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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.

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