Seismic model of the Earth's crust and upper mantle for the east European platform

Abstract

A model is presented of the P-wave velocity distribution for the east European platform from the sedimentary layer through the crystalline complex of the Earth 's crust, the lower lithosphere and the asthenosphere, down to a depth of 900 km in the upper mantle. The structure of the Earth 's crust for the east European platform is complex and heterogeneous. The crystalline basement usually lies at a depth of 2 -3 kin, outcropping in the area of the Baltic and Ukrainian shields. A characteristic of the marginal zones of the Precambrian Platform is the existence of large sedimentary basins, where the thickness of the sediments exceeds 10 km (e.g. P o l i s h G e r m a n D a n i s h ' trough, Donbass basin, Pre-Caspian foredeep, Pechora basin and Barents Sea depression. For the sedimentary cover we have determined the dependences of the velocities on depth from the results obtained from 261 boreholes in the Polish part of the east European platform. In the sediments, a distinct increase in velocity with increasing depth is observed and there is also a large velocity differentiation, from about 1.8 km s for the Tertiary and Quaternary to about 5.0-5.5 km s a for Permian, Devonian and Cambrian formations. The j ump in velocity at the boundary of the Cenozoic-Mesozoic cover and the Permian exceeds 1.0 km s i , and consequently it is the most distinct seismic discontinuity in the sedimentary layer. For the Cenozoic-Mesozoic cover and for the whole sedimentary complex, we determined the dependences of the mean velocities on thickness. To determine the dependence of the mean velocity on the thickness of the whole sedimentary layer, we used data from 71 boreholes which reached the crystalline basement. For these data, up to a depth of 5.6 km we obtained the linear dependence Vsed(z) = 2.13 + 0.34 z. For a greater depth, for which there are no data from direct measurements in boreholes, we used the results of interpretation and modelling of shallow refraction seismic data. With the formula obtained from these data, Vsed(z) = 2.35~/1 + 0.227 z , we can estimate the mean velocity of sediments for thicknesses up to 10 km. In the Polish part of the east European platform the thickness of the Earth 's crust is 42-47 kin, and the boundary velocity of Pn waves at the Moho is 8.1-8.3 km s -1. The crystalline complex of the crust consists of three parts, with velocities of 6.1-6.3, 6.7-6.9 and 7.2-7.3 km s -a (Grad, 1986). In the other regions of the platform, the Earth 's crust thickness varies over a wide range from 35 to 55 km. The mean crust thickness is 40-45 kin. In the crystalline complex we can distinguish three different parts in the layer velocity and the velocity gradient. The first, upper part is characterized by velocities of 5.8-6.4 km s -a , corresponding to acid rocks. In this part there are zones of low velocities, horizontal heterogeneities and also fractures and tectonic disturbances. The second, transition part, is characterized by velocities of 6.3-6.6 km s -1, corresponding to basic rocks with an admixture of acid rocks. The lower part with velocities of 6.8-7.5 km s-1 is built from basic rocks with the addition of ultrabasic rocks (Pavlenkova, 1984). The mean velocity of the whole crystalline complex is 6.4-6.5 km s -1, and the velocity beneath the Moho is 8.1-8.3 km S -1 Over the last decade, within the east European platform, a number of long-range seismic profiles were made (Fig. 1). Their purpose was to study the Earth 's crust and the lower lithosphere. In this study, we have determined one-dimensional seismic models of the lithosphere for two travel-time branches. For the shot-point P4 on the Baltic Sea-Black Sea profile, we used UkrainiaJa and Polish recordings from the distance intervals 0-400 and 600-900 kin, respectively. Refracted Pn waves from the Moho occur as the first arrivals, beginning at a distance of 220 km, and are