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Russian Geology and Geophysics

2019 year, number 3

1.
A POSSIBLE RELATIONSHIP BETWEEN DEEP EARTHQUAKES AND THE STRUCTURAL TRANSITION OF SUBMOLECULAR SiO2 FRAGMENTS IN ROCKS OF A SUBDUCTING OCEANIC SLAB

M.I. Kuzmin1, R.G. Khlebopros2,3, A.N. Didenko4, S.G. Kozlova5,6, V.E. Zakhvataev2,3
1A.P. Vinogradov Institute of Geochemistry, Siberian Branch of Russian Academy of Sciences, ul. Favorskogo 1A, Irkutsk, 664033, Russia
2Federal Research Center Krasnoyarsk Scientific Center, Siberian Branch of the Russian Academy of Sciences, pr. Svobodnyi 78, 660041, Krasnoyarsk, Russia
3Siberian Federal University
4Pacific National University, ul. Tikhookeanskaya 136, Khabarovsk, 680035, Russia
5A.V. Nikolaev Institute of Inorganic Chemistry, Siberian Branch of the Russian Academy of Sciences, pr. Akademika Lavrenteva 3, Novosibirsk, 630090, Russia
6V.G. Shukhov Belgorod State Technological University, ul. Kostyukova 46, Belgorod, 308012, Russia
Keywords: Phase transitions, deep-focus earthquakes, upper-lower mantle, lithospheric plate, subduction

Abstract >>
Quantum-chemical calculations show a tendency of SiO2 molecule to transform from linear to isomeric cyclic (bent) form and back. In the latter case, the energy released during the transition isomeric SiO2 ® linear SiO2 is about 240 kJ/mole. This hypothetic structural transition of submolecular SiO2 fragments in mantle minerals is supposed to initiate deep-focus high-energy earthquakes at the upper-lower mantle boundary. It is at this depth (600-670 km) that the subducting oceanic slab is delaminated: Its upper part moves «horizontally along the upper-lower mantle boundary, while its lower part separated into blocks subsides into the lower mantle and reaches the D″ layer to accumulate there.

DOI: 10.15372/RGG2019038
																				  																		



2.
GEODYNAMIC COMPLEXES AND STRUCTURES OF TRANSBAIKALIA: RECORD IN GRAVITY DATA

N.L. Dobretsov1,2, M.M. Buslov2,3,4, A.N. Vasilevsky1,2
1A.A. Trofimuk Institute of Petroleum Geology and Geophysics, Siberian Branch of the Russian Academy of Sciences, pr. Koptyuga 3, Novosibirsk, 630090, Russia
2Novosibirsk State University, ul. Pirogova 2, Novosibirsk, 630090, Russia
3V.S. Sobolev Institute of Geology and Mineralogy, Siberian Branch of the Russian Academy of Sciences, pr. Koptyuga 3, Novosibirsk, 630090, Russia
4Kazan Federal University, ul. Kremlevskaya 18, Kazan, 420008, Russia
Keywords: Tectonics, geodynamics, correlation, free-air and Bouguer gravity anomalies, Baikal rift system, Mongolia-Okhotsk orogen, Angara-Vitim batholith

