GSA Annual Meeting in Phoenix, Arizona, USA - 2019

Paper No. 291-11
Presentation Time: 4:20 PM


KRONER, Uwe1, HALLAS, Peter1, STEPHAN, Tobias1 and ROMER, Rolf L.2, (1)Geological Department, TU Bergakademie Freiberg, Bernhard-von-Cotta Straße 2, Freiberg, 09599, Germany, (2)GFZ German Research Centre for Geosciences, Potsdam, 14473, Germany

The architecture of the Central European Variscides resulted from prolonged convergence of the Gondwana and Laurussia plates between 400-300 Ma. First order geometries inside the orogen, like the pervasive fabric of allochthonous units or the orientation of fold and thrust belts, are either compatible with or highly oblique to small circle trajectories of the relative plate motions. Because highly oblique particle paths are predominantly related to later orogenic stages, we propose a principal change of the tectonic style during the orogeny. Early Variscan deformation is mainly controlled by the interaction of the rigid plates of Gondwana and Laurussia. Mid- and late-orogenic processes are additionally influenced by extrusion tectonics of decoupled Peri-Gondwana lithosphere. The principal change of the direction of the maximum horizontal compressional stress axis (σh max) from an early Variscan NE-SW to a late Variscan NNW-SSE direction is responsible for the finite strain pattern. Exemplified by the Bohemian Massif and adjacent areas, we subdivide the following crustal-scale deformation stages responsible for the complex finite strain geometry: (i) Subduction-accretion tectonics during initial plate convergence resulted in the formation of NW-SE and NE-SW striking subduction zones and transform faults, respectively. Devonian, upper allochthonous units are pervasively deformed and exhibit a dominantly NE-SW oriented stretching lineation. (ii) Ongoing crustal stacking in deeper crustal levels resulted in final (U)HP metamorphism that terminated at 340 Ma. The subsequent emplacement of deeply buried rocks beneath the existing nappe pile of upper parts of the orogen occurred during NNW-SSE oriented σh max and resulted in clockwise rotation of the particle path from WSW-ENE to NNW- SSE, as revealed by the fabric of exhumation-related shear zones. (iii) Late Variscan NNW-SSE shortening in the Carboniferous resulted in the formation of external fold and thrust belts thereby reactivating early Variscan structures: former NE-trending transform systems and NW-trending anisotropies acted as oblique ramps and strike slip faults, respectively. The interplay of different shortening directions with mechanical anisotropies resulted in a complex architecture far from finite plane strain geometry.