RECONSTRUCTING EOCENE PALEOTOPOGRAPHY OF THE ROCKY MOUNTAINS USING STABLE ISOTOPE PALEOALTIMETRY OF HYDRATED VOLCANIC GLASS

Paleoelevation data is crucial to reconstructing past crustal thicknesses and intraplate processes. In the complex tectonic setting of the northwestern U.S., we can use elevation records to interpret the timing and processes behind Rocky Mountain and Great Plains uplift and regional extension. During the Eocene (55-35 Ma), the region experienced a shift from compression and Laramide-style thrusting to extension, recorded in extensional basin formation, exhumation of metamorphic core complexes, and southwestward migration of magmatic activity. Within a precise sanidine ⁴⁰Ar/³⁹Ar geochronologic framework, we use stable isotope paleoaltimetry of Eocene-age hydrated volcanic glass shards, preserved in basins in Montana and Wyoming, to determine the temporal and spatial patterns of Rocky Mountain surface uplift. Results will ultimately guide our interpretations of regional tectonic drivers of basin formation and orogenic collapse.

Stable isotope ratios of hydrogen (δD) in precipitation become progressively D-depleted as air masses rainout due to increasing elevation and decreasing atmospheric temperature. Silicic volcanic glass hydrates with meteoric water within ~10,000 years after deposition and preserves this hydration water on geologic timescales (10⁷ years). Thus, volcanic glass acts as a proxy for the long-term average δD value of paleo-meteoric water at the time of glass deposition. Samples of ash fall tuff, ignimbrite, and tuffaceous sandstone were collected from intermontane basins in eastern Idaho, southwestern Montana, and western Wyoming, along with stratigraphic data from basin sections. δD values from a preliminary subset of samples from fluvial and alluvial sections range from -195.2‰ to -132.4‰ ± 3.3‰_VSMOW. The modern precipitation δD values for the same region range from -160‰ to -63‰, and both Eocene and modern δD values are the most D-depleted in southwest Montana. From this preliminary data, we confirm the presence of an orographic barrier during the early Eocene, as the most D-depleted δD values require substantial rainout due to high topography. The Eocene hothouse climate would have reduced isotopic lapse rates, which could lead to underestimations of paleoelevation, so our future work will incorporate Eocene climate model comparisons to quantify paleoelevations.