India-Eurasia Collision Research Articles

The dispersal and diversification of earthworms (Annelida: Oligochaeta) related to paleogeographical events in the Hengduan Mountains

The Hengduan Mountains (HMs), China, is a biodiversity hotspot that uplifted mainly between Late Miocene and Late Pliocene, the uplift movement might have a significant impact on the evolution of earthworms. Here, we investigated the species richness and the distribution of the earthworms in the HMs. A total of 170 species/subspecies belonging to four families and 12 genera were preliminary identified, species number in Megascolecidae account for more than 88%. Molecular phylogeny and divergence times analysis based on concatenated genes suggest that the divergence time of Moniligastridae (Outgroup) and Lumbricidae (Group 1) in the HMs was 215.84 million years ago (MYA). The divergence of first two clades in Megascolecidae (M405 and Group 2) can be date to around 44.42 MYA (95% HDP: 38.14–52.62 MYA). Dispersal of Megascolecidae from outside of the HMs to this region coincides with Indian-Eurasian collision between Eocene and Oligocene. The reconstruction of ancestral-area reveals the clades in Megascolecidae (Group 2–8) may have experienced different evolutionary histories following their divergence time during Cenozoic, which suggests that paleogeographical events might promote the dispersal and diversification of earthworm in the HMs. Unearthing the biogeographical pattern of earthworms provide insight into the evolution of soil invertebrate fauna in the temperate northern hemisphere and can be used for tracing the complex paleogeographical history of the HMs.

Slow Strain Waves in Rocks as Potential Precursors to Seismic Hazard

The article reports on instrumental monitoring of rock deformation in the South Baikal geodynamic test site. Based on the analysis of the time series of data sets, two groups of strain components are distinguished by differences in their origin and wave properties. Strain components in the first group are caused by external factors, such as lunar-solar tides and atmospheric pressure variations. The second group includes non-periodic, random and periodic (constant) components generated by internal tectonics. Non-periodic components are manifested as single deformation impulses that differ in intensity and form, and are related to slow displacements along large active faults, which occur outside the monitoring sites. Besides, non-periodic components are related to fast displacements at block interfaces within the monitoring sites. The periodic (constant) component is related to the India-Eurasian collision. It is represented by slow strain waves, which lengths amount to 400–500 m, with amplitudes of few microns, and periods of (1-3)·10−4 Hz. The strain wave directions and rates of their migration in space vary with time. The identified trends of changes of the wave parameters in the South Baikal geodynamic polygon can be related to the preparation of a strong earthquake.

Thermochronological insights into the intracontinental orogeny of the Chinese western Tianshan orogen

The Chinese western Tianshan orogen (CWTS) experienced multiple tectono-thermal events since the late Paleozoic. In order to better understand the intracontinental orogeny, we integrate new thermochronologic data to reconstruct the long-term cooling history of the CWTS. Our new data reveal that individual tectonic units within the Tianshan orogen show spatial/temporal disparities in their cooling histories. The CWTS experienced a rapid cooling episode during a late Paleozoic orogeny. The Cimmerian collisional orogeny (i.e., the Qiangtang-Kunlun and Lhasa-Qiangtang collision) and the closure of the Mongol-Okhotsk ocean were not strictly coincident with the exhumation events in the CWTS, and thus these distal events were likely not the sole orogenic drivers. The thermal curves reflect that lower crustal rocks had exhumed to middle-upper crustal levels in the late Permian-Triassic, and a rapid cooling phase during this period records the collapse of the CWTS. Furthermore, magmatism related to the melting of subducted Paleozoic plates was probably a crucial factor in the Mesozoic thermal evolution. The age-elevation relationship produced here emphasizes the importance of faulting during intracontinental reactivation and implies that the CWTS has expanded from its central part to both the south and north sides since the late Mesozoic. Low exhumation rates and sedimentological analysis indicate that the relief gradually decreased in the Jurassic-early Cretaceous. The initial reactivation time of the CWTS could be as old as ~38 Ma based on a rapid exhumation event and sedimentary evidence. In the Cenozoic, the crust-mantle interaction, tectonic compression, and continuous northward pushing of the Indian plate rather than the India-Eurasia collision are interpreted as the driving forces that have rebuilt the CWTS. The variable tectono-thermal evolution of individual tectonic units ascribes to the discrepant thermal-rheological properties, complex tectonic boundaries, and the various components of the Tianshan orogen.