Abstract >>
The Transbaikalian region comprises several known geologic structures: the Mesozoic Mongolia-Okhotsk orogen, the Cenozoic Baikal rift system, and the world largest Angara-Vitim granitic batholith. They all formed upon heterogeneous Neoproterozoic-Early Paleozoic continental-margin complexes of the Siberian craton. The region is subject to the influence of mantle plumes, which induced Meso-Cenozoic volcanism and controlled structural and lithological changes in the crust in the early history. Transbaikalia, which has been a scene of multiple tectonic events, is a model area for geophysical (in particular, gravity) surveys for various geological and geodynamic applications. As a novel approach, we interpret geological and geodynamic data from the region with reference to the pattern of free-air and Bouguer gravity anomalies revealed by satellite altimetry. Bouguer anomalies highlight large structures in the lithospheric mantle which were produced in the Cenozoic mainly by the activity of mantle plumes. Basaltic lava fields were confirmed to be almost coeval with mantle anomalies and to record the presence of the plume head at the crustal base. However, the origin of the Late Paleozoic Angara-Vitim granitic batholith was only tentatively attributed to the plume activity, based on gravity data. Mesozoic metamorphic core complexes (MCC) and basins that formed during the evolution of the Mongolia-Okhotsk orogen show up clearly on the map of free-air anomalies. Most of the MCC revealed in Transbaikalia coincide with oval gravity highs and border negative elongate features corresponding to Mesozoic basins. The zone of Cenozoic tectonism stands out in the pattern of free-air anomalies as maximum gravity contrasts, with the values changing from -110 to -120 mGal in basins to +90 or +100 mGal in ranges. This zone encompasses rift basins filled with Cenozoic or, locally, Mesozoic sediments, which jointly form a domino-like system of rhomb-shaped structures typical of the Baikal rift system and, in general, of Cenozoic Central Asia resulted from the far-field effect of the India-Eurasia collision.

DOI: 10.15372/RGG2019021
																				  																		



3.
STRUCTURE OF THE MONGOL-OKHOTSK OROGENIC BELT AND THE PROBLEM OF RECOGNITION OF THE AMUR MICROCONTINENT

I.V. Gordienko1, D.V. Metelkin2,3, L.I. Vetluzhskikh1
1Institute of Geology, Siberian Branch of the Russian Academy of Sciences, ul. Sakhyanovoi 6a, Ulan-Ude, 670047, Russia
2Novosibirsk State University, ul. Pirogova 2, Novosibirsk, 630090, Russia
3A.A. Trofimuk Institute of Petroleum Geology and Geophysics, Siberian Branch of the Russian Academy of Sciences, pr. Akademika Koptyuga 3, Novosibirsk, 630090, Russia
Keywords: Neoproterozoic, Vendian, Early Cambrian, paleomagnetism, sedimentary basins, paleotectonic reconstructions, Mongol-Okhotsk belt, Amur microcontinent, Argun terrane, Siberian paleocontinent

Abstract >>
A new concept of the geologic structure and geodynamic evolution of the Mongol-Okhotsk Orogenic Belt is proposed. The problems of recognition of the Amur composite microcontinent (Amuria superterrane) and substantiation of its existence are considered. We present recent data on the geologic structure, composition, age, and paleomagnetism of the Neoproterozoic-Paleozoic complexes, one of the main elements of Amuria: Argun terrane and adjacent Transbaikalian and Mongolian structures. In particular, we refine the age of Precambrian and Paleozoic stratified and igneous units. The absence of an Archean-Paleoproterozoic crystalline basement is shown. Using our paleomagnetic and paleontological data, we substantiate the equatorial position of the Argun terrane in the immediate vicinity of Siberia in the period 560-525 Ma. The results of the research and analysis of available geological data on the Argun terrane and adjacent Transbaikalian and Southeast Asian structures clearly show the fallacy of the concept of the Amur composite microcontinent as a single tectonic element whose collision resulted in the folded structures of the Mongol-Okhotsk belt. This conclusion is of crucial importance for the reconstruction of the Neoproterozoic, Paleozoic, and Mesozoic geodynamic evolution of the eastern part of the Central Asian Orogenic Belt.

DOI: 10.15372/RGG2019018
																				  																		



4.
AGE AND ISOTOPE-GEOCHEMICAL FEATURES OF THE MURZINKA-ADUI METAMORPHIC COMPLEX IN CONNECTION WITH THE PROBLEM OF FORMATION OF THE MURZINKA INTERFORMATIONAL GRANITE PLUTON

G.B. Fershtater1, A.A. Krasnobaev1, P. Montero2, F. Bea2, N.S. Borodina1, M.D. Vishnyakova1, N.G. Soloshenko1, M.V. Streletskaya1
1Institute of Geology and Geochemistry, Ural Branch of the Russian Academy of Sciences, ul. Akademika Vonsovskogo 15, Yekaterinburg, 620016, Russia
2Departamento Mineralogía y Petrología, Campus Fuentenueva, Univ. Granada, 18002, Granada, Spain
Keywords: Paragneisses, orthogneisses, granites, protolith, paleocontinental sector of northwestern megablock, zircon isotope parameters, Rb-Sr age, Middle Urals

