Concepts of the evolution of the Austroalpine basement complex (Eastern Alps) during the Caledonian-Variscan cycle
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Keywords:
Basement
Orogeny
Flysch
Anatexis
Continental Margin
Allochthon
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Orogeny
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Allochthon
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Structural variability in rocks of the upper plate of the Roberts Mountains thrust indicates a complex history of thrust emplacement during the mid-Paleozoic Antler orogeny. The Roberts Mountains allochthon consists predominantly of a highly deformed assemblage of structurally imbricated lower continental-slope and continental-rise sedimentary and volcanic rocks of early Paleozoic age. It has been interpreted variously as a back-arc thrust and as a grounded accretionary prism that overrode the upper Precambrian and lower Paleozoic passive margin of western North America. Current understanding of structural and stratigraphic relations supports the second interpretation.
The trace of the Roberts Mountains thrust trends south-southwesterly across Nevada from near the Idaho border to the vicinity of Tonopah (38°N lat.), where it is interpreted to swing westerly and extend to the Sierra Nevada. Subparallel to the thrust trace is the trend of the western boundary of the late Precambrian passive continental margin as deduced from lower Paleozoic facies patterns and the 87Sr/86Sr = 0.706 line. The westerly deflection of these and younger trends has been attributed by some workers to crustal deformation of late Mesozoic and/or Cenozoic age, and by others to an original bend in the continental margin. Results presented here support the latter interpretation.
Structures in the northern and central segments of the Roberts Mountains thrust (characterized by south-southwest trace) consist of a single phase of folds and indicate easterly transport. The inferred southern segment of the thrust, with a westerly trend, exhibits two and locally three phases of folds interpreted as being genetically related to emplacement of the thrust. First-phase folds in the south are correlated with folds in the central and northern segments of the thrust and, after the effects of superimposed structures are geometrically removed, also yield an easterly transport direction. Cross folds in the southern part of the allochthon and their absence to the north indicate different structural histories for the regions.
Structures in the allochthon are interpreted as having developed during the formation of an accretionary prism. Differences in structural, history are related to differential rates of slip on the allochthon's basal thrust during transport and emplacement. Development of cross folds in the south is thought to be due to a local reduction in the rate of displacement and corresponding internal shortening of the upper-plate rocks caused by resistance to subduction of a west-facing promontory of continental crust.
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Granitic dikes intruding the upper thrust sheets of the Jotun Nappe Complex, southwest Norway, previously assigned a Sveconorwegian age based on Rb‐Sr data, are redated to 427 ± 1 Ma by U‐Pb. The erroneous Rb‐Sr age can be explained by mixing of magmas with different isotopic compositions. Similar geochemical signatures, evidence of Sr mixing, inherited Proterozoic zircons and synkinematic modes of emplacement are also seen in Silurian granites in the Lifjorden Complex as well as in the Lindås Nappe and the Totland unit of the Bergen Arc Complex. We propose that these granites are related and derived from anatexis of Rb‐poor sediments of Baltica affinity overridden by the nappes during Scandian thrusting. This interpretation is compatible with the presence of the granites in nappes of both oceanic and of continental affinities and their absence in the autochthonous basement.
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Anatexis
Baltic Shield
Dike
Isochron dating
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Halostransk~ n4m. 19, 6SSR ABSTRACT. A review of the stratigraphy, facies, palaeo geography, and tectonic development of the Permian of the Bohemian Hassif is given. The Permian rocks of Bohemia and Moravia are of terrestrial origin and belong to the Variscan (central European) province according to the division of H. Fal ke. Regionally they are represented in the following areas: 1/ in the Sudetic region, 2/ in the Central and western Bohemian area, and 3/ in the Boskovice and Bla nice furrows. The Carboniferous/permian boundary lies partly in a continuous sequence, but in some areas a sedimentary break is developed. The palaeogeography is defined as the share of se diments of the different sedimentary environments on the composition of the sedimentary fill. The character of the sedimentary area and of its fill was mainly influenced by diastrophism and climate. The filling of the basins had a cyclic character. Diastrophic movements were connected with the final evolution of the Variscan orogeny. Volcanic activity was controlled also by cyclicity, and is manifested by the alternation of basic to intermediate and acid vol canism. The Late Palaeozoic volcanism culminated in the Autunian period, but in that time also was terminated. The volcanic centres are associated with the zones of
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Underthrusting of Laurentia by the continental margin of Baltica during Caledonian orogeny resulted in the lateral emplacement of Iapetus Ocean-related terranes of the Upper Allochthon at least 500 km onto Baltica. The underlying Lower and Middle allochthons of the Baltoscandian margin mostly comprise Cryogenian, Ediacaran and Cambro-Silurian sedimentary successions; basement to these formations are present only as minor, isolated fragments, except at the base of the Middle Allochthon and within the underlying windows. The upper parts of the Middle Allochthon are notable for the presence of early Ediacaran dyke-swarms and other components of the Baltoscandian continent–ocean transition zone (COT). New data are presented here on the c. 610 Ma age of the COT-related dolerites in the Kalak Nappe Complex in Northern Norway and also on detrital zircons in the underlying Laksefjord and Gaissa nappes. The former confirms that the Baltoscandian COT has a similar age along the length of the orogen; the latter shows that the detrital zircon signatures in the Lower and Middle allochthons are comparable throughout the orogen. These sedimentary rocks have dominating populations of Mesoproterozoic to latest Palaeoproterozoic zircons similar to those from southern parts of the orogen, where Sveconorwegian complexes comprise the basement to the Caledonides. Thus, they help define the probable character and age of the crystalline basement that existed along this outer margin of Baltica during the Neoproterozoic, continental lower crust that was partly subducted during Ordovician continent-arc collision and subsequently lost beneath Laurentia during the 50 million years of Scandian collisional orogeny.
