North-South Gravity Lineament Research Articles

Shear wave splitting and mantle dynamics of the southern Great Xing'an orogenic belt

We deployed a linear seismic array consisting of 50 broadband stations in the southern part of the Great Xing'an orogenic belt to investigate seismic anisotropy in the Songliao Basin, the orogenic belt and the Erlian Basin by making SKS wave splitting measurements. Different features of anisotropy are revealed on the two sides of the north-south gravity lineament. The anisotropy on the east side of the gravity lineament primarily results from mantle flows in the big mantle wedge above the flat Pacific slab in the mantle transition zone, and the SKS fast polarization direction is generally NW-SE, being consistent with the subduction direction of the western Pacific plate. On the west side of the gravity lineament, the SKS splitting is attributed to freezing anisotropy in the stable lithosphere under the Erlian Basin. The SKS splitting delay times show some changes with azimuths of the teleseismic events, but the fast polarization directions are generally the same, indicating that the anisotropies below these seismic stations have the same origin in the continental lithosphere and active flows in the big mantle wedge.

Anisotropic Rayleigh wave tomography revealed lithospheric modification and tectonic deformation mechanisms in Hubei Province, central China

The collision of the North China and Yangtze blocks, along with multiple orogenic movements, has caused complex tectonic deformation of the lithosphere in Hubei Province. In order to reveal the stress distribution features and tectonic deformation mechanisms of the regional lithosphere, we inverted an azimuthal anisotropic structural model of the Hubei lithosphere from anisotropic Rayleigh wave tomography with data obtained from 48 broadband seismometers deployed in and around the province. Our model revealed strong lateral variations of azimuthal anisotropy in the crust and upper mantle, providing new seismological evidence for the lithospheric modification of the Yangtze block and the layered deformation of the lithosphere east of the North-South Gravity Lineament. We speculate that the primary dynamic factor causing lithospheric modification in the Yangtze block is related to the rifting events associated with the super-mantle plume, triggered by the deep subduction of the Pacific and Philippine plates. Meanwhile, the rifting event is also the main cause of the lithospheric structural layered deformation east of the North-South Gravity Lineament. The inverted anisotropic model, together with the results of previous geodetic and seismologic studies, revealed the causes and tectonic deformation mechanisms of anisotropy generation in the crust and upper mantle in Hubei Province. Crustal deformation in the Hubei region may be related to the eastward expansion of materials from the Tibetan Plateau and the ultra-high pressure metamorphic zone formed by the subduction and collision of the Yangtze and North China blocks, while upper mantle deformation is associated with subduction and collision of Yangtze and North China blocks, preserved fossil anisotropy from previous orogenic movements, and rifting events tied to the super-mantle plume induced by the deep subduction of the Pacific and Philippine plates. Moreover, we analyzed the regional crust-mantle coupling mechanism and concluded that the crust-mantle in the Dabie orogenic belt is coupled, whereas it is decoupled in other regions.

High-resolution lithospheric structure of continental China from joint inversion of surface wave and gravity data

The lithospheric structure of continental China has been previously determined by seismic travel time tomography, surface wave tomography, and joint inversion of body wave and surface wave data. However, due to the inherent limitations of seismic data, the lithospheric structure of continental China is still not well resolved in the shallow part and in some regions where the station coverage is relatively sparse. In this study, we aim at improving the lithospheric structure by joint inversion of seismic surface wave data and satellite gravity data to take advantage of the uniform distribution and complementary strength of the gravity data. The empirical relationship between velocity and density is used as a bridge for joint inversion of surface wave and gravity data. The joint inversion shear-wave velocity Vs (density) model, named as USTClitho1.0g, can fit both surface wave and gravity data well. This high-resolution Vs model can better fit the active airgun source seismic arrival times and better delineate some features in continental China, such as the velocity contrast across the north–south gravity lineament (NSGL), the lithosphere thinning in eastern China, the crustal footprint of the Hainan mantle plume, the likely magma chamber beneath volcanos in northeast China, the middle-lower crust low velocity layer beneath the Tibetan plateau, and the tearing of subducted Indian mantle lithosphere. Our joint inversion Vs model can provide a reference model for geosciences in continental China and surrounding areas.

