TheVariscan orogeny orHercynian orogeny was a geologic mountain-building event caused by LatePaleozoiccontinental collision betweenEuramerica (Laurussia) andGondwana to form thesupercontinent ofPangaea. It remains visible today as a series of isolatedmassifs, including theArdennes,Bohemian Massif,Vosges-Black Forest,Odenwald,Armorican Massif,Cornubian Massif,Massif Central, andIberian System. These are interspersed withMesozoic andCenozoicsedimentary basins. The chain also crops out in southernIreland and was later incorporated into theAlpine orogeny (external crystalline massifs) andPyrenean orogeny. These ancient massifs form the pre-Permianbasement of western and Central Europe, part of a larger mountain system stretching from theUral Mountains in Russia to theAppalachian Mountains inNorth America.
The chain originated from the convergence and collision of three continental masses: themicrocontinentArmorica and thesupercontinentsProtogondwana andLaurussia (a union ofLaurentia andBaltica from theCaledonian orogeny). This convergence contributed to the formation of the supercontinentPangaea.
Today, the chain is heavilyeroded, with most geological evidence consisting of metamorphic rocks and granites, which once formed the deep roots of the massif.
The nameVariscan comes from the MedievalLatin name for the districtVariscia, the home of a Germanic tribe, theVarisci;Eduard Suess, professor of geology at theUniversity of Vienna, coined the term in 1880. (Variscite, a rare green mineral first discovered in theVogtland district ofSaxony in Germany, which is in the Variscan belt, has the same etymology.)
Hercynian, on the other hand, derives from theHercynian Forest. Both words were descriptive terms ofstrike directions observed by geologists in the field,variscan for southwest to northeast,hercynian for northwest to southeast.[2] Thevariscan 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 geologistFranz Kossmat, establishing a still valid division of the European Variscides in 1927.[3]
The other direction,Hercynian, for the direction of theHarz Mountains in Germany, saw a similar shift in meaning. Today,Hercynian is often used as asynonym forVariscan but is somewhat less used than the latter in the English speaking world.[4][5] In the United States, it is used only for European orogenies; the contemporaneous and genetically linked mountain-building phases in theAppalachian Mountains have different names.[6][7] "Variscan" is preferred for the orogenic cycle, and "Hercynian" for the resulting massifs, though both describe related geological entities.[8]
The regional termVariscan 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 termVariscan for fold belts of even younger age,[9] deviating from the meaning as a term for the North American and European orogeny related to the Gondwana-Laurasia collision.
The European Variscan Belt includes the mountains of Portugal, Spain (Galicia, and thePyrenees), southwestern Ireland (i.e.Munster),Cornwall,Devon,Pembrokeshire, theGower Peninsula and theVale of Glamorgan. In France, the belt occurs fromBrittany, below theParis Basin to theArdennes, in theMassif Central, the Pyrenees, theVosges and inCorsica.
The Variscan Belt reappears inSardinia in Italy and in Germany where theRhine Massif (Ardennes,Eifel,Hunsrück,Taunus and other regions on both sides ofMiddle Rhine Valley), theBlack Forest, theOdenwald andHarz 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-grademetamorphic rocks and lower grade sedimentary rocks in a wide belt north of theAlgarve and extending into the northernmost part the autonomous region ofAndalusia and southernExtremadura.[12][13]
In the Czech Republic and southwestern Poland theBohemian Massif is the eastern end of the unmodified Variscan belt of crustal deformation in Europe. Further Variscan developments to the southeast are partly hidden andoverprinted by theAlpine orogeny. In theAlps a Variscan core is built byMercantour,Pelvoux,Belledonne,Montblanc andAar Massif.Dinaric, Greek and Turkish mountain chains are the southeastern termination of the Variscan proper.[14]
The Variscan was contemporaneous with theAcadian andAlleghenian orogeny in the United States and Canada, responsible for forming theOuachita andAppalachian Mountains. North American areas with Variscan foldbelts includeNew England,Nova Scotia andNewfoundland and Labrador. TheMoroccan Meseta and theAnti-Atlas in northwestern Africa show close relations to the Appalachian Mountains and used to form the eastern part of theAppalachian orogeny before the opening of theAtlantic Ocean inJurassic times.[15] 'Variscan' mountains in a broad chronological sense include theUrals, thePamir, theTian Shan and other Asian foldbelts.[16][17]
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.[18][19] Key regions include:
The Variscan orogeny partly overlapped with theAcadian orogeny, which shaped theAppalachians. Its northwest-southeast (Armorican) and northeast-southwest (Variscan) branches form a characteristic "Hercynian V" pattern.[20] European Hercynian massifs primarily consist ofCarboniferousgranites,metamorphic rocks (gneiss,micaschist), and locallyquartzite and Carboniferouscoal deposits.
The Variscan orogeny unfolded over several phases, broadly divided into pre-collision and post-collision stages.[21] During the pre-Variscan phase, from theCambrian toOrdovician (550–450Ma), widespreadextension fragmented the supercontinentRodinia, separating Northern Europe fromGondwana. This created a vast marine region, thinning thecontinental crust (e.g.,Laurentia,Baltica,Kazakhstania,Siberia) and formingoceanic crust in theIapetus,Rheic, and Centralian oceans.[21]
In the eo-Variscan phase, from the late Ordovician toSilurian (450–400 Ma), extension gave way toplate convergence, leading to the collision of Gondwana in the south with the Euro-American continent (Laurentia-Baltica) in the north, involving intermediate plates likeAvalonia andArmorica. Subduction of the African plate margin beneath the Euro-American plate closed theRheic Ocean and Centralian Ocean, producingarc magmatism and high-pressure, high-temperaturemetamorphism as continental and oceanic lithosphere was buried beyond 100 km.[22] Basicmagmatic rocks transformed intoeclogites, and acidic rocks intogranulites.[22]
During the meso-Variscan phase, from the early to mid-Devonian (380–340 Ma), continental collision betweenLaurussia and Gondwana causedobduction of oceanic material onto continental crust. This phase featured high-pressure, medium-temperature metamorphism and significant deformation, includingthrusting andnappe tectonics.[23]
In the neo-Variscan phase, from the late Devonian to lateCarboniferous (380–290 Ma), nappe tectonics stacked metamorphic units, creating relief comparable to the modernAlps. The thickened crust—nearly double its normal thickness—caused thermal perturbations,[note 1] leading topartial melting (anatexis) and widespreadplutonism (granite formation), alongside medium-pressure, medium-temperature metamorphism.[23] The unstable, thickened crust underwentisostatic thinning, driven by gravitational collapse or changes in plate kinematics. This late-orogenic extension, lasting into thePermian, involved tangential tectonics, intense erosion exposing lower crustal rocks, and the formation ofsedimentary basins filled with material from bordering faults, volcanic flows, andcalderas.[24]
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