We investigate the crustal structure and Moho topography beneath the northwest Ordos rift zone by employing receiver functions analysis at dense broadband seismic stations. The results delineate a depressed Moho zone (by similar to 5 km) beneath the Cenozoic rifts, which is unconventional for classical continental rift zones. We interpret this depressed Moho zone as a relict of the thickened crustal root formed during the intracontinental orogeny in late Mesozoic. The Cenozoic rift has not yet reequilibrated the crustal-mantle boundary possibly due to the lack of decompressional melting during the lithospheric extension.

Constraining variations of lithospheric structure and deformation across southeastern Tibet and the surrounding region are crucial for understanding the lateral growth of the plateau. Using data from two dense seismic arrays in the SE Tibetan Plateau, we obtained fine-scale Rayleigh-wave phase velocity maps by Eikonal tomography, and further inverted for 3-D shear wave velocity and azimuthal anisotropy of the crust and upper mantle. High velocities and weak lithospheric anisotropy are revealed under Dianzhong Block (DZB), where is covered by the relics of the Emeishan Large Igneous Province, while strong anisotropy is imaged around. We propose that the DZB strengthened by the paleo igneous activity plays an essential role in the evolution along the southeastern plateau margin. On the one hand, the DZB obstructs the migration of plateau material to the southeast. On the other hand, strong deformation is localized along the DZB margins, while the stress due to the plate expansion is transferred further to the Xiaojiang fault, which induced significant deformation within the Yangtze Craton. Furthermore, the lithosphere of DZB deflects the mantle flow driven by the eastward Indian subduction under the northern Indochina block.(c) 2022 Elsevier B.V. All rights reserved.

The uplift of the Tibetan Plateau in the Cenozoic has changed the tectonics of the whole Asia and affected the climate and environment significantly. However, the mechanism of the plateau uplift and expansion is still in debate. The southeastern margin of the Tibetan Plateau, as the one of the expansion frontiers, provides the key to understand the tectonic evolution of the plateau. In this study, we review the progress of the researches on the crustal structures in the southeastern plateau margin, especially those achieved after the ChinArray project since 2011. The latest results show that the thickness of the crust descends from the plateau to its southeastern margin with an abrupt change under the plateau boundary. Two independent low-velocity zones are revealed in the lower crust: one under the Songpan-Ganzi terrane which is part of the high plateau, and another orientating NE-SW under the Xiaojian fault zone. We propose that the lower crustal materials extruded from the high plateau are blocked around the plateau margin (i.e., Lijiang-Xiaojinhe fault) by a strong crustal block in the western margin of the Yangtze block. However, the stress is further transferred to the Xiaojiang fault zone and induced the crustal deformations there.

The uplift of the Tibetan Plateau in the Cenozoic has greatly affected the tectonics in Asia. But the evolution of the plateau is still unclear. The northeastern Tibetan Plateau is under ongoing expansion and is therefore essential for understanding the tectonic evolution of the plateau. Using data from 675 stations deployed by the ChinArray project, we obtained an anisotropic shear-wave velocity model by Rayleigh-wave phase-velocity Eikonal tomography and further investigated layered anisotropy and deformation patterns in the northeastern Tibetan Plateau. In the upper crust (<20 km depth), the fast directions are parallel with major strike-slip faults. In the lower crust, the fast directions are mostly margin-parallel along the plateau margin (at depths of ∼20 km to Moho), but they are perpendicular to the plateau margin while parallel to the topographic gradient under the high plateau (at depths of ∼40 km to Moho). Our result indicates that there is lower crustal flow under the high plateau due to the differential lateral gravity potential. However, the deep crustal flow is blocked by the surrounding blocks, which results in pure shear along the plateau margin in the northeastern Tibetan Plateau.

The tectonic uplift of the Tibetan Plateau is a focus in the geosciences. Middle-lower crustal flow is a popular model to interpret the geodynamic mechanism on the margin of the Tibetan Plateau. The model predicts different surface and Moho topographies across the plateau boundary due to the different strengths of the surrounding blocks, that is, sharp boundaries on the eastern plateau boundary and gentle variations in the southeastern plateau boundary. Here, we employ receiver function and common conversion point stacking analysis with the seismic waveforms recorded by the dense ChinArray and other local seismic stations to accurately define the Moho topography in southeastern (SE) Tibet. We find that the Moho under the Tibetan Plateau is much deeper than that under the surrounding Yangtze Craton and Indochina block; abrupt Moho changes are found across the southeastern plateau margin, similar to that under the eastern plateau margin. We interpret these sharp Moho variations across the plateau margin to have developed when the Tibetan Plateau was extruded southeastward in the late Miocene. Subsequent gravity collapse resulted in crustal extension and gentle topographic variation, while the sharp Moho slope was preserved.

