New evidence for Alpine overprint of poly-deformed gneisses in the ‘Median Dacides’ of SE Europe: restoring polyphase deformations and transposition cycles

The outcrop and microscale structural constraints on metamorphosed fragments of Carpathian-Balkan exotic north Gondwanan continental crust show sequential recurrent peripheral subduction processes which produced a series of collisional orogens: (i) Variscan suturing of ‘Median Dacides’ (western Danubian/Moesian realm), and (ii) post-dating polystage Alpine overprinting. The ‘Median Dacides’ comprise the two gneiss-bearing amphibolite-facies, Serbo-Macedonian and Getic/Kučaj basements, embedded into Southern Carpathians (Romania) and Carpathian-Balkans (Serbia, North Macedonia, Bulgaria). To unravel polyphase deformation processes, we have incorporated the data available on Neoproterozoic-Lower Palaeozoic protoliths and tectonometamorphic events, coupling these with new extensive structural field-based analysis. The deformation patterns expose polystage deformation history (D1-2), fashioned by the youngest Alpine overprinting (D3-4). The structural data and protolith analyses suggest a Lower Palaeozoic Cadomian-to-Cenerian geodynamic linkage between the two gneissic basements. Notably, the pre-Variscan anatexis, migmatitization, metamorphism (D1-1), and the formation of Cenerian (axial planar) foliation (D1-2) are precursors of the tight isocline Variscan folds (D2-1). Progressive deformation of the initial Variscan structural fabric (metre-scale folds) produced complete transposition (D2-2) with scarcely preserved metre-scale folds. The successor Alpine shortening (D3) (re)activated the existing remnants of Variscan axial plane cleavages (or overlapped foliation-to-cleavage planes), further producing schistosity and folding pattern preserved due to the partial transposition.