O. V. Legostaeva

3-D gravity analysis of the Dniepr–Donets Basin and Donbas Foldbelt, Ukraine

Abstract The Dniepr–Donets Basin (DDB) is a linear, NW–SE trending, late Palaeozoic and younger sedimentary basin on the East European Platform separating the Ukrainian Shield from the Voronezh Massif. Its northwestern (Dniepr) segment has the characteristics of a typical rift basin. To the southeast, through a transition zone of approximately 200 km length, the DDB is progressively uplifted and compressionally deformed into the correlatable Donbas Foldbelt (DF). Along the axis of the Dniepr segment a series of gravity highs has been previously explained by high-density crystalline crust beneath the axis of the basin caused by intrusion of mafic and ultramafic rocks. In this paper, the results of a 3-D gravity analysis, using a gravity backstripping technique, is described that investigates the crustal and upper mantle structure in the region of the DDB–DF transition zone and DF. A residual gravity field I, obtained by subtracting the gravity influence of the sedimentary succession of the DDB from the observed field, reveals a distinct positive anomaly along the axis of the rift basin increasing in amplitude to the southeast in concert with increasing sedimentary thickness. A residual gravity field II, derived by removing the gravity effects of a modelled homogeneous crystalline crust from residual field I, reaches 200 and 100 mGal amplitude in the DF for two respective Moho models based on different interpretations of the published crust and upper mantle seismic velocity models. The first of these (model A) assumes crustal thickening beneath the transition zone and DF (to a Moho depth up to 50 km) whereas the second (model B) assumes a Moho shallowing (to depths in the range 35–37 km) along the whole basin axis. For each residual anomaly II, the best-fitting 3-D distribution of average density in the crystalline crust has been computed. Both models indicate the existence of a high-density body in the crystalline crust along the DDB axis, increasing in density from the Dniepr segment to the DF, with higher average crustal density required in the case of Moho model A (3.17×103 kg m−3 versus 3.06×103 kg m−3 for model B). The preferred interpretation of the density models is one in which the denser crystalline crust underlying the DDB–DF transition zone and DF is explained by intrusion of mafic and ultramafic rocks during late Palaeozoic rifting processes. Invocation of processes related to the uplift and inversion in the DF are not required to explain the observed gravity field although reworking of the crust during Permian and younger tectonism in the DF cannot be ruled out.

Magnetic fields of 3-D anisotropic bodies: Theory and practice of calculations

The general theory of the distribution of the volume and surface magnetic mass within 3-D anisotropic bodies and solving the forward problem is given in this paper. An algorithm for calculating the magnetic fields of monoclines of complex shape and folded structures with uniform anisotropy is constructed. The algorithm is based on the regularities in the relationship between the magnetic susceptibility of anisotropy, tectonic structure, and the anomalous magnetic field established experimentally by Zavoisky. These regularities not only simplify the solution of the problem, but significantly facilitate the preparation of original field data necessary for solving it. The latter circumstance is of especial importance. The algorithm is designed for wide practical application in the construction of 3-D magnetic models of local and regional geological structures.We draw attention to the fact that the use of a curvilinear coordinate system is reasonable in cases when the distribution of the magnetic mass density in anisotropic geological formations is studied.The features of the relationship between the intensity and induction of a magnetic field in different unit systems are pointed out in their application to magnetology problems.

