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    Not found Special Section on The India-Eurasia Collision and Its Long-Range Effect

    Wang Xiaolong

    The special issue of "India-Eurasia Continental Collision and Its Remote Effects" in the Earth Science Frontiers gathers important papers on the lithospheric structure, geological structure, and geodynamic processes of the Tibetan Plateau and its surrounding areas. These articles explore the impacts of the collision between the Indian and Eurasian continents and the uplift of the Tibetan Plateau on the formation and evolution of the Earth's geodynamic system.

    This special issue covers in-depth research on the deep exploration and lithospheric structure of the Tibetan Plateau, revealing the structure of the crust and upper mantle, and delving into the mechanisms of plateau uplift. The studies primarily focus on the geological structure and evolution of the Tibetan Plateau, providing profound insights into the deep crustal structures and geodynamic processes in different regions or tectonic zones. Furthermore, the research highlights the driving mechanism for the northward drift of the Indian Plate and the effects of continental collision and plateau uplift on the Baikal region. Additionally, the special issue includes technical innovations conducted in the Tibetan Plateau, such as distributed acoustic sensing for shallow exploration, presenting new research approaches for a deeper understanding of the geodynamic processes in the Tibetan Plateau.

     Overall, this special issue emphasizes the geological evolution and tectonic processes of the Tibetan Plateau, providing essential theoretical foundations for environmental resources and geological evolution studies. Its publication will promote interdisciplinary development and collaboration in Tibetan Plateau research and offer new insights for the advancement of global geoscience.

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    Geological-geophysical models of the Earth’s crust along the Russian-Mongolian geotransects
    Evgeny Kh. TURUTANOV, Evgeny V. SKLYAROV, Valentina V. MORDVINOVA, Anatoly M. MAZUKABZOV, Viktor S. KANAYKIN
    Earth Science Frontiers    2021, 28 (5): 260-282.   DOI: 10.13745/j.esf.sf.2021.3.10
    Abstract （1287）   HTML （32）    PDF（pc） （7300KB）（311）       Save

    Transects are vertical sections of the Earth’s crust, which reveal the nature of tectonic zones, as well as their spatial relationships through a combined analysis of their geology and geophysics. Transect documents contain a geological map for a strip of land 100 km wide, a geological section of the upper crust, gravity and magnetic maps (and/or corresponding profiles along the transect), and a geophysical profile of the crust, differentiated by seismic velocities, densities and other geophysical properties. These data are used to compose a combined cross-section (the resulting section), which shows a set of rocks typical of various geodynamic conditions (rifts, oceans, collision zones, orogenic basins, continental platforms and magmatic arcs, including Andean island arcs, active continental outskirts, trenches, basins of front and rear arcs). The objective of this project was to build deep sections according to unified legends based on the interpretation of all available geological and geophysical data in order to determine the spatial relationship of terranes and their geodynamic nature in terms of plate tectonics.
    A number of terranes have been discriminated in the territory of the southern part of Eastern Siberia and the territory of Mongolia, and their geodynamic nature and space-time relations were analysed. The terranes were found out to be Vendian-Early Paleozoic, Middle-Late Paleozoic and Late Paleozoic-Early Mesozoic island arcs and microcontinents. Moreover, Middle-Late Paleozoic and Late Paleozoic-Early Mesozoic Andean-type active continental margins and Late Paleozoic-Early Mesozoic passive margins and Early Cretaceous rifts were identified and studied. The rock complexes related to the island arcs and Andean-type active continental margins are thrust over the bordering continents and microcontinents, the width of the respective tectonic nappes attaining 150 km. Schematic paleogeodynamic reconstructions for the area of the Mongolia-Okhotsk ocean have been performed, spanning the period from Devonian to Late Jurassic.
    “Non-geosyncline” granitoid magmatism finds straightforward and sound explanation in terms of plate tectonics where provinces of Devonian-Carboniferous and Permian-Triassic magmatism correspond to Andean-type active continental margins and Middle-Late Jurassic magmatism is associated with Siberia/Mongolia-China collision. The presence of a subalkaline (mantle) element in collisional magmatism and the great extent of the area it occupies can be explained by suggesting that an oceanic rift (a mantle hotspot) was buried under thick continental lithosphere after closure of the Mongolia-Okhotsk ocean. In the Early Cretaceous, the setting of collision gave way to that of continental rifting.
    The existence of an Andean-type active margin over the great extent of the southern border of Siberia is likewise responsible for minor abundance of ophiolites along the Mongolia-Okhotsk suture. When one colliding continent has an Andean-type active margin and the other has a passive margin, the continental crust of the former thrusts over the latter, and no conditions arise for ophiolites to expose. Blocks of dismembered ophiolites, that are remnants of truncated seamounts, can be part of chaotic complexes building accretion-subduction wedges. However, accumulation of such wedges in the Late Permian-Early Jurassic was not typical of the active margin of Siberia because of rapid subduction.
    An analysis of geological and geophysical data on transects shows that the Asian continent was formed in the Phanerozoic as a result of accretion of terranes, some of which were microcontinents with a Precambrian foundation. Precambrian blocks are separated by deformed and strongly eroded Phanerozoic igneous arcs of various widths, also classified as specific terranes.

