Variscan orogeny

The Variscan (or Hercynian) orogeny is a geologic mountain-building event caused by Late Paleozoic continental collision between Laurasia and Gondwana to form the supercontinent of Pangea.

Naming

The name, "Variscan", comes from the Medieval Latin name for the district "Variscia", the home of a Germanic tribe, the Varisci, and was coined in 1880 by Vienna geologist Eduard Suess. Variscite, a rare green mineral found in the region and first discovered in the Vogtland district of Saxony in Germany, which is in the Variscan belt, has the same etymology. Hercynian, on the other hand, derives from the Harz Mountains, in northern Germany. 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. [Kossmat, F.: "Gliederung des varistischen Gebirgsbaus". Abh. Sächs. Geol. L.-A. 1, 1-39, Leipzig 1927]

The other direction "hercynian" for the direction of the Harz Mountains in Germany saw a similar shift in meaning. Today it is often used as a synonym for "variscan", being somewhat less used than the latter. [Google search on December 29, 2007: ca. 44.500 for "variscan orogeny", ca. 15.000 "hercynian orogeny". In German: 1.170 for "variszische Orogenese", 154 for "herzynische Orogenese".] In the USA it is only used for European orogenies, the contemporaneous and genetically linked orogenetic phases in the Appalachian Mountains have different names. [ [http://www.ucmp.berkeley.edu/devonian/devtect.html "Tectonics of the Devonian"] . Website of University of California Museum of Paleontology. Accessed on December 29, 2007.] ^, [http://courses.unt.edu/hwilliams/GEOL_3020/exam3review.htm "Historical Geology: The Hercynian Orogeny"] . Historical Geology, University of North Texas. Accessed on December 29, 2007.]

The regional term "variscan" underwent a further meaning shift since the 1960s. It now is generally used for late paleozoic fold belts and orogenetic phases having an age of approximately 380 to 280 Ma. Some publications use the term "variscan" for fold belts of even younger age, [ [http://pubs.er.usgs.gov/usgspubs/b/b1871 Geology of petroleum and coal deposits in the North China Basin, Eastern China] . Lee, K. Y., USGS Bulletin 1871, 1989, table 1, p. 3] deviating from the meaning as a term for the North American and European orogeny related to the Gondwana-Laurasia collision.

Distribution

The North American and European Variscan Belt includes the mountains of Portugal and western Spain, south-western Ireland (i.e Munster), Cornwall, Devon, Pembrokeshire, the Gower peninsula and the Vale of Glamorgan. Its effects are present in France from Brittany, below the Paris Basin to the Ardennes, the Massif Central, the Vosges and Corsica. It shows in Sardinia in Italy and in Germany where the Rheinisches Schiefergebirge (Ardennes, Eifel, Hunsrück, Taunus and other region on both sides of Middle Rhine Valley), the Black Forest and Harz Mountains remain as testimony. In the Czech Republic and eastern Poland the Bohemian Massif is the eastern end of the 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. Dinaric, Greek and Turkish mountain chains are the southwestern termination of the Variscan proper. [ [http://www.unil.ch/igp/page22669_fr.html Tectonic Map of the western Tethysides] . Institute of Geology and Paleontology of Université de Lausanne, Switzerland. Accessed on December 29, 2007.]

The Variscan was contemporaneous with the Acadian and Alleghenian orogeny in the United States, forming the Ouachita and Appalachian Mountains. Other North American areas with variscan foldbelts include New England, Nova Scotia and Newfoundland. The Morroccan Meseta and the Anti Atlas in northestern Africa show close relations to the Appalachian Mountains and have been the eastern part of the appalachian orogeny before the opening of the Atlantic Ocean in jurassic times.

Variscan mountains in a broader sense are the Urals, the Pamir, the Tianshan and other Asian foldbelts. [ [http://www.unil.ch/igp/page22666_en.html "Paleotethys". Paleogeographic reconstructions for the Devonian and Carboniferous] . "Tethyan Plate Tectonic Working Group" of Université de Lausanne, Switzerland. Accessed on December 29, 2007.] ^, [ [http://www.scotese.com/newpage4.htm Paleogeographic configuration Lower Carboniferous] . "Paleomap Project" by C.Scotese. Accessed on December 29, 2007.]

Formation

The Variscan orogogeny involved a complicated heterogeneous assembly of different microplates and heterochronous collisions, making the exact reconstruction of the plate tectonic processes difficult. Plate convergence that caused the Caledonian orogeny in the Silurian continued to form the Variscan orogeny in the succeeding Devonian and Carboniferous Periods. Both orogenies resulted in the assembly of a super-continent, Pangaea, which was essentially complete by the end of the Carboniferous.

In the Ordovician Period, a land mass, which has been named Gondwana (present day South America, Africa, Antarctica and Australia), straddled the space between the South Pole and the Equator on one side of the globe. Off to the west were three other masses: Laurentia, Siberia and Baltica, located as if on the vertices of a triangle. To the south of them was a large archipelago, the terrane Avalonia, rifted off the north Gondwana margin in early Ordovician.

By the end of the Silurian and in Early Devonian times, Baltica and Laurentia drifted towards each other, closing the Iapetus Ocean between them. They collided in the Caledonian orogeny and formed the Caledonide mountains of North America, Greenland, the British Isles and Norway. Seafloor spreading to the south of Avalonia pushed the latter into north Laurentia and thrust up the northern Appalachian Mountains in the acadian phase of the Caledonian orogeny. Contemporaneously the Tornquist Ocean between Avalonia and Baltica was entirely closed. Thus Avalonia formed the southern coast of the new continent Laurasia (the Old Red Sandstone continent in present day North America, British Isles, northern Germany, Scandinavia and western Russia). In late Devonian and in the Carboniferous the archipelago Armorika of southern Europe, which had rifted off Gondwana after Avalonia later in the Ordovician, was pushed into Avalonia, creating a second range, the North American/European Variscan, to the east of the Caledonide/Appalachian. The collision of Gondwana proper with Laurasia followed in the early Carboniferous, when the Variscan belt was already in place and actively developing.

By the end of the Carboniferous, Gondwana had united with Laurasia on its western end through northern South America and northwestern Africa. Siberia was approaching from the northeast, separated from Laurentia only by shallow waters. Collision with Siberia produced the Ural Mountains in the latest Paleozoic and completed the formation of Pangea. Eastern Laurasia was still divided from Gondwana by the Paleotethys Ocean.

In the Triassic Period of the Mesozoic Era, animals could move without oceanic impediment from Siberia over the North Pole to Antarctica over the South Pole. In the Mesozoic Era, the opening of the Atlantic split Pangea. As a consequence, the Variscan Belt around the then periphery of Baltica ended up many hundreds of miles from the Appalachians.

References

External links

* [http://www.scotese.com/newpage3.htm Early Devonian Map]
* [http://www.scotese.com/newpage4.htm Early Carboniferous Map]
* [http://www.scotese.com/late.htm Late Carboniferous Map]
* [http://www.scotese.com/newpage8.htm Triassic Map]