The 28 March 2019 Mw 5.04 Mangya earthquake damaged eight ongoing drilling boreholes in the oil‐production Yingxiong Ling (YXL) area, southwestern Qaidam of northern Tibet. The borehole damages provide an opportunity to measure directly the coseismic slips, the rupture area, and the seismic moment. The damages reveal the underground rupture area of 45.30 ± 10.24 km 2 , the maximum slip of 400 ± 13 mm, and the seismogenic fault dip of ~38.6°. These parameters generate a seismic moment of (1.81 ± 0.47) × 10 17 Nm and a moment magnitude of 5.47 ± 0.16. Seismic exploration reveals that the geometry of the SZG ramp, the uppermost part of the multibend Yingxiong Ling thrust system, agrees primarily with the rupture plane derived from the borehole damages and one plane of the focal mechanism solution. This suggests that this earthquake resulted from slipping on the ramp. The hanging wall of the YXL thrust system forms the complex fault‐bend fold YXL anticlinorium. Active thrusting and folding along both edges of YXL attest to the southwestern vergence of this thrust system. Growth strata demonstrate average slip rates of the thrust system ranging from ~0.2 to ~0.3 mm/yr. The thrusted and folded recent alluviums along the southwestern edge indicate two thrusting events with coseismic slips of 1.7 ± 0.15 and 3.5 ± 0.15 m at 6.16 ± 0.52 and ~35.91 ka, respectively. The entire rupturing of the thrust system can produce Mw 7.65 ± 0.03 earthquakes.

The Indian plate began to subduct under the Eurasian plate in the late Mesozoic. Since the Eocene, continuous continental collision has resulted in the uplift of the Tibetan Plateau with extensive volcanic activities. Geophysical and geochemical studies have suggested that the Indian plate subducted and that volcanism originated in the deep upper mantle. However, the depth range of the subduction and volcanism and the relationship between them are still unclear. Here, to image the mantle transition zone (MTZ) structures, we deployed new seismic stations and collected as much seismic data as possible from the Tibetan Plateau. We then calculated the receiver functions and analyzed them using the common conversion point stacking method. We found that the MTZ is thickened by similar to 20 km under the Lhasa block, where high-velocity anomalies are obvious in the MTZ. Apparent thinning of the MTZ by similar to 20 km was revealed under the Qiangtang and Songpan-Ganze blocks, confirming a component of depleted mid-oceanic ridge basalt in the magmatic rocks. Our results suggest that the Indian Plate has at least subducted to the MTZ, which has further induced mantle upwelling and volcanism in the Tibetan Plateau.

The eastern margin of the Tibetan plateau is under ongoing expansion. Rise and growth of the Longmenshan are results of the interaction between the Songpan-Ganzi Terrane and the Sichuan Basin. In order to understand the block interactions in the eastern plateau margin, we studied the crustal structures using P wave receiver functions (including H-k staking, CCP stacking, and velocity inversion) with the waveforms recorded by 115 portable stations. The Moho, indicated by clear Ps phases, is generally deeper in the Songpan-Ganzi Terrane than in the Sichuan Basin, while a step is notable under the Longmenshan fault zone. We found a low velocity anomaly in the mid-lower crust below the Songpan-Ganzi Terrane and Longmenshan fault zone, which may imply the accumulated partial melting materials in the deep crust under the eastern plateau margin. Clear P-to-s converted phases are found within the crust of the Sichuan Basin. They look as apparent huge anticline structures, which may be caused by ancient crustal deformation under the stable craton.

The southeastern (SE) Tibetan plateau is uplifting accompanying strong seismicity, but the relationship between the crustal structure and tectonic process in this region is still unclear. Seismic anisotropy provides important clues for the deformation mechanism. Here we applied the harmonic decomposition approach and further conducted a Markov-Chain-Monte-Carlo (MCMC) inversion to analyze the crustal anisotropy from receiver functions using waveforms recorded by dense stations in the SE Tibetan plateau. More spatial details on the crustal anisotropy and interface are determined consequently. While the anisotropies in the upper crust are relatively weak (~2%), the anisotropies in the mid-lower crust are mostly stronger than 4%. The fast polarization directions (FPDs) in the mid-lower crust show a dominant circular pattern, in good coherence with the topographic contours, indicating that the crustal deformations in the SE Tibetan plateau may be mainly controlled by the gravity potential. Because of the significant Moho steps along the plateau margin, the extruded deep crustal materials are blocked by the strong crust around so that the minerals are aligned to be sub-parallel to the plateau margins.

