The Cretaceous-Tertiary (K-T) transition in eastern Bulgaria (Bjala) was analyzed in terms of lithology, mineralogy, stable isotopes, trace elements, and planktic foraminifera. The sequence represents a boreal-Tethyan transitional setting, spans from the last 300 k.y. of the Maastrichtian (zone CF1) through the early Danian (zones P0-Plc), and contains several short hiatuses. It differs from low-latitude Tethyan sequences primarily by lower diversity assemblages, pre-K-T faunal changes, a reduced K-T d 13 C shift, and the presence of two clay layers with platinum group element anomalies. The first clay layer marks the K-T boundary impact event, as indicated by an iridium anomaly (6.1 ppb), the mass extinction of tropical and subtropical planktic foraminifera, and cooling. The second clay layer is stratigraphically within the upper Parvularugoglobigerina eugubina (Pla) zone and contains a small Ir enrichment (0.22 ppb), a major Pd enrichment (1.34 ppb), and anomalies in Ru (0.30 ppb) and Rh (0.13 ppb) that suggest a volcanic source.

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Brittle tectonics analysis and stress tensor reconstructions allow us to better depict the Mesozoic and Cenozoic geodynamical evolution of the Eastern Balkanides which is characterized by a series of overimposed basin-systems. The Late Permian–Triassic corresponds to a wide carbonate platform with local embryonic troughs. During the Jurassic–Early Cretaceous period, the area, limited by regional unconformities, was at first dominated by the existence of a deep basin until the latest middle Jurassic, then by its gradual closure during the Late Jurassic–Early Cretaceous. Traces of these tectonic events are found as a result of brittle tectonic data analysis, especially in the Strandja Zone where NE–SW extension and ENE–WSW compression may be related to the Early Alpine phases of rifting and shortening, respectively. The Late Cretaceous–middle Eocene period was governed by the back-arc basin/island-arc system of East Balkan/Srednogorie zones. The inversion of these extensional zones occurred after the Maastrichtian, then important shearing and thrusting developed during the major shortening in the latest Middle Eocene. These Late Alpine tectonic phases were well characterized by brittle tectonics and the analysis of faulting in terms of stress tensors allows us to identify the main directions of extension of the rifting stage which is north–south to NNE–SSW, and the directions of compression of the Laramian and Illyrian phases, which are NE–SW and north–south, respectively. The relationships between folding and reverse and strike–slip faults are established as well as the occurrence of post-folding normal faults assigned to collapse process following the shortening.

The Balkan Fold-Thrust Belt is a part of the northern branch of the Alpine-Himalayan orogen in the Balkan Peninsula and represents a Tertiary structure developed along the southern margin of the Moesian Platform. The thrust belt displays of two clearly distinct parts: an eastern one dominated exclusively by thin-skinned thrusting and a western part showing ubiquitous basement involvement. A wide transitional zone is locked between both parts where the structural style is dominantly thin-skinned, but with significant pre-Mesozoic basement involvement in the more internal parts. For the western thick-skinned part the poorly developed syn-orogenic flysch is a characteristic feature that along with the very restricted development of foreland basin suggests a rather limited orogenic shortening compared to the eastern part of the belt. The Tertiary Balkan Fold-Thrust Belt originated mainly through a basement-driven shortening and this is explained by the occurrence of compatibly oriented reactivated basement weak zones of pre-Carboniferous, Jurassic and Early Cretaceous ages. The proposed re-definition of the Balkan thrusts system and internal structure of the allochthons also call for significant re-assessment of the existing schemes of tectonic subdivision.

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