Coupling between Cenozoic extensional exhumation in North China and the subduction of the Pacific Plate

Both the timing and mechanism for the formation of the topographic stepping in East Asia along the North-South Gravity Lineament (NSGL) are critical to understand the coupled influence of tectonics, climate, and surface processes on landscape evolution. The Yanshan Mountains lie in the central portion of the NSGL and offer a chance to reveal the link between surficial tectonic deformation and the underlying subduction to understand the continental dynamics and paleoclimate of NSGL. In this study, we report 22 new apatite (UTh)/He (AHe) data from the Yanshan Mountains. Age-elevation profiles indicate a period of rapid exhumation initiated at ∼27–26 Ma, while the AHe ages generally increase with the distance from the fault coincides with northwestward tilting of the fault footwall. Combining a series of shallow extensional events in East Asia with deep process of the Pacific plate subduction, we suggest that the widely-distributed late Oligocene rapid exhumation represents a phase of Cenozoic re-destruction of the North China Craton (NCC). The Cenozoic re-destruction of the NCC and topographic stepping along the NSGL may be related to the retreat, dehydration, and decarbonization of the underlying stagnant Pacific slab.

Undulating electrical lithosphere–asthenosphere boundary beneath Northeast China; as revealed by long–period magnetotelluric data

The geometry of continental lithosphere–asthenosphere boundary (LAB) significantly affects the movements of tectonic plates and flow of the upper mantle. However, the properties and geometry of this boundary beneath tectonically active regions in Phanerozoic continents are mysterious and elusive. Ninety–three long–period magnetotelluric (LMT) observations across Northeast China (NEC) provide a unique opportunity to image the shape of the LAB. Probabilistic resistivity–depth model inversion was employed to obtain the one–dimensional resistivity structure beneath each LMT site. The depth of the electrical LAB is constrained by the most probable interface at which the resistivity changes rapidly from high to low. The inversion results show that the electrical LAB is undulating beneath NEC. The North–South Gravity Lineament marks the transition from deep to shallow LAB. The thickest lithosphere occurs beneath the Great Xing'an Range (GXAR), where the LAB is ∼200 km deep. The electrical lithosphere is relatively thin in the east, with the thinnest lithosphere (∼50 km) being found beneath the Changbai Mountain volcano. We speculate that melt depletion, together with possible mantle downwelling are the likely causes for the formation of the thick GXAR lithosphere. The unresolved lithosphere beneath the northern Songliao Basin could be ascribed to the strong attenuation of electromagnetic waves in highly conductive sedimentary layers or the weak difference in the resistivity of the lithosphere and asthenosphere. The lithosphere steps that can form the edge-driven convection in the asthenosphere may play key roles in forming the widely-spread intraplate magmatism in NEC.

Electrical resistivity structure across the Late Cenozoic Abaga-Dalinor volcanic field, eastern China, from 3-D magnetotelluric imaging and its tectonic implications

