Imaging the Northeastern Crustal Boundary of the Tibetan Plateau With Radial Anisotropy

We obtained a 3‐D crustal radial anisotropy model beneath northeastern (NE) Tibet by joint inversion of Rayleigh and Love dispersion curves from ambient noise tomography. Positive crustal radial anisotropy and significant low velocity are dominant beneath the Songpan‐Ganzi Terrane (SGT), indicating the presence of crustal channel flow. The Qilian orogen is characterized by negative radial anisotropy, which could be caused by folding and thrusting due to crustal shortening. This difference in radial anisotropy suggests that crustal shortening deformation may have occurred at a relatively early stage of the plateau evolution (the Qilian orogen) and crustal channel flow at the later stage (the SGT). The crustal radial anisotropy clearly defines the NE crustal boundary of the expanding Tibetan Plateau, which is roughly situated along the north Qilian frontal thrust from 97° to 103.5°E in the north and turns north‐south, passing the Zhuanglanghe Fault and terminating at the Kunlun Fault in the south. Plain Language Summary How did Tibetan Plateau uplift has been controversial for decades. The growing process of the plateau could be recorded by the crustal deformation, especially in its northeastern (NE) margin. The discrepancies between the horizontally and vertically polarized shear waves, named radial anisotropy, give important constrain on crustal deformation at depth. We presented a high‐resolution 3‐D model of crustal S‐wave velocity and radial anisotropy in the NE Tibetan Plateau using ambient noise tomography. We found distinct crustal deformation mechanisms for the Songpan‐Ganzi Terrane (SGT) and Qilian Shan, respectively, which are related to different growth stages of NE Tibet. The crustal horizontal shortening and vertical thickening may take place beneath the Qilian Shan, which may represent a relatively early stage of the plateau growth, and the crustal channel flow plays a dominant role in the crustal deformation in the SGT, which is in the later stage of plateau evolution. A clear anisotropy polarity reversal boundary between the Qilian Shan and the Alax block delineates the current crustal expansion frontier of NE Tibet. Key Points We construct a high‐resolution radial anisotropic velocity model for northeastern (NE) Tibet and its surroundings Different crustal radial anisotropy in Qilian orogen and Songpan‐Ganzi Terrane indicate different stages of crustal deformation in NE Tibet The distribution of crustal radial anisotropy clearly defines the