Russian Journal of Pacific Geology最新文献

This article describes the peculiarities of the deep-seated structure of sedimentary depressions in the West Kamchatka coastal zone and in adjacent Sea of Okhotsk offshore areas, which are manifested in the observed potential fields and in their transforms. To reveal geophysical regularities reflecting the old structure of the deepest part of the crystalline basement within the analyzed objects, a pseudo-gravity field calculated on the basis of analytical approximation was used. Regional and local components of the decomposition of the observed potential fields calculated by 2D adaptive energy filtering and presumably associated with different-depth structural-tectonic objects were analyzed. Based on comparison of the investigated Pustoretsky Trough with the “reference” Kolpakovsky Trough, which includes known gas-condensate fields (West Kamchatka Oil and Gas Region), conclusions were made about the similarity of their formation conditions and subsequent evolution. A regional geophysical model of the Pustoretsky Trough was compiled, which made it possible to reasonably assume its formation in the zone of Cenozoic subduction processes caused by the interaction of lithospheric plates. This model is thought to be associated with possible presence of oil and gas prospective structures.

The paper presents and analyzes new and previously published data on track dating of detrital apatite grains isolated from different-age lithotectonic complexes of Eastern Kamchatka. The investigations were carried out in areas with different geodynamic settings: in Kronotsky Peninsula which is situated in the convergence zone of the Kamchatka margin with the relatively warm and light oceanic crust of the Pacific plate with the Obruchev Rise; in the Kamchatsky Peninsula situated in the collision zone of the Komandorsky block of the Aleutian volcanic arc and the Kamchatka continental margin; and the Kumroch Range, complexes of which belong to the Ozernoy–Valagin and Vetlovsky terranes. The analysis allowed us to distinguish several exhumation events: 29.9–34.9, 19.3–24.1, 10.2–15.4, 6.2–8.6, and 3.3–5.1 Ma. The highlighted age intervals correlate well with the main tectonic events and indicate that the accretionary and collisional processes in Eastern Kamchatka continued from the Oligocene to the Late Miocene–Pliocene.

Abstract

This work deals with the methodology and results of forecasting sandstone-type uranium deposits in the territory of the Republic of Niger, based on the use of a set of regional geological and geophysical materials and a new technology for spatial analysis of data by the method of group accounting of arguments. The necessary information on the genesis and localization of the sandstone-type uranium deposits in the study area is briefly presented. Examples of identified formal geological, tectonic, and geophysical features that characterize the location of known large objects of uranium mineralization are given. Using the example of reference ore and inherently ore-free objects, an identification analysis of the features focused on determining their total information content has been carried out. The results of the identification analysis made it possible to assess the weights of the features and to perform a comprehensive regional forecasting of sandstone-type uranium deposits in the territory of the Republic of Niger. Based on the results of the forecast, the three most promising sites where detailed work on the detection of industrial uranium mineralization is recommended have been defined.

Abstract

In the 90-m river bank of the middle reaches of the Icha River, thick strata of volcano-sedimentary deposits represented by pyroclastic rocks and siliceous tuffites with layers of diatomites were studied. Representative diatom assemblages assigned to the Middle Miocene Denticulopsis hyalina Zone (14.9–13.1 Ma) indicate relatively deep-water conditions of the marine basin. Comparison with the data on diatom assemblages from reference sections of West Kamchatka indicates that the deposits of the Tyrkachyn section belong to the upper part of the Kakert Formation. The petrographic composition and lithological structure of volcanogenic formations (members 1, 2, and 4) suggest that they are the product of pyroclastic flows associated with volcanic activity in central Kamchatka. It is supposed that members 1 and 2 were formed in subaerial conditions, as indicated by the complete absence of marine microfossils, signs of primary volcanic stratification, and the absence of layering. The sediments of member 4 with eroded contacts and pronounced layering apparently accumulated in a subaqueous setting.

Abstract

This paper presents the results of gravimetric studies carried out at a potash deposit to obtain the information on negative engineering–geological processes in rocks caused by subsoil development. The developed high-precision monitoring gravimetric observation method makes it possible to determine field variations over time. The created physicogeological gravimetric monitoring model is a homogeneous geological medium with an isolated area with a variable rock density. The dynamic gravity anomaly processing and interpreting based on the synthesis of qualitative and quantitative methods for obtaining the geological information from the gravimetric data have been adapted for the gravimetric monitoring purposes. The interpretation resulted in a distribution area and a probable depth interval of rock decompaction, as well as rock density variations, which characterize the decompaction rate. Examples are given in relation to testing the developed technology for dividing the decompressed zones into natural and man-made zones at the Verkhnekamskoe potash deposit in order to improve its operation safety. The monitoring gravimetric observation results can be used to predict the dangerous geological process areas and to control the subsidence of the earth’s surface.

