How many extensional stages marked the Variscan gravitational collapse in the Bohemian Massif?

Ondřej Bárta, Rostislav Melichar, Jan Černý


Tectonic development of the Variscan belt in Central Europe included, besides important compression, also an extensional phase related to gravitational collapse, which governed the origin of many sedimentary basins and magmatic bodies. One of these bodies is the Benešov pluton, featuring primary magmatic fabrics as well as deformational fabrics, related to subsequent extensional stages. Recognition of these fabrics and their links to other significant extension-induced structures in the Bohemicum and Moldanubicum not only sheds new light on the pluton itself but also extends a general knowledge of deformational stages, accompanying gravitational collapse of the Variscan orogen. The authors found that this pluton was strongly strained in a normal-faulting regime under brittle-ductile conditions. The age of deformation is constrained by a magmatic age of 347 ±3 Ma and by the age of Carboniferous sedimentary cover. New data indicate a three-stage extensional history during the phase of gravitational collapse: (1) Tournaisian extension (~350–345 Ma) within arc-related tonalitic intrusions; (2) late Viséan to Serpukhovian extension (~332–320 Ma), connected to the brittle-ductile unroofing and origin of a NE–SW basin system; and (3) Gzhelian to Cisuralian extension (~303–280 Ma), related to normal faulting and sedimentation in “Permo–Carboniferous” troughs, elongated NNE–SSW. Consequently, the gravitational collapse studied involved a complex succession of individual extensional stages, rather than a simple process.

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