Asian Lithosphere Research Articles

Gravity anomalies and deep structure of the northeastern Tibetan Plateau and adjacent areas

The northeastern Tibetan Plateau is a region having experienced subduction, collision, and intracontinental extension. In this study, a gravity model (SGG-UGM-2) was used to investigate the distribution of gravity anomalies, Moho depth, and lithospheric effective elastic thickness in the northeastern Tibetan Plateau and adjacent areas. The results were compared with the regional heat flow, crustal magnetic anomalies, and seismicity. The Bouguer gravity anomalies correlate with the surrounding stable blocks (i.e., the Tarim Basin, Alxa Block, Ordos Basin, and Sichuan Basin) and lateral crustal extrusion of the Tibetan Plateau. The isostatic gravity anomalies, Moho depth, and lithospheric effective elastic thickness are –80 to 80 mGal, 35–70 km, and 5–105 km, respectively. Isostatic disequilibrium occurs mainly near boundaries between blocks and is associated with seismic activity. The distribution of Bouguer gravity anomalies and the Moho surface record the northeastward expansion of the Tibetan Plateau. The lithospheric effective elastic thickness decreases gradually from west to east in the Bayan Har Block, reaching its lowest values (<10 km) in the Longmenshan tectonic belt, which confirms the eastward extrusion along the block. The lithospheric effective elastic thickness in the Qaidam Basin is higher than that in the Bayan Har Block and Qilian orogenic belt, which reflects underthrusting of the East Asian lithosphere beneath the Qilian orogenic belt and the fact that extrusion from the Qiangtang Block is blocked by the Qaidam Basin.

Cold deep subduction of Indian continental crust and release of ultrahigh‐pressure fluid during initial exhumation: Insights from coesite‐bearing eclogite‐vein systems in Kaghan Valley, Pakistan

AbstractThe ultrahigh‐pressure (UHP) eclogites from the Kaghan Valley in Pakistan, which formed by the deep subduction of the Indian plate beneath the Asian plate in the Eocene, contain complex metamorphic vein systems (including both isolated veins and vein networks), with mineral assemblages of epidote + quartz + kyanite + phengite ± omphacite ± garnet. The investigations on the Kaghan UHP eclogite‐vein systems provide important insights into the mechanism and timing of metamorphic dehydration, fluid flow, and fluid–rock interaction in the deeply subducted Indian continental slab as well as the chemical characteristics of slab‐derived, aqueous fluids. Abundant lawsonite pseudomorphs, characterized by prismatic aggregates of epidote, kyanite, and quartz porphyroblasts, are first recognized in the Kaghan eclogites. This observation, in combination with the occurrence of coesite pseudomorphs in epidote porphyroblasts as well as the coexistence of epidote and coesite in the eclogite zircon, indicates the previous existence of UHP lawsonite in these eclogites. Petrological studies and phase equilibrium modelling reveal clockwise P–T trajectories for the Kaghan eclogites that are featured by prograde vectors in lawsonite‐stability regions with peak conditions of 3.0–3.4 GPa/650–690°C, followed by isothermal decompression and lawsonite breakdown under UHP conditions during the initial exhumation stage. The results of metamorphic evolution, together with in situ epidote and bulk Sr isotopic analyses, indicate that the fluids responsible for vein systems are most likely derived from the breakdown of UHP lawsonite in the eclogites. SIMS U–Pb dating of metamorphic zircons from the eclogites, integrated with the Raman analysis of inclusions in zircons, indicates that the UHP dehydration of eclogites occurred at 46.4 ± 1.2 and 46.8 ± 0.9 Ma. Analyses of hydrothermal zircons from the veins yielded slightly younger ages of 44.7 ± 1.0 and 44.9 ± 1.4 Ma, which represent the timing of fluid flow and/or vein crystallization during exhumation of the UHP rocks. Mass‐balance calculation results, in combination with the vein compositions, show that the fluid flow and fluid‐eclogite interaction led to the transfer of Si, Al, Ca, K, and incompatible trace elements from the eclogites into the fluids, from which the vein systems crystallized. This study indicates cold deep subduction of Indian continental crust along low geothermal gradients (6–7°C/km). The UHP fluid liberation and channelized fluid flow occurred during the initial exhumation of the cold Indian slab and are expected to induce the transfer of H2O and incompatible trace elements from the Indian slab to the Asian lithosphere, which potentially contributes to the formation of post‐collisional magmas. Moreover, we suggest that metamorphic vein systems in UHP lawsonite eclogites offer important constraints on the occurrence and timing of fast slab exhumation in continental subduction‐collision zones.

