Amashukeli T. The structure of the lithosphere of the south-western margin of the East European Platform according to the wide-angle deep seismic soundings profiles

The thesis is performed to obtain the deep structure of the lithosphere of the south-western margin of the Eastern European Platform on the basis of experimental seismic data obtained from wide-angle reflection and refraction RomUkrSeis profile. The results, based on techniques of controlled-source WARR seismic data acquisition and interpretation, add new constraints to the existing geophysical data set of the area and have contributed to a better understanding of the structure of the lithosphere of the south-western margin of EEP. The seismic data modelling was done by trial-and-error using the 2-D ray-tracing SEIS83 package with the graphical user interfaces MODEL and ZPLOT. P-wave velocity model was prepared using existing geological and geophysical information about the structure of the uppermost crust. According to velocity model was prepared the interpretation model of the RomUkrSeis profile. The most striking element of the velocity model along the profile on the margin of the EEP is a sedimentary basin consisting of two velocity layers: the upper layer – 5 km deep with P-wave velocities of ~ 4.9 km/s takes part in the Carpathian orogeny; the underlying layer – 15 km deep with P-wave velocities of ~ 5.35 km/s represents Paleozoic-Mesozoic sediments; this sedimentary basin has a width of ~ 40 km and dips at about 40 degrees to the southwest. Similar sedimentary basins, but with different width and depth, are observed on other WARR profiles, which cross the south-western margin of the EEP. The deep nature of the Fore-Carpathian Fault (the most pronounced fault of the south-western margin of the EEP) is substantiated on the RomUkrSeis and PANCAKE profiles and its spatial connection with the structure of the Moho is established. The south-western margin of the EEP on the RomUkrSeis velocity profile exhibits reduced P-wave velocities (from 6.2 to 6.3 km/s at a depth of 10 to 40 km), which are observed under the sedimentary basin at the all thickness of the crust. At km 360, Vp 6.3 km/s, occurs immediately above the Moho. The 6.20 km/s isoline is also significantly deepened on south-western margin of the EEP. Such reduced P-wave velocities and changes of geophysical fields in this area may indicate that the crystalline crust has been extended here. The geometry of the Moho within south-western margin of the EEP of the profile is profoundly variable, displaying a narrow keel dipping northeast on its southwestern side and almost vertical on its northeastern side. The maximum Moho depth at the apex of this keel is about 50 km and is the greatest Moho depth observed along the RomUkrSeis profile as a whole. The keel structure on the Moho (up to a depth of 50 km) in the area between the Inner and Outer Carpathians divides the crust of different thickness: 32–35 km under the younger area to the south-west and up to 42 km below the platform to the north-east. An upper mantle reflector is observed at 50–55 km depth and change in mantle thickness is observed on this part of profile too. There are a number of common features for velocity models crossing south-western margin of the EEP: sedimentary basins of different ages, depths and widths, located on the margin of the EEP; generally low velocity crust; significant changes in Moho depth beneath the margin of the EEP; profoundly variable geometry of the Moho (a keel structure on the Moho boundary on some profiles, in particular on the RomUkrSeis profile). The structure of the crust and upper mantle revealed along the RomUkrSeis profile echoes many disparate geodynamic processes involved in its formation and deformation of this part of the EEP from the Archaean through to the present-day. It is characterized by significant lateral heterogeneity, as well as by complex Moho topography. There is a significant thickening of the lithosphere in the transition zone from the Tisza-Dacia and Alcapa to EEP: from 70 km along RomUkrSeis under the Transylvanian Basin and from 60 km along PANCAKE under the Pannonian Basin to 200 km below the platform. In this transition zone, which is associated with TTZ, changes are also observed in geophysical fields: strongly negative gravity anomaly (less than –120 mGal); transition from quiescent to more differentiated magnetic field; reduction of heat flow towards EEP. Key words: lithosphere, Earth's crust, mantle, Eastern European Platform, wide-angle reflection and refraction profile, seismic velocity.