Electrical Conductors in Earth Crust Subsoil and Upper Earth Mantle. Ukrainian Shield.

In the subsoils of Earth crust a sufficient amount of abnormalities of high electric conductivity were revealed, which are defined by the characteristic features – length (hundreds and thousands of kilometers), depth of cover bedding (10-15 km). Some abnormal structures have a propensity for zones of present tectonic activity, areas of lithosphere plates’ subduction, others – for joint zones between blocks inside crystal boards. One more feature is the fact that zones of high electric conductivity are often accompanied with deposits of zinc, lead, gold and copper (Australia, New Zealand), diamonds (Africa, North America).

Some of them are stretched on thousands of kilometers, regional and interregional high-conductive structures, revealed on the majority of continents, can be result of geodynamic processes and limits of regions with their different displays. A few examples of this phenomenon can be given.

Abnormality of electric conductivity in Australia begins in the area of Carpentaria bay and is stretched into the depth of the continent for the length more than 1000 km. High electric conductivity’s body was found at the depth of 10 km under basic sediments of Eromang basin. This structure is also fixed in gravitation field and according to data of aeromagnetic researches and is considered by scientists as inner plate’s joint. As for the seismic materials, it acts as the main zone of sharp contrast of the seismic velocities, which is stretched deeply to the mantle. There exists a hypothesis that main Australian abnormalities of electric conductivity - Carpenthariam South-Eastern Queensland and Flieders correspond to the continental joint and are important links in the continent formation. This block is known by its deposits of zinc, lead, gold and copper.

Geographical correspondence of high electric conductivity abnormality in the crust subsoils and the highest gradient of seismic waves’ velocity are fixed not only in this region, but also in the south of Australia.

Within the limits of Central plains of North America (Southern part of North-American platform) abnormality of high electric conductivity is stretched from the East, Hudson Bay, to the US territory and is stretched on more than 2000 km. This abnormality may be longer and wider, than it is shown on the maps. Depth of abnormal body bedding is 10 km. There exists an assumption that it appeared due to the presence of graphite shales and it is possible that it controls the edge of submerged tectonic plate. Alan Johns called it “huge and mysterious structure of continental scale, revealed as a result of electromagnetic induction researches”.

Through the southern Scotland area of high electric conductivity is stretched on H=4-12 km, which correlates with negative gravitational Buge abnormality.

In Central Africa with the help of electromagnetic researches in the Earth crust in the region of Kenyan reef an area of high electric conductivity with the upper limit of H=25 km was revealed.

Analysis shows that the majority of transcontinental crust abnormalities of high electric conductivity correspond to the subduction areas, which last during the previous 120 mln of years. Such correlation is observed on both American continents, in Australia and Africa.

Numerous stretched areas of high electric conductivity of the Eurasian continent also correspond to the subduction areas, for instance, Carpathian.

In alpine structures abnormality of West Carpathians is timed to the joint area of Flish Carpathians and Inner covers, including Pennine and Marmaros zones.

Abnormality of South Carpathians has a propensity for the joint zone of the Inner covers, which divide Pannonia and Transylvania, and Southern Carpathians, bit not to the Precarpathians deflection.

Chernivetsko-Korosten abnormality is galvanically connected with Flish zone of Eastern Carpathians and Marmaros belt.

Western branches of the abnormality are situated in the area of deep Podil rupture and joint of the South-Western SEP edge with Scythian plate.

Geoelectrical model doesn’t always correspond to the surface geology. Pennine and Marmaros belt as well as Flish Carpathians are not an infinite zone of high electric conductivity in the Earth crust. Abnormality of the Precambrian UB is wedged into the alpine Carpathians.

Correlation of the Transeuropean seam area with series of high electric conductivity abnormalities in the Earth crust of Eastern Carpathians and Dobruja is worth special mentioning.

But in Eurasia there exist local abnormalities, which are not connected with areas of modern subduction. They are submeridional high electric conductivity abnormalities in the Earth crust: Kirovograd and Ural. Kirovograd abnormality is stretched to the North in the form of Ladog and Chudskoi abnormalities and can be stretched to the Scandinavian Peninsula.

Today huge experimental material is collected, which showed the fact that as far as geoelectrical aspect is concerned, both Earth crust and upper mantle on the territory of Ukraine are heterogeneous.

