Big Mantle Wedge Research Articles

The 2022 Luding, Sichuan, China, M 6.8 earthquake: A fluid-related earthquake?

We determine a detailed 3-D crustal S-wave velocity structure in and around the Xianshuihe (XSH) fault zone using the direct ambient-noise surface-wave tomography. The continuous waveform data recorded at 22 portable seismic stations of our newly deployed around the XSH fault zone and 13 Chinese provincial stations are used in the data collection during September to December 2020. Our results show obvious low-velocity (low-V) anomalies under the Sichuan basin and high-velocity (high-V) anomalies under the Songpan-Ganzi block at 5–10 km depths but high-V anomalies under the Sichuan basin and low-V anomalies under the Chuandian block at 20–40 km depths. The XSH faut zone shows a clear boundary of low-V and high-V anomalies. Furthermore, there is a clear relationship between velocity structure and Luding mainshock. The Luding mainshock and its aftershocks occurred at the boundary of high-V anomalies in the upper crust but underlain by low-V anomalies in the mid-lower crust. These low-V anomalies indicate that the fluids have reached in the source area. These results suggest that fluids might reduce the friction force due to the decrease of both frictional coefficient and effective normal stress on the fault planes and trigger the strike-slip movement of the fault zone and the occurrence of the Luding earthquake. These fluids could be contained in the hot and wet mantle upwelling flow in the big mantle wedge caused by the deep subduction of the India slab down to the mantle transition zone, in addition to the crustal channel flow due to the Indo-Eurasian collision.

Shear-wave tomography of the Changbai volcanic area in NE China derived from ambient noise and seismic surface waves

A detailed 3-D shear-wave velocity (Vs) model of the crust and upper mantle beneath the Changbai volcano and adjacent areas in NE China is obtained by conducting a joint inversion of group-velocity dispersion curves at periods of 5–30 s and phase-velocity dispersion curves at periods of 5–100 s from ambient noise and earthquake surface waves. We use 1602 group-velocity dispersion curves and 3125 phase-velocity dispersion curves, which were recorded at 57 permanent stations of the China Earthquake Administration and 27 portable stations of Jilin University during 2010 to 2014. Our results reveal strong heterogeneities in the crust and upper mantle beneath the study area. A remarkable low-Vs zone is revealed in the crust beneath the Changbai volcano, indicating a hot magma chamber. The Longgang volcano is also characterized by a low-Vs anomaly reflecting partial melt in the crust. In contrast, there is no notable low-Vs anomaly in the crust beneath the Jingpohu volcano. Previous geological results and our tomographic images suggest that Changbai is a composite volcano, whereas Jingpohu and Longgang are monogenetic volcanoes. Low-Vs zones are revealed in the upper mantle beneath the three active volcanoes, which are interconnected at different depths. Combining the present results with previous findings, we deem that the three active intraplate volcanoes are caused by hot and wet upwelling flows in the big mantle wedge associated with deep subduction of the western Pacific plate beneath NE Asia.

Direct surface-wave tomography under Northeast China: New insights into 3-D crustal S-wave velocity structure and dynamics of intraplate volcanism

A new three-dimensional shear-wave velocity structure of the crust beneath Northeast China is constructed by inversion of phase velocity dispersion curves between 5 and 40 s using the direct surface-wave ambient noise tomography method. In addition to 127 NECESSArray (NorthEast China Extended SeiSmic Array) seismic stations, 19 SCVDSZ (Study of Changbai Volcanoes and Deep Subduction Zone) plus 3 CDSN (China Digital Seismograph Network) stations are added in the data collection. The addition of these data greatly improves the ray path coverage and imaging results around the Zhangguangcai range and the Changbaishan mountain. Our results show an obvious low-velocity anomaly extending from the surface down to uppermost mantle under the Changbaishan, suggesting that the volcano formation could be related to mantle upwelling. One more striking feature is a widely distributed mid-lower crustal low-velocity layer under the entire Northeast China, which is connected to those in the upper crust under the volcanoes, indicating that these volcanoes could have exchanges of material and energy in the mid-lower crust. In addition, under the Shuangliao and Erke volcanoes, low-velocity anomalies are visible in the upper and middle crust but high-velocity anomalies in the lower crust, whereas under the Abaga and Halaha volcanic groups, the low-velocity anomalies extend from the crust down to uppermost mantle, suggesting their continuous mantle magma provision. Integrating with previous global and regional body-wave tomographic models, our results suggest that the volcanoes in Northeast China could be related to hot and wet mantle upwelling in the big mantle wedge caused by the deep subduction of the subducted Pacific slab in the mantle transition zone.

