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Russian Geology and Geophysics

2022 year, number 12

1.
THE MIDDLE PALEOZOIC MAGMATISM OF THE CENTRAL TUVINIAN TROUGH (eastern Altai-Sayan fold area): PETROGENESIS, TECTONICS AND GEODYNAMICS

E.V. Vetrov1, A.N. Uvarov2, E.S. Andreeva2, N.I. Vetrova1, F.I. Zhimulev1, A.S. Stepanov3, I.A. Vishnevskaya4,5, M.V. Chervyakovskaya6
1V.S. Sobolev Institute of Geology and Mineralogy, Siberian Branch of the Russian Academy of Sciences, pr. Akademika Koptyuga 3, Novosibirsk, 630090, Russia
2Siberian Research Institute of Geology, Geophysics and Mineral Resources, Krasnyi pr. 67, Novosibirsk, 630091, Russia
3China University of Geosciences (Wuhan), 430074 Wuhan, 388 Lumo, Hubei, China
4Vernadsky Institute of Geochemistry and Analytical Chemistry, Russian Academy of Sciences, ul. Kosygina 19, Moscow, 119334, Russia
5Novosibirsk State University, ul. Pirogova 1, Novosibirsk, 630090, Russia
6A.N. Zavaritsky Institute of Geology and Geochemistry, Ural Branch of the Russian Academy of Sciences, ul. Akademika Vonsovskogo 15, Yekaterinburg, 620016, Russia
Keywords: Tuvinian trough, Devonian, Carboniferous, volcanism, magmatism, U-Pb dating, geochemical studies, Nd isotope geochemistry, Kendei Formation, Torgalyk Complex

Abstract >>
The Tuvinian trough is one of the large grabens of the rift system formed in the Devonian-Carboniferous in the eastern part of the Altai-Sayan fold area. Based on the results of comprehensive studies, the age was refined, and the geochemical features of igneous rocks formed during two stages of tectonic and magmatic activity within the Tuvinian trough were studied. In the Early Devonian (397 Ma, Emsian), at the stage of the initiation of the Tuvinian trough in the stretching setting, the volcanic and subvolcanic rocks of the Kendei Formation formed, which make a bimodal series. The Early Devonian igneous rocks of mafic composition have geochemical features of both intraplate (low values of Mg#, high contents of K2O (up to 2.9 wt. %) and TiO2 (up to 2.2 wt. %), and enrichment in LREE relative to HREE) and suprasubductional (enrichment in Pb and Sr and depletion in Ta and Nb) formations and are characterized by high values of εNd(T) (+5.9 to +8.0). They are assumed to have formed from a mixed source including the depleted mantle and components modified by subduction. The Early Devonian felsic volcanic rocks, which are the extreme member of the bimodal sequence, also combine the geochemical features of rocks of intraplate (high Fe and low Sr, P, and Ti contents, Zr and Hf enrichment) and island arc (Ta and Nb depletion) origin. These rocks with εNd(T) values from +4.0 to +7.0 resulted from the melting of a heterogeneous source corresponding in composition to the lower continental crust. In the Middle Devonian-early Carboniferous (390-350 Ma), the Tuvinian rift trough evolved to a mature stage, at which the mafic rocks of the Torgalyk Complex were intruded. The Middle Devonian-early Carboniferous mafic rocks are similar in isotope and geochemical characteristics, including the Nd isotope composition (εNd(T) = +6.7), to the Early Devonian formations. In contrast to the Early Devonian rocks, the magmas for the Middle Devonian-early Carboniferous mafic rocks were generated from a relatively homogeneous mantle source without significant metasomatic transformations, the features of which are better manifested in the Kendei rocks.



