Northwestern Anatolia contains three main tectonic units: (a) the Pontide Zone in the north which consists mainly of the Istanbul-Zonguldak Unit in the west and the Balhdag-Kure Unit in the east; (b) the Sakarya Zone (or Continent) in the south, which is juxtaposed against the Pontide Zone due to the closure of Paleo-Tethys prior to Late Jurassic time; and. (c) the Armutlu-Ovacik Zone which appears to represent a tectonic mixture of both zones. These three major tectonic zones are presently bounded by the two branches of the North Anatolian Transform Fault. The two tectonic contacts follow older tectonic lineaments (the Western Pontide Fault) which formed during the development of the Armutlu-Ovacik Zone. Since the earliest Cretaceous, an overall extensional regime dominated the region. A transpressional tectonic regime of Coniacian/Santonian to Campanian age caused the welding of the Istanbul-Zonguldak Unit to the Sakarya Zone by an oblique collision. In the Late Campanian, a transtensional tectonic regime developed, forming a new basin within the amalgamated tectonic mosaic. The different tectonic regimes in the region were caused by activity of the Western Pontide Fault. Most of the ophiolites within the Armutlu-Ovacik Zone belong to the Paleo-Tethyan and/or pre-Ordovician ophiolitic core of the Istanbul-Zonguldak Unit. The Late Cretaceous ophiolites in the eastern parts of the Armutlu-Ovacik Zone were transported from Neo-Tethyan ophiolites farther east by left-lateral strike-slip faults along the Western Pontide Fault. There is insufficient evidence to indicate the presence of an ocean (Intra-Pontide Ocean) between the Istanbul-Zonguldak Unit and the Sakarya Zone during Late Cretaceous time.