Home – Home – Jornals – Russian Geology and Geophysics 2023 number 5

We performed a petrochemical study and determined the Sm-Nd isotope composition of the Sumian (early Paleoproterozoic) mafic metavolcanic rocks in the Vodlozero domain (Semch’, Kumsa, and Kamennye Ozera structures) and the Central Karelian domain (Lake Vottomuks and Lake Kogu structures, the western flank of the Lekhta structure) of the Karelian province of the Fennoscandian Shield and compared them with the coeval volcanic rocks of the Kola-Norwegian province and the intrusive rocks of the Karelian and Belomorian provinces. The Sumian mafic rocks intruding the rocks of the Archean domains with different crust ages (>3.1 and 2.7-2.8 Ga) have similar geochemical characteristics (SiO₂ = 44-55 wt.%, MgO = 5-14 wt.%; LREE enrichment, (La/Yb)_N = 5-17) and similar values of ε_Nd( T ) (from -2.7 to -0.5) and T_Nd(DM) (from 3.0 to 2.7 Ga), which distinguishes them from their Archean analogues and indicates a single large magma source. The crustal contamination of the primary melts did not exceed 15% and could not determine the isotope-geochemical specifics of the Sumian mafic rocks. Their specific composition is better explained by the model of the formation of primary melts through the low-pressure (2.5-3.0 GPa) melting of the Neoarchean metasomatized zones of the subcontinental lithospheric mantle, with the addition of the primitive-mantle melts. In the period 2.41-2.45 Ga, the early Paleoproterozoic mafic rocks of the Fennoscandian Shield formed in the intracontinental magma generation setting.

The Novaya Zemlya archipelago is an early Cimmerian folded structure located on the northern periphery of the Ural-Mongolian Fold Belt. The geological development of the archipelago is well-studied and serves as a benchmark for interpolating its features to the shelf of the surrounding water areas, whose sedimentary cover contains large accumulations of hydrocarbon raw materials. In addition to the stratified section of essentially sedimentary strata, it is of interest to study igneous formations that clearly mark tectonic events in the geological history of the archipelago. In the light of the latter, the age of these formations remains important. The age of some magmatic complexes of the archipelago remains debatable. This paper touches upon a lamprophyre complex in the extreme south of Novaya Zemlya, which until recently has been dated to the late Proterozoic or late Proterozoic-early Paleozoic (?). The results obtained for the U-Pb age of apatites contained in the rocks of this complex confirm the Devonian age of lamprophyres, and it is suggested by estimates based on the presented data that the emplacement time lies in a range from 360 to 398 Ma. This means that they can be associated with riftogenic processes on Novaya Zemlya since the second half of the Early Devonian, when the stage of short-term stable carbonate sedimentation (in the first half of the Devonian period) was replaced by the initiation of a new structural-formational zonal sequence.

We report results of a detailed study of the paleomagnetic record in the sedimentary rocks of the Taseeva Group of the Yenisei Ridge in three typical sections in the lower courses of the Angara, Taseeva and Irkineeva rivers. Our results confirm that the geomagnetic field was in an anomalous state at the Precambrian-Phanerozoic boundary. It is well known that Ediacaran rocks in general have preserved several different paleomagnetic directions that do not conform to the geocentric axial dipole model. For example, Siberian sections display two equally valid groups of paleopoles that cause many debates over the geometry of the geomagnetic field and whether any of the components correspond to its dipole configuration. The paleomagnetic record we studied is unique in that the rocks of the Chistyakovka and Moshakovka formations have captured both these components, which is factual evidence of a synchronous existence of two sources. To explain these findings, we propose an original hypothesis in which the bipolar component that is widely present in the rocks and corresponds to the Madagascar group of paleomagnetic poles is associated to the field of the geocentric axial dipole. The less widespread monopolar component corresponding to the Australian-Antarctic group of poles is reflective of a stationary anomalous source. The recording of this source became possible due to the abrupt decrease in the strength of the virtual dipole moment that probably was at its lowest during the accumulation of the Chistyakovka and Moshakovka formations. The new paleomagnetic pole calculated for the bipolar component - 39.2°N, 61.1°E - plots on the apparent polar wander path for Siberia and can be considered a key determination for the age ~570 Ma.

The Salair fold-thrust orogenic belt (Salair orogen, Salair) is located in the northwestern Altai-Sayan fold area within the Central Asian Orogenic Belt. The Salair orogen is an allochthon overriding the Kuznetsk Basin on a system of imbricate thrusts. The southern flank of the Salair thrust system is tectonically juxtaposed against the Gornaya Shoria terrane which differs markedly from Salair in its geological setting. The Salair and Gornaya Shoria terranes are separated by the Nenya-Chumysh Basin, a deep Mesozoic trough. The Salair orogen is composed of Cambrian-Early Ordovician island arc volcanic and sedimentary rocks, widespread garnet amphibolites and gneisses of the Angurep complex in its southern flank, and the Shalap subduction-related melange in the Alambai ophiolite suture. The southern Salair orogen and its junction with Gornaya Shoria have been imaged down to the lower crust by magnetotelluric (MT) soundings, which is an efficient tool for investigating the deep structure and tectonic history of orogenic areas. The MT surveys were performed at 25 stations on a 120 km long profile. MT data revealed an up to 70 km wide low-resistivity zone (a conductor) traceable till a depth of 20 km between the Salair and Gornaya Shoria terranes. The low-resistivity zone has a complex structure with its outer and interior boundaries dipping almost vertically. The conductor lies under several major geological structures: the Shalap melange, the Nenya-Chumysh Basin, and the NE trending Altai-Salair right-lateral strike-slip fault. The Altai-Salair fault, along which the Salair allochthon was displaced relative to Gorny Altai and Gornaya Shoria, joins the Salair system of imbricate thrusts. The Nenya-Chumysh Basin at the Salair-Gornaya Shoria junction is a deep trough having an asymmetric transversal profile with a steep western side and a shallower-dipping stepped eastern side. The southeastern flank of the basin is a wide area of thin sediments over the Paleozoic basement dipping gently in the northwestern direction. The revealed deep structure of the Nenya-Chumysh trough is consistent with its tectonic model implying an Early Cretaceous basin superposed over an early Jurassic pull-apart basin. Early Mesozoic motions on major faults is a regional-scale phenomenon known from many areas of southern West Siberia.

