Late Paleogene paleotopographic evolution of the northern Cordilleran orogenic front: Implications for demise of the orogen

The paleotopographic history of the North American Cordilleran orogen holds the key to understanding mechanisms of orogenesis and subsequent orogenic collapse. It has been suggested that the orogenic front in western Montana (USA) and Alberta (Canada) was more than 4 km high during Late Cretaceous-early Eocene contractional deformation and during the initial phase of extension in the middle Eocene; however, the late Eocene-Oligocene topographic evolution during continued extensional collapse remains poorly constrained. Here we extend the paleotopographic record in the Kishenehn Basin in northwestern Montana and southeastern British Columbia (Canada) to the late Oligocene by studying delta O-18 values of fossil mollusks and cement and paleosol carbonates. The molluscan taxa changed from three sympatric groups with preferred habitats ranging from tropical wet, semi-arid subtropical, and temperate during the middle and late Eocene, to mainly a single group associated with temperate environment during the Oligocene, reflecting a decline in molluscan biodiversity induced by climate cooling across the Eocene-Oligocene transition. Reconstructed delta O-18 values of alpine snowmelt and basinal precipitation decreased by 1.4% and 3.8 parts per thousand, respectively, from the middle to late Eocene, reflecting climate cooling and similar to 1 km surface uplift of the basin floor. The reconstructed alpine snowmelt delta O-18 values then increased by 2.9 parts per thousand in the Oligocene suggesting a similar to 0.5 km drop in elevation of the orogenic front. Collectively, the results of our new and previously published delta O-18 data chronicle the paleotopographic response to the change from flat-slab subduction to slab rollback over a 45 m.y. period. These data suggest that the orogenic front was characterized by high elevation (>4 km) in the ancestral Lewis-Clark-Livingston ranges during latest Cretaceous-early Eocene (ca. 75-52 Ma) contraction. The initial phase of extension related to the Kishenehn Basin created a lowland basin with a surface elevation of only similar to 1.5 km during the early middle Eocene (ca. 46-44 Ma) whereas the ranges remained >4 km high. The high range elevations were sustained for at least 12 m.y. in the middle to late Eocene concurrent with extension, while the basin floor elevation was uplifted to similar to 2.5 km by the latest Eocene (ca. 36-34 Ma). Basin aggradation can explain at most half of the 1 km basin floor uplift. The remaining amount (at least 0.5 km) and sustained high range elevation suggest that range denudation and crustal extension was compensated by the isostatic and thermal effects of slab rollback and/or passage of a slab window and infusion of hot asthenosphere beneath the continent. The range elevation in the orogenic front decreased similar to 0.5 km by the late Oligocene (ca. 28 Ma), associated with a decrease in rock uplift rate associated with extension. A post-Oligocene elevation drop of similar to 1 km resulted in both the ranges and basin floor reaching modern topography in the Kishenehn Basin drainage, likely due to the regional effect of Neogene Basin and Range extension. This study, along with the previous investigation of the Kishenehn Basin by Fan et al. (2017), are the first studies that systematically investigate paleorelief of the orogenic belt by reconstructing paleoelevations of the mountains and the basin at the same time. The results highlight that the Cordilleran orogenic front of northern Montana and southern British Columbia sustained its high elevation edifice for at least 12 m.y. after the start of extension. We suggest that initial crustal extension did not result in orogenic demise because of concurrent thermal and isostatic uplift.

Automated summary

The study reveals the paleotopographic changes in the North American Cordilleran orogen from Late Cretaceous to Late Oligocene, showcasing the biodiversity decline of molluscan taxa in response to climate cooling. The reconstructed delta O-18 values suggest a 1 km surface uplift followed by a 0.5 km drop in elevation of the orogenic front, indicating the effects of climate change and tectonic processes on the mountain system.