Search

The Khangai plume is situated under Central and Eastern Mongolia and is a mantle volume with significantly reduced longitudinal (P) wave velocities. The plume has been identified as a result of the analysis of the MITP08 volumetric model of P-wave velocity variations, representing the deviations of P-wave velocities from the average values (δVp), given as a percentage. The lithospheric mantle is thinned to ca. 50 km above the plume. Especially low velocities (δVp ≤ –0.6%) are found in the sublithospheric mantle up to a depth of 400 km. The main body of the plume is located under the Khangai Highland and extends northward to the edge of Southern Siberia. The Khentei branch of the plume that is located SE of the Khentei Highland is connected with the main plume body at depths of 800–1000 km. Branches of the plume and its Khentei branch extend into Transbaikalia. The area of the plume decreases with depth, and its deepest part (1250–1300 km) is located under the southern Khangai Highland. The main body of the Khangai plume is expressed on the land surface by the Cenozoic uplift reaching 3500–4000 m in the southern Khangai Highland. From the SE, the Khangai plume and its Khentei branch territory are limited by Late Cenozoic troughs stretching along the southeastern border of Mongolia. From other sides, the Khangai uplift is bounded by a C-shaped belt of basins. The belt includes the southwestern part of the Baikal Rift Zone, the Tunka and Tuva Basins in the north, the Ubsu-Nur Basin and the Basin of Big Lakes in the west, and the Valley of Lakes in the south. The basins are filled with lacustrine and fluvial deposits of the Late Oligocene to Pliocene. In the Quaternary, the South and Central Baikal Basins, which existed as early as the Early Paleogene, became a part of the Baikal Rift, and the other basins were involved in the general uplift of the region. The structural paragenesis of the Khangai uplift and the surrounding basins is caused by the influence of the Khangai plume. On the territory above the plume, including its Khentei and Transbaikalia branches, the Cenozoic basaltic plume volcanism occurred, inheriting the Cretaceous volcanic manifestations in some places. The structural paragenesis associated with the Khangai plume is combined with the structural paragenesis produced by lithospheric plate interaction. The latter is expressed the best of all by active faults, but developed synchronously to the plume paragenesis. The active fault kinematics shows that the eastern and central parts of the region developed in the transpression conditions and the north-eastern part developed in conditions of extension and transtension. The Khangai plume is connected at depth with the Tibetan plume, which is situated under the central and eastern Tibetan Plateau north of the Lhasa block. The Tibetan plume has the shape of a funnel rising from depths of 1400–1600 km and is accompanied by thinning of the lithosphere and uplift of the land surface. The Khangai and Tibetan plumes represent a specific category of plumes. They rise from the upper Lower Mantle and differ by this from the Upper Mantle plumes and the African and Pacific superplumes rising from the core-mantle boundary. Data are presented on the possible connection of the Khangai and Tibetan plumes with the superplume branches, but independent origin of the plumes is also admitted. [ABSTRACT FROM AUTHOR]

The Tannuola Terrane of Tuva, which is located in the northern part of the Central Asian Fold Belt, formed as a result of island-arc and accretionary-collision events in the Early Paleozoic. Further tectonic evolution of the terrane is related to a multiple reactivations of large and mostly normal fault structures. In the middle Paleozoic, the extension of the crystalline basement of the terrane led to the active uplift of mafic melts along the faults to the surface and formation of subalkaline igneous rocks. The Mesozoic and Cenozoic igneous complexes within the Tannuola Terrane are unknown; thus, its tectonic evolution in the Mesozoic and Cenozoic could be considered only after a sedimentary record, which is preserved in the Mesozoic intermountain depressions of Tuva and adjacent Cenozoic Ubsunur Basin. This geological information, however, is insufficient for an exhaustive consideration of the regional tectonic regime taking into account the lack of confirmation of present-day precise methods. The understanding the Mesozoic–Cenozoic tectonic evolution of the Tannuola Terrane is impossible without the analysis of data on low-temperature thermochronology for rocks of the crystalline basement. In our study, fission-track analysis of apatite from 12 samples of the early Paleozoic granitoids of the Tannuola Terrane is conducted in order to recognize the stages of activation and tectonic stability within the absolute time scale. The analysis showed a wide range of ages from 83.4 ± 4.7 (Late Cretaceous) to 35.5 ± 2.2 (late Eocene) Ma at variation of the mean fission-track length from 11.4 to 12.3 µm. The modeling of the thermal evolution of the basement of the Tannuola Terrane, which is based on these data, indicated three stages of tectonic activation of various origins and intensities divided by stages of tectonic quiet periods for the last ~185 million years: (i) ~185–135 Ma (Jurassic–Cretaceous), (ii) ~90–35 Ma (Cretaceous–Paleogene), and (iii) ~15–0 Ma (Neogene–Quaternary). [ABSTRACT FROM AUTHOR]