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    Deep seismic reflection evidence on the deep processes of tectonic construction of the Tibetan Plateau
    GAO Rui, ZHOU Hui, GUO Xiaoyu, LU Zhanwu, LI Wenhui, WANG Haiyan, LI Hongqiang, XIONG Xiaosong, HUANG Xingfu, XU Xiao
    Earth Science Frontiers    2021, 28 (5): 320-336.   DOI: 10.13745/j.esf.sf.2021.8.10
    Abstract （819）   HTML （283）    PDF（pc） （14121KB）（866）       Save

    The collision between the Indian and Asian plates uplifted the Himalayan- Tibetan Plateau, thickening and expanding the crust. It is a scientific mystery of global concern as how the two continents collide and how the continent-continent collision deforms the continent. Deep seismic reflection profile detection is one of the most effective ways to unlock this scientific mystery. For more than 20 years using this technology, we have detected fine structures of the thick crust of the Tibetan plateau after overcoming technical bottlenecks to access the lower crust and Moho thus revealing the continental collision processes. This paper systematically summarizes the deep behaviors of the India-Asia collision and subduction beneath the Tibetan Plateau, from south to north, east to west and further into the hinterland of the plateau. The Indian crust undergoes underthrusting beneath the Himalayan orogenic belt on the southern margin of the plateau. Meanwhile, the lithosphere of the Alxa block in the Asian plate subducts southward beneath the Qilian Mountain in the north of the plateau, driving the northward overthrusting of the Qilian crust. Additionally, the Tarim and West Kunlun blocks undergo face-to-face collision in the northwestern margin of the plateau. In the easternmost part of the plateau, the Longriba fault, instead of the Longmen Shan fault zone, marks the western margin of the Yangtze block. It is also seismically evidenced that the Moho geometry in the plateau’s hinterland appears thin and flat, indicating lithospheric collapse and extrusion. Multiple deep reflection profiles revealed the collisional behavior under the Yalung-Zangbo suture zone and longitudinal variation in subducting geometry of the Indian crust from west to the east. In the middle of the suture zone, it shows a decoupling between the upper and lower crusts of the Indian plate, where the upper crust undergoes a northward overthrusting while the lower one experiences a northward underthrusting. It is also seismically evidenced a down-and southward crustal duplexing of the subducting Indian crust thickening the northern Himalayas, leaving over a thinning subducting lower crust of the Indian slab. The subduction front of the Indian crust collides with the lower crust of the Asian plate at the mantle depth. A near-vertical collision boundary is seen between the Gangdese batholith and the Tethyan Himalayas, where the Gangdese batholith shows almost transparent weak reflections in the lower crust with localized bright spot reflection that indicates partial melting. Additionally, the near-flat Moho geometry implies an extensional tectonic environment of the southern margin of the Asian plate.

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    Neotectonics of the Altai-Sayan Mountains and reactivation of regional faults controlling seismicity
    Mikhail M. BUSLOV, Lyudmila P. IMAEVA
    Earth Science Frontiers    2021, 28 (5): 301-319.   DOI: 10.13745/j.esf.sf.2021.9.9
    Abstract （279）   HTML （16）    PDF（pc） （2408KB）（157）       Save

    The formation of mountain system and neotectonic structure of the Altai-Sayan region is regarded to be a result of intercontinental deformations, related to a distant effects of tectonic stress from the Indo-Eurasian collision. Within this tectonic model we carried out the joint analysis of the geology, seismicity data and topographic materials enable to assume that the maximum changes in the relief and seismic activity in the northern part of Central Asia mountain belt are confined to the zones of intersections of the Late Paleozoic regional faults. The intersections and junctions of faults should be considered as one of the most important structural factors that increase the fragmentation of the substrate, affect changes in the local stress field and predetermine the localization of large earthquake foci with a magnitude M≥5. Some regularities were revealed, based on the example of helium and travertine manifestations in the junction zone of the Charysh-Terekta and Kurai regional faults, which can be used as precursors of earthquakes.

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    Crustal deformation, long-term plate motion and earthquake occurrence process of the Shan Plateau region, Northern Sunda Arc: Constraints from geodetic measurements
    Raja SEN, Dibyashakti PANDA, Bhaskar KUNDU
    Earth Science Frontiers    2021, 28 (5): 283-300.   DOI: 10.13745/j.esf.sf.2021.9.10
    Abstract （286）   HTML （20）    PDF（pc） （2529KB）（156）       Save

    In the present article, we explore the present day crustal deformation and long-term plate motion of the Shan Plateau and surrounding region by constraining geodetic measurements to provide an updated status of geodynamics and associated seismic hazard of this region. The Shan Plateau is laterally bounded by two prominent master faults on either sides (i.e., Sagaing fault in the western side and Red River fault in the eastern side), where extrusion of the ductile flow the Tibetan crust has been considered to be a predominant factor of the deformation in this sandwiched deformable unit. Geodetic measurements clearly indicate a dextral motion of 18 mm/yr and 4-5 mm/yr across the Sagaing fault and Red River fault segments, respectively. Moreover, the cumulative geodetic slip-rate across the networks of faults within the Shan Plateau indicates an overall sinistral motion of 12-13 mm/yr. We argue that the distributed deformation and long-term plate motion of the Shan Plateau region, with respect to the rigid (undeformed) Sundaland block, is primarily controlled by the regional bookshelf faulting, which is evident by the differential fault motion along the two master faults on the either sides (i.e., the Sagaing fault and Red River fault).

