The tectonics of the Strandja Mssif: Late-Variscan and mid-Mesozoic deformation and metamorphism in the Northern Aegean

The Strandja Massif is a mid-Mesozoic orogenic belt in the Balkans build on a late-Variscan basement of gneisses, migmatites and granites. New single-zircon evaporation ages from the gneisses and granites indicate that the high-grade metamorphism and plutonism is Early Permian in age “Ο271 Ma”. The late- Variscan basement was unconformably overlain by a continental to shallow marine sequence of Early Triassic-Mid-Jurassic age. During the Late Jurassic-Early Cretaceous “Oxfordian-Barremian” the lower Mesozoic cover and the basement were penetratively deformed and regionally metamorphosed in greenschist facies possibly due to a continental collision. An Rb-Sr biotite whole-rock age from a metagranite dates the regional metamorphism as Late Jurassic “155 Ma”. Deformation involved north-vergent thrust imbrication of the basement and the emplacement of allochthonous deep marine Triassic series over the Jurassic metasediments. The metamorphic rocks of the Strandja Massif are unconformably overlain by the Cenomanian shallow marine sandstones. During the Senonian, the northern half of the Strandja Massif formed a basement to an intra-arc basin and to a magmatic arc generated above the northward-subducting Tethyan oceanic lithosphere. The Srednogorie arc closed during the early Tertiary through renewed northward thrusting of the Strandja Massif.