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T. Yegorova, Institute of Geophysics of National Academy of Sciences of Ukraine (Ukraine)
V. Gobarenko, Institute of Geophysics of National Academy of Sciences of Ukraine (Ukraine)
E. Baranova, Institute of Geophysics of National Academy of Sciences of Ukraine (Ukraine)
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The back-arc Black Sea (BS) Basin, originated at the hinterland of the Pontide Arc at the end of Cretaceous, consists of two deep sub-basins - West-Black Sea (WBS) and East-Black Sea (EBS) basins filled with thick (up to 12-14 km) Cenozoic sediments. They are separated by Mid-Black Sea Ridge (MBSR) - a NW linear stripe of basement uplift. In order to get new information on the deep structure of the study region we undertook reinterpretation of some existed profiles of deep seismic refraction study (DSS) using the ray-tracing method and local seismic tomography. Reinterpretation of the DSS data shows presence of thin (5-7 km) high-velocity oceanic crust below the WBS and EBS. The built-up of the transition from oceanic to continental domain (on northern shelf) of Scythian Platform and southern slope of the Ukrainian Shield has been studied. In western part of the BS it is associated with rifted continental margin related, most probably, to the opening of the WBS. Reinterpretation of seismic data in central part of the BS have shown structure of the MBSR, which represents a bridge of thin (29 km) continental crust separating the two oceanic domains.
Local seismic tomography has been performed using the arrival times of P-waves from the earthquakes inside and around the BS, recorded by seismic stations on the BS coast. As a result, rather heterogeneous structure of the BS lithosphere has been distinguished for the depth range 40-60 km. Upper section (40 km) demonstrates tectonic units of different crustal thickness and structure - high-velocity uppermost mantle below thin oceanic crust in the WBS and EBS, separated by the low-velocity stripe of the MBSR. The lowest P-wave velocities outline domains of thick crust and deep-rooted Moho (45-50 km) in the adjusting to the BS regions of Crimea-Caucasus coast and in Pontides. Combined velocity section, constructed for the depth range 40-200 km indicates that lithosphere of the WBS has in general higher velocities than in the EBS. Different lithospheric domains in western and eastern parts of the Black Sea are also evident in the distribution of recent seismicity. Unloading of stress accumulated within the Black Sea lithosphere occurs in two sub-basins in different way. In western part of the BS it is the North-Anatolian seismic zone, characterized by high density of strong earthquakes with hypocenters in the upper crust. Main seismic zone in the eastern part of the BS locates along the northern, Crimea-Caucasus, coast, where deeper earthquakes with hypocenters in the lower crust-upper mantle and magnitude M=4-5 occur. Revealed distinctions in the velocity structure of the crust and upper mantle of the WBS and EBS and of their seismicity character are determined, first of all, by different affinity of micro-plates, on which they were originated, by the peculiarities of their rift and post-rift history and recent geodynamic activity and kinematics of Africa-Arabia system.
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