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Abstract Deposition of the Late Jurassic Morrison Formation in a back‐bulge depozone and formation of the overlying sub‐Cretaceous unconformity above a forebulge mark the birth of the foreland basin system in the central U.S. Cordillera. In the southern U.S. Cordillera, the Morrison Formation is either anomalously thick or absent and the sub‐Cretaceous unconformity cuts out progressively older stratigraphy toward the south on the Colorado Plateau. Based on results of 2D and 3D flexural modeling, we suggest that flexural uplift of the northern rift flank of the Bisbee segment of the Borderland Rift Belt can explain these observations. Structural restoration of the sub‐Cretaceous unconformity indicates a minimum of 1.5 km of uplift and flexural models with an effective elastic thickness of 55 ± 5 km can reproduce the geometry of the unconformity and rift flank. This implies that effective elastic thickness has decreased between the Jurassic and the present, consistent with hypotheses for uplift and modification of the Colorado Plateau lithosphere during the Late Mesozoic to Cenozoic. Modeling results also predict the presence of a rift‐related flexural trough in the Four Corners region of the Colorado Plateau, which may explain above‐average thickness of the Morrison Formation. Constructive interference between a flexural back‐bulge depozone and a flexural rift‐flank trough may help explain anomalously high Late Jurassic subsidence. 

Granitic rocks, interpreted to be related to crustal melting, were emplaced into regions of thickened crust in southern Arizona during the Laramide orogeny (80–40 Ma). Laramide-age anatectic rocks are exposed as plutons, sills, and dike networks that are commonly found in the exhumed footwalls of metamorphic core complexes. This study investigates newly discovered exposures of granodioritic–leucogranitic rocks from three intrusive phases in the footwall of the Pinaleño–Jackson Mountain metamorphic core complex of southeastern Arizona, called the Relleno suite. Zircon U–Pb geochronology indicates that the suite was emplaced from 58 to 52 Ma. Zircon Lu/Hf isotope geochemistry, whole-rock Sr and Nd isotope geochemistry, and mineral O isotope geochemistry were used to investigate the source of these rocks and evaluate whether they are related to crustal anatexis. Average zircon εHf(t) values of the suite range from −4.7 to −7.9, whole-rock εNd(i) and 87Sr/86Sr(i) values range from −9.4 to −11.8 and 0.7064 to 0.7094 respectively, and quartz δ18OVSMOW values range from 6.8 to 9.4 ‰. Isotopic and geochemical data of these rocks are consistent with derivation from and assimilation of intermediate–mafic (meta)igneous rocks, at deep crustal levels, and are supported by thermodynamic melt models of Proterozoic igneous rocks equivalent to those exposed in the Pinaleño Mountains. In comparison with other Laramide-age anatectic granites in SE Arizona, those exposed in the Pinaleño Mountains are temporally similar but present compositional and isotopic differences that reflect melting and assimilation of different lithologies, producing distinct mineralogical and isotopic characteristics. The results suggest that crustal melting during this interval was not limited to metasedimentary protoliths and may have affected large portions of the deep crust. The early Paleogene Relleno suite in the Pinaleño Mountains strengthens the relationship between crustal melting and regions of thickened crust associated with the Sevier and Laramide orogenies. 

The Orocopia Schist and related schists are sediments subducted during the Laramide orogeny and are thought to have been underplated as a laterally extensive layer at the base of the crust in the southwestern United States Cordillera. This concept is hard to reconcile with the existence of continental mantle lithosphere in southeastern California and western Arizona. Analytical solutions and numerical modeling suggest that the Orocopia Schist may have ascended through the mantle lithosphere as sediment diapirs or subsolidus crustal plumes to become emplaced in the middle to lower crust. Modeled time-temperature cooling paths are consistent with the exhumation history of the Orocopia Schist and explain an initial period of rapid cooling shortly after peak metamorphism. The Orocopia Schist represents a potential example of relaminated sediment observable at the surface. 

Abstract 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.