 

Three-dimensional geoelectrical models of the Earth crust and upper mantle of the Ukrainian board

1 – High electric resistance block; 2 – subvertical zones of high electric conductivity, which have galvanic connection with surface sediments; High electric conductivity abnormalities: I – Volyn, II – Korosten, III – Chernivtsi-Korosten, IV – Kirovograd, V – Preasov, VI – Donbass.

 

S.M. Kulik and T.K. Burachtovych at their time created 2D and quasi-3D deep geoelectrical models of the earth crust and upper mantle of the UB and its slopes, which include zones with abnormally low values of electric resistance: Korosten (depth of object bedding is H=15 km, total lengthwise electric conductivity of the object is S=500cm), Chernivtsi-Korosten (Н=15 km, S=1000 sm; Н=70 km, S=2000 m), Gaivoron-Dobrovelychkivska (Н=0,1 km, S=2000 sm), Kirovograd (Н=10 – 25 km, S=100 – 20 000 sm), Preasov (Н=1 – 2 km, S=2000 sm), Volyn (Н=2,5 km, S=1000 sm), Donbass (Н=2 km, S=500 – 20 000 sm; Н=10 km, S=1000 – 10 000 sm). Thee different in configuration and geoelectrical parameters areas of high electric conductivity in the earth crust and mantle were basis for the creation of the first 3D models.

Distribution of the specific electric resistance of the surface sediments was found by the values of the lengthwise electric conductivity and the capacity of sedimentary depth. Electric conductivity of the surface sediments has to correlate naturally with the stretch of the main sedimentary structures. But on the territory of Ukraine this regularity is marked only for Dnieper-Donetsk (S>2000 sm), Prychernomorska (S up to 1000 sm) vugs and structure of the crystal plate (0.5<S>100 sm). At the same time surface electric conductivity of the Donetsk basin, where capacity of the sedimentary laminations can reach 10 km, doesn’t exceed 300 sm.

Value of the specific electric resistance of the UB crystal rocks in this research is assumed to be 1000 Ohm?m, against which background abnormalities of both high and low electric conductivity were singled out.

Some stretched areas of high electric conductivity, which have galvanic connection with conductive surface formations, spacely correlate with separate parts of deep intermegablock rupture zones – Talnov, Pervomaisk, West-Ingulets, Kryvorizko-Kremenchutskyi and some other ruptures of other rank. Because of the fact that these structures of electric conductivity are singled out only by the MT sounding without use of profiling, penetration depth and slope angle are very hard to estimate with the collection of experimental data.

Characteristic feature of the Serednyoprydniprovsky megablock and the part of Orihovo-Pavlogradsky seam zones are high values of the imaginary specific electric resistance, which reach several thousands Ohm*m against the background of the average value of 1000 Ohm*m for the whole UB. Magnetotelluric profiles for the period of 2500 seconds are characterized by the only ascendant branches, which can be explained by the induction currents flow around high resistance block along the more electroconductive peripheric parts. As a result of researches high resistance block was revealed, where isolator’s capacity is estimated to be 10-20 km, at the bigger depths fragments of Kirovograd electric conductivity abnormality are observed.

On the western slope of UB within the depths’ interval of 2.5-6 km Volyn electric conductivity abnormality is situated, which has complicated space outline and is characterized by the specific electric resistance value of ρ=10 Ом?м.

At the North-Western part of the board on the border of Volyn and Rosyn megabloks in the Earth crust at the depth between 15 and 30 km Korosten electric conductivity abnormality is situated. Average value of the specific resistance ρ is equal to 30 Ohm*m. Spacely this structure correlates with Korosten pluton.

At the west of the board within the same depth interval Chernivtsi-Korosten electric conductivity abnormality was revealed, which is characterized by big square and complicated structure. Abnormality part with ρ=5 Оhm*m is situated within the limits of Rosyn and Podil megabloks at depths between 15 and 30 km and contains high resistance object at 1000 Ohm*m. Western part of this structure, which has average specific resistance of 20 Ohm*m, exceeds the limits of UB and stretches in two directions – to the South and to the Southern East alongside the Podil rupture zone up to Golovan seam zone. Chernivtsi-Korosten abnormality contains at its South-East a branch, which is characterized by the lowest resistance – about 1 Ohm*m within the interval of 3 to 30 km. This branch is situated in the area of Rosyn, Bug megablocks and Golovan seam zone joint.

