Tectonic evolution of the Kraishte area (southwestern Bulgaria and southeastern Serbia) - an overview

In the Kraishte area, straddling the borders of Bulgaria, Serbia and North Macedonia, two major Alpine tectonic units are exposed – Vlasina-Morava and Struma. They exhibit quite differing geological evolution until the late Early Cretaceous compressional phase during which the Vlasina-Morava (Supragetic) was thrust onto the Struma (Getic) unit. The Vlasina-Morava Unit consists of Ediacaran-early Cambrian volcano-sedimentary sequence intruded by Cadomian age granites and covered by early Paleozoic marine sediments. The Struma Unit is built of tectonically imbricated Ediacaran–early Cambrian calc-alkaline magmatic rocks and ophiolites covered by Permian to Early Cretaceous sedimentary sequences. Part of the Ograzhden complex, consisting of high-grade ortho- and para-sequences of Ediacaran to Silurian age, is also considered as belonging to Struma autochthon. Post late Early Cretaceous orogenic extension was related to formation of low-angle normal faults associated with partial cooling and denudation in the area and volcanic-free sedimentation. The following Late Cretaceous to Paleogene compressional phase did not affect significantly the Kraishte area. Since the middle Eocene the area was affected by important crustal scale extension related to the formation of detachment faults and sedimentary basins characterised by continental to marine deposits and accompanied by magmatic activity. The following extensional phases, in the late Oligocene–earliest Miocene and middle Miocene–Quaternary time, were also related to faulting and formation of sets of sedimentary basins. These two phases were separated by a late early to middle Miocene strike-slip to transpressional phase characterised by formation of some minor thrust faults associated with major strike-slip fault zones.