Crustal structure and deformation mechanism of the western northeast Tibetan Plateau

The collision between the Indian and Eurasian plates continues to drive significant deformation and uplift within the interior of the Tibetan Plateau, together with its outward expansion along the margins. In particular, the North Qilian Shan fold-thrust belt (NQLS) and the Hexi Corridor basins (HXBS) represent the northernmost region of the northeastern Tibetan Plateau. This area serves as a natural laboratory for deciphering mechanisms of crustal deformation and thickening along the plateau’s margins. Specifically, the northeastern Tibetan Plateau has been attributed to 1) southward underthrusting of the Asian lithosphere, 2) distributed shortening and crustal thickening, 3) vertical inflation of the Tibetan crust due to mid-lower crustal channel flow, and 4) intracontinental subduction facilitated by large-scale strike–slip faults. The exact mechanism underlying the most concentrated convergent stress in the western segment of NQLS–HXBS remains a subject of debate. To address this uncertainty, we gathered seismic data along a 130-km-long linear array that extends northward from NQLS, traversing the Jiuquan Basin and reaching the Huahai Basin. Our analysis, conducted through the receiver function method, reveals intriguing findings. The Moho depth deepens from 45–50 km beneath the Huahai Basin to 55–60 km beneath NQLS. Notably, a double Moho structure emerged, marked by a distinctive near-flat positive amplitude at a depth of 45–50 km beneath NQLS within a distance of 0–50 km. Our study presents a comprehensive analysis of the crust-scale deformation mechanism, shedding light on the following key aspects: 1) the development of a decollement at 12–20 km depth decoupling the upper and lower crust; 2) deformation of the upper crust occurring through south-dipping brittle thrust faults, while the lower crust features imbricate structures and duplexes; 3) evidence pointing to the underthrusting of the Beishan Block beneath NQLS, indicated by the double Moho beneath NQLS; and 4) the formation of a Moho ramp beneath the Jiuquan Basin, facilitating the transfer of shortening stress from beneath NQLS and HXBS to the north. In the context of the western segment of NQLS and HXBS, our speculation is that coupled distributed shortening and Beishan Block subduction beneath NQLS work in tandem to accommodate crustal deformation.

Electrical Response of a Reactivated Ancient Suture Revealed by Three‐Dimensional Magnetotelluric Inversion Across the Jinsha River Suture, Northern Tibetan Plateau

AbstractThe northern Tibetan Plateau offers an excellent opportunity to investigate continental collision involving the Indian and Asian plates in this remote, high‐altitude region. However, the deep structure of the region remains incomplete and controversial, which limits our understanding of the processes of continental convergence. This study presents a new 350‐km‐long magnetotelluric (MT) profile across the Northern Qiangtang and Hoh Xil terranes in the western part of the northern Tibetan Plateau to elucidate the lithospheric structure of the reactivated Jinsha River suture. We carry out a three‐dimensional inversion to acquire the electrical structure beneath the Jinsha River suture, and reveal the presence of large‐scale conductive anomalies in the middle‐lower crust of the Northern Qiangtang and Hoh Xil terranes. The high‐conductivity anomaly beneath the Jinsha River suture extends to &gt;80 km depth and is coincident with a zone of high melt fraction (&gt;5%) in the middle‐lower crust beneath the Jinsha River suture, thereby indicating that this suture has become a rheological channel. The convergence of Indian and Asian lithosphere beneath the northern Tibetan Plateau may be the driving mechanism for the reactivation of this ancient suture. Continental subduction leads to enhanced convection in the mantle. This reactivated suture, which is a tectonically weak zone, provides a conduit for the upwelling of hot mantle material to result in the subsequent melting of the middle‐lower crust. Moreover, our findings provide valuable insights into the geodynamic evolution of the northern Tibetan Plateau, as well as contribute to our understanding of continental convergence processes.