Crustal density structures and isostasy beneath the Western North China craton, Trans-North China Orogen, and surrounding regions

To determine the lateral and vertical variations in crustal structure and their influence on the seismicity of the Western North China Craton, the Trans-North China Orogen, and the surrounding regions, the wavelet multi-scale structures, Moho depth, crustal density structures, and isostatic state are modelled using Bouguer gravity anomaly data, topography, and earthquake focal mechanisms. We obtained homogeneous crustal densities and deviations of <1 ​km between the crustal thicknesses estimated from the isostatic model and those inverted from the Bouguer gravity anomalies in the Ordos Block, the Inner Mongolia Suture Zone, the Sichuan Basin, and the Jizhong Depression. These results provide new evidence for relatively simple and stable continental crustal structures, and indicate that these regions will remain stable in both the vertical and lateral directions. The Hetao Graben, Yinchuan Graben, Weihe Basin, and Shanxi graben system have heterogeneous crustal densities and are isostatically over-compensated. In contrast, the crust beneath the Yinshan Uplift, Lvliang Uplift, and northern and central Taihang Uplift is thin and under-compensated. The heterogeneous crustal densities and non-isostatic state beneath the Tibetan Plateau and Qinling Central China Orogen indicate that these two blocks are unstable in the vertical and lateral directions. Although Cenozoic deformation of the North China Craton is thought to be driven by lithospheric stresses related to the India-Eurasia collision and Pacific slab retreat in South East Asia, we suggest that gravitational potential energy created by the heterogeneous crustal structure modulates these first-order forces. The results of this study could constrain the causes of seismicity in systems surrounding the Ordos Block.

Magma system beneath Tengchong volcanic zone inferred from local earthquake seismic tomography

The Tengchong volcanic zone (TCVZ) located in the southeastern part of the India-Eurasia collision zone is characterized by large-scale volcano groups, significant thermal activities and active seismicity. To reveal the magma system, we present new 3-D models of Vp, Vs and Vp/Vs ratio to 10 km depth beneath the TCVZ based on the tomographic inversion of P- and S-wave arrival times from local earthquakes recorded by 10 broadband seismic stations. We use arrival times to construct a reference velocity model. Checkerboard and jackknife tests are carried out to assess the reliability of the inversion results. At shallow depths, we suggest that the complex distribution of high Vp/Vs ratio is the result of a combination of water and magmatic activity. At depth of 10 km, only three areas show obvious high Vp/Vs ratio corresponding to areas of high relative geothermal value observed through hot springs at the surface. We suggest that the three high Vp/Vs ratio areas are magma chambers. The upwelling volatiles heat the water in the shallow layer, inducing the corresponding high relative geothermal value at the surface. Meanwhile, the magma chamber beneath Rehai spring, which corresponds to high He3/He4 ratio, may be fed by mantle magma. We suggest that the Heikongshan volcano and Dayingshan volcano are fed by the magma chambers and that Maanshan volcano is separated from the magma. We also infer that the significant geothermal activity and active seismicity beneath Rehai spring are the result of interaction between the magma and hot-water reservoir.

