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.

中文翻译：

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藏北的生长：从祁连山造山带地壳剪切波速度结构看

我们收集了 2014 年至 2015 年在 288 个宽带站记录的连续环境噪声数据。瑞利波的相速度图是从 2 到 35 s 周期构建的。结合从地震数据中获得的长周期（40、45、50、55 和 60 s）的相位弥散，我们构建了一个三维地壳S- 祁连造山带（QLOB）及其周边地区下方横向分辨率为~20公里的波速（Vs）模型。20km以上的速度结构受断层约束，与子块体吻合较好。QLOB 中下地壳横波速度较低，在 20～40 km 深度存在不均匀分布的低速带（LVZ，Vs < 3.4 km/s），而阿拉善地块除为穿过龙首山的狭窄区域。青藏高原上地壳和下地壳变形解耦，藏北的生长通过韧性下地壳增厚和上地壳逆冲作用发生。QLOB 中地壳 LVZ 的出现可能是由于高热流、中下地壳的更多长英质成分，特别是剪切加热。