Abstract The geological understanding of the opening of the Western Black Sea Basin appears to be quite far from being reasonably resolved. The main faults used in the existing map-view reconstruction schemes are either very poorly defined (West Black Sea fault) or simply nonexistent as interpreted earlier (West Crimean fault) and therefore they need be redefined or replaced by other structural elements. Various kinematic elements and facies boundaries on the conjugate margins of the Western Black Sea ( i.e. , the Bulgarian, Romanian and Ukrainian margin in the northwest versus the Turkish margin in the southeast) appear to be a key in constraining the opening geometry of the basin. The along-strike changes in the synrift structural pattern of the Bulgarian-Romanian margin, reflecting contrasting crustal rheologies inherited from prerift deformational phases, do appear to have their counterparts in the offshore part of the conjugate Turkish margin including the Pontides. A correlation of regional 2D reflection seismic and well data, and the critical review of the relevant onshore geology did provide some preliminary corresponding tie-points to constrain the kinematics of the basin opening. If the European margin is fixed in a kinematic reconstruction, the clockwise opening of the rift basin occurred along northwest–southeast trending transform faults around an Euler rotation pole positioned to the southwest of the present Black Sea. The rotational element in the opening of the Western Black Sea Basin, as opposed to the dominantly translational kinematics used in some of the existing kinematic models, is also supported by the broadly triangular shape of oceanic crust imaged in the basin center.

Abstract A regional, long-offset 2D reflection seismic grid that images the basin to a depth of ~30–40 km has been studied across the entire Western Black Sea Basin (WBSB). Mapping the structure and the stratigraphy of the basin on these transects provides valuable insights into the basin dynamics. An approximately 50–150 km wide zone (Turkish margin and Ukrainian sector, respectively) that roughly agrees with the present-day shallow shelf area corresponds to unstretched continental crust having a thickness of 35 km. Normal faults detach at a depth of about 15–20 km, marking the brittle-ductile transition zone. Some of the rift-related normal faults can be shown to be a re-activation of the older structural grain. Basinward, there is a distinct segment of the margin consisting of stretched continental crust and the interpreted Moho reflection located at about 20 km. The width of this zone is fairly uniform in the Bulgarian-Romanian-Ukrainian sector (80–110 km) but is much less on the Turkish side (30 km). In the central part of the basin, we interpret two distinct basement types. In the East, between the Ukraine and Turkey, there is a transparent 7 km thick seismic facies interpreted to consist of oceanic crust. The zone occupied by this crust has a broadly triangular shape. The center of the basin in the Bulgaria-Turkish sector shows a strikingly different seismic facies: rotated fault blocks, fault planes, magmatic intrusions, and large paleo-volcanoes representing extremely stretched continental crust, very much akin to that described in other passive margins ( i.e. , offshore Iberia). Assuming plane strain deformation and constant crustal area on the 2D lines during and after the rifting, we calculate approximately 250 km of extension in the eastern part of the WBSB, and progressively smaller values to the west; i.e. , ~110 km at the westernmost seismic line. High extension values also correlate well with the position of the oceanic crust. This systematic variation in stretching values and crustal types is best explained by assuming clockwise rotation of Turkey away from the conjugate margins on the northwest.