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 similar to 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.
