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1. Laramide Deformation and Flexural Effects in the Upper Cretaceous: A Basin in Transition

   Rudolph, Kurt Saylor (July 2018, AAPG Annual Convention and Exhibition)

   The nature of subsidence in the Western Interior evolved in the Late Cretaceous from a contiguous (Sevier) foreland to partitioned (Laramide) basins coeval with an increase in long-wavelength “dynamic” subsidence. This evolution is interpreted by many as indicators of flat slab subduction. However, the timing and geographic location of changing subsidence mechanisms remains poorly documented. To better assess the geodynamic mechanisms responsible for this transition, we have mapped active elements versus time, including classic foredeeps, intra-basinal uplifts, long-wavelength subsidence, and local flexural wedges adjacent to rising Laramide structures. Criteria include isopachs, paleogeography, geohistory analysis, unconformities/exhumation, and sediment dispersal patterns. The analysis identifies a continuous foredeep along the Sevier Thrust Front through the Santonian, but not subsequently. Long-wavelength “dynamic” subsidence in the basin commences in the Coniacian, but is spatially and temporally quite variable. Short-wavelength Laramide structures first begin growing in the Ceno-Turonian. The influence of Laramide uplifts increases over time, with associated flexures becoming a dominant subsidence mechanism by the Maastrichtian. Thirteen flexural stratigraphic wedges, associated with both Sevier and Laramide uplifts, have been used to quantitatively model loads (uplift height/width) and effective elastic thicknesses (EET). EET is a measure of the integrated strength of the lithosphere. Results indicate that EET decreases over time, enhancing Laramide basin partitioning. The decrease in effective elastic thickness of the lithosphere is consistent with lithospheric weakening by the introduction of volatiles during flat slab subduction. Calculated Maastrichtian EET’s are consistent with modern EET, supporting the hypothesis that flat slab subduction preconditioned the lithosphere for subsequent Cenozoic tectonic and magmatic events. Large-scale petroleum system play elements are correlated with the distribution of these tectonic elements and associated subsidence. Examples include the Lance reservoir at Pinedale Field, Lewis source/seal in the Washakie Basin and the Niobrara source/reservoir in the Sand Wash, eastern Piceance and Denver Basins. 

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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. Effects of contemporaneous orogenesis on sedimentation in the Late Cretaceous Western Interior Basin, northern Utah and southwestern Wyoming

   https://doi.org/10.1111/bre.12623  

   Davis, Elizabeth M.; Rudolph, Kurt W.; Saylor, Joel E.; Lapen, Thomas J.; Wellner, Julia S. (November 2021, Basin Research)

   Abstract Three drivers of subsidence are recognized in the Western Interior Basin: Mesozoic–early Cenozoic flexure adjacent to the thin‐skinned, eastward propagating Sevier Orogeny, Late Cretaceous–Eocene flexure associated with thick‐skinned Laramide Uplifts and Late Cretaceous dynamic subsidence. This study combines outcrop lithofacies, palaeocurrent measurements, detrital zircon geochronology, biostratigraphy, stratigraphic correlations and isopach maps of Coniacian–Maastrichtian (89–66 Ma) units to identify these subsidence mechanisms impact on basin geometry and stratigraphic architecture in the northern Utah to southwestern Wyoming segment of the North American Cordillera. Detrital zircon maximum depositional ages and biostratigraphy support that the Maastrichtian Hams Fork Conglomerate was deposited above the Moxa unconformity in the wedgetop and foredeep depozones. The Moxa unconformity underlies the progradational Ericson Formation in the distal foredeep. The Hams Fork, however, is younger than the Ericson Formation, and instead equivalent to upper Almond Formation. Therefore, the hiatus associated with the Moxa unconformity continued for several million years longer in the fold belt and proximal basin than in the distal foredeep, with Ericson Formation‐equivalent strata onlapping the Moxa unconformity towards the west. Regional thickness patterns record and constrain the timing of the transition from Sevier to Laramide‐style tectonic regimes. From 88 to 83 Ma (upper Baxter Formation) a westward‐thickening stratigraphic wedge characterized the foredeep developed by lithospheric flexure by thrust‐belt loading. Nevertheless, the presence of >500 m of subsidence >200 km from the thrust front suggests a long‐wavelength subsidence mechanism consistent with dynamic subsidence. By 83 Ma (Blair Formation) the long‐wavelength depocentre shifted away from the thrust belt, with no evidence of a Sevier foredeep. This depocentre continued migrating eastward during the early‐mid Campanian (ca. 81–77 Ma). The late Campanian–Maastrichtian (ca. 74–66 Ma) is marked by narrow sedimentary wedges adjacent to the Wind River, Granite and Uinta Mountain uplifts and attributed to flexural loading by Laramide deformation. 

