The purpose of the work is the analysis and geological-tectonic interpretation of the anomalous gravity field of the Ukrainian Carpathians and adjacent territories, as well as the construction of a density model of the Earth's crust and upper mantle according to the international PANCAKE seismic profile. The need to build a density model along the PANCAKE profile is due to the significant interest of a number of geologists and geophysicists in the results of seismic research along this profile. It is also caused by certain discrepancies in the seismological models of different authors. The gravity modeling technique, used in the work, includes the analysis of geological-geophysical maps and models. They are related to the geological-tectonic structure of the research region, to the creation of the initial structural part of the model and to the determination of the densities of strata and blocks of the model. The geometry and densities of the model are refined by the selection method, which is based on the interactive solution of the direct problem of gravimetric and the analysis of the reasons for the inconsistency of the calculated gravity field and Bouguer anomalies. A qualitative correspondence of the density model to the tectonic interpretation of the seismic section along the PANCAKE profile was achieved by using the methods of gravity modeling. The modelling results confirm the four-layer structure of the Earth's crust: the sedimentary cover, the upper, middle, and lower parts of the crust, which differ significantly in density. There is also evidence of the difference of the ALCAPA lithospheric plate, Flysch Carpathians and Precambrian Craton in Earth's crust and upper mantle structure. The ALCAPA plate is characterized by a small thickness (up to 29 km) and a low density of the Earth's crust. The density of the ALCAPA upper mantle is lower (3.20-3.21×103 kg/m3) compared to the upper mantle under the Ukrainian Carpathians and the East European Craton (3.28-3.30×103 kg/m3). This may be related to a change of a mantle composition and increased heat flow under ALCAPA. The Ukrainian fragment of the East European craton in the PANCAKE profile zone is characterized by a typical thickness of the crust (~41-45 km). The upper part of the crystalline crust, in contrast to the middle (2.86-2.90×103 kg/m3) and the lower part (2.98-3.10×103 kg/m3), is characterized by a lower density and greater differentiation in horizontal direction and with depth (from 2.66×103 kg/m3 to 2.86×103 kg/m3). The complex transition zone (subduction zone, Carpathian Orogen) between the ALCAPA microplate and the East European Craton causes an intense negative Bouguer anomaly – the Carpathian gravity minimum, which reaches -90×10-5 m/s2. It has a complex nature: Neogene and Paleogene-Cretaceous flysch rocks low density (≤2.50×103 kg/m3) of the Boryslav-Pokuttia cover, the main huge Precarpathian sub-vertical fault (>4 km) on the extreme southwestern slope of the platform (relatively local factors) and significant deepening of the MOHO surface under the Carpathian structure (regional factor). According to our density model, the depth of the MOHO under the front of the Carpathian thrust reaches 56 km.
ДЕЯКІ ОСОБЛИВОСТІ СЕЙСМІЧНОСТІ ЗАКАРПАТТЯ Мета. Дослідження особливостей сейсмічності Закарпатського прогину та її зв'язку з розломноблоковою структурою земної кори. Методика. Для аналізу сейсмічності Закарпаття використано дані інструментальних спостережень Карпатської сейсмічної мережі за 2001-2012 рр. Проведено зіставлення карт епіцентрів землетрусів із розломно-блоковою структурою регіону. Проаналізовано розподіл гіпоцентрів землетрусів з глибиною. Виконано аналіз сейсмічної активності основних розломів Закарпатського прогину, досліджено зміни середньорічної кількості землетрусів та виділеної сумарної сейсмічної енергії за 2001-2012 рр. Проаналізовано зв'язок просторово-часового розподілу сейсмічності з тектонікою земної кори Закарпатського прогину. Результати. Досліджено просторово-часові особливості сейсмічності Закарпатського прогину за 2001-2012 рр. Встановлено, що найвища сейсмічність за розглянутий період була характерна для Закарпатського та Панонського розломів, в зонах яких відбулися відчутні землетруси поблизу м. Берегове 23.11.2006 р. (К = 12,1, М = 4,2) та с. Угля 14.12.2010 р. (К = 9,7, М = 3,2). Серед поперечних розломів за рівнем сейсмічності виділяються Латорицький, Боржавський, Виноградівський, Оашський розломи, Тячівський лінеамент, а також вузли їх перетину із Закарпатським та Панонським глибинними розломами. У розподілі вогнищ землетрусів із глибиною у Закарпатському прогині виявлено три поверхи (рівні) їх концентрації: товща осадового шару та фундаменту до глибини гранітного шару (2-10 км), верхня частина базальтового шару (15-22 км) та шар поблизу поверхні Мохо (25-35 км). Зроблено загальний висновок, що сейсмічність Закарпатського прогину зумовлена взаємодією Карпатської складчастої споруди з мікроплитами АЛЬКАПА та ТИСІЯ-ДАКІЯ. Наукова новизна. Проаналізовано просторово-часовий розподіл землетрусів з енергетичним класом K = 7-12 за даними інструментальних спостережень 2000-2012 рр. та показано їх зв'язок з розломно-блоковою структурою земної кори. Досліджено особливості розподілу вогнищ землетрусів з глибиною у зоні Закарпатського глибинного розлому. Показано, що Закарпатський розлом проявляється як субвертикальна зона і маркується гіпоцентрами землетрусів у діапазоні глибин від 2-3 км до 30-35 км (поверхня Мохо). Практична значущість. Виявлені особливості просторово-часового розподілу гіпоцентрів землетрусів, їх зв'язку з розломно-блоковою структурою земної кори можуть бути використані для вивчення сейсмічно активних зон, сейсмічного районування та оцінки сейсмічної небезпеки окремих територій та населених пунктів Закарпаття.
