青藏高原及邻近地区地壳与上地幔剪切波三维速度结构

3-D SHEAR VELOCITY MODEL OF CRUST AND UPPER MANTLE BENEATH THE TIBETAN PLATEAU AND ITS ADJACENT REGIONS

本文利用30个基准台所记录的238条长周期面波资料,经过适配滤波和分格频散反演,得到中国大陆及邻区147个分格10—105s的纯路径频散,进而反演出青藏高原及邻近地区深至170km的剪切波三维速度结构.研究表明,青藏高原中西部地区和东部地区的地壳平均厚度分别为70±7km和65±7km,地壳平均剪切波速度分别为3.55和3.62km/s,上地幔顶盖平均速度分别为4.63和4.61km/s; 岩石层厚度均为120±10km;东部地区下地壳内30—40km深度处普遍存在低速层;青藏高原及其东侧的上地幔低速层内有横贯东西且明显向上隆起的低速腔.滇西缅北地区的地壳厚45±5km,上地壳及下地壳内都有低速层;上地幔顶盖的速度为4.42km/s,比青藏高原本体及恒河平原都低.恒河平原地壳厚34±2km,速度平均为3.45km/s;上地幔顶盖厚86±10km,速度平均为4.63km/s,顶盖内55—83km深处有一个低速夹层.

In the present study, 238 long-period surface wave records from 30 seismic network stations are selected, Matched-Filtering and Grid Dispersion Inversion are performed on these data to give the Rayleigh pure-path dispersion curves ranging from 10 to 105 seconds for 147 composing grids of China and the adjacent area, then the 3-D shear velocity model of the crust and upper mantle beneath the Tibetan Plateau and its adjacent regions to a depth of 170km is in-versed. It is found that: the average crust thicknesses of the Mid-Western and Eastern Tibetan Plateau are 70 ±7km and 65±7km respectively, the average shear velocities Vs in the crusts of the two areas are 3.55km/s and 3.62km/s re spectively, the average Vs of the lid in the upper mantle of the two areas are 4.63km/s and 4.61km/s respectively. The thickness of the li-thosphere of the Tibetan Plateau is about 120±10km. There are extensive low velocity layers in the lower crust of depth from 30 to 40km in the eastern part of the Plateau. There exists a well-observed swelling-up low velocity channel within the upper mantle low velocity zone beneath the Tibetan Plateau and its eastern flank. In the area of the western Yunnan and northern Burma the crust thickness is 45 ±5km, there are low velocity layers in both the lower and upper crusts, and Vs of the lid in the upper mantle is 4.42km/s which is less than tha± in the Tibetan Plateau and in the Ganges area. In Ganges area the crust thickness is 34± 2km, the crustal Vs is 3.45 km/s, the lid thickness of the upper mantle is 86 ±10km, Vs of the lid is 4.63km/s, within the lid there is an inserted low velocity section of depth from 55 km to 83 km.