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1. Regional exhumation of the Laramide Province

   https://doi.org/10.1130/B37625.1  

   Jepson, Gilby; Carrapa, Barbara; Reeher, Lauren J; DeCelles, Peter G; Afonso, Walter D; Howlett, Caden J; Caylor, Emilia A; Sherpa, Tshering ZL; Wang, Jordan W; Constenius, Kurt N (February 2025, Geological Society of America Bulletin)

   Western North America is the archetypical Cordilleran orogenic system that preserves a Mesozoic to Cenozoic record of oceanic Farallon plate subduction-related processes. After prolonged Late Jurassic through mid-Cretaceous normal-angle Farallon plate subduction that produced the western North American batholith belt and retroarc fold-thrust belt, a period of low-angle, flat-slab subduction during Late Cretaceous−Paleogene time caused upper plate deformation to migrate eastward in the form of the Laramide basement-involved uplifts, which partitioned the original regional foreland basin. Major questions persist about the mechanism and timing of flat-slab subduction, the trajectory of the flat-slab, inter-plate coupling mechanism(s), and the upper-plate deformational response to such processes. Critical for testing various flat-slab hypotheses are the timing, rate, and distribution of exhumation experienced by the Laramide uplifts as recorded by low-temperature thermochronology. In this contribution, we address the timing of regional exhumation of the Laramide uplifts by combining apatite fission-track (AFT) and (U-Th-Sm)/He (AHe) data from 29 new samples with 564 previously published AFT, AHe, and zircon (U-Th)/He ages from Laramide structures in Arizona, Utah, Wyoming, Colorado, Montana, and South Dakota, USA. We integrate our results with existing geological constraints and with new regional cross sections to reconstruct the spatial and temporal history of exhumation driven by Laramide deformation from the mid-Cretaceous to Paleogene. Our analysis suggests a two-stage exhumation of the Laramide province, with an early phase of localized exhumation occurring at ca. 100−80 Ma in Wyoming and Montana, followed by a more regional period of exhumation at ca. 70−50 Ma. Generally, the onset of enhanced exhumation occurs earlier in the northern Laramide province (ca. 90 Ma) and later in the southern Laramide province (ca. 80 Ma). Thermal history models of selected samples along regional cross sections through Utah−Arizona−New Mexico and Wyoming−South Dakota show that exhumation occurred contemporaneously with deformation, implying that Laramide basement block exhumation is coupled with regional deformation. These results have implications for testing proposed migration pathway models of Farallon flat-slab and for how upper-plate deformation is expressed in flat-slab subduction zones in general. 

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2. Laramide Orogenesis Driven by Late Cretaceous Weakening of the North American Lithosphere

   https://doi.org/10.1029/2020JB019570  

   Saylor, Joel E.; Rudolph, Kurt W.; Sundell, Kurt E.; van Wijk, Jolante (August 2020, Journal of Geophysical Research: Solid Earth)

   Abstract This paper investigates the causes of the Late Cretaceous transition from “Sevier” to “Laramide” orogenesis and the spatial and temporal evolution of effective elastic thickness (EET) of the North American lithosphere. We use a Monte Carlo flexural model applied to 34 stratigraphic profiles in the Laramide province and five profiles from the Western Canadian Basin to estimate model parameters which produce flexural profiles that match observed sedimentary thicknesses. Sediment thicknesses come from basins from New Mexico to Canada of Cenomanian–Eocene age that are related to both Sevier and Laramide crustal loads. Flexural models reveal an east‐to‐west spatial decrease in EET in all time intervals analyzed. This spatial decrease in EET may have been associated with either bending stresses associated with the Sevier thrust belt, or increased proximity to attenuated continental crust at the paleocontinental margin. In the Laramide province (i.e., south of ~48°N) there was a coeval, regional decrease in EET between the Cenomanian–Santonian (~98–84 Ma) and the Campanian–Maastrichtian (~77–66 Ma), followed by a minor decrease between the Maastrichtian and Paleogene. However, there was no decrease in EET in the Western Canada Basin (north of ~48°N), which is consistent with a lack of Laramide‐style deformation or flat subduction. We conclude that the regional lithospheric weakening in the late Santonian–Campanian is best explained by hydration of the North American lithosphere thinned by bulldozing by a shallowly subducting Farallon plate. The weakening of the lithosphere facilitated Laramide contractional deformation by focusing end‐loading stresses associated with flat subduction. Laramide deformation in turn may have further reduced EET by weakening the upper crust. Finally, estimates of Campanian–Maastrichtian and Paleogene EET are comparable to current estimates indicating that the modern distribution of lithospheric strength was achieved by the Campanian in response to flat subduction. 