S-Wave Velocity Images of the Crust in the Southeast Margin of Tibet Revealed by Receiver Functions

The southeast margin of Tibet is the region in clockwise rotation around the Eastern Himalayan Syntaxis due to the India–Eurasia collision and the resistance of the stable Sichuan Basin and South China block. However, the dynamic processes involved in the evolution and deformation of the region remain poorly understood due to a lack of reliable geophysical observations. We collected abundant seismic data recorded by 108 permanent broadband stations deployed in the SE margin of Tibet since 2000, and obtained 4536 pairs of P-wave receiver functions (PRFs) with high signal-to-noise ratio. In this study, we have implemented a novel two-step data inversion procedure that can reduce the dependence of the inversion results on the initial model. We first use low-frequency PRFs obtained by iterative deconvolution in the time domain, and then an initial model consisting of a series of 2-km-thick isotropic layers to fit velocity models, and thus determine an overall statistical solution by means of the bootstrap resampling technique. This statistical solution is then regarded as a new initial model to adjust high-frequency PRFs. Hence, the same resampling process is executed again to estimate the optimal S-wave velocity structure below each station. The results provide an accurate 3D image of the crust and uppermost mantle in the SE margin of Tibet. We infer a wide intra-crustal low-velocity zone that varies laterally and in depth, which is thinner or even absent in the most southern part of Yunnan. Our hypothesis is that this low-velocity zone is the result of the accumulation of lower crustal flow coming from central Tibet. Furthermore, we show that this lower crustal flow extends largely through the Sichuan–Yunnan diamond-shaped block, and that there are significant variations in both crustal velocity structure and deformation mechanism across the great strike-slip faults of the Jinshajiang–Red River and Xiaojiang fault systems.

Paleogene crustal extension in the eastern segment of the NE Tibetan plateau

The Tibetan plateau experienced lateral growth in response to the India–Eurasia collision, but uncertainties remain about when and how its present-day margins were formed. It was commonly viewed that crustal shortening in the NE Tibetan plateau had commenced since the Eocene and led to reverse faulting and compressional basins. Our recent investigations of some Cenozoic intermontane basins reveal that synsedimentary normal faults occur widely in the eastern segment of the NE Tibetan plateau. Normal faulting at basin bases obviously controlled basin sedimentation and presumably resulted in footwall uplift and basement exhumation. Small-scale synsedimentary normal faults are very common in basin fill sequences although they decrease in intensity upwards and eventually vanish in the uppermost Paleogene sequences. Measurement of the attitudes of synsedimentary normal faults indicates that crustal stretching was in NW–SE direction. The NW–SE extension was consistent with Paleogene stress field of the whole North China craton, which were thought responsible for the NE–SW-trending rift basins in the interior of the North China craton, such as the Yinchuan, Weihe, and Bohai Bay rift basin systems. A broad unconformity separates Paleogene and Neogene sequences in the eastern segment of the NE Tibetan plateau, contrasting strikingly with conformable Oligocene to Middle Miocene sequences in the neighboring Ningnan and Yinchuan rift basins. It thus follows that the eastern segment of the NE Tibetan plateau had not been formed in Paleogene when it was basically dictated by crustal extension, possibly as a result of far-field effect of subduction of the western Pacific plate. End-Oligocene termination of normal faulting and the presence of a regional unconformity indicate that the eastern segment of the NE Tibetan plateau began rising at the beginning of the Miocene.

Biotic interchange through lowlands of Tibetan Plateau suture zones during Paleogene

A substantial biotic interchange took place between Gondwanan India and Laurasia immediately following the India-Eurasia collision. Located just north of the collision zone, the role of the Tibetan Plateau in the geographic interchange of biota is still unknown because lack of fossil evidence. Here, well-preserved samara fossils of Ailanthus (the family Simaroubaceae) are described from the Lunpola and Nima basins, central Tibetan Plateau, ranging in age from the latest Paleocene to the Late Oligocene. These fossils are the largest samara fossils of this genus found so far. Characteristics such as the main ventral vein located in the intramarginal part of the samara, and the stylar scar located on the midline of the seed suggest that they represent a new species, which we herein name Ailanthus maximus J. Liu, T. Su et Z.K. Zhou, sp. nov. These findings greatly expand the known spatial distribution of Ailanthus in the Paleogene, and suggest that the Tibetan Plateau has played an important role in the migration of biota between Gondwanan India and the Northern Hemisphere. In addition, A. maximus is morphologically similar to A. triphysa, a living species which is naturally distributed in tropical and subtropical regions in Southeast Asia at elevation no more than 600 m, indicating a tropical or subtropical lowland environment in the central Tibetan Plateau during the latest Paleocene to Late Oligocene. Moreover, together with previous paleoaltimetric estimates, we propose that tropical or subtropical lowlands along the Bangong-Nujiang Suture Zone and the Yarlung-Zangpo Suture Zone existed in some parts of the Tibetan Plateau during the Paleogene only obtained their current elevation during the Neogene.

