板块汇聚带的造山和重力垮塌作用及其力学成因:以雅鲁藏布江-喜马拉雅山汇聚带为例

Orogeny and gravitational collapse along the convergent plate boundary and their mechanical origin:a case study on the Yarlung Tsangpo-Himalaya belt

从大洋底部磁异常条带的宽度变化可以看出,大洋的扩张速率是时常变化的,这种变化与板块俯冲角度的变化一样,对板块汇聚带的应力和应变场有重要的控制作用。中国存在众多不同特征、不同年代的板块汇聚带,根据其中发生的构造作用可以反演汇聚带在板块扩张速率和俯冲角度控制下的演化。有着巨大高差的喜马拉雅山构造带和雅鲁藏布江缝合带在喜马拉雅山东、西构造结逐渐交汇在一起,其平均海拔高度随之增大而宽度不断变小。喜马拉雅山中段的推覆发生在中新世早期,在推覆的过程中,其北缘沿藏南拆离系还发生了大规模的南北向伸展。这表明在中新世前,在雅鲁藏布江缝合带和喜马拉雅山之间可能存在一个规模很大的造山带,在这里称之为喜马拉雅山—雅鲁藏布江造山带,它在中新世初发生了垮塌。作为这个造山带的前缘,喜马拉雅山中段发生向南的推覆,这就是喜马拉雅山中段的推覆时间要远远滞后于印度和欧亚大陆的碰撞时间的原因。造山带的垮塌可能是印度与欧亚大陆间水平汇聚速率的突然减小造成的。发生在古近纪的日本海和中国的松辽盆地的弧后扩张与喜马拉雅山—雅鲁藏布江造山带的重力垮塌作用可以对比,可能是太平洋和欧亚大陆汇聚速率的突然减小造成的。在白垩纪,太平洋和欧亚大陆汇聚速率很大,所以,欧亚大陆东缘,包括日本海和中国的松辽盆地,在当时可能是规模很大的造山带。位于秦岭南侧,上覆在四川盆地之上的大巴山推覆带的形成机制与喜马拉雅山在中新世的推覆成因类似,与晚白垩世—古近纪秦岭的垮塌有成因关联。秦岭的垮塌可能是华南—华北汇聚速率减小造成的,在此之前秦岭要比现今高得多。

The rapid increase or decrease in plate convergence rates plays a controlling role for deformation within convergent plate boundaries. The Himalayas range with high topography and the Yarlung Tsangpo suture zone with lower elevations come together at the east and west syntaxes of the Himalayas. Therefore, the two features should have belonged to a unified orogenic belt, resulting from the collision between Indian and Eurasia during the early Tertiary. This belt is here referred to as the Himalayas-Yarlung Tsangpo orogenic belt. During the early Tertiary, the Himalayas were located on the southern edge of the orogenic belt and had a lower elevation than at present. Its rapid uplift did not occur until the early Miocene, much after the time of collision of India and Eurasia. In the Miocene, large-scale north-south extension occurred in the middle section of the belt along the south Tibet detachment system. Thus, it can be inferred that the middle section of this orogenic belt experienced major gravitational collapse during that period, leading to an increase in its width and a decrease in its average height. Therefore, the initial uplift of the Himalayas during Miocene is interpreted to have trigged by gravitational collapse of the Himalayas-Yarlung Zangbo orogenic belt. The cause of the collapse was a decrease in the convergence rate between India and Eurasia and resultant releasing of compressional stress. The forming of the Himalayas-Yarlung Tsangpo orogenic belt occurred prior to the Miocene and therefore it can be confirmed that during the Paleogene there must have been a high convergence rate occurred between India and Eurasia. The back-arc spreading of the Japan sea and the Songliao basin during Oligocene and Miocene is comparable to the gravitational collapse of the Himalayas-Yarlung Tsangpo orogenic belt, caused by the decrease in the convergence rate between the Pacific ocean and Eurasia. As a result of a higher convergence rate between the Pacific ocean and Eurasia continent in the early Cretaceous, the eastern edge of the Eurasia, including what is now the Japan sea and the Songliao basin, might have been a broad orogenic belt with a high altitude. With a decreasing convergence between the Pacific ocean and Eurasia during the Tertiary, a gravitational collapse occurred in the orogenic belt which resulted in the landforms of today. The Daba Shan thrust belt located at the south side of the Qinling orogenic belt and tectonically underlain by the Sichuan basin, experienced a similar evolutionary history during late Cretaceous-early Paleogene time as the Himalayas during the Miocene and the eastern margin of Eurasia during the Tertiary, and its southwestward thrusting was associated with the gravitational collapse of the Qinling orogenic belt owing to rapid decrease in convergence rate between south and north China blocks.