The first signs of the widely-known Kirovograd electric conductivity abnormality on the UB slopes appear beginning from the depth of 10 km. The structure is completely situated in the earth crust of the UB central part and stretches far from its limits to the North into the Voronezh massive and to the south into Prychernomorska vug. In the north of Ingulets-Kryvorizka seam zone, UB slope and DDZ at the depths of 10-13 km two zones are singled out: the first one – in the form of S-shaped abnormality ρ=1 Оhm*m, which is situated within the space between Kryvorizko-Kremenchutska and West-Inguletska rupture zones; the second one – in the G-shaped form with the intensity of ρ=30 Оhm*m within the DDZ limits.

At the depths of 13-17 km its form and intensity doesn’t practically change. Only the middle part of the S-shaped abnormality disappears. Within the interval of 17-20 km the first abnormal zone shortens and it can be traced only in the form of square, connected with the northern part of West-Inguletska rupture zone, which borders with UB edge and DDZ. At the South of Kirovograd abnormality, beginning from the depth of 10 km, complicated sublatitudinal zone is singled out, which is already situated within the area of Prychernomorska vug. The most electroconductive parts with ρ=5 Оhm*m are situated in the area of Pervomaisk and Kryvorizko-Kremenchutska rupture zones. It should be mentioned that separate parts of these two rupture zones are traced as electroconductive at different depths. If within the UB limits Pervomaisk rupture zone corresponds to the conductor from the surface, then within the limits of Prychernomorska vug it is characterized by high electric conductivity only at depths beginning from 10 km. Between the Southern and Northern UB slopes there exists a difference in occurrence of electroconductive abnormalities at different depths. If at the Northern slope high electroconductivity disappears at 20 km, at the South abnormality exists at these depths, although in less special sizes.

Character of specific electric resistance distribution within the limits of Kirovograd abnormality changes greatly at the depths of 20-25 km. Hear full-featured abnormality is fixed in the form of almost isometric flat figure with two outlines of different values of the specific resistance: internal with ρ=1 Оhm*m and external with ρ=5 Оhm*m. Internal outline is at the edge of Ingulsk megablock and Ingulets-Kryvorizka seam zone practically along all its length within the UB. This structure covers also South-Eastern part of Korsun-Novomyrgorodskyi pluton. External outline of high electric conductivity abnormality contains almost all the Eastern part of Ingulsk megablock, Ingulets-Kryvorizka seam zone and western part of the Serednyoprydniprovskyi UB megablock.

High electric conductivity (ρ=5 Оhm*m), which is mostly revealed in the Kirovograd abnormality at the depth interval of 25-30 km. It is stretched from south to north-east flat figure, which completely covers the space of Ingulets-Kryvorizka seam zone, eastern part of Ingulsk megablock and eastern part of Korsun-Novomyrgorodskyi pluton. This whole deep structure within the limits of Ingulets-Kryvorizka seam zone continues to the north-east to DDZ and further to the North. At the south of UB it changes its stretch into sublatitudinal and covers a part of Serednyoprydniprovskyi UB megablock and northern part of Prychernomorska vug. Further to the south abnormality extends alongside Pervomaisk ruptures zone.

At the east of the UB at depth of 2.5 km the covering of Preasov abnormality is fixed, which almost completely covers East-Preasov massive. In the subsurface of earth crust of the Preasov UB megablock within the depth interval of 2 to 20 km high electroconductivity zone with specific resistance of 50-100 Ohm*m is situated. It is most probable that Preasov geoelectrical structure in its upper part at the depths of 2 to 10 km is galvanically connected with Donbass high electroconductivity abnormality.

High electroconductivity abnormalities penetrate all megablocks and UB seam zones. They are situated at different depths, beginning from surface up to crust lower strata (more than 30 km) and are characterized by the specific electric resistance from 1 to 100 Ohm*m.

UB mantle is not uniform by the geoelectrical parameters. In the south-western part conductor at the depth interval of 70-120 km with specific resistance of 25 Ohm*m was revealed, while in the UB eastern part mantle is characterized by the comparatively high electric resistance of approximately 1000 Ohm*m.

Thus, the main result of geoelectrical research, which is based on the use of natural source of electromagnetic energy, is the special natural occurrence, observed in the subsurface of UB earth crust – high electric conductivity, which is concentrated in the separate parts of the earth crust and upper mantle and forms zones of different intensity and depth of the bedding. These zones differently characterize separate geological regions.