The Abaga-Dalinor volcanic field (ADVF) in Inner Mongolia, China, located to the west of the North-South Gravity Lineament (NSGL), is the largest and less known area of Late Cenozoic intraplate volcanism in Northeast China. Knowledge of the subsurface structure beneath the ADVF will improve our understanding of its evolution process and formation mechanisms. New broadband magnetotelluric (MT) data were collected along a profile of about 300 km that crosses the major basaltic areas exposed at the surface in the ADVF and a series of east-north-east (ENE) trending faults. The electrical resistivity structure of the crust and uppermost mantle across the ADVF was generated by 3-D inversion of full impedance tensor and tipper data. This model reveals a thick high-resistivity layer (>1000 Ωm) in the upper crust that may represent the Precambrian basement and Late Paleozoic-Mesozoic granitic plutons. In contrast, high conductivity anomalies dominate the middle-lower crust. The northward-dipping high-conductivity feature at depths of 20–40 km below sea level under the Chagan Obo fault is suggested to be the involvement of the Early Paleozoic subducted oceanic crust. A remarkably high-conductivity zone (1–20 Ωm) exists in the middle-lower crust beneath the Abaga and Dalinor volcanic areas. It is explained to be a saline fluid zone of ∼0.45%–0.48% that exsolved from the partial melts as magma ascent rapidly. The impermeable upper crust traps these fluids in the middle-lower crust. A moderately high-conductivity anomaly in the uppermost mantle below near the Solonker Suture Zone (SSZ) is attributed to a minimum partial melt of ∼3–4% associated with the localized asthenospheric upwelling, which feeds the Abaga and Dalinor volcanos as a common magma source. Integrating our resistivity model with previous geophysical and geochemical studies, we suggest that intraplate volcanism in the ADVF is mainly controlled by the pre-existing tectonic weak zones. The lithosphere within the SSZ is susceptible to thermal perturbation from the upwelling mantle, providing the preconditions for melt generation. A set of ENE trending faults and an inferred NW-SE trending fault jointly dominated the vertical transport of magma and lateral extension along the faults, which may have influenced the spatial distribution of basalts and volcanic cones at the surface in the ADVF. In addition, we tend to suggest that Late Cenozoic intraplate volcanism in the ADVF could be the result of decompressing melting within the localized small-scale asthenospheric upwelling induced by the edge-driven convection near the NSGL, which is indirectly related to the westward subduction and retreat of the (Paleo-)Pacific plate during the Late Mesozoic to Cenozoic.

Thermal and rheological structure of lithosphere beneath Northeast China

We investigate 3-D thermal and rheological structures of the lithosphere beneath Northeast China using detailed P- and S-wave velocity models following mineral physics as well as geothermal methods. Small-scale temperature changes and thermal lithosphere thickness variations between different tectonic blocks are revealed. Our results show that strong lateral heterogeneities exist in the lithospheric thermal structure and rheological structure on both sides of the North-South Gravity Lineament (NSGL). The Changbai volcanic area and the central part of the Songliao Basin in the eastern side of the NSGL exhibit higher temperatures, thinner thermal lithosphere and lower rheological strength, which are closely associated with the western Pacific plate subduction under the Eurasian continent, resulting in upwelling of wet and hot asthenospheric material above the stagnant Pacific slab in the mantle transition zone. The thermo-chemical erosion of the upwelling asthenospheric material may induce delamination of partial lithosphere under the Songliao Basin. In addition, the western and southern edges of the Songliao Basin are characterized by lower temperature, thicker thermal lithosphere and higher rheological strength, which may indicate a relatively stable lithosphere. The Halaha and Abaga volcanic areas in the western side of the NSGL exhibit higher temperature, thinner thermal lithosphere and lower rheological strength, which could be caused by small-scale upwelling of hot asthenospheric material associated with delamination of partial lithosphere beneath the Songliao Basin.

Two-dimensional electrical resistivity structure of the crust and upper mantle across the North-South Gravity Lineament in NE China

The North-South Gravity Lineament (NSGL) is a large anomalous gravity zone that traverses several tectonic units with different properties in northeastern China. Its deep structural features and formation mechanism have long been a topical area of deep geophysical research. This study presents a magnetotelluric profile that extends from the Dongwuqi to Tongliao in Inner Mongolia and passes through geological structures such as the NSGL. Dimensional analysis of the profile data reveals good overall 2D features. 2D nonlinear conjugate gradient inversion of the profile data produced a model of the deep electrical structure. Horizontally, the electrical structure along the profile displays different characteristics in different blocks. The NSGL serves as a boundary for the change in the depth of the Moho discontinuity. Specifically, the Moho discontinuity is deep on the west side of the NSGL and shallow on its east side. Overall, the thickness of the lithosphere varies nonsignificantly, except in the western part of the profile, where the presence of a large low-resistivity anomaly in the shallow layer obscures the boundary of the lithosphere. The large high-resistivity anomaly in the center of the profile may be an indication that the partial delamination of the lithosphere caused by the convection of mantle materials and the upwelling of high-density mantle materials under the action of the westward subduction of the Pacific plate were the major causes of large positive gravity anomalies on the east side of the NSGL. The west side of the NSGL was predominantly controlled by the closure of the Mongol-Okhotsk Ocean, resulting in an increase in the thickness of the crust, which led to the formation of large negative gravity anomalies. These two factors collectively led to the formation of the NSGL.

