The Alpine evolution of the Ograzhden unit (Serbo-Macedonian Massif, SW Bulgaria)

Available ages and pressure-temperature estimates of the rocks from the Ograzhden unit of the Serbo-Macedonian Massif (SMM) in Bulgaria indicate a Variscan ultra-high-pressure metamorphic event at ~334 Ma (Trapp et al., 2021, Terra Nova 33, 174-183). The bulk of the Ograzhden unit are Variscan migmatites. Cretaceous amphibolite-facies metamorphism and deformation were reported from the Vertiskos unit in the Greek part of the SMM (Kilias et al., 1999, Int. J. Earth Sci. 88, 513-531) whereas the Ograzhden unit is considered to be largely unaffected by Alpine orogeny (Kounov & Gerdjikov, 2024, Geologica Balcanica 53, 29-85).

We present new structural data from the Ograzhden unit collected along a profile through the Triassic Igralishte pluton, documenting that along with at least two Variscan high-grade deformation events a substantial Alpine upper greenschist to lower amphibolite-facies overprint is present in the Ograzhden unit. Alpine ductile deformation is inhomogeneous and localised along several high-strain shear zones. This is in line with interpretations that the Ograzhden unit, and the SMM in general, forms part of the high-grade Rhodope Metamorphic Complex, although the Ograzhden unit is situated within the hanging wall of the extensional system that exhumed the Rhodopes in the Late Eocene to Neogene time.