Stress field modelling of the Late Oligocene tectonic inversion in the Liaodong Bay Subbasin, Bohai Bay Basin (northern China): Implications for geodynamics and petroleum accumulation

The Liaodong Bay Subbasin (LDBS), Bohai Bay Basin, is one of the most petroliferous basins in China. The basin experienced a strong tectonic inversion in the Late Oligocene, which has been proven to have had a major influence on petroleum accumulation. However, the dynamic origin of this tectonic event is poorly understood. In this paper, we present a finite element-based dynamic modelling study of the Late Oligocene tectonic inversion in the basin based on geological, geophysical and geomorphological data with different boundary conditions. We compared the modelling results with actual geological data and concluded that the NE-SW-directed compression model (labelled with BC-1) is the preferred model, suggesting that the effects of the India-Eurasia collision were the main cause of the Late Oligocene tectonic inversion. The results suggested that the maximum horizontal principal compressive stress direction (σHmax) generally trends EN E-W SW in the whole basin. The majority of the basin features a strike-slip faulting stress regime; the normal fault regimes are limited to the major faults, and the thrust faulting regimes are present only in the northern area of the Liaodong Uplift and Liaozhong Depression. The predicted fault slip rates occurring along the LZ1 fault in the Liaozhong area are significantly higher than those in the Liaoxi and Liaodong areas. Two high shear strain rates belts are present along the major faults in the Liaoxi and Liaozhong areas, indicating that the major faults accommodated the majority of the strain rates. This tectonic inversion play a significant positive role in the accumulation of petroleum in the basin due to the favourable relationship among the high strain rates, the formation of inversion-related traps and the timing of hydrocarbon charging. The high strain rate belts are suggested as the preferred targets for oil and gas exploration.

An assessment of present-day crustal deformation in the Kumaun Himalaya from GPS observations

The India-Eurasia collision has created high mountains in the Himalayan plate boundary system, which has led to numerous earthquakes in this region and released part of the elastic strain accumulated over years. In this study, the present-day deformation pattern is derived from geodetic mode Global Positioning System (GPS) measurements obtained during 2005–2016 from 16 campaign sites in the Kumaun province of the central Himalaya. GPS derived horizontal velocity vectors are in the range of 1–9 mm/yr and 27–38 mm/yr with respect to stable India and Eurasia reference frames respectively. To understand the interseismic strain partitioning in the study region, we consider the uniform slip dislocation model over the fault in an elastic, isotropic and homogeneous half-space medium. The model indicates the Main Himalayan Thrust (MHT) fault is locked to a depth of ∼20 km and slips at a rate of 17.2 ± 1 mm/yr with a dip angle of ∼7°. This MHT slip rate build-up corresponds to a moment deficit of 8.4 ± 1 × 1018 Nm/yr due to locking of the fault beneath the Kumaun Himalaya. The total seismic strain accumulation of 61 × 10−6 estimated from the geodetic data suggests the possibility of large earthquakes (typically magnitude 8 and above) with the recurrence interval of around 600 years in the Kumaun Himalayan region.

