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1. Detrital zircon record of Phanerozoic magmatism in the southern Central Andes

   https://doi.org/10.1130/GES02346.1  

   Capaldi, T.N.; McKenzie, N.R.; Horton, B.K.; Mackaman-Lofland, C.; Colleps, C.L.; Stockli, D.F. (May 2021, Geosphere)

   null (Ed.)

   Abstract The spatial and temporal distribution of arc magmatism and associated isotopic variations provide insights into the Phanerozoic history of the western margin of South America during major shifts in Andean and pre-Andean plate interactions. We integrated detrital zircon U-Th-Pb and Hf isotopic results across continental magmatic arc systems of Chile and western Argentina (28°S–33°S) with igneous bedrock geochronologic and zircon Hf isotope results to define isotopic signatures linked to changes in continental margin processes. Key tectonic phases included: Paleozoic terrane accretion and Carboniferous subduction initiation during Gondwanide orogenesis, Permian–Triassic extensional collapse, Jurassic–Paleogene continental arc magmatism, and Neogene flat slab subduction during Andean shortening. The ~550 m.y. record of magmatic activity records spatial trends in magma composition associated with terrane boundaries. East of 69°W, radiogenic isotopic signatures indicate reworked continental lithosphere with enriched (evolved) εHf values and low (<0.65) zircon Th/U ratios during phases of early Paleozoic and Miocene shortening and lithospheric thickening. In contrast, the magmatic record west of 69°W displays depleted (juvenile) εHf values and high (>0.7) zircon Th/U values consistent with increased asthenospheric contributions during lithospheric thinning. Spatial constraints on Mesozoic to Cenozoic arc width provide a rough approximation of relative subduction angle, such that an increase in arc width reflects shallower slab dip. Comparisons among slab dip calculations with time-averaged εHf and Th/U zircon results exhibit a clear trend of decreasing (enriched) magma compositions with increasing arc width and decreasing slab dip. Collectively, these data sets demonstrate the influence of subduction angle on the position of upper-plate magmatism (including inboard arc advance and outboard arc retreat), changes in isotopic signatures, and overall composition of crustal and mantle material along the western edge of South America. 

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2. Subduction Duration and Slab Dip

   https://doi.org/10.1029/2019GC008862  

   Hu, Jiashun; Gurnis, Michael (April 2020, Geochemistry, Geophysics, Geosystems)

   Abstract The dip angles of slabs are among the clearest characteristics of subduction zones, but the factors that control them remain obscure. Here, slab dip angles and subduction parameters, including subduction duration, the nature of the overriding plate, slab age, and convergence rate, are determined for 153 transects along subduction zones for the present day. We present a comprehensive tabulation of subduction duration based on isotopic ages of arc initiation and stratigraphic, structural, plate tectonic and seismic indicators of subduction initiation. We present two ages for subduction zones, a long‐term age and a reinitiation age. Using cross correlation and multivariate regression, we find that (1) subduction duration is the primary parameter controlling slab dips with slabs tending to have shallower dips at subduction zones that have been in existence longer; (2) the long‐term age of subduction duration better explains variation of shallow dip than reinitiation age; (3) overriding plate nature could influence shallow dip angle, where slabs below continents tend to have shallower dips; (4) slab age contributes to slab dip, with younger slabs having steeper shallow dips; and (5) the relations between slab dip and subduction parameters are depth dependent, where the ability of subduction duration and overriding plate nature to explain observed variation decreases with depth. The analysis emphasizes the importance of subduction history and the long‐term regional state of a subduction zone in determining slab dip and is consistent with mechanical models of subduction. 

