青藏高原碰撞、固结的过程及隆升机理

THE COLLISION,SOLIDIFICATION PROCESSES AND UPLIFT MECHANISM OF QINGHAI-XIZANG PLATEAU

青藏高原的地质构造十分复杂,自南而北不同块体曾经历过裂解、漂移、拼合、碰撞、固结的过程。本文仅探讨拼合后碰撞、固结期间青藏高原地质、地球物理特征及其成因机制。 就亚东—格尔木GGT地学断面研究的范围而言,现可分为4个构造带,4个构造带又分别处于不同的构造发育阶段:高喜马拉雅带处于碰撞早期阶段,以逆冲、叠覆作用为主,促使地壳加厚;北喜马拉雅带处在碰撞中期阶段,地壳在持续挤压应力作用下,褶皱缩短并加厚;冈底斯带处在碰撞晚期阶段,以走向滑动和物质侧向流动为特征;羌塘-巴颜喀拉带进入碰撞期后的稳定阶段,地壳活动趋于稳定,以地表剥蚀为主,高原逐渐夷平。

The evolution of Qinghai-Xizang Plateau in time and space dome may be classified into four stages and four structural belts (fig. 1 and tab. 1). The main Himalaya belt in early collision stage, mainly thrusting and over-laping as a result of the plastic deformation, is appeared in the shallow part of the crust. Downward extension comes to the ductile shear zone. The high temperature dynamic-thermal metamorphism and crust-derived granites can be found there. Because of stacking, the crust is thickened and the previous isostatic status is destrayed. The North Himalaya belt has already been the middle stage of collision. Under the continuous compression, the upright fold and cleavage were produced in the shallow crust accompanying with the metamorphism of prethnite-chlorastrolite, while the mountain root subsides and enlarges. Partial melting gives rise to deep magma activity and increase of heat-flow value. Because of the isostatic balance action, the crust is thickened and shortened again. The Gandise belt is in the last stage of collision. When the crust-fold is shortened to a limitation, further compression will produce slip-fault. This forces material to move laterally. In the direction perpenticular to the compressive stress, the weaker stretching stress is well-developed. This gives rise to shallow focus-source earthquake and minor depression basins. Owing to stress locally lessened, the deep-source material melt in a very low degree, and sub-alkaline-alkaline volcanics are produced. The exchanging of heat in the subsided crust mountain root with that in mantle creates a crust-mantle mixed zone. Heat-flow value varies in a wide range, which belongs to disturbance zone of high heat-flow. It tends to be in isostatic balance. The Qiangtang-Bayan Har belt has been solidified, and it is in the steady stage after collision. At this stage, it is in isostatic balance, iso-thermal surface goes down and the lithosphere is thickened. The upper crust is abundant with radiation elements. Because of eroding and heat radiating for a long time, the heat flow here is lower and it has become a cold crust and also stable on mechanics. The surface of crust suffers mainly erosion and the highland is graduatly eroded to peneplain. In the period of Mesozoic and Cenozoic, Qinghai-Xizang Plateau, from south to north, has suffered from several stages of spreading, drifting combination, collision and solidification. At the part of combining zone of terrane, plasticity is very large from crust to the upper mantle. Therefore, when India Ocean spreads, the India plate compresses northward. This will make the crust where the plasticity is large, thrust, stack and thicken. The compressed fold and intrusion of deep-source magma give rise to slip fault and mixed zone of crust and mantle, while orogenic zone spread from inside to outside and the area inside tends to be stable, then newly solidified belt is formed.