The Hangay Dome in central Mongolia, a typical intraplate plateau far from plate boundaries, is characterized by rapid uplift and widespread volcanic activities in the Cenozoic. However, thermodynamics process for the landform in Cenozoic remains mysterious. Seismic Lg-wave, which is sensitive to crustal thermal status, can provide constraints on the crustal attenuation structure; thus, may be helpful to understand the thermodynamics. In this study, we estimate the Lg-wave quality factor QLg across the Hangay Dome and its surroundings from 231 crustal earthquakes recorded by 136 regional broadband seismic stations distributed throughout north China and Mongolia. Using a joint tomographic method, we construct a broadband Lg attenuation model at 58 discrete frequencies distributed evenly in log scale between 0.1 and 20.0 Hz. The obtained QLg model provides new insights into crustal attenuation in the region. The strong Lg wave attenuation observed in the northeastern Hangay Dome and western Gobi-Altai, associated with crust low-resistivity anomalies, low S-wave velocity region, heat flow and volcanic activities, suggest the existence of possible partial melting due to asthenospheric upwelling. The widespread moderately-low QLg values around the perimeter of the Hangay Dome imply that the margin of the Hangay Dome has been weakened. The center part of the uplift is a small high QLg nucleus, which is possibly the residue of the Precambrian basement. The above observations suggest that the Hangay Dome is lifted by small-scale asthenospheric upwellings. During this process, the very hot mantle materials strongly modified the Hangar Dome and also caused the Cenozoic volcanism and major earthquakes. [ABSTRACT FROM AUTHOR]

Intracontinental deformation is an inseparable element of the evolutionary history of orogenic belts. It can reactivate and intensely modify the structures of previous orogenic belts due to remote effects of plate margin orogenesis. As an important part of the Phanerozoic Central Asian Orogenic Belt (CAOB), the Alxa Block and its vicinity experienced multistage intracontinental deformation during the Mesozoic and Cenozoic and became a representative region to study remote effects from plate margins. Both the previous structures of the CAOB and continuous accretion to the Eurasian continent from different directions controlled and affected the Mesozoic evolution of the Alxa Block. However, mantle loading can't pertinently explain the geological phenomena alone. In the Late Triassic, the Alxa Block was deformed by large-scale left-lateral shearing along largely northeast-southwest-trending faults and rotated anticlockwise caused by the collision between the North China Craton and Yangtze Craton and the northward movement of the North China Craton. In the Late Jurassic, the low-angle westward subduction of the Paleo-Pacific Ocean beneath the Eurasian Plate, the closure of the Mongol-Okhotsk Ocean to the north and/or the Lhasa-Qiangtang collision to the south combined to deform the Alxa Block from different directions, forming contraction structures with various strikes and invert previous extensional basins. In the Late Cretaceous, the Eurasian Plate experienced oblique collision along its southeastern margin and the subduction of the Neo-Tethys along its southern boundary, an intraplate sinistral transpressional megashear system developed along the eastern Alxa Block, and the uplift of mountain ranges along the basin margins. The multistage Mesozoic intracontinental deformation of the Alxa Block supports the view that they are driven by stresses generated at plate boundaries largely, and controlled by previous complicated orogenic structures in the upper crust. [ABSTRACT FROM AUTHOR]