The surface pattern of permafrost area in the Tibetan Plateau, with 15-20 m polygons, resembles the patterned ground of the Arctic periglacial loess plains in northeastern Eurasia and North America. However, unlike the Arctic plains, it consists of semi-stabilized modern sand dunes, up to 2.5-3.0 m high, and U-shaped epigenetic ice wedge casts inherited from an ancient polygonal network on the surface of a 10-12-m terrace of the Yangtze River. The polygonal dunes and the U-shaped sand wedges were studied in the Yangtze headwaters in the vicinity of the high-altitude research station Bei-Lu-Xe. The polygons have desert pavement floors with ventifacts, composed of Late Pleistocene alluvial gravel and debris. The dunes and wedges consist of well sorted quartz-carbonate sand with an average grains size of 2.0-2.2 mm. Ground Penetrating Radar surveys in the area revealed the permafrost table at depths varying from 2 m to 4 m below the dunes. The polygonal dunes may have formed by biogenic-aeolian deposition over an ancient system of ice wedges.

The rarely visited islands of the Kara Sea in its Pritaymyr zone have been studied during a geological survey on the scale 1: 1,000,000. The Quaternary deposits of the islands are represented by the Middle Neopleistocene to Holocene marine, lacustrine-marsh, alluvial and slope sediments. New radiocarbon and uranium-thorium dating has been obtained for the Quaternary sediments, using organic remains (wood, peat, mollusk shells). For the first time, dating on sandy sediments was obtained for the islands, using the method of infrared optical-stimulated luminescence (IR-OSL). The sediments of marine terraces are of the greatest distribution on the islands; these sediments can be correlated with the Karginsky (MIS 3) horizon, lacustrine-marsh sediments and deposits of the cover complex, containing flora remains, mainly the Holocene.

The paper focuses on the surface topography and deposits associated with the Late Pleistocene ice-dammed lake in the valley of the Kharal River dammed by the Azas ice sheet in a depression between the East Sayan and Akademik Obruchev Range. The study includes paleogeographic reconstructions for the time spans before the lake formation and drainage; reconstruction of vegetation in the lake area based on pollen and paleocarpological data; and dating by optically stimulated luminescence (OSL) and ⁴⁰Ar/³⁹Ar methods. The OSL and Ar-Ar dates provide more rigorous constraints on the history of deposition and topography in the western Serligkhem Basin. The territory was affected by erosion and subsequent basaltic volcanism at 6.9 ± 0.7 and 2.2 ± 0.7 Ma. The erosion valleys were dammed by the edge of an ice sheet and then by its moraine, which produced large Upper Kharal damlake. According to OSL data, the lake existed between 36 ± 4 and 14 ± 3 ka BP. The related glacial, glaciolimnic, and glaciofluvial facies in the area were deposited at the end of the Late Pleistocene (stages III and IV of regional stratigraphy).

Magnetic and density depth models were calculated for the Malmyzh, Pony-Muli, and Anadzhakan porphyry-copper ore clusters emplaced in the northern part of the Middle Amur sedimentary basin. Based on analysis of anomalous magnetic field (magnetic model) data, a magmatogenic ring structure determining spatial patterns in localization of porphyry-copper mineralization in the Malmyzh and Pony-Muli ore clusters was established. It is shown that similar structures in the anomalous magnetic field are reported for many porphyry copper deposits. The area of the identified geologic structure can be considered as a high-potential region for detection of gold-copper-porphyry ore mineralization. Within the ring structure, perspective areas containing granitoid and diorite intrusions developed along its periphery and seated beneath the Middle Amur basin sedimentary cover can also be identified. Geophysical depth models are calculated for individual ore clusters with Au-Cu porphyry mineralization, to determine subsurface lithology and structures that may be useful for assessing regional ore potential and deducing areas for metallogenic exploration. The geophysical data-based deep subsurface model for the Malmyzh ore cluster is found to be consistent with the integrated petrological models developed by J.P. Richards and R.H. Sillitoe for giant Cu-porphyry deposits.

Based on the analysis of the 2017-2021 recordings from the Kharino seismic station (the Altai-Sayan seismic stations network) and the 2002-2019 satellite imagery, the trigger of the July 18, 2020 earth-slide at Elbashinsky waste rock dump of the Kolyvan Anthracite Deposit (Novosibirsk region) has been established. The study of natural oscillations of the mine waste dump allowed estimating its stability and thus contributing to environmental and industrial safety in surface mining of solid minerals.