Magma migration and surface uplift in Pamir–western Tibet driven by deep lithospheric dynamics

Abstract There are two parallel &amp;gt;1200-km-long semi-continuous (ultra)potassic magmatic belts in the southern (Karakorum-Lhasa) and the northern (Central Pamir–western Kunlun) parts of Pamir–western Tibet. The southern belt is widely attributed to northward subduction of the Indian plate, while it has been suggested that the northern belt relates to the southward subduction of the Asian plate. We report new zircon U-Pb ages and isotopic data for the northern belt that show eastward magma migration between ca. 20 Ma and the present, which are contemporaneous with continental-scale thermochronometric cooling ages. Whereas magma migration in the south was caused by progressive west-to-east Indian lithosphere break-off, magma generation in the north is shown to be related to asthenospheric mantle upflow through a small mantle window (~100 km width) forced by Indian lithosphere underthrusting, Pamir–western Tibet lithosphere mantle dripping, and resistance of the Tarim lithosphere. Northern belt magma migration relates to progressively eastward underthrusting of the Indian lithosphere that took ~15 m.y. to move northward across ~350 km to meet Asian lithosphere. Accordingly, both belts of (ultra)potassic magmatism relate to the northward subduction of the Indian plate that was responsible for plateau uplift in Pamir–western Tibet.

Was cratonic Asia deeply subducted beneath the Pamir? Evidence from P–T conditions and tectonic affinities of Cenozoic Pamir crustal xenoliths

AbstractOne of the most striking geological features of the Pamir is the south‐dipping lithospheric slab beneath the orogen characterized by an intracontinental Wadati‐Benioff zone. A widely accepted hypothesis over the past 40 years interprets the slab to represent southward subducted cratonic Asian continental lithosphere, which predicts significant cratonic Asia‐sourced crustal materials (e.g., Tarim Basin) beneath the Pamir. Alternatively, recent studies have interpreted the slab to be lithosphere delaminated from the base of the Pamir. To test these hypotheses, depth–tectonic affinity relations of crustal xenoliths carried by Miocene volcanic rocks in the eastern Pamir, interpreted to be sourced from the Pamir deep lithosphere, are used to determine whether they represented Asian affinity cratonic crust. Thermodynamic calculations, zircon U–Pb geochronology combined with rare earth element analysis, and whole‐rock major‐trace element and Sr–Nd isotopic analyses document that (1) eclogite and pyroxenite xenoliths (~31–43 kbar/~960–1170°C) are the deepest sourced portions of the lithosphere from ~100 to 140 km depth, the protoliths of which represent the mid‐lower crustal rocks of the Cretaceous Pamir magmatic arc, rather than material from cratonic Asia, and (2) granulite xenoliths (~20 kbar/~900°C) represent the Cenozoic lower crustal rocks of Pamir terranes from ~70 km depth. These results indicate the south‐dipping slab represents delaminated Pamir lower crust and mantle lithosphere, rather than intracontinental subduction of Asian lithosphere, and further support the hypothesis of minimal Cenozoic northward translation of the Pamir.

