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We provide teleseismic constraints on the continental collision zone along the SW-NE profile from the Tibetan Plateau to the Alxa block, traversing Lenglong Ling, a topographic boundary of the northeastern Tibetan Plateau. Our seismic profile crosses the Qilian Shan thrust belt, a zone of intense earthquake activity that includes a pair of M W 5.9 earthquakes (in 1986 and 2016) with a 30-year time interval occurring at the city of Menyuan near Lenglong Ling. A high degree of similarity between the two waveforms and their similar hypocenters indicate that the two events are repeating earthquakes. Both P - and S -wave receiver functions show contrasting lithospheric structure and crustal seismic properties across the repeating earthquake region, suggesting that Lenglong Ling can be the crustal boundary between the Tibetan Plateau and the Alxa block. High-frequency P receiver functions clearly show a SW-dipping underthrusted upper crust beneath the foreland of the Qilian Shan thrust belt. A strong low-velocity feature above the intracrustal underthrust beneath southwest Lenglong Ling indicates a mechanically weak zone developed coeval with the main compressional deformation during the last major tectonic event. Our observed, localized upper-crustal deformation beneath Lenglong Ling can be direct evidence of the lateral growth of the northeastern Tibetan Plateau, and can control the seismicity of the Qilian Shan thrust belt, including the Menyuan repeating earthquakes. [ABSTRACT FROM AUTHOR]

A dense linear array of 38 broadband seismograph stations was deployed to traverse the northeastern margin of Tibetan plateau (NE Tibet). Shear wave splitting measurements show significant lateral variations of seismic anisotropy across NE Tibet. Combined with previous tomography studies, the SKS travel-time analysis along the array supports the inference that a cold/rigid Asian lithosphere resides beneath the Qilian and Alxa blocks while a hot/soft Tibetan lithosphere resides beneath the Songpan-Ganzi (SPGZ) and Kunlun-West Qinling (KL-WQL) blocks. The observed variations of anisotropy along the array indicate the important role the major faults have been playing in the process of lithospheric deformation in NE Tibet. The West Qinling fault (WQLF) is reckoned as the boundary between the Tibetan lithosphere and the Asian lithosphere in the study area. The Kunlun fault to West Qinling fault (KLF-to-WQLF) zone may constitute a boundary accommodating the eastward extrusion of the Tibetan lithosphere, with the rigid Asian lithosphere in the north barrier to the northeastward tectonic flow of central-eastern Tibet. A significant character of two-layer anisotropy was identified in the Qilian orogen, which was inferred to be associated with the low velocity layer (LVL) acting as a thrust decollement in the mid-to-lower crust. A thorough analysis involving crustal anisotropy and the regional XKS splitting results in NE Tibet, in association with the crustal LVL feature, indicates that decoupling deformation may dominate the lithosphere beneath the Qilian orogen while coherent deformation may dominate the lithosphere beneath the WQL and SPGZ blocks. Anisotropy beneath the Qilian orogen seems consistent with recent deformation under the boundary forces related to possible lithosphere underthrusting from its northern margin. Our shear wave splitting analysis, combined with published results, reflects a regional anisotropic regime that emphasizes the dominance of eastward extrusion of lithospheric blocks on the present-day deformation in NE Tibet. [ABSTRACT FROM AUTHOR]

Abstract: The southeastern margin of China is an ideal area to study the modern plate interactions. The previous models, however, are poorly constrained with respect to the geometry due to the sparse coverage. Here we present new observations from a temporary array with 20 stations in southeastern China. We isolated converted seismic phases in the P and S wave coda to generate receiver function from the structure of the crust and upper mantle. By using H–K stacking and common conversion point stacking, the geometry of discontinuities above was imaged along the southeastern Chinese coastline. The CCP section shows Moho dips gently northeastward with a mean depth of 30km. The 410 and 660km discontinuities are at the depth close to IASP91 model. This means that we have not seen any significant anomaly from Moho and mantle transition zone that corresponded with the geodynamics of plate subducting, instead, we found that the crust was ruptured by Min River fault to depth of the Moho. We provide a 2D map of Moho topography by combining our crustal thickness to that from permanent stations, active seismic and OBS offshore profiles. The map is consistent with previous studies and shows a crust that thins coastward and southwestward in the rate of ~1.5km per 100km. We image the lithosphere–asthenosphere boundary at a depth of 60 to 70km by using S-wave to combine with the P-wave. Our crustal structure suggests that Min River fault certainly plays an important role in adjusting regional stress field induced by plate interactions in the study area. Our lithospheric thickness is more consistent with a very strongly attenuated continental lithosphere. Given the close proximity to the coast this might suggest that the lithospheric mantle transitions to oceanic before the crust does. Alternatively, we may infer that the continental lithosphere has been thermally eroded in this region. [Copyright &y& Elsevier]