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3. Exhumation of the Coyote Mountains metamorphic core complex (Arizona): implications for orogenic collapse of the southern North American Cordillera.

   Gottardi, R. (October 2020, Tectonics)

   A microstructural and thermochronometric analysis of the Coyote Mountains detachment shear zone provides new insight into the collapse of the southern North American Cordillera. The Coyote Mountains is a metamorphic core complex that makes up the northern end of the Baboquivari Mountains in southern Arizona. The Baboquivari Mountains records several episodes of crustal shortening and thickening, and regional metamorphism, including the Late Cretaceous-early Paleogene Laramide orogeny which is locally expressed by the Baboquivari thrust fault. Thrusting and shortening were accompanied by magmatic activity recorded by intrusion of Paleocene muscovite-biotite-garnet peraluminous granites such as the ~58 Ma Pan Tak Granite, interpreted as anatectic melts representing the culmination of the Laramide orogeny. Following Laramide crustal shortening, the northern end of the Baboquivari Mountains was exhumed along a top-to-the-north detachment shear zone, which resulted in the formation of the Coyote Mountains metamorphic core complex. Structural and microstructural analysis show that the detachment shear zone evolved under a strong component of non-coaxial (simple shear) deformation, at deformation conditions of ~450 ± 50°C, under a differential stress of ~60 MPa, and a strain rate of 1.5 ×10-11 s-1 to 5.0 × 10-13 s-1 at depth of ~11–14 km. Detailed 40Ar/39Ar geochronology of biotite and muscovite, in the context of the deformation conditions determined by quartz microstructures, suggests that the mylonitization associated with the formation of the Coyote Mountains metamorphic core complex started at ~29 Ma (early Oligocene). Apatite fission track ages indicate that the footwall of the Coyote Mountains metamorphic core complex experienced rapid exhumation to the upper crust by ~24 Ma. The fact that mylonitization and rapid extensional exhumation post-dates Laramide thickening by ~30 Myr indicates that crustal thickness alone was insufficient to initiate extensional tectonic and required an additional driving force. The timing of mylonitization and rapid exhumation documented here and in other MCCs are consistent with the hypothesis that slab rollback and the effect of a slab window trailing the Mendocino Triple Junction have been critical in driving the development of the MCCs of the southwest. 

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

   https://doi.org/10.1130/G47117.1  

   Chapman, James B.; Greig, Roy; Haxel, Gordon B. (November 2019, Geology)

   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. 

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5. Miocene Construction of the High Andes Recorded by Exhumation of the Frontal Cordillera, La Ramada Massif of Western Argentina (32°S)

   Howlett, C J; Ronemus, C B; Carrapa, B; DeCelles, P G (January 2025, Tectonics)

   Abstract The Frontal Cordillera is a first‐order geologic feature of the southern central Andes, hosting the highest hinterland topography above the modern Pampean flat‐slab segment. The timing of Frontal Cordillera exhumation is important for testing models of Andean tectonics, yet large latitudinal gaps exist between structural and thermochronological constraints for the region. We conducted a thermochronometric study using a 4.4 km age‐elevation transect along the northeast ridge of Cerro Mercedario, the highest peak in the La Ramada massif at ∼32°S. Zircon (U‐Th)/He dates indicate partial resetting, supporting a limited magnitude of exhumation in even the most extreme Andean topography. Single grain apatite (U‐Th‐Sm)/He dates range from 8.5 ± 0.9 to 35.8 ± 3.6 Ma, with median dates of ∼10.5 to ∼15.7 Ma with increasing elevation. Integrated with geologic mapping and thermal history modeling, these data suggest Early to Middle Miocene exhumation along the Santa Cruz and Espinacito faults concomitant with uplift of the La Ramada massif. New apatite helium data from the Cordillera del Tigre segment of the Frontal Cordillera are partially reset and preferred modeling interpretations suggest exhumation ca. 11–9 Ma, coeval with shortening in the eastward adjacent Precordillera. These data add to accumulating regional evidence for out‐of‐sequence deformation during the Miocene, consistent with internal (hinterland) growth of a subcritical orogenic wedge contemporaneous with surface uplift and crustal thickening in the south‐central Andes. 

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