A STABLE ISOTOPIC RECORD OF THE PALEOGENE TOPOGRAPHY OF THE WESTERN UNITED STATES (Invited Presentation)

High elevations result from and record the deformation of continental tectonic plates. Large changes in elevation drive the reorganization of river systems, alter global climate and atmospheric moisture transport, and change the evolution of landscapes and life. But geologic records often lack the spatial and temporal detail to determine the complexities of past elevation change in continental interiors, or the deep earth mechanisms that drive those changes. Across the western U.S., Paleogene sedimentary basins, extensional complexes, and widespread volcanism record a complicated tectonic transition as the stress field changed. To determine the progress of and controls on this transition, we have reconstructed the height and distribution of elevations across the western U.S., from the Pacific to the Great Plains, at multiple latitudes and time slices. The hydrogen isotope ratios (δD) of paleo-precipitation, preserved in Paleogene (55-20 mya) volcanic glass, decrease considerably with elevation due to rainout. δD value profiles document moisture transport from paleo-coastal locations across the Sierra Nevada, Cascades, and Rocky Mountains, providing a record of the ancient topography of the American west. Results show an extensive high elevation Eocene U.S. Cordillera, with the lowest δD values, interpreted as the highest elevation waters, sourced from the eastern culmination of Sevier thrusting. δD values progressively decrease over 120‰ from the paleo-Pacific in Oregon and California to central Montana and Wyoming. δD values across eastern Wyoming, Colorado, and the Dakotas exhibit an inverse isotopic gradient, as Pacific-derived moisture mixed with less D-depleted moisture sourced from the Gulf of Mexico, mimicking a pattern seen across the western Andes today. Comparisons to climate-isotope distillation models show that a broad area of high topography exceeding 3 km elevation existed west of the Sevier thrust front, but the highest elevations resided near the thrust front and coincided with high magnitude silicic volcanism and the initiation of metamorphic core complex extension. These paleoelevation data lead to fundamental insights into the timing of uplift of the Rocky Mountains, the drivers for Paleogene intraplate volcanism, and the tectonic mechanisms that build and sustain topography.