Predicting Dynamic Effects of Large Earthquakes in Areas of Degrading Permafrost (Baikal-Mongolia Region)

Abstract—The study addresses basic problems of modern geodynamics and seismicity of the Central Asian lithosphere. It aims at predicting dynamic effects of large earthquakes for seismic safety of the Baikal–Mongolia region. Special focus is made on the seismic behavior of areas where seismic risk zoning is problematic because of permafrost. The paper presents synthesis of previous and new data on ground responses to large earthquakes in specific territories of the Baikal–Mongolia region with complex cryological conditions. The results include shaking intensity patterns and predicted seismic parameter values for degraded permafrost in zones of different climates and seismicity levels. The obtained prediction maps can make reference in studies of variable permafrost responses to temperature, seismic, and mining-related impacts.

Interplay between oceanic subduction and continental collision in building continental crust

Generation of continental crust in collision zones reflect the interplay between oceanic subduction and continental collision. The Gangdese continental crust in southern Tibet developed during subduction of the Neo-Tethyan oceanic slab in the Mesozoic prior to reworking during the India-Asia collision in the Cenozoic. Here we show that continental arc magmatism started with fractional crystallization to form cumulates and associated medium-K calc-alkaline suites. This was followed by a period commencing at ~70 Ma dominated by remelting of pre-existing lower crust, producing more potassic compositions. The increased importance of remelting coincides with an acceleration in the convergence rate between India and Asia leading to higher basaltic flow into the Asian lithosphere, followed by convergence deceleration due to slab breakoff, enabling high heat flow and melting of the base of the arc. This two-stage process of accumulation and remelting leads to the chemical maturation of juvenile continental crust in collision zones, strengthening crustal stratification.

Deep Electrical Structure of the Qilian Orogenic Belt with Dynamic Implications for the Northeastern Margin of the Tibetan Plateau: Revealed by 3D Magnetotelluric Inversion Using Unstructured Tetrahedral Elements

Abstract We present the results of a magnetotelluric (MT) array across the Qilian orogenic belt to elucidate the uplift mechanism of the northeastern Tibetan Plateau (Qinghai-Tibet Plateau). The array extends from the Qaidam basin in the south to the southern edge of the Alxa block in the north. Using the three-dimensional (3D) inversion of MT data based on unstructured tetrahedral elements, the electrical structure 100 km below the orogenic belt is obtained. The results show that there are high-resistivity bodies in the lithospheric mantle of the North Qilian and Hexi Corridor, which may represent the trace of southward subduction of the Asian lithosphere. Besides, there are partially molten bodies with low resistivity in the middle and lower crust below the Qilian orogenic belt, which may be caused by tectonic heat. The melt fraction of low-resistivity bodies is 2-5%, which indicates that the crustal flow from the Qiangtang and Songpan-Ganzi blocks is unable to penetrate beneath the Qilian orogenic belt. The low-resistivity bodies beneath the Qilian orogenic belt decouples the upper crust from the middle-lower crust. Owing to the continuous compression, the decoupled middle-lower crust has subsequently driven the northward movement of the upper crust, resulting in the uplift of the Qilian orogenic belt.

Abnormally low heat flow in Southeast China resulted from remnant slab subducted beneath the east Asian lithosphere

AbstractSubducted remnant slabs play an important role in the geodynamics and evolution of the Earth, but their behaviour in the mantle is not well understood. Terrestrial heat flow and thermal lithosphere structure provide crucial constraints on lithospheric dynamics. Based on whole‐well steady‐state temperature logs and conductivity measurements of core samples from a scientific borehole, we obtained a new high‐quality terrestrial heat flow values for East Asia. The heat flow was calculated at 60.6 ± 16.0 mW/m2 with a high crust/mantle heat flow ratio of 1.41, suggesting a hot‐crust‐cold‐mantle lithospheric structure. The thermal lithospheric thickness of eastern Fujian is estimated to be 152–166 km, indicating a thick and cold lithosphere. We demonstrate that the cooling of the lithosphere is caused by a remnant palaeo‐slab of the Palaeo‐Pacific plate subducted beneath SE China. The remnant slab blocked heat convection into the bottom of the lithosphere of the Eurasian Plate.