Lithospheric Structure in Central-East China from Joint Inversion of Surface-Wave Dispersion and CCP-Derived Receiver Function: Implications for Regional Tectonics

Abstract To investigate the lithospheric deformation beneath central-east China, we constructed a 3D crustal and uppermost-mantle velocity model of this region by joint inversion of Rayleigh-wave phase velocities and common conversion point-derived receiver functions. The velocity images at the east and west sides of Xuefeng Mountain presented quite different features and a sharp change in lithospheric thickness beneath this mountain, suggesting that this mountain may be the southwestern segment of the boundary between the Yangtze craton (YZC) and the Cathaysia block (CAB). We observed extensive lithospheric thinning in the eastern South China block (SCB), including the CAB and lower YZC. Considering that the eastern SCB is within the influence of the circum-Pacific system and features intensive Mesozoic magmatic activity, we deduced that subduction of the Pacific plate was the main controlling factor of lithospheric thinning. The westward extension of lithospheric thinning in the SCB terminates in the area east of the north–south gravity lineament. We observed a large-scale, low-velocity zone in the uppermost mantle of the southern Trans-North China orogen that penetrates northwestward into the southeastern corner of the Ordos block. The low-velocity zone intrudes into the bottom of the crust and results in crustal thinning in the region. We proposed that both the southern Trans-North China craton and the southeastern Ordos block are suffering ongoing lithospheric mantle modification.

Distinct Lithospheric Structure in the Xing'an‐Mongolian Orogenic Belt

AbstractDetailed knowledge of the lithospheric structure is essential for a better understanding of the tectonic evolution of the Xing'an‐Mongolian Orogenic Belt (XMOB). We constrained the lithospheric structure across the XMOB by receiver function imaging and shear wave splitting analysis from a dense seismic array. The lithosphere‐asthenosphere boundary (LAB) is coherently imaged in different tectonic blocks. The mid‐lithospheric discontinuity is also identified at ∼75–100 km depth over the deeper LAB (∼110–130 km) beneath the western side of the North‐South Gravity Lineament (NSGL), roughly at the same depth as the LAB beneath the eastern side of the NSGL. The distinct variations in both the lithospheric structure and fast shear‐wave polarization direction across the NSGL are likely caused by different tectonic processes in the two sides, indicating that the NSGL might represent the western boundary influenced by the subduction of the (Paleo‐)Pacific Plate. Lithosphere thinning in the XMOB is evidenced to have been limited mainly to the east of the NSGL.

Panoptic View of Mantle Flow Beneath Trans‐Continental Northeast Asia: Distinct Variation Detected From ∼2,000 km Shear Wave Splitting Profile

AbstractWe obtained a panoptic view of the pattern of mantle flow in the “Big Mantle Wedge” associated with the stagnation of the subducting Pacific slab. We applied a simultaneous inversion of multiple waveforms method to measure SKS‐wave splitting parameters for a dense seismic array in trans‐continental Northeast Asia. We identify distinct patterns separated by the North‐South Gravity Lineament (NSGL): to the west, the ENE fast axis orientation with small δt reflects limited frozen anisotropy in the stable lithosphere; while to the east, local variation of anisotropy is superimposed on the NNW‐dominated fast direction, suggesting contribution of other mechanisms rather than the only asthenospheric flow. Within a distance of 200 km crossing the NSGL, we reveal a gradual clockwise rotation in the fast direction, and the keel‐deflected flow caused by the 60–80 km reduction in the lithosphere thickness across the NSGL might contribute to the variation of anisotropy.

