Lithosphere structure of the Black Sea from 3-D gravity analysis and seismic tomography

Abstract

The backarc Black Sea (BS) basin was formed in the Late Cretaceous-Palaeocene at the hinterland of the Pontide magmatic arc due to subduction of the Neotethys ocean below the southern Eurasian continental margin. At present the BS consists of two large depressions—the West- and East-Black Sea basins (WBS and EBS) filled with thick (up to 12 km) Cretaceous and younger sediments and underlain by a crust of oceanic/suboceanic type. The sediments mask poorly investigated crystalline crust that is thought to comprise an accretional collage of microplates and terranes of different affinities. To investigate the lithospheric structure of the BS we performed a 3-D gravity analysis and local seismic tomography study. 3-D gravity backstripping analysis allowed us to separate the gravity signal from different parts of the crustal model and then, by subtracting the crustal effect from the observed field, to obtain gravity anomalies of presumed mantle origin only. The broad positive long wavelength component of this might be indicative of good isostatic equilibrium of the deep structure of the Black Sea, that is, that the negative gravity effect of sediments is almost totally compensated by the strong positive gravity impact of Moho shallowing. Velocity structure of the BS lithosphere has been studied by P-wave local seismic tomography. It uses the traveltimes of the earthquakes occurring inside the study region and recorded by permanent seismic stations around the BS. Initial data were corrected for the effect of the crust. The resulting model shows the BS lithosphere as being rather heterogeneous with two domains of increased velocity in its western and eastern parts. The gravity analysis and seismic tomography approaches were integrated by calculating the upper-mantle gravity effect of the tomography model and comparing this to the mantle gravity signature inferred from the gravity analysis itself. The integrated results suggest the presence of rheologically strong and cold continental lithosphere below the BS, similar to Precambrian lithosphere of the East European Platform.