青藏高原及周边现今构造变形的运动学

KINEMATICS OF PRESENT-DAY TECTONIC DEFORMATION OF THE TIBETAN PLATEAU AND ITS VICINITIES

青藏高原现今构造变形的定量化研究是理解其动力过程的基础 ,近年来高速发展的GPS(全球定位系统 )技术为测量大尺度现今构造变形提供了最有效的手段。我们利用青藏高原及周边的5 5 3个GPS观测数据给出了其现今构造变形的速度场 ,表明印度和欧亚板块之间的相对运动主要被青藏高原周边的地壳缩短和内部的走滑剪切所调整吸收。其中 ,喜马拉雅山系吸收了青藏高原总缩短量的 4 4%～ 5 3% ,北部的阿尔金山、祁连山和柴达木盆地吸收了 1 5 %～ 1 7% ,高原内部吸收了 32 %～4 1 %。青藏高原的“向东挤出”实际上是地壳物质的向东流动而不是刚性地块的挤出。这一地壳物质流动带在高原西部以地表张性正断层和共轭剪切走滑断层为特征 ,到高原中东部转换为巨型的弧形走滑断裂带 ,再到高原东北缘转换为地壳缩短和绕东喜马拉雅构造结的顺时针旋转。

As the most prominent example of large scale continental deformation, Tibetan Plateau offers an ideal natural laboratory for quantifying such deformation and understanding the relevant dynamic processes. Global Positioning System (GPS) provides a powerful means to directly measure the kinematics of present day deformation. Our synthesis of GPS velocities from 553 stations in Tibetan Pla￣teau and its margins quantitatively show that most of the relative India/Eurasia motion has been accommodated primarily by crustal shortening along the margins, strike slip and normal faulting in the pla￣teau interior, and clockwise rotation around the eastern end of Himalayas. Taken 36～40mm/yr as total relative motion between India and Eurasia, the eastern Tibetan Plateau and its margins accommodates 85%～94% of the total motion, whereas western Tibet absorbs 70%～91% of total convergence and the rests are taken up by shortening across the Tianshan in the north. The NNE SSW shortening of the plateau interior is accommodated by conjugate strike slip faulting and orthogonal normal faulting, which do not require crustal thickening or thrust faulting. The eastward extrusion of Tibetan Plateau out of India's northward pass is carried out by roughly eastward flow of crustal material rather than by rigid block rotation. The flow of Tibetan crustal material rotates around the eastern Himalayan syntaxis, causing southeastward to southward and even southwestward velocities observed in southern and western Yunnan Province of China. To the east, the eastward flow of crustal material causes shortening across the eastern margin of the plateau and clockwise rotations of the region where resistance to such flow is weak. To the west, the westward motion of the western margin of the plateau is observed with only 4mm/yr slip rate. Components of velocity of Tibetan Plateau in both parallel and perpendicular directions to the relative motion between India and Eurasia can not be attributed to slips along a few faults either strike slip or thrust because of their distributed nature. Thus, the present day tectonics in the Tibetan Plateau is best described as deformation of a continuous medium, at least when averaged over distances of ～100km.