Seismic imaging of the crust and uppermost mantle structure in the Qinling orogenic belt and its surroundings: Geodynamic implications

The Qinling orogenic belt and surrounding areas are a junction zone in Central China that connects the North China craton, Yangtze craton and northeastern Tibetan plateau. This area is in a key position within the more general framework where there is eastward extrusion of the Tibetan plateau and westward subduction of the Pacific plate. There is an ongoing scientific debate on how these two main processes are driving the geodynamic evolution of the Qinling orogenic belt and surrounding areas. Specific mechanisms have been proposed, such as lower crustal flow, crustal shortening and big mantle wedge tectonics. Knowledge of the seismic velocity structure helps us understand how these two driving forces affect the tectonic blocks of the study area, as well as test how well the proposed mechanisms to fit the regional velocity structure. For this purpose, a high-resolution P-wave velocity model of the crust and uppermost mantle was obtained by inverting arrival times from local earthquakes that occurred in the Qinling and surrounding region. Our three-dimensional (3D) model suggests that crustal channel flow cannot exist in the whole Qinling orogenic belt, and the localized low-velocity zone generated by the horizontal compressive stress caused by the collision of the Indian and Eurasian plates, is a possible explanation for the growth of the northeastern Tibetan plateau. The contrast in seismic velocities found under the east-Qinling and the west-Qinling orogenic belts implies a different response of these two terranes to the eastward extrusion of the Tibetan plateau. Our velocity model points to relatively stable structural features in the east-Qinling and the Weihe graben, while a low-velocity anomaly in the uppermost mantle of the southernmost Ordos plateau suggests that the lithosphere of the southern Ordos plateau is undergoing destruction.

Anatomy of large earthquakes in North China

• Most of the large events occurred in high-V or close to the boundary between low-V and high-V zones. • Prominent low-V and high-σ anomalies in the middle and lower crust reflect crustal fluids . • Crustal fluids may originate from hot and wet upwelling flows in the big mantle wedge. • Crustal fluids affected the nucleation and generation of intracontinental large earthquakes. We present detailed 3-D isotropic P and S wave velocity (Vp, Vs) images and Vp azimuthal anisotropy in source areas of 26 large crustal earthquakes (M5.6–7.8) that occurred in North China during 1966 to 2021. Most of the large earthquakes are located in high-velocity (high-V) zones or close to the boundary between low-velocity (low-V) and high-V anomalies. Prominent low-V and high Poisson’s ratio (high-σ) anomalies are revealed in the middle and lower crust under almost all the source areas, which may reflect crustal fluids. The low-V and high-σ anomalies may cause local thinning and weakening of the brittle seismogenic layer above them. The crustal fluids may originate from hot and wet upwelling flows in the big mantle wedge (BMW) beneath East Asia due to corner flows in the BMW and deep dehydration reactions of the stagnant Pacific slab in the mantle transition zone.

