The Palaeozoic Variscan oceans revisited |
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| Institution: | 1. Institut für Geowissenschaften der Goethe-Universität Frankfurt, Altenhöferallee 1, 60438 Frankfurt a. Main, Germany;2. Department of Earth Sciences, The Natural History Museum, Cromwell Road, London SW7 5BD, UK;3. Centre for Earth Evolution and Dynamics (CEED), University of Oslo, 02316 Oslo, Norway;4. Geodynamics, Geological Survey of Norway, 7040 Trondheim, Norway;5. School of Geosciences, University of the Witwatersrand, Johannesburg, South Africa;1. Institute of Geological Sciences, Polish Academy of Sciences, Research Centre in Wroc?aw INGPAN, Podwale St. 50-449, Wroc?aw, Poland;2. Centre of Isotopic Research, All-Russian Geological Research Institute, Sredny prospect 74, 199 106 St. Petersburg, Russia;1. Czech Geological Survey, Klárov 3, 118 21 Prague 1, Czech Republic;2. Institute of Petrology and Structural Geology, Charles University, Albertov 6, 128 43 Prague 2, Czech Republic;3. Institute of Geological Sciences, Faculty of Science, Masaryk University, Kotlá?ská 2, 611 37 Brno, Czech Republic;4. Institut für Geowissenschaften, Mineralogie, Abt. Geochemie & Petrologie, Altenhöferallee 1, D-60438 Frankfurt am Main, Germany;5. Centre for Geobiology, Department of Earth Science, University of Bergen, Allégaten 41, N-5007 Bergen, Norway;6. UCD School of Geological Sciences, University College Dublin, Belfield, Dublin 4, Ireland;1. Department of Earth Sciences, St. Francis Xavier University, Antigonish, B2G 2W5, Canada;2. Instituto Geológico y Minero de España (IGME), Ríos Rosas 23, 28003 Madrid, Spain;3. Universidad de Salamanca, Departamento de Geología, Plaza de los Caídos s/n, 37008 Salamanca, Spain;4. Geology and Geography Department, Tomsk State University, Lenin Street 36, Tomsk 634050, Russian Federation;5. Department of Earth and Atmospheric Sciences, University of Alberta, Edmonton, Alberta T6G 2E3, Canada;6. Dept. of Geology, Bryn Mawr College, Bryn Mawr, PA 19010, USA |
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| Abstract: | Geological evidence, supported by biogeographical data and in accord with palaeomagnetic constraints, indicates that “one ocean” models for the Variscides should be discarded, and confirms, instead, the existence of three Gondwana-derived microcontinents which were involved in the Variscan collision: Avalonia, North Armorica (Franconia and Thuringia subdivided by a failed Vesser Rift), and South Armorica (Central Iberia/Armorica/Bohemia), all divided by small oceans. In addition, parts of south-eastern Europe, including Adria and Apulia, are combined here under the new name of Palaeo-Adria, which was also Peri-Gondwanan in the Early Palaeozoic. Oceanic separations were formed by the break-up of the northern Gondwana margin from the Late Cambrian onwards. Most of the oceans or seaways remained narrow, but – much like the Alpine Cenozoic oceans – gave birth to orogenic belts with HP-UHP metamorphism and extensive allochthons: the Saxo-Thuringian Ocean between North and South Armorica and the Galicia-Moldanubian Ocean between South Armorica and Palaeo-Adria. Only the Rheic Ocean between Avalonia and peri-Gondwana was wide enough to be unambiguously recorded by biogeography and palaeomagnetism, and its north-western arm closed before or during the Emsian in Europe. Ridge subduction under the northernmost part of Armorica in the Emsian created the narrow and short-lived Rheno-Hercynian Ocean. It is that ocean (and not the Rheic) whose opening and closure controlled the evolution of the Rheno-Hercynian foldbelt in south-west Iberia, south-west England, Germany, and Moravia (Czech Republic). Devonian magmatism and sedimentation set within belts of Early Variscan deformation and metamorphism are probably strike-slip-related. The first arrival of flysch on the forelands and/or the age of deformation of foreland sequences constrains the sequential closure of the Variscan seaways (Galicia-Moldanubian in the Givetian; Saxo-Thuringian in the Early Famennian; Rheno-Hercynian in the Tournaisian). Additional Mid- to Late Devonian and (partly) Early Carboniferous magmatism and extension in the Rheno-Hercynian, Saxo-Thuringian and Galicia-Moldanubian basins overlapped with Variscan geodynamics as strictly defined. The Early Carboniferous episode was the start of episodic anorogenic heating which lasted until the Permian and probably relates to Tethys rifting. |
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