2.
PHYSICOCHEMICAL PARAMETERS OF ULTRABASIC MAGMATIC SYSTEMS OF THE NORTHEASTERN SIBERIAN PLATFORM (data on melt inclusions in chromian spinel)

V.A. Simonov1,2,3, Yu.R. Vasil'ev1,2, A.V. Kotlyarov1,3, E.I. Nikolenko4, T.A. Alifirova1, V.V. Sharygin1, S. Aulbach5,6
1V.S. Sobolev Institute of Geology and Mineralogy, Siberian Branch of the Russian Academy of Sciences, pr. Akademika Koptyuga 3, Novosibirsk, 630090, Russia
2Novosibirsk State University, ul. Pirogova 1, Novosibirsk, 630090, Russia
3Kazan Federal University, ul. Kremlevskaya 18, Kazan, 420008, Russia
4ALROSA (ZIMBABWE) Ltd, 19 Van Praagh Ave., Milton Park, Harare, Zimbabwe
5Institut für Geowissenschaften, Goethe-Universitat, Frankfurt am Main, 60323, Frankfurt, Germany
6Frankfurt Isotope and Element Research Center (FIERCE), Goethe- Universitat Frankfurt, Frankfurt am Main, 60323, Frankfurt, Germany
Keywords: Ultrabasic magmatic systems, northeastern Siberian Platform, melt inclusions in chromian spinel, P-T conditions of magmatic processes

Abstract >>
Experimental studies and analysis of silicate inclusions testify to the magmatic genesis of part of chromian spinel from the Triassic deposits of the northeastern Siberian Platform. The compositions of melt inclusions in the chromian spinel show the participation of alkaline (potassic) magmas in their crystallization. Most data indicate the presence of magmatic systems similar to melts in the Guli ultrabasic massif in the northern Siberian Platform. Studies of the distribution of trace and rare-earth elements in the melt inclusions show the existence of several magmatic systems. These are, first of all, magmas that formed Guli-type ultrabasic massifs and gave rise to meimechites and picrites in the Maimecha-Kotui province. There are also plume-related magmatic systems with kimberlites, lamprophyres, and/or other continental hot spots. The composition of the melt inclusions suggests the existence of several types of the primary sources of chromian spinel in the northeastern Siberian Platform, which confirms the earlier data on the heterogeneous composition of the deposits of the diamondiferous Carnian (Upper Triassic) Stage. Applying computer modeling with the well-known COMAGMAT, PETROLOG, and WinPLtb programs as well as the Ol-Sp geothermometers based on the melt inclusions in chromian spinel from the Triassic deposits of the northeastern Siberian Platform, we have determined the P - T conditions of crystallization of minerals in the igneous rocks being the sources of the examined chromites. The temperature of liquidus crystallization of chromian spinel is 1324-1275 ºC. The P - T conditions of formation of olivine and clinopyroxene inclusions in it are estimated at ca. 4.5-4.1 kbar, 1510-1150 ºC and 3.2-1.0 kbar, 1285-1200 ºC, respectively.



3.
KUVAEVITE, Ir5Ni10S16, A NEW MINERAL SPECIES, ITS ASSOCIATIONS AND GENETIC FEATURES, FROM THE SISIM RIVER PLACER ZONE, EASTERN SAYANS

A.Y. Barkov1, N.D. Tolstykh2, R.F. Martin3, N. Tamura4, Ma Chi5, A.A. Nikiforov1
1Cherepovets State University, pr. Lunacharskogo 5, Cherepovets, 162600, Russia
2V.S. Sobolev Institute of Geology and Mineralogy, Siberian Branch of the Russian Academy of Sciences, pr. Akademika Koptyuga 3, Novosibirsk, 630090, Russia
3Department of Earth and Planetary Sciences, McGill University, 3450 University Street, Montreal, Quebec H3A 0E8, Canada
4Advanced Light Source, 1 Cyclotron Road, Lawrence Berkeley National Laboratory, Berkeley, CA 94720-8229, USA
5Division of Geological and Planetary Sciences, California Institute of Technology, 1200 East California Blvd., Caltech, 170-25 Pasadena, CA 91125, USA
Keywords: Kuvaevite, platinum-group minerals, iridium and nickel sulfide, placer deposits, Ko River, the Sisim placer zone, the Lysanskiy mafic-ultramafic complex, Eastern Sayans, Krasnoyarsk Territory