Adjoint Tomography of the Lithospheric Structure beneath Northeastern Tibet

Abstract Northeastern Tibet is still in the primary stage of tectonic deformation and is the key area for studying the lateral expansion of the Tibetan plateau. In particular, the existence of lower crustal flow, southward subduction of the Asian lithosphere, and northward subduction of the Indian lithosphere beneath northeastern Tibet remains controversial. To provide insights into these issues, a high-resolution 3D radially anisotropic model of the lithospheric structure of northeastern Tibet is developed based on adjoint tomography. The Tibetan plateau is characterized as a low S-wave velocity lithosphere, in contrast with the relatively high S-wave velocities of the stable Asian blocks. Our tomographic result indicates that the low-velocity zone (LVZ) within the deep crust extends northeastward from Songpan–Ganzi to Qilian, which is interpreted as a channel flow within the crust. The upper mantle of Alxa and Qinling–Qilian are dominated by a rather homogeneous LVZ, which is inconsistent with the hypothesis that the Asian lithospheric mantle is being subducted southward beneath northeastern Tibet. Furthermore, high-velocity regions are observed in the southern Songpan–Ganzi region at depths ranging from 100 to 200 km, indicating that the northward-subducting Indian plate has probably reached the Xianshuihe fault.

Lateral growth of NE Tibetan Plateau restricted by the Asian lithosphere: Results from a dense seismic profile

We present high-resolution receiver function images along a 700-km long dense seismic array extending from northern Tibetan Plateau to the Alxa block, crossing the entire Qilian thrust belt (QTB). The dense stations, with less than ~2 km station intervals, allow the receiver functions to unveil unprecedented details of crustal structures across the northern frontier of the growing Tibetan Plateau. The migration image shows a thickened and strongly deformed QTB crust, with an uneven Moho and complex internal structures that are indicative of pure-shear shortening. The Alxa block, in contrast, has a thinner crust, a flat Moho, and little internal crustal deformation. These results suggest that the lateral growth of NE Tibetan Plateau is restricted by the strong Asian lithosphere, which shows no visible subduction beneath the Tibetan Plateau as previously suggested.

Miocene Olivine Leucitites in the Hoh Xil Basin, Northern Tibet: Implications for Intracontinental Lithosphere Melting and Surface Uplift of the Tibetan Plateau

Abstract The generation of Miocene–Pliocene post-collisional magmatic rocks in northern Tibet was coeval with surface uplift, meaning that understanding the petrogenesis of these rocks should provide clues to the mechanism of uplift of the Tibetan Plateau. However, the nature of the source(s) of Miocene–Pliocene post-collisional rocks is unresolved, especially for potassic–ultrapotassic rocks. This study focuses on 16 Ma olivine leucitites in the Hoh Xil Basin of northern Tibet, which display the lowest SiO2 (43·4–48·8 wt%) contents of all Miocene–Pliocene magmatic rocks in northern Tibet and have high MgO (4·85–8·57 wt%) contents and high K2O/Na2O (&amp;gt;1) ratios. Whole-rock geochemical compositions suggest that the olivine leucitites did not undergo significant fractional crystallization or crustal assimilation. All samples are enriched in large ion lithophile elements relative to high field strength elements, and they exhibit uniform whole-rock Sr–Nd isotope [(87Sr/86Sr)i = 0·7071–0·7077 and εNd(t) = −3·1 to −3·9] and olivine O isotope (5·8–6·6 ‰, mean of 6·2 ± 0·2 ‰, n = 21) compositions. We propose that the olivine leucitites were derived by low-degree partial melting of phlogopite-lherzolite in garnet-facies lithospheric mantle. Given the tectonic evolution of the Hoh Xil Basin and adjacent areas, we suggest that southward subduction of Asian (Qaidam block) lithosphere after India–Asia collision transferred potassium and other incompatible elements into the lithospheric mantle, forming the K-enriched mantle source of the Miocene–Pliocene potassic–ultrapotassic rocks. Removal of lower lithospheric mantle subsequently induced voluminous Miocene–Pliocene magmatism and generated &amp;gt;1 km surface uplift in the Hoh Xil Basin.

