Extending a thickened crustal bulge: toward a new geodynamic evolution model of the paleozoic NW Bohemian Massif, German Continental Deep Drilling site (SE Germany)

Abstract

Fault-bounded (tectonic) metamorphic complexes assembling the NW Bohemian Massif around the German Continental Deep Drilling (KTB) site are seen to be extremely heterogeneous in tectonic and metamorphic histories. In current models, the different complexes were supposed to reflect a puzzle of small pre-Devonian microplates, and the related collision events supposedly lasted until the Carboniferous. Opposed to these models, it will be shown that all the boundaries among the complexes were formed by detachment, late in a prolonged overall geodynamic history of a thickened crustal bulge, during extensional tectonics and associated thermal events that outlasted the onset of collision in the Silurian/Lower Devonian by about 70–80 Ma. (Micro-)structures, petrological and geochronological data of individual complexes predominantly preserve the late stages rather than the unbroken record of their tectonometamorphic histories. Such partial histories strongly different among individual complexes, depict diverse snapshots taken at different places in the evolving thickened crustal bulge and at different instants in its overall evolution, and do not define different precollisional microplates. Predominantly P– T and deformation episodes after terrane juxtaposition are preserved. This article presents an integrated view of the structural geology, microscopic fabrics, P– T data and geochronology of such diverse metamorphic complexes. This integrated view provides a new understanding of (1) the tectonic evolution during Upper Silurian/Devonian collision of the Gondwana-derived Central European lithosphere with Laurussia, (2) the postaccretionary events that lasted through the Upper Carboniferous and (3), the earlier (Lower Ordovician) metamorphic and magmatic history, which is only locally recorded. Metamorphic complexes occupying the structurally highest position (upper tectonic complexes) record Devonian and earlier tectonometamorphic and magmatic events. After the Mid-Devonian, such complexes did not experience any metamorphism. The recorded Devonian events consist of subduction and exhumation of HP-rocks and their exhumation involved thrusting and extensional tectonics. Upper tectonic complexes comprise fragments of both the plate that was subducted in this period and the overriding plate. In the footwall, Carboniferous extension has brought upper tectonic complexes against metamorphic complexes that essentially record Lower and Upper Carboniferous tectonometamorphic and magmatic events (lower tectonic complexes). In the lower tectonic complexes, such events are (1) consecutive extensional stages that created at least three sets of ductile normal detachment systems intersecting each other, and various associated thermal pulses, as well as (2) predetachment events that are only recorded as petrologic and structural relics transposed during extension. Inferred as predetachment events are crustal subduction as well as stacking that outlasted thrusting and exhumation of the upper tectonic complexes. In the deeper portion of the thickened crustal bulge represented by the lower tectonic complexes, spatial variations of P– T– t– d histories during decompression occurred as a result of continued differential exhumation on the three sets of detachment systems, and also from various thermal pulses that have affected different parts of this section during progressive shearing and exhumation to various degrees. Whereas the lower tectonic complexes of the Erzgebirge Gneiss Dome preserve the record of a Lower Carboniferous history of HP-metamorphism and crustal thickening followed by extension, in those of the Oberpfalz region (KTB site), a major Upper Carboniferous thermal pulse mostly erased the pre-Upper Carboniferous strain fabrics and metamorphic record. In the Oberpfalz region, ongoing extension emplaced mid-crustal rocks that pervasively equilibrated in the Upper Carboniferous at high temperatures and low pressures (low P/T ratio) against rocks not exposed to high temperatures at that time. In summary, a prolonged postaccretionary history concealed large-scale structural relationships linked to crustal subduction and thickening associated with former juxtaposition of Gondwana lithosphere to Laurussia. Polyphase crustal extension and strike slip created the variety of different metamorphic tectonic complexes observed in the NW Bohemian Massif.