GEOLOGICAL AND GEOPHYSICAL TRANSECT OF THE MIDDLE TIEN SHAN ACROSS THE NARYN AND ATBASHI DEPRESSIONS

The article presents the results of integrated geological and geophysical research works dealt with detailed magnetotelluric (MT) sounding and the study of the morphology and spatial position of the sedimentary cover and basement structures along the key transect of the Middle Tien Shan crossing the Naryn and Atbashi depressions. The data on the distribution of deep electrical conductivity of the crust and upper mantle were compared with the seismic profiling data. The compilation of the results of structural-geological and geophysical studies provided the opportunity to draw 2D upper-crust geological structure, consistent with the structure of electrical conductivity of the crust to depths of about 10 km. The detailed geological cross-sections and the structural and geological data allow us to characterize the deformations of the Cenozoic sedimentary complex and Paleozoic basement surface associated with the alpine activation of the key segment of the Tien Shan. It is shown that the Cenozoic structural parageneses emerged during a relatively short-term phase of deformation and orogeny under conditions of horizontal compression and transpression, which did not manifest themselves during sagging that occurred previously

TIME CONSTRAINTS ON THE FORMATION OF THE KANDALAKSHA AND KERETSK GRABENS OF THE WHITE SEA PALEO-RIFT SYSTEM FROM NEW ISOTOPIC GEOCHRONOLOGICAL DATA

Initially, the age and stratigraphic position of the Tersk formation were determined with respect to the fact that this formation overlaps the Early Proterozoic granitoids. Its top was marked by the rocks penetrated by the Late Devonian alkaline intrusions, including explosion pipes.This article presents the U-Pb isotopic dating of detrital zircon grains (dZr) from sandstones of the Tersk formation. It describes the geochemical compositions of the rocks and the Sm-Nd study results. In our study, the weighted average age of four youngest dZr grains from the sandstones of the Tersk formation is 1145±20 Ma, which suggests that the rocks above the studied rock layer (see the Tersk formation cross-section) are is not older than the end of the Middle Riphean. The U-Pb isotopic ages of dZr grains (provenance signals) from the sandstones of the Tersk formation were compared to the ages of other Upper Precambrian clastic strata in the northeastern East European platform (EEP) and adjacent areas. Our comparative analysis shows that these rocks significantly differ in age. This conclusion is in good agreement with the idea that at the end of the Middle and during the Late Riphean, several small (mainly closed) basins separated by uplifts dominated in the paleogeographic setting of the area wherein the White Sea rift system (WSRS) formed and developed. Temporal connections of these basins with the ocean were possible. Such paleogeographic setting does not favour the development of large rivers; this is why the grabens are mainly filled with local rock materials. The Keretsk and Kandalaksha grabens (WSRS) are filled with marine sediments eroded from the grabens walls. The local sediment sources include eclogite complexes (~1.9 Ga), which basic magmatism is dated at ~2.4–2.5 and ~2.7–2.9 Ga. Any potential primary sources for dZr grains are lacking in the area near the Keretsk graben. We suggest that such grains occurred due to recycling of the secondary sources of zircon, i.e. originated from ancient local sedimentary formations

ГЕОЛОГО-ГЕОФИЗИЧЕСКИЙ ТРАНСЕКТ СРЕДИННОГО ТЯНЬ-ШАНЯ ЧЕРЕЗ НАРЫНСКУЮ И АТБАШИНСКУЮ ВПАДИНЫ

The article presents the results of integrated geological and geophysical research works dealt with detailed magnetotelluric (MT) sounding and the study of the morphology and spatial position of the sedimentary cover and basement structures along the key transect of the Middle Tien Shan crossing the Naryn and Atbashi depressions. The data on the distribution of deep electrical conductivity of the crust and upper mantle were compared with the seismic profiling data. The compilation of the results of structural-geological and geophysical studies provided the opportunity to draw 2D upper-crust geological structure, consistent with the structure of electrical conductivity of the crust to depths of about 10 km. The detailed geological cross-sections and the structural and geological data allow us to characterize the deformations of the Cenozoic sedimentary complex and Paleozoic basement surface associated with the alpine activation of the key segment of the Tien Shan. It is shown that the Cenozoic structural parageneses emerged during a relatively short-term phase of deformation and orogeny under conditions of horizontal compression and transpression, which did not manifest themselves during sagging that occurred previously.В статье представлены результаты комплексных геолого-геофизических исследований вдоль ключевого трансекта, пересекающего Нарынскую и Атбашинскую впадины в Срединном Тянь-Шане, которые включали детальные магнитотеллурические (МТ) зондирования и изучение морфологии и пространственного положения структур осадочного чехла и фундамента. Данные о распределении глубинной электрической проводимости коры и верхней мантии сопоставлялись с материалами сейсмического профилирования. Компиляция результатов структурно-геологического и геофизического изучения дала возможность создать 2D-модель верхнекоровой геологической структуры, согласующейся со структурой электрической проводимости коры до глубины около 10 км. Построенный детальный геологический разрез и данные структурно-геологических исследований позволяют полно охарактеризовать деформации кайнозойского осадочного комплекса и поверхности палеозойского фундамента, связанные с альпийской активизацией Тянь-Шаня. Показано, что кайнозойские структурные парагенезы возникли в течение относительно кратковременной фазы деформации и горообразования в условиях горизонтального сжатия и транспрессии, не проявившихся на стадии предшествующего прогибания

