FORMATION AND EVOLUTION OF THE MESOZOIC AND CENOZOIC DEEP-WATER SEDIMENTARY BASINS ALONG THE YARLUNG ZANGBO RIVER (Ⅲ): TECTONIC SUBSIDENCE OF THE HIMALAYAN PASSIVE CONTINENTAL MARGINS

The dynamic processes of the formation and evolution of the Himalayan passive continental margins are researched with the aid of the "backstripping" method. On the basis of tectonic subsidences in the areas from Nyalam to Yarlung Zangbo suture zone, the dynamic evolution of the sedimentary basins on the Himalayan passive continental margins are "summarized as follows. The continental rifts and continental margin rifts were controlled by the extensional faulting during the Triassic to Middle Jurassic. The rapid thermal subsidence occurred in the early formation stage of the passive continental margins in the Late Jurassic, while the slower linear thermal cooling subsidence was developed in the complete maturation stage of the passive continental margins in the Early Cretaceous. The subduction of the Indian plate towards the Gangdise block in the Late Cretaceous gave rise to an additional tectonic subsidence, as indicated by the tectonic subsidence curves for the Kangmar-Yarlung Zangbo zone. On the contrary, there was a slight tectonic uplift in the Nyalam-Gamba region-due to the compression of the crust in this region. The tectonic subsidences of the sedimentary basins still lasted in the Palaeocene probably because of the continent-continent collision. During this period, the amount of the subduction of the Indian plate was not very large, therefore the sedimentary wedges of the foreland sedimentary areas were less developed, and the sediment thickness was not very great. All these characteristics show that the mountains were not so high during the early stage of the plate collision. In the Eocene, the tectonic uplift occurred in the whole study area, with greater uplift rates on the northern Indian plate margin. Finally, the submarine topography was overturned and the sea water retrograded from the Himalayan region from north to south, suggesting the compression and uplift of the plate tectonics.