| 藏东、川西及青海南部二叠—三叠纪时期地质构造特征的讨论——地块自身的解体和敛合 |
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| A DISCUSSION ON CHARACTERISTICS OF GEOLOGICAL STRUCTURE IN EAST XIZANG (TIBET) AND WEST SICHUAN AND SOUTH QINGHAI DURING PERMIAN-TRIASSIC -DISINTEGRATION AND CONVERGENCE OF THE MASSIF ITSELF |
| The triangular region including Longmenshan, the southern margin of east Kunlun-west Qinling Mountains and the Hengduan Mountains is situated in the central part of China, which is part of the Qinghai-Xizang (Tibet) Plateau and regarded as a key region to solve some geological problems. In this paper, the author begins with analyzing and correlating the sedimentary lithofacies, and attempts to make an exposition to the disintegration and convergence of the massif itself during the Permian-Triassic and a period of later time, and gives a division of its structural units.1. The disintegration and convergence of the massif itselfIn this region, the Upper Carboniferous is mainly composed of carbonate facies; the Lower Permian is of carbonate facies intercalated with clasolites; and the Upper Permian, of clasolite facies intercalated with carbonates and a lot of volcanics.The sedimentary characteristics of the Upper Carboniferous to the Lower Permian are the same on the Changdu landmass, the Zhongza micro-landmass, the Zayu landmass, the Himalayan landmass, the western margin of Long-menshan and the Lijiang region. In the graben-trenches, they also have the same sedimentary facies although appearing in the form of exotic block in the ophiolitic melanges.The Triassic, especially the Lower-Middle Triassic, is quite different in various places. The lithofacies are strictly controlled by the geological structures. It may be divided into miogeosynclinal and eugeosynclinal sediments. On some landmasses, the Triassic is absent. And there are many discontinuities below and within the Triassic.It has been considered that the continental crust of this region was probably an entire block or a massif during the Late Paleozoic. It was due to the influence of the Late Variscan movement with violent volcanic eruption that the massif itself had been gradually split, forming many graben-trenches inside. As the graben-trenches were deepened, ophiolite materials had at last appeared on the bottom. According to the latest data, the oldest age of ra-diolarites from ophiolites or ophiolitic melanges of all the graben-trenches is P or P2, indicating that the oldest ophiolite is Permian in age. So there was a period of great change during the Permian in the geological history of the massif itself.Besides P or P2 radiolarins contained in silicolites, there are those of Ti, T3, J3-K1 and K1. It could be considered that the trenches were not developed simultaneously and nor was each single graben-trench itself. The real ages of formation of ophiolite materials are different among these graben-trenehes, although the massif started to split in the Permian. Accordingly the convergent period of each graben-trench inside was not identical either. Generally speaking, Jishishaff, Zhidoi-Litang-Sanjiaagkou and Batang-Weixi (Jisha River) graben-trenches converged in the Late Triassic; Zogang-Dengqen (Dengqen-Dongqiao) in the Late Jurassic; and Yarlung Zangbo in the Eate Cretaceous.2. The division of structural unitsBased on the processes of split and; convergence from the Permian to the Late Cretaceous, the massif may be divided inth the following structural units from north to south: Ⅰ. Jishishan graben-trench, Ⅱ. Bayan Har sea basin,Ⅲ;Zhidoi-Litang-Sanjiangkou graben-trench, Ⅳ. Yidun island-arc, ZⅤ. Batang-Weixi (Jinsha River)graben-trench,Ⅵ. Zhongza micro-landmass,Ⅶ. Changdu landmass,Ⅷ. Zogang-Dengqen (Dengqen-Dongqiao) gEaben-trench,Ⅸ. Zayu landmass,Ⅹ. Yarlung Zangbo graben-trench, and Himalayan landmass situated to the south of Yarlung Zangbo graben-trench.3. Concluding remarksThe disintegration and convergence of the massif in the above-mentioned region belong to intra-pJate tectonic movements in nature. The collision-association zone between the so-called Gondwanaland and Eurasianland has found no existence here. The presence of ophiolites and ophiolitic melanges is only the manifestation of some stages of geological development inside the massif itself. |
| QU Jing-chuan.1984.A DISCUSSION ON CHARACTERISTICS OF GEOLOGICAL STRUCTURE IN EAST XIZANG (TIBET) AND WEST SICHUAN AND SOUTH QINGHAI DURING PERMIAN-TRIASSIC -DISINTEGRATION AND CONVERGENCE OF THE MASSIF ITSELF[J].Acta Geoscientica Sinica,6(3):117-127. |
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