In northwestern Turkey, the pre-Eocene basement of the Thrace Basin consists of the Strandja-Rhodope metamorphics in the north/northwest, the A degrees stanbul-Zonguldak Paleozoic sequence in the northeast, a tectonic mixture of the fragments from the Sakarya Continent and pre-Upper Cretaceous ophiolites in the southeast, and the uppermost Cretaceous-Paleocene transgressive sediments. The ophiolites belong to a Dogger-Early Cretaceous oceanic basin (the Northeastern Vardar Ocean) between the Rhodope and Sakarya Continents. The NE-trending oceanic branch closed diachronously during the early Late Malm to the northeast but during latest Early Cretaceous to Late Cretaceous time toward the main Vardar Ocean in the southwest. During the final closure, the suture zone acted as a strike-slip fault zone. The latest Cretaceous-Paleocene transpressional to transtensional activities caused the juxtaposition of different basement types and the development of a tectonic mixed zone (the Biga-Armutlu-Ovacik Zone) between the Rhodope and Sakarya Continents. Under the regional N-S compression, on the plate overlying the north/northeastward-dipping Vardar subduction zone, the southwestward escape of an area of crust bounding by the WNW-trending Balkan-Thrace Dextral Fault Zone to the north and the NE-trending Biga-Armutlu-Ovacik Sinistral Zone to the southeast caused a triangular-shaped and isolated depression area for the deposition of the uppermost Cretaceous-Paleocene transgressive sediments in a transtensional setting. The tectonically bounded depression area constitutes also a necessary accommodation space for the deposition of the initial deposits of Early Eocene age in the Thrace Basin.