Prolonged high relief in the northern Cordilleran orogenic front during middle and late Eocene extension based on stable isotope paleoaltimetry

Abstract

The paleoelevation and size of the North America Cordilleran orogen during the late Cretaceous-Paleogene contractional and subsequent extensional tectonics remain enigmatic. We present new estimates of paleorelief of the northern Cordilleran orogenic front during the middle and late Eocene using oxygen isotope compositions of unaltered molluscan fossils and paleosol carbonates in the Kishenehn basin. Bounded by several mountains ranges to the east, the Kishenehn basin was a half graben developed during middle Eocene to early Miocene collapse of the Cordilleran orogen. These mollusk taxa include three sympatric groups with affinities to wet tropical, semi-arid subtropical, and temperate environments. Our reconstructed surface water  18 O values vary between-19.8 ‰ and-6.3 ‰ (VSMOW) during the middle and late Eocene. The large differences in paleoenvironments and surface water  18 O values suggest that the catchment of the Kishenehen basin was at variable elevation. The estimated paleorelief between the basin and the surrounding mountains, based on both Rayleigh condensation model and predictions of Eocene precipitation isotope values using an isotope-enabled global climate model, is ~4 km, and the basin floor was < 1.5 km high. This high topography and high relief paleogeography suggest that the Cordilleran orogenic front reached an elevation of at least 4 km, and the crust thickness may have reached more than 55 km before Eocene gravitational collapse. We attribute the maintenance of high Eocene

The Sensitivity of Terrestrial δ18O Gradients to Hydroclimate Evolution

“…Journal of Geophysical Research: Atmospheres most reliable signal of orographic precipitation in an isotopic record (e.g., Criss & Taylor, 1983;Fan et al, 2017;Garzione et al, 2000Garzione et al, , 2006Mulch et al, 2006;Saylor et al, 2009).…”

“…Additionally, terrain blocking is more likely to influence distal, leeward records where blocked flow, with minimal distillation, reconverges and dampens the signature of distilled overland flow (Galewsky, ). For these reasons, paleoaltimetry studies that sample directly within or behind the mountain range are likely to capture the most reliable signal of orographic precipitation in an isotopic record (e.g., Criss & Taylor, ; Fan et al, ; Garzione et al, , ; Mulch et al, ; Saylor et al, ).…”

Oligocene Deformation of the Chuandian Terrane in the SE Margin of the Tibetan Plateau Related to the Extrusion of Indochina

“…Several stable isotope paleoaltimetry studies provide seemingly contradictory constraints on the surface elevation of the SE margin of the Tibetan Plateau (Gourbet et al, 2017; Hoke et al, 2014). Stable isotope paleoaltimetry of mountains and orogenic plateaus (e.g., Fan et al, 2017; Rowley & Garzione, 2007) relies heavily on appropriate assumptions of source water isotopic composition, air parcel trajectories, isotope elevation gradients, and paleoclimate, all of which are age dependent (Spicer et al, 2020). However, most of the Cenozoic basins in the SE margin of the Tibetan Plateau lack absolute age constraints, and the ages for these basins in the Chinese geologic maps have mainly relied on biostratigraphic and lithostratigraphic correlations.…”

Modernisation of the Hymenoptera: ants, bees, wasps, and sawflies of the early Eocene Okanagan Highlands of western North America

“…Insect-bearing localities of the Kishenehn Formation (Montana) and at Florissant (Colorado) of the United States of America also represent uplands, but these were younger, after the Ypresian and the early Eocene Climatic Optimum (Meyer 2003; Fan et al . 2017).…”

“…The Tadushi, however, contains fewer and less diverse Hymenoptera than the Okanagan Highlands despite large collections, a fact that cannot easily be explained as a taphonomic artefact, and may then represent a community with a genuinely smaller component of the order. Insect-bearing localities of the Kishenehn Formation (Montana) and at Florissant (Colorado) of the United States of America also represent uplands, but these were younger, after the Ypresian and the early Eocene Climatic Optimum (Meyer 2003;Fan et al 2017).…”