青藏高原现今构造变形特征与GPS速度场

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

文章以青藏高原的GPS观测数据为基础 ,结合活动地质构造资料 ,研究了青藏高原的现今构造变形状态和机制 ,并探讨青藏高原现今构造变形所反映的大陆内部动力学过程。GPS观测的速度矢量揭示了青藏高原整体向北和向东运动的趋势 ,平行于印度和欧亚板块碰撞方向上的地壳缩短量约是 38mm/a ,而青藏高原周边主要断裂带的滑动速率均在 10mm/a以下。大约 90 %的印度与欧亚板块相对运动量被青藏高原的地壳缩短所吸收和调节。GPS速度矢量由南向北逐渐向东偏转 ,向东的分量也增加 ,形成了以羌塘地块北部 (或玛尼—玉树—鲜水河断裂 )和祁连山中部为中心的两个地壳物质向东流动带。青藏高原的向东挤出实际上是地壳物质在印度板块推挤下和周边刚性地块阻挡下围绕东构造结发生的顺时针旋转。

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.