帕米尔东北侧地壳结构研究

1998年在帕米尔东北侧伽师及其周边地区完成了两条深地震宽角反射/折射剖面. 结果表明,西昆仑、塔里木和天山在地壳速度结构、构造特征上显示出较大差异. 塔里木块体具有稳定地块的地壳结构特征,地壳平均速度较高(6.5km/s). 向南进入西昆仑,地壳明显增厚,厚度可达0km左右,且地壳平均速度偏低(6.0-6.2km/s),偏低的地壳平均速度主要来源于相对低速度的下地壳结构,反映了西昆仑褶皱系下地壳介质的特征. 向北进入天山后,地壳同样明显增厚,但增厚的程度低于西昆仑下,约为50-55km. 天山地壳同样具有明显低的平均速度(6.2km/s),显示了天山地壳相对"软"的特征,但天山地壳偏低的平均速度来源于广泛分布于中地壳的低速度层和速度偏低的下地壳. 在印度块体向北强烈推挤的作用下,该区地壳遭受强烈的不均匀变形,塔里木块体向南插入西昆仑下,向北插入天山下,形成了该区强烈地震频繁发生的深部构造环境.

CRUSTAL STRUCTURE AT THE NORTHEAST SIDE OF THE PAMIRS

Two profiles of wide angle reflection and refraction were carried out in 1998 around Jiashi and its adjacent area, the northeast side of the Pamirs. The results show marked differences of crustal structures in West Kunlun Mountains, Tarim basin and Tianshan Mountains. The crustal structures of Tarim block shows the features of a stable block, its average crustal velocity is relatively high (6.5km/s). Going southward into West Kunlun Mountains, the crust thickens distinctly, and the thickness is about 70km. Its average crustal velocity is lower (6.0-6.2km/s), mainly resulted from the relatively low velocity in the lower crust. It implies special medium features of the lower crust in West Kunlun fold zone. Going northward into Tianshan fold region, the crust also thickens, but the extent of thickening is weaker, the thickness of the crust is about 50-55km. The Tianshan Mountains are also characterized by low crustal velocity (6.2km/s). It implies that the crust of Tianshan Mountains is relatively "soft". But the low average crustal velocity of Tianshan block results from both lower crust and middle crust, where low velocity is distributed widely. Under strong compressional actions of India plate, the crust of this area has undergone inhomogeneous deformation and Tarim block interpolates below West Kunlun and Tianshan blocks in south and north directions, respectively, which formed a deep tectonic setting for frequent occurrence of strong earthquakes in this area.