Contrasting fates of subducting carbon related to different oceanic slabs in East Asia

Subduction transfers surface carbon into the Earth’s interior in a main form of carbonates that influences the global carbon cycles and surface climate through geologic time. Nevertheless, whether the fate of downgoing carbonates significantly varies in past subduction zones is rarely constrained by natural observations. Marine carbonates have remarkably higher zinc isotopic ratios (expressed as δ66ZnJMC-Lyon) relative to the mantle (0.99 ± 0.24‰ vs. 0.18 ± 0.05‰), making zinc isotopes a sensitive tracer for subducting carbonates. Here we examine this issue through a comparative zinc isotope study on basalts across the North-South Gravity Lineament (NSGL) in East Asia that were genetically related to two different oceanic slabs. Together with existing data, we show that all basalts in the east of the NSGL have high δ66Zn (∼0.3–0.6‰; n = 134) that do not vary with distances to the trench and are spatially coupled with the horizontally stagnated slab in the transition zone (410–660 km). This indicates that subducting carbonates survived shallow dissolution and were deeply buried during westward subduction of the Paleo-Pacific slab. By contrast, basalts in the west of the NSGL display a gradual decline of δ66Zn from 0.50 ± 0.04‰ to 0.28 ± 0.03‰ (n = 35) with increasing distances to the trench. No known magmatic processes (e.g., partial melting, crystal-melt differentiation, melt-peridotite interaction, and degassing) can account for the spatial Zn isotopic variation. The role of slab-derived sulfate rich fluids is also excluded because of the mantle-like Cu isotopic compositions of these basalts. Instead, the gradual decrease of δ66Zn, together with the coupled decline of CaO/Al2O3, are best explained as the diminished amounts of dissolved carbonates in their mantle sources. Thus, substantial carbonate dissolution must have occurred during southeastward subduction of the Paleo-Asian slab, which prevents deep burial of subducting carbon. The main differences between the two large slabs include: (i) the Paleo-Asian slab has an extended longevity (∼1.1 Ga) and slow spreading rate in comparison with the Paleo-Pacific slab, leading to the main incorporation of carbonate minerals into the altered oceanic crust, and (ii) the younger Paleo-Pacific slab contains abundant deep-sea Mg-rich carbonates that were not sufficiently dissolved at shallow depths. These differences demonstrate that subduction of different oceanic slabs can lead to contrasting fates of subducting carbon in ancient subduction zones, depending on the contents and species of carbonate sediments in the oceanic crust.

Regional lithospheric deformation beneath the East Qinling-Dabie orogenic belt based on ambient noise tomography

SUMMARY The Qinling–Dabie orogenic belt, which contain the arc-shaped Dabbashan orocline and is the world's largest belt of HP/UHP metamorphic rocks, formed by a long-term complex amalgamation process between the North China Block and the Yangtze Block. To understand the collision processes and tectonic evolution, we constructed a 3-D S-wave velocity model from the surface to a depth of ∼120 km in the eastern Qinling-Dabie orogenic belt and its adjacent region by inverting 5–70 s phase velocity dispersion data of Rayleigh waves extracted from ambient noise data. Our 3-D model reveals low velocities in the middle–lower crust and high velocities in the upper mantle beneath the orogenic belt, suggesting the delamination of the lower crust. Our results support a two-stage exhumation model for the HP/UHP rocks in the study area. First-stage exhumation was caused by the slab breaking away from the subducted Yangtze Block during the Early–Middle Triassic. Partial melting of the lithospheric mantle caused by slab breakoff-related asthenospheric upwelling weakened the lithospheric mantle beneath the orogenic belt, and continued convergence of the two continental blocks led to further thickening of the lower crust. Such processes promoted lower-crust delamination, which triggered the second-stage exhumation of the HP/UHP rocks. In the Dabbashan orocline, two deep-rooted high-velocity domes, that is, Hannan–Micang and Shennong–Huangling domes, acted as a pair of indenters during the formation stage. High-velocity lower crust was observed beneath the Dabbashan orocline. In addition, our 3-D model reveals that high-velocity lithospheric mantle extends from the Sichuan Basin to the Dabbashan orocline, with a subhorizontal distribution, providing strong support for the high-velocity lower crust. We also observed the destruction of lithospheric mantle beneath the Yangtze Block; the destruction area is bounded by the North–South Gravity Lineament, suggesting that the destruction mechanism of the Yangtze Block may be similar to the North China Block.