Subvertical zones of high electric conductivity correlate with intermegablock UB ruptures. The majority of such objects are in Golovanov and Ingulets-Kryvorizka seam zones. Electric conductivity abnormalities of this type may testify about high penetration for fluids of seam zones in the process of their formation and great graphitization of the intergrain space. As opposed to this, Orihovo-Pavlogradska seam zone is characterized by big values of specific electric resistance.

Within the limits of megablocks – Podil, Bug, Ingulsk, Serednyoprydniprovskyi subvertical electroconductive zones are also observed. They are characterized by the shorter length and correlate with deep ruptures of the second rank.

At the north-west of UB I.B. Scherbakov assume presence of the fourth seam zone. This structure is also characterized by the high electric conductivity abnormality – Volyn. But the electroconductive object is not subvertical structure and is not a part of any deep rupture.

Three parts can be singled out in geoelectrical connection in the UB: western, central and eastern. Western parts include Volyn, Podil, Rosyn, and Bug megablocks. Central part contains Ingulsk megablock, Golovan and Ingulets-Kryvorizka seam zones. Eastern part – Serednyoprydniprovskyi megablock, Orihovo-Pavlogradska seam zone and, probably, Preasov megablock. Western part is characterized by low resistance of the earth crust rocks in comparison with eastern one. This can testify about abnormally low penetration of the earth crust rocks of the eastern part. Central part is defined by the high electric conductivity along the whole crust cut and this testifies both about high penetration of paleoproterozoic block rocks.

Characteristic feature of the western part of the board is the presence of conductor at the depth of more than 70 km. Similar geoelectric picture is observed in archaic Slave craton in the North-Western territories of Canada, where in the central part of the craton sharp reduction of electroresistance at the depth from 80 to 100 km (r = 30 Ohm*m) is revealed. Geophysical anomaly coincides with geochemical anomaly (based on Cr-pyrop chemistry), which is caused by harzburgite layer depleting. Negative Buge abnormality with spatial wave length of about 100 km that correlates with the main kimberlite field and geoelectric abnormality is also defined in here.

It is necessary to underline connection between different deep electroconductivity anomalies (Korosten, Kirovograd, Preasov) with separate parts of Korosten, Korsun-Novomirgorodskiy and East-Preasov massives. Within the UB Kherson-Smolensk the transregional tectonic seam coincides with the western branch of deep Kirovograd abnormality, while transregional tectonic Donetsk-Bryansk seam is crossed by different electroconductivity abnormalities - Preazov and Donbass.

Approximately 95 % of Antarctica's surface is covered in ice layer with capacity of 1-3 km. But this continent is still too little studied in geological sense, though Antarctica is the important element of Cainozoic activization and occupies essential position at studying of processes which occurred before formation of stable lithosphere cratons.

Under the conditions of polar areas sources of induction field differ essentially from the model of flat electromagnetic wave, for example, "bay". Nevertheless, the review shows, that it is possible to obtain high-quality MT sounding and GVP data for studying both the earth crust and mantle even under such circumstances.

Italian scientists conducted MW researches, which should solve the problems connected with continental rifting. Induction vectors which can answer a conductor along coast and Transantarctic Mountains were obtained, which has probably been caused by partial fusion of breeds of the upper mantle.

Antarctic kept an appreciable place in geological history of southern hemisphere. Testimony of structure and geological structure of continent were and are essential for acknowledgement or refutation of Gondvani's concept. Tectonic reconstruction and construction of tectonic Gondvani's map before fragmentation (approximately 150 million years, average mesozoic) are well documented, but are still too inconsistent. Australia and Antarctica were uniform in late mesozoic era, and they were divided only about 50 million years ago.

Alongside Rose orogen the border of sharp change in the seismic speeds in the upper Antarctic mantle passes. Eastern part of Antarctic preserves craton-like structure up to the depths of 250 km. The highest velocities are revealed around Enderby Land, where samples of old rocks were found, and low velocities are localised along active tectonic zones, which surround the continent. Low speeds of the western part of Transantarctic Mountains can characterise rift zone, similar to the African one.

The interesting fact consists in the electroconductive abnormality placing in relation to the revealed oregon area. There is an assumption of electroconductive abnormality existence in the earth crust of Antarctica in the area of Transantarctic Mountains.

The probability of such assumption increases if we consider the message of the Italian scientists about МW research in the region of Northern Victoria land, where at 20-50 km by the preliminary data the taken area of high electroconductivity is revealed, and F.Vannameker's remarkable review, written together with colleagues, about MT sounding near the South Pole, where the conductive layer at depth of about 30 km is revealed.