Abstract >>
Kuvaevite ((Ir,Rh)5(Ni,Fe,Cu)10S16) forms small grains (up to 20 m across) in globular inclusions hosted by grains of Os-Ir-(Ru) alloys (up to 0.5 mm) in ore occurrences along the Ko River in the Sisim placer zone, Eastern Sayans. Rh-bearing pentlandite or oberthürite (or both), the minerals of the laurite-erlichmanite series and Pt-(Pd)-Fe alloys are the main associated minerals. Kuvaevite is gray to brownish gray in color in reflected light. Its bireflectance is weak to absent. It is slightly pleochroic in gray to light brown shades, and slightly anisotropic, from gray to light yellow shades. Its calculated density is 6.37 g/cm3. According to results of microprobe analyses ( n = 3) carried out using wavelength-dispersive spectrometry, WDS, the composition of kuvaevite is: Cu 5.94 (4.39-6.89), Ni 13.95 (13.80-14.24), Fe 10.95 (10.18-11.97), Co 0.07 (0.06-0.10), Ir 32.38 (32.19-32.73), Rh 7.27 (7.22-7.31), Pt 1.91 (1.67-2.06), Os 0.05 (0-0.09), Ru 0.05 (0.04-0.05), S 27.06 (26.77-27.41), total 99.63 wt.%. The empirical formulae calculated using the mean results of analyses are: (Ir3.22Rh1.35Pt0.19Ru0.01Os0.01)Σ4.78 (Ni4.54Fe3.75Cu1.79Co0.02)Σ10.10S16.13 (WDS) and (Ir3.23Rh1.43Pt0.25)Σ4.91 (Ni4.49Fe3.57Cu1.86Co0.06)Σ9.98S16.11 (SEM/EDS; n = 56). These are based on a total of 31 atoms according to structural data obtained for torryweiserite, the rhodium-dominant analogue. Kuvaevite forms solid-solution series with torryweiserite, tamuraite and ferrotorryweiserite, all these being isostructural. The symmetry of kuvaevite was determined using the synchrotron Laue microdiffraction; the results are in good agreement with the trigonal crystal system and give the following unit-cell parameters: a = 7.079(5) Å, c = 34.344(12) Å, V = 1490(2) Å3; Z = 3. The ratio c/a is 4.852. The probable space-group, R3m (#166), is based on structural results for torryweiserite. The strongest eight reflections in the X-ray diffraction pattern derived from the microdiffraction study [d in Å(hkl) (I)], are the following: 3.0530(20 1)(43), 3.0103(2 1 6)(100), 2.9962(1010) (53), 2.7991(205)(50), 2.4946(208)(31), 1.9208(3 1 10)(41), 1.7697(4 2 0)(73), 1.7582(20 16)(66). The results of the electron backscatter diffraction study (EBSD) of two kuvaevite crystals are well-indexed based on the R3m space group. Kuvaevite and related sulfides significantly vary in composition in the Ko River placer, in the entire Sisim zone, and in some other ore occurrences worldwide. Associations of platinum-group minerals observed in ore occurrences at Ko River and in the Sisim zone seem to be genetically related to bedrock zones of chromite-bearing ultramafic rocks (serpentinites) of the Lysanskiy complex. Kuvaevite and other minerals present in the polymineralic inclusions, hosted by Os-Ir-(Ru) alloys, formed from droplets of residual melt. This melt accumulated the incompatible elements, which could not be incorporated into the structure of the host alloy, including lithophile elements, chalcogens (S, Te), semimetals (As, Sb, Bi), base metals (Fe, Ni, Cu), as well as relatively low-temperature PGE species (Pt, Pd) and Rh. There are local data on metastable crystallization and undercooling of the silicate melt, as well as effective differentiation and fractionation of S and ore components during the crystallization of these inclusions. Kuvaevite is named after O.M. Kuvaev (1934-1975), a prominent geologist, geophysicist and writer.



