ГЛУБИННОЕ СТРОЕНИЕ И ПАЛЕОГЕОДИНАМИКА СИБИРИ И ЦЕНТРАЛЬНОЙ АЗИИ В РАБОТАХ ИНСТИТУТА ЗЕМНОЙ КОРЫ СО РАН (2009–2013 ГГ.)

The deep structure and paleogeodynamics of the Siberian craton and the Central Asian folded belt are studied by two specialized laboratories of the Institute of the Earth’s Crust, SB RAS – the Laboratory of Complex Geophysics and the Laboratory of Paleogeodynamics. A variety of modern geophysical methods is applied. Surface wave tomography is focused on determination of 3D velocity structure of the upper mantle. Seismic, gravimetric and electrical surveys are aimed at stu­dies of structures of the crust and the upper mantle. Geothermic parameters of the lithosphere in Central Asia are measured. In search for mineral resources, new geophysical prospecting technologies are developed. Quality paleogeodynamics reconstructions require a proper understanding of the structural evolution of the Siberian craton and the Central Asian folded belt. Researches in this scientific field are conducted by the Laboratory of Paleogeodynamics. Besides, the Laboratory studies new minerals and conditions of their formation. Some of the scientific research projects are jointly implemented by the two laboratories, and research results are regularly published in Russia and abroad.   Изучением глубинного строения и палеогеодинамики Сибирского кратона и Центрально-Азиатского складчатого пояса занимаются две специализированные лаборатории ИЗК СО РАН: лаборатория комплексной геофизики и лаборатория палеогеодинамики. Использование геофизических методов в рамках данной тематики ориентировано на определение трехмерной скоростной структуры верхней мантии методом поверхностно-волновой томографии, изучение строения земной коры и верхней мантии сейсмическими, гравиметрическими и электроразведочными методами, измерение геотермических параметров литосферы Центральной Азии и разработку новых технологий поиска месторождений полезных ископаемых. Выполнение качественных палеогеодинамических реконструкций требует расшифровки сценария становления структуры Сибирского кратона и Центрально-Азиатского складчатого пояса. Исследованием этой проблемы занимаются сотрудники лаборатории палеогеодинамики. Отдельным направлением в данной лаборатории является изучение новых минералов и выяснение условий их образования. Сотрудники лабораторий ведут скоординированные научные исследования, результаты которых активно публикуются в ведущих российских и зарубежных научных изданиях.

