Growth of Northern Tibet: Insights From the Crustal Shear Wave Velocity Structure of the Qilian Shan Orogenic Belt.

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]