Exhumation of high-grade rocks in the Saxo-Thuringian Belt: geological constraints and geodynamic concepts
Published:January 01, 2000
W. Franke, E. Stein, 2000. "Exhumation of high-grade rocks in the Saxo-Thuringian Belt: geological constraints and geodynamic concepts", Orogenic Processes: Quantification and Modelling in the Variscan Belt, Wolfgang Franke, Volker Haak, Onno Oncken, David Tanner
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The Saxo-Thuringian Belt on the northern flank of the European Variscides resulted from SE-ward subduction of a Cambro-Ordovician rift basin under the Teplá– Barrandian (Bohemian) margin. It contains ultra-high-pressure (UHP) metamorphic rocks, which are now exposed in different tectonic settings, and were exhumed in different modes, under different thermal regimes. Eclogites of the upper allochthon (tectonic klippen of Münchberg, Wildenfels and Frankenberg) originated from early Devonian (c. 400 Ma) subduction. Cooling ages in the klippen, combined with clast spectra and ages of detrital minerals in the foreland flysch record exhumation in Famennian time. The high-pressure (HP) rocks rose in a narrow corridor along the suture zone, were retrogressed under amphibolite facies conditions, and rapidly recycled into the foreland flysch. Rocks of the lower allochthon are exposed in dome structures emerging from under the relative autochthon. The HP Saxonian Granulites were formed at c. 20 kbar/1050°C, and the Erzgebirge contains diamond-bearing quartzo-feldspathic rocks, associated with eclogites. Peak temperatures of the lower allochthon were attained at c. 340 Ma. Immediately afterwards, these rocks were emplaced under the floor of the relative autochthon, which, at this time, was a foreland basin receiving clastic sediments from the orogen adjacent to the SE. Emplacement of the lower allochthon is most clearly documented in the Saxonian Granulite Dome, in which HP granulites are juxtaposed against low-pressure–high-temperature (LP–HT) rocks of the hanging wall. The interface is a zone of HT shear, which cuts out ≥60 km of crustal thickness. In more internal parts of the belt (Erzgebirge), the newly emplaced lower allochthon was subsequently reworked by thrusting and polyphase refolding. Emplacement of the hot, low-viscosity lower allochthon was probably driven by buoyancy and the hydraulic gradient between the crustal root to the SE and the lower crust of the foreland. Unlike the earlier HP rocks exposed in the upper allochthon, the 340 Ma HP rocks of the lower allochthon were thermally softened, and, instead of piercing their cover, intruded into the foreland. Therefore, the 340 Ma rocks do not appear in the clastic record of the flysch. The two contrasting mechanisms of exhumation observed in the upper and lower allochthon are apparently due to different thermal regimes.
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Orogenic Processes: Quantification and Modelling in the Variscan Belt
Research into the orogenic processes that shaped the continental crust of Europe has a long-standing tradition. Why the need to quantify and model? It is not just satisfactory to identify subduction zones, accretionary prisms, island arcs, extensional collapse and other standard items of the geodynamic menu. Such interpretations need to be quantified: extent and composition of subducted crust, angle and speed of subduction, amount and composition olmelts produced, heat sources for metamorphism. All such interpretations have to conform to first principles, and also to stand the test of quantitative balancing – a concept first developed for the conservation of length or volume in tectonic cross sections. Also in other fields, the correlation of causes and effects and the internal consistency of dynamic models requires a numerical approach.
The present volume combines review articles with reports on recent progress in an attempt to address these aims. There is a foldout map of the region, which locates the main areas of outcrop and tectono-stratigraphic units, and a reassesment of the Palaeozoic time scale permits correlation of tectonic, metamorphic and magmatic events with the sedimentary record of the upper crust.