Contrasting tectono-metamorphic evolution of orogenic lower crust in the Bohemian Massif: A numerical model

Abstract

The Bohemian Massif, located at the eastern margin of the European Variscan belt, is characterised by an exceptional accumulation of felsic high-pressure granulites. The petrological, structural and geochronological studies of this region revealed systematic differences between the tectonometamorphic evolution of the southern (Moldanubian) and northern (West Sudetes) parts of the orogen. Two contrasting tectonic scenarios have been proposed: gravity-driven vertical mass exchanges followed by continental indentation in the Moldanubian domain, and crustal-scale folding leading to gneiss dome formation in the West Sudetes. We present a numerical model in order to correlate the apparent differences between these two regions with the variations in the dynamics of the modelled system. We model two colliding blocks: an orogenic root, where a felsic lower crust is overlain by a mafic layer and a middle crust, and a continental indentor. We examine the role of the rate of convergence of the two blocks, radiogenic heat production within the felsic lower crust and efficiency of erosion. The prograde part of the metamorphic evolution is controlled by the rate of convergence and the peak temperature depends on the heat production. The retrograde evolution is controlled mostly by erosional processes. In the models, where the material is weakened due to the heating in the felsic lower crust, the gravitational instability of the mafic and felsic layers causes their complete vertical exchange followed by a flow above the indentor. In colder and/or faster models, the thickening is dominated by the buckling of the mafic layer. These two styles of deformation, i.e. gravity-dominated and fold-dominated models, correspond to the structures observed in the Moldanubian and the West Sudetes. Moreover, the calculated pressure–temperature paths of the felsic lower crust are in agreement with available data.