Tajik Basin and Southwestern Tian Shan, Northwestern India‐Asia Collision Zone: 3. Preorogenic to Synorogenic Retro‐foreland Basin Evolution in the Eastern Tajik Depression and Linkage to the Pamir Hinterland

AbstractThe Tajik basin archives the orogenic evolution of the Pamir hinterland. Stratigraphic‐sedimentologic observations from Cretaceous‐Pliocene strata along its eastern margin describe the depositional environment and basin‐formation stages in reaction to hinterland exhumation and basin inversion. During the Late Cretaceous‐Eocene (preorogenic stage: ~100–34 Ma), a shallow‐marine to terrestrial basin extended throughout Central Asia. An alluvial plain with influx of conglomerate bodies (Baljuvon Formation) indicates a first pulse of hinterland erosion and foreland‐basin formation in the late Oligocene‐early Miocene (synorogenic stage Ia: ~34–23 Ma). Further hinterland exhumation deposited massive alluvial conglomerates (Khingou Formation) in the early‐middle Miocene (synorogenic stage Ib: ~23–15 Ma). Westward thickening growth strata suggest transformation of the Tajik basin into the Tajik fold‐thrust belt in the middle‐late Miocene (synorogenic stage IIa: ~15–5 Ma). Increased water supply led to the formation of fluvial mega‐fans (Tavildara Formation). Latest Miocene‐Pliocene shortening constructed basin morphology that blocked sediment bypass into the central basin from the east (Karanak Formation), triggering drainage‐system reorganization from transverse to longitudinal sediment transport (synorogenic stage IIb: &lt; ~5 Ma). Accelerated shortening (~27–20 Ma) and foreland‐directed collapse (~23–12 Ma) of Pamir‐plateau crust loaded the foreland and induced synorogenic stages Ia and Ib. Coupling of Indian and Asian cratonic lithospheres and onset of northward and westward delamination/rollback of Asian lithosphere (i.e., lithosphere of the Tajik basin) beneath the Pamir at ~12–11 Ma transformed the Tajik basin into the Tajik fold‐thrust belt (synorogenic stage IIa). The timing of the sedimentologically derived basin reconfiguration matches the thermochronologically derived onset of Tajik‐basin inversion at ~12 Ma.

Simultaneous and extensive removal of the East Asian lithospheric root

Much evidence points to a dramatic thinning of East Asian lithosphere during the Mesozoic, but with little precision on when, or over what time scale. Using geochemical constraints, we examine an extensive compilation of dated volcanic samples from Russia, Mongolia and North China to determine when the lithosphere thinned and how long that process took. Geochemical results suggest that magmatism before 107 Ma derived from metasomatised subcontinental lithospheric mantle (SCLM), whereas after 107 Ma, melt predominantly derived from an asthenospheric source. The switch to an asthenospheric magma source at ~107 Ma occurred in both Mongolia and North China (>1600 km apart), whereas in eastern Russia the switch occurred a little later (~85 Ma). Such a dramatic change to an asthenospheric contribution appears to have taken, from beginning to end, just ~30 Myrs, suggesting this is the duration for lithospheric mantle weakening and removal. Subsequent volcanism, through the Cenozoic in Mongolia and North China does not appear to include any contribution from the removed SCLM, despite melts predominantly deriving from the asthenosphere.

