CRUSTAL STRUCTURE FROM P-SEISMIC-WAVE AND S-SEISMIC-WAVE AND PETROLOGICAL MODELS OF THE UKRAINIAN SHIELD

Presented are the results of reinterpretation of data from deep seismi c sounding profiles VIII and XXIV, located in the central part of the Ukrainian shield. Seismic models of the crustal structure were determi ned by 1-D and 2-D modelling using refracted and reflected P and S wav es. Velocities of the uppermost crystalline basement vary between 5.8- 6.0 km/s at the surface and 6.3-6.4 km/s at a depth of 5-8 km, underla in by a 4-9 km thick low-velocity layer. The boundary below the low-ve locity layer is situated at a depth of 10-16 km, and characterized by a velocity of 6.25-6.4 km/s, which increases towards the lower crust r eaching up to 6.8-7.3 km/s with a gradient of 0.015-0.025 s(-1). The M oho depth varies greatly, from 30 to 56 km and the velocity below Moho is 8.1-8.35 km/s. These results show that the crustal structure is he terogeneous and differentiated. Apart from a distinct division into th e upper and lower crust, crustal blocks are observed. The deep fractur e zones correlate on the surface with faults which are well known from geology. A large range in the velocity ratio k = upsilon(P)/upsilon(S ) is observed. In the upper crust k is usually from 1.65 to 1.71. The lower crust is characterized by a low gradient and a relatively low va lue of the S-wave velocity with k reaching 1.8. Petrologically, we int erpret the uppermost part of the crust as represented by rocks croppin g out at the surface - migmatites, granitoids, gneisses and granites. The middle and lower parts of the crust consist of enderbites and meta morphic rocks of granulite facies (chamockites). The rocks of the lowe r crust with velocities of 7.2 km/s and higher are interpreted as a su bfacies of eclogitelike rocks.