Crustal structure from P and S seismic waves and petrological models of the Ukrainian shield

Abstract

Presented are the results of reinterpretation of data from deep seismic sounding profiles VIII and XXIV, located in the central part of the Ukrainian shield. Seismic models of the crustal structure were determined by 1-D and 2-D modelling using refracted and reflected P and S waves. 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, underlain by a 4–9 km thick low-velocity layer. The boundary below the low-velocity layer is situated at a depth of 10–16 km, and characterized by a velocity of 6.25–6.4 km/s, which increase towards the lower crust reaching up to 6.8–7.3 km/s with a gradient of 0.015–0.025 s −1. The Moho 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 heterogeneous and differentiated. Apart from a distinct division into the upper and lower crust, crustal blocks are observed. The deep fracture zones correlate on the surface with faults which are well known from geology. A large range in the velocity ratio k = ν P ν 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 value of the S-wave velocity with k reaching 1.8. Petrologically, we interpret the uppermost part of the crust as represented by rocks cropping out at the surface — migmatites, granitoids, gneisses and granites. The middle and lower parts of the crust consist of enderbites and metamorphic rocks of granulite facies (charnockites). The rocks of the lower crust with velocities of 7.2 km/s and higher are interpreted as a subfacies of eclogitelike rocks.