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4. Causes of Variable Shortening and Tectonic Subsidence During Changes in Subduction: Insights From Flexural Thermokinematic Modeling of the Neogene Southern Central Andes (28–30°S)

   https://doi.org/10.1029/2022TC007334  

   Mackaman‐Lofland, Chelsea; Horton, Brian K.; Ketcham, Richard A.; McQuarrie, Nadine; Fosdick, Julie C.; Fuentes, Facundo; Constenius, Kurt N.; Capaldi, Tomas N.; Stockli, Daniel F.; Alvarado, Patricia (August 2022, Tectonics)

   Abstract The Andes of western Argentina record spatiotemporal variations in morphology, basin geometry, and structural style that correspond with changes in crustal inheritance and convergent margin dynamics. Above the modern Pampean flat‐slab subduction segment (27–33°S), retroarc shortening generated a fold‐thrust belt and intraforeland basement uplifts that converge north of ∼29°S, providing opportunities to explore the effects of varied deformation and subduction regimes on synorogenic sedimentation. We integrate new detrital zircon U‐Pb and apatite (U‐Th)/He analyses with sequentially restored, flexurally balanced cross sections and thermokinematic models at ∼28.5–30°S to link deformation with resulting uplift, erosion, and basin accumulation histories. Tectonic subsidence, topographic evolution, and thermochronometric cooling records point to (a) shortening and distal foreland basin accumulation at ∼18–16 Ma, (b) thrust belt migration, changes in sediment provenance, and enhanced flexural subsidence from ∼16 to 9 Ma, (c) intraforeland basement deformation, local flexure, and drainage reorganization at ∼12–7 Ma, and (d) out‐of‐sequence shortening and exhumation of foreland basin fill by ∼8–2 Ma. Thrust belt kinematics and the reactivation of basement heterogeneities strongly controlled tectonic load configurations and subsidence patterns. Geo/thermochronological data and model results resolve increased shortening and combined thrust belt and intraforeland basement loading in response to ridge collision and Neogene shallowing of the subducted oceanic slab. Finally, this study demonstrates the utility of integrated flexural thermokinematic and erosion modeling for evaluating the geometries, rates, and potential drivers of retroarc deformation and foreland basin evolution during changes in subduction. 

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5. Investigating the formation of the Cretaceous Western Interior Seaway using landscape evolution simulations

   https://doi.org/10.1130/B35653.1  

   Chang, Ching; Liu, Lijun (June 2020, GSA Bulletin)

   Abstract Transient intraplate sedimentation like the widespread Late Cretaceous Western Interior Seaway, traditionally considered a flexural foreland basin of the Sevier orogeny, is now generally accepted to be a result of dynamic topography due to the viscous force from mantle downwelling. However, the relative contributions of flexural versus dynamic subsidence are poorly understood. Furthermore, both the detailed subsidence history and the underlying physical mechanisms remain largely unconstrained. Here, we considered both Sevier orogenic loading and three different dynamic topography models that correspond to different geodynamic configurations. We used forward landscape evolution simulations to investigate the surface manifestations of these tectonic scenarios on the regional sedimentation history. We found that surface processes alone are unable to explain Western Interior Seaway sedimentation in a purely orogenic loading system, and that sedimentation increases readily inland with the additional presence of dynamic subsidence. The findings suggest that dynamic subsidence was crucial to Western Interior Seaway formation and that the dominant control on sediment distribution in the Western Interior Seaway transitioned from flexural to dynamic subsidence during 90–84 Ma, coinciding with the proposed emplacement of the conjugate Shatsky oceanic plateau. Importantly, the sedimentation records require the underlying dynamic subsidence to have been landward migratory, which implies that the underlying mechanism was the regional-scale mantle downwelling induced by the sinking Farallon flat slab underneath the westward-moving North American plate. The simulated landscape evolution also implies that prominent regional-scale Laramide uplift in the western United States should have occurred no earlier than the latest Cretaceous. 

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