青藏高原东北缘是青藏高原隆升的前缘地带,其构造变形特征一直是研究的热点.前人在该区域进行过面波成像研究,但不同结果存在较明显的差异,可能与不同成像方法的分辨率有关,此次研究分别使用基于射线的方法和基于程函方程的方法进行了瑞利波相速度成像.笔者对比了20 s、30 s和60 s的成像结果,两种方法的结果表现出相似的速度特征.总体的速度特征与研究区域的主要构造单元分布相吻合,青藏高原东北缘表现出低速异常,鄂尔多斯块体表现出高速异常.在银川河套地堑,基于程函方程的成像在20 s和30 s得到更明显的低速异常,低速异常体的分布与地堑的轮廓吻合更好,说明基于程函方程的方法对数据利用更充分.

SUMMARYFocal mechanism solutions (FMSs) reflect the stress field underground directly. They provide essential clue for crustal deformations and therefore improve our understanding of tectonic uplift and expansion of the Tibetan Plateau. In this study, we applied generalized Cut and Paste and P-wave first-motion methods to determine 334 FMSs (2.0 ≤ Mw ≤ 6.4) with the data recorded by a new temporary network deployed in the NE Tibetan Plateau by ChinArray project. We then used 1015 FMSs (including 681 published FMSs) to calculate the regional stress field with a damped linear inversion. The results suggest dominant thrust and strike-slip faulting environments in the NE Tibetan Plateau. From the Qilian thrust belt to the Qinling orogen, the maximum horizontal stress orientations (${S_\mathrm{ H}}$) rotate clockwise from NNE to NE, and further to EW, showing a fan-shaped pattern. The derived minimum horizontal stress orientations (${S_\mathrm{ h}}$) are parallel to the aligned fabrics in the mantle lithosphere indicated by shear wave splitting measurements, suggesting vertically coherent deformation in the NE Tibetan Plateau. Beneath the SW Qinling adjacent to the plateau, however, the stress orientations in the shallow and deep crust are different, whereas the deep crustal stress field indicates possible ductile crustal flow or shear.

The SE Tibetan Plateau is located between the eastern Himalayan syntaxis and the stable Yangtze craton; its tectonic evolution is important for understanding the plateau expansion. However, interactions between the plateau and its adjacent subduction zone and craton are still unclear. In this study, we determine updated high‐resolution P and S wave tomographic models of the crust and upper mantle beneath the SE Tibetan Plateau. We confirmed the existence of a high‐velocity layer in the upper mantle down to 200‐km depth beneath the Sichuan basin, the core of the Yangtze craton, representing the stable cratonic root. Another high‐velocity body is located beneath Burma, reflecting the subducting Indian slab in the upper mantle and possibly the remnant Burman slab in the mantle transition zone. A strong low‐velocity column exists above the high‐velocity body in the transition zone and reaches the Moho under the Tengchong volcano. In addition, we find high‐velocity fragments in the upper mantle beneath the Yangtze craton and northern Indochina, which may reflect delaminated lithosphere beneath the SE Tibetan Plateau. The lithospheric delamination may be closely related to mantle flow extruded from the high plateau and possible thermal upwelling from the lower mantle. Our results provide new insight into the deep interactions between the Tibetan plateau and its surrounding blocks near the SE plateau margin.

青藏高原东南部作为板块碰撞的前缘地带一直是地球科学研究的热点,为了揭示碰撞前缘地带地壳结构特征,作者利用布设在中国青藏高原东南部的38个宽频带流动台站记录的2487条远震P波接收函数,采用接收函数CCP叠加(共转换点叠加)和H-κ叠加两种方法获得了研究区域详细的地壳厚度图像和泊松比值.研究结果显示:两种方法获得的地壳厚度特征具有较好的一致性;青藏高原东南部地壳厚度存在明显的东西差异和南北差异;喜马拉雅构造区内莫霍面深度变化较大,介于65～80 km之间;拉萨地体内莫霍面深度介于72～80 km之间;雅鲁藏布缝合带两侧地壳厚度突变,缝合带北侧和南侧地壳厚度相差约8 km.研究区域平均泊松比值较小,为0.24,和大多数造山带泊松比偏低的特征类似.研究区域中下地壳广泛存在强转换界面,该界面可能对应中下地壳高速层的上界面,埋深40～70 km,表明壳内发生深熔或部分熔融作用,导致壳内发生重力分异,在中下地壳形成了高速薄层.

2016年8月24日意大利中部发生MW6.0地震，2个月之后，震中附近相继发生MW5.5、MW5.9、MW6.5地震。研究这几次较大地震间的相互触发作用及机制十分必要，然而在大地震之后传统地震目录通常缺失很多余震事件，缺失的余震事件包含着早期余震时空分布和迁移规律的信息，为完善余震目录本文利用匹配滤波方法对MW6.0地震后80天内的连续数据进行余震检测，得到了数十倍于模板数量的新检测事件，检测事件与模板事件组成的新余震目录完备震级为1.0，提高了地震目录的完备性。依据新余震目录进行余震时空分布研究，结果显示MW6.0、MW5.9、MW6.5地震的早期余震迁移规律不同。MW6.0地震的早期余震沿着断裂走向同时朝两侧迁移；而MW5.9、MW6.5地震的早期余震向南、北迁移表示出不对称的特征。通过拟合余震迁移前端发现，MW6.0、MW5.5地震的早期余震朝着随后较大地震的方向迁移，且较符合lgt的特征，表明余震迁移可能与慢滑动有关。