The purpose of the work is to study the features of seismotectonics of the junction zone of the Oash and Transcarpathian faults in the Ukrainian Transcarpathians. The research methodology combines a complex analysis of geological-tectonic, seismological, geomorphological and geodetic data on the studied area. For the clarification of coordinates and depths of local earthquake foci we applied methods of their hypocenters specification, using a calculated seismological hodograph and kinematic corrections. Geological and geophysical data, in particular on regional profiles, were used to link seismic events to specific geological structures. It was established that a number of seismic events were recorded at depths of 40-52 km in the studied zone, in its northeastern part. They occurred simultaneously with traditional deep localization of seismic sources in the crust of the Transcarpathian depression (0-27 km deep), including at the depths of the sole of thrust sedimentary strata / roof of the pre-Neogene basement of the Transcarpathian depression (2-3 km deep) in the southwestern part of the studied area. Since these earthquakes are spatially localized to the northeast of the zone where the Transcarpathian deep fault is traced by geological and geomorphological data on the surface, they indicate the subduction of the crustal structures of the Transcarpathian trough in this direction under the thrusts of the Folded Carpathians. This is also evidenced by the corresponding slope of the seismofocal zone at the intersection of the Transcarpathian Deep Fault at greater (15-30 km) depths. These and other features of local seismotectonics reflect the so-called "crocodile" tectonics in the Ukrainian Carpathians. The features encompass the reverse, south-west slope of the seismofocal zone at shallower (0-12 km) depths, as well as the features of the relief of the Carpathians in the studied seismogenic zone. They fully correspond to the characteristics of the "alpine" and "terrain" geodynamics of the region, i.e. compression and displacement in the northeastern and eastern directions, respectively. For the first time, the research established the presence of relatively deeper (30-55 km) seismic activity in the earth's crust of the Ukrainian Transcarpathians – in the zone of subduction of the Moho border under the Carpathians in the area of junction of the Transcarpathian and Oash deep faults. Additionally, the geodynamic and tectonophysical justification for its presence was given in terms of combination of "alpine", terrane and asthenolitic geodynamics of the region. Taking into account the features of seismotectonics of the Oash and Transcarpathian faults junction zone will contribute to clarifying the assessment of the characteristics and peculiarities of the spatial distribution of natural geoecological, in particular, seismotectonic risks and hazards in the central part of the Ukrainian Transcarpathians.
Purpose. To investigate structure-tectonic and faults features of the central part of the Transcarpathian internal depression and their relations with earthquake epicenters localization and peculiarities of seismic waves spreading on the base of instrumental observations in regime geophysical observatories of the Carpathian geodynamic polygon. Methods and results. The study developed methodology for specification of local earthquake focus depths by means of neuron-network modeling, computer processing and systematization of observation data at the Carpathian geodynamic network and adjusting areas on the basis of recent mapping and graphical information. On the examples of earthquakes registered by the regime geophysical station "Mukachevo", focal mechanisms of earthquakes were determined using first arrival polarities. Scientific novelty. The research determined the relationships between structural and tectonic features in the central part of the Carpathian backdeep, the recent geodynamics of the region's basement, peculiarities of seismic waves spreading, and formation of local earthquake focuses. It was stated that the influence of the igneous rocks of the Vyhorlat-Hutyn volcanic ridge significantly reduced the time of seismic waves passing in the sedimentary layer and partly in the basement. To determine a focal mechanism of the earthquake, the velocity of seismic waves in the layers should be taken into account on a case-by-case basis, taking into account the location of seismic stations with respect to deep and surface faults and the impact of volcanic rocks, and not using standard velocity models for seismic stations resulting from the stations that give significant results in calculations of output seismic wave angle and facilitate the selection of nodal surfaces. Practical value. Detailed investigations allow determining characteristics of earthquake epicenters, velocities and direction of seismic waves spreading in accordance with the structure of the basement and sedimentary layer. This will, in turn, provide an opportunity to supplement the data of long-term monitoring of natural and technological hazards in the region.
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