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Regional Structure of the Western Black Sea Basin: Constraints From Cross-section Balancing

Abstract

The past few decades have seen several models proposed for the opening and evolution of the Western Black Sea Basin (WBSB). These models made assumptions about the nature of the underlying basement types and were mostly relying on gravity and magnetics data. However, just recently, a regional, long-offset 2D seismic reflection grid was acquired across the entire WBSB and these data image the entire basin down to about 30–40 km depth, for the first time. Mapping the structure and the stratigraphy of the basin on these new seismic transects provided valuable insights into the basin dynamics. About a 50–150 km wide zone (Turkish margin and Ukrainian sector, respectively) that roughly fits with the present day shallow shelf area corresponds to the un-stretched continental crust with a thickness of 35 km. Normal faults that were active in earlier tectonic phases detach at a depth of about 15–20 km, marking the brittle-ductile transition zone. In the Romanian sector we interpreted a Late Jurassic period of transtension followed by Early Cretaceous extension, both related to reactivated pre-rift basement faults. Basinward, there is a distinct segment of the margin zone with stretched continental crust and a reflection Moho located at about 20 km depth. In this stretched domain, some of the rift-related normal fault traces are in most cases clearly visible. We interpret multiple phases of normal faulting during rifting similarly to other rift basins. In this zone, the rift-related normal faults have a NE-SW strike. The width of the zone that is underlined by stretched continental is fairly uniform in the Bulgarian-Romanian-Ukrainian sector (80–110 km) and is much less on the Turkish side (30 km). In the central part of the basin we intepreted two distinct basement types. In the East, between Ukraine and Turkey, we observed a transparent, 7 km thick seismic facies unit that we interpret as oceanic crust. The zone occupied by oceanic crust has a broadly triangular shape. The centre of the basin in the Bulgaria-Turkish sector shows a different seismic facies with rotated fault blocks, fault planes and magmatic intrusions and large paleo-volcanoes. We interpret this as extremely stretched continental crust, very much akin to that described in other passive margins (i.e. offshore Iberia). We use simple area balance of the continental crust material on these cross-sections to investigate the amount of stretching and the dynamics of the opening of the WBSB.