Formation mechanism of the North–South Gravity Lineament in eastern China

Gravity lineaments are well recognized along plate boundaries and in the interiors of oceanic plates. However, the formation mechanism of intracontinental gravity lineaments remains poorly understood. The North–South Gravity Lineament (NSGL), which extends >4000 km from Russia to South China, is characterized by an abrupt change in the observed Bouguer gravity anomaly, ranging from −100 mGal in the west to −40 mGal in the east over a distance of <100 km. Here we employ gravity modeling along a deep seismic sounding profile that transects the NSGL to delineate the relative contributions of the thickness and composition of the crustal and upper mantle and the stagnant slab in the mantle transition zone to the observed gravity anomaly, and to ultimately constrain the formation mechanism of the NSGL. Our results indicate that crustal thickness is the dominant factor in the formation of the NSGL, with lithospheric composition also playing a role. Combining our results with those of previous studies, lateral variations in the lithospheric mantle composition and crustal/lithospheric thickness across the NSGL can be attributed to lithospheric thinning and mantle replacement in the eastern part of the North China Craton during the late Mesozoic. This was triggered by the subduction of the Paleo-Pacific plate and subsequently led to the persistence of a stagnant slab and big mantle wedge. This study highlights the fundamental need for integrated geophysical–petrological analyses to better understand the surface response to deep tectonic processes.

Detailed Moho variations under Northeast China inferred from receiver function analyses and their tectonic implications

In this study we investigate detailed Moho variations beneath Northeast China by applying the arithmetic mean, back-projected and Fresnel-zone migration imaging methods to a total of 169,602 high-quality P-wave receiver functions from seismograms of 2903 teleseismic events recorded at 127 NECESSArray stations and 321 China Earthquake Administration stations. Our results show that the Moho depth variations are correlated with the surface geology in the study region. The Moho is deeper (~34.0–42.0 km) under the Great Xing'an range, the Lesser Xing'an range, the Zhangguangcai range, and the Changbaishan mountain, whereas it is shallower (~26.0–32.0 km) under the Songliao basin. Our results also reveal obvious Moho variations across the North-South Gravity Lineament. The Moho offsets up to ~5.0 km are clearly observed under the Nenjiang-Balihan, Yilan-Yitong, and Dunhua-Mishan faults, indicating that they are lithospheric-scale faults. A deeper Moho is revealed under the volcanoes, such as the Jingpohu, Wudalianchi, and Changbaishan volcanoes and Abaga and Halaha volcanic groups. In particular, the Moho under the Changbaishan volcano reaches ~40.0 km depth and the observation varies with the teleseismic back-azimuths, suggesting a complicated magma system in the crust. In addition, the Moho under the Songliao basin varies significantly from ~26.0 km depth in the east to ~32.0 km depth in the west, which could be related to the lithosphere extension and thinning. All these results suggest that there exists a hot and wet mantle upwelling in the big mantle wedge formed by the deep dehydration of the long stagnant Pacific slab in the mantle transition zone under Northeast China.