Структура земной коры Улан-Баторского района Монголии по гравиметрическим данным

Establishing relationshipsbetween the regional seismicity andthe depth geometry and compositions of large-size density inhomogeneities of the upper crust is one of the ways to determine the parameters of mean long-term seismicity. Theoriginof geological bodies and mechanisms of penetration of intrusive bodiesinto the upper crust aredetectable from the shapes and sizes of such bodies.Knowledge on the shapes and sizes of large intrusive bodies is important for prospecting and exploration of mineral resources. Based on the medium-scale areal gravimetric survey data, a regional map of Bouguer gravity anomalies was constructed for theUlaanbaatar region, Central Mongolia.Interpreted gravimetric data from the solutions of the inverse problem of gravimetry based on the body-shape selection were used to developthe 3D models showing the largest crustal density inhomogeneities of the study area – the Tolskaya (Ulaanbaatar) basin, Nalaiha depression and Bogdoulin granite massif. It is confirmed that the basement of these depressionshas a block structure. The basement of the Tolskaya basinincludes two depressions, the western and eastern ones.In the western depression, the sediment thickness is 150 m. It amounts to 400 m in the eastern depression at the intersection of the Selbin and Tolskaya deep fault systems. The information on the shapes and thicknesses of the sediment bodiesisused in the seismic studies aimed at obtaining a more precise assessment of seismic hazard of the Ulaanbaatar city and the region. In the Nalaiha depression, the sediments are 800 m thick.The morphology of the basement and the thickness of sediments in this depression can be used to update the mining prospects of the Nalaiha coal deposit. The maximum thickness of granitoids in the Bogdoulin massif is 9.0 km. The area of its projection to the ground surface is twice as big as the area of the outcrops. Our research results can be useful for assessing the metallogenic specialization of granitoids, which indicators in relation to some minerals are related to the specific features of the Bogdoulinsky massif. Such indicators have beendiscovered by the geological, geophysical and geochemical methods. Our study shows that the geophysical methods (in particular, gravimetry) can considerably expand the knowledge of the morphology of geological objects at depth and facilitate reaching a new level in structural analysis, which is critical for developingnew ideas to clarify the geological history and tectonic conditions of the formation of the Mongolia-Siberian region.Одним из способов оценки параметров долговременной средней сейсмичности является установление связи последней с глубинной геометрией и составом крупных плотностных неоднородностей верхней части земной коры. Кроме того, сведения о форме и размерах геологических тел дополняют информацию о их образовании и механизме проникновения (для интрузивных тел) в верхние части земной коры. Не менее важны знания о форме и размерах крупных интрузивных тел для поисков и разведки полезных ископаемых. По результатам площадной гравиметрической съемки среднего масштаба, выполненной на территории Улан-Баторского района (Центральная Монголия), построена карта аномалий силы тяжести в редукции Буге. На основе интерпретации полученных гравиметрических данных (решение обратной задачи гравиметрии методом подбора формы тел) определены 3D-модели крупнейших плотностных неоднородностей земной коры района – Улан-Баторской (Тольской) впадины, Налайхинской котловины и Богдоулинского гранитного массива. Установлено, что фундамент депрессий имеет блоковое строение. В Тольской впадине он осложнен двумя котловинами – западной и восточной. Мощность осадочных отложений в западной котловине составляет 150 м, в восточной котловине (в районе пересечения Сэльбинской и Тольской систем глубинных разломов) может достигать 400 м. Сведения о форме и размерах осадочных отложений будут использованы при инженерно-сейсмологических работах по уточнению сейсмической опасности города Улан-Батора. Мощность осадочных отложений в Налайхинской котловине достигает 800 м. Данные о морфологии фундамента и мощности осадков во впадине могут быть использованы для уточнения перспектив эксплуатации Налайхинского угольного месторождения. Максимальная толщина гранитоидов Богдоулинского массива составляет 9.0 км, площадь его проекции на земную поверхность в два раза превосходит площадь выходов. Результаты исследований могут быть полезными для оценки металлогенической специализации гранитоидов, признаки которой в отношении отдельных минералов связаны с особенностями формы Богдоулинского массива и установлены геолого-геофизическими и геохимическими исследованиями. Как показали выполненные исследования, геофизические методы (в частности, гравиметрический), значительно расширяя наши знания о морфологии геологических объектов на глубине, помогают достичь нового уровня в структурном анализе, без которого невозможно разработать полноценные представления об истории развития и тектонических условиях формирования Монголо-Сибирского региона

THE DEEP STRUCTURE AND PALEOGEODYNAMICS OF SIBERIA AND CENTRAL ASIA IN STUDIES OF THE INSTITUTE OF THE EARTH’S CRUST, SB RAS (2009–2013)

The deep structure and paleogeodynamics of the Siberian craton and the Central Asian folded belt are studied by two specialized laboratories of the Institute of the Earth’s Crust, SB RAS – the Laboratory of Complex Geophysics and the Laboratory of Paleogeodynamics. A variety of modern geophysical methods is applied. Surface wave tomography is focused on determination of 3D velocity structure of the upper mantle. Seismic, gravimetric and electrical surveys are aimed at stu­dies of structures of the crust and the upper mantle. Geothermic parameters of the lithosphere in Central Asia are measured. In search for mineral resources, new geophysical prospecting technologies are developed. Quality paleogeodynamics reconstructions require a proper understanding of the structural evolution of the Siberian craton and the Central Asian folded belt. Researches in this scientific field are conducted by the Laboratory of Paleogeodynamics. Besides, the Laboratory studies new minerals and conditions of their formation. Some of the scientific research projects are jointly implemented by the two laboratories, and research results are regularly published in Russia and abroad

Deep structure of the Ulaanbaatar area (Mongolia) from the gravimetric data

The medium-scale gravity survey over the Ulaanbaatar area (Central Mongolia) has yielded the Bouguer gravity anomaly map. Based on interpretation of these data, 3D models have been built for large density inhomogeneities in the Earth’s crust - Ulaanbaatar (Tola) depression and Bogdo granite massif. It has been identified that the basement of the depression has a block structure and complicated by two basins: western and central (Selbe) The sediment thickness averages about 150 m in the western basin and may reach 300 m in the Selbe. The thickness of granitoids in the Bogdo massif reaches 4.5 km, the area of its projection on the earth's surface is twice the area of the exits. The data on the form and dimensions of these density inhomogeneities may be used in engineering seismology to update seismic hazard estimates for the town of Ulaanbaatar and to conclude about the prospects for metallogenic specialization of granitoids whose features concerning some minerals are related to peculiar massif form and identified by geological-geophysical and geochemical studies

