EOCENE SKYSCRAPERS: HIGH TOPOGRAPHY OF THE NORTHERN U.S. CORDILLERA FOLLOWING MESOZOIC COMPRESSION

Abstract

Estimates of paleotopography can inform the timing and drivers of surface deformation in complex tectonic settings. During the Eocene (ca. 56-34 Ma), the northern U.S. Cordillera experienced a shift from compression and thrusting to extension and magmatism, as recorded in supradetachment basin formation, exhumation of metamorphic core complexes, and changes in surface topography. Researchers disagree on the extent, lifespan, and elevation of the U.S. Cordillera over this time and how the region evolved into the modern Rocky Mountains. We use stable isotope paleoaltimetry of Eocene-age hydrated volcanic glass shards preserved in basins along a transect from the western paleoshoreline in Oregon, across the Rocky Mountains, and to the Great Plains to determine the distribution and magnitude of high topography at this time. Results help identify the contributing tectonic and geodynamic factors of Eocene and later high elevations. Paleogene high topography would have additionally influenced the delivery of Pacific moisture to the Bighorn Basin, where the PETM has been extensively studied. 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 dD value of paleo-meteoric water, which responds to positive elevation gradients. We analyzed ancient meteoric water preserved in Paleogene volcanic glasses across the study area to create paleotopographic profiles across the range. dD values from samples from fluvial and alluvial sections range from -223.6‰ ± 2.4‰ to -89.0‰ ± 6.9‰ (VSMOW). The lowest dD values are in west-central and southwest Montana and yielded early Eocene paleoelevation estimates of 4,640 +720/-440 m, based on a one-dimensional airmass lifting and rainout model calibrated for early Eocene climate. The high pCO2 of this time period may have reduced isotopic lapse rates, making these values minimum paleoelevation estimates. The highest elevations resided at the thrust front, not in the hinterland of central Idaho, as previously thought. By the Oligocene, surface lowering to 3070 +270/-245 m had occurred, likely due to extensional collapse based on coincident extensional basin formation and reduced volcanism.