Kinematic and rheological model of exhumation of high pressure granulites in the Variscan orogenic root: example of the Blanský les granulite, Bohemian Massif, Czech Republic |
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Authors: | J Franěk K Schulmann O Lexa |
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Institution: | (1) Centre de Géochimie de Surface, UMR CNRS 7516, Strasbourg, France;(2) Institute of Petrology and Structural Geology, Prague, Czech Republic |
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Abstract: | Summary A large-scale relict domain of granulite facies deformation fabrics has been identified within the Blansky les granulite body.
The granulite facies mylonitic fabric is discordant to the dominant amphibolite facies structures of the surrounding retrograde
granulite. The complex geometry of retrograde amphibolite facies fabric indicates a large-scale fold-like structure, which
is interpreted to be a result of either crustal-scale buckling of an already exhumed granulite sheet or active rotation of
a rigid granulite facies ellipsoidal domain in kinematic continuity with the regional amphibolite facies deformation. We argue
that both concepts allow similar restoration of the original granulite facies fabrics prior to the amphibolite facies deformation
and “folding”. The geometry of the granulite facies foliations coincides with the earliest fabrics in the nearby mid-crustal
units suggesting complete mechanical coupling between the deep lower crust and the mid-crustal levels during the vertical
movements of crustal materials. Microstructures indicate grain-size sensitive flow enhanced by the presence of silicate melts
at deep crustal levels and a beginning of an exhumation process of low viscosity granulites through a vertical channel. The
amphibolite facies fabrics developed at middle crustal levels and their microstructures indicate significant hardening of
feldspar-made rigid skeleton of the retrograde granulite. Increase in the strength of the granulite allowed an active buckling
or a rigid body rotation of the granulite sheet, which acted as a strong layer inside the weaker metasediments. |
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