4.
GEOCHEMISTRY OF ACCESSORY APATITE FROM THE Cu-Ni-SULFIDE-BEARING ULTRAMAFIC-MAFIC ROCKS OF THE KHUDOLAZ COMPLEX (South Urals) AS A MONITOR OF MAGMATIC AND METASOMATIC PROCESSES

I.R. Rakhimov1,2, I.A. Gottman2, V.V. Kholodnov2, V.S. Chervyakovskiy2
1Institute of Geology, Ufa Federal Research Center of the Russian Academy of Sciences, ul. K. Marksa 16/2, Ufa, 450077, Russia
2Zavaritsky Institute of Geology and Geochemistry, Ural Branch of the Russian Academy of Sciences, ul. Akademika Vonsovskogo 15, Yekaterinburg, 620016, Russia
Keywords: Apatite, ultramafic-mafic rocks, Cu-Ni-sulfide ores, geochemistry, melt, hydrothermal fluid

Abstract >>
We present results of mineralogical and geochemical studies (optical and electron microscopy, probe microanalysis, and LA-ICP-MS) of accessory apatite from the rocks of four ore-bearing massifs of the Khudolaz Complex: Vostochnyi Buskun, Severnyi Buskun, Malyutka, and Tashly-Tau. Two groups of apatite are recognized by morphology and chemical composition: (1) magmatic unaltered apatite (Ap-1) in unaltered and slightly altered rocks and (2) metasomatized apatite (Ap-2) surrounded by secondary silicates that replaced primary minerals. Ap-1 is represented by euhedral and subhedral hexagonal crystals with a high content of chlorine (0.7-1.2 wt.% Cl) and an extremely low content of sulfur (<0.05 wt.% SO3). It is enriched in REE (ΣREE = 2.2-3.0 wt.%), shows a pronounced negative Eu anomaly (Eu/Eu* = 0.36-0.58), and is depleted in Co, Ni, and chalcophile trace elements (Zn, Pb, and Bi). The distribution of major and trace elements in Ap-1 made it possible to identify its two generations, early and late, in the Khudolaz Complex. The early generation, characterized by a low content of Ca ( ≥ 1000 C, CaO < 54 wt.%), crystallized almost simultaneously with Ca-plagioclase (An81-61). The late generation, characterized by a high content of Ca ( ≥ 700 C, CaO > 55 wt.%), is associated with Na-plagioclase (An28-8), amphibole, and phlogopite. Ap-2 is represented by highly cracked grains and is characterized by a low content of chlorine (0.0n-0.5 wt.% Cl) and a high content of sulfur (0.06-0.93 wt.% SO3). The content of fluorine in both groups of apatite varies in the same range (0.6-1.7 wt.% F), which determines its relative inertness during metasomatic processes. Ap-2 is less enriched in REE (ΣREE = = 1.1-2.1 wt.%) and shows a less pronounced Eu anomaly (Eu/Eu* = 0.61-0.77) but has high contents of Co, Ni, and chalcophile trace elements. The Ap-1 and Ap-2 show different variations in contents of major elements (Fe, Mg, Na, etc.), which is due to the impact of hydrothermal fluid on the mineral at the postmagmatic stage. It is shown that the low content of sulfur in apatite from unaltered rocks does not indicate a low sulfide ore potential of the Khudolaz Complex. The Cu-Ni sulfide signature of the complex is more clearly evidenced by the composition of metasomatized apatite enriched in S, Co, Ni, and chalcophile elements as a result of their hydrothermal removal from primary sulfide minerals. The high Eu/Eu* value indicates an increase in oxygen fugacity at the postmagmatic stage.



