred during the late Eocene‐earliest Oligocene (∼40–32 Ma) when global and regional climate experienced cooling, and the shift can be mostly accounted for by climate change. The remaining 16‰ negative shift occurred during the early Oligocene (∼32–30 Ma), which can be best explained by regional surface uplift of 0.9 ± 0.5 km. This inferred surface uplift occurred more than 17 Myr after the initiation of the Cordilleran extension in this region, and thus challenges the traditional assumption of progressive topographic lowering during the post‐orogenic collapse. This surface uplift, if occurred, may reflect lower mantle lithosphere removal in southwestern Montana.