Timing of deformation in the Moravosilesian Foreland Basin – new insights from detrital zircon U-Pb dating

The Moravosilesian Foreland Basin preserves information of an important interval of the evolution of the eastern European Variscan Orogen. The basin largely was deposited on the Brunovistulian microplate, which underthrusts east-vergent Moldanubian and Moravosilesian units. It hosts an up to 7.5 km thick suite of siliciclastic marine sediments, recording the unroofing of the adjacent Moldanubian zone and the Moravosilesian nappes. Moreover, the basin records polyphase, late-stage Variscan, foreland-affecting deformation occurring between 330 Ma and 310 Ma. Outcrops of the basin can be found in the Drahany upland, southeast of the Czech town of Olomouc and the Nízký-Jeseník mountains, northeast of Olomouc. We conducted zircon U-Pb dating on four detrital samples of a suite of three marine formations in the Drahany upland (Yao et al., 2024). Maximum depositional ages (MDA) of the allochtonous Protivanov (328.7 ± 1.8 Ma), and the parautochthonous Rozstání (326.1 ± 1.0 Ma) and Myslejovice (335.1 ± 2.4 to 329.8 ± 2.4 Ma) formations are coeval to the depositional age of tuff layers within the shallow marine to continental, synorogenic, coal-rich Ostrava formation of the Nízký-Jeseník mountains, which were deposited between 329.2 ± 0.5 Ma and 324.2 ± 0.5 Ma.This finding challenges the established stratigraphy of the Protivanov, Rozstání and Myslejovice formations, which were previously based on detrital fossiliferous limestone pebbles. The MDAs of all three formations suggest a Serpukhovian rather than Visean depositional age. This has strong implications on the timing of NE-verging basin folding and thrusting. It constraines late-stage Variscan deformation propagation into the foreland to a timespan between <326 Ma (youngest MDA in the Drahany upland) and 303 Ma (opening of the adjacent Boskovice graben (Nehyba et al., 2012)). This timespan postdates time estimates made for compressive tectonics in the region (e.g. Tomek et al., 2019), which proposed the termination of Brunovistulian underthrusting at ~330 Ma. Nehyba S, Roetzel R and Maštera L 2012 Geologica Carpathica 63 365–82 Tomek F, Vacek F, Žák J, Petronis M S, Verner K and Foucher M S 2019 Tectonophysics 766 379–97 Xiao Y, Rembe J, Čopjaková R, Aitchison J C, Chen Y and Zhou R 2024 Gondwana Research 128 141–