Structural evolution of the eastern Balkans (Bulgaria)

Abstract

The Balkans are an E-W trending thrust belt mainly of Palaeogene age, generated by a left-lateral transpression with maximum stress-oriented N30°E. The transpressive nature of the Balkans may be deduced from the en echelon pattern of folds and thrusts which is due to oblique shortening with respect to pre-existing Mesozoic E-W trending crustal anisotropies between the Moesia Platform and the Balkans Basin. The front of the eastern Balkans is marked by the Chudnite Skali-Ajtos thrust. 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 margin of the Balkans (e.g. the Emine Line). The steep transtensional faults disconnected the thrust planes of the Balkans from the thrusts of the Srednogorie to the south. Grabens are in general asymmetrical, with master faults dipping 80-40° south-westward and probably flattening into the middle crust. The structural evolution of the Balkans may also be traced into the Black Sea, but the structural trends gradually change from the E-W trend to a NW-SE strike, indicating pure Palaeogene 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 the pre-existing compressive fabric. The eastern Balkans and their continuation in the Black Sea are morphologically and structurally deeper and they plunge eastward and south-eastward towards the Black Sea. This is due both to the deeper position of the Balkans de´collement planes in the Black Sea and to the thermal subsidence along the Black Sea continental margin.