Nd, Pb, Sr, and O isotopic characterization of Saudi Arabian Shield terranes
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Radiogenic nuclide
Island arc
Back-arc basin
Back-arc basin
Batholith
Continental Margin
Island arc
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Abstract Earth’s continental crust has evolved through a series of supercontinent cycles, resulting in a patchwork of Archean cores surrounded by terranes, fragments, and slivers of younger crustal additions. However, the dispersal (and/or stranding) of continental fragments during breakup is not well understood. Inherited structures from previous tectonic activity may explain the generation of continental terranes by controlling first-order deformation during rifting. Here, we explored the influence of lithospheric deformation related to ancient orogenesis, focusing on the impact of the Torngat orogen in the genesis of the Nain Province continental fragment in Eastern Canada. We present three-dimensional continental extension models in the presence of an inherited lithospheric structure and show that a narrow continental terrane could be separated and stranded by deep lithospheric scarring. The results show that continental terranes formed by this method would be limited to a width of 100–150 km, imposed by tectonic conditions during continental suturing. The findings have broad implications, demonstrating an original theory on the fundamental geologic problem of terrane generation and continent breakup.
Supercontinent
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continental collision
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The Anadyr‐Koryak region in northeastern USSR consists of a series of exotic terranes, most of which arrived from the Pacific and accreted to the Eurasian margin in Cretaceous and Tertiary times. Eight major terranes can be recognized: (1) The Kanchalan and (2) Pekulnei terranes include possible ancient blocks in their cores. The (3) Penzhina, (4) Ust' Belaya, and (5) Vaega terranes (which are parts of the Talovo‐Mainsky zone) have chaotic structure incorporating Paleozoic and early Mesozoic oceanic, island arc, continental rise, and other lithologie assemblages indicative of a subduction melange environment. The (6) Mainits and (7) Ekonai terranes (two parts of the Khatyrka megaterrane) contain late Paleozoic and Triassic island arc and back‐arc assemblages with faunas of Tethyan provenance. The (8) Olyutorsky terrane is composed mostly of Late Cretaceous island arc assemblages which accreted to the Eurasian margin in the middle Tertiary. Computed trajectories of Kula/Eurasia and Pacific/Eurasia relative motions suggest that some (or most) of these terranes traveled thousands of kilometers from the central Pacific until they eventually collided with, and were attached to, the Eurasian margin.
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Recent models for the chemical composition of the continental crust rely on various geophysical, geological, and petrological constraints and display a wide range in composition, with $$SiO_{2}$$ varying between 57 and 64 wt % and $$K_{2}O$$ varying between 1.1 and 2.4 wt %. Models that predict relatively high levels of the most incompatible elements imply extreme depletion (>70%) of these elements in the "depleted mantle." Crustal heat flow constrains the abundances of heat-producing elements (HPE: K, Th, U) within the continental crust and thus provides a relatively unambiguous and important test for these various models. The crustal radiogenic component of continental heat flow must lie within the range of $$18-48 mW/m^{2}$$ and almost certainly lies within the range of $$21-34 mW/m^{2}$$. The latter constraints limit the abundances of HPE within the crust (assuming K/U = 10,000, Th/U = 3.8, and a crust 41 km thick) to $$K_{2}O = 0.96-1.57\%$$; Th = 3.0-4.94 ppm, and U = 0.80-1.30 ppm. Models of crustal composition based largely on seismological or geological mapping data, and that predict relatively siliceous and incompatible element enriched compositions, exceed the upper limits of these constraints.
Radiogenic nuclide
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Radiogenic nuclide
Metamorphic core complex
Noble gas
Crustal recycling
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Back-arc basin
Basement
Continental Margin
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Allochthonous upper Paleozoic volcanic island-arc suites of the U.S. Cordillera have previously been interpreted as parts of a single island arc. New lithologic, age, structural, biogeographic, and geochemical data indicate the presence at least of two arcs, one built on a basement with continental crustal affinities including that of the Northern Sierra terrane, and the other built on an oceanic basement including that of the Eastern Klamath terrane. In the Garfield Hills, western Nevada, the Lower Permian Black Dike Formation appears to be a remnant of the Northern Sierra Paleozoic island arc. It consists of calc-alkaline rocks with low-to-negative ratios. Volcanic rocks share similar ages, magmatic affinities, and ratios with lavas of the Goodhue Formation in the Northern Sierra terrane. Our new data suggest that these rocks are part of the Northern Sierra terrane, and we therefore infer that part of western Nevada was originally underlain by a basement with continental crustal affinities.
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Basement
Lithology
Volcanic arc
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The recognition of terranes and terrane accretion has fundamentally changed the way we view the development of continental crust. The terrane concept originated from studies of the western Cordillera of North America, where it was demonstrated in the 1970s that microplates had travelled substantial distances before being amalgamated to cratonic North America. Since these early studies, the terrane concept has been widely applied to older orogenic belts, including the Appalachians and most of the Precambrian shields. Despite the acceptance of the terrane concept, a number of fundamental questions remain regarding the process of terrane accretion and its interaction with transform faulting and igneous and metamorphic events. Still not fully understood are: the coupling processes of mantle with deeper crust, and of deeper crust with upper crust; how pieces of continental crust with different histories respond to juxtaposition; the mechanism of the Mono's formation; and the behavior of fluids (melt and aqueous) with the changing stress field during accretion.
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Continental Margin
Island arc
Felsic
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Recent models for the chemical composition of the continental crust rely on various geophysical, geological, and petrological constraints and display a wide range in composition, with $$SiO_{2}$$ varying between 57 and 64 wt % and $$K_{2}O$$ varying between 1.1 and 2.4 wt %. Models that predict relatively high levels of the most incompatible elements imply extreme depletion (>70%) of these elements in the "depleted mantle." Crustal heat flow constrains the abundances of heat-producing elements (HPE: K, Th, U) within the continental crust and thus provides a relatively unambiguous and important test for these various models. The crustal radiogenic component of continental heat flow must lie within the range of $$18-48 mW/m^{2}$$ and almost certainly lies within the range of $$21-34 mW/m^{2}$$. The latter constraints limit the abundances of HPE within the crust (assuming K/U = 10,000, Th/U = 3.8, and a crust 41 km thick) to $$K_{2}O = 0.96-1.57\%$$; Th = 3.0-4.94 ppm, and U = 0.80-1.30 ppm. Models of crustal composition based largely on seismological or geological mapping data, and that predict relatively siliceous and incompatible element enriched compositions, exceed the upper limits of these constraints.
Radiogenic nuclide
Heat flow
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