Electrical conductivity in the mantle transition zone beneath Eastern China derived from L1-Norm C-responses

SUMMARY Constraining the distribution of water in different regions of the mantle remains one of the significant challenges to comprehend the global deep water cycle. Geomagnetic depth soundings can provide such constraint through the electrical conductivity structure. Hence, this study aims to propose a regularization technique that can estimate previously unavailable C-response. In the method, the objective function comprised an L1-norm measured data prediction error and a spectral smoothness constraint term. We used the data error of C-response to weight the predicted error. The L-BFGS method was introduced to determine the minimum point of the objective function, and the regularization parameter decreased adaptively during inversion. Thus, the geomagnetic data processed yielded high-quality C-responses in 31 stations in Eastern China. In addition, we obtained 1-D electrical conductivity profiles in the mantle transition zone (MTZ) beneath Eastern China from C-responses using the L-BFGS method. Compared with the global 1-D model, the conductivity–depth profiles revealed that the MTZ beneath Eastern China is more conductive in the east but more resistive in the west. The conversion of these conductivities to water content based on the mineral physics suggested that the MTZ beneath Eastern China is characterized by a high water concentration, approximately 0.2 and 1 wt per cent in the upper and lower MTZ, respectively. Owing to the inclusion of more stations, the water-rich region could be constrained roughly to the east of the North–South Gravity Lineament (NSGL). Further considering seismic images in the same area, this water content distribution pattern suggested that the front of the stagnant Pacific Plate in the lower MTZ might have reached the NSGL. However, the dehydration reactions in the stagnant slab were more active in the eastern part. Perhaps, some of these fluids migrated into the upper MTZ and could be the source of the trapped water found in the xenoliths from the deep upper mantle beneath Eastern China.

Structure of the Sediment and Crust in the Northeast North China Craton from Improved Sequential H-k Stacking Method

AbstractH-k stacking method is a standard receiver-function method to detect crustal thickness. But this method can not be applied in low-velocity sedimentary basins. To solve this problem, we propose an improved sequential H-k stacking method. The improved method needs two sequential stacks. Firstly, sediment structure is calculated using converted waves and multiples on the bottom boundary of sediments. Secondly, the sedimentary results are applied to calculate the crustal structure. Theoretical calculations and “recovery tests” indicate that the improved method can obtain accurate estimates in sedimentary basins. With the teleseismic data of North China Craton, the structure of sediments is thick in the depression and thin in the uplifted area, which is consistent with Deep Seismic Sounding results. The crust to the west of the North-South Gravity Lineament is relatively thick and has a low average Poisson ratio, whereas the east is relatively thin and has a high average Poisson ratio. This result and the structural feature from data regression imply that the eastern crust of the North China Craton has experienced wide extension, which reflect the crustal response to the severe destruction and deformation in that area compared to the western crust.

Onset of the North-South Gravity Lineament, NE China: Constraints of Late Jurassic bimodal volcanic rocks

The North-South Gravity Lineament (NSGL) is an important geophysical boundary within NE China. Gravity anomaly, heat flow, crustal and lithospheric thickness, terrain and altitude all change dramatically across the NSGL. However, the timing of onset of the NSGL remains enigmatic, and the Mesozoic lithospheric thinning mechanism to the east of the NSGL is still unclear in NE China. Here we study a Late Jurassic bimodal volcanic suite located on the NSGL including N-MORB-like basalts, which constrains the initial formation of the NSGL. The bimodal volcanic rocks consist mainly of basalts and basaltic andesites with subordinate rhyolites. Our results show that the bimodal volcanic rocks are products of crust-asthenosphere interaction. Calculations indicate that lithospheric thicknesses in NE China have significantly changed from ~90–100 km in the west to 49–62 km in the east since the Late Mesozoic. The variation of lithospheric thicknesses on both sides of the NSGL could be ascribed to lithospheric delamination. Zircons from the bimodal volcanic rocks yield Late Jurassic ages of 147 Ma, nearly synchronous with the onset of the Songliao Basin and marking the start of extensional tectonics in NE China. Combined with previous studies, we confirm that the delamination induced by Paleo-Pacific subduction gave rise to the formation of the NSGL, which initiated at the Late Jurassic in NE China.

