Russian Journal of Pacific Geology最新文献

Abstract

The paper provides a detailed description of the topographical distribution characteristics, along-strike morphologic variations, and tectonic implications of the Kyushu-Palau Ridge (KPR) in the 13°–17° N regions. The description is based on multibeam bathymetric data that was recently collected in the mid-southern segment of KPR and its surrounding areas. The findings indicate that the KPR in the studied region comprises several discontinuous seamounts and seamount chains. Along the ridge running from east to west, there is significant variation in the topography of the seafloor. Visible on this ridge are numerous deep-sea depressions and apparent topographical discontinuities. Geological tectonic processes such as plate subduction and seafloor spreading largely determine the topography of the seafloor. During the beginning of the Parece Vela Basin (PVB) spreading, the pre-existing volcanic arcs fractured under east-west extensional stress, resulting in the development of prominent seamounts, sharp ridges, relief alternations, and rift basins in the KPR. The east-west stress was subsequently transmitted to the weak and inactive mid-ocean ridge in the West Philippine Basin (WPB), which was reactivated and resulted in an episode of magma-poor spreading during 30–26 Ma. This episode caused the KPR to move eastward, stretched, and destroyed its pre-existing structure.

Abstract

The issues of the environmental evolution of the western sector of the Khanka valley and the evolution of Lake Khanka are considered for the maxima of the last glaciation and Late Pleistocene lacustrine transgression. The lithological indicators and results of diatom analysis of lacustrine sediments of Lake Khanka indicate two sedimentation conditions over the period of 24.0–19.0 cal ka BP: an oligotrophic–mesotrophic moderately swampy shallow reservoir with an unstable level and a relatively deep lake. The maximum of the transgression occurred at ~19.6 cal ka BP, when the lake level was 1.5–2.0 m higher than at present for the first time. Four phases of the vegetation evolution in the western sector of the Khanka valley are distinguished: I, birch–larch mari with alder and frigid shrubs (23.8–22.8 cal ka BP); II, dark coniferous taiga with dwarf pine and a small amount of broad-leaved trees (22.8–20.6 cal ka BP); III, spruce forests with dwarf pine, fir, larch, and rare oak (20.6–20.1 cal ka BP); and IV, light birch forests and larch mari with frigid shrubs (20.1–19.0 cal ka BP). Two cooling phases are registered at the Khanka valley (~24.0 cal ka BP (cooling maximum) and 20.6 cal ka BP) and one warming phase (∼22.8 cal ka BP), which are correlated with the detailed global Greenland glacier records. The driest periods occurred at 21.7–21.2 cal ka BP. The quantitative characteristics of climate events are reconstructed from modern plant analogs.

Abstract

This article focuses on the issues related to the primary nature and metamorphism of gneisses from the Larba block of the Dzhugdzhur–Stanovoy fold area, which are based on the study of the mineralogical composition of supracrustal rocks. The PT-parameters for the highest temperature and pressure metamorphic transformations of rocks of the Larba block are estimated as P = 6.5–8.5 kbar and T > 800°C. Relatively low-temperature garnet–biotite and garnet–biotite–chlorite mineral parageneses were formed at the regressive stage of metamorphism at ~4–6 kbar and temperatures up to 500–600°C. Wide variations in paragneiss compositions indicate that the studied gneisses were derived from protoliths of different composition. Overall, Larba metasedimentary rocks had weakly and moderately weathered source areas. Most of the compositions of paragneisses are located in the field of chlorite–montmorillonite, montmorillonite–kaolinite–hydromica, and chlorite clays formed from the mafic protolith. The low alkalinity in combination with a high MgO content indicates the presence of rock erosion products with a significant admixture of basic volcanic material in the detrital material. Migmatization of the rocks is the most significant factor that distorts the reconstruction of the original rock composition.

Abstract

The structural features of rifting are reviewed during the formation of Cenozoic basins of the Western Pacific marginal seas. The examples of studied basins confirmed a hypothesis that rifting always starts from a passive phase and is only interrupted by episodes of active rifting. Rifting occurred under NE- and NNE-trending horizontal compression with the formation of either a chain of pull-apart basins or fields of diffuse rifting between the large strike-slip faults (the main part of the South China Sea). The NE and NNE directions of horizontal compression are probably related to the upper mantle convective flows from a spreading ridge of the Indian Ocean, which bear the plate fragments variously deformed during the transportation. The related NNE drift of the Indian, Australian, and Eurasian plates has no link with subduction processes from adjacent Paleopacific plates, which continue to move to northwest. The Western Pacific marginal basins mainly formed without any subduction influence from the Pacific Ocean and can be called backarc basins only based on geographical features.

Abstract

This paper discusses the problems of interpreting the study results obtained for metal and mineral microinclusions by the scanning electron microscopy with energy-dispersive X-ray microanalysis, which are related to the matrix material effect. The method was shown to work well when studying “relatively large” (5–10 μm) microinclusions for the purpose of phase identification and semi-quantitative determination of their composition normalized to 100%. The potential for “cleaning” analysis results from “additives” caused by matrix elements, as well as the criteria for assessing a distance from boundaries of a micro-object at which the matrix effect becomes minimum, are considered. The results we obtained by means of these approaches are illustrated by examples of petrology and metallogeny of igneous rocks studied in the Kamchatka region and the Stanovoy fold belt. This study made it possible to obtain new data on petrological processes in the mantle–crust substrate under the active volcanic arc and on the ore–magmatic system evolution under the ancient active continental margin.

