Wuling movement also caused a large-scale thrust nappe. On the one hand, the crust is shortened and heated by deep shear extrusion, so that the silicon and aluminum in the crust are partially melted and transformed granite is formed; On the other hand, deep faults occur, and mantle materials (basaltic magma) invade along the deep faults, forming basic and ultrabasic rock belts and basalts formed by some associated volcanic eruptions. Therefore, Wuling Movement is the first major thermal event in Hunan.
Xuefeng movement Xuefeng movement is a tectonic movement in the late Neoproterozoic, which caused the unconformity between Banxi Group and Sinian system. This unconformity contact is mostly manifested as the superposition of different horizons, which Chen Xincai (1996) thinks is "the result of the rotational erosion of extensional fault blocks or rock slices, not the compression of folding movement". Xuefeng magmatic rocks are mainly distributed in the northeast of Hunan Province, which is the western extension of Jiuling rock mass in Jiangxi Province. There are many rock masses, mostly compound rock masses with multiple intrusions.
The Yangtze tectonic domain from Middle Sinian to Middle Ordovician in Caledonian movement is dominated by carbonate deposits, which is a stable and coherent continental slope deposit. From the late Ordovician to the late Ordovician, the sedimentary characteristics changed greatly, that is, on the basis of the middle Ordovician siliceous rocks deposited on the continental slope, the Hunan orogenic belt in the Yangtze tectonic domain deposited a very thick (nearly 1000 meters) flysch rhythmic deposit composed of feldspar-time complex, time complex, debris complex, siltstone and slate; Similarly, in the Cathaysian tectonic domain, the late Ordovician sediments were transformed into middle Ordovician siliceous argillaceous rocks, thick layered sandstone clasts and syngenetic conglomerates containing sandstone and slate debris. The stable and coherent continental slope deposits from Sinian to Middle Ordovician accumulated rapidly in the Late Ordovician, which indicated that the Yangtze and Cathaysian tectonic domains began a new tectonic movement-Caledonian movement after long-term stability.
Figure 6-2 subduction-collision splicing relationship between Yangtze plate and South China plate
(According to Rao Jiarong 1993)
Ⅰ —— the interface between sedimentary rocks and metamorphic rocks; Ⅱ —— bottom interface of metamorphic rocks; Ⅲ —— Upper mantle; Ⅳ —— Decomposition surface of lithosphere and asthenosphere
The early Caledonian movement was characterized by the contact and collision between the protruding parts of the Yangtze plate and the Cathaysian plate, which formed a rapid accumulation of flysch deposits. Due to the convergence of collision zones, basins with uplift on both sides of thick and light shells (Xuefeng-shogunate and Luo Xiao-Wuyi uplift) were formed in Hunan, and thin and heavy shells sank in the collision zones. This basin straddles the margin of Cathaysian plate and Yangtze plate, and is called the overlying basin. After the late Paleozoic sedimentation, the general outline of the basin is "two kinds of basement and one kind of caprock". The Caledonian movement at the end of Silurian finally formed the South China fold orogenic belt, and the Yangtze block and the South China fold belt were spliced to form a unified southern plate. And began the history of the activity stage on the chessboard. During the Hercynian-Indosinian period, the intraplate activities of the southern plate were mainly characterized by uneven uplift and rifting, while most of Hunan Province was located in the depression area, which was a continental basin with widely developed shallow-sea carbonate deposits. During this period, a small rift zone appeared, forming a deep and relatively closed detention sea environment. For example, carbonate flysch deposits appear in Ningyuan area of eastern Hunan, and a complete gravity flow combination appears in Li Antang of Guiyang. There is a steep slope between it and the adjacent shallow-sea carbonate platform, showing the structural characteristics of a banded submarine canyon (with faults on both sides) or a dustpan basin (with faults on one side) controlled by synsedimentary fault.
