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We collected continuous ambient noise data recorded between 2014 and 2015 at 288 broadband stations. Phase velocity maps of Rayleigh waves are constructed from 2 to 35 s periods. Combining phase dispersions at long periods (40, 45, 50, 55, and 60 s) obtained from earthquake data, we construct a three‐dimensional crustal S‐wave velocity (Vs) model with a lateral resolution of ∼20 km beneath the Qilian orogenic belt (QLOB) and its surrounding areas. The velocity structure above 20 km is constrained by faults and agrees well with subblocks. The middle‐lower crustal shear wave velocity in the QLOB is relatively low and has an unevenly distributed low‐velocity zone (LVZ, Vs < 3.4 km/s) at depths of 20–40 km, while that in the Alxa block is normal except for a narrow region protruding through the Longshou Shan. Deformation of the upper and lower crust in the QLOB is decoupled, and the growth of northern Tibet occurs through ductile lower crustal thickening and upper crustal overthrusting. The emergence of the mid‐crust LVZ in the QLOB could be due to high heat flow, more felsic composition of the mid‐lower crust, and especially shear heating. The weak lower crust from the QLOB to the Alxa block through the Longshou Shan is conducive to the expansion of the plateau, and the active front of northern Tibet is north of the Longshou Shan. Plain Language Summary: The magnificent Tibetan Plateau (TP) was formed by the collision of the Indian and Eurasian plates during the Cenozoic. Although various models of TP uplift have been proposed, the growth and lateral expansion of the northern TP remain uncertain. The Qilian orogenic belt (QLOB), as the young edge of the TP, has become of major interest to researchers. In this study, we obtain a high‐resolution 3‐D image of the crustal S‐wave velocity (Vs) structure in the QLOB and its surrounding areas using continuous ambient noise data recoded by a dense seismic array via an ambient noise imaging technique. The results show that the middle‐lower crustal shear wave velocity in the QLOB is relatively low and has an unevenly distributed low‐velocity zone (Vs < 3.4 km/s) at depths of 20–40 km, while that in the Alxa block is normal except for the narrow region protruding through the Longshou Shan. The results shed new light on the crustal structure in the study area and improve our understanding of the growth of northern Tibet. Key Points: High‐resolution 3‐D crustal shear wave velocity images beneath the Qilian orogenic belt are obtained via ambient noise imagingThe growth of northern Tibet in the crust occurs through ductile lower crustal thickening and upper crustal overthrustingThe active front of northern Tibet is north of the Longshou Shan [ABSTRACT FROM AUTHOR]

We present high‐resolution 3D images of the crustal S‐wave velocity structure beneath northeastern Tibet, via ambient noise imaging techniques. The data come from continuous recordings taken in one year by a seismic array consisting of 366 broadband stations in the study region. The results show a substantial and pervasive middle crust low‐velocity structure (<3.4 km/s) beneath the plateau region, which partially protrudes northeastward across the West Qinling fault, reflecting channel flow from inside the plateau and material extrusion in the periphery. The growth of the northeastern Tibetan Plateau in the study region is manifested in its northeastward expansion and thickening driven by crustal channel flow, accompanied with upper crust shortening. Plain Language Summary: The growth mechanisms of the Tibetan Plateau (TP) have been considered controversial for decades. Expansion of the northeastern TP and the interaction with its peripheral area have become of major interest to researchers. In this study, we present a high‐resolution 3D image of the crustal S‐wave velocity structure beneath this area by analyzing the continuous ambient noise data recoded by a dense seismic array via ambient noise imaging techniques. The results show a substantial and pervasive middle crust low‐velocity structure beneath the plateau region, which partially protrudes northeastward across the West Qinling fault, reflecting channel flow from inside the plateau and material extrusion in the periphery. The channel flow slows down gradually with its outward protrusion. The upper and lower crust is decoupled. In addition to the crustal channel flow, upper crust shortening takes part in the growth of the northeastern Tibet. Key Points: High‐resolution 3D crustal shear‐wave velocity images beneath the northeastern Tibetan margin via ambient noise imaging techniquesSubstantial and pervasive middle crustal channel flow is found beneath the northeastern TibetThe northeastward expansion of the northeastern Tibet is driven by crustal channel flow, accompanied with upper crust shortening [ABSTRACT FROM AUTHOR]