We determined three-dimensional P and S wave velocities and P wave azimuthal anisotropic tomography of the Northwest Pacific subduction zones by inverting 1,225,086 P wave and 335,117 S wave arrival times from 13,413 earthquakes. Our results show some differences between P and S wave images for the stagnant Pacific slab in the mantle transition zone (MTZ) beneath Northeast China. The stagnant slab looks thicker in the P wave image than that in the S wave image, which may reflects the effects of both hydration and lower temperature in the MTZ, though differences in the resolution of P and S wave tomography may also have some effects. The Changbai intraplate volcanism is caused by hot and wet upwelling in the big mantle wedge above the stagnant Pacific slab. Our P wave anisotropy tomography shows that the fast velocity direction (FVD) in the subducting Philippine Sea plate beneath the Ryukyu arc is NE-SW (trench parallel), which is consistent with the spreading direction of the West Philippine Basin during its initial opening stage, suggesting that it may reflect the fossil anisotropy. A striking variation of the FVD with depth is revealed in the subducting Pacific slab beneath the Northeast Japan arc, which may be caused by slab dehydration that changed elastic properties of the slab with depth. The FVD in the mantle wedge beneath the Northeast Japan and Ryukyu arcs is trench normal, which reflects subduction-induced convection. Beneath the Kuril and Izu-Bonin arcs where oblique subduction occurs, the FVD in the mantle wedge is nearly normal to the moving direction of the downgoing Pacific plate, suggesting that the oblique subduction together with the complex slab morphology have disturbed the mantle flow.

High‐resolution images of P‐wave anisotropic tomography beneath the Eastern Mediterranean and Middle East are determined by inverting a large number of high‐quality P‐wave arrival times. Our results clearly show along‐strike variations of subducting slabs in the Hellenic subduction zone. At least three high‐velocity anomalies are imaged beneath and behind the north Hellenides, which are associated with the subducting Hellenic slab at different stages, while in regions further south, a single slab is revealed that has subducted down to a depth of ~1,100 km. The slab feature is not prominent in the Cyprus subduction zone, but a fine‐scale high‐velocity structure is revealed beneath the central Anatolia, which may reflect the detached Cyprus slab. Trench‐parallel fast‐velocity directions occur in the forearc mantle wedge of the Hellenic subduction zone, which may reflect mantle flow induced by slab curvature. In the backarc north Aegean and westernmost Anatolia, dominant NNE‐SSW to N‐S fast‐velocity directions exist in the shallow mantle, which accord well with the extensional direction of present deformation at the surface, suggesting a vertically coherent deformation in the lithosphere. Our results reveal a large‐scale mantle flow beneath western Arabia, which is possibly related to the Zagros and Caucasus orogens and westward motion of the Anatolia lithosphere. The Arabian lithospheric plate is thinner beneath the Zagros than that under the Mesopotamian Foredeep, suggesting that a large portion of the crust was scrapped off from the subducted Arabian lithosphere during the Zagros orogen.

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Northern Hainan Island and Leizhou Peninsula volcanic fields (Leiqiong), the southernmost continental Cenozoic volcanism in China, cover an area of ∼8000 km2 with 177 volcanoes recognized. Far from the subduction areas, volcanoes in this area provide an ideal opportunity to study the geodynamics of intraplate volcanoes. Here, we review the geochronological and geochemical data of the volcanic rocks in Leiqiong volcanic fields and discuss their magma sources and geodynamics on the basis of the geological and geophysical observations. Chronological data (34.78-0.01 Ma) show that the volcanic activities started approximately in Miocene and continued to Quaternary. These basalts show typical geochemical characteristics of oceanic island basalts (OIB), and tomographic images reveal that a mantle plume is situated beneath Hainan Island and extends down to the core-mantle boundary. Thus, we suggest Hainan mantle plume is responsible for the Cenozoic volcanism in Leiqiong volcanic fields and this plume is sourced from the lower mantle with additions of dehydrated slab fragments. These mixed plume materials were brought to the upper mantle and produce solid pyroxenites, which are the major source of Leiqiong magmas. Although there is no documental record of volcanic eruptions in Leiqiong volcanic fields, the volcanic danger cannot be neglected.Supplementary material at https://doi.org/10.6084/m9.figshare.c.5227601