After the Hercynian movement in the early Hercynian movement, the South China continent quickly entered a relatively uniform settlement stage. The early Permian reached its climax, and the tectonic palaeogeographic pattern of Maokou period was re-differentiated. By the end of the early Indosinian period, there were no major structural changes in the whole area. Since the late Hercynian, the rifting along the basement fault tends to be relatively weak, and it is generally in the state of a large depression basin. For example, a typical deep-water sedimentary basin was formed in the west of Chaling-Chenxian fault in Hunan.
Indosinian Caledonian movement from late Ordovician to Silurian made the Yangtze and Cathaysian ancient plates converge into a whole after early collision, forming the main body of South China ancient plate. Late Paleozoic entered the stage of continental evolution. Late Paleozoic strata were deposited in the central basin. From the Early Triassic to the end of the Middle Triassic, the ancient plate in South China was deeply influenced by the subduction of the western Pacific in the east, the convergence of the Tethys in the south and the simultaneous collision of the Qinling-Dabie Mountains in the north.
The Indosinian movement made the structural deformation of the Yangtze-Cathaysian convergence zone and its adjacent areas very complicated. As far as Hunan is concerned, the most important feature is that "the base slides and the block is hairy". The so-called basement sliding is due to the different shortening mechanisms of brittle layers in the upper crust, lower crust and mantle. Its location is estimated to be about 20 kilometers; Depth of low velocity layer reflecting earthquake from Taiwan Province-Heishui geoscience section. The flattening of the block is due to the inclusion of deep and large fault structures in the early stage of the super-shell, such as the collision zone at the convergence edge of two plates and the sliding of the block cut by these faults. In the process of "basement sliding and block napping", due to friction and shear heat, crustal materials partially melted to form magma chamber, and magma invaded the late Paleozoic strata along the fault to form magmatic rocks. Therefore, Indosinian rock masses are mostly distributed in fault zones or their intersections.
This movement mechanism of Indosinian Movement also resulted in the phenomenon that Wugongshan (Luo Xiao) block was pushed to the northwest from east to west in turn, and the Lengjiaxi Group in northeast Hunan moved to the south near Hengshan Mountain. The central Hunan block overthrusts to the west, forming the Qiyang arc protruding to the west. The existence of Qiyang arc shows that there is north-south compression while pushing from east to west. This also proves that the Indosinian movement of the ancient land block in South China is the result of the east-west and north-south compression during the subduction and convergence of the surrounding ancient land blocks.
The Late Triassic Indosinian Movement of Yanshan Movement turned the stable continent in East Asia into an active continental margin, and the history of transgression in Hunan and eastern China basically ended. The early Yanshanian movement at the end of Jurassic not only folded the Banxi Group of Middle Jurassic in western Hunan, but also led to the subsidence of Mayang-Yuanling area on the west side of Xuefeng Uplift, forming the Ma Yuan depression basin. In order to maintain the crustal balance, the upper mantle is depressed at the topographic uplift around the basin, while the upper mantle uplift area is formed inside the basin. After the subduction and convergence of Indosinian, the ancient plates around South China still existed and were strengthened in the Late Triassic-Cretaceous. The crustal movement that occurred during this period was the Yanshan movement. According to Shi Mingkui, the Yanshan movement was mainly in the Pacific plate, followed by the compression-shear convergence environment under the influence of Indian plate subduction. In this environment, a series of NE-trending fault basins were produced in Hunan, and the red beds were formed in Cretaceous-Tertiary. At the same time, Yanshanian magmatism was very intense, including large-scale intrusion into bedrock and small-scale intrusion into rocks and dikes; In addition, some medium-acid and acid porphyry small rocks are closely related to the very important rare and non-ferrous metal mineralization in this area.
During the Yanshan and Himalayan periods, due to the hardening of the southern plate, the structural deformation was characterized by the uplift of basement folds, the melting and pulping of the crust and the development of faults. Therefore, the Mesozoic and Cenozoic sedimentary rocks in Hunan Province since the Late Triassic are mostly terrigenous clastic rocks and argillaceous rocks, accompanied by a small amount of iron rocks, carbonate rocks and evaporites.