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1. 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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2. 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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3. 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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4. Neogene Retroarc Foreland Basin Evolution, Sediment Provenance, and Magmatism in Response to Flat Slab Subduction, Western Argentina

   https://doi.org/10.1029/2019TC005958  

   Capaldi, Tomas_N; Horton, Brian_K; McKenzie, N_Ryan; Mackaman‐Lofland, Chelsea; Stockli, Daniel_F; Ortiz, Gustavo; Alvarado, Patricia (July 2020, Tectonics)

   Abstract Understanding the effects of flat slab subduction on mountain building, arc magmatism, and basin evolution is fundamental to convergent‐margin tectonics, with implications for potential feedbacks among geodynamic, magmatic, and surface processes. New stratigraphic and geochronological constraints on Cenozoic sedimentation and magmatism in the southern Central Andes of Argentina (31°S) reveal shifts in volcanism, foreland/hinterland basin development, sediment accumulation, and provenance as the retroarc region was structurally partitioned during slab flattening. Detrital zircon U‐Pb age distributions from the western (Calingasta basin), central (Talacasto and Albarracín basins), and eastern (Bermejo foreland basin) segments of the retroarc basin system preserve syndepositional volcanism and orogenic unroofing of multiple tectonic provinces. Initial shortening‐related exhumation of the Principal Cordillera at 24–17 Ma was recorded by the accumulation of distal eolian deposits bearing Oligocene–Eocene zircons from the Andean magmatic arc. The Calingasta basin chronicled volcanism and basement shortening in the Frontal Cordillera at ~17–11 Ma, as marked by an upward coarsening succession of fluvial to alluvial fan deposits with a sustained zircon U‐Pb age component that matches pervasive Permian‐Triassic bedrock in the hinterland. An ~450 km eastward inboard sweep of volcanism at 11 Ma coincided with the inception of flat slab subduction, and subsequent thin‐skinned shortening in the Precordillera fold‐thrust belt that exhumed wedge‐top deposits and induced cratonward (eastward) advance of flexural subsidence into the Bermejo foreland basin. This foreland basin was structurally partitioned as basement uplifts of the Sierras Pampeanas transformed a fluvial megafan sediment routing network into smaller isolated alluvial fan systems fed by adjacent basement blocks. 

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5. Location, extent, and magnitude of dynamic topography in the Late Cretaceous Cordilleran Foreland Basin, USA : New insights from 3D flexural backstripping

   https://doi.org/10.1111/bre.12706  

   Li, Zhiyang; Aschoff, Jennifer (February 2023, Basin Research)

   Abstract Mantle‐induced dynamic topography (i.e., subsidence and uplift) has been increasingly recognized as an important process in foreland basin development. However, characterizing and distinguishing the effects (i.e., location, extent and magnitude) of dynamic topography in ancient foreland basins remains challenging because the spatio‐temporal footprint of dynamic topography and flexural topography (i.e., generated by topographic loading) can overlap. This study employs 3D flexural backstripping of Upper Cretaceous strata in the central part of the North American Cordilleran foreland basin (CFB) to better quantify the effects of dynamic topography. The extensive stratigraphic database and good age control of the CFB permit the regional application of 3D flexural backstripping in this basin for the first time. Dynamic topography started to influence the development of the CFB during the late Turonian to middle Campanian (90.2–80.2 Ma) and became the dominant subsidence mechanism during the middle to late Campanian (80.2–74.6 Ma). The area influenced by >100 m dynamic subsidence is approximately 400 by 500 km, within which significant (>200 m) dynamic subsidence occurs in an irregular‐shaped (i.e., lunate) subregion. The maximum magnitude of dynamic subsidence is 300 ± 100 m based on the 80.2–74.6 Ma tectonic subsidence maps. With the maximum magnitude of dynamic uplift being constrained to be 200–300 m, the gross amount of dynamic topography in the Late Cretaceous CFB is 500–600 m. Although the location of dynamic subsidence revealed by tectonic subsidence maps is generally consistent with isopach map trends, tectonic subsidence maps developed through 3D flexural backstripping provide more accurate constraints of the areal extent, magnitude and rate of dynamic topography (as well as flexural topography) in the CFB through the Late Cretaceous. This improved understanding of dynamic topography in the CFB is critical for refining current geodynamic models of foreland basins and understanding the surface expression of mantle processes. 

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