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Thermo-mechanical role of a cambro-ordovician paleorift during the Variscan collision: The NE margin of the Bohemian Massif

Abstract:

The Staré Město (SM) belt (NE margin of the Bohemian Massif) represents a preserved example of an intracontinental Cambro-Ordovician rift that has been shortened during the Variscan orogeny. The rifted sequence consists of gabbroic intrusions and a leptyno-amphibolite complex of Cambro-Ordovician protolith age. The latter suffered medium pressure, granulite facies, Cambro-Ordovician metamorphism associated with extension in a continental protorift. The Variscan tectonometamorphic event is manifested by convergent orogenesis with syn-convergent intrusion of a Carboniferous tonalitic sill, high temperature-medium pressure compressional deformation of gabbros and high temperature metamorphism of periodotites. Numerical thermal and rheological modelling suggests that the SM paleorift after 140 Ma of cooling represented a stiff heterogeneity between an easterly situated Cadomian continent and a westerly lying Variscan orogenic root zone. Carboniferous underplating of the paleorift by tonalitic magma was probably responsible for the thermal weakening of both of the upper mantle and the mafic lower crust, and the subsequent uplift of the remaining rigid part of the Cambro-Ordovician crust. All the units were later deformed in transpressional heterogeneous greenschist facies regime at a supracrustal level. © 2001 Elsevier Science B.V. All rights reserved.