5.
UPPER CRETACEOUS AND CENOZOIC STRATIGRAPHY OF THE GYDAN PENINSULA

V.A. Marinov1,2, A.N. Kurchatova3,4, Z.N. Gnibidenko5, O.B. Kuzmina5, E.A. Potapova1, V.V. Rogov3,6, R.B. Khamzin7
1Tyumen Petroleum Research Center, ul. Osipenko 79/1, Tyumen, 625002, Russia
2Tyumen State University, ul. Volodarskogo 6, Tyumen, 625003, Russia
3Institute of the Earth's Cryosphere, Tyumen Science Center, ul. Malygina 72, Tyumen, 625000, Russia
4AO Messo-Yakhaneftegaz, ul. Kholodilnaya, Tyumen, 625048, Russia
5Trofimuk Institute of Petroleum Geology and Geophysics, Siberian Branch of the Russian Academy of Sciences, pr. Akademika Koptyuga 3, Novosibirsk, 630090, Russia
6Lomonosov Moscow State University, Leninskie Gory 1, Moscow, 119991, Russia
7OOO Gazpromneft-Yamal, ul. 50 let Oktyabrya 8b, Tyumen, 625048, Russia
Keywords: Upper Cretaceous, Paleogene, Quaternary, microfossils, palynology, paleomagnetism, paleogeography, Gydan Peninsula, northern West Siberia

Abstract >>
The post-Cenomanian sedimentary sequence of the East Messo-Yakha oil and gas field in the southern Gydan Peninsula comprises twelve lithological units (beds) correlated to Upper Cretaceous, Paleogene, and Quaternary regional stratigraphic stages and formations. The facies of the sedimentary units are constrained from lithology and microfossils. The stratigraphic division is based on bio-stratigraphy (fauna and spore-pollen assemblages), paleomagnetism, and geochronology (quartz and feldspar grains dated by optically stimulated luminescence). The deposition record is interrupted by six large stratigraphic unconformities. The obtained data provide new insights into the Late Cretaceous, Paleogene, and Quaternary paleogeography of the area.



6.
THE INSKIE SPRINGS: NEW INSIGHTS INTO LOW-RADON WATERS

D.A. Novikov1,2, Yu.G. Kopylova3, A.F. Sukhorukova1, L.G. Vakulenko1,2, A.N. Pyryaev2,4, A.A. Maksimova1, A.S. Derkachev1, A.N. Faguet1, A.A. Khvashchevskaya3, F.F. Dultsev1, A.V. Chernykh1, M.S. Melgunov4, P.N. Kalinkin2, S.A. Rastigeev5
1Trofimuk Institute of Petroleum Geology and Geophysics, Siberian Branch of the Russian Academy of Sciences, pr. Akademika Koptyuga 3, Novosibirsk, 630090, Russia
2Novosibirsk State University, ul. Pirogova 1, Novosibirsk, 630090, Russia
3Research Laboratory for Hydrogeochemistry of the School of Earth Sciences and Engineering, Tomsk Polytechnic University, pr. Lenina 30, Tomsk, 634050, Russia
4V.S. Sobolev Institute of Geology and Mineralogy, Siberian Branch of the Russian Academy of Sciences, pr. Akademika Koptyuga 3, Novosibirsk, 630090, Russia
5G.I. Budker Institute of Nuclear Physics, Siberian Branch of the Russian Academy of Sciences, pr. Akademika Lavrentieva 11, Novosibirsk, 630090, Russia
Keywords: Radon waters, hydrogeochemistry, radionuclides, stable isotopes, radiocarbon dating, origin, fractionation, Inskie springs, West Siberia