О ВРЕМЕНИ ФОРМИРОВАНИЯ КАНДАЛАКШСКОГО И КЕРЕЦКОГО ГРАБЕНОВ ПАЛЕОРИФТОВОЙ СИСТЕМЫ БЕЛОГО МОРЯ В СВЕТЕ НОВЫХ ДАННЫХ ИЗОТОПНОЙ ГЕОХРОНОЛОГИИ

Initially, the age and stratigraphic position of the Tersk formation were determined with respect to the fact that this formation overlaps the Early Proterozoic granitoids. Its top was marked by the rocks penetrated by the Late Devonian alkaline intrusions, including explosion pipes.This article presents the U-Pb isotopic dating of detrital zircon grains (dZr) from sandstones of the Tersk formation. It describes the geochemical compositions of the rocks and the Sm-Nd study results. In our study, the weighted average age of four youngest dZr grains from the sandstones of the Tersk formation is 1145±20 Ma, which suggests that the rocks above the studied rock layer (see the Tersk formation cross-section) are is not older than the end of the Middle Riphean. The U-Pb isotopic ages of dZr grains (provenance signals) from the sandstones of the Tersk formation were compared to the ages of other Upper Precambrian clastic strata in the northeastern East European platform (EEP) and adjacent areas. Our comparative analysis shows that these rocks significantly differ in age. This conclusion is in good agreement with the idea that at the end of the Middle and during the Late Riphean, several small (mainly closed) basins separated by uplifts dominated in the paleogeographic setting of the area wherein the White Sea rift system (WSRS) formed and developed. Temporal connections of these basins with the ocean were possible. Such paleogeographic setting does not favour the development of large rivers; this is why the grabens are mainly filled with local rock materials. The Keretsk and Kandalaksha grabens (WSRS) are filled with marine sediments eroded from the grabens walls. The local sediment sources include eclogite complexes (~1.9 Ga), which basic magmatism is dated at ~2.4–2.5 and ~2.7–2.9 Ga. Any potential primary sources for dZr grains are lacking in the area near the Keretsk graben. We suggest that such grains occurred due to recycling of the secondary sources of zircon, i.e. originated from ancient local sedimentary formations.На начальном этапе исследований возраст и стратиграфическое положение терской свиты ограничивались снизу ее налеганием на раннепротерозойские гранитоиды, а сверху – тем, что породы терской свиты вмещают позднедевонские щелочные интрузии (в том числе и трубки взрыва).В статье представлены результаты U-Pb изотопного датирования зерен детритового циркона (dZr) из песчаников терской свиты, дополненные определениями химического состава пород и результатами их изотопного Sm-Nd изучения. Полученный средневзвешенный возраст четырех наиболее молодых зерен dZr из песчаников терской свиты составил 1145±20 млн лет, что означает, что часть разреза терской свиты, залегающая выше изученного уровня, не может быть древнее конца среднего рифея. Сравнительный анализ полученного набора U-Pb изотопных датировок зерен dZr (провенанс-сигнала) из песчаников терской свиты и аналогичных данных по другим верхнедокембрийским обломочным толщам, распространенным на северо-востоке Восточно-Европейской платформы и в сопредельных регионах, выявил существенные различия. Это находится в хорошем соответствии с представлениями о том, что палеогеографическую обстановку, господствовавшую в конце среднего и в позднем рифее в области заложения и развития рифтовой системы Белого моря (РСБМ), можно рассматривать как совокупность небольших (часто бессточных) бассейнов, разделенных поднятиями. Эти бассейны могли время от времени иметь связь с Мировым океаном. Такая палеогеографическая ситуация не благоприятствовала развитию крупных рек, поэтому грабены заполнялись преимущественно материалом местного происхождения. Кандалакшский и Керецкий грабены РСБМ заполнены осадками морского генезиса, представляющими собой преимущественно продукты размыва вещественных комплексов, участвующих в строении бортов этих грабенов. Среди местных локальных источников идентифицированы эклогитовые комплексы с возрастом метаморфизма ~1.9 млрд лет и базитовового магматизма с возрастом ~2.4–2.5 и ~2.7–2.9 млрд лет. Попадание зерен dZr, для которых нет потенциальных близких первичных источников, расположенных около Керецкого грабена, произошло за счет рециклинга из вторичных источников циркона – местных древних осадочных образований

Postmagmatiс tectonics of granites and morphostructure the Northern Tien Shan

The granites are represented significant volumes (up to 50% or more) of the continental upper crust, and, certainly, they should influence on the formation of the crust structure and peculiarities of its evolution. Is generally assumed that after cooling and including in a basement composition, the granites represent the passive volumes and they are not subject to major structural changes. However, some geological data refute this view. In particular, the study of the Northern Tien Shan granites revealed that after cooling and entering the basement they have undergone intense structural reworking. Deformation expresses in cross-scale 3D disintegration of rocks, eventually leading to the emergence of huge masse of kataklasites. That tectonic reworking fundamentally changes the rheology of rocks, causing their 3D kataklastic flow. Going spatial displacement of the granite masses, the formation of domes and vertical and horizontal crystalline protrusions. Postmagmatic tectonics of the granite basement has a significant influence on the formation of tectonic and modern morphological plan of the Tien Shan region