Terrestrial heat flow of continental China: Updated dataset and tectonic implications

Terrestrial heat flow is a crucial parameter to indicate the thermal state within the Earth, containing integrated information of the ground temperature, thermal conductivity, and crustal/mantle heat flow. We present an updated heat flow dataset and map in continental China. The data increases from 862 to 1230 observations since the year 1999 and the sites cover the major tectonic units, although their spatial distribution remains uneven. Excluding the local anomalies related to hydrothermal activities, the background heat flow values display a range from 30 to 140 mW m–2 with a mean of 60.4 ± 12.3 mW m–2. The updated heat flow map exhibits a heat flow pattern consisted of four heat flow provinces in continental China: eastern mantle-induced high flow province, southwestern crust-induced high heat flow province, central normal heat flow province and northwestern low heat flow province. The major heat flow provinces correspond to the Cenozoic lithosphere-scale tectonic units. The heat flow shows good correlation with crustal thickness and elevation, agreeing with the thermal isostasy. The heat flow distribution also corresponds with the Meso-Cenozoic tectonic activities. The present heat flow-tectonic pattern in continental China is formed by the Meso-Cenozoic geodynamic processes, including the Cenozoic India-Eurasian collision and the Mesozoic-Cenozoic western subduction of the Pacific Plate.

Mid-Miocene uplift of the northern Qilian Shan as a result of the northward growth of the northern Tibetan Plateau

The northern Tibetan Plateau, north of the Qaidam Basin and south of the Hexi Corridor (China), consists of a series of WNW- to NW-trending elongated mountain ranges. Deciphering the time-space deformation pattern of these ranges is central to understanding the mechanism of plateau formation and to the controversial issue of whether Tibet has undergone progressive northward growth or synchronous growth since the India-Eurasia collision. Here, we report new constraints on the timing of accelerated uplift of the Tuolai Shan, one of the elongated mountain ranges in the northern Tibetan Plateau. New apatite fission-track data from an elevation transect in the Tuolai Shan provide a definitive tie to rapid cooling that began at 17–15 Ma. We attribute this rapid cooling to accelerated exhumation resulting from thrusting in the hanging wall of the Haiyuan fault in response to progressive northward growth of the plateau. Combining these fission-track data and the published geologic, sedimentological, and thermochronologic data from the northern Qilian Shan and Hexi Corridor, we propose a progressively north-northeastward growth model for the northernmost part of Tibet, suggesting that deformation in the inner Qilian Shan occurred synchronously in the middle Miocene, and subsequently, increasingly further north.

Collisional processes between the Qiangtang Block and the Lhasa Block: Insights from structural analysis of the Bangong–Nujiang Suture Zone, central Tibet

The Bangong–Nujiang Suture Zone (BNSZ) is one of the main suture zones in the Tibetan Plateau and indicates the existence of the Bangong–Nujiang Neo‐Tethys Ocean (BNO). It was formed by the collision between the Qiangtang and Lhasa blocks after the closure of the BNO. However, its evolutionary processes and subduction polarity remain controversial. Research on the BNSZ is of great significance for exploring plate tectonic evolution and ocean–continent connection. The scarcity of the BNSZ structural data is one of the most important reasons for the debate of the BNO tectonic evolution. Based on structural analysis in the field and combined with previous petrology and palaeomagnetic data, this paper has determined that the closure time of the BNO is progressive and scissor‐like, from Middle Jurassic in the east to Early Cretaceous in the middle and late Early Cretaceous to early Late Cretaceous in the west. There was a three‐stage deformation in the BNSZ: (a) N–S‐directed compression induced by the initial collision; (b) WNW–directed transpression related to the stress adjustment; and (c) NE–SW‐directed compression caused by the low‐angle NE‐directed subduction of the Indus–Yarlung Zangbo Ocean. The different structural pattern along the eastern segment of the BNSZ may have resulted from orogenic bending due to the later Himalayan Orocline caused by the India–Eurasia collision in the Cenozoic.