Abstract >>
The chemical composition of rocks of the Murzinka-Adui metamorphic complex and the Murzinka granite pluton, a reference interformational granite pluton in the Urals, is considered. A detailed comparative analysis of ancient gneisses and related granite veins included an isotope-geochemical study of zircons from both groups of rocks. Zircons are subdivided into seven age groups (I, 1588 20 Ma; II, 1060 28 Ma; III, 530 11 Ma; IV, 380 6 Ma; V, 330 9 Ma; VI, 276 3 Ma; and VII, 260 3 Ma). The first four groups are apparently zircons from gneisses, reworked to different extents, and the other three groups are zircons crystallized during granite genesis. The gneisses and most of the granite samples contain zircons of all the above age populations, which is evidence of trapping zircons from gneisses by granite melts, on the one hand, and the occurrence of «granite-derived zircons in gneisses, on the other. The granitoids and gneisses of all types differ considerably in geochemical features. The behavior of trace elements and the Rb-Sr ages indicate that the formation of granites of the Murzinka massif was a discrete episode of magmatic activity. The Sr isotope ratios in the granites and gneisses indicate different degrees of the mantle-crust interaction and the participation of the material of the crystalline basement and newly formed crust in their formation.

DOI: 10.15372/RGG2019039
																				  																		



5.
HIGH-REE GABBROIDS AND HORNBLENDITES OF THE ILMENY MOUNTAINS (Urals)

V.G. Korinevskii1,2, E.V. Korinevskii1,2
1Institute of Mineralogy, Ural Branch of the Russian Academy of Sciences, Miass, Chelyabinsk Region, 456317, Russia
2Ilmeny State Reserve, Miass, Chelyabinsk Region, 456317, Russia
Keywords: Zoisite gabbro, anorthite-amphibole gabbroids, hornblendites, metaultrabasic rocks, REE, Ilmeny complex, Urals

Abstract >>
Chaotically localized isolated small bodies of metaultrabasic rocks have been found in the quartzite-schist strata of the Ilmeny metamorphic complex in the South Urals. These are metamorphosed rootless blocks and lumps of serpentinite melange within the so-called Urazbaevo olistostrome. Sometimes they contain lumpy inclusions of massive anorthite gabbroids with gabbro, ophitic, and cumulative textures, free of crystallization schistosity, and of different mineral compositions. The rocks have abnormally high contents of Al2O3, CaO, MgO, and REE and low contents of SiO2 and are characterized by weak secondary alteration. Seldom, inclusions of hornblendites, along with anorthite, spinel, apatite, enstatite, diopside, and rutile, are present. Some gabbroid and hornblendite bodies have abnormally high contents of REE, with a strong predominance of LREE (81-93% of the total REE). The maximum contents of REE have been established in zoisite amphibolites (170-850 ppm) and apatite-garnet-containing hornblendites (up to 450 ppm). The conclusion has been drawn that the rocks formed in the basement of the Earths crust and got with protrusions of serpentinite melange to the surface.

DOI: 10.15372/RGG2019023
																				  																		



6.
PALYNOLOGICAL CHARACTERISTICS OF MIOCENE DEPOSITS FROM THE SUBMARINE YAMATO RISE (Sea of Japan)

M.T. Gorovaya, N.G. Vashchenkova
V.I. Ilichov Pacific Oceanological Institute, Far Eastern Branch of the Russian Academy of Sciences, ul. Baltiyskaya 43, Vladivostok, 690041, Russia
Keywords: Spore and pollen complex, Miocene, Yamato Rise, Sea of Japan

