Tectonic controls on post-subduction granite genesis and emplacement: The late Caledonian suite of Britain and Ireland
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Transtension
Abstract. Continents are formed by the amalgamation of numerous micro-terranes and island arcs, so they have spatially varying lithosphere strengths. The Crème brȗlée (CB) model and the Jelly sandwich (JS) model have been commonly used to describe continental lithosphere strength-depth variations. Depending on the strength of continental lower crust, the CB and JS models can be further subdivided into two subclasses, in which the I subclass (CB-I and JS-I) and II subclass (CB-II and JS-II) respectively have a strong or weak lower crust. During continental collision, lithosphere deformation is the byproduct of the comprehensive interaction of multiple terranes. Here we used 2-D thermo-mechanical numerical models that contain three continental terranes to systematically explore the effects of terranes with various strengths on continental deformation, and studied the effects of different rheological assumptions terrane deformation. We find four types of lithosphere deformation patterns: collision, subduction, thickening and delamination, and replacement. Lithosphere structures, especially local pre-existing weaknesses, also have nonnegligible influences on lithosphere deformation. These simulation patterns are seen in observed deformation patterns and structures in East Asia, suggesting they are likely to be naturally occurring modes of intracontinental orogenesis.
Collision zone
Delamination
continental collision
Obduction
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Transtension
Basement
Classification of discontinuities
Transpression
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Based on the great thickness crustal struture and complex geophysical character,this paper points out the principle for dividing terranes in Qinghai─Xizang Plateau according to earthquake activities and wave field sign,lithospheric structure and velocity character,palemagnetic mark,potential field sign ,temperature sign, geological and tectonics character. From north to south of the Qinghai─Xizang Plateau and its neighbouring regions can be devided in seven terranes, ie, Qaidam terrane, Kunlun terrane, Hon Xil-Bayan Har terrane, Qangtang terrane, Lhasa-Gangdise terrane, Himalayan terrane and Ganges Plain terrane. The distribution and character of these terranes have important rools for the studies of the formation and evolution.of the plateau and plate movement and dynamic mechanism.
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Granitoid plutons are a major component of pre-Carboniferous rocks in Cape Breton Island and knowledge of the time and tectonic setting of their emplacement is crucial for understanding the geological history of the island, guiding exploration for granite-related economic mineralization, and making along-orogen correlations. The distribution of these plutons and their petrological characteristics have been used in the past for recognizing both Laurentian and peri-Gondwanan components in Cape Breton Island, and for subdividing the peri-Gondwanan components into Ganderian and Avalonian terranes. However, ages of many plutons were assumed on the basis of field relations and petrological features compared to those of the relatively few reliably dated plutons. Seventeen new U–Pb (zircon) ages from igneous units reported here provide enhanced understanding of the distribution of pluton ages. Arc-related plutons in the Aspy terrane with ages of ca. 490 to 475 Ma likely record the Penobscottian tectonomagmatic event recognized in the Exploits subzone of central Newfoundland and New Brunswick but not previously recognized in Cape Breton Island. Arc-related Devonian plutonic activity in the same terrane is more widespread, continuous, and protracted (445 Ma to 395 Ma) than previously known. Late Devonian magmatism in the Ganderian Aspy terrane is similar in age to that in the Avalonian Mira terrane (380 to 360 Ma) but the tectonic settings are different. In contrast, magmatic activity in the Bras d’Or terrane is almost exclusively arc-related in the Late Ediacaran (580 to 540 Ma) and rift-related in the Late Cambrian (520 to 490 Ma). The new data support the terrane distinctions previously documented.
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Rubidium and strontium concentrations and (87)Sr/(86)Sr values are documented herein for samples of plutons and associated rocks from 238 locations in the southern Sierra Nevada and vicinity. The goals of the investigation were to determine ages of rock units, to aid in the separation of plutons in poorly exposed areas, to determine the pattern of variation of initial (87)Sr/(86)Sr (hereafter called Sr(i)) for plutons, and to constrain more rigorously the boundaries of the continental Sierran and Salinian-western Mojave terranes defined on the basis of the Sr(i) of their plutons by Kistler (1978) and by Kistler and Peterman (1978). These new data expand the boundaries of the Salinian-western Mojave terrane and make it part of the Panthalassan lithosphere that lies west of the tectonic boundary, whereas the Sierran terrane is made part of the North American lithosphere that lies east of the tectonic boundary.