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    Foreword to a spacial section on The India-Eurasia Collision and Its Long-Range Effect (with an illustration of programmatic themes
    Earth Science Frontiers    2021, 28 (5): 226-229.  
    Abstract （236）   HTML （39）    PDF（pc） （300KB）（227）       Save
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    Crust-mantle structure and geodynamic processes in western China and their constraints on resources and environment: Research progress of the ANTILOPE Project
    ZHAO Junmeng, ZHANG Peizhen, ZHANG Xiankang, Xiaohui YUAN, Rainer KIND, Robert van der HILST, GAN Weijun, SUN Jimin, DENG Tao, LIU Hongbing, PEI Shunping, XU Qiang, ZHANG Heng, JIA Shixu, YAN Maodu, GUO Xiaoyu, LU Zhanwu, YANG Xiaoping, DENG Gong, JU Changhui
    Earth Science Frontiers    2021, 28 (5): 230-259.   DOI: 10.13745/j.esf.sf.2021.9.38
    Abstract （518）   HTML （37）    PDF（pc） （13033KB）（555）       Save

    In order to systematically and thoroughly study the crust-mantle structure and deep geodynamic processes of basins, mountains and plateaus of western China, we proposed and led the implementation of the ANTILOPE Project (Array Network of Tibetan International Lithospheric Observation and Probe Experiments) in 2003. So far, we have completed four 2D broadband arrays, ANTILOPE-I to ANTILOPE-IV, on the Tibetan Plateau, and deployed two 3D broadband arrays, ANTILOPE-V and ANTILOPE-VI, at the eastern and western Himalayan syntaxis, respectively. In addition, we included in our study framework nine comprehensive geophysical observation profiles previously obtained from the Junggar Basin, Tienshan Orogenic Belt, Tarim Basin, Altyn Orogenic Belt, and Qaidam Basin.
    Through the implementation of the ANTILOPE Project, we collected a large amount of high-quality, comprehensive first-hand observational data from western China (including the basin-mountain system surrounding the Tibetan Plateau in the northwest and the Tibetan Plateau in the southwest). The fine crust-mantle structure systematically reveals the deep geodynamic processes of the basin-mountain-plateau geosystem in western China. The up-to-date main research progress can be summarized as follows. The structure and properties of the basement of the Junggar Basin have been determined, and the basement structural framework has been optimized. A new intracontinental orogenic model of lithospheric subduction with crustal interlayer intrusion in the Tienshan Orogenic Belt has been established, which reveals the fate of the 44% shortened Tienshan lithosphere after the India-Eurasia collision and the conversion mechanism from ocean-continent subduction to continent-continent collision and subduction. Our results reveal the basin-mountain contact relationship between the Tarim Basin, Altyn Orogenic Belt and Qaidam Basin. We have obtained the deep geometric, kinematic and geodynamic evidence for the clockwise rotation of the Tarim Basin, and determined the collision boundary between the Indian and the Eurasian Plates under the Tibetan Plateau. We also found that the current Tibetan Plateau consists of the Indian Plate in the south, the Eurasian Plate in the north, and the giant crush zone—also called the “Tibetan Plate”—between them. For the first time, the respective lithospheric bottom boundaries are determined; two end-member models of plateau deformation are corrected; and the constraints of deep structures on the surface topography are established. Our result systematically reveals the changing pattern and controlling factors of the horizontal advancing distance and the subduction angle of the Indian Plate along the Himalayan Orogenic Belt.
    By combining a huge observation network with comprehensive geophysical detection technologies, the ANTILOPE Project adopts different methods, including geophysical, geological and geochemical methods, to reveal the subduction of the Indian continent, the development of the giant crush zone in Tibet, the clockwise rotation of the Tarim Block, the accelerated closure of the western water vapor channel, and the advance of aridification and desertification in northwest China and their constraints on surface topography, oil and gas resources, and environmental variations. The above results have promoted the development of the Earth system theory in the Tibetan Plateau.

