No 4 (2024)
- Year: 2024
- Articles: 4
- URL: https://journals.eco-vector.com/0016-853X/issue/view/11441
- DOI: https://doi.org/10.31857/S0016-853X20244
Full Issue
Articles
Subduction Style at Different Stages of Geological History of the Earth: Results of Numerical Petrological-Thermomechanical 2D Modeling
Abstract
In this article we examine the effects of impact of slab rocks eclogitization on the subduction regime under the continent. Eclogitization of rocks in high-pressure metamorphic complexes occurs only in the areas of penetration of hydrous fluid. In the absence of hydrous fluid, the kinetic delay of eclogitization preserves low-density rocks under P‒T conditions of eclogite metamorphism, delaying the weighting of a slab and reducing the efficiency of the slab-pull mechanism which contributes to the steep subduction into the deep mantle. The results of numerical petrological-thermomechanical 2D modeling of subduction under the continent in a wide range of eclogitization parameters of oceanic crust rocks (discrete eclogitization) are presented. The effects of a lower kinetic delay of eclogitization in the water-bearing basalt layer, compared to the drier underlying gabbro layer, have been tested. Based on results of 112 numerical experiments with 7 variants of eclogitization ranges (in range 400–650°C for basalt and 400–1000°C for gabbro) at different potential mantle temperatures (ΔT = 0–250°C, above modern value), and steep, flat and transitional subduction regimes were identified. The mode of steep subduction occurs under modern conditions (ΔT = 0°C) with all ranges of eclogitization. Here it is characterised by an increase in the angle of subduction of the slab as the plate descends, and above the boundary of the mantle transition zone there is a flattening or and then tucking of the slab. Subduction is accompanied by the formation of felsic and mafic volcanics and their plutonic analogues. At elevated temperatures of the mantle (ΔT≥150°С) and discrete eclogitization over a wide range, the flat subduction regime is observed with periodic detachments of its steeper frontal eclogitized part. The flat subduction regime is accompanied by significant serpentinization of the mantle wedge and episodic, scarce magmatism (from mafic to felsic), which occurs at a significant distance (≥500 km) from the trench. During the transition regime, which is also realised in models with elevated mantle temperatures, there is a characteristic change occurs from flat to steep subduction, resulting in a stepped shape of the slab. As the kinetic shift of eclogitisation increases, flat subduction develops. An increase in the thickness of the continental lithosphere from 80 km to 150 km contributes to the implementation of steep subduction, while the influence of the convergence rate (5–10 cm/year) is ambiguous.
Discrete eclogitization of thickened oceanic crust and depletion of lithospheric mantle in the oceanic plate are the main drivers of flat subduction. In modern conditions, their influence becomes insignificant due to the decrease in the thickness of the oceanic crust and the degree of depletion of the oceanic mantle lithosphere. As a result, the less frequent flat movement of slabs is determined by other factors.
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Modeling of Stress-Strain State and Coseismic Effects of Epicentral Zone of Tangshan Earthquake (Southeastern China)
Abstract
The paper presents the results of numerical modeling and analysis of stress-strain state of the epicentral zone of the strong earthquake in the north-east of China, which occurred on 27.07.1976 with Ms=7.8. Many present-day works continue to discuss the reasons for such a strong earthquake, which occurred in tectonic conditions ‒ far from interplate boundaries, inside the Tangshan tectonic block bounded by tectonic faults. However, published new geodynamic, seismological, geophysical and geodetic data provide confidence in the determining role of fault tectonics in this region.
Based on the analysis of the results of modeling of the stress-strain state preceding the Tangshan earthquake with coseismic geophysical and geodetic data, we propose a model of earthquake rupture formation. The results of comparison of independent estimates of shear stresses with the results of modeling in the sources of strong earthquakes suggest that the areas of tectonic stress concentration are localized in the interfault rupture of the Tangshan fault, reaching maximum values at the termination of fault ruptures σi ≈ 50 MPa и τxy ≈ 20 MPa. The hypocenter of the main seismic event (taking into account the error of coordinate determination) is located in the region of stress intensity 35‒50 MPa and the ratio of main stresses σxx/σyy ≈ 8–10. It should be expected that these zones are the starting site of rupture, the extent of which depends on the amount of accumulated elastic potential energy of tectonic stresses in the adjacent region. For the Tangshan earthquake, this area corresponds to a high intensity of stresses exceeding 30 MPa in a band with a length more than 30 km and a width reaching 4.5 km.
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Application of Neural Network Technologies for Tectonic Earthquake Forecast
Abstract
Successful earthquake prediction includes statistical, tectonic and physical forecasting. The main requirements for this are the establishment of the laws of earthquake mechanics and control of the geodynamic state in the region at the right times. However, resolving this issue faces difficulties of both theoretical and practical nature. Despite the fact that specialists all over the World have collected the fairly complete database on earthquakes, tectonic, electromagnetic, hydrological, etc. signs of earthquakes, the very nature of predicting the future source remains uncertain. The results obtained in the world on statistical forecasting using artificial intelligence give hope for the possibility of predicting earthquakes if we combine tectonic forecasting with the destruction of materials under experimental conditions and numerical modeling under the roof of deep learning neural network technologies. The paper provides the first results of predicting medium-term tectonic earthquakes using artificial intelligence for the Fergana depression in Uzbekistan.
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Tectonic Evolution of Tuvinian Trough (Northern Part of Central Asian Orogenic Belt): Synthesis of Geological Data and Results of Feldspar Ar‒Ar Dating
Abstract
The Tuvinian rift trough, located in the northern part of the Central Asian orogenic belt (CAOB), was formed in the Early Devonian on late Proterozoic (?)‒Early Paleozoic terranes as a result of the activity of the Altai-Sayan mantle plume. The sedimentary record from the middle Paleozoic to the middle Mesozoic, preserved in the Tuvinian trough, and the middle Paleozoic igneous complexes confined to the structures of the trough, reflect the stages of evolution of the Earth’s crust in the Tuva segment, that necessary for understanding the history of the geological development of the CAOB as a whole. Dating of accessory and rock-forming minerals from igneous rocks using low-temperature geochronology methods allows us to obtain additional information about post-magmatic processes and thereby update the model of tectonic evolution of the region.
In this study, we have reconstructed the stages of tectonic development of the Tuvinian trough in the northern part of the CAOB based on the analysis of geological data and new Ar‒Ar dating data on feldspars from mafic intrusions. As a result of this study, the chronology of the previously known stages of post-magmatic processes manifested in the Tuvinian trough was clarified, and new stages were identified according to the tectonic evolution of the CAOB. Ar‒Ar dating of feldspars carried out on eight samples showed four age groups: (i) Late Devonian, (ii) middle Carboniferous, (iii) early Permian and (iv) Early Jurassic. Late Devonian (~377 and 375 Ma) ages record an impulse of mafic magmatism, widely manifested in the northern segments of the CAOB (~380‒365 Ma). Middle Carboniferous (~320 and 319 Ma) dates may be associated with the closure of the Ob-Zaisan branch of the Paleo-Asian ocean as a result of the Kazakhstan-Siberian collision. Early Permian (~290–279 Ma) ages are consistent with the formation of late Carboniferous–Early Permian (~305–275 Ma) large igneous provinces in connection with rifting processes in the northern segments of the CAOB. Finally, a single Early Jurassic (~188 Ma) age marks tectonic reorganization of the CAOB in Late Triassic‒Early Jurassic in response to (i) closure of the Paleotethys ocean with subsequent collision of the Cimmerian blocks and the southern margin of the Eurasian continent and/or (ii) activity of the Mongolian mantle plume.
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