THE CORDILLERAN OROGEN IN THE EOCENE: TOPOGRAPHY, DRAINAGE SYSTEMS, AND TECTONIC IMPLICATIONS

The Cordilleran system is often compared to the modern Central Andes, and many studies have shown that a hinterland orogenic plateau of ~3 km elevation spanned what is now the central Basin and Range from the Late Cretaceous through Eocene. Although flattening of the subducting Farallon slab at ca. 80 Ma and its subsequent removal beginning at ca. 55 Ma are recognized as key drivers for a variety of tectonic uplift and magmatic models, the surface expression of slab rollback remains unresolved, particularly across the northern US. Detrital records from Paleogene basins spanning from the Pacific coast to the Laramide foreland contain mineral compositions characteristic of the Challis volcanic province or the Idaho batholith, suggesting that these units had relatively higher elevations than surrounding regions. Here we present preliminary findings from hydrogen stable isotope ratios (δD) from volcanic glass, single crystal sanidine ⁴⁰Ar/³⁹Ar geochronology, and detrital zircon U-Pb geochronology data to reconstruct paleoelevations and drainages across the US Cordillera during Farallon slab removal.

We collected ignimbrite, air fall tuff, and basal sandstone and conglomerate samples from across the Intermountain West, as well as tuffs from paleocoastal locations in California, Washington, and Texas. New and legacy ⁴⁰Ar/³⁹Ar ages show that sampled units range in age from 50 to 29 Ma. All samples show significant decreases in δD_glass values compared to both western and southeastern paleo-nearshore values. Progressive westward decreases in Eocene δD_glass values of samples across Idaho, Montana, and northwest Wyoming suggest continued distillation of precipitation, and define an elevated landscape in the study region by as early as 50 Ma, confirming early inferences based on paleobotanical observations. Late Eocene and Oligocene δD_glass values across a transect to the south show lower magnitude decreases in δD_glass values from both paleo-shorelines, with the lowest values located in eastern Nevada and western Utah. These topographic relationships are confirmed by new detrital zircon provenance data, which also distinguish between Challis-sourced southward drainages and Idaho-batholith-sourced eastward drainages.