Seismic velocity structure across the Fennoscandia–Sarmatia suture of the East European Craton beneath the EUROBRIDGE profile through Lithuania and Belarus

Abstract

During EUROBRIDGE'96 seismic data were acquired along a 544-km NW–SE profile, from the East Lithuanian Belt (EL) to the Ukrainian Shield. Explosive sources from 16 shotpoints at 30-km intervals were recorded by 114 three-component seismographs deployed at 3–4km intervals along the profile. Tomographic inversion and raytrace modelling, integrated with results from the EUROBRIDGE'95 experiment, established a two-dimensional P-wave velocity lithospheric model and the spatial variation of Vp/Vs. Sedimentary cover in Belarus consists of two principal layers with P-wave velocities of about 2.3 and 4.0km/s. Upper, middle and lower crystalline crust exhibit velocities of 6.1–6.3, 6.4–6.8 and 6.9–7.2km/s, and are characterised by low velocity gradients and small contrasts at boundaries. The crust below Belarus is about 50km thick with Moho elevations of a few kilometres. Mantle P-wave velocities immediately beneath the Moho are generally 8.2–8.4km/s. A lower lithosphere reflector occurs at 65–70km depth. S-wave velocities are high in the upper crust and low in the lower crust. Our crustal model shows similarities to results from Scandinavia. High lower-crustal velocities and a crustal thickness of about 50km (common features of Proterozoic crust) are observed throughout the EUROBRIDGE'96 profile. The boundary between the EL and West Lithuanian Granulite Domain (WLG) is associated with pronounced crustal velocity changes, and a thinning of crust towards the northwest. The WLG may be part of a larger southern Baltic Sea tectonic unit. Correlation of our seismic structure with near-surface geology tentatively suggests that contact zones between the EL, Belarus–Baltic Granulite Domain, Central Belarussian Belt (CB), and the Osnitsk–Mikashevichi Igneous Belt all dip slightly to the southeast, consistent with successive docking of these terranes during craton growth. A spectacular feature of our model is high velocities throughout the CB crust, which marks the Fennoscandia–Sarmatia suture. Here we observe a change from typical shield/platform crust in the northwest to highly heterogeneous crust with pronounced lower crustal reflectivity in the southeast. Our results are consistent with CB uplift during continental collision. Our model implies significant tectonic involvement of middle crust in the formation of the Pripyat Trough.