TECTONICS AND GEODYNAMICS OF THE ALTAI-JUNGGAR OROGEN IN THE VENDIAN-PALEOZOIC: IMPLICATIONS FOR THE CONTINENTAL EVOLUTION AND GROWTH OF THE CENTRAL ASIAN FOLD BELT

In the end of the 20th century folded structures of central Asia were regarded as formed by accretion and collision of the Paleo-Asian oceanic plate and Siberia continent.In the end of the 20th century folded structures of central Asia were regarded as formed by accretion and collision of the Paleo-Asian oceanic plate and Siberia continent

ТЕРРЕЙНОВАЯ ТЕКТОНИКА ЦЕНТРАЛЬНО-АЗИАТСКОГО СКЛАДЧАТОГО ПОЯСА

The terrain analysis concept envisages primarily a possibility of approximation of fragments / terrains of various geodynamic settings which belong to different plates. The terrain analysis can supplement the theory of plate tectonics in solving problems of geodynamics and tectonics of regions of the crust with complex structures. The Central Asian belt is among such complicated regions. Terrain structures occurred as a result of combined movements in the system of 'frontal' and/or oblique subduction – collision. In studies of geological objects, it is required first of all to prove their (vertical and horizontal) autochthony in relations to each other and then proceed to paleogeodynamic, paleotectonic and paleogeographic reconstructions. Obviously, such a complex approach needs data to be obtained by a variety of research methods, including those applied to study geologic structures, stratigraphy, paleontology, paleogeography, lithothlogy, geochemistry, geochronology, paleomagnetism etc. Only by correlating such data collected from inter-disciplinary studies of the regions, it is possible to establish reliable characteristics of the geological settings and avoid mistakes and misinterpretations that may be associated with the 'stratigraphic' approach to solutions of both regional and global problems of geodynamics and tectonics of folded areas. The terrain analysis of the Central Asian folded belt suggests that its tectonic structure combines marginal continental rock complexes that were formed by the evolution of two major oceanic plates. One of them is the plate of the Paleo-Asian Ocean. As the analogue of the current Indo-Atlantic segment of Earth, it is characterised by the presence of continental blocks in the composition of the oceanic crust and the formation of oceanic basins resulting from the breakup of Rodinia and Gondvana. In the course of its evolution, super-continents disintegrated, and the blocks were reunited into the Kazakhstan-Baikal continent. The base of the Kazakhstan-Baikal continent was formed in the Vend-Cambrian due to subduction of the oceanic crust of the Paleo-Asian Ocean, including the Precambrian microcontinents and terrains of the Gondvana group, underneath the south-eastern margin of the Siberian continent (in the current coordinates). Due to subduction followed by collision of the microcontinents with the Kazakhstan-Tuva-Mongolia island arc, the crust had consolidated, and a complex continent was formed. Another major plate is the plate of the Paleo-Pacific Ocean. It is characterized by the long-term tectono-magmatic evolution without any involvement of the continental crust and by complex processes of the formation of the continental margins. Its evolution resulted in the formation of the Vend-Paleozoic continental margin complexes of the western segment of the Siberian continent which comprise the Vend-Cambrian Kuznetsk-Altai island arc and a complex of rocks of the Ordovic-Early Devonian passive margin and the Devon-Early Carbonic active margin. In the accretional wedges of the Kuznetsk-Altai island arc, abundant are only fragments of the Vend-Early Cambrian oceanic crust including ophiolites and paleo-oceanic uplifts. The contemporary analogue of the Central Asian folded belt is the south-eastern margin of Asia, represented by the junction area of the Indo-Australian and Pacific plates.  