Abstract
Stable isotope paleoaltimetry has emerged as a powerful tool by which to reconstruct the past elevations of Earth’s orogens, enabling geodynamic investigation into the mechanisms of mountain-building and providing a test of feedbacks between tectonism and climate. Here we reconstruct Oligocene-Miocene topography of the Oregon Cascades using δD values of volcanic glass hydration water (Ch. 1) and evaluate the theoretical foundation of stable isotope paleoaltimetry in the western U.S. using modern streamwater δD/δ18O values (Ch. 2).
Chapter 1 presents stable isotope paleoaltimetric data from the Oregon Cascades, where both geodynamic models of arc evolution and regional tectono-climatic feedbacks remain debated. New hydrogen stable isotope ratios of hydrated volcanic glass, interpreted using a 1-D Rayleigh distillation model, suggest that arc paleoelevation increased by over one km from the early Oligocene to early Miocene, reaching above-modern elevations just prior to eruption of the Columbia River Basalts. This uplift is coeval with high volcanic flux and geochemical evidence of crustal thickening, suggesting that magmatic additions to the crust likely drove gradual surface uplift of ~0.1 km/Ma from 33 to 16 Ma. Uplift is also synchronous with independent evidence of drying in the range lee, indicating that an orographic rainshadow similar to the present was likely established by the late Oligocene.
Chapter 2 presents new small-catchment streamwater δD, δ18O, and d-excess values and compiles existing data across a longitudinal profile spanning the Cascade Range, northern Rocky Mountains, Laramide foreland, and Great Plains. We input stable isotope data into a simple Rayleigh distillation model to test whether variability in δ18O values reflects progressive orographic distillation of a uniform air parcel, an implicit assumption of most paleoaltimetric studies. We also interpret the regional controls on small-catchment streamwater H and O isotope ratios within each physiographic province using new back-trajectory modeling and interpolated δ18O and d-excess isoscapes. We find that broad-scale patterns in δ18O values in the Cascades and northern Rockies are explained by Rayleigh distillation of a Pacific-sourced airmass, though further east, moisture mixing and recycling violate Rayleigh assumptions. These results suggest that Rayleigh distillation modeling of paleometeoric water δ18O/δD values, such as that presented in Ch. 1, likely provides an accurate and precise method by which to reconstruct paleoelevation in the northern Rocky Mountains, Cascades, and similar geographic settings, provided that moisture was derived from a single source.