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    Basement structure of the Junggar Basin
    Xiaojun WANG, Yong SONG, Baoli BIAN, Junmeng ZHAO, Heng ZHANG, Maodu YAN, Shunping PEI, Qiang XU, Shuaijun WANG, Hongbing LIU, Changhui JU
    Earth Science Frontiers    2021, 28 (6): 235-255.   DOI: 10.13745/j.esf.sf.2021.11.13
    Abstract （499）   HTML （17）    PDF（pc） （3904KB）（340）       Save
    Five comprehensive geophysical profiles, I-I, II-II, III-III, IV-IV and Emin-Hami, have been completed across the Junggar Basin and surrounding areas. A preliminary understanding of the geodynamic problems in the greater Junggar Basin is achieved through comprehensive research. The basement of the Junggar Basin is composed of the Wulungu Terrane in the north and the Manas Terrane in the south. The boundary between the two is the Dishuiquan-Sangequan suture in the NWW direction. It is connected to the NE-trending Dalbutte suture in the west and the NW-trending Cranamary suture zone in the east. The basement of the Wulungu Terrane in the northern Junggar Basin has a double-layered structure, where the upper layer is a folded basement composed of Devonian and Lower Carboniferous rocks, generally thick (3-5 km) in the north and thin (1-2 km) in the south. The Manasi Terrane south of the suture line has a single layer basement, namely the crystallization basement of the middle to upper Proterozoic. The crust in the Junggar Basin, 44-52 km thick, is thin in the north and thick in the south, while the crust in the surrounding mountainous area is thicker compared to the basin area. The crust in the basin area is divided into upper, middle (generally thinner) and lower layers and contains several deep faults. Six major deep faults are in the north-south direction: Hongche, Delunshan, Shixi, Hutubi, Cainan and Fukang. These faults have large dips, extending upward to the lower part of the upper crust and cutting down through the basement interface of the crust. The horizontal structure and structural plane of the crust have no obvious vertical fault or seem to have the feature of “open fault”. These faults are good channels for the upper mantle material squeezing into the Earth's crust. In addition, there are two main transverse deep faults, one is the Dishuiquan-Sangequan deep fault with an NWW strike. It dips to the south and has the property of reverse fault, and it may break the Dishuiquan-Sangequan suture. The other is the near EW Changji-Manas deep fault dipping to the south. It is mainly developed in the middle and lower crust and resembles a reverse fault. These deep faults play a role in controlling the development of basin structure. The Moho interface in the western Junggar Basin extends to the deeper Moho interface in the Tianshan Mountains, while the Moho interface in the eastern part of the basin is not connected to but underneath the one in the Bogda Mountains, indicating crustal subduction. This observation helps to explain the tectonogeomorphologic phenomenon in the eastern part of the Tianshan Mountains, where the tectonic activity is relatively weakened but the Bogda Mountains are uplifted to the north. The surrounding Junggar Basin is characterized by a compression basin-mountain tectonic coupling pattern, especially the Bogda-Zhundong basin-mountain tectonic coupling in the eastern part of the southern margin. The Bogda Mountains, which now separate the Junggar Basin from the Tuha Basin, are a young and still rising mountain range. The uplift of the Bogda Mountain is a reflection of multiple nappe orogeny since the Indo-China Movement, and its present appearance is the result of neotectonic movement since the Neogene. In the Junggar Basin the cap rock developed in three stages: the Permian-Triassic foreland basin stage associated with the formations of the Tianshan and Songpan-Ganzi orogenic belts; the Jurassic-early Eocene intracontinental depression stage when the regional compression was weak; and the Neogene rejuvenated foreland-basin stage related to the uplift of Tianshan.
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    The key question of the Aral Sea evolution important for understanding its economic, social and ecological values
    S.K.KRIVONOGOV, T.I.KENSHINBAY, R.Kh.KURMANBAEV, B.S.KARIMOVA
    Earth Science Frontiers    2021, 28 (6): 196-204.   DOI: 10.13745/j.esf.sf.2021.9.36
    Abstract （333）   HTML （13）    PDF（pc） （831KB）（103）       Save
    The paper reviews the up-to-date knowledge on the changes of the Aral Sea level to make it useful for decision makers in social and economic projects. We propose a new vision of the ecological standard for the Aral Sea: small to medium size lake instead of huge one as it was before 1960, when modern human-induced shrinking started. Change of the vision may turn many expensive initiatives based on extreme scenarios to be more realistic. The conclusion is based on the multiproxy data obtained from geological, archaeological, historiographical and hydrological studies. These results gave a quite reliable scheme of the changes during about 20 thousand-year history of the Aral, which experienced multiple abrupt transgressions and regressions. However, understanding of the reasons for these changes, except for the modern one, is still not complete and requires special investigation. In this paper, we discuss approaches for correlating the water level changes with climate change and variations of the Aral Basin water supply from its feeding—the Syr Darya and Amu Darya rivers.
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    Subduction reversal in the accretion complex drives the exhumation of deep subducted mélange in southern Qiangtang, Tibet: Insights from the Mao'ershan detachment fault
    LI Dian, WANG Genhou, LIU Zhengyong, LI Pengsheng, FENG Yipeng, TANG Yu, LI Chao, LI Yang
    Earth Science Frontiers    2021, 28 (6): 205-226.   DOI: 10.13745/j.esf.sf.2021.11.12
    Abstract （278）   HTML （10）    PDF（pc） （12737KB）（156）       Save
    The subduction and accretion of oceanic lithosphere may be accompanied by a complex slab motion in depth, and HP rocks are certainly a good proxy to reflect this deep process. Recent studies show that subduction reversal can occur, in particular, in a divergent double subduction zone when a slab pull of one slab exceeds that of a shorter slab, the shorter slab then experiences a net upward pull. This study prompted us to pay attention to this ‘abnormal’ plate movement during oceanic subduction and accretion in objective and comprehensive analysis of collisional orogen belt. Most of the current mechanisms of the exhumation of HP rocks emphasize a single rapid exhumation process, except the ‘corner flow’ model. It can be expected that the exhumation process should be different in the case of subduction reversal. Exhumation of a single HP rock is still a rapid process; but exhumation of an entire HP rock belt must occur during the entire subduction reversal period, so the exhumation process lasts longer. Research on the exhumation related upper crustal structural deformation has the potential to unveil the above process.|||This subduction reversal hypothesis is first proposed for Triassic HP rocks exposed in the southern Qiangtang mélange belt in central Tibet. Therefore, we chose to study the Mao'ershan accretionary complex located in the northernmost part of the Qiangtang mélange belt. We analyzed its crustal structural characteristics, geometric structure, movement style and active period related to subduction reversal, based on geological mapping and structural and chronological studies. The field geological mapping results show that the Mao'ershan complex has similar characteristics as the metamorphic core complex. Subduction-accretion complex forms its core that is surrounded by Late Paleozoic strata from the top, with a detachment fault system separating the two. Beneath the brittle detachment fault, a shear zone develops southward from the top of the subduction-accretion complex. Three-dimensional strain and kinematic vorticity results indicate that the strain type in the Mao'ershan shear zone is elongate strain, dominated by simple shear strain. Mineral deformation analysis and fractal dimension measurements show that the shear temperature is associated with low greenschist and lower amphibolite facies. Based on the new 40Ar-39Ar geochronology data, we conclude that the Mao'ershan shear zone was active at ~260 Ma. Based on the above study and combined with the geological features of the central Qiangtang mélange belt, we believe that the Mao'ershan complex was exhumed during the early stage of the subduction reversal. We thus infer that the movement rate of subduction reversal is about 3.5 mm/a, which is similar to the exhumation rate of HP rocks in central Qiangtang mélange belt. Our research may provide a new perspective on the exhumation mechanism for other HP rocks around the world.
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    Application of distributed acoustic sensing in structural investigation of Lake Yigong in Tibet
    ZHANG Heng, XU Tuanwei, PEI Shunping, ZHAO Junmeng
    Earth Science Frontiers    2021, 28 (6): 227-234.   DOI: 10.13745/j.esf.sf.2021.11.10
    Abstract （200）   HTML （6）    PDF（pc） （3160KB）（106）       Save
    Seismic data are essential for seismological detection of underground structures. However, it is difficult to obtain long-term, high-density continuous seismic data using traditional instruments in extreme environments such as underwater or a plateau. Compared with instrument manufacturers abroad, the development of DAS in China is relatively late. Since 2016, domestic DAS has been gradually applied to petroleum logging and underground structure detection in urban areas. However, there is much less DAS application in extremely harsh environments. After years of collaboration among the Institute of Semiconductors, Chinese Academy of Sciences (CAS) and Institute of Tibetan Plateau Research, scientists from CAS deployed the self-developed DAS in April 2021 for collecting field data in the Tibetan Plateau. During testing, an armored optical cable (deployed both in the water and above ground) was used to record continuous ambient noise data and active source signals. In this study, the ambient noise tomography was applied to DAS data, and a near-surface (less than 70 m deep) S-wave velocity structure along a two-dimensional survey line was obtained in the Yigong Lake in southeastern Tibetan Plateau. This study provided both theoretical and experimental evidence for high-density data collection and underground structure detection in harsh environments.
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    Active faulting along the western boundary of the Amur plate (territory of Mongolia)
    Vladimir A. SANKOV, Anna V. PARFEEVETS, Andrey I. MIROSHNITCHENKO, Aleksey V. SANKOV, Amgalan BAYASGALAN, Sodnomsambuu DEMBEREL
    Earth Science Frontiers    2022, 29 (1): 245-265.   DOI: 10.13745/j.esf.sf.2021.12.16
    Abstract （554）   HTML （17）    PDF（pc） （10891KB）（149）       Save