National-Scale Cobalt Geochemical Mapping of Exposed Crust in China

Geochemical background is the basis of anomaly evaluation in mineral exploration and environmental investigation. However, the background value obtained from secondary media is inevitably affected by weathering, transportation, and sorting, which leads to secondary depletion or enrichment of chemical elements. This problem can be avoided by the geochemical mapping of the exposed crust. In this paper, more than 38,000 samples of rocks data were collected, and an original method was presented for the first time to produce the cobalt geochemical map of exposed crust across the whole of China. Using a geological map grid of 1:200,000 as the basic calculation unit, the area and content of rock types in each unit were calculated, and then the grid was assigned using the area-weighted average method. Based on this, an geochemical contour map was drawn in ArcGIS. The results show that the median of the exposed crust is 9.74 mg/kg, which is lower than that of the whole crust and soils. This value is explained by the vertical heterogeneity of the crust and the secondary enrichment, respectively. Co anomalies and anomalous centers of exposed crust are distributed in southwest, north, and northwest China, where they are primarily related to Permian Emeishan Large Igneous Province (ELIP), Pacific plate, and the big mantle wedge (BMW), Archean metamorphic basic rocks, and ophiolite belts.

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.

Subducting slabs, Hainan plume and intraplate volcanism in SE Asia: Insight from P-wave mantle tomography

We present a high-resolution 3-D model of P wave tomography of the crust and mantle down to 1600 km depth under Southeast Asia (SE Asia), which is determined by inverting more than 1.3 million P wave arrival time data of local, regional and teleseismic events recorded at 1241 seismic stations deployed in SE Asia and mainland China. Strong and weak smoothing inversions are conducted to reveal different characteristic scales of velocity anomalies in the mantle. Both seismic and aseismic portions of the subducted plates in different parts of the circular subduction system in SE Asia are clearly revealed as high-velocity bodies in our tomographic images. The intraplate volcanoes in Borneo are underlain by significant low-velocity anomalies in the upper mantle, whereas a clear high-velocity anomaly is visible in the mantle transition zone and the lower mantle. We deem that hot and wet upwelling flows in the big mantle wedge above the stagnant slab in the mantle transition zone may have caused the intraplate volcanism in Borneo. A large low-velocity anomaly consisting of a flat head and a twisted tail extending to the bottom of our model is imaged beneath the southeastern Asian basalt province. The low-velocity anomaly possibly represents a hot zone whose shape may have been modified by mantle wind associated with a plate rotation. The hot zone has a lower-mantle origin and probably contains some small mantle upwellings inside, with the Hainan plume being a major trunk.

Anisotropic Tomography and Dynamics of the Big Mantle Wedge

AbstractWe determine high‐resolution 3‐D tomographic images of P‐wave isotropic velocity, radial anisotropy and azimuthal anisotropy beneath NE Asia down to 800 km depth. Our results show negative radial anisotropy (i.e., Vhorizontal < Vvertical) in the asthenosphere of the big mantle wedge (BMW), which may reflect mineral alignment caused by vertical flow in the asthenosphere. Across the Tanlu fault zone (TLF), the western and eastern parts of the BMW exhibit high and low P‐wave velocities, respectively. Combining our tomographic results with surface geological features, we speculate that convection in the BMW includes upwelling asthenosphere beneath the Japan Sea and the Korean Peninsula and downwelling asthenosphere beneath the Songliao and North China basins. The downwelling asthenosphere beneath the two basins is associated with diminishing volcanism and anomalous tectonic subsidence since ∼110 Ma. The great TLF is an important boundary for the BMW structure and dynamics.

Dynamics of the Subducted Izanagi-Pacific Plates Since the Mesozoic and Its Implications for the Formation of Big Mantle Wedge Beneath Eastern Asia

The slab dynamics of the subducted Izanagi-Pacific plate is still a subject of controversy and its relationship with the tectonic evolution of Eastern Asia remains not well explored. Here, we perform 3-D global convection models to investigate the slab dynamics of the Izanagi-Pacific plate beneath Eastern Asia since the Mesozoic time. We introduce a tracking technique in numerical models to explicitly distinguish the Izanagi slab and the Pacific slab during their subduction processes. We find that all subducted Izanagi slabs have completely fallen into the lower mantle until the late Cenozoic and the stagnant slabs currently observed at the mantle transition zone depth beneath Eastern Asia are entirely from the Pacific plate. We also find that multiple slab stagnation events have occurred during the subduction of the Izanagi plate in the Mesozoic time (∼150–120 Ma, 90–70 Ma) with a timescale of tens of million years. The stagnation of the subducted slabs facilitates the formation of a big mantle wedge beneath the overriding lithosphere and the time periods of the mantle wedge are consistent with the episodes of magmatic activities in Eastern Asia.