The upper crust in the SE Tibetan Plateau is rotating around the eastern Himalayan syntax clockwise, and the western margin of the Yangtze Craton has been involved in the active tectonics. However, it is still unclear whether and how the deep crust and upper mantle respond to the plateau expansion. In this study we present a high‐resolution three‐dimensional model of P wave velocity tomography and azimuthal anisotropy in the crust and uppermost mantle beneath the SE Tibetan Plateau determined using traveltime data recorded by a dense seismic network. Widespread low‐velocity zones are revealed around a high‐velocity body in the deep crust and uppermost mantle beneath the southwest Yangtze Craton, where fast‐velocity directions of the azimuthal anisotropy are mostly parallel to the contour lines of the surface topography and the Moho depth along the plateau margin. These results indicate that gravitational potential plays an important role in the crustal and uppermost‐mantle deformations in the SE Tibetan Plateau. Meanwhile, the shallow crust may drop down to the deep crust and drive the ductile deep‐crustal material to intrude into the adjacent regions. The extruded crust is trapped in a deep‐crustal corner and obstructed by the surrounding strong blocks. The trapped crustal material may rise up, causing significant uplift and high heat flow at the surface.

A high-resolution model of 3-D P-wave velocity (Vp) tomography of the crust in the source area of the 2016 Kumamoto earthquake (M 7.3) in West Japan is determined using a large number of arrival times of first P waves and reflected Pwaves from the Moho discontinuity (PmP). The PmP data are collected from original seismograms of the Kumamoto aftershocks and other local crustal events in Kyushu. Detailed resolution tests show that the addition of the PmP data can significantly improve the resolution of the crustal tomography, especially that of the lower crust. Our results show that significant low-velocity (low-V) anomalies exist in the entire crust beneath the active volcanoes, which may reflect the pathway of arc magma. The 2016 Kumamoto earthquake occurred at the edge of a small low-V zone in the upper crust. A significant low-V anomaly is revealed in the lower crust beneath the source zone, which may reflect the arc magma and fluids ascending from the mantle wedge. These results suggest that the rupture nucleation of the 2016 Kumamoto earthquake was affected by fluids and arc magma.

The subduction of the Indian Plate beneath SE Tibet and its related volcanism in Tengchong are important geologic processes that accompany the evolution of the Tibetan Plateau. However, it is still not clear whether the subduction and volcanism are confined to the upper mantle or if they extend deep into the mantle transition zone (MTZ). Here, we imaged MTZ structures by using receiver function methods with the waveforms recorded by more than 300 temporary stations in SE Tibet. The results show significant depressions of both the 410-km and 660-km discontinuities and a thickened MTZ (260–280 km) beneath SE Tibet. The depression of the 660-km discontinuity (by 10–30 km) and the thickened MTZ correlate well with high P-wave velocity anomalies in the MTZ, indicating the presence of a subducted Indian slab within the MTZ. Significant depression of the 410-km discontinuity (by 10–20 km) beneath the Tengchong volcano indicates that the volcano originates from the MTZ and is closely related to the subducted Indian slab. Our results confirm the deep subduction of the Indian plate and the deep origin of the Tengchong volcano. However, it remains unknown whether a slab gap exists and contributes to the Tengchong volcano.

The northeastern Tibetan plateau margin is the current expansion border, where growth of the plateau is ongoing. We analyze shear-wave splitting at ChinArray stations in the NE Tibetan Plateau and its margin with the stable North Chine Craton. The measurements provide important information on the seismic anisotropy and deformations patterns in the crust and upper mantle, which can be used to constrain the expansion mechanism of the plateau. Along the margin and within the craton, the dominant NW–SE fast polarization direction (FPD) is NW–SE, subparallel to the boundary between the plateau and the North China Craton. The shear-wave splitting measurements on the NE Tibetan Plateau itself generally reflect two-layer anisotropy. The lower-layer anisotropy (with NW–SE FPDs) is consistent in the whole region and FPDs are the same as those in the North China Craton. The upper-layer FPDs are parallel to crustal motion rather than surface structures within the high plateau. The two-layer anisotropy implies the presence of deformed Tibetan lithosphere above the underthrusting North China Craton. The NE Tibetan shows similar deformation patterns at the surface (inferred from GPS) and within the mantle (inferred from shear-wave splitting), but significant crustal anisotropy (parallel to crustal motion) requires mid-lower crustal channel flow or detachment to drive further tectonic uplift of the plateau.