    The spatial position of the western boundary of the Amur plate within the territory of Mongolia is still not clear; active fault tectonics and the stress state of the Earth’s crust along it have been poorly studied. Within three regions along this border—the Hangay-Khentiy tectonic saddle, the Burgut block (Orhon-Tola interfluve) and the Selenga block, which includes the Selenga depression and the Buren-Nuruu uplift, studies of active faults were carried out using space imagery interpretation, relief analysis, geological structural data and reconstruction of tectonic paleostresses from tectonic fracturing and displacement along with fractures data. It is shown that active faults inherit ancient structural heterogeneities of the Paleozoic and Mesozoic ages. The faults do not form a single zone along the plate boundary, but form clusters. Their kinematics depend on the strike: sublatitudinal faults are left-lateral strike-slip faults with an obligatory reverse component; NW-strike faults are reverse faults or thrusts, most often with a right-lateral strike-slip component; submeridional faults are right-lateral strike-slip faults; and NE-strike faults are normal faults. The activation of fault structures localized in the Selenga depression and in the eastern part of the Hangay began in the Pliocene. Revers and strike-slip faults are not conformal to the Pliocene, and often to the Pleistocene relief, which indicates a younger, Late Pleistocene, age of their activation. Reconstructions of the stress-strain state of the last stage of deformation in zones of active faults, using tectonic fracturing and displacements along fractures, indicate the predominance of compression and strike-slip conditions with the N-NE and NE direction of the axis of maximum compression. Only within the Selenga depression is the prevalence of stress tensors of extension and strike-slip types with the NW strike of the axis of minimum compression noted. To the south, a local area with a predominance of the extension regime is located within the Eastern Hangay (Orhon graben). It is concluded that the activation of faults in the central part of Mongolia at the Pleistocene-Holocene stage, as well as modern seismicity, are mainly controlled by additional horizontal compression in the NE direction associated with the process of convergence of Hindustan and Eurasia. An additional factor that allows the implementation of strike-slip deformations in the crust of the study area and explains the divergent movements in the Baikal Rift, as well as the SE movement of the Amur plate, is the impact on the base of the lithosphere of the asthenospheric flow in the SE direction. The boundary between the Amur plate and the Mongolian block in the tectonic structure is expressed fragmentarily and represents the marginal part of the deformation zone covering the whole of Western Mongolia.

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    Structural characteristics of the Nima Basin in the Bangong-Nujiang tectonic belt, central Tibet
    ZHONG Linglin, ZHONG Kanghui, QIN Qin, YAN Zhao, YANG Xiong, HE Zhiyuan, ZHANG Hongjie, PENG Jie, Johan De GRAVE, Stijn DEWAELE, ZHOU Huiwen, HE Xingjie, HAN Wenwen, GONG Xiaobo, YANG Hairui, DONG Suiliang, CHANG Yupeng, LI Kaizhi, DOU Jie, LI Lin, HE Mingfeng, LIU Yilong
    Earth Science Frontiers    2022, 29 (1): 266-284.   DOI: 10.13745/j.esf.sf.2021.11.45
    Abstract （336）   HTML （26）    PDF（pc） （23781KB）（225）       Save

