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The Variscan orogeny, Hercynian orogeny or Variscan chain was a geologic mountain-building event caused by Late Paleozoic continental collision between Euramerica (Laurussia) and Gondwana to form the supercontinent of Pangaea. It remains visible today as a series of isolated massifs, including the Ardennes, Bohemian Massif, Vosges-Black Forest, Armorican Massif, Cornubian Massif, Massif Central, and Iberian System. These are interspersed with Mesozoic and Cenozoic sedimentary basins. The chain also crops out in southern Ireland and was later incorporated into the Alpine orogeny (external crystalline massifs) and Pyrenean orogeny. These ancient massifs form the pre-Permian basement of western and Central Europe, part of a larger mountain system stretching from the Ural Mountains in Russia to the Appalachian Mountains in North America.
The chain originated from the convergence and collision of three continental masses: the microcontinent Armorica and the Gondwana and Laurussia (a union of Laurentia and Baltica from the Caledonian orogeny). This convergence contributed to the formation of the supercontinent Pangaea.
Today, the chain is heavily erosion, with most geological evidence consisting of metamorphic rocks and granites, which once formed the deep roots of the massif.
Hercynian, on the other hand, derives from the Hercynian Forest. Both words were descriptive terms of strike directions observed by geologists in the field, variscan for southwest to northeast, hercynian for northwest to southeast. The variscan direction reflected the direction of ancient fold belts cropping out throughout Germany and adjacent countries and the meaning shifted from direction to the fold belt proper.
One of the pioneers in research on the Variscan fold belt was the German geologist Franz Kossmat, establishing a still valid division of the European Variscides in 1927.
The other direction, Hercynian, for the direction of the Harz Mountains in Germany, saw a similar shift in meaning. Today, Hercynian is often used as a synonym for Variscan but is somewhat less used than the latter in the English speaking world.Google search on December 29, 2007: approximately 44,500 for Variscan orogeny, approximately 15,000 Hercynian orogeny. In German: 1,170 for "variszische Orogenese", 154 for "herzynische Orogenese". In the United States, it is used only for European orogenies; the contemporaneous and genetically linked mountain-building phases in the Appalachian Mountains have different names. Tectonics of the Devonian. Website of University of California Museum of Paleontology. Accessed on December 29, 2007. "The Hercynian Orogeny". Historical Geology, University of North Texas. "Variscan" is preferred for the orogenic cycle, and "Hercynian" for the resulting massifs, though both describe related geological entities.
The regional term Variscan underwent a further meaning shift since the 1960s. Geologists generally began to use it to characterize late Paleozoic fold-belts and orogenic phases having an age of approximately 380 to 280 Ma.
Some publications use the term Variscan for fold belts of even younger age, deviating from the meaning as a term for the North American and European orogeny related to the Gondwana-Laurasia collision.
The Variscan Belt reappears in Sardinia in Italy and in Germany where the Rhine Massif (Ardennes, Eifel, Hunsrück, Taunus and other regions on both sides of Middle Rhine Valley), the Black Forest and Harz Mountains remain as testimony. In southern Iberia it is marked by a classic strike-slip suture zone between very distinct suspect terranes, and clear evidence can be seen of ductile shearing between high-grade and lower grade sedimentary rocks in a wide belt north of the Algarve and extending into the northernmost part the autonomous region of Andalusia and southern Extremadura.
In the Czech Republic and southwestern Poland the Bohemian Massif is the eastern end of the unmodified Variscan belt of crustal deformation in Europe. Further Variscan developments to the southeast are partly hidden and overprinted by the Alpine orogeny. In the Alps a Variscan core is built by Mercantour, Pelvoux, Belledonne, Montblanc and Aar Massif. Dinarides, Greek and Turkish mountain chains are the southeastern termination of the Variscan proper. Tectonic Map of the western Tethysides . Institute of Geology and Paleontology of the University of Lausanne, Switzerland. Accessed on December 29, 2007.
The Variscan was contemporaneous with the Acadian orogeny and Alleghenian orogeny in the United States and Canada, responsible for forming the Ouachita and Appalachian Mountains. North American areas with Variscan foldbelts include New England, Nova Scotia and Newfoundland and Labrador. The Moroccan Meseta and the Anti-Atlas in northwestern Africa show close relations to the Appalachian Mountains and used to form the eastern part of the Appalachian orogeny before the opening of the Atlantic Ocean in Jurassic times. 'Variscan' mountains in a broad chronological sense include the Ural Mountains, the Pamir Mountains, the Tian Shan and other Asian foldbelts. Paleotethys. Paleogeographic reconstructions for the Devonian and Carboniferous . Tethyan Plate Tectonic Working Group of the University of Lausanne, Switzerland. Accessed on December 29, 2007. Paleogeographic configuration Lower Carboniferous. Paleomap Project by C.Scotese. Accessed on December 29, 2007.
The Variscan chain, stretching long, wide, and initially reaching in elevation, is evident across Europe and beyond. Key regions include:
In the eo-Variscan phase, from the late Ordovician to Silurian (450–400 Ma), extension gave way to plate tectonics convergence, leading to the collision of Gondwana in the south with the Euro-American continent (Laurentia-Baltica) in the north, involving intermediate plates like Avalonia and Armorica. Subduction of the African plate margin beneath the Euro-American plate closed the Rheic Ocean and Centralian Ocean, producing volcanic arc and high-pressure, high-temperature metamorphism as continental and oceanic lithosphere was buried beyond 100 km. Basic igneous rock transformed into , and acidic rocks into .
During the meso-Variscan phase, from the early to mid-Devonian (380–340 Ma), continental collision between Laurussia and Gondwana caused obduction of oceanic material onto continental crust. This phase featured high-pressure, medium-temperature metamorphism and significant deformation, including thrust fault and nappe tectonics.
In the neo-Variscan phase, from the late Devonian to late Carboniferous (380–290 Ma), nappe tectonics stacked metamorphic units, creating relief comparable to the modern Alps. The thickened crust—nearly double its normal thickness—caused thermal perturbations,The abundance of radioactive elements (uranium, thorium) in crustal material generated significant heat, increasing the geothermal gradient and causing post-thickening thermal relaxation. leading to partial melting (anatexis) and widespread plutonism (granite formation), alongside medium-pressure, medium-temperature metamorphism. The unstable, thickened crust underwent isostasy thinning, driven by gravitational collapse or changes in plate kinematics. This late-orogenic extension, lasting into the Permian, involved tangential tectonics, intense erosion exposing lower crustal rocks, and the formation of sedimentary basins filled with material from bordering faults, volcanic flows, and calderas.
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