VARISCIDES BETWEEN THE APPALACHIANS AND THE URALS: SIMILARITIES AND DIFFERENCES BETWEEN PALEOZOIC SUBDUCTION AND COLLISION BELTS

Abstract

The Variscan Belt of western Europe is part of a large Paleozoic mountain belt, 1000 km wide and 8000 km long, that extended in Permian time from the Caucasus and Urals in Europe to the Appalachian and Ouachita Mountains in America. The tectonic evolution of these Paleozoic belts bordering Laurentia and Baltica is closely linked, but their geometry, tectonothermal history, and the amount of shortening are different. The Appalachians and the Variscides are typical collision belts. They were built before the Triassic, between 480 and 250 Ma, and resulted from the diachronous collision of two major continents, Laurentia-Baltica to the northwest and Gondwana to the southeast. Between these two large continents, smaller intermediate continental plates (Avalonia, Armorica, Carolina) show a peculiar Late Proterozoic (650-550 Ma, Cadomian), soft, volcanic and sedimentary basement. These microplates detached from Gondwana during the Cambrian-Ordovician and were accreted to Baltica and Laurentia before the final Carboniferous collision of Gondwana with Baltica and Laurentia. The two belts show very large thrusts and recumbent folds evidencing low- to high-grade polymetamorphism. They were strongly eroded; the Moho is flat and between 30 and 40 km deep. The Urals separating Europe from Asia were built at approximately the same time, but are not a typical collision belt between two large continents. The belt results from eastward subduction of continental crust beneath oceanic and/or island-arc lithosphere. This stage, contemporaneous with high-pressure metamorphism, was followed by westward subduction, accretion, and soft collision of island arcs and micro-continents. The amount of shortening is much less, as are the grade of metamorphism and the amount of erosion. A peculiar feature of the Urals is the preservation of an orogenic root corresponding to the mafic core of the orogen, the Moho deepening down to 55-60 km in the center of the orogen. Differences in geometry, amount of shortening, grade of metamorphism, production of granites of these three belts depend on differences in collision processes and the nature of the preorogenic basement involved.