The Mesta half-graben is one in a series of extensional basins in SW Bulgaria that record the onset of extension within the Rhodope Zone in the Late Eocene. Tectonic activity on a continuous detachment along the eastern margin was a major control on subsidence, accommodation space creation, sediment supply and facies distribution in the basin. The sedimentary architecture was complicated by synsedimentary rotation, the presence of intrabasinal faults and the resulting compartmentalization, as well as synsedimentary volcanic activity. Facies and structural analysis of a key transverse section in the central part of the basin, together with supporting observations from other parts of the basin, indicate a pulsed tectono-sedimentary evolution of the basin with three distinct stages. The first stage (Late Eocene) is a phase of rapid extension with an initial alluvial setting. Basin margin fans and an axial fluvial through-drainage system were the major depositional systems in this stage. The second stage (Early Oligocene) marks the onset of volcanic activity within the Mesta Basin and is characterized by the formation of volcanic centres, an intense phase of explosive volcanism and rapid infilling of the previous basin topography with volcanic material deposited from pyroclastic density currents. The third stage (Late Oligocene) represents waning volcanic activity in a mixed alluvial–volcaniclastic environment. This stage is characterized by alternating alluvial and volcaniclastic depositional cycles, as well as partial reworking of volcanic material.

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