中国大陆现今构造运动的GPS速度场与活动地块

GPS VELOCITY FIELD AND ACTIVE CRUSTAL BLOCKS OF CONTEMPORARY TECTONIC DEFORMATION IN CONTINENTAL CHINA

GPS观测结果给出了在欧亚参考框架下周边板块的运动状态 ,印度板块的运动方向约NE2 0° ,速度是 40～ 42mm/a ;北美板块的运动方向约NW 2 80°～ 2 90° ,速度是 2 1～ 2 3mm/a ;菲律宾板块的运动方向是NW 2 90°～ 310° ,速度是 37～ 45mm/a ;哈萨克—西伯利亚地盾的运动方向约NE130° ,速度是 3～ 5mm/a。GPS所揭示的中国大陆现今运动场清晰地表现出了以活动地块为单元的分块运动特征。文中给出了各主要活动地块的运动方向和速度。大部分活动地块内部结构完整 ,以整体性的运动为主 ;个别活动地块内部发生构造变形 ,地块的整体性不好。中国大陆以活动地块为单元的现今构造变形可能与大陆岩石圈的结构和性质有关 ,上地壳以脆性变形为主 ,下地壳和上地幔以粘塑性的流变为特征 ,从底部驱动着上覆脆性地块的整体运动。

On the basis of velocity field constrained by GPS measurements, relative motions of lithospheric plates around continental China can be determined under a common reference framework of the fixed Eurasia. The Indian plate moves toward N20°E direction at a rate of 40～42mm/a. The Philippine plate, southeast of China, moves to about N300°W direction at a rate of 37～45mm/a. GPS stations located in North America plate show N280°～290°W motion at a rate of 21～23mm/a. Stations on the Kazakhstan and Siberian block move in a completely different fashion from that those located in continental China, suggesting that continental China may not be part of stable Eurasia, and that the boundary between these two blocks may be the active tectonic zone from Tianshan Mountains to Baikal.Widespread active faulting and folding, uplifting of mountain range, and high seismicity within the belt attest to continued crust shortening throughout the Tianshan Mountains. A velocity profile across the Tianshan between 76°E and 78°E shows about 20mm/a crustal shortening across Tianshan Mountains. Total shortening rate between longitude 83°E and 85°E is about 7 mm/a. Farther east to the longitude about lmqi City, the rate of crustal shortening across the Tianshan decreases to about 2 mm/a. The crustal shortening across the Tianshan shows a tendency of eastward decrease, which is consistent with that inferred from theoretical modeling and geological investigations. GPS measurements indicate oblique convergence across the Qilianshan Mountains region. Crustal shortening, perpendicular to the Qilian Mountains (N30°E), is (4.0±1.0)mm/a. Left\|lateral shearing, parallel to the Qilian Mountains, is (7.5±1.5)mm/a. The entire Qilian Mountain behaves as a huge left\|lateral shear zone.The North China region includes the Ordos block and northern China Plain. Despite variations in rate and direction of each individual station, GPS measurements in the North China region exhibit a relatively homogeneous strain field with most stations moving at rates between 8 and 13mm/a, and in orientations varying from 90 to 110 with respect to stable Eurasia. Stations located in the Ordos block move consistently eastward about 8～9mm/a. There has been about 5mm/a eastward extension from Shanxi graben to the northern China Plain. The South China region has been geologically and seismologically stable and behaves as a coherent block. Based on 22 GPS stations distributed over the block, we obtain a velocity of 11～14mm/a with respect to stable Eurasia in the orientations of N90°～120°E. Thus, the South China block indeed behaves as a rigid block without internal deformation. The Chuandian active crustal block consists of western Sichun and central Yunnan (Yunnan and Sichuan provinces). The GPS stations in the northern part of the block, especially along the Xianshuihe fault, move in the direction of about N120°E.The stations in the southern part of the block, however move to about N160°E. The Chundian block itself exhibits a clockwise rotation on a background of southeastward movement. The velocity field revealed by GPS measurements shows that present\|day deformation of China continent is characterized by active crustal blocks. Some blocks are stable with little or no internal deformation. Others show internal deformation. In any case, the magnitude of deformation in the block interior is much less than that along the block boundaries. The observed slip rates along the margins of Tibetan Plateau are significantly less than those predicted by 'continental extrusion' model. Rheological flow in the lower crust and upper mantle may play an important rule in controlling deformation of upper crust of China continent.