Geodynamic Complexes and Structures of Transbaikalia: Record in Gravity Data

Abstract —The Transbaikalian region comprises several known geologic structures: the Mesozoic Mongolia-Okhotsk orogen, the Cenozoic Baikal rift system, and the world largest Angara–Vitim granitic batholith. They all formed upon heterogeneous Neoproterozoic– Early Paleozoic continental-margin complexes of the Siberian craton. The region is subject to the influence of mantle plumes, which induced Mesozoic–Cenozoic volcanism and controlled structural and lithological changes in the crust in the early history. Transbaikalia, which has been a scene of multiple tectonic events, is a model area for geophysical (in particular, gravity) surveys for various geological and geodynamic applications. As a novel approach, we interpret geological and geodynamic data from the region with reference to the pattern of free-air and Bouguer gravity anomalies revealed by satellite altimetry. Bouguer anomalies highlight large structures in the lithospheric mantle which were produced in the Cenozoic mainly by the activity of mantle plumes. Basaltic lava fields were confirmed to be almost coeval with mantle anomalies and to record the presence of the plume head at the crustal base. However, the origin of the Late Paleozoic Angara–Vitim granitic batholith was only tentatively attributed to the plume activity, based on gravity data. Mesozoic metamorphic core complexes (MCC) and basins that formed during the evolution of the Mongolia–Okhotsk orogen show up clearly on the map of free-air anomalies. Most of the MCC revealed in Transbaikalia coincide with oval gravity highs and border negative elongate features corresponding to Mesozoic basins. The zone of Cenozoic tectonism stands out in the pattern of free-air anomalies as maximum gravity contrasts, with the values changing from –110 to –120 mGal in basins to + 90 or +100 mGal in ranges. This zone encompasses rift basins filled with Cenozoic or, locally, Mesozoic sediments, which jointly form a domino-like system of rhomb-shaped structures typical of the Baikal rift system and, in general, of Cenozoic Central Asia resulted from the far-field effect of the India–Eurasia collision.

Normal faulting and viscous buckling in the Tibetan Plateau induced by a weak lower crust

Flow of weak lower crust has been invoked to reconcile observed topographic gradients, uniform elevations, slow seismic velocity, and high conductivity measured in the Tibetan Plateau, with viscosity estimates of 1016–1021 Pa·s. Here we investigate the dynamic response resulting from a range of lower crust viscosities in a 3-D lithospheric-scale geodynamic simulation of the India–Eurasia collision zone to determine bounds of physically viable lower crustal strengths. We show that thickening of the plateau is accommodated through viscous buckling of the upper crust in response to lower crustal flow for a lower crustal viscosity on the order of 1020 Pa·s. This generates two east–west trending bands of surface subsidence and dilatation consistent with observed normal faulting and estimates of vertical velocity. These results suggest viscous buckling of the upper crust, induced by lower crustal flow from gravitational pressure gradients due to high topography, is responsible for the observed extension in Tibet.

Melt inclusions in phenocrysts track enriched upper mantle source for Cenozoic Tengchong volcanic field, Yunnan Province, SW China

The Tengchong Volcano Field (TVF), located at the southeastern margin of the Tibetan Plateau, is the only Cenozoic volcanic-related geothermal region around the Himalayan geothermal belt that extends from southwestern Tibet to western Yunan along the Indo-Asian suture zone. The volcanic rock suites from this field are important proxies for the India-Eurasia convergence since the Late Cretaceous. Here, we investigate the melt inclusions hosted in the Tengchong volcanoes from different locations including Qingliangshan, Heikongshan and Dayingshan based on petrological and geochemical data with a view to gain insights on the magma sources and petrogenesis. Olivine (Ol), clinopyroxene (Cpx), orthopyroxene (Opx) and plagioclase (Pl) phenocrysts from basalts and andesites of the Tengchong volcanic rocks carry abundant melt inclusions. These melt inclusions are high in silica and alkali contents, and enriched in light rare earth elements (LREEs) and large ion lithophile elements (LILEs) such as Rb, Th, U, K, Pb, but depleted in high field strength elements (HFSEs) including Nb, Ta, Ti, P, with arc-like signature. The geochemical features of the melt inclusions suggest that the TVF volcanic rocks are possibly cogenetic, but with a progressive increase in the degree of magma evolution from Qingliangshan, through Heikongshan to Dayingshan. Melt and trace element modeling indicates that the magmas probably originated through variable degrees of partial melting of LREE-enriched upper mantle within garnet stability field with significant contributions from subducted oceanic crust. The melts were contaminated with continental crustal during ascent, followed by fractional crystallization.The volcanic activity was controlled by major Late Cenozoic dextral strike-slip faults within an extensional setting related to the geodynamic evolution of India plate. Our study provides insights into the tectonics accompanying the India-Eurasia collision.