Barium isotope fractionation during dehydration melting of the subducting oceanic crust: Geochemical evidence from OIB-like continental basalts

Continental basalts commonly show ocean island basalts (OIB)-like geochemical compositions, and their origin is geochemically linked to metasomatism of the big mantle wedge by the deeply subducting oceanic slab at postarc depths of >200 km. This linkage is tested by a combined study of Ba isotopes with major-trace elements and Sr–Nd isotopes in Cenozoic continental basalts from eastern China. These basalts show OIB-like trace element distribution patterns and depleted to weakly enriched Sr–Nd isotope compositions, suggesting incorporation of the dehydrated slab component into their mantle sources. Their δ138Ba values vary from −0.05‰ to +0.06‰, which are lower than the depleted mid-ocean ridge basalts (MORB) mantle but similar to island arc basalts, OIB and OIB-like continental flood basalts. The variation of δ138Ba values cannot be ascribed to fractional crystallization and crustal contamination of mantle-derived magmas, but it results from mixing of the depleted mantle with the recycled crustal material of low δ138Ba values. In other words, the geochemical deviations from the depleted MORB mantle are attributable to the source mixing at mantle depths rather than the crustal contamination at crustal depths. Although the δ138Ba values of the dehydrated oceanic crust would be lower than the initially subducting crust based on the lower coordination number of Ba in liquid phases relative to solid phases in the crust, the low δ138Ba recycled crustal material including igneous oceanic crust (IOC) and seafloor sediment is still required to produce the low δ138Ba values for the studied basalts. The similarity in δ138Ba to island arc basalts not only indicates the similarly low δ138Ba values for the recycled crustal components in these basalts, but also suggests the limited Ba isotope fractionation during dehydration melting of the subducting oceanic crust at postarc depths.

Carbonated Big Mantle Wedge Extending to the NE Edge of the Stagnant Pacific Slab: Constraints from Late Mesozoic-Cenozoic Basalts from Far Eastern Russia

Carbonated Big Mantle Wedge Extending to the NE Edge of the Stagnant Pacific Slab: Constraints from Late Mesozoic-Cenozoic Basalts from Far Eastern Russia

Pn Anisotropic Tomography of Northeast Asia: New Insight Into Subduction Dynamics and Volcanism

AbstractWe present a new high‐resolution model of P‐wave anisotropic tomography of the uppermost mantle beneath NE Asia obtained by inverting 88,303 high‐quality manually picked Pn arrival times. Our model reveals strong lateral heterogeneities in both isotropic velocity and azimuthal anisotropy. Distinct high Pn velocities are visible under the basins in NE China, and eastern and western Japan basins. Very high Pn velocities with trench‐normal fast Pn anisotropy are revealed near the Kurile‐Japan trench, representing the incoming Pacific plate. Prominent low Pn velocity anomalies are revealed under active intraplate volcanoes in NE China and arc volcanoes on the Japan Islands. Very low Pn velocities with trench‐parallel fast Pn anisotropy are observed along the volcanic front in Japan, reflecting compressed mantle flows. An NW‐SE oriented low‐velocity (low‐V) zone from the Wudalianchi volcano to the Changbaishan volcano meets the E‐W oriented low‐V zone around the Changbaishan volcano and the N‐S low‐V zone east of the Sanjiang basin, which converge to a broad N‐S low‐V zone. This zone passes through the central Japan basin to the junction of the Japan and Izu‐Bonin trenches, where it meets the nearly E‐W low‐V zone from the Changle volcano. These low‐V anomalies exhibit zone‐parallel fast Pn anisotropy and may indicate channels for material and energy exchanges among the volcanoes, driven by mantle flows in the big mantle wedge. These results provide new insights into interplate and intraplate volcanism and subduction dynamics in the study region.

