THE STRANDJA MASSIF AND THE ISTANBUL ZONE WERE ONCE PARTS OF THE SAME PALAEOTECTONIC UNIT: NEW DATA FROM TRIASSIC DETRITAL ZIRCONS

Spatially continuous rock assemblages that share similar environmental evolution or structural features can be classified as a single tectonic unit. This approach enables us to link dispersed units or massifs with each other and sometimes can be subjective, depending on the classification criteria. The relationship between the Strandja Massif and the İstanbul Zone and the nature and place of their mutual contact have been controversial due to the Cainozoic cover. The timing of amalgamation of the Strandja Massif and the İstanbul Zone was claimed as early as Aptian-Albian.

Lower Triassic sedimentary rocks, which are overlain by the Carboniferous flysch with a thrust fault exposed in the NW of the İstanbul Zone. This study reveals the spatial relationship between the Strandja Massif and the İstanbul Zone deduced from the U-Pb dating and Lu-Hf isotopes of the detrital zircons from the early Triassic clastics. Our results show that the early Triassic basin was fed from a source area that included arc-related Upper Carboniferous-Lower Permian magmatic rocks. These features of the provenance is much more similar to the Strandja Massif than to the İstanbul Zone. In the light of these findings, the early Triassic rocks can be assigned to the Strandja Massif and the boundary between the Strandja Massif and the İstanbul Zone is thus a N-verging thrust fault. The second outcome of this study is that a unit that previously assigned to Paleozoic turned out to be Triassic, which brings some parts of the Strandja massif further to the east, in the north of the Istanbul Zone.