【摘要】 本文首次采用Rayleigh面波双台法研究中国西部及其邻域的三维横波速度结构.共处理了超过3000条双台资料,经仔细挑选共获得110条高质量的双台Rayleigh波相速度频散资料.采用Tarantola的概率方法反演得到研究区域内15-120 s的Rayleigh相速度分布图像.采用Tarantola非线性问题的最小二乘反演方法反演得到研究区域内2°×2°的三维横波速度结构.利用不同周期的Rayleigh面波相速度大致对λ/3波长附近深度的横波速度最为敏感这一物理特性,在反演过程中引入一种层速度自适应调整的技巧,可以较好地加快收敛和提高反演的稳定性.反演得到的横波速度结构的主要结论为:(1)青藏高原的西部地区下地壳和上地幔顶部横波速度很高,软流层不发育;而青藏高原东缘地区的下地壳和上地幔顶部速度明显偏低,很可能是青藏高原地壳低速物质沿青藏高原东部边缘地区向南运动、形成经川滇地区连接缅甸北部低速区的低速物质运移通道;在青藏高原东北部边缘地区,下地壳的速度明显低于中地壳的速度;(2)青藏高原南部的拉萨地块具有较高速度的上地幔顶盖层,从南向北拉萨地块的软流层埋深约从130 km减至100 km,软流层厚度约从40 km增至80 km;北部羌塘地块的下地壳速度偏低,上地幔顶盖层缺失,速度很低,软流层的厚度较大;(3)塔里木盆地和准噶尔盆地都表现出较高的上地幔横波速度结构,软流层不明显,准噶尔盆地下地壳的厚度和速度都比塔里木盆地的高;(4)蒙古高原西部的下地壳上地幔顶部速度明显低于蒙古高原东部地区的,且在蒙古高原中西部地区存在巨厚的低速软流层.该软流层越往蒙古高原东部厚度越小,上覆顶盖层的速度和厚度越大.对上述反演结果作了地质解释.更多还原

【Abstract】 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 adjacent area.More than 3000 interstation great circle paths were processed to determine the phase velocity for 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 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 Rayleigh fundamental 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 V-S velocity in the lower-crust and top of upper mantle,and the asthenosphere is not prominent.But the eastern margin of Qinghai-Tibet clearly shows low velocity anomaly in the lower-crust and top of upper mantle,which probably created a southward extrusion channel for low velocity crust material of Qinghai-Tibet along the eastern margin of Qinghai-Tibet through the western Sichuan and Yunnan region and then linking the low velocity zone in northern Burma. In the northeastern margin of Qinghai-Tibet Plateau,the V-S of the lower crust is apparently lower than that of the mid-crust;(2) The Lhasa block of southern Qinghai-Tibet Plateau has a high velocity lid in the upper mantle and from south to north the depth of 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 northern Qinghai-Tibet Plateau 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 is not clear and the thickness and velocity of the lower-crust in Junggar basin is apparently higher than that in Tarim basin;(4) The velocity of the lowercrust and uppermost mantle in western Mongolia Plateau is apparently lower than that in eastern Mongolia Plateau.And in the central and western Mongolia Plateau a thick and large-scale asthenosphere is clearly shown.This thick asthenosphere gradually becomes thinner from central to eastern Mongolia Plateau and above which the higher velocity lid becomes thicker and thicker.Geological explanations are accomplished based on results of inversion.更多还原