P-wave anisotropic tomography of NE China: insight into lithospheric deformation, mantle dynamics and intraplate volcanism

SUMMARY We determine robust 3-D P-wave anisotropic tomography of the crust and upper mantle beneath NE China using high-quality traveltime data of local earthquakes and teleseismic events recorded at 334 network and portable stations. In the upper crust, nearly E-W fast-velocity directions (FVDs) of azimuthal anisotropy are revealed in the central Songliao basin, which is surrounded by circular-shaped FVDs along the basin edges. The E-W FVDs may reflect microcracks or fractures in the upper crust, which are aligned under the control of regional tectonic stress. In the lower crust, low-velocity (low-V) anomalies with NE-SW FVDs exist along the Tanlu fault zone, which may reflect NE-SW trending ductile deformation or viscous flow along the fault zone. The FVDs are mainly NNW-SSE to N-S in the uppermost mantle beneath most of the study region, which may reflect fossil deformation of the mantle lithosphere caused by the Palaeo-Pacific plate subduction. High-velocity anomalies with NE-SW FVDs are revealed at ∼100 km depth directly beneath the Great XingAn Mountain, which may reflect an old lithospheric remnant. In the deep upper mantle, the FVDs are mostly NW-SE, consistent with SKS splitting measurements, which may be closely related to the intermittently retreating stagnant Pacific slab beneath East Asia. Under the Changbai and other intraplate volcanoes in NE China, prominent low-V anomalies exist in the big mantle wedge (BMW) above the slab in the mantle transition zone, suggesting that these volcanoes are caused by hot and wet upwelling flows in the BMW closely associated with the deep subduction and dehydration of the western Pacific plate.

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.

Lithospheric structure beneath the boundary region of North China Craton and Xing Meng Orogenic Belt from S-receiver function analysis

The boundary region of the North China Craton (NCC) and Xing-Meng Orogenic Belt (XMOB), including the northern boundary of the NCC and the southern XMOB, is the ideal area to investigate the spatially uneven lithospheric deformation and associated tectonic evolution in the boundary region of the craton and orogen. The depths and structures of the lithosphere–asthenosphere boundary (LAB) and mid-lithosphere discontinuity (MLD) are the key information to describe the nature and deformation pattern of the lithosphere, and thus can provide basic constraints on the associated tectonic processes and mechanism of the lithospheric deformation. Based on waveform data collected from 226 broadband seismic stations, high resolution spatial variations in the LAB and MLD depths were obtained by using the wave equation migration method for the S-receiver functions. The migrated images show a lateral variation in the LAB depth from 100–120 km in the northern boundary of the NCC to 120–140 km in the southern XMOB. The imaged LAB within the NCC is much shallower than that revealed by mantle xenoliths enclosed in the nearby Early Paleozoic kimberlites (~180 km), suggesting that the cratonic lithosphere of the area may have undergone significant thinning during the Phanerozoic evolution. Furthermore, a coherent MLD is identified at depths of 70–90 km beneath the northeastern NCC, which corroborates the speculation that the MLD probably existed in the eastern NCC before lithospheric destruction in the Mesozoic. The spatial variations in the LAB and MLD depths probably reflect complex lithospheric deformation and thinning around the boundary region of NCC and XMOB. The properties, including the gradient thickness and S-wave velocity contrast, of the two discontinuities further indicate that lithospheric deformation and thinning might be related to partial melting induced by the Big Mantle Wedge associated with the subducting Paleo-Pacific plate and its stagnation during the Late Mesozoic.