Ambient noise Love wave tomography of China

We first report on the Love wave tomography of China based on ambient noise cross-correlations. We used 3 years of continuous waveform data recorded by 206 broadband seismic stations on the Chinese Mainland and 36 neighboring global stations and obtained Love wave empirical Green’s functions from cross-correlations of the horizontal components. The Love wave group velocity dispersion measurements were used to construct dispersion maps of 8- to 40-s periods, which were then inverted to obtain a three-dimensional horizontally polarized S-wave (SH) velocity structure. The resolution was approximately 4° × 4° and 8° × 8° for eastern and western China, respectively, and extended to a depth of approximately 50 km. The SH model was generally consistent with a previously published vertically polarized S-wave (SV) model and showed large-scale features that were consistent with geological units, such as the major basins and changes in the crustal thickness across the north-south gravity lineament. The SH and SV models also showed substantial differences, which were used to examine the subsurface radial anisotropy. We define the radial anisotropy parameter as \begin{document}$\psi = 2\left( {{V_{\rm SH}} - {V_{\rm SV}}} \right)/\left( {{V_{\rm SH}} + {V_{\rm SV}}} \right)$\end{document} . At a shallow depth, we observed significant radial anisotropy under major basins, which may be related to thick sedimentary layers. At the mid to lower crust, most of the Chinese continent showed strong positive radial anisotropy (SH > SV). Central and southern Tibet showed strong positive anisotropy, whereas the radial anisotropy was relatively weak at the northern and eastern margins, which suggests a change in deformation style from the plateau interior to its margins. The North China craton showed prominent positive radial anisotropy, which may be related to decratonization and strong extension since the Mesozoic Era. Love waves are less well retrieved than Rayleigh waves from ambient noise cross-correlations. Increasing the duration of the cross-correlation data beyond 4 to 8 years may not aid in retrieving Love waves of longer periods, for which improved methods need to be explored.

Deciphering the origin of the Cenozoic intracontinental rifting and volcanism in eastern China using integrated evidence from the Jianghan Basin

Intracontinental rifting and low-volume volcanism are a globally common phenomenon, yet the underlying driving mechanisms and whether they can be explained through classic plate tectonic concepts, remain hotly debated. A prominent example is the Cenozoic rift and volcanic province in eastern China. Using an integration of geological, geophysical and geochemical data, we unravel the spatial and temporal variations of the rifting and volcanism in the Jianghan Basin. Both rifting and volcanism in the Jianghan Basin show two intense-to-weak cycles (65–50 Ma and 50–26 Ma, respectively) with significant enhancement in activity during the late rift phase. Moreover, rifting and depocentres progressively migrated eastward. The Jianghan basalts all share an asthenospheric origin while the source of the late phase basalts is slightly more enriched and heterogenous in Nd-Hf isotopes than that of the early phase basalts. The late phase basalts also display a smaller extent of partial melting even under a thinner lithosphere, likely indicating a significant decrease of volatile content in the mantle source. Based on regional tectonic correlations, the main stages of tectonic evolution of the Jianghan Basin and eastern China are not synchronous with changes in Pacific plate motion, while they are coincident with India-Asia collision processes. These observations lead us to propose that the asthenospheric flow driven by India-Asia collision rather than the rollback of the subducted Pacific slab has caused the widespread rifting and volcanism in eastern China. The variations of rifting and volcanism in the Jianghan Basin suggest a multiphase and eastward asthenospheric flow beneath eastern China driven by India-Asia collision, with an intense upwelling when passing through the North-South Gravity Lineament (NSGL). The much more intense rifting and volcanism during the late rift phase may indicate a much larger scale of volatile-poor asthenospheric flow than the early rift phase which could result in a more intense erosion of ancient enriched lithospheric mantle and the volatile content in the mantle source dropping sharply. This study provides an improved model based on our multidisciplinary observations for asthenospheric flow which may be an alternative driving mechanism for intracontinental rifting and low-volume volcanism in the regions where there are step changes in lithospheric thickness globally.