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The Huamenlou pluton, is an elongated granite intrusion with high aspect ratio, emplaced within the southern margin of the North Qinling (central China). Here we investigate this pluton through multiple techniques including the fabric study, microstructural observation and zircon geochronology. Our zircon U–Pb data confirm that the granite crystallized at ca. 462 Ma which is consistent with the ages of other linear plutons in North Qinling. Microstructural observations of the Huamenlou granites illustrate that the pluton has undergone superimposed deformation during its emplacement, from magmatic to high-temperature solid state conditions. The internal fabric obtained by anisotropy of magnetic susceptibility (AMS) and shape preferred orientation (SPO) show similar results. The fabrics are relatively concordant and generally vary from NE–SW to NEE–SWW which are roughly oblique to the trend of the pluton elongation and the regional structures. Meanwhile, scalar parameters reflect two completely different strain regimes for the pluton and its host rocks, i.e., the fabrics within host rocks are mainly oblate while the central part of the intrusion displays mainly prolate fabrics. It is inferred that the structural pattern recorded in this pluton was caused by local dextral transtension in consequence of oblique convergence between the South and North China Blocks. We propose that the local transtension in convergence setting probably evolved from vertical extrusion tectonics that provided room for the magma emplacement and imparted prolate fabrics in the Huamenlou pluton.
Transtension
Dike
Transpression
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In southern Alaska, multiple terranes—slabs that have broken off from larger tectonic plates and shuffed around—create a complex patchwork that makes it challenging for scientists to untangle the tectonic history and structure of the region. One of these, the Yakutat terrane, which lies just offshore southern Alaska in the Gulf of Alaska, is converging with the North American plate and driving the growth of the Chugach–St. Elias mountains. The structure of this terrane has not been well studied until now. Worthington et al. conducted seismic studies to create a two‐ dimensional seismic velocity model of the Yakutat terrane. The model allows them to constrain the crustal thickness and composition of the terrane.
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Large igneous provinces (LIPs) are formed by enormous (i.e., frequently >106 km3) but short-lived magmatic events that have profound effects upon global geodynamic, tectonic, and environmental processes. Lithospheric structure is known to modulate mantle melting, yet its evolution during and after such dramatic periods of magmatism is poorly constrained. Using geochemical and seismological observations, we find that magmatism is associated with thin (i.e., ≲80 km) lithosphere and we reveal a striking positive correlation between the thickness of modern-day lithosphere beneath LIPs and time since eruption. Oceanic lithosphere rethickens to 125 km, while continental regions reach >190 km. Our results point to systematic destruction and subsequent regrowth of lithospheric mantle during and after LIP emplacement and recratonization of the continents following eruption. These insights have implications for the stability, age, and composition of ancient, thick, and chemically distinct lithospheric roots, the distribution of economic resources, and emissions of chemical species that force catastrophic environmental change.
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The Tibetan plateau is manifested by contrasting along-strike lithospheric structures, but its formation mechanism and the relationship with the heterogeneous multi-terrane configuration is a challenging problem. Here we conduct systematic numerical modeling to explore the roles of width, density, and rheological properties of the multiple terranes in the lithospheric evolution of the Tibetan plateau, which reveals two distinct collision modes. In Mode-I, the lithospheric mantles of both the strong and weak terranes in the Tibetan plate are completely detached, followed by the underthrusting of Indian lithosphere beneath the whole plateau. Alternatively, Mode-II is characterized by full detachment of the weak terranes, but (partial) residue of the strong terranes during collision. These two contrasting modes, broadly consistent with the lithospheric structures of western and central-eastern Tibetan plateau, respectively, are strongly dependent on the along-strike variation of the width of the strong Lhasa-Qiangtang terranes.
Collision zone
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Transtension
Pull apart basin
Strain partitioning
Transpression
Choreoathetosis
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