Shear Wave Velocity Structure of the Crust and Upper Mantle in Western China and Its Adjacent Area

It's the first time that interstation method for Rayleigh wave is used to study the 3‐D V S structure in the crust and upper mantle beneath western China and its adjacent area. More than 3000 interstation great circle paths were processed to determine the phase velocity of the fundamental mode of Rayleigh wave, and finally 110 paths of high quality dispersion data were selected which show good spatial coverage in western China and its neighboring regions. Phase velocity maps from 15 s to 120 s were obtained by Tarantola's probability method. The shear wave velocity structure in the crust and upper mantle on 2° × 2° grid points in our studied area was inverted by Tarantola's least square method for nonlinear problem. Based on the physical property that the phase velocity of fundamental Rayleigh wave at period T is most sensitive to the V S structure around λ /3 depth, we provided a layer‐velocity self‐adaptive technique in the structure inversion, which can well accelerate converging speed and improve stability in the inversion. The main conclusions concerning the V S structure of the researched area are: (1) The western Qinghai‐Tibet Plateau shows very high VS value in the lower‐crust and top of the upper mantle, and the asthenosphere is not developed. But the eastern margin of the Qinghai‐Tibet Plateau clearly shows low‐velocity anomaly in the lower crust and top of the upper mantle, which probably created a southward extrusion channel for the low velocity crust material of the Qinghai‐Tibet Plateau along the eastern margin of the plateau through the western Sichuan and Yunnan region and then linking the low velocity zone in northern Burma. In the northeastern margin of the Qinghai‐Tibet Plateau, the VS of the lower crust is apparently lower than that of the mid‐crust. (2) The Lhasa block of the southern Qinghai‐Tibet Plateau has a high velocity lid in the upper mantle and from south to north the depth of the asthenosphere decreases from 130 km to 100 km, the thickness of the underlying asthenosphere increases from about 40 km to 80 km. The Qiangtang block of the northern Tibet shows lower velocity in lower crust, lacks high velocity lid in the upper mantle and displays much thicker asthenosphere. (3) The Tarim basin and Junggar basin both show high velocity structure in the upper mantle, asthenosphere layer does not clearly appear. Thickness and velocity of the lower crust in the Junggar basin is apparently higher than that in the Tarim basin. (4) Velocity in the lower crust and uppermost mantle beneath the western Mongolia plateau is apparently lower than its eastern part. In the middle‐western Mongolia plateau a thick and large scale asthenosphere is clearly shown. This thick asthenosphere gradually becomes thinner from middle to eastern Mongolia plateau and above which the higher velocity lid becomes thicker and thicker. Geological explanations are accomplished based on results of inversion.