Geodynamic Interpretation of the Mid-European Variscides

Abstract

The geological history of different parts of the Mid-European Variscides points to an intracontinental, polyphase development during Late Precambrian — Early Palaeozoic up to the end of Permo-Carboniferous time. Continental rift-processes, which reached their maximum during the Ordovician, characterize the beginnings of this history. They might be interpreted as an effect tending to compensate to some extent the closure of the Iapetus Ocean in the neighbouring Caledonian Orogen. Rifting of the continental lithosphere and concomitant ascent of hot asthenosphere is interpreted as the cause of high grade metamorphism inside the lower crust under extensional strain. This “continental rift metamorphism” is a concept which would accomodate the observation of widespread granulite facies metamorphism and contemporaneous calcalkaline to peralkaline magmatism during continuous sedimentation in parts of the European Variscides. Due to the rheological properties of dry granulite facies rocks, the granulites form a layer of higher strength inside the deep crust. During later crustal shortening such granulite layers will tend to form large scale folds which pierce the overlying crust during increasing amplification. The further development of these “intrusive folds” may result in the formation of imbricate structures and thrust sheets of granulite facies rocks. The main phase of Variscan crustal shortening, which can be observed in the higher grade metamorphic (amphibolite grade) rocks is pre-Middle Devonian in age and post-dates the Caledonian rifting event. Radiometric dates indicate Upper Silurian to Lower Devonian ages. Unmetamorphic to very low grade metamorphic sedimentary rocks of Middle to Upper Devonian ages, which unconformably cover this older amphibolite grade basement indicate deep denundation of the earlier metamorphosed rocks. The weakly metamorphosed units of the Saxothuringian and Rhenohercynian Zones have been folded and metamorphosed during Carboniferous time. The present pattern of metamorphic zones and deformation styles is strongly influenced by large-scale crustal imbrications that are accompanied by nappe formation. Major crustal overthrusts follow the boundaries of the Moldanubian and Saxothuringian Zones. The orogenic crustal shortening took place in a thermally and mechanically highly inhomogeneous continental lithosphere subjected to convergent movements between the Laurentian, Fennosamartian and African plates. For mechanical and geometrical reasons, such convergence seems possible only if, during progressive crustal shortening, the lithospheric mantle becomes detached from the overlying crust and will be subducted underneath the continental crust (A-subduction). Such subcrustal subduction of the lithospheric mantle introduces the possibility of a differential movement between crust and lithospheric mantle. Such relative movement is necessary for understanding the symmetry and polarity of the orogenic movement picture. Descent of large volumina of lithospheric mantle into the asthenosphere would require a compensating upward transfer of similar volumina of asthenospheric material. However, the lack of any mid-ocean ridge at which mass-equilibration could be effected, requires that this mass-equilibration takes place in front of a subducting slab, and that the subducting slab of lithospheric mantle has to retreat in order to allow further mass-equilibration. A possible means of return is available in Andrews and Sleeps (1974) model of forced convection induced by the subducting slab. In the case of the bilateral Variscides one has to assume two such Andrews-Sleep cells operating on both sides of the orogen in an opposite sense. The mechanisms of subduction of lithospheric mantle and Andrews-Sleep convection orerating below a region of crustal convergence offers the possibility for interpreting several geodynamic processes such as folding, backfolding and late tectonic rifting as well as several other peculiarities in the geological history of the Variscides.