Hercynian Folding

also Variscan or Variscian folding, the sum total of processes of the second half of the Paleozoic era (end of the Devonian to the beginning of the Triassic)— that is, processes of intensive folding, orogeny, and granitoid intrusive magmatism, manifested in Paleozoic geosynclines and responsible for folded mountain systems, or Hercynides. Geosynclinal systems subjected to Hercynian folding arose mainly in the Ordovician, Silurian, and beginning of the Devonian period on an older, Baikalian, base and comprised thick series of marine sedimentary and volcanic rock. The name “Hercynian folding” was given by M. Bertrand, after a mountain group in central Europe known to the ancient Romans as the Hercynian Forest (Hercynia Silva, Saltus Hercynius). The term “Variscan” or “Variscian” folding was introduced by E. Suess, after the old name for regions of present-day Saxony, Thuringia, and Bavaria (Cur Variscorum); this term predominates in German writings, where it is used to denote dislocations in a northwesterly direction.

The first epoch of Hercynian folding is the Bretonian (in America, the Acadian), from the end of the Devonian to the beginning of the Carboniferous. It is evident in the Appalachians, the Canadian Archipelago, the Andes, the central parts of the Paleozoic geosyncline of western Europe, and in central Asia (the Kunlun Mountains). The main epoch of Hercynian folding is the Sudetic, from the end of the early Carboniferous to the beginning of the middle Carboniferous. It was of primary importance in the formation of the fold structure of the European Hercynides and the transformation of Paleozoic geosynclines into folded mountain structures. Middle Carboniferous deposits (Westphalian) were crumpled into folds by movements of the so-called Asturian epoch (phase) of folding, while deposits of the Upper Carboniferous (Stephanian) and lower strata of the Permian were crumpled by movements of the Saalian epoch. From the middle of the early Permian or from the late Permian, a platform regimen was established in the greater part of the Hercynian fold regions of central and western Europe, whereas in southern Europe processes of folding and orogeny still continued, and in eastern Europe, in the Urals and in the Donets Ridge, these processes were only beginning. For the Donbas, Ciscaucasia, Urals, and Appalachians, the main epoch of folding is the end of the Carboniferous and the beginning of the Permian; locally, uplifts and fold formation (the Cisuralic foredeep, the Tien-Shan, the Cordilleras of North and South America, the Australian Alps) continued to the beginning or even the middle of the Triassic. In the Carpathian-Balkan region, the Greater Caucasus, the Altai, and the Mongolian-Okhotsk system, orogeny began at the end of the early Carboniferous, and the orogenic period lasted through the entire late Paleozoic and the beginning of the Triassic.

After the Hercynian folding had ended, there appeared for the first time the fold mountains, or Hercynides, of northwest Aftica (the Moroccan Meseta), the Northern Caucasus and Ciscaucasia, the Urals, the Tien-Shan, the Altai, Mongolia, the Greater Khingan Mountains, the Appalachians, the Ouachitas, the Canadian Archipelago, the South American Andes, the Australian Alps, and western, central, and southern Europe. The Hercynian folding caused a number of internal uplifts in the North American Cordillera. Hercynian orogeny also extended to the region of the Caledonian folding of northwestern Europe, the western part of central Kazakhstan, the eastern part of the Altai-Saian region, northern Mongolia, and northern Transbaikalia. In the south and east of the Mediterranean belt (the Dinaride-Hellenides, the mountains of Anatolia, the southern slope of the Caucasus and of the Hindu Kush, and the central Pamirs), the Hercynian folding dies away; in the part of the belt that is within the limits of Southwest and Southeast Asia, as far as the Himalayas, Burma, and the Malay Peninsula, the Hercynian movements were expressed only as weak uplifts and a break in the accumulation of sediments. In this part of Tethys the tectonic regimen in the Paleozoic and early Mezozoic was close to a platform regimen.

The subsequent history of the Hercynian fold regions was varied. Considerable areas of western and central Europe, a large part of the Iberian Peninsula, the plains part of the Crimea and Ciscaucasia, the Urals and parts of the Western Siberian Plain, central Kazakhstan and the Tien-Shan, the Altai-Saian region and Mongolia, the region of the Canadian Archipelago, the coast of the Gulf of Mexico, the Appalachians, the Atlantic Coastal Plain, and eastern Australia experienced platform development with its slow, smooth uplifts and subsidences. As a result of more recent Neocene-Anthropogene uplifts, however, many sections of these Epihercynian platforms again emerged in the form of mountain ranges: the Ardennes, the Rhenish Slate Mountains, the Harz, the Ore Mountains, the Sudetes, the Świętokrzyskie Mountains, the Urals, the Tien-Shan, the Altai, the Greater Khingan Mountains, the Kunlun Mountains, the Tsinling Mountains, the Appalachians, and the Australian Alps. After experiencing considerable Hercynian orogeny, the Pyrenees, the southern part of the Iberian Peninsula, the Atlas (partially), the Alps, Apennines, Carpathians, Balkans, the Greater Caucasus, and the Andes were again involved in intensive subsidence in the beginning of the Mesozoic, thus experiencing the regeneration of a geosynclinal regimen.

Underwater volcanism in the epoch of geosynclinal subsidence which preceded Hercynian orogeny was accompanied by the formation of pyritic deposits of copper, lead, and zinc in the Urals, Altai, Northern Caucasus, and elsewhere, and the introduction of basic and ultrabasic intrusions accounted for the formation of industrial concentrations of platinum, chromites, titanomagnetites, and asbestos in the Urals and elsewhere. Granite formation in the orogenic period of the Hercynian cycle created deposits of ores of lead, zinc, copper, tin, tungsten, gold, silver, and uranium in Europe, Asia (the Tien-Shan and elsewhere), and eastern Australia. The large coalfields (South Wales, France-Belgium, Ruhr-Westphalia, Saar, Upper Silesia, and Hither Appalachia abroad; the Donets, Pechora, and Kuznetsk basins in the USSR) are concentrated on the forward and intermontane troughs of the Hercynides, as are also basins of rock and potassium salts (the Cisuralic trough).