EXTENDING A THICKENED CRUSTAL BULGE - TOWARD A NEW GEODYNAMIC EVOLUTION MODEL OF THE PALEOZOIC NW BOHEMIAN MASSIF, GERMAN CONTINENTAL DEEP DRILLING SITE (SE GERMANY)

Fault-bounded (tectonic) metamorphic complexes assembling the NW Bohem ian Massif around the German Continental Deep Drilling (KTB) site are seen to be extremely heterogeneous in tectonic and metamorphic histori es. In current models, the different complexes were supposed to reflec t a puzzle of small pre-Devonian microplates, and the related collisio n events supposedly lasted until the Carboniferous. Opposed to these m odels, it will be shown that all the boundaries among the complexes we re formed by detachment, late in a prolonged overall geodynamic histor y of a thickened crustal bulge, during extensional tectonics and assoc iated thermal events that outlasted the onset of collision in the Silu rian/Lower Devonian by about 70-80 Ma. (Micro-)structures, petrologica l and geochronological data of individual complexes predominantly pres erve the late stages rather than the unbroken record of their tectonom etamorphic histories. Such partial histories strongly different among individual complexes, depict diverse snapshots taken at different plac es in the evolving thickened crustal bulge and at different instants i n its overall evolution, and do not define different precollisional mi croplates, Predominantly P-T and deformation episodes after terrane ju xtaposition are preserved. This article presents an integrated view of the structural geology, microscopic fabrics, P-T data and geochronolo gy of such diverse metamorphic complexes. This integrated view provide s a new understanding of (1) the tectonic evolution during Upper Silur ian/Devonian collision of the Gondwana-derived Central European lithos phere with Laurussia, (2) the postaccretionary events that lasted thro ugh the Upper Carboniferous acid (3), the earlier (Lower Ordovician) m etamorphic and magmatic history, which is only locally recorded. Metam orphic complexes occupying the structurally highest position (upper te ctonic complexes) record Devonian and earlier tectonometamorphic and m agmatic events. After the Mid-Devonian, such complexes did not experie nce any metamorphism. The recorded Devonian events consist of subducti on 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 overridin g plate. In the footwall, Carboniferous extension has brought upper te ctonic complexes against metamorphic complexes that essentially record Lower and Upper Carboniferous tectonometamorphic and magmatic events (lower tectonic complexes). Ln the lower tectonic complexes, such even ts 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 event s that are only recorded as petrologic and structural relies transpose d during extension. Inferred as predetachment events are crustal subdu ction as well as stacking that outlasted thrusting and exhumation of t he upper tectonic complexes. In the deeper portion of the thickened cr ustal bulge represented by the lower tectonic complexes, spatial varia tions of P-T-t-d histories during decompression occurred as a result o f continued differential exhumation on the three sets of detachment sy stems, and also from various thermal pulses that have affected differe nt parts of this section during progressive shearing and exhumation to various degrees. Whereas the lower tectonic complexes of the Erzgebir ge Gneiss Dome preserve the record of a Lower Carboniferous history of HP-metamorphism and crustal thickening followed by extension, in thos e of the Oberpfalz region (KTB site), a major Upper Carboniferous ther mal pulse mostly erased the pre-Upper Carboniferous strain fabrics and metamorphic record. In the Oberpfalz region, ongoing extension emplac ed mid-crustal rocks that pervasively equilibrated in the Upper Carbon iferous at high temperatures and low pressures (low P/T ratio) against rocks not exposed to high temperatures at that time. In summary, a pr olonged postaccretionary history concealed large-scale structural rela tionships linked to crustal subduction and thickening associated with former juxtaposition of Gondwana lithosphere to Laurussia. Polyphase c rustal extension and strike slip created the variety of different meta morphic tectonic complexes observed in the NW Bohemian Massif. (C) 199 8 Elsevier Science B.V. All rights reserved.