    The Neo-Tethyan subduction and subsequent India-Eurasia collision resulted in significant contractional deformation of the continental lithosphere and the uplifting of the Tibetan Plateau. Revealing the formation mechanism and process of giant fracture zone in Tibet is crucial for understanding the propagation of the deformation toward the hinterland of the Eurasian continent. Several continental basins of Cretaceous-Cenozoic age developed along the Bangong-Nujiang suture zone, providing a splendid record of the tectonic-sedimentary evolution of the central Tibet. So far, competing geodynamic models are proposed to explain the structural characteristics and the formation of these basins, including strike-slip faulting, extensional rifting and foreland flexural depression. The key to testing these models includes (1) fully documenting the structural characteristics of the basin basement and sedimentary infills and (2) clarifying the structural evolution of the Nima Basin in a regional tectonic context. With these goals in mind, we conducted large-scale geological mapping and structural analyses on the Nima Basin in the Bangong-Nujiang suture. Together with previous research findings, we discussed the tectonic settings, structural characteristics and structural evolution of the Nima Basin and reached the following conclusions: (1) The basement of the Nima Basin mainly consists of metamorphic rocks and marine sedimentary sequences within the Bangong-Nujiang suture zone formed through “soft” collision, and is fractured, with several E-W striking northerly/southerly dipping reverse faults. (2) The Nima Basin is infilled with upper Cretaceous-Neogene multi-cycle fluvial and lacustrine deposits; the terrigenous sedimentary infills later deformed into asymmetric folds with E-W trending axial planes and locally involved in the reactivated basement faults, while multi-phase contractional deformation propagated from the southern edge toward the basin center. (3) The surface structural pattern of the Nima Basin features “two depressions between three uplifts”, where the terrigenous sediments are mostly sourced from the northern imbricate thrust system and the southern fan-shaped extrusion structure, and, along with clasts derived from the central thrust nappes, accumulate in the two depressions—a northern one at the footwall of the imbricate thrusts and a southern one sandwiched between two reverse faults with opposing polarities. (4) The demise of the Bangong-Nujiang Ocean resulted in regional N-S crustal shortening and syncontractional development of the Nima basin in the suture zone, after which the Neo-Tethyan subduction and subsequent India-Asia continental collision triggered repeated reactivation of the basement faults within the Nima region, which invoked both multi-cycle deposition of molasse sequences and propagative deformation. In short, the Nima Basin is a contractional depression basin superimposed over a “soft-collision” type suture zone.

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    Tectonostratigraphic properties and evolution of the Yeba volcanic arc in South Gangdese, Tibet
    TANG Yu, WANG Genhou, FENG Yipeng, CI Dan, LI Dian, FAN Zhengzhe, GAO Xi, WEI Yufei, HU Jixin, ZHANG Peilie
    Earth Science Frontiers    2022, 29 (1): 285-302.   DOI: 10.13745/j.esf.sf.2021.7.33
    Abstract （288）   HTML （12）    PDF（pc） （23749KB）（422）       Save

    In the South Gangdese magmatic belt a set of exposed Early-Middle Jurassic volcanic-sedimentary formations underwent multi-stage structural deformation, which resulted in intense foliation displacing the volcanic-sedimentary sequence and forming the typical tectonic-rock strata. According to the stratigraphic division scheme for orogenic belt, the Early-Middle Jurassic volcanic arc is defined as Yeba rock group, which is further divided into the Bangdui, Yeba and Jiama rock formations based on its internal lithologic assemblages and structural deformation modes. The three-stage deformation is characterized as below. In D1 stage, the brittle-ductile shear deformation is pure shear dominant general shear deformation, and beddings (S0) are mostly replaced by penetrative foliations (S1) (S1∥S0). The deformation is characterized as pure westward shearing, from the top, with a steep stretching lineation to 85-100° according to kinematic observations. EBSD results showed the deformation temperature is no more than 380 ℃, and quartz particles are clearly fine-grained, formed by sub-particle rotational recrystallization.40Ar-39Ar dating results suggests the D1 tectonic deformation happened around 79 Ma, therefore it could represent an extrusion structure formed by the low angle (flatten) Neo-Tethys ocean plate subduction in the back-arc compression background. In D2 stage, longitudinal folding of S1 led to the axial-plane cleavage (S2) that dips to N or S, with an inclination of 40-70°, and hinges toward W or NWW. Combining with the regional geological evolutionary history, we believe the Southern Gangdse back-arc basin was extruded upward under continuous N-S compressional stress during the Late Cretaceous (79-68 Ma), which resulted in upper crustal shortening and thickening, then led to folding. In D3 stage, deformation is mainly kink-bands and near E-W normal faults. The maximum principal compressive stress is in the vertical direction, while the minimum (extension direction) is in near N-S direction. Combining with the regional tectonic evolutionary history, we suggest that the near N-S extensional event during the late Oligocene-early Miocene (23.74-21.1 Ma) may represent the uplift of the Gangdese batholith caused by the delamination of the Indian lithosphere and/or Tibetan plateau lithosphere (the main dynamic mechanism) and the Gangdese counter thrust activity.

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    The exhumation mechanism of eclogites in continental orogenic belts: Metamorphic petrology and geophysical constraints
    ZHANG Dingding, ZHANG Heng
    Earth Science Frontiers    2022, 29 (1): 303-315.   DOI: 10.13745/j.esf.sf.2021.12.37
    Abstract （428）   HTML （38）    PDF（pc） （6415KB）（337）       Save

    The deep subduction of the continental lithosphere is a frontier hotspot in the field of earth science, and the exhumation mechanism of eclogite is a key scientific issue in plate tectonics and continental dynamics. The p-T paths of eclogites from the world-famous continental orogenic belts show differential exhumation characteristics. To explore the exhumation mechanism in this study, metamorphic petrology and geophysical investigations were conducted on eclogites from three typical continental orogenic belts-Mesozoic-Cenozoic Alps, Mesozoic Sulu-Dabie, and Cenozoic Himalayas. The geophysical investigation found that, in the Alpine orogenic belt, subduction of the European plate resulted in large variations in lithospheric thickness beneath the Adria area. At the same time, the exhumation histories of eclogites are not the same in the Doria Maria and Pohorje areas of the Alpine orogenic belt versus in the greater Pohorje area, which, then, is probably due to the differential oblique extrusion of thrusting nappes from different periods after the break-off of the Adriatic ocean lithosphere. In the Sulu-Dabie orogenic belt, rapid exhumation of eclogites is likely be caused by the delamination or break-off of the lithosphere after the collision of the South and North China blocks. In the Himalayan orogenic belt, there are differences in the exhumation histories of eclogites in the middle Himalayas, and so are the differences in the p-T path and exhumation rate of eclogites from Naran versus from the Upper Kaghan Valley in the Western Himalayan Syntaxis. The differential exhumation, according to both metamorphic petrology and geophysical studies, is likely related to the tectonic extrusion and the break-off of the Indian continental lithosphere during the collision of the Indian and European plates. The Himalayan Orogen is a young and ongoing orogeny, thus it is more suitable for comprehensive metamorphic petrology and geophysical studies compared to the ancient Sulu-Dabie Orogen. Therefore, the exhumation mechanism of (ultra)-high-pressure eclogites in the Western Himalayan Syntaxis, i.e., tectonic compression and the break-off of subducting plate, can be applied to global orogenic belts.