Early Paleogene strike‐slip transition of the Tan–Lu Fault Zone across the southeast Bohai Bay Basin: Constraints from fault characteristics in its adjacent basins

The Tan–Lu Fault Zone (TLFZ) is the largest continental‐scale strike‐slip fault zone in East China. It experienced a complex Meso‐Cenozoic deformation and controlled the development and evolution of the Bohai Bay Basin (BBB). Though its Mesozoic sinistral and Cenozoic dextral motions have been well documented, its strike‐slip transition history and mechanism from sinistral to dextral motions in the Early Paleogene remain poorly understood. To investigate these issues, we made a thorough analysis on the fault geometry and kinematics as well as the response of depocentre in the Early Paleogene of the sags adjacent to the TLFZ across the southeast BBB. The results show that the WNW‐/NW‐, E‐W‐, and ENE‐/NE‐trending extensional faults developed in the Early Paleogene of those sags share almost the same geometrical and kinematic features. Such features indicate that the TLFZ is sinistral in Ek3‐Ek2 depositional stage, extension dominated in Ek1 depositional stage, and dextral in Es4L depositional stage. Such strike‐slip transition of the TLFZ is a comprehensive effect of the plate events around the Eurasian Plate. The NNW‐directed subduction of the Pacific to the Eurasian plates triggers the sinistral motion of the TLFZ during 65–55 Ma, but this sinistral motion is terminated by the far‐field effect of the NNE‐directed India–Eurasia collision initiating at 55 Ma. Then, the kinematic adjustment of the Pacific Plate from NNW to WNW breaks the stress balance exerted on the TLFZ by the subduction of the Pacific to Eurasian plates and the India–Eurasia collision and makes it turn into dextral motion gradually during 48–42 Ma.

Geodynamic conditions for Cenozoic activation of tectonic structures in Southeastern Mongolia

The knowledge of the neotectonic structures inSoutheastern Mongolia, that is considerably distant from the active plate boundaries, is important for determining a source of tectonic deformation and regular features of activation in the intracontinental setting. Our research was focused on the East Gobi and South Gobi depressions located inSoutheastern Mongolia, which developed since the Mesozoic and were activated to various degrees in the neotectonic stage. The study aimed to assess the paleostress state of the crust inSoutheastern Mongolia, identify the stages, factors and mechanisms of the Cenozoic activation of the regional structures of different strike, and determine the sources of activation. The analysis of the available literature suggests a similar history of their development in the Late Jurassic – Early Cretaceous (rifting) and Late Cretaceous – Paleogene (tectonic quiescence). In the Cenozoic stage, the depressions experienced activation of completely different styles. In theEast Gobidepression, left-lateral strike-slip faults were activated in the Tertiary, and the post-Late Cretaceous thrusting took place along the northeastern faults on the northern slope of the Totoshan uplift. In the Early Cenozoic, the N-S and N-W compression was dominant as evidenced by the deformed Late Cretaceous sediments and the reconstructed stress tensors typical of the compression and transpression regimes. An overview of the published data suggests that the most probable cause of such deformation was the impact of the Western Pacific zone of plate interaction. However, a potential influence of compression at the early stages of the Indo-Asian collision cannot be completely excluded. TheEast Gobidepression was low active in the second half of the Cenozoic. In contrast to the East Gobi depression, theSouth Gobiactivation began in the Late Cenozoic (Late Miocene – Early Pliocene). Young uplifts and forbergs (Gobi Altai eastern termination) developed actively and ‘cut’ the sediments of the basins originating from the Mesozoic. The W-E and N-W strike-slip and thrust faults were active in the Pliocene–Quaternary. The stress field reconstructions show compression, transpression and strike-slip regimes with the NE-trending axis of compression. Deformation in the East Goby Altay (as well as in Western andSouthwestern Mongolia) is driven by the India-Eurasia collision.