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Volcanic eruptions, despite causing large-scale disasters, also provide important natural resources and are an effective way to understand the Earth's internal structure and its evolution. Herein, a comprehensive review is presented on recent progress in geophysical imaging of the structure and origin of intraplate volcanoes in Mainland China. We primarily focus on the Changbaishan, Wudalianchi, Tengchong, Hainan, and Ashikule volcanoes as they are currently active and hence, likely potential hazards during future eruptions, particularly the Changbianshan volcano. The Changbaishan and Wudalianchi volcanoes are widely believed to be caused by the dehydration of the stagnant Pacific slab in the mantle transition zone (MTZ) along with wet upwelling in the big mantle wedge (BMW). There are a number of different views regarding the formation mechanism of the Tengchong volcano. Some studies suggest that a BMW structure is also present under eastern Tibet, and the Tengchong volcano has a deep origin, similar to volcanism in northeast China. Others suggest that the Tengchong volcano is caused by a local and shallow process. Most tomographic studies suggest that the Hainan volcano is a hotspot, and its track has been located in Southeast China by combining seismological, geochemical, and numerical modeling data. A gap exists between the subducted Indian Plate and the Tarim lithosphere beneath the Ashikule volcano, which provides a channel for asthenospheric upwelling to give rise to intralplate volcanism in the Ashikule basin. The interactions of lithospheres may produce shear heating of the subcontinental lithospheric mantle (SCLM), which can generate localized melting. This process has been proposed as an explanation for the intraplate volcanism in Ashikule.

We investigate the youngest volcanic activity on the Tibetan Plateau by combining observations from petrologic, geochemical and seismic tomography studies. Recent (from 2.80 Ma to present) post-collisional potassium-rich lavas from the Ashikule Volcanic Basin (AVB) in northwestern Tibet are characterised by remarkably enriched light rare earth elements (LREE) relative to heavy rare earth elements (HREE), and enriched large ion lithophile element (LILE) relative to high field strength elements (HFSE). Strontium and neodymium isotopic compositions are surprisingly restricted, and show little evidence for mixing or crustal contamination, despite the thick crust upon which they are erupted. Geochemical characteristics indicate a homogeneous source, highly enriched in trace elements, which is most consistent with derivation from long-lived subcontinental lithospheric mantle (SCLM). P-wave anisotropy tomography documents a gap between the north-subducting Indian slab and south-subducting Tarim slab directly beneath the AVB. We propose that volcanism in northwestern Tibet is associated with the progressive closure of this gap, during which shear heating of the SCLM can generate localised melting, with deep-seated faults providing a mechanism for erupted lavas to escape large-scale crustal contamination and fractionation in magma reservoirs. Thus, shear heating may provide an explanation for the restricted range of radiogenic isotope compositions from a SCLM source that should be, by its nature, heterogeneous on a large scale

Following decades of geological surveys and studies, 14 active volcanic field have been identified in China. Evidence for Holocene volcanism in several of these areas highlights the need to understand and monitor volcanic hazards in those regions. Six volcano observatories have been established in the past 40 years accordingly. This work reviews China's national capability and history of volcano monitoring, with emphases on the Changbaishan-Tianchi Volcano Observatory and the Tengchong Volcano Observatory. The Changbaishan-Tianchi Volcano Observatory (CTVO) was constructed in 1996 and began monitoring in 1999, with limited recorded observations dating back to 1973. Currently, CTVO is the largest and most advanced observatory in China. The monitoring network of the CTVO incorporates 11 seismic and 15 GPS stations, 2 leveling routes, 3 gas geochemistry sampling points. The Changbaishan-Tianchi volcano experienced unrest during 2002-2005, evidenced in elevated levels of seismicity and ground deformation, as well as shifts in gas geochemistry. After 2006, the volcano returned to quiescence, with activities at background levels as recorded in 1973-2001. The monitoring network of Tengchong Volcano Observatory (TVO) incorporates 8 seismic stations, 20 GPS points, 95 leveling points, and 3 gas geochemistry sampling points. The observations made since 1965 indicate significant seismicity, with more than 3000 events recorded in 2011, mostly related to regional tectonics. Tengchong is known for its widespread hot springs, with temperatures up to 105 °C recorded at Dagunguo spring. The four other observatories are Longgang Volcano Observatory (LVO), Jingbohu Volcano Observatory (JVO), Wudalianchi Volcano Observatory (WVO) and Qiongbei Volcano Observatory (QVO). They are equipped with seismic, geodetic, and geochemical monitoring equipment. These areas saw only low levels of activity over the past several decades, but related fault systems are relatively active. In a relatively short time, China has gained considerable experience in observatory design and volcano monitoring and has trained up a sizeable task force, laying the foundation for sustained volcano monitoring at the national level. Future efforts must focus on maintaining and expanding observational capacity, as well as gaining better dynamic understanding to inform volcano hazard assessment.