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    Detailed structure of the Earth’s crust and upper mantle of the Severomuysk segment of the Baikal rift zone according to teleseismic data
    Valentina V. MORDVINOVA, Maria A. KHRITOVA, Elena A. KOBELEVA, Mikhail M. KOBELEV, Evgeniy Kh. TURUTANOV, Victor S. KANAYKIN
    Earth Science Frontiers    2022, 29 (2): 378-392.   DOI: 10.13745/j.esf.sf.2022.2.1
    Abstract （290）   HTML （9）    PDF（pc） （8420KB）（99）       Save

    The records of distant strong earthquakes, obtained by way of a dense linear network of seismic stations in the Severomuysk segment of the Baikal rift system, revealed a complex layered-block structure of the Earth’s crust and subcrustal mantle using the longitudinal receiving function. The distribution of cross-wave velocities indicates that the properties of the blocks that make up the Severomuysk Earth’s crust differ. The western vergence of these blocks and the stratification of the lower part of the Earth’s crust confirm the accretion-collision origin of the uplift. The intensity of the collision effect on the Earth’s crust of the region is explained by the location of the Severomuysk segment on the thinned inclined edge of the Siberian Craton. A convincing correlation was found between the focal depths of earthquakes in 2015 and contrasting velocity heterogeneities in the upper part of the Earth’s crust of the Muyakan depression.

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    Electrical resistivity structure beneath the central Cona-Oiga rift, southern Tibet, and its implications for regional dynamics
    XUE Shuai, LU Zhanwu, LI Wenhui, WANG Guangwen, WANG Haiyan, LIANG Hongda
    Earth Science Frontiers    2022, 29 (2): 393-401.   DOI: 10.13745/j.esf.sf.2022.2.3
    Abstract （245）   HTML （13）    PDF（pc） （4628KB）（160）       Save

    As one of the most significant extensional structural styles of the Tibetan Plateau, the rifts in southern Tibet are an important window for studying the growth of the plateau. However, the formation mechanism of these deep rifts remains controversial. In this paper, magnetotelluric data from the central part of the Cona-Oiga rift were used to study the rifts in southern Tibet. The Magnetotelluric sounding curves and phase tensors were calculated and analyzed, and the electrical resistivity structure beneath the Qiongduojiang and Oiga grabens was obtained through 3D MT inversion. The 3D inversion result showed that an obvious continuous high-conductivity anomaly develops beneath the Cona-Oiga rift in a “subduction” pattern overlaying with high-resistivity structures, while low-resistivity anomalies distribute in the relatively shallow parts beneath either side of the Qiongduojiang graben. Combined with the previous studies, the continuous high-conductivity anomaly beneath the Cona-Oiga rift is believed to be originated from crustal partial melting, probably related to the southward crustal flow. We suggest that, under the N-S compression driven by the India-Eurasia collision, the weakened crust promotes the development of the rifts in southern Tibet by decoupling the upper crust and the lower lithosphere.

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    2D Tomographic imaging of the P-wave velocity structure in the upper crust beneath the southern Beishan tectonic belt
    WU Guowei, XIONG Xiaosong, GAO Rui, CHEN Xuanhua, LI Yingkang, WANG Guan, WANG Xiaocheng, REN Haidong
    Earth Science Frontiers    2022, 29 (2): 402-415.   DOI: 10.13745/j.esf.sf.2022.2.2
    Abstract （207）   HTML （19）    PDF（pc） （16930KB）（269）       Save

    The Huahai Basin-Beishan tectonic belt is located north of the northeastern margin of the Qinghai-Tibetan Plateau and is the junction zone connecting the Tethys and Paleo-Asian Ocean domains. Since the Late-Neoproterozoic it has undergone multi-era, multi-stage evolution involving multiple plate splitting, subduction, collision and merging events. The subsequent overthrusting and strike-slip faulting since the Mesozoic, in particular the northward expansion of the northeastern margin of the Tibetan Plateau caused by the far-field effect of the collision between the Indian and Eurasian plates in the Cenozoic, formed the present complex geological/geomorphological structure. The crustal structure records the overprinting of the tectonics, whereas the upper crust is a natural notebook valuable for understanding the outward growth of the NE Tibet and its role in the transformation of the adjacent tectonic units. In this paper, based on the first arrive seismic wave (Pg phase) data from the 180 km-long deep seismic reflection profile completed by the Chinese Academy of Geological Sciences in 2018, we applied the tomographic inversion method to determine the P-wave velocity structure in the upper crust beneath the Huahai Basin-southern Beishan tectonic belt, 0-4 km deep underground. It was found that the three basins, Huahai, Zongkouzi and Zhagehao Basins, have a relatively low P-wave velocity and small vertical velocity gradient; the Late-Paleozoic granite outcrop shows obvious high-velocity anomalies and large vertical velocity gradient; and the left strike-slip Altyn fault zone across the southern margin of the Huahai Basin is a north-dipping high-angle strike-slip fault as deep as cutting through the basin basement at the least. In addition, many low-velocity anomalies revealed the extend of fault development in the southern Beishan.