Abstract >>
Four heterochronous spore and pollen complexes (SPC) have been identified in the deposits of the submarine Yamato Rise: SPC-1 (Early Miocene), SPC-2 (the end of Early and the beginning of the Middle Miocene), SPC-3 (Middle-Late Miocene), and SPC-4 (Late Miocene). Pollen of various woody plants dominates in SPC-1; the families of gymnosperms (Pinaceae) also prevail in this complex. The climate was moderate and humid because of the proximity to the water area. The complex SPC-2 differs from SPC-1 in the highly increased role of thermophile angiosperms due to the impact of the climatic optimum between the Early and Middle Miocene. The complex SPC-3 is distinguished by the diversity and predominance of broad-leaved woody plants. Gymnosperms are mainly represented by pine families. The composition of palynoflora points to a dissected relief and the burial of pollen not far from the places of its growth. Gymnosperms with a predominance of the Taxodiaceae family are characteristic of SPC-4. The SPC-4 composition is indicative of humid habitat conditions, marshy shoreland, and pollen burial in places of plants growth. The climate was moderate and humid. Palynological investigation suggests the existence of a dry land in the area of the current Yamato Rise throughout the Miocene. The relief was highly dissected and mountainous in the Early and early Middle Miocene; then, the relief roughness and firm-land area reduced dramatically.

DOI: 10.15372/RGG2019019
																				  																		



7.
DYNAMICS OF THE ENERGY STRUCTURE OF SEISMICITY ON THE SOUTHWESTERN FLANK OF THE BAIKAL RIFT SYSTEM: ANDRONOV-HOPF BIFURCATION

A.V. Klyuchevskii, V.M. Demyanovich
Institute of the Earths Crust, Siberian Branch of the Russian Academy of Sciences, ul. Lermontova 128, Irkutsk, Russia, 664033
Keywords: Baikal Rift System, southwestern flank, seismicity, energy structure, dynamics model, Andronov-Hopf bifurcation

Abstract >>
The dynamics of the energy structure of seismicity on the southwestern flank of the Baikal Rift System (BRS) was analyzed at three hierarchical levels based on annual and accumulated total data on KP ≥ 8 earthquakes that took place in 1964-2013. The state changes are characterized by three main parameters: the maximum energy class (Kmax), the slope of the earthquake recurrence plot (γ), and seismic activity A10. With an increase in the period and number of analyzed earthquakes, the parameters reach the limiting values reflecting stable regularities in the long-term distribution of shocks by classes. Two attractors are recognized in the phase pattern, which reflect the total quasi-stationary state (limit cycle) and local instability (focus) of the energy structure of seismicity. The specifics of the attractor formation suggest that the dynamics of the system includes a cycle birth bifurcation (Andronov-Hopf bifurcation). For a detailed analysis of the dynamics of formation of the energy structure of seismicity, we calculated the slopes of the earthquake recurrence plots in the model scenarios of aftershock and swarm series of shocks. Comparison of the model and real changes in the slopes shows that the aftershock model corresponds to the dynamics of the energy structure of seismicity of the lithosphere on the southwestern flank of the BRS.

DOI: 10.15372/RGG2019020
																				  																		



8.
MODIFICATIONS OF THE TREFFTZ METHOD FOR ESTIMATING THE CONTRIBUTION OF THE HALL EFFECT ON MAGNETOTELLURIC SOUNDING

V.V. Plotkin1, V.S. Mogilatov1,2, V.V. Potapov1
1Trofimuk Institute of Petroleum Geology and Geophysics, Siberian Branch of the Russian Academy of Science, pr. Akademika Koptyuga 3, Novosibirsk, 630090, Russia
2Novosibirsk State University, ul. Pirogova 2, Novosibirsk, 630090, Russia
Keywords: Magnetotelluric sounding of a three-dimensional heterogeneous earth, Hall effect, Trefftz method, electrical conductivity

Abstract >>
Possible manifestations of the Hall effect in the Earths magnetic field during magnetotelluric sounding are considered. Numerical calculations are made for the magnitude of the effect for a three-dimensional heterogeneous earth, using modifications of the Trefftz method suitable for accounting for anisotropy. Versions of the measurement that allow easy detection of manifestations of the Hall effect are analyzed.

DOI: 10.15372/RGG2019017