Variscan chain

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The Variscan chain, also known as Hercynian orogeny, especially in France, is a major mountain chain formed from the Devonian to Permian periods during the Variscan cycle. 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.

Contents

The chain originated from the convergence and collision of three continental masses: the microcontinent Armorica and the supercontinents Protogondwana 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 eroded, with most geological evidence consisting of metamorphic rocks and granites, which once formed the deep roots of the massif.

Naming

The term "Variscan" was introduced by geologist Eduard Suess in 1888 to describe mountain ranges he studied in southern Germany. It derives from the Varisci, inhabitants of present-day Vogtland, whose main town, Hof, was called Curia Variscorum in Latin (the mineral variscite, found in the same region, shares this etymology). [1]

Concurrently, French geologist Marcel Bertrand used "Hercynian" in 1892 (from Latin Hercynia Silva, the Hercynian Forest spanning central Germany) to describe the same structural framework of Europe. Initially, the terms referred to distinct fold and fault directions: southwest to northeast for Variscan and northwest to southeast for Hercynian. [1]

Suess focused on paleontological and structural differences between mountain ranges, while Bertrand sought correlations among massifs. Today, "Variscan" is preferred for the orogenic cycle, and "Hercynian" for the resulting massifs, though both describe related geological entities. [2]

Formation

The Variscan orogeny unfolded over several phases, broadly divided into pre-collision and post-collision stages. [3] During the pre-Variscan phase, from the Cambrian to Ordovician (550–450 Ma), widespread extension fragmented the supercontinent Rodinia, separating Northern Europe from Gondwana. This created a vast marine region, thinning the continental crust (e.g., Laurentia, Baltica, Kazakhstania, Siberia) and forming oceanic crust in oceans like Iapetus, Rheic, and Centralian. [3]

In the eo-Variscan phase, from the late Ordovician to Silurian (450–400 Ma), extension gave way to plate 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 arc magmatism and high-pressure, high-temperature metamorphism as continental and oceanic lithosphere was buried beyond 100 km. [4] Basic magmatic rocks transformed into eclogites, and acidic rocks into granulites. [4]

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 thrusting and nappe tectonics. [5]

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, [note 1] leading to partial melting (anatexis) and widespread plutonism (granite formation), alongside medium-pressure, medium-temperature metamorphism. [5] The unstable, thickened crust underwent isostatic 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. [6]

Distribution

Hatched areas show the distribution of Variscan chains in Europe, North America (Appalachians), North Africa (Mauritanides), and other contemporaneous chains (Urals, Eurasian Steppe). The Himalayan model suggests the Variscan chain resembled parts of the modern Himalayas. Simplistically, 350 million years ago, there might have been an "Everest" in Lyon, an "Annapurna" in Clermont-Ferrand, and a "Tibet" where the Paris Basin now lies. Distribution of varisican orogenies.png
Hatched areas show the distribution of Variscan chains in Europe, North America (Appalachians), North Africa (Mauritanides), and other contemporaneous chains (Urals, Eurasian Steppe). The Himalayan model suggests the Variscan chain resembled parts of the modern Himalayas. Simplistically, 350 million years ago, there might have been an "Everest" in Lyon, an "Annapurna" in Clermont-Ferrand, and a "Tibet" where the Paris Basin now lies.

The Variscan chain, stretching 5,000 kilometres (3,100 mi) long, 700 kilometres (430 mi) wide, and initially reaching 6,000 metres (20,000 ft) in elevation, is evident across Europe and beyond. [9] [10] Key regions include:

The Variscan orogeny partly overlapped with the Acadian orogeny, which shaped the Appalachians. Its northwest-southeast (Armorican) and northeast-southwest (Variscan) branches form a characteristic "Hercynian V" pattern. [11] European Hercynian massifs primarily consist of Carboniferous granites, metamorphic rocks (gneiss, micaschist), and locally quartzite and Carboniferous coal deposits.

See also

Notes

  1. ↑ The abundance of radioactive elements (uranium, thorium) in crustal material generated significant heat, increasing the geothermal gradient and causing post-thickening thermal relaxation.

References

  1. 1 2 Rast, Nicholas (1988). "Tectonic implications of the timing of the Variscan orogeny" . Geological Society, London, Special Publications. 38: 585–595. doi:10.1144/GSL.SP.1988.038.01.38 . Retrieved April 13, 2025.
  2. ↑ "Variscan or Hercynian Chains". Encyclopædia Universalis (in French). 29 January 2025. Retrieved 2025-04-13.
  3. 1 2 Autran, Albert; Chiron, J. C. (1980). Introduction à la carte tectonique de la France à 1/1 000 000 [Introduction to the 1:1,000,000 tectonic map of France] (in French). BRGM. p. 23. ISBN   978-2-7159-5014-6.
  4. 1 2 Renard, Maurice; Lagabrielle, Yves; Martin, Erwan; Saint Sauveur, Marc de Rafelis (2015). Éléments de géologie [Elements of geology] (in French). Dunod. p. 458. ISBN   978-2-10-072066-8.
  5. 1 2 Dercourt, Jean (2002). "Modèle d'évolution de la chaîne hercynienne du Massif Central" [Model of the evolution of the Massif Central Hercynian chain]. Géologie et géodynamique de la France : outre-mer et européenne [Geology and geodynamics of France: overseas and European] (in French). Paris: Dunod. ISBN   978-2-10-006459-5. Archived from the original on 30 April 2007.
  6. ↑ Burg, Jean-Pierre; Van Den Driessche, Jean; Brun, Jean-Pierre (1994). "Syn- to post-thickening extension in the Variscan Belt of Western Europe: Modes and structural consequences". Géologie de la France (3): 33–51.
  7. ↑ Mattauer, Maurice (1974). "Existe-t-il des chevauchements de type himalayen dans la chaîne hercynienne du Sud de la France ?". 2° Réunion Annuelle des Sciences de la Terre: 279.
  8. ↑ Rebeyrol, Yvonne (June 17, 1981). "Un Anapurna à Clermont-Ferrand et un Everest à Lyon" [An Anapurna in Clermont-Ferrand and an Everest in Lyon]. Le Monde (in French). Retrieved April 13, 2025.
  9. ↑ Behr, H.-J.; Engel, W.; Franke, W.; Giese, P.; Weber, K. (1984). "The Variscan Belt in Central Europe: Main structures, geodynamic implications, open questions" . Tectonophysics. 109 (1–2): 15–40. Bibcode:1984Tectp.109...15B. doi:10.1016/0040-1951(84)90168-9.
  10. ↑ Matte, Philippe (June 15, 1986). "Tectonics and plate tectonics model for the Variscan belt of Europe" . Tectonophysics. 126 (2–4): 329–374. Bibcode:1986Tectp.126..329M. doi:10.1016/0040-1951(86)90237-4.
  11. ↑ Faure, Michel (February 24, 2021). "La chaine varisque en France, un édifice multi-collisionnel et poly-cyclique" [The Variscan chain in France: a multi-collisional and poly-cyclic edifice]. Ressources Scientifiques Pour l'Enseignement des Sciences de la Terre et de l'Univers (in French).

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