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    Continent-continent collision at the southwestern margin of the Cretaceous Qiangtang terrane: Constraints from granite in the western Bangong-Nujiang Suture Zone
    FU Shun, ZHAO Yingquan, WANG Jinjun, YU Yu, ZHU Yingtang, FU Xingzhe
    Earth Science Frontiers    2022, 29 (2): 416-430.   DOI: 10.13745/j.esf.sf.2021.10.50
    Abstract （221）   HTML （7）    PDF（pc） （6973KB）（143）       Save

    A large number of moderately felsic to basic intrusive rocks in the forms of rock strains or rock branches are distributed in the southwestern margin of the Qiangtang terrane. These intrusive rocks are characterized by petrographic analysis as peraluminous granite with low SiO2 (67.99%-73.32%) and high Al2O3 (12.82%-15.02%) contents, and defined as calc alkaline based on the Ritmann serial indices of 2.09-2.63 (σ<3.3). They show strong enrichment of light rare earth elements (REE) and strong depletion of heavy REE, with a high degree of magma crystallographic differentiation. The geochemical characteristics of the rocks and REE in granite indicate its tectonic environment is mainly type I, with some variability, and its formation and intrusion should be related to the closure of the ancient Tethys Ocean and the expansion of the new Tethys Ocean, which suggests an intracontinental collisional orogenic granite origin. The zircon laser ablation inductively coupled plasma-mass spectrometry (LA-ICPMS) dating of several granite masses in this area yielded a formation age of 107-116 Ma, which is consistent with the time of continent-continent collision at the southern margin of the Qiangtang terrane in the late Early Cretaceous.

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    Crustal structure of the Cona rift, eastern Himalaya
    WU Jiajie, XU Xiao, GUO Xiaoyu, LU Zhanwu, WU You, XIANG Bo, YU Yang, LI Chunsen, YU Jiahao, TONG Xiaofei, LUO Xucong
    Earth Science Frontiers    2022, 29 (4): 221-230.   DOI: 10.13745/j.esf.sf.2022.4.66
    Abstract （268）   HTML （11）    PDF（pc） （6467KB）（187）       Save

    The Himalayan mountain range is the result of continental-continental collision between the Indian and Eurasian plates. It has been under great debate as to why the rifts of southern Tibet are formed at the front of the collision zone. To answer this question, it is necessary to understand the crustal structure of the rifts. The age of each rift zone tends to be younger from west to east. In this study, we revealed the crustal structure of the Cona rift, a relatively young continental rift, using the P-wave receiver function calculated from teleseismic data received by a dense array across the rift, and analyzed the rift formation process based on the crustal structure. We showed that the Cona rift is a crustal-scale rift, where Moho offsets beneath the rift and significant lateral variations develops. We suggest that the formation of the rifts may be associated with regional tectonic activities, and further studies are needed to ascertain whether a single gravity collapse can form crustal-scale rifts. Based on the previous studies of magmatic rocks and geophysical observations, we infer that the asthenospheric upwelling caused by tearing of the subducting Indian plate weakened the lower crust of the Cona rift region where the middle and upper crust is also weakened as shown by the study of Himalayan leucogranites. Considering all the study results, we hypothesize that the formation of crustal-scale rift requires crustal weakening at different depths.

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    Fold-thrust belt of South Qiangtang, Tibet and the double-layer structure of the South Qiangtang accretionary complex
    LI Dian, WANG Genhou, LIU Zhengyong, LI Pengsheng, FENG Yipeng, TANG Yu, LI Chao, LI Yang
    Earth Science Frontiers    2022, 29 (4): 231-248.   DOI: 10.13745/j.esf.sf.2022.3.30
    Abstract （203）   HTML （9）    PDF（pc） （6045KB）（131）       Save

    The accretionary orogens formed in the active continental margin are represented by extensive and stable accretionary complexes. During the slow and complex oceanic subduction and collision, oceanic plate, intra-oceanic arc, seamount, and continental fragments are accreted onto the retreating oceanic trench through off-scrapping, underplating, and tectonic erosion at the leading edge of the overriding plate. Continental crust thus grows laterally as significant accretion of oceanic crusts at the inner wall of the trench. Via a similar tectonic process during the continental collision passive margin are incorporated into the subduction channel, where a crustal accretionary wedge similar to but much larger than oceanic accretionary prisms are expected to form. Therefore the composition, structure, and evolution of accretionary complex in orogenic belt play a key role in understanding the complex geodynamic process during the ocean to continent transformation. The accretionary complex of South Qiangtang, Tibet was recently recognized through corridor geological mapping and multidisciplinary research. However, the composition and structure of the complex are not well studied, which greatly hinders the understanding of its formation mechanism and evolution. Therefore in this paper, focusing on its spatiotemporal evolution, we researched in detail the composition and structure of the South Qiangtang accretionary complex to understand its formation and evolutionary processes. We show that (1) the South Qiangtang accretionary complex has a double-layer structure, with the subduction complex at the bottom, the fold-thrust belt at the top, and a regional detachment fault system separating the upper and lower layers. (2) The subduction complex contains not only the ocean plate stratigraphy but also a large part of the South Qiangtang passive margin. (3) Although the fold-thrust belt is mainly deformed passive continental margin, it also contains ocean plate stratigraphic units like seamounts and intra-oceanic arcs. Based on the spatiotemporal distribution of fore-arc basin and wedge-top basin during subduction and Late Triassic syn-collisional magmatism, the double-layer structure of the South Qiangtang accretionary complex should mainly be induced by subduction of passive continental margin during continental collision, and probably also by subduction reversal during oceanic subduction. The double-layer structure of the South Qiangtang accretionary complex and its continental subduction origin proposed in this paper are of great significance for understanding the crustal structure of the South Qiangtang terrane and the evolution of Mesozoic basement.

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