MIDDLE TO LATE CENOZOIC HIGH TOPOGRAPHY AND CLIMATE COOLING IN THE CENTRAL ROCKY MOUNTAINS: CONSTRAINTS FROM CLUMPED ISOTOPE GEOTHERMOMETRY

The tectonic history and geodynamic drivers leading to the high mean elevation of the central Rocky Mountains in the western interior of North America remain controversial. We use clumped isotope geothermometry of carbonate cements to constrain temperature change and paleorelief between the central Rocky Mountains and the adjacent Great Plains during the middle and late Cenozoic. Our petrographic observations show that the studied carbonate cements are primarily micritic low-magnesium calcite, which were formed during eodiagenesis in near-surface conditions and retain primary paleoclimate and paleoelevation signals. Our data show that carbonate clumped isotope temperatures decrease ~10 °C from ~50 Ma to ~5 Ma in the central Rocky Mountains and ~5 °C from ~34 Ma to ~8 Ma in the western Great Plains, following middle and late Cenozoic global cooling trends. Our data also show that persistent longitudinal gradients in clumped isotope temperature and calculated paleowater δ¹⁸O value existed between the central Rocky Mountains and the Great Plains during the Oligocene and Miocene, with temperatures 4-6 °C higher, and water δ¹⁸O values 3-6 ‰ higher in the Great Plains. These gradients are slightly steeper than the gradients of Quaternary soil carbonate formation temperature, modern air temperature and precipitation δ¹⁸O values. Because the eastward increase of air temperature and precipitation δ¹⁸O values reflect the decreases of mean surface elevation from the central Rocky Mountains to the Great Plains, our observations of steep gradients in temperature and δ¹⁸O suggest the mean elevation of the central Rockies was at least ~ 1 km higher than the western Great Plains during the middle and late Cenozoic. When placed in the context of other paleoaltimetry studies and geological observations, our findings support the hypothesis that the high mean topography of the Rockies was developed during the late Eocene, possibly related to isostatic adjustment, or dynamic uplift caused by foundering of lower mantle lithosphere.