Magnesium isotope constraints on contributions of recycled oceanic crust and lithospheric mantle to generation of intraplate basalts in a big mantle wedge

Despite numerous studies, the origin of intraplate basalts from eastern Asia is still elusive. This includes the relative importance of subducted oceanic crust and metasomatized lithospheric mantle in generation of these lavas. To address this important issue, we have carried out an integrated Mg-Sr-Nd isotope study of Jiaohe garnet pyroxenite xenoliths and Cenozoic basalts from the Changbaishan-baoqing volcanic belt (CVB), NE China. The Jiaohe garnet pyroxenite xenoliths exhibit low Hf/Hf* ((HfN/(SmN × NdN)0.5, 0.7–0.9), depleted SrNd isotopic compositions (87Sr/86Sri = 0.7035–0.7041; εNd = +2.9 − +8.6) and low δ26Mg (−0.61 to −0.39‰). These features reflect both protolith heterogeneity and the effect of isotopic exchange with carbonates during subduction. The CVB basalts display variable Mg isotopic compositions. Low-Si basalts are characterized by low δ26Mg (−0.27 to −0.44‰), variable Hf/Hf* (0.7–1.4) and depleted SrNd isotopic compositions (87Sr/86Sri = 0.7039–0.7050; εNd = +1.0 − +4.7), which were attributed to partial melting of upwelling asthenospheric mantle containing recycled oceanic crustal components. In contrast, high-Si basalts display low Hf/Hf* (0.6–0.9), EM1-like SrNd isotopic compositions (87Sr/86Sri = 0.7047–0.7054; εNd = −2.4 - -0.2) and normal mantle-like δ26Mg values (−0.18 to −0.33‰). Such features can be explained by participation of metasomatized lithospheric mantle in their source. The transition from low-Si melts with low-δ26Mg to high-Si melts with normal mantle-like δ26Mg for the CVB basalts are genetically related to continental rifting, asthenospheric mantle upwelling and mechanical-chemical erosion of basal metasomatized lithosphere induced by subduction of the Pacific oceanic slab. Recycled oceanic crust and basal lithospheric mantle both played a fundamental role in generation of the intraplate basalts in eastern Asia.

The big mantle wedge and decratonic gold deposits

The Circum-Pacific subduction zone is a famous gold metallogenic domain in the world, with two important gold metallogenic provinces, the North China Craton and Nevada, which are related to the destruction of the North China Craton and the Wyoming Craton, respectively. Their ore-forming fluids were possibly derived from the stagnant slab in the mantle transition zone. The oceanic lithospheric mantle usually contains serpentine layers up to thousands of meters thick. During plate subduction, serpentine is dehydrated at depths of 300 km. The overlying big mantle wedge is hydrated during the breakdown of these hydrous facies in the mantle transition zone. The dehydration of the subducted slab in the big mantle wedge releases sulfur-rich fluid, which extracts gold and other chalcophile elements in the surrounding rocks, forming gold-rich fluid. Because the cratonic geotherm is lower than the water-saturated solidus line of lherzolite, the fluid cannot trigger partial melting. Instead, it induces metasomatism and forms pargasite and other water-bearing minerals when it migrates upward to depths of less than 100 km in the cratonic lithospheric mantle, resulting in a water- and gold-rich weak layer. During the destruction of craton, the weak layer is destabilized, releasing gold-bearing fluids that accelerate the destruction. The ore-forming fluids migrate along the shallow weak zone and are accumulated at shallow depths, and subsequently escape along deep faults during major tectonic events, leading to explosive gold mineralization. The ore-forming fluids are rich in ferrous iron, which releases hydrogen at low pressure through iron hydrolysis. Therefore, decratonic gold deposits are often reduced deposits.

Seismic imaging of Northwest Pacific and East Asia: New insight into volcanism, seismogenesis and geodynamics

