Paleocene Paleoelevations of the Basin and Range from Clumped Isotope Thermometry and Paleosol Geochemistry: A Modest Nevadaplano?

Nathan Niemi, University of Michigan

The Basin and Range Province of the western United States is one of the premier examples of diffuse continental extension in the world. Estimates of extension across the Basin and Range during the Cenozoic range from 200% province-wide to locally as great as 400%. However, crustal thicknesses across this region, as derived from a variety of geophysical methods, are remarkably uniform and, at ~35 km thick, are similar to global averages. Reconciling large-magnitude crustal extension with observed crustal thicknesses is difficult without calling on one of three possible alternatives: (1) an Andean-plateau crustal thickness of ~60 km at the termination of the Sevier Orogeny and prior to extension; (2) substantial addition of material to the crust by syn-extensional magmatism or (3) mobilization and redistribution of fluid lower crust during extension. Quantitative paleoelevation histories can help discriminate between these competing mechanisms for widespread Cenozoic extension. New estimates of pre-extensional paleoelevations for the northern and central Basin and Range are presented using clumped isotope (Δ47) thermometry of lacustrine carbonates that suggest modest (~2-3 km) pre-extensional elevations for the northern Basin and Range and quite low (< 1 km) elevations for the southern Basin and Range. These paleoelevations are incompatible with mass balance considerations based on the observed magnitude of crustal extension and modern crustal thicknesses, and imply that crustal mass was added to the Basin and Range during extension, either from magmatism or crustal flow.