Geochemical evidence for an orogenic plateau in the southern U.S. and northern Mexican Cordillera during the Laramide orogeny

Previous studies of the central United States Cordillera have indicated that a high-elevation orogenic plateau, the Nevadaplano, was present in Late Cretaceous to early Paleogene time. The southern United States Cordillera and northern Mexican Cordillera share a similar geologic history and many of the same tectonic features (e.g., metamorphic core complexes) as the central United States Cordillera, raising the possibility that a similar plateau may have been present at lower latitudes. To test the hypothesis of an elevated plateau, we examined Laramide-age 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 data set, 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 may have been present in the southern United States Cordillera. This result raises questions about the mechanisms that thickened the crust, because shortening in the Sevier thrust belt is generally not thought to have extended into the southern United States Cordillera, south of ∼35°N latitude. INTRODUCTION High-elevation orogenic plateaus like the modern Tibetan, Anatolian, and AltiplanoPuna plateaus commonly develop in the interior, or hinterland, of convergent orogenic systems. Construction of such plateaus is arguably among the most significant tectonic phenomena of Cenozoic time. The plateaus influence Earth systems in a wide variety of ways, including disrupting atmospheric circulation patterns (Molnar et al., 1993), driving past climate change (Raymo and Ruddiman, 1992; Strecker et al., 2007), concentrating metallic and other natural resources (Hou and Cook, 2009), and altering plate motions (Patriat and Achache, 1984; Iaffaldano et al., 2006). A similar plateau—the Nevadaplano, analogous to the Altiplano in the central Andes—was present in the Great Basin region of the central United States Cordillera (∼35°N– 45°N latitude; Fig. 1) during Late Cretaceous to early Paleogene time (DeCelles, 2004; Best et al., 2009; Snell et al., 2014). The Nevadaplano was isostatically supported by thickened continental crust formed by retroarc shortening in the Sevier thrust belt and in precursor thrust belts such as the Luning-Fencemaker (Jones et al., 1998; DeCelles, 2004). Previous estimates for crustal thickness in the Nevadaplano using igneous geochemical proxies, similar to the technique employed in this study, range from 55 to 65 km (Chapman et al., 2015). Structural restoration of late Paleogene to Holocene extension, including zones of highmagnitude extension associated with the Cordilleran metamorphic core complexes (Fig. 1), also suggests that the Nevadaplano was supported by thick (40–65 km) crust (Coney and