Концепция террейнового анализа предусматривает прежде всего возможность сближения фрагментов (террейнов) самых различных геодинамических обстановок, принадлежащих различным плитам. В связи с этим террейновый анализ дополняет теорию литосферных плит в решении вопросов геодинамики и тектоники сложнопостроеных регионов земной коры, к числу которых относится Центрально-Азиатский складчатый пояс. Сформированные террейновые структуры являются результатом комбинированных движений в системе «фронтальная» и (или) «косая» субдукция – коллизия. При изучении конкретных геологических объектов в первую очередь нужно доказать их автохтонность (вертикальную и латеральную) относительно друг друга, а затем выполнять палеогеодинамические, палеотектонические и палеогеографические реконструкции. Несомненно, такой подход является очень сложным и требует разноплановых исследований (структурных, палеонтолого-стратиграфических, палеогеографических, литологических, геохимических, геохронологических, палеомагнитных и др.). Лишь на основе корреляции данных, полученных при междисциплинарном изучении регионов, можно получить качественную характеристику геологического строения и избежать ошибок, связанных со «стратиграфическим» подходом в решении как региональных, так и глобальных проблем геодинамики и тектоники складчатых областей. Террейновый анализ структуры Центрально-Азиатского складчатого пояса позволяет утверждать, что в нем тектонически совмещены окраинно-континентальные комплексы пород, сформированные при эволюции двух крупнейших океанических плит. Одна из них, плита Палеоазиатского океана, аналог современного Индо-Антлантического сегмента Земли, характеризуется наличием континентальных блоков в составе океанической коры и формированием океанических бассейнов в результате деструкции Родинии и Гондваны. В результате ее эволюции происходили процессы распада суперконтинентов и повторное объединение блоков в составе Казахстано-Байкальского континента. Фундамент Казахстано-Байкальского континента сформирован в венде–кембрии в результате субдукции под юго-восточную окраину Сибирского континента (в современных координатах) океанической коры Палеоазиатского океана, включающей докембрийские микроконтиненты и террейны гондванской группы. Субдукция и последующая коллизия микроконтинентов и террейнов с Казахстано-Тувино-Монгольской островной дугой привели к консолидации земной коры и формированию составного континента. Другая плита, Палеопацифики, аналог современного Тихоокеанского сегмента Земли, характеризуется длительной тектономагматической эволюцией без участия континентальной коры и сложными процессами формирования материковых окраин. В результате его эволюции созданы венд-палеозойские окраинно-континентальные комплексы западной части Сибирского континента, состоящие из венд-кембрийской Кузнецко-Алтайской островной дуги, комплексов пород ордовикско-раннедевонской пассивной окраины и девонско- раннекарбоновой активной окраины. В аккреционных клиньях Кузнецко-Алтайской островной дуги широко представлены только фрагменты вендско-раннекембрийской океанической коры, состоящей из офиолитов и палеоокеанических поднятий. Современным аналогом Центрально-Азиатского складчатого пояса является юго-восточная окраина Азии, представленная зоной сочленения Индо-Австралийской и Тихоокеанской плит.