Abstract

The paper presents findings of tectonic-stress field reconstruction in the central part of Sakhalin Island as a result of field tectonophysical studies between 2020 and 2022. The measurements made it possible to determine the predominant orientation of the main slickenside systems with different kinematics. The territory of Central Sakhalin is characterized by mainly subvertical orientation of the axis of maximum compression and by the subhorizontal and mostly submeridionally directed axis of minimum compression/extension. The main detected feature of Central Sakhalin is the predominance of horizontal extension as the stress state type for the entire area. Horizontal shear and compression settings are significant in its western part. The number of horizontal extension settings increases in the near-axial and eastern part of Central Sakhalin towards the Sea of Okhotsk. We found a cardinal difference between the stress field reconstruction data obtained there and the results of similar studies in other geodynamically active regions of Russia.

The paper presents original and published geochronological data on the volcanic rocks of the Magellan Seamounts. These data and available geological materials allow us to distinguish five major volcanic complexes of different ages in the guyots of the Magellan Seamounts: (1) Late Jurassic–Early Cretaceous (earliest Cretaceous); (2) Early Cretaceous (Aptian–Albian); (3) Late Cretaceous (Late Cenomanian–Turonian–Early Campanian); (4) Late Cretaceous (Late Campanian–Maastrichtian); (5) Cenozoic. Each of these age complexes corresponds to a peculiar tectonomagmatic stage of the evolution of the Magellan Seamounts and characterizes a specific geomorphological space (pedestal, main body, small complicating superimposed structures of the second order). This division is based on the numerous isotopic age determinations for 11 guyots obtained by K-Ar and Ar-Ar methods. The tectonomagmatic evolutionary stages of the Magellan Seamounts correspond, in general, to the periods of tectonic activation for other structures of the Pacific seafloor.

Abstract

In the northeastern Atlantic (from south to north) the archipelagoes of the volcanic islands of Cape Verde, Canary, Selvagens, Madeira, and Azores occur. These were formed on the oceanic crust in the Miocene-Quaternary times. The analysis of the geological structure of the thirty islands leads to the conclusion that half of them were destroyed at different times in the course of their evolution by catastrophic landslide processes. Some of these islands were affected by such events repeatedly. These events caused tsunamis and formed large landslide masses in the adjacent parts of the ocean. Based on the data, volcanic edifices that might have been destroyed by landslide processes are proposed. A prerequisite for landslides to appear on a volcanic edifice could be a shift of its center of gravity.

Abstract

The detailed three-dimensional density modeling results obtained for the upper part of the earth’s crust in the area located in the northeastern part of the Voronezh crystalline massif at the junction of Losevo and Vorontsovka terranes are considered. The model obtained by solving the inverse gravimetry problem characterizes the rock density distribution in the crystalline basement to a depth of 16 km. An important role in such constructions is given to the starting model of the studied environment taking into account the available a priori geological and geophysical information: the regional density model of the East European Platform lithosphere and gravity field, data on rock density in the sedimentary cover and crystalline basement, estimated “gravity” layer thickness, the geological map of the crystalline basement, and digital elevation models of the region. The three-dimensional density model made it possible to specify the geological structure of the upper crust in the study area and to trace a deep continuation of surface structures of the basement. The obtained results are reliable due to the consistency of the accepted a priori data on the environment and agreement of model and observed gravity fields in the study area.

Abstract

The tectonosphere and formation conditions of the Mozambique Ridge are considered. The Mozambique Ridge is located in the southwestern part of the Indian Ocean between the Natal and the Mozambique Mesozoic basins. The formation of the ridge is still debatable issue. The anomalous structure of the crust of the Mozambique Ridge can be explained either by underplating (thickening of the oceanic crust from below due to magmatism) or by stretching and thinning of the continental crust. Based on gravity and magnetic field anomalies, seismotomography and other geological and geophysical data, density modeling along four profiles was carried out. Physical modeling determined the formation conditions of the Mozambique Ridge, which was formed due to the splitting of the African–Antarctic continent, the presence of structural heterogeneities in the lithosphere of the African continent, and the influence of the Karoo plume.