Crustal and upper mantle velocity model along the DOBRE-4 profile from North Dobruja to the central region of the Ukrainian Shield: 2. geotectonic interpretation

This part of the paper addresses the geotectonic interpretation of the velocity model obtained from the results of seismic studies under the DOBRE-4 project in Ukraine. The velocity field does not show distinct lateral changes from the Precambrian platform towards the younger tectonic structures in the southwest. Hence, based on the seismic data alone, it is not possible to recognize the tectonic units that are known on the surface. The Moho has an undulating pattern over an interval with a length of ~150 km. The amplitude of the undulations reaches 8 to 17 km. The similar wavelike behavior, although on a shorter spatial scale and lower amplitude, is also typical of the upper crust and upper mantle. The presence of several separate horizons in the folded crust revealed by the velocity model is consistent with the presence of the folded systems which have different extensions on the different depth levels in the Earth’s crust. This situation is believed to be typical of folding on the lithospheric scale and to reflect the rheological stratification of the crust. The DOBRE-4 velocity section of the crust and adjacent part of the mantle promotes a clearer view of the geodynamical model describing the formation of the southwestern part of East European Platform in the Early Precambrian from the plate’s tectonic standpoint.

Crustal and upper mantle velocity model along the DOBRE-4 profile from North Dobruja to the central region of the Ukrainian Shield: 1. seismic data

For studying the structure of the lithosphere in southern Ukraine, wide-angle seismic studies that recorded the reflected and refracted waves were carried out under the DOBRE-4 project. The field works were conducted in October 2009. Thirteen chemical shot points spaced 35–50 km apart from each other were implemented with a charge weight varying from 600 to 1000 kg. Overall 230 recording stations with an interval of 2.5 km between them were used. The high quality of the obtained data allowed us to model the velocity section along the profile for P- and S-waves. Seismic modeling was carried out by two methods. Initially, trial-and-error ray tracing using the arrival times of the main reflected and refracted P- and S-phases was conducted. Next, the amplitudes of the recorded phases were analyzed by the finite-difference full waveform method. The resulting velocity model demonstrates a fairly homogeneous structure from the middle to lower crust both in the vertical and horizontal directions. A drastically different situation is observed in the upper crust, where the Vp velocities decrease upwards along the section from 6.35 km/s at a depth of 15–20 km to 5.9–5.8 km/s on the surface of the crystalline basement; in the Neoproterozoic and Paleozoic deposits, it diminishes from 5.15 to 3.80 km/s, and in the Mesozoic layers, it decreases from 2.70 to 2.30 km/s. The subcrustal Vp gradually increases downwards from 6.50 to 6.7–6.8 km/s at the crustal base, which complicates the problem of separating the middle and lower crust. The Vp velocities above 6.80 km/s have not been revealed even in the lowermost part of the crust, in contrast to the similar profiles in the East European Platform. The Moho is clearly delineated by the velocity contrast of 1.3–1.7 km/s. The alternating pattern of the changes in the Moho depths corresponding to Moho undulations with a wavelength of about 150 km and the amplitude reaching 8 to 17 km is a peculiarity of the velocity model.

Methods for reconstructing harmonic functions from the magnetic field ΔT and V.N. Strakhov’s function ΔS: A review

We give an overview of the methods designed for reconstructing close-to-harmonic functions from the magnetic field ΔT. The formula of Yu.P. Tafeev is refined. It is shown that this refined formula directly leads to the relation derived by V.M. Gordin and his colleagues that allows isolating the harmonic component in the function ΔT. V.N. Strakhov’s linearized representation of the function ΔT is immediately derived from the main approximate Tafeev formula for Q ΔT. The experience of using Strakhov’s ΔS function in the interpretation of the magnetic anomaly ΔT generated by the Krivoi Rog structure is described. It is noted that the problem of reconstructing the corresponding harmonic functions from the data of magnetic and gravity surveys has much in common. The specific features of measuring the magnetic field H and magnetic induction B in the material media are considered, and the physical interpretation of these fields is presented.

Generalization of the Rayleigh-Tikhonov stationary geothermal problem for a horizontal layer

The Rayleigh-Tikhonov generalized stationary geothermal problem is formulated and solved exactly for a set of homogeneous horizontal infinite plane-parallel layers located in a lower half-space. The known solutions follow as particular cases of the solutions found. The results obtained in this work are usable for 1-D approximations of many important problems of geophysics, tectonophysics, and geology.