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Variations of lithospheric structure across the northeastern Tibetan Plateau and its bounding Asian blocks, the Alxa block to the north and the Ordos block to the east, are crucial for understanding the rise and lateral growth of the Tibetan Plateau. Using waveforms from high-density seismic arrays in northeastern Tibetan Plateau and the surrounding regions, we investigated the lithospheric structure with S- and P-wave receiver functions. The results show strong and relatively simple negative velocity gradients in the depth range of mantle lithosphere (∼70–150 km) under the Ordos and Alxa blocks, similar to those under typical stable continental lithosphere. In contrast, under northeastern Tibetan Plateau including its marginal regions, the velocity gradients are weak and diffusive for the mantle lithosphere, which may be explained by elevated temperature and presence of partial melts. The changes of lithospheric structures are sharp between the Tibetan Plateau and the bounding Ordos and Alxa blocks, suggesting that these two blocks have restricted the lateral growth of the Tibetan Plateau as rigid boundaries. However, across the northeastern corner of the Tibetan Plateau to the Yinchuan rift, the lithospheric mantle structures are similar, suggesting a lateral mantle flow from the Tibetan Plateau to the gap between the Ordos and the Alxa blocks. The crustal structures along this transition show evidence of lateral growth of the Tibetan Plateau. In particular, the edge of thickened crust and evidence of Moho superposition are found between the Haiyuan Fault and the Tianjin-shan Fault, which may have replaced the Haiyuan Fault as the front boundary of the laterally growing Tibetan Plateau in its northeastern corner. [ABSTRACT FROM AUTHOR]

Whether or not the Asian lithosphere has underthrusted beneath the Tibetan Plateau is important for understanding the mechanisms of the plateau's growth. Using data from the permanent seismic stations in northeastern Tibetan Plateau, we studied seismic structures of the lithosphere and upper mantle across the plateau's northeastern margin using P and S receiver functions. The migrated P- and S-receiver function images reveal a thick crust and a diffuse lithosphere–asthenosphere boundary (LAB) beneath the Tibetan Plateau, contrasting sharply with the relatively thin crust and clear, sharp LAB under the bounding Asian blocks. The well-defined LAB under the Asian blocks tilts toward but does not extend significantly under the Tibetan Plateau; this is inconsistent with the model of Asian mantle lithosphere underthrusting beneath the Tibet Plateau. Instead, our results indicate limited, passive deformation of the bounding Asian lithosphere as it encounters the growing Tibetan Plateau. [ABSTRACT FROM AUTHOR]

Lithospheric deformation of the Tibetan plateau is caused by subduction of the Indian (northward) and Asian (southward) plates. The effects of this interaction on inner reaches of the Asian continent, between the Tibetan plateau and the North China craton (NCC) for example, remain uncertain due to poor geophysical data coverage in northeastern Tibet (NE Tibet). We provide here detailed knowledge of the lithospheric structure beneath NE Tibet as determined from a dense broadband seismic profile traversing NE Tibet to the westernmost NCC. Receiver function imaging reveals several significant features, including a north-dipping intracrustal converter (NC), Moho offset/overlap beneath major fault zones, and a low velocity layer (LVL) in the middle-lower crust. The lithosphere–asthenosphere boundary (LAB) is clearly defined and appears as a south-dipping interface that runs continuously from the Alxa interior to the Qilian orogen. Interpretation of these observations, combined with other seismic evidence, implies that the NCC lithospheric mantle has been persistently underthrust beneath the Qilian orogen. This process forms the thick-skinned crustal accretionary wedges, which develop above a middle-lower intracrustal decollement. Our results provide further deep-geophysical constraints on the Cenozoic post-collisional evolution of the convergent boundary between NE Tibet and the NCC and help clarify the mechanism of plateau growth in this boundary area. [ABSTRACT FROM AUTHOR]