Intramontane lacustrine basins in the Siberian Altai: recorders of Cenozoic intracontinental tectonic and climatic events

The Altai Mountains are part of the vast intracontinental Central Asian orogenic system that formed during the Cenozoic as a distal effect of continued indentation of the Indian plate into the Eurasian continent. In the Siberian part of the Altai Mountains there is ample evidence to suggest that the pre-Cenozoic structural fabric of its basement is a controlling factor in the Cenozoic deformation and development of this intracontinental orogen. We give evidence that important Paleozoic fault zones were reactivated during the Cenozoic, particularly the Late Cenozoic and play a key role in the formation, evolution and current morphology of the Siberian Altai Mountains. Some of these faults are still active and recent and historic movements along them have triggered large seismic events. Late Cenozoic reactivation was expressed as pure thrust, oblique thrust to pure strike-slip faulting, resulting in an overall transpressive tectonic regime. In some cases, as for the graben basin of Lake Teletskoye, local, pure extensional stresses are responsible for its formation as we show in this contribution. Various other intramontane, tectonic basins developed. Some of these are very recent structures (the Teletskoye Basin) and are Pleistocene in age or younger, others have a prolonged history and contain a relatively complete Cenozoic sedimentary section, with evidence of Late Mesozoic precursor basins (Chuya Basin, Dzhulukul Basin). Some of these exhibit indications of marine incursions, but the basins are predominantly continental. The development of these basins is clearly associated with the location and Cenozoic reactivation of aforementioned long-lived fault zones in the Altai tectonic assemblage. Many of these basins accommodated fresh water lakes during their evolution and some are still the site of contemporary mountain lakes. Their stratigraphy, as well as the sedimentary architecture and basin morphology is manifestly influenced by and progresses with the stages of (Late) Cenozoic intracontinental mountain building and erosive denudation of the growing mountain ranges. Together with the clastic sedimentary input and the provenance characteristics, the intramontane Altai basin deposits are affected by evolving climatic conditions. These conditions dictate the main mode of erosion and transport, influence the sedimentary facies and supply rate and create the framework for a specific biocoenosis signature found in the fossil record. Our contribution reviews the data obtained over the last years from a selection of intramontane lacustrine basins in the Siberian Altai Mountains. We direct our attention in particular to the Teletskoye basin, the Chuya-Kurai Basin and the Dzhulukul Basin. We combine sedimentologic-stratigraphic data with basin architecture and morphology, and with basement geochronologic-thermochronologic constraints (apatite fission-track, U/Pb and Ar-dating) in order to show the potential of these basins as recorders of Cenozoic tectonic and climatic events in relation with basement features. While for example the data obtained from the Chuya Basin yields a continuous Cenozoic picture of deformation and climatic evolution of the Altai area, data from the Teletskoye Basin zooms in with higher resolution on the Pleistocene to Recent history. In general, all data point towards intensifying tectonic reactivation and mountain building of the Siberian Altai Mountains since the Middle Cenozoic, with clear peak activity in the Pliocene to Recent. This is demonstrated by the molassetype deposits in these basins, and by thermochronologic constraints. This activity is ongoing and structural, (paleo)seismic, geomorphologic and sedimentologic data corroborates this. The lacustrine Altai basins provide a record for a more or less continuous progressive cooling and aridification of the Altai area during the Cenozoic as manifested in the pollen fossil assemblages found in the Altai sediments

ТЕКТОНОТЕРМАЛЬНАЯ ЭВОЛЮЦИЯ ЗАГАНСКОГО КОМПЛЕКСА МЕТАМОРФИЧЕСКОГО ЯДРА ЗАБАЙКАЛЬЯ: РЕЗУЛЬТАТ ПОСТКОЛЛИЗИОННОГО РАЗРУШЕНИЯ МОНГОЛО-ОХОТСКОГО ОРОГЕНА В МЕЛУ – ПАЛЕОЦЕНЕ

Thermochronological reconstructions of the Zagan metamorphic core complex were carried out using samples from the central part of the core, mylonite zone detachment and lower nappe with U/Pb zircon dating, 40Ar/39Ar amphibole and mica dating, and apatite fission-track dating. In the tectonothermal evolution of the metamorphic core, there was distinguished an active phase (tectonic denudation) of the dome structure formation during the Early Cretaceous (131–114 Ma), which continued in the Late Cretaceous – Paleocene (111–54 Ma) in passive phase (erosive denudation). During an active phase, there was initiated a large-amplitude gently dipping normal fault (detachment), which was accompanied by tilting (sliding of rocks along subparallel listric faults). As a result, about 7 km thick rock strata underwent denudation over 17 Ma at a rate of about 0.4 mm/year. In passive phase, about 6 km thick rock strata were eroded over 57 Ma, with a denudation rate of about 0.1 mm/year. Thus, the Zagan metamorphic core complex was tectonically exposed from the mid-crust to depths of about 9 km in the Early Cretaceous as a result of post-collisional collapse of the Mongol-Okhotsk orogen. Further cooling of the rocks in the metamorphic core to depths of about 3 km occurred in the Late Cretaceous – Pliocene as a result of destruction of more than 6 km high mountains.Термохронологические реконструкции Заганского комплекса метаморфического ядра проводились по образцам центральной части ядра, зоны милонитов из детачмента и нижней части покрова с использованием U/Pb датирования циркона, 40Ar/39Ar датирования амфибола и слюд, трекового датирования апатита. В тектонотермальной эволюции метаморфического ядра выделена активная фаза (тектоническая денудация) в период раннего мела (131–114 млн лет), которая продолжилась в позднем мелу – палеоцене (111–54 млн лет) пассивной фазой (эрозионная денудация). В активную фазу произошла инициация крупноамплитудного пологопадающего сброса (детачмента), которая сопровождалась сползанием пород по субпараллельным листрическим сбросам. В результате за 17 млн лет было денудировано около 7 км мощности пород со скоростью около 0.4 мм/год. В пассивную фазу за 57 млн лет было размыто около около 6 км со скоростью денудации около 0.1 мм/год. Таким образом, тектоническая экспозиция Заганского метаморфического ядра со средних уровней коры до глубин около 9 км осуществлялась в раннем мелу в результате постколлизионного растяжения Монголо-Охотского орогена. Дальнейшее охлаждение пород метаморфического ядра до глубины около 3 км происходило в позднем мелу – плиоцене в результате разрушения горного поднятия, имеющего высоту более 6 км