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Laurentia
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Rodinia
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Devonian
Laurentia
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Lithology
Baltica
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Viséan
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Late Devonian extinction
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The pattern and characteristics of thrust‐related very low grade metamorphism in the marginal part of an orogenic wedge have been investigated by combining a clay mineral crystallinity survey with detailed structural mapping of the thin‐skinned foreland thrust belt along the external part of the Scandinavian Caledonides. This external part is composed of late Neoproterozoic to Ordovician sedimentary sequences of the autochthonous cover and Lower Allochthon, which are overlain by the higher Caledonian nappes (Middle and Upper Allochthonous units). Stages of the Scandian phase of thrust wedge development are described which are related to the very low grade metamorphic history. The initial stage involved the emplacement of cooled nappes belonging to the Middle and Upper Allochthons, with very low grade peak metamorphic conditions attained within the underlying Lower Allochthon (cover) sediments as they were progressively buried and deformed beneath the thrust wedge. During this initial emplacement the isotherms are considered to have been undisturbed and dipping parallel to the wedge surface. The following stages of wedge development consisted of extensive post‐metamorphic imbrication of the underthrusted cover sediments, with a transition from basal accretion and uplift at the rear, to accretion and forward propagation at the wedge's toe. During accretion into the wedge, the externally dipping isograd surfaces were extensively displaced from deeper levels toward higher tectonic horizons. The last stage of wedge development considered here was characterized by late out‐of‐sequence thrusting with enhanced (epizonal) metamorphic grades developed in the vicinity of the fault zones, which either resulted from further displacement of the isograds toward higher levels, or from localized heating via intense fluid activity. Overall, the pattern of metamorphic grade, fabric relationships, and physical calculations of heat transfer based on the geometry of the thrust wedge, suggest that neither inverted temperature gradients nor shear heating were likely causes of the metamorphism in this flat‐lying part of the orogenic wedge. The description of inverted very low grade metamorphic isograds in other marginal parts of the Scandinavian Caledonides, which have been previously attributed to either the rapid emplacement of hot thrust nappes, or the effects of dissipative shear heating, are discussed in terms of variations in both the critical wedge geometry and its controlling boundary conditions.
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Abstract 40 Ar/ 39 Ar geothermochronology is especially useful within the Scandinavian Caledonides, which experienced deformation and metamorphism as a result of two major tectonothermal pulses during the Early and Middle Palaeozoic. This detailed 40 Ar/ 39 Ar geochronological study of the Central Norrbotten Caledonides allows examination of the timing of tectonothermal activity within the Lower Köli Nappe, the Seve–Köli shear zone, the Seve Nappe and the shear zone rocks of the Middle Allochthon. The results obtained from a suite of 19 samples from the study area can be incorporated with structural and metamorphic constraints to establish the geological history of the region. Results from this study include: (1) the high grade metamorphism and associated deformation of the Seve units was a Late Cambrian to Early Ordovician event (Finnmarkian) in which the rocks cooled below the respective closure temperatures for hornblende at ≈ 490 Ma and muscovite at ≈ 454 Ma; (2) assuming a simple linear cooling model a cooling rate of 3–6°C/Ma was obtained for the older tectonothermal event; (3) there is evidence for late stage Finnmarkian (450 Ma), relatively high grade shear zones separating different tectonostratigraphic elements within the Seve; (4) the Scandian phase of deformation and metamorphism partially reset some of the Seve hornblendes and most of the muscovites, which indicates that the rocks affected by the Finnmarkian event felt a second tectonothermal pulse of more than 350°C beginning at ≈ 430 Ma; and (5) during the Scandian event all of the far travelled allochthonous tectonic units were juxtaposed and the Middle Allochthon mylonites were formed as these nappes were emplaced above the Baltic Shield. The tectonic units of the Singis‐Nikkaluokta transect were assembled before regional cooling through the closure temperature of muscovite.
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Closure temperature
Hornblende
Thermochronology
Titanite
Geochronology
Muscovite
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