Abstract >>
In August 2019, four descending (gravity) springs in the Inya River valley were sampled during the field works conducted within the study of low-radon waters of the Novosibirsk urban agglomeration in the area lying away from known granite massifs. Laboratory analyses have revealed enhanced radon activity concentrations (from 5 to 149 Bq/dm3). It has been established that these waters are fresh, of HCO3 Mg-Ca chemical composition, and have a TDS value of 413 to 548 mg/dm3 and a silicon content of 4.1-8.6 mg/dm3. They are characterized by neutral to slightly alkaline pH (7.1-8.4) and oxidizing geochemical conditions with Eh from +205.3 to +231.8 mV and O2 dissolv. = 6.24-12.26 mg/dm3. The revealed predominance of SO42- over Cl- concentrations in the waters of the study area was probably due to the presence of sulfides in the water-bearing sediments, in particular, pyrite in the surface sediments. More than tenfold proportional excess of Ca concentrations over Si in the Inskie spring waters indicates the predominantly carbonate composition of the water-bearing sediments. The gross α-activity of waters is 3-4 mBq/dm3, and gross β-activity is 11-15 mBq/dm3. Natural radionuclides are found in the spring waters within the following limits (mg/dm3): 238U, from 2.83 10-3 to 4.13 10-3; 232Th, from 2.39 10-6 to 1.16 10-5, and 226Ra, from 3.83 10-10 to 4.93 10-10. The value of the 232Th/238U ratio for the waters ranges from 5.79 10-4 to 3.61 10-3, as a result of the oxidative geochemical migration-arresting capability of thorium. The uranium isotope ratio (γ) 234U/238U varies from 2.6 to 3.2 for the waters, with the uranium isotope activity determined as 117-124 mBq/dm3 for 234U and 38-48 mBq/dm3 for 238U. This indicates shallow circulation of the studied waters as compared with those of the Svyatoi spring in Verkh-Tula Village, for which γ = 1.3; the 234U activity, 147 mBq/dm3; and the 238U activity, 115 mBq/dm3. By isotopic composition, the origin of the spring waters is assigned to the infiltration type, and they are characterized by relatively narrow distribution of δ18O (from -17.5 to -16.7) and δD (from -128.4 to -126.2) values. The δ13CDIC values are from -10.3 and -10.9 in springs 3 and 2 and become lighter (-11.2 and -12.1) in springs 1 and 4, respectively. This is due to significant participation of surface waters in the recharge mechanism of springs 1 and 4, which is also consistent with the δ18O and δD data and 14C dating. The estimated age of water-dissolved carbon is 1478 81 years for the waters of spring 3 (the oldest), while it is found to be only 651 53 years for spring 4 and is estimated as modern for spring 1. The reported decline in the age of water-dissolved carbon down to recent age is indicative of increased contribution of surface water to the spring recharge area. The C and O isotope compositions of calcite of the host aquifer rocks are characterized by close values for most of the samples: δ13 varies within narrow limits (from -3.1 to -2.7), and δ18 varies from 17.2 to 18.4. The isotopic composition becoming lighter for carbon (up to -11.0) and oxygen (up to 13.9) was noted for weathered schist samples. Results of the carbon isotope analysis of rock samples, their organic component, and water indicate an active isotopic exchange in the water-rock-organic matter system.



7.
STRESS STATE OF THE EARTHS CRUST, SEISMICITY, AND PROSPECTS FOR LONG-TERM FORECAST OF STRONG EARTHQUAKES IN UZBEKISTAN

T.U. Artiko1, R.S. Ibragimov1, T.L. Ibragimova1, M.A. Mirzaev1, Yu.L. Rebetsky2
1G.A. Mavlyanov Institute of Seismology, Academy of Sciences of the Republic of Uzbekistan, ul. Zulfiyakhonim 3, Tashkent, 100128, Uzbekistan
2Shmidt Institute of Physics of the Earth, Russian Academy of Sciences, ul. Bol. Gruzinskaya 10, Moscow, 123995, Russia
Keywords: Earthquake source mechanism, stress field reconstruction, principal stress axes, geodynamic stress state, long-term earthquake prediction, seismic regime parameters, seismic activation, seismic quiescence

