Tectonic Evolution Models for the Black Sea
The Black Sea is a 423,000-km2 (163,000-mi2)-large Cretaceous-Tertiary basin surrounded by Alpine fold belts. It consists mainly of two large subbasins separated by the northwest-southeast-trending mid-Black Sea ridge (Figure 1). The west Black Sea basin is floored by oceanic crust overlain by more than 3-km (1.8-mi)-thick flat-lying sediments probably of Cretaceous and younger age (Letouzey et al., 1977; Finetti et al., 1988; Okay et al., 1994; Robinson, 1996). Thenorthwest-trending east Black Sea basin has a thinned continental or oceanic crust overlain by less than 10-km (6-mi)-thick sediments, which are intersected by a large number of faults. It is generally accepted that the Black Sea opened during the Mesozoic as a back-arc basin above the northward-subducting Tethyan oceanic lithosphere (e.g., Bocceletti et al., 1974; Şengör and Yilmaz, 1981).
A kinematic model for the opening of the Black Sea, based largely on data from onshore areas, was suggested in 1994 by Okay et al. The model involved separate mechanisms for the origin of the west and east Black Sea basins. The west Black Sea basin was believed to have opened by orthogonal rifting of a continental fragment from the odessa shelf starting in the Albanian-Cenomonian (Okay et al., 1994). This continental fragment, called the Istanbul zone (Figure 1), drifted south, bounded by two major strike-slip faults, opening the oceanic west Black Sea basin in the north and closing the Tethyan Ocean in the south (Figure 2). During the early Eocene, the Istanbul zone collided with the Sakarya zone in the south, thereby causing a change-over from extension to compression in the Black Sea. Okay et al. (1994) suggested that the eastern half of the Black Sea, including the east Black Sea basin, mid-Black Sea ridge, and the easternmost part of the west Black Sea basin (Figure 1), opened through the anticlockwise rotation of a large continental block around a pole situated in Crimea (Figure 2). Such a mode of opening explained the Tertiary compression in the Caucasus, which diminishes northwestward toward the pole of rotation, as well as the segmented southern boundary of the eastern Black Sea. The rotation was believed to have been contemporaneous with the rifting in the west Black Sea basin (Figure 2).