Moho Beneath Tibet Based on a Joint Analysis of Gravity and Seismic Data

AbstractWe use the improved Parker‐Oldenburg's formulas that include a reference depth into the exponential term and employ the Gauss‐fast Fourier transform method to determine Moho depth beneath the Tibetan Plateau. The synthetic models demonstrate that the improved Parker's formula has high accuracy with the maximum absolute error less than 0.25 mGal compared to the analytical solution. Two inversion parameters, that is, the reference depth and the density contrast, are essential for the Moho estimation based on the gravity field, and they need to be determined in advance to obtain correct results. Therefore, the Moho estimates derived from existing seismic studies are used to reduce the nonuniqueness of the gravity inversion and to determine these parameters by searching for the maximum correlation between the gravity‐inverted and seismic‐derived Moho depths. Another critical issue is to remove beforehand the gravity effects of other factors, which affect the observed gravity field besides Moho variations. In addition to the topography, the gravity effects of the sedimentary layer and crystalline crust are removed based on existing crustal models, while the upper mantle impact is determined based on the seismic tomography model. The inversion results show that the Moho structure under the Tibetan plateau is very complex with the depths varying from about 30–40 km in the surrounding basins (e.g., Ganges basin, Sichuan basin, and Tarim basin) to 60–80 km within the plateau. This considerable difference up to 40 km on the Moho depth reveals the substantial uplift and thickening of the crust in the Tibetan Plateau. Furthermore, two visible “Moho depression belts” are observed within the plateau with the maximum Moho deepening along the Indus‐Tsangpo Suture and along the northern margin of Tibet bounding the Tarim basin with the relatively shallow Moho in central Tibet between them. The southern “belt” is likely formed in compressional environment, where the Indian plate underthrusts northward beneath the Tibetan Plateau, while the northern one could be formed by the southward underthrust of the Asian lithosphere beneath Tibet.

Deep‐seated lithospheric geometry in revealing collapse of the Tibetan Plateau

Abstract The Tibetan Plateau's enigmatic collapse, which was indicated based on extensional faulting in the interior of the plateau and large eastward-directed strike-slip faults, has inspired intensive geologic inquiry and debate. Interaction of the subducting Indian plate with the overlying Asian lithosphere is one factor that may be affecting the plateau. A more detailed image of the subducting Indian plate can help constrain its role, but regional high-resolution seismic data have not been widely available at a relevant scale. Here we present an integrated interpretation based on two types of seismic datasets: (1) two N-S oriented deep seismic-reflection profiles that cross the Yarlung-Zangbo suture, and (2) two E-W receiver function profiles that cross the Xainza-Dingjye and Nyima-Tingri rifts, respectively. These images reveal different crustal-scale structures in the dominant collision zone between the western and central regions, as well as distinctly offset Moho discontinuities across the N-S trending rift grabens in southern Tibet. These crustal variations, combined with previous tomographic studies in Tibet, outline an easterly tilt of the subducting Indian slab, along which crust-mantle decoupling occurred. Together with the spatio-temporal distribution of synchronous potassium-rich volcanics exposed within the grabens in southern Tibet, this offset Moho structure beneath the grabens lends support to the hypothesis that eastward steepening of the subducting Indian slab was accompanied by slab tearing beneath each north-south striking rift. This overall crustal geometric variation suggests a stepwise flattening of the torn Indian slab from west to the east, a process that brought easterly migration of gravitational instability to the overriding Tibetan crust that drove collapse of the Tibetan Plateau once gravitational instability reached a critical level to the east.

Eastward extrusion and northward expansion of the Tibetan plateau—discussions for the deep processes of the plateau uplift