Zircon U–Pb geochronology and geochemistry of Early Jurassic granodiorites in Sumdo area, Tibet: Constraints on petrogenesis and the evolution of the Neo-Tethyan Ocean

The Gangdese magmatic belt is oriented E–W and located between the Indus–Yarlung Tsangpo and Bangong–Nujiang suture zones in Tibet. It records the subduction of the Neo-Tethyan oceanic crust, Indian–Eurasian collision, and uplift of the Tibetan Plateau. Here we present new zircon U–Pb geochronology, Hf isotopic compositions, and whole-rock geochemistry of the granodiorites in the Sumdo area of the Gangdese Belt. Our aim was to identify the petrogenesis of granitoid rcoks and to clarify the early-stage evolution of the Neo-Tethyan Ocean during the Late Triassic to Early Jurassic. The granodiorites yield zircon U–Pb ages of 193–201 Ma, indicating an Early Jurassic magmatic event. They are metaluminous, with medium- to high-K calc-alkaline geochemistry, and show characteristics of I-type granitoids. They show enrichment in light rare earth elements (LREEs) and depletion in high-field-strength elements (HFSEs) in primitive-mantle-normalized spider diagrams, indicating that their initial magma was generated in a subduction-related active continental margin. They have negative εHf(t) values (−11.3 to −4.1) and crustal model ages (TC DM) of 1.50–1.95 Ga. Hf isotopes and whole-rock geochemistry indicate that the granodiorites were derived from partial melting of ancient lower-crust material mixed with mantle material. Our data, in combination with published geochronological and geochemical data, show that the Late Triassic–Early Jurassic arc magmatism in the Gangdese magmatic belt was formed by the northward subduction of the Neo-Tethyan oceanic crust beneath the Lhasa Terrane.

The low-temperature thermo-tectonic evolution of the western Tian Shan, Uzbekistan

The Kyzylkum-Nurata region represents a key area in understanding the tectonic evolution of the western Tian Shan. In this study, we present new thermochronological data (apatite fission track and apatite (U-Th-Sm)/He) and associated thermal history models for 45 igneous samples from the Kyzylkum-Nurata Segment of South Tian Shan on the territory of Uzbekistan and Tajikistan. Our data show that the Kyzylkum-Nurata Segment experienced a multi-phase Mesozoic thermal history that differs from previously studied segments of the Tian Shan. A Triassic (~220–200 Ma) cooling signal is widespread throughout the Tian Shan and is interpreted as being associated with exhumation following the closure of the Palaeo-Asian Ocean. Following this period of fast cooling, the Kyzylkum-Nurata Segment experienced a period of slow cooling and erosion in the Early Jurassic (~190–160 Ma). However, in contrast to other parts of the Tian Shan, our study area preserves evidence for rapid cooling during the Late Jurassic–Early Cretaceous (~160–120). Given that this rapid cooling signal was only recorded for samples in association with major structures (e.g., relic suture-shear zones), we interpret this event as a period of fault reactivation related with tectonic processes at the Eurasian margin of the Tethys Ocean. Subsequently, the Late Cretaceous–early Palaeogene (~100–50 Ma) is characterised by slow cooling and erosion. Since the late Palaeogene, the basement of the Tian Shan experienced cooling related to the India-Eurasia collision. The thermal signal of this collision has been extensively recorded in the high-altitude Kyrgyz Tian Shan. Within the low-relief Kyzylkum-Nurata Segment, this Cenozoic overprint is not recorded, allowing for a detailed assessment of the Mesozoic thermal and landscape evolution of the western Tian Shan. Our study demonstrates that the Cretaceous Uzbek Tian Shan was characterised by a series of parallel, linear mountain belts that formed along suture zones during fault reactivation.