The mid-European segment of the Variscides: tectonostratigraphic units, terrane boundaries and plate tectonic evolution

Abstract

Abstract The mid-European segment of the Variscides is a tectonic collage consisting of (from north to south): Avalonia, a Silurian-early Devonian magmatic arc, members of the Armorican Terrane Assemblage (ATA: Franconia, Saxo-Thuringia, Bohemia) and Moldanubia (another member of the ATA or part of N Gondwana?). The evolution on the northern flank of the Variscides is complex. Narrowing of the Rheic Ocean between Avalonia and the ATA occurred during the late Ordovician through early Emsian, and was accompanied by formation of an oceanic island arc. By the early Emsian, the passive margin of Avalonia, the island arc and some northern part of the ATA were closely juxtaposed, but there is no tectonometamorphic evidence of collision. Renewed extension in late Emsian time created the narrow Rheno-Hercynian Ocean whose trace is preserved in South Cornwall and at the southern margins of the Rhenish Massif and Harz Mts. Opening of this ‘successor ocean’ to the Rheic left Armorican fragments stranded on the northern shore. These were later carried at the base of thrust sheets over the Avalonian foreland. Closure of the Rheno-Hercynian Ocean in earliest Carboniferous time was followed by deformation of the foreland sequences during the late lower Carboniferous to Westphalian. Closure of narrow oceanic realms on both sides of Bohemia occurred during the mid- and late Devonian by bilateral subduction under the Bohemian microplate. In both these belts (Saxo-Thuringian, Moldanubian), continental lithosphere was subducted to asthenospheric depths, and later partially obducted. Collisional deformation and metamorphism were active from the late Devonian to the late lower Carboniferous in a regime of dextral transpression. The orthogonal component of intra-continental shortening produced an anti-parallel pair of lithospheric mantle slabs which probably joined under the zone of structural parting and became detached. This allowed the ascent of asthenospheric material, with important thermal and rheological consequences. The strike slip displacements were probably in the order of hundreds of kilometres, since they have excised significant palaeogeographic elements.