Tectonostratigraphic units in the Variscan belt of central Europe

Abstract

The Variscan belt of Europe resulted from the collision of Africa with Baltica and intervening microplates. In early Paleozoic time, Baltica was probably separated from the areas farther south by a North-German/Polish ocean, whose trace is now under younger cover in the northern part of continental Europe. To the south followed an assembly of microplates with African affinities: a Cadomian (Pan-African) orogen (550 to 600 Ma); traces of the Late Ordovician glaciation (South Pole in the Sahara); and, correspondingly, relatively high paleolatitudes. Some of these units also contain fragments of much older crust (up to 2.5 Ga). These microplates were separated from Africa during an important rifting episode in Cambro-Ordovician time. Rifting is reflected in several belts with spilite-keratophyre volcanism, thick sedimentary sequences, and a thermal event at deeper crustal levels. Small yet widespread occurrences of MOR-type basalts (now mostly eclogites) suggest that rifting attained at least the narrow-ocean stage. To the south of the North-German/Polish ocean, in Ordovician time, the following elements can be recognized (Fig. 1): mid-European microplate; Saxothuringian rift/narrow ocean; Tepla/Barrandian microplate; Barrandian/Moldanubian basin, which is possibly continued westward in the Southern Vosges/Black Forest basin; Vindelician microplate; Massif Central/Western Alps narrow ocean; and southern Europe, possibly separated by an additional basin from Gondwanaland. Closure of all these basins was accompanied by northward drift of the African assembly, during Late Ordovician through Carboniferous time. Opening of the Rhenohercynian basin within the mid-European plate, during Devonian time, contrasts with the general trend of convergence. The Rhenohercynian basin possibly opened when the mid-European plate overrode the spreading-center of the pre-Caledonian ocean farther north, or alternatively, due to back-arc spreading related to northward subduction of oceanic crust farther south. Subduction zones developed in each of these basins. They exhibit a bilateral symmetry, with the Moldanubian block acting as a median zone of structural parting. Subduction of oceanic crust started in Late Ordovician–Early Devonian time (depending on the region), and was largely terminated by Late Devonian or Early Carboniferous time; convergence continued during Late Carboniferous time and involved the subduction of continental crust. Crustal shortening has brought about alpinotype nappe-thrusting, commonly resulting in an inversion of early Variscan, pressure-dominated metamorphic facies. The tectonic assemblages were welded together in Carboniferous time by extensive low-pressure metamorphism and widespread post-tectonic granites.