STRUCTURAL EVOLUTION OF THE EASTERN BALKANS (BULGARIA)

The Balkans are an E-W trending thrust belt mainly of Palaeogene age, generated by a left-lateral transpression with maximum stress-oriented N30 degrees E. The transpressive nature of the Balkans may be deduced from the en echelon pattern of folds and thrusts which is due to obli que shortening with respect to pre-existing Mesozoic E-W trending crus tal anisotropies between the Moesia Platform and the Balkans Basin. Th e front of the eastern Balkans is marked by the Chudnite Skali-Ajtos t hrust. During the Neogene and Quaternary the Balkans were inverted by right-lateral transtensional tectonics which cross-cut and dismembered the thrust belt. The main transtensional segment is the southern marg in of the Balkans (e.g. the Emine Line). The steep transtensional faul ts disconnected the thrust planes of the Balkans from the thrusts of t he Srednogorie to the south Grabens are in general asymmetrical, with master faults dipping 80-40 degrees south-westward and probably flatte ning into the middle crust. The structural evolution of the Balkans ma y also be traced into the Black Sea, but the structural trends gradual ly change from the E-W trend to a NW-SE strike, indicating pure Palaeo gene compression and more recent Neogene extension inverting and cross -cutting the compressive features. It is also interesting to note that the Neogene tensional or transtensional Inversion Is not cylindrical; it exhibits undulations of graben and transfer faults controlled by t he pre-existing compressive fabric. The eastern Balkans and their cont inuation in the Black Sea are morphologically and structurally deeper and they plunge eastward and south-eastward towards the Black Sea, Thi s is due both to the deeper position of the Balkans decollement planes in the Black Sea and to the thermal subsidence along the Black Sea co ntinental margin.