Two Cenozoic prominent features are spatio-temporally associated in central Mongolia, i.e., the continental basalts and regional uplift, but their genesis and relationship remain unclear. This study presents major- and trace-element compositions for olivine phenocrysts and xenocrysts, as well as data of bulk-rock geochemistry and Sr-Nd-Hf isotopes for the host basalts. The studied basalts mostly have trachybasalt compositions with high total alkali (Na2O + K2O = 5.1–8.2 wt%) contents and all display OIB-like trace element patterns (e.g., spikes of Ba, Nb, and Ta and troughs of Th and U) and EM1-like Sr-Nd-Hf isotopic compositions. Compared to the partial melting products of mantle peridotite, these basaltic samples have higher FeO/MnO, Zn/Mn, and Zn/Fe ratios. Meanwhile, phenocrystic olivines are characterized by lower Ca, Mn, Mn/Zn, and Mn/Fe but higher Ni than their counterparts in the peridotitic melts, indicating a pyroxenite-rich mantle source. The above geochemical data suggest that the source of the studied basalts was mainly made up of secondary pyroxenite produced by the reaction of recycled oceanic crust with its ambient mantle peridotite. The calculated magma oxygen fugacities (ΔFMQ-0.26 to +0.42) and mantle melting temperatures (1343–1430 °C) do not support a genetic link with the stagnant Pacific slab or with a deep mantle plume. Instead, the far-field effect of India-Eurasia convergence possibly tapped the upper asthenospheric mantle, subsequent melting of which gave rise to the dispersive Cenozoic basalts. On the other hand, the xenocrystic olivines exhibit zoned textures with high-Fo (up to 92) cores and low-Fo (down to 76) rims, reflecting the melt-rock interaction. Preservation of zoned olivine xenocrysts indicates rapid magma ascent and widespread melt-rock reaction in the mantle lithosphere, which may modify the rheology and accelerate the mechanical erosion of mantle lithosphere. Consequently, mass deficit in the lithosphere could have caused isostatic uplift of central Mongolia in the Cenozoic. [ABSTRACT FROM AUTHOR]

The West Siberian Basin (WSB) is one of the largest intracratonic Meso-Cenozoic basins in the world. Its evolution has been studied over the recent decades; however, some fundamental questions regarding the tectonic evolution of the WSB remain unresolved or unconfirmed by analytical data. A complete understanding of the evolution of the WSB during the Mesozoic and Cenozoic eras requires insights into the cooling history of the basement rocks as determined by low-temperature thermochronometry. We presented an apatite fission track (AFT) thermochronology study on the exposed parts of the WSB basement in order to distinguish tectonic activation episodes in an absolute timeframe. AFT dating of thirteen basement samples mainly yielded Cretaceous cooling ages and mean track lengths varied between 12.8 and 14.5 μm. Thermal history modeling based on the AFT data demonstrates several Mesozoic and Cenozoic intracontinental tectonic reactivation episodes affected the WSB basement. We interpreted the episodes of tectonic activity accompanied by the WSB basement exhumation as a far-field effect from tectonic processes acting on the southern and eastern boundaries of Eurasia during the Mesozoic–Cenozoic eras. [ABSTRACT FROM AUTHOR]

The relative importance of global cooling and Tibetan Plateau uplift in driving the aridification of Asia during the late Cenozoic is debated, largely due to the lack of appropriate proxy indicators. Here we address this problem by investigating changes in the source of Chinese loess and Red Clay, which is directly controlled by changes in the extent and distribution of the arid zone of Asia and the intensity of the East Asian winter monsoon, using zircon U‐Pb dating of 27 levels in a near‐continuous eolian sedimentary sequence in southern Chinese Loess Plateau. The results show that source regions expanded stepwise at ~7.2, ~2.6, 1.2–0.9 Ma, and at the Last Glacial Maximum. These changes were synchronous with global cooling and ice cover expansion in the Northern Hemisphere, suggesting that the drivers of aridification of the Asian interior were intimately related to global cooling in the late Cenozoic. Plain Language Summary: The arid and semiarid regions of Central Asia are one of the most important dust sources on Earth. Dust emitted from these regions is deposited in the Chinese Loess Plateau, the North Pacific Ocean, and even in Greenland, with broad environmental impacts. Although there is evidence that the drying of the Asian interior occurred gradually over several tens of million years, the driving factor remains unclear. Here we investigate the evolution of the sediment sources of the Chinese Loess Plateau and find that the changes in the source regions of wind‐blown sediment were synchronous with global cooling since ~7.2 Ma. This indicates that the long‐term aridification of the Asian interior was most likely driven by global cooling. Key Points: Dust source areas of the Chinese Loess Plateau expanded at ~7.2, 2.6, 1.2–0.9 Ma, and at the Last Glacial MaximumExpansion of dust source areas since ~7.2 Ma was mainly controlled by the aridification of the Asian interior and the strengthening of the East Asian Winter MonsoonAridification of the Asian interior and strengthening of the East Asian Winter Monsoon were mainly driven by global cooling [ABSTRACT FROM AUTHOR]