Crustal structure of the Ulaanbaatar region, Mongolia according to gravimetric data

Establishing relationshipsbetween the regional seismicity andthe depth geometry and compositions of large-size density inhomogeneities of the upper crust is one of the ways to determine the parameters of mean long-term seismicity. Theoriginof geological bodies and mechanisms of penetration of intrusive bodiesinto the upper crust aredetectable from the shapes and sizes of such bodies.Knowledge on the shapes and sizes of large intrusive bodies is important for prospecting and exploration of mineral resources. Based on the medium-scale areal gravimetric survey data, a regional map of Bouguer gravity anomalies was constructed for theUlaanbaatar region, Central Mongolia.Interpreted gravimetric data from the solutions of the inverse problem of gravimetry based on the body-shape selection were used to developthe 3D models showing the largest crustal density inhomogeneities of the study area – the Tolskaya (Ulaanbaatar) basin, Nalaiha depression and Bogdoulin granite massif. It is confirmed that the basement of these depressionshas a block structure. The basement of the Tolskaya basinincludes two depressions, the western and eastern ones.In the western depression, the sediment thickness is 150 m. It amounts to 400 m in the eastern depression at the intersection of the Selbin and Tolskaya deep fault systems. The information on the shapes and thicknesses of the sediment bodiesisused in the seismic studies aimed at obtaining a more precise assessment of seismic hazard of the Ulaanbaatar city and the region. In the Nalaiha depression, the sediments are 800 m thick.The morphology of the basement and the thickness of sediments in this depression can be used to update the mining prospects of the Nalaiha coal deposit. The maximum thickness of granitoids in the Bogdoulin massif is 9.0 km. The area of its projection to the ground surface is twice as big as the area of the outcrops. Our research results can be useful for assessing the metallogenic specialization of granitoids, which indicators in relation to some minerals are related to the specific features of the Bogdoulinsky massif. Such indicators have beendiscovered by the geological, geophysical and geochemical methods. Our study shows that the geophysical methods (in particular, gravimetry) can considerably expand the knowledge of the morphology of geological objects at depth and facilitate reaching a new level in structural analysis, which is critical for developingnew ideas to clarify the geological history and tectonic conditions of the formation of the Mongolia-Siberian region

Low Seismic Velocity Layers in the Earth\u27s Crust of Eastern Siberia (Russia) and Mongolia: Receiver Function Data and Geological Implication

Analysis of teleseismic receiver functions at digital stations along the Bratsk-Irkutsk-Ulanbaatar-Undurshil profile suggests that low-velocity layers in the Earth\u27s crust exist not only beneath the Baikal rift zone, where such a layer was found earlier by Deep Seismic Sounding (DSS), but also beneath Early Paleozoic Sayan-Baikal, Paleozoic Mongolian, Early Mesozoic Mongolia-Okhotsk fold areas, and beneath the Siberian platform. The reliability of detection of the low-velocity layers by receiver function analysis has been checked by numerical modeling. The results of this modeling demonstrate that receiver functions can reveal the low- velocity layers in the crust if the initial model (starting approximation) is close to real velocity distribution, and if the model medium is divided into thin layers. Averaged DSS velocity model without low-velocity layers was used as starting approximation for the inversion of observed receiver functions. The low-velocity layers are interpreted to reflect inhomogeneities of the Earth\u27s crust formed during its evolution. Most of these layers are presumed to correspond to thick mylonite zones related to large pre-Cenozoic thrusts. The mylonites possess a great seismic anisotropy caused by the mineral orientation formed by the ductile flow in large thrust zones. They can result in low-velocity layers only for seismic waves whose rays are oriented perpendicular to the mylonite foliation, i.e., in the direction of the minimum velocity; the velocities along the foliation direction can be rather high. Therefore, the low-angle mylonite zones can be distinguished by the receiver function method, which uses the waves from the teleseismic events with nearly vertically oriented rays. The suggestion that the low-velocity layers mark low-angle thrusts is in agreement with gravity and geological data. The amount of overthrusting is estimated to be as large as several hundred kilometers. Multichannel seismic profiling can be used to verify the existence and the deep geometry of the presumed thrusts