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

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.

Intraplate seismicity in the western Bohemian Massif (central Europe): A possible correlation with a paleoplate junction

“…This result confirms a separate development of the Cadomian TBU block, an "undigested" microcontinent, which was only little affected by Variscan shortening, metamorphism and granitoid intrusion (Franke, 2000). A dense network of mobile and permanent seismic stations distributed in the western BM within the international BOHEMA experiment (Babuška et al, 2003) allowed Plomerová et al (2006) to delineate boundaries of the three mantle lithospheres characterized by different orientations of mantle A c c e p t e d M a n u s c r i p t olivine fabrics.…”

“…We suggest that the TBU, the best preserved fragment of the Cadomian orogen in central Europe (Franke, 2000), played a key role in the platetectonic development of the region (Fig. 5).…”

“…The western BM represents a junction of three first-order tectonometamorphic units (Franke, 2000): Saxothuringian (ST), Teplá-Barrandian (TBU) and Moldanubian (MD, Fig. 1a,b).…”

Basement structure of the Paleozoic Platform in Poland

“…The west ern ex ten sion of this zone turns to the SW, pass ing into the Mid-Ger man Crys tal line High (Wonik et al, 2001;Ga briel et al, 2011). The lat ter struc ture is in ter preted as the NW mar gin of the Saxothuringian Zone of the Cen tral Eu ropean Variscides (Franke, 2000;Kro ner et al, 2008) cor respond ing to the Rheic Su ture be tween the Saxothuringian and Rhenohercynian belts.…”

“…The in ter nal struc ture of the Sudetic base ment is a com plex tec tonic col lage of sev eral amal gam ated tectonostratigraphic units gen er ally as cribed to the Gond wana-de rived Armori can Terrane As sem blage (Franke, 2000). They in cor po rate heavily tectonized and vari ably meta mor phosed frag ments of the Neoproterozoic crust, Early-Mid dle Pa leo zoic oce anic crust and accretionary prisms, and syn-to post-orogenic Late De vonian to Early Perm ian piggy-back and intramontane bas ins and granitoid plutons.…”