Abstract >>
The current stress state of the Earths crust on the territory of Uzbekistan is studied using algorithms from the cataclastic analysis of displacements (CAD) with the application of a combined catalog of earthquake source mechanisms. The axis of maximum compression is nearly horizontal and orthogonal to the strike of tectonic structures for the larger part of the area under study, and the angle of penetration of the axis of minimum compression varies greatly for different parts of the territory. The areal distribution of the Lode-Nadai coefficient indicates the predominance of a stress state close to pure shift for the area under study. Without differentiation of the seismically active bed by depth, almost the entire territory of Uzbekistan is characterized by a geodynamic stress state corresponding to horizontal compression. There are significant differences in the stress state parameters for different deep beds of the Earths crust. With account for the representative recording periods of seismic events of different energy levels, the earthquake recurrence parameters for the entire territory as a whole and large seismically active zones are determined. It has been established that the epicenters of strong earthquakes recorded since the historical time are grouped in compact areas with linear dimensions of 50-80 km. The tectonophysical interpretation of the selected areas is given within the framework of the CAD, in which it is noted that strong earthquakes occur mainly in areas of lower effective confining pressure and maximum tangent stresses. This is due to the fact that the values of friction forces at faults in such areas are low, which creates the most favorable conditions for large-scale destruction. According to the results of the reconstruction of the natural stresses of the CAD, such areas on the territory of Uzbekistan are identified within faults and flexural-fault zones in South Fergana and North Fergana, as well as in the Gazli region. The current seismological situation in the selected areas of long-term forecast is estimated using a set of prognostic parameters of the seismic regime, and a map of the areas of expected seismic activity is compiled. The analysis of previous forecast maps developed within the framework of the presented approach shows high value in their information.



8.
GEOELECTRIC SECTION OF THE COASTAL REGION OF THE CHUKCHI SEA NEAR THE CAPE SCHMIDT OBSERVATORY

S.S. Starzhinskii1, A. Yoshikawa2, S.Yu. Khomutov3
1V.I. Ilichev Pacific Oceanological Institute, Far Eastern Branch of the Russian Academy of Sciences, ul. Baltiiskaya 43, Vladivostok, 690041, Russia
2International Center for Space Weather Science and Education, Kyushu University, Fukuoka 819-0395, Motooka 744, Nishi-ku, Japan
3Institute of Cosmophysical Research and Radio Wave Propagation, Far Eastern Branch of the Russian Academy of Sciences, ul. Mirnaya 7, Paratunka, 684034, Russia
Keywords: Magnetovariational sounding, 3D inversion, ModEM, Cape Schmidt, geoelectric section

Abstract >>
This paper presents the results of 3D inversion of magnetovariational tippers obtained at the Cape Schmidt observatory via digital processing of geomagnetic variation records performed by the MAGDAS-I magnetometer. 3D tipper frequency inversion is carried out using the ModEM software. The result is horizontal and vertical sections in the coastal strip of the Chukchi Sea in a spatial region with a dimension of 300 × 300 × 200 km along the X , Y , and Z axes, respectively. The geoelectric section of the investigated region contains both surface and deep conductive blocks located at different azimuths relative to a measurement point. The surface conductive anomaly near the observatory with a resistivity of ≈3 Ohmm is located in the southeast direction at depths of a few hundred meters. The other conducting inhomogeneity is larger and located in the same direction, but at a distance of about 25-30 km and depths of 4-16 km. The resistivity of its central region at a depth of 8 km is equal to 1 Ohmm. From the side of the South Chukchi Depression, there is an inclined conductive block in the Chukchi Sea, located closer to the coast to the bottom of the Earths crust, and the maximum resistivity of this block in the central part at a depth of 20 km is equal to ≈7 Ohmm. The most noticeable feature of the section is a massive upper-mantle conductive block under the continent at depths of 50-120 km with a resistivity of 3-4 Ohmm in the central part at depths of 70-85 km. Deeper into the continent, the block is slightly inclined to the southeast. There are visible conductive regions that connect this block with upstream conductive formations. The epicenters of weak crustal earthquakes recorded in the region are located above the northwestern marginal part of the upper-mantle block, including its central region. Possible mechanisms of high electrical conductivity of anomalous blocks are discussed.