GPS VELOCITY FIELD AND ACTIVE CRUSTAL DEFORMATION IN AND AROUND THE QINGHAI-TIBET PLATEAU

GPS stations located in the northern Ganges plains, south of the Himalaya Mountains, show a northward movement (N19°～22°E) at a rate of 39～41 mm/a with respect to stable Eurasia. The maximum velocity of these sites (about 40 mm/a) may represent the rate of collision between the Indian and Eurasian plates. This total convergence rate is intermediate between that estimated using the NUVEL\|1A model and that re\|estimated recently from a revised plate configuration in the Indian Ocean. The GPS measurements indicate that the Lhasa Block moves to N30°～47°E, at a rate between 27 and 30 mm/a. The rate of extension between Shiquanhe in the west and Lhasa in the east is measured to be about 21 3 mm/a. This rate is similar to the seismological rate of (18±9) mm/a. Central Tibet is cut by several NWW\|trending left\|lateral strike slip faults. These faults divide the central Tibet into four active crustal blocks from south to north as follows:the Qiangtang block, the Kunlun block, the Qaidam block, and the Qilianshan block. The Qiangtang block moves to N60°E direction at a rate of (28±5) mm/a. One station in the Kunlun block shows similar velocity vector to those in the Qiangtang block. Stations in the Qaidam block show similar direction of movement to the Qiangtang block, but the rates decrease to 12～14 mm/a. Northward to the Qilianshan block, most stations move to NEE direction, and the rates decrease to 7～14 mm/a. It appears that different active crustal block within the Qinghai\|Tibet Plateau behaves differently. The Qinghai\|Tibet Plateau undergoes substantial internal shortening, with the direction of maximum shortening being ～N20°E, the inferred India\|Eurasia plate convergence direction. Along a N20°E profile passing through the eastern part of the Qinghai\|Tibet Plateau and the Qilianshan Mountains, the ～39 mm/a convergence rate between the Indian plate and the coherent Alashan block north of the Qilianshan Mountains represents approximate 90% of the total collision rate between Indian and Eurasian plates. The N20°E velocity gradient is strikingly linear except for a high velocity gradient across the Himalaya at the southern margin of the plateau. The average contraction rate along this direction is about 2×10 \|8 strain / a. Taking 10～13 mm/a for the contraction rate across the Himalaya Mountains, 16～18 mm/a of N20°E contraction, almost 1/2 of the total India\|Eurasia plate convergence, is absorbed by internal shortening of the plateau. An additional 10 mm/a of contraction occurs across the Qaidam basin and the Qilianshan Mountains.Although contraction appears to accommodate most of the convergence of India, the eastern Qinghai\|Tibet Plateau, south of the left\|lateral Kunlun and/or Ganzi\|Yushu strike slip faults, these blocks are being extruded to the east relative to both India and Eurasia plates. The N110°E component of velocities (orthogonal to the convergence direction) increases steadily northward from the Himalaya across the breadth of the Qinghai\|Tibet Plateau, then decreases rapidly. Sites in the eastern Qinghai\|Tibet Plateau show a prominent clockwise rotation. In the eastern margin of the Qinghai\|Tibet Plateau, stations move eastward in western Sichuan, southeastward in northern Yunnan Provinces, and south southeastward in southern Yunnan. Together with stations in southern Tibet, the velocities show a clockwise rotation around the Eastern Himalayan Syntaxis.