中轴构造带(南段)的变形及缩短速率计算

THE CALCULATION OF DEFORMATION AND SHORTENING RATE OF THE CENTRAL AXIS BELT (SOUTHERN SEGMENT)

中轴构造带是欧亚大陆的一个重要的构造带 ,具有地震震中密集分布、重力梯级、岩石圈厚度或结构变异等特点 ,但没有突兀连贯的山链 ,其原因是多地块的大规模走滑活动的抑制。以往对于中轴构造带的宏观运动和块体之间的相对运动研究得比较深入 ,但是对其缩短变形的研究尚有明显欠缺 ,这使得在讨论青藏高原东向运动是任其“自由逃逸”还是有阻挡性约束的问题时缺乏必要的依据。针对此问题 ,笔者以地块的宏观变形与块体内变形的透入程度为切入点 ,通过编绘横切中轴构造带的平衡剖面 ,计算出该带的缩短率为 2 2 .37% 7.4 3% ,中新世以来的平均缩短速率为 3.6 34.4 5mm/a ,并对中轴构造带形成的变形机制进行了初步探讨。提出青藏高原东向运动受阻 ,中轴构造带内块体东向汇聚下 ,上下地壳拆离并呈现出上叠式仰冲褶皱缩短的认识。

The Central Axis Belt (CAB) is a special tectonic belt within Eurasian continent, along which the epicenters of earthquake are densely distributed; the belt is an important gradient of gravity; the thickness and structure of lithoshpere along the belt change greatly; however, along the belt no continued mountain chains are developed owing to the restrain of the large-scale strike-slip faulting among many blocks. Many researches on the microscopic movement of the CAB and the relative movements among blocks have been carried out before, but the shortenings of deformation have rarely been carried out yet, which leads to the lack of essential basis for the discussion on whether the eastward moving of Qinghai-Tibet Plateau is free or obstructed. Aiming at this problem, the microscopic deformation and degree of penetration of the internal deformation of blocks were taken into account, and with the drawing of the balanced sections across the CAB, the rate of the shortening of this belt was obtained (22.37%~ 27.43%); the average velocity of shortening since the Miocene was also calculated (3.63~4.45mm/a). Based on these, the mechanism of the deformation of the CAB has been discussed preliminarily. Furthermore, we propose that the upper and lower crust of the CAB was detached and the shortening caused by the upstacked overthrusting and folding was presented when the eastward movement of Qinghai-Tibet Plateau was blocked along the CAB.