Flexural deformation and basin-mountain coupling in the northern Kyrgyz Tien Shan: transition from the Issyk-Kul basin to the Kumtor plateau

During the late stage of the India-Asia collision, deformation propagated northwards into the Asian foreland. North of the stable Tarim plate, the Tien Shan range – an old Palaeozoic fold belt – was strongly reactivated. It now accommodates more than one third of the total shortening rate between Stable Eurasia and the Indian continent.In the northern part of the Kyrgyz Tien-Shan Range, the 600 m deep Lake Issyk-Kul occupies a lense-shaped tectonic depression elongated in an E-W direction and bordered on its northern and southern sides by high mountain ranges (> 4000 m high). To the north, the Kungey Alatau range has the structure of an active positive flower structure with the Chon-Kemin – Chilik fault in its middle (location of several Ms > 8.0 historic earthquakes). To the south, the Terskey range forms the frontal scarp of the high and relatively flat Kumtor Plateau whose surface is undulating between 3800 and 5200 m high.Multidisciplinary investigation was performed during several summer campaigns, involving structural geology, paleostress reconstructions, tectono-stratigraphy and paleoseismology in the mountain ranges and lake shore, as well as high-resolution seismic profiling and heat flow measurements in Lake Issyk-Kul. Investigations included also the seismotectonics analysis of a large number of earthquake focal mechanisms determined from the local seismic network.All the results are best integrated in a model of lithospheric deformation by flexural folding and basin-mountain coupling. The Issyk-Kul basin probably formed as a flexural downwarp of the lithosphere rather than as a symmetric ramp basin, as once proposed. South of the Issyk-Kul depression, the Kumtor plateau still show large remains of the pre-Cainozoic flat erosion surface that is widespread in the Central Asia, now strongly uplifted and slightly undulating. Deformation at mountain-basin interface occurs mainly by tilting around a horizontal axis of the pre-Cainozoic basement (up to 60°from the horizontal), and top-to-the south reverse faulting at the southern margin of the basin (basin towards the range), in an opposite sense to what could be expected in the case of a ramp basin. The focal mechanism of a recent earthquake along the southern mountain-basin interface confirms this interpretation (25 December 2006 Ms 5.8)

Mеsozoic Tectonothermal Evolution of the Zagan Metamorphic Core Complex in Western Transbaikalia: 40Ar/39Ar and FTA Dating

A model of tectonothermal evolution of the Zagan metamorphic core complex (MCC) based on the new data from 40Ar/39Ar dating of amphibole, mica, and apatite fission-track dating is discussed. A relationship with the long-range impact of processes from the collision of the North China (Amurian–North China) block with the Siberian continent in the Mesozoic era is proposed. The Zagan MСС was formed in the Cretaceous period on the southern flank of a high mountain uplift of Western Transbaikalia, composed of late Paleozoic granitoids of the Angara–Vitim batholith. According to 40Ar/39Ar dating of amphiboles and micas from the mylonite zone, the active development time of the Zagan MCC corresponds to the early Cretaceous epoch (131, 114 Ma). The tectonic exposure of the core from about 15 km to the depths of about 10 km occurred at a rate of tectonic erosion of 0.4–0.3 mm/year as a result of post-collisional extension of the Mongol–Okhotsk orogen. Apatite fission-track dating shows that further exhumation and cooling of the rocks to about 3 km occurred in the lower-upper Cretaceous epoch (112, 87 Ma). The erosional denudation rate was about 0.3 mm/year.MCC- metamorphic core complexes, AFT- apatite fission-trac

