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It has long been established that plastic flow in the asthenosphere interacts constantly with the overlying lithosphere and plays a pivotal role in controlling the occurrence of geohazards such as earthquakes and volcanic eruptions. Unfortunately, accurately characterizing the direction and lateral extents of the mantle flow field is notoriously difficult, especially in oceanic areas where deployment of ocean bottom seismometers (OBSs) is expensive and thus rare. In this study, by applying shear wave splitting analyses to a dataset recorded by an OBS array that we deployed between mid-2019 and mid-2020 in the South China Sea (SCS), we show that the dominant mantle flow field has a NNW–SSE orientation, which can be attributed to mantle flow extruded from the Tibetan Plateau by the ongoing Indian–Eurasian collision. In addition, the results suggest that E–W oriented flow fields observed in South China and the Indochina Peninsula do not extend to the central SCS. [ABSTRACT FROM AUTHOR]

The lateral expansion of the southeastern Tibetan Plateau causes devastating earthquakes, but is poorly understood. In particular, the links between regional variations in surface motion and the deeper structure of the plateau are unclear. The plateau may deform either by movement of rigid crustal blocks along large strike-slip faults, by continuous deformation, or by the eastward flow of a channel of viscous crustal rocks. However, the importance of crustal channel flow was questioned in the wake of the 2008 Wenchuan earthquake. Controversies about the style of deformation have persisted, in part because geophysical probes have insufficient resolution to link structures in the deep crust to the observed surface deformation. Here we use seismic data recorded with an array of about 300 seismographs in western Sichuan, China, to image the structure of the eastern Tibetan Plateau with unprecedented clarity. We identify zones of weak rocks in the deep crust that thicken eastwards towards the Yangtze Craton, which we interpret as crustal flow channels. We also identify stark contrasts in the structure and rheology of the crust across large faults. Combined with geodetic data, the inferred crustal heterogeneity indicates that plateau expansion is accommodated by a combination of local crustal flow and strain partitioning across deep faults. We conclude that rigid block motion and crustal flow are therefore not irreconcilable modes of crustal deformation. [ABSTRACT FROM AUTHOR]

SUMMARY We present S receiver functions and SKS splitting measurements from the China Seismograph Network located in the Qinghai and Gansu provinces. Teleseismic data are used to interpret the structure of the lithosphere-asthenosphere boundary (LAB) and upper-mantle deformation beneath the northeastern Tibetan Plateau (NETP) in regions north of the east Kunlun Fault. Based on our observations, the LAB lies at a depth of 125-135 km beneath the northeastern Songpan-Ganzi block and the west Qinling orogen, between 145 and 175 km beneath the Kunlun and Qilian orogen, and deepens below the Qaidam Basin (175-190 km), Ordos Craton (170 km) and Alashan platform (200 km). The NETP is characterized by a nearly uniform fast NW-SE S-wave direction. These observations are different from those to the south of the Kunlun Fault where fast S directions are rotating clockwise from the inner plateau. The change in fast directions across the Kunlun Fault implies a sudden variation of upper-mantle deformation. Shear wave splitting delay times vary from 0.8 to 1.9 s. Data from beneath regions north of the Kunlun-Ayimaqin suture showed that delay time was positively correlated with lithospheric thickness with an increase of 0.7 s per 100 km. This indicates that the anisotropy may develop in the uppermost mantle, such as the lithosphere, beneath the NETP. [ABSTRACT FROM AUTHOR]

• We image how the crustal stucture nature vary in the SE Yellow Sea. • The Gunsan Basin is underlain by relatively thin upper and much thick lower crustal layers. • A transition in crustal structure is recognized from the South China Block to the Korean Peninsula. • The crustal structure is inherited from the growth of Rodinia. Two major tectonic units in NE Asia are the Sino-Korean (or North China) and South China Blocks that collided in the Permo-Triassic periods. The South China Block is suggested to extend eastward to the Korean Peninsula across the southern Yellow Sea. The Gunsan Basin is a well-defined, fault-bounded sedimentary basin in the SE Yellow Sea and is regarded as a structural link of the South China Block to the Korean Peninsula. We collected deep seismic sounding data recorded on ocean bottom seismometers to investigate how the crustal structure and nature of the South China Block vary in the Gunsan Basin toward the middle part of the Korean Peninsula. The main part of the Gunsan Basin is underlain by relatively thin (∼10 km thick) upper and much thicker (∼20 km thick) lower crustal layers, indicating a close affinity to the South China Block; whereas it shows distinction from the Korean Peninsula underlain by upper and lower crustal layers with equal (∼15 km) thickness. A change in crustal structure is recognized under the eastern margin of the Gunsan Basin toward the Korean Peninsula, that features a transition to the Korean Peninsula. The thick lower crustal layer in the Gunsan Basin appears to be inherited from the evolution of the South China Block including crustal shortening associated with the assembly of Rodinia in the Neoproterozoic before the collision between the Sino-Korean and South China Blocks. [ABSTRACT FROM AUTHOR]