CRUSTAL THICKNESSES OF THE NORTH AMERICAN CORDILLERA SINCE THE LATE CRETACEOUS

In the North American Cordillera, the mechanisms governing changes in crustal thickness and surface elevation since the Late Cretaceous remain debated. Previous work used whole-rock geochemical paleocrustal thickness proxies to shed light on the thickening and thinning trends of orogenic plateaus in Nevada and Arizona. The spatial and temporal resolution of these paleocrustal thickness results, however, were limited by the availability of geochemical data for the La/Yb and Sr/Y crustal thickness calibrations. The recent calibration of new paleocrustal thickness proxies expands the potential number of paleocrustal thickness results that can be obtained using existing data. Here we apply the Geochemical Arc Moho Estimator (GAME) tool to calculate crustal thickness in the North American Cordillera from existing whole-rock geochemical data (Luffi and Ducea, 2022). Using this high-resolution dataset (n > 7,000), we create interpolated maps of crustal thickness within six times slices from the Late Cretaceous to Miocene. We also present maps of paleoelevation calculated assuming crustal isostacy and compare these predictions to existing stable isotope paleoaltimetry results. Paleocrustal thickness results corroborate previous interpretations of thick-crusted (>55 km) plateaus in Nevada and Arizona during the Late Cretaceous and early Cenozoic. We also observe thickened features in the northern Cordillera (45-47°N) and west-central Colorado, the latter of which appears to form during the Oligocene. Within their respective uncertainties, isostacy-calculated paleoelevations are similar to previous stable isotope paleoaltimetry results in Nevada and Idaho, suggesting that first-order variations in elevation largely reflected changes in crustal thickness. Broadly, these results highlight that thick-crusted and presumably high-elevation orogenic plateaus prevailed over 1,500 km along strike of the North American Cordillera prior to the Eocene onset of Farallon slab rollback.