Abnormal gravitational field. Abnormal Earth crust and upper mantle thickness.
Earth gravitational field or field of gravity force – is a force field caused by Earth gravity and centrifugal force as a result of the Earth axial rotation. It is conventionally divided into abnormal and basic-normal parts, calculated according to the distribution formulae of acceleration of normal gravity. Abnormal part of field is considerably smaller in magnitude, has complicated structure and reflects Earth form characteristics and its subsoil inhomogeneity. On a map abnormal gravitational field structure is shown by the similar size lines of its values, which are called isoanomalous lines.
Abnormal gravitational field changes in wide ranges in connection with great diversity of Earth crust and upper mantle material thickness distribution along the lateral when natural gradient stratified increasing at the same time with depth; in some regions thickness inhomogenuities take place in the upper mantle also. Gravitational field morphology and intensity visually in the first place reflect thickness distribution characteristics in the subsurface Earth crust part.
In the Western Ukraine (Precarpathian bending), where sedimentary strata is thick and bounder crust-mantle (or the Mokhorovichich discontinuity) is immersed greatly, minimal magnitude of the field in Bouguer reduction (up to -100mGal). To the West from Carpathians Moho discontinuity rises sharply, and field becomes positive (up to +30 mGal). In the north-western part of the territory, where the sediment thickness reaches 1-3 km, the field is weakly negative (about -10-20mGal).
Central part of Ukraine is situated on the Ukrainian Shield. Field level there is weakly positive. And granitic massifs are characterized by the subzero field (except anomaly in -30mGal over the massif in the region of Novoukrainka-Kirovohrad), and basic materials and granulites are characterized by positive field magnitude, the most intensive of which is (higher than +60 mGal) are observed in the region of Golovanivsk. On the south East of the Shield great positive anomalies are made by the ferruginous structures in the regions of Kryvyi Rih, Dnipropetrovs’k, Nikopol’, Zaporizhzhia, Melitopol’, Mariupol’.
In the Eastern part of Ukraine the Dniprovs’ko-Donets’kyi avlakogene (DDA) is situated. The field there depends on correlation of sedimentary strata effects and basic Devonian material implanted along the axis of the avlakogene. In general, between the depressions due to the sedimentary strata thickness decreasing lifting of the Moho discontinuity is observed. High field is observed in Donbass (higher than +40mGal), where thick sedimentary strata materials are very metamorphized and have high thickness. From the North DDA is characterized by the minimal field, that is caused by light sedimentary strata, and the local intensive minimums in the regions of Sieverodonets’k (-40 mGal), Svatove, Shevchenkove, Okhtyrka are caused by the sole material low thickness.
On the South, in Crimea, very high field is observed, which maximum (over than +150mGal) is situated in the Crimean Mountains. Earth crust there usually has the basic structure and there might be ultrabasic materials.
In the tectonically active structures (Carpathian, Donbass, Crimea) in gravitational field effects of mantle material thickness decrease, caused by high temperature influence.
Gravity isoanomalies (mGal)
The connection between structure characteristics and Earth crust substantial structure (upper mantle in some regions) and gravitational field is in most cases inverse. That is why features of abnormal Earth crust and upper mantle thickness distribution were discovered only with the help of methodology and technology of gradient stratified three-dimensional deep structure gravitational modeling worked out in Ukraine, which is based on rapid openings, correlation dependences r=f (Vp) for crystalline materials, which are based on experiments under PT conditions of deep structures subject to material substantial structure specific amendments, studying of theoretical models of different types of blocks, modern software.
Due to that, Earth crust and upper mantle abnormal thickness areas are singled out and regularities in its distribution were established. It was found out that due to isostatic compensation tendency of the separate blocks and its groups heavier blocks has immersing of the Earth crust bottom, and the lighter ones – lifting of the Moho surface. That is, on the Ukrainian Shield the Golovanivs’k structure, separate blocks on Volhynia-Podolya and in other regions have Earth crust raised thickness when bending, and granitoid blocks (Kirovohrad, Korosten’ and others) are characterized by the material lightened thickness and lifting of the Moho bounder.
By results of gravitational modeling areas of upper mantle materials with raised thickness under the DDA, Crimea, Precarpathian, and in Transcarpathian region- area of upper mantle material with lightened thickness are predicted.
Authors: V. I. Starostenko, S.S Krasovsky