Several depocentres have developed along the NNW–SSE-striking Strouma (Strymon) Lineament since the Miocene, forming the Strouma/Strymon graben system. Some of these basins, including the Dzherman and Simitli, strike at high angles to the lineament. The rhomboid-shaped Simitli basin is limited in the south by the NE–SW-striking, seismically active Kroupnik fault and contains terrestrial clastic sediments, which have been deposited since the Sarmatian. Its formation is attributed to ‘wrench-dominated transtension’ (DB event) which activated the basin's NE–SW-striking boundary faults as left-lateral strike-slip structures during the Early–Middle Miocene. This deformation was associated with NNE–SSW contraction and WNW–ESE extension and succeeded the NNE–SSW contraction caused by ‘pure shear-dominated transpression’ (DA event), which governed the region in Late Oligocene–Early Miocene times. During the Middle–Late Miocene, the DB event was followed by WNW–ESE ‘pure shear-dominated extension’ (D1 event), which caused further widening of the basin. The transtensional origin of the Simitli basin is explained by lateral extrusion of the crustal mass squeezed between the Apulian–Adriatic microplate and the European foreland towards the North Aegean Sea. The subduction retreat of the Hellenic orogen possibly enhanced this lateral extrusion, and from the Late Miocene onwards the NE–SW-oriented back-arc extension balanced the continuing retreat.

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