The weak materials beneath the Tibetan Plateau have been escaping and extruding eastward from beneath the thickened and elevated central plateau driven by continuous convergence between India and Eurasia. This paper presents a detailed shear wave velocity cross-section image of crustal and uppermost mantle structure across the Tibet-Qinling transition zone that was generated from a dense linear seismic array. Based on our inverted seismic image together with previous geological and geophysical studies, we discussed about the material extrusion of the Tibetan Plateau, unveiling the depth extent in which the Qinling belt (which could serve as a flow channel) may accommodate the extrusion of the ductile material beneath the Tibetan Plateau, and suggested that how these dynamic processes might accommodate plateau uplift and expansion in the eastern margin of the Tibetan Plateau; furthermore, we compared the mechanisms of crustal deformation/thickening and the behavior patterns of lithospheric mantle between the eastern and northern margins of the Tibetan Plateau. Integrating our seismic image with previous observations, we can obtain the following findings: (1) A vigorous low velocity zone (LVZ) ( V s V s<4.2 km/s) resides beneath the Tibet-Qinling/Sichuan boundary area, contrasting to the high-velocity upper mantle to the east beneath the Qinling block and Sichuan basin. A synthetic analysis based on these findings and previous geological/geophysical observations leads to the corresponding conclusions: (1) Middle to lower crustal flow accompanied by regional fault-related strike-slip/thrusts may be occurring beneath the eastern margin of the Tibetan Plateau, and may be a significant deep seismogenic mechanism of the large earthquakes occurrence in the eastern margin of the plateau, but it has not vastly extruded farther beyond the Tibet-Qinling boundary into the lower crust of the Qinling belt. (2) The hot asthenospheric mantle material beneath the Tibetan Plateau may escape eastward driven by convergence/squeezing between India-Eurasia continents and flow to the Qinling belt, accelerating the delamination or thermal erosion of the base of the lithosphere beneath the Tibet-Qinling/Sichuan boundary area. Combination of the preceding two mechanisms, i.e., the extrusion of ductile middle to lower crustal materials accompanied by regional fault-related strike-slip/thrusts and isostatic buoyancy resulting from lithospheric detachment (triggered by asthenospheric flow), may have jointly engendered the plateau uplift and expansion in the Tibet-Qinling and Tibet-Sichuan transition zones. In contrast, uplift of the Qilian Shan in the northern margin of the Tibetan Plateau significantly depends upon upper crustal shortening processes above a weak midcrustal decollement, which is subject to the North China plate underthrusting beneath the plateau. Obviously, the deep process of material extrusion in the eastern margin of the Tibet Plateau is distinguishable from the deep mechanism responsible for the northward expansion within the Tibet-Alxa transition zone in the northern margin of the plateau. The fundamental geodynamic mechanism leading to the mode discrepancy of plateau uplift between the eastern and northern margins of the Tibetan Plateau may be attributed to the lateral variation of the nature of the lithospheres from central and eastern Tibet (interior part of the plateau) to the northern margin of the plateau. A relatively hot/weak Tibetan lithosphere underlies the Songpan-Ganzi and Qiangtang blocks in the central and eastern Tibet, while a relatively cold/rigid Asian lithosphere underlies the Qaidam and Qilian blocks (as well as the northern bordering Alxa block) in the northern margin of the Tibetan Plateau, with the boundary extending along the Kunlun-West Qinling range on a large-scale view.

Depth-dependentPnvelocities and configuration of Indian and Asian lithosphere beneath the Tibetan Plateau

Depth-dependent<i>Pn</i>velocities and configuration of Indian and Asian lithosphere beneath the Tibetan Plateau

Little Geodetic Evidence for Localized Indian Subduction in the Pamir‐Hindu Kush of Central Asia

AbstractGeodetically derived velocities from Central Asia show that Northern Afghanistan, the Tajik Pamir, and northwestern Pakistan all move northward with comparable large velocities toward Eurasia. Steep velocity gradients, hence high strain rates, occur only across the Main Pamir Fault zone and with lesser magnitude between the northernmost Hindu Kush and the south and southeast margins of the Tajik Depression. Localized shortening is not apparent on any active India‐Hindu Kush crustal boundary; hence, crustal convergence between India and Eurasia in Central Asia is absorbed primarily on the northern and western margins of the Pamir. This concentrated strain on the Pamir margins is consistent with one, geometrically complex, interface between subducting Asian lithosphere and the Pamir. That interface might curve westward such that the Hindu Kush seismic zone is a continuation of the Pamir seismic zone, or alternatively, Hindu Kush earthquakes might occur in convectively unstable mantle lithosphere mechanically detached from surface faults.