THE ARIZONAPLANO: GEOCHEMICAL EVIDENCE FOR THICK CRUST DURING THE LARAMIDE OROGENY

Previous studies of the central U.S. Cordillera have indicated a high-elevation orogenic plateau, the Nevadaplano, was present during the Late Cretaceous to Early Paleogene. The southern U.S. and northern Mexican Cordillera share a similar geologic history and many of the same tectonic features (e.g., metamorphic core complexes) as the central U.S. Cordillera, which raises the possibility that a similar plateau may have been present at lower latitudes. To test this hypothesis, we examined Laramide-age (80–40 Ma) continental arc geochemistry and employed an empirical relation between whole rock La/Yb and Moho depth as a proxy for crustal thickness. Calculations of crustal thickness from individual data points range between 45 and 72 km with an average of 57 ± 12 km (2σ) for the entire dataset, which corresponds to 3 ± 1.8 km paleoelevation assuming simple Airy isostasy. These crustal thickness and paleoaltimetry estimates are similar to previous estimates for the Nevadaplano and are interpreted to suggest that an analogous high-elevation plateau, the “Arizonaplano” may have been present in the southern U.S. Cordillera. The results raise questions about the geodynamic mechanisms that thickened the crust because shortening in the Sevier thrust belt is generally not thought to have extended south of ca. 35 °N latitude.