Позднепалеозойские деформации пород Курайского блока: структурно‐кинематический анализ (верховья реки Курайка, Горный Алтай)

In the Kurai ridge located in the southeastern Gorny Altai (Altai Mountains, Russia) metamorphic rocks of the Kurai complex are widely developed: granite‐gneisses, crystalline schists and amphibolites, including widespread Early Carboniferous mylonites and blastomylonites. Oriented samples of blastomylonites were taken from the upper reaches of the Kuraika river for the microstructural study aimed to determine the kinematics of movements. The analyses of thin cut samples show structural‐kinematic indicators that suggest two deformation events (left‐lateral shearing, then right‐lateral shearing).В пределах Курайского хребта юго‐восточной части Горного Алтая широко распространены метаморфические породы курайского комплекса. Он представлен гранитогнейсами, кристаллическими сланцами и амфиболитами, среди которых развиты раннекарбоновые милониты и бластомилониты. По отобранным в верховьях реки Курайка ориентированным образцам бластомилонитов проведено микроструктурное исследование с целью определения кинематики движений. В шлифах по структурно‐кинематическим индикаторам зафиксировано наличие двух деформационных событий: более раннее – левостороннее и более позднее – правостороннее

ГЕОХРОНОЛОГИЯ МЕТАМОРФИЧЕСКИХ ПОРОД КУРАЙСКОГО АККРЕЦИОННОГО КЛИНА (ЮГО-ВОСТОЧНАЯ ЧАСТЬ ГОРНОГО АЛТАЯ)

The paper provides a review of the published and new geological and geochronological data on the metamorphic rocks in the Kurai accretionary prism, considering the evolution of the Kuznetsk-Altai island-arc paleosubduction channel of the Siberian continent. The following two stages are distinguished by 40Ar/39Ar and U/Pb dating: (1) 636–619 Ma and earlier: sinking of ophiolites into the subduction zone; (2) 604–585 Ma: sinking of the large bo­dies of oceanic uplifts into the subduction zone. These processes led to the exhumation of the high-pressure rocks and the hot Chagan-Uzun peridotites. The dynamo-thermal effect of the latter on the basalts is reflected in the inverted metamorphic zoning and the occurrence of garnet amphibolites and plagiogranite migmatites. It is probable that the paleo-seamounts collided with the island arc during the Vendian–Early Cambrian. The data reviewed in this paper give evidence of an active margin of the West Pacific type in the western Altai-Sayan folded region in the Vendian–Cambrian.Представлен обзор опубликованных и новых геолого-геохронологических данных по метаморфическим породам Курайского аккреционного клина, характеризующих эволюцию палеосубдукционного канала Кузнецко-Алтайской островной дуги Сибирского континента. По результатам 40Ar/39Ar и U/Pb датирования выделяются: 1) этап в интервале 636–619 млн лет и древнее, когда в зону субдукции погружались преимущественно офиолиты; 2) этап в интервале 604–585 млн лет, характеризующийся погружением в зону субдукции крупных тел океанических поднятий, приведшим к эксгумации высокобарических пород и горячего тела перидотитов Чаган-Узунского массива. Его динамотермальное воздействие на базальты отразилось в формировании инвертированной метаморфической зональности с образованием гранатовых амфиболитов и плагиогранитовых мигматитов. Столкновение палеосимаунтов с островной дугой, по-видимому, продолжалось в течение венда – раннего кембрия. Изложенные в статье данные свидетельствуют о том, что в западной части Алтае-Саянской складчатой области в венд-кембрийское время существовала активная окраина западно-тихоокеанского типа

RT-2 Detection of Quasi-Periodic Pulsations in the 2009 July 5 Solar Hard X-ray Flare

We present the results of an analysis of hard X-ray observations of the C2.7 solar flare detected by the RT-2 Experiment onboard the Coronas - Photon satellite. We detect hard X-ray pulsations at periods of ~12 s and ~15 s. We find a marginal evidence for a decrease in period with time. We have augmented these results using the publicly available data from the RHESSI satellite. We present a spectral analysis and measure the spectral parameters.Comment: 12 pages, 8 figures and 3 tables, accepted for publication in The Astrophysical Journa