Abstract: In order to monitor and research crustal movement and deformation in northeast margin of Qinghai-Tibet Plateau and their relationship with strong earthquake activity, a reexamination of GPS (C) Net in Hexi region was carried out in 1999. Basing on horizontal displacement velocity field obtained from GPS (C) Net in 1993 and 1999, this paper provides the spatial distribution map of strain field in northeast margin of Qinghai-Tibet Plateau, and presents the results of preliminary study of the strain field and tectonic deformation of this region. By using Discontinuous Deformation Analysis (DDA) method, the tectonic stress field background of crustal movement and deformation in this region is discussed in this paper as well. The main results are as follows: (1) The strain field in this region is dominated mainly by nearly NE-trending principal compressive strain, accompanied with NW-trending principal tensile strain. The amplitude of the later, however, is generally much smaller than that of the former. The strongest compressive strain occurs in Wuwei block, which is located in the eastern section of the Hexi corridor; (2) The shear strain produced by the strain field of this region is dominated by nearly EW-trending sinistral shear, indicating that the NWW-trending major fault on the northeast border of the Qinghai-Tibet block is dominated by sinistral strike-slipping, accompanied with compressive movement. The areas of high shear strain value are mainly distributed at Qilian-Wuwei region, which is located on the northeast boundary of the Qinghai-Tibet Plateau; (3) The rate of the bulk sinistral twisting between Ganqing block and Alashan block is about 6mm/a. Nearly EW-trending tensile strain is predominant in entire Alashan block (5.6?10^-9?0.4?10^-9), but it is weaker in Ganqing block (3.0?10^-9 ?2.0?10^-9); (4) Based on numerical simulation by using discontinous deformation analysis method (DDA), and the analysis of horizontal movement background in China continent and adjacent areas, it is suggested that the distribution features of relative horizontal movement and tectonic deformation in this region are not only the result of compressive stress field produced by the squeezing of the Indian Plate, but also related to the eastward action of the dynamic force from the west, which is stronger in the south and becomes weaker in the north. This dynamic force may be produced under larger spatial background. The pushing by Indian Plate from the south produces mainly the compressive stress field, which controls the crustal movement and strain distribution in this region; while the inhomogeneous pushing from the west may be closely related to the sinistral strike-slip motion of the NWW-trending major boundary faults of the blocks in this region.

Key words: Qinghai-Tibet Plateau, GPS survey, Crustal movement, Tectonic deformation

摘要： 利用青藏块体东北缘地区1993与1999年GPS观测获得的地壳水平运动速度场结果,初步研究了该区的应变场与构造变形。该区应变场以近NE向的主压应变为主体,伴随着近NW向的张性应变。河西走廊中、东段,尤其是武威断块是压应变最强的区域。应变场形成的剪应变以近EW向的左旋剪切为主体,表明该区NWW向的块体边缘主干断裂的活动方式是左旋走滑兼挤压。剪应变高值区主要分布于青藏块体东北边界带的武威、祁连一带。甘青块体与阿拉善块体之间整体左旋扭动速率约为6mm/a。配合非连续变形分析法(DDA)数值模拟,初步分析了该区的构造应力场背景,认为该区相对水平运动和构造变形分布特征不仅是印度板块推挤应力场作用的结果,还可能与来自西侧南强北弱的向东的动力作用有关。

关键词: 青藏块体, GPS观测, 地壳运动, 构造变形