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3. 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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4. Temporal changes in diamond formation by subduction throughout Earth history: thermal modeling, seismology, and petrology evidence

   Shirey, Steven B; Smit, Karen V; Pearson, D Graham; Walter, Michael J; Stachel, Thomas (July 2024, Journal of International Kimberlite Conference Abstracts (JIKCA))

   In the current plate tectonic regime, thermal modeling, petrology, and seismology show that subsurface portions of cold slabs carry some of their volatiles into the deep upper mantle, mantle transition zone, and uppermost lower mantle avoiding the devolatilization occurring with normal arc and wedge subduction. Slab crustal remnants at these depths can melt by intersecting their carbonated solidus whereas slab mantle remnants can devolatilize by warming and metamorphosing to ‘dryer’ mineral assemblages. Since fluid release and earthquake production (“dehydration embrittlement”) operates down to ~300 km depths in all subduction zones, we propose, that deep-focus earthquakes trace the places of fluid release at deeper levels (350 to 750 km). Fluids in faults related to earthquake generation will become diamond-forming as they react with mantle rocks along the fault walls. Diamonds thus formed will record deformation produced by mantle convection and slab buckling during mantle storage. Lithospheric diamonds, stored in static ancient continental keels, lack the connection to this type of geodynamic regime that is evident for sublithospheric diamonds. However, a comparison between the two diamond types suggests a geologic model for lithospheric diamond formation in the ancient past. Lithospheric diamonds and sublithospheric diamonds both contain evidence for the recycling of sediments or surficial rocks that have equilibrated at low temperatures with seawater. The known way to inject these materials into diamond-forming regions is slab subduction. Hence both diamond types may have formed by variants of this same process that differ in depth and style over geologic time. Lithospheric diamonds are different from sublithospheric diamonds in critical ways: higher average N content, ages extending into the Paleoarchean, inclusion assemblages indicating formation at lower pressure, and lack of ubiquitous deformation features. Nitrogen content is the key to relating lithospheric diamonds to the subducting slab. Nitrogen occurs in clays and sediments at the slab surface or uppermost crust. Regardless of whether the slab is hot or cold during subduction, nitrogen will be removed into a mantle wedge if one exists. Additionally, diamonds will not survive in the melts/fluids generated in the wedge under oxidizing conditions. For sublithospheric diamonds, their low to non-existent nitrogen content occurs because they are derived from slab fluids/melts once nitrogen has been largely removed or from rocks deeper in the slab where nitrogen is scarce. The much higher nitrogen in lithospheric diamonds suggests that they formed from fluids/melts derived near the slab surface that contained N. In the Archean, such slabs must have subducted close to the nascent mantle keel with no mantle wedge so the fluids could be directly reduced by the mantle keel. We propose a gradual temporal change from shallow, keel-adjacent, mantle-wedge-poor subduction that produced lithospheric diamonds starting in the Paleoarchean to wedge-avoiding, cold and deep subduction that produced sublithospheric diamonds in the Paleozoic. This temporal change is consistent with many geologic features: an early stagnant lid and a buoyant Archean oceanic lithosphere; the slab-imbrication, advective thickening, and diamond-richness of portions of mantle keels; and anomalously diamond-rich ancient eclogites. 

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5. Role of subduction dynamics on the unevenly distributed volcanism at the Middle American subduction system

   https://doi.org/10.1038/s41598-023-41740-y  

   Liu, Meng; Gao, Haiying (September 2023, Scientific Reports)

   A typical subduction of an oceanic plate beneath a continent is expected to be accompanied by arc volcanoes along the convergent margin. However, subduction of the Cocos plate at the Middle American subduction system has resulted in an uneven distribution of magmatism/volcanism along strike. Here we construct a new three-dimensional shear-wave velocity model of the entire Middle American subduction system, using full-wave ambient noise tomography. Our model reveals significant variations of the oceanic plates along strike and down dip, in correspondence with either weakened or broken slabs after subduction. The northern and southern segments of the Cocos plate, including the Mexican flat slab subduction, are well imaged as high-velocity features, where a low-velocity mantle wedge exists and demonstrate a strong correlation with the arc volcanoes. Subduction of the central Cocos plate encounters a thick high-velocity feature beneath North America, which hinders the formation of a typical low-velocity mantle wedge and arc volcanoes. We suggest that the presence of slab tearing at both edges of the Mexican flat slab has been modifying the mantle flows, resulting in the unusual arc volcanism. 

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