Abstract

Brachiopods from the upper part of the Khalpirkian Regional Stage of the Tiryakh–Kobyume section of the Kobyume structural–facies zone in Southern Verkhoyanie have been studied. The collection is dominated by Strophalosia? vollossovitschi (Fredericks) and Cancrinella? sp. Single spiriferids are assigned to Crassispirifer monumentalis Abramov et Grigorjeva and Cr. aff. monumentalis Abramov et Grigorjeva, and, thus, host deposits are referred to the Crassispirifer monumentalis Beds. We found these beds earlier only in the lower part of the Khalpirkian Regional Stage. Reliable findings of the terminal brachiopod assemblage confined to the Khalpirkian of the Upper Permian have been reviewed. The assemblage is dominated by strophalosiids represented by Wyndhamia gijigensis (Zavodowsky), Marginalosia? magna Abramov et Grigorjeva, Subtaeniothaerus lungersgauzeni Solomina, and Strophalosia? aff. vollossovitschi (Fredericks). “Magadania” sp. and Cancrinella? sp. have been identified from linoproductids. Crassispirifer monumentalis spiriferids are an important element of the terminal assemblage. Athyridids of the Khalpirkian Regional Stage, known only in its lower part, are represented by Cleiothyridina nikolaevi Grunt, Cleiothyridina sp., and Bajtugania sp. The studied regional stage also contains findings of terebratulids attributed to Marinurnula? aff. mantuanensis (Campbell) and M.? aff. chivatschense (Zavodowsky). No brachiopods are known at the top of the Khalpirkian Regional Stage, while rhynchonellids Piarorhynchia sp. appear at the base of the Nekuchan Regional Stage. At the beginning of the Khalpirkian, the brachiopod community had an extended natural habitat in the shelf zone of the Verkhoyan Sea along the Angaraland coastline. Subsequently, this area was reduced and initially divided into two independent subareas confined to the East Kharaulakh and South Verkhoyan sectors. In the Late Khalpirkian, the brachiopod community was concentrated only in the Kobyume zone, where it ended up under the end-Permian mass extinction. Some taxa of the identified brachiopod assemblage have been traced in the Okhotsk region, the Ayan—Yuryakh anticlinorium, and the Omolon massif.

The study of the magnetic susceptibility of igneous rocks from the Sikhote-Alin orogenic belt has shown that zones with the predominant development of ilmenite- or magnetite-series rocks formed during the Albian–Cenomanian magmatic stage. This zonation is independent of the petrochemical composition, type, and age of rocks and has been preserved until at least the Paleocene. It also shows no correlation with terranes. According to the authors, the redox zonation in the post-Cenomanian period was mainly determined by the granite-metamorphic layer of the new continental crust of the Sikhote-Alin orogen, which formed at the beginning of the Late Cretaceous.

Original analytical data on trace elements and radiogenic Nd and Pb isotopes in the volcanic rocks of the Southern and southwestern part of the Northern plateaus of the underwater Vityaz Ridge are presented. Interpretation of these data and a comparison with published materials on the volcanic rocks from the southern and northern parts of the Kuril Island Arc (KIA), which formed on two basement blocks of different genetic nature, allow us to draw the following conclusions. The tholeiite varieties of volcanic rocks of the Southern Plateau and the southern part of the KIA have similar isotope-geochemical features, which point to the similar geodynamic conditions of the formation and the identical influence of low-temperature fluid on magma-generating processes. The geochemistry of the volcanic rocks of the Northern Plateau, which are mainly represented by subalkaline varieties, indicates a more pronounced contribution of the mantle component to the magmagenesis and a greater degree of influence of high-temperature melt compared to the rocks of the Southern Plateau, but a lesser degree compared to the rocks of the northern part of the arc. The volcanics of both plateaus are derivatives of a single mantle source, the MORB of the Indian Ocean (Indian MORB), and were formed together with the rocks of the southern part of the KIA within the lithospheric block transformed by tectonomagmatic processes that accompanied the opening of the Kuril Basin.

Abstract

This paper reports on results of bathymetric and seismoacoustic studies of the shelf in the western part of Peter the Great Bay. Five flooded coastlines, formed in the Late Pleistocene–Holocene, were revealed on the shelf. It was found that a significant part of the shelf is occupied by a zone of irregular sedimentation. This zone is underlain by an erosion surface buried under sediments of ebb–flood tidal deltas in the areas of active sedimentation and is exposed on the seafloor within the area of active modern erosion. The abrasion and formation of the irregular sedimentation zone on the shelf intensified about 11 500–11 700 years ago. Acoustic anomalies, associated with gas occurrences in sediments and gas plumes in the water column, were recorded on the shelf of the bay. Types of anomalies were classified, and a map of their areal distribution was compiled. It is concluded that the trigger mechanism that provides gas migration into sediments and to the water column is associated with a group of factors: the change of the postglacial sea level and abrasion processes, as well as the meteorological and hydrological regimes.

Abstract

This work reports on data on a moderate effusive-explosive eruption of a summit crater of Alaid volcano (Atlasova Island, Northern Kuril Islands) observed from September 10 to December 1, 2022. Based on satellite and visual data, it was determined that the strombolian-vulcanian activity resulted in two lava flows, 2.6- and 1-km long, that outflowed onto the southern slope of the volcano and at least 16 emissions occurred to heights of 2.5 to 6 km a.s.l. The ash plumes spread out mainly in the southeast and east-southeast directions with their maximum length reaching 300–530 km. The eruption posed a danger to local airlines. The lava outflow provoked the formation of lahars that descended along the southern slope of the volcano and reached the island coast. The alluvial fan formed from mudflow masses partially deposited beyond the coastline, causing accretion of the island land.