Recent studies on seismic imaging of the Northwest Pacific and East Asian region are reviewed. High-resolution tomographic images reveal significant lateral heterogeneities in the crust and upper mantle, which shed new light on interplate and intraplate volcanism, earthquake mechanism, and mantle dynamics. Significant recent advances in seismic imaging are tomographic inversions for 3-D distribution of seismic anisotropy and attenuation in the crust and upper mantle, which provide important new information on lithospheric deformation, mantle convection, melts and fluids associated with plate subductions and mantle dynamics. The results show that intraplate volcanism in NE Asia is caused by hot and wet upwelling flows in the big mantle wedge above the stagnant Pacific slab in the mantle transition zone. The subducting Pacific and Philippine Sea slabs exhibit mainly trench-parallel fast-velocity directions (FVDs) of azimuthal anisotropy, which may reflect frozen-in lattice-preferred orientation of aligned anisotropic minerals and/or shape-preferred orientation, such as transform faults formed at the mid-ocean ridge and normal faults produced at the outer-rise area near the trench axis. Trench-normal FVDs are generally revealed in the mantle wedge under the volcanic front and back-arc region, which may reflect corner flows in the mantle wedge due to the plate subduction and dehydration. Trench-normal FVDs also appear in the subslab mantle, which may reflect asthenospheric shear deformation associated with the overlying slab subduction. The nucleation of large earthquakes, including both crustal and megathrust events, is controlled by structural heterogeneities in and around seismogenic faults. The cause of deep-focus earthquakes is a subject of ongoing debate. A recent example is the 2015 Bonin deep earthquake (M 7.9, ~670 km depth), which was probably caused by joint effects of several factors, including the Pacific slab's fast deep subduction, slab tearing and thermal variation, stress changes and phase transformations in the slab, and complex interactions between the slab and ambient mantle. The key to future advances in seismic imaging is instrumentation. Gradual deployment of seismometers on seafloors and in those less instrumented land areas will be the most important task for seismologists from now. For this purpose, international cooperation of seismologists in different countries will be necessary

Oxidation of the deep big mantle wedge by recycled carbonates: Constraints from highly siderophile elements and osmium isotopes

Widespread Cenozoic intraplate basalts from eastern China offer the opportunity to investigate the consequences of interaction between the stagnant Pacific slab and overlying asthenosphere and chemical heterogeneity within this “big mantle wedge”. We present and compile a comprehensive study of highly siderophile elements and Mg-Zn isotopes of this magmatic suite (60 samples including nephelinites, basanites, alkali basalts and tholeiites). The large-scale Mg-Zn isotopic anomalies documented in these basalts have been ascribed to mantle hybridization by recycled Mg-carbonates from the stagnant western Pacific plate. Our results reveal that the nephelinites and basanites are characterized by unfractionated platinum-group element (PGE) patterns normalized to primitive upper mantle (PUM) (e.g., PdN/IrN normalized to PUM = 1.1 ± 0.8, 1σ), relatively high total PGE contents (e.g., Ir = 0.25 ± 0.14 ppb) and modern mantle-like 187Os/188Os (0.142 ± 0.020). These characteristics are coupled with lighter Mg isotope (δ26Mg = −0.48 ± 0.07‰) and heavier Zn isotope (δ66Zn = +0.46 ± 0.06‰) compositions compared to the mantle values (δ26Mg: −0.25 ± 0.07‰; δ66Zn: +0.18 ± 0.05‰). Together, these data are interpreted to reflect the oxidative breakdown of low proportions of mantle sulfides in the sources of these small-degree melts, likely caused by recycled carbonates, which then release chalcophile-siderophile elements into carbonatitic melts. By contrast, the contemporaneous alkali basalts and tholeiites are characterized by highly fractionated PGE patterns (e.g., PdN/IrN = 4.4 ± 3.3; Ir = 0.037 ± 0.027 ppb) and radiogenic 187Os/188Os (0.279 ± 0.115) coupled with less fractionated Mg-Zn isotope compositions (δ26Mg: −0.39 ± 0.05‰; δ66Zn: +0.35 ± 0.03‰). In combination with other isotopic (e.g., Sr-Nd) and chemical (SiO2, Ce/Pb, Ba/Th, Fe/Mn) constraints, the alkali basalts and tholeiites were derived from higher degree melting of ancient pyroxenite-bearing mantle in addition to mixing with the aforementioned nephelinitic and basanitic melts. Collectively, we suggest that deep recycled carbonates promoted melting within the “big mantle wedge” leading to the generation of Cenozoic intraplate basalts across eastern China and the “redox freezing of carbonates” may cause the oxidation of Fe0 and S2-. This process may provide an important mechanism to oxidize mantle sulfides and transfer precious metals from deep mantle to crust.