Caledonian Structural Characteristics And Dynamics In South China

This paper selects some regions with many typical Lower Paleozoic strata and intensive Caledonian orogeny. The regions mainly include the Yuanbaoshan-Yuechengling area, the Damingshan-Dayaoshan area in Guangxi province, the Sandu area in southeast Guizhou province, the Lianyuan area in central Hunan province, the Hunan-Jiangxi border area and so on. By field geological observation, indoor geological analysis and geophysical data interpretation, this thesis detailedly describes and analyses not only the formation of folds and fractures but also the spatial-temperal distibution of angular disconformities and magmatism in these regions.The major conclusions are presented as follows:1) Two-stage Caledonian deformation has been identified. The main structures of Caledonian have E-W-trending at early stage, while the NE-NNE-striking fold belts of Lower Paleozoic strata are formed at late stage. The Caledonian orogeny is weaker west of the Xuefeng Mountains, where deformation mechanism is similar to Micang Mountain range. Both the mountains have a similar foreland-like basin in front of them, but the formation ages of the two basins are different. The Caledonian deformation is constrained at the east of the Xiushui-Yuanling-Mayang-Sandu line.2) The'Caledonian deformation is considered as intercontinental geological setting. Analysis of Caledonian granite and palaeogeography data imply that the Caledonian Movement in South China is intracontinental event, rather than ocean-subduction-related, nor continent-continent collisional event.3) The Caledonian deformation regime is a collision among multiple blocks. At the early stage of Caledonian orogeny, the E-W-striking tight fold belts of Cambrian strata in the Damingshan-Dayaoshan area are caused by the Late Cambrian-Early Ordovician nappe thrusting and pushing between the Yunkai Block and the Guangxi-Yunnan-North Vietnam Block as well as the Yangtze Block from south to north. At the same time, the northern margin of the Yangtze Block is compressed by the extension of the Qinling Block. There are some E-W-striking broad-tempered folds in the Yangtze Block, which are caused by the force from the north and south ends. The deformation strength decreases gradually from south to north at this stage. At the late stage of Caledonian orogeny, the NE-NNE-striking fold belts of Lower Paleozoic strata in the Yuanbaoshan-Yuechengling area and Hunan-Jiangxi border area as well as the eastern margin of Xuefeng Moutains area are caused by the Late Ordovician-Early Silurian shortening of the Cathaysian Block to the Yangtze Block along the Chenzhou-Lingwu fault zone. The E-W-striking folds of early stage are superimposed by this deformation, whose strength decreases gradually from east to west. Meanwhile, a large number of planar distribution of Caledonian S-type granites in South China and the tectonic-magmatic-thermal event in Yunkai Mountains area are also caused by this shortening.4) Caledonian thrust system is clarified first. In the eastern margin of Xuefeng Moutains, the Jingzhou-Xupu fault, the Sanjiang-Rongshui fault as well as the Longsheng-Yongfu fault are characterized by the eastward thrusting, however, tie Chengbu-Xinhua fault that on the east side of them is characterized by the westward thrusting. Therefore, ramp-triangle zones develop in this place. Besides, owing to the blocking of the Xuefeng Moutains, some folds of Lower Paleozoic strata whose ax al planes are west-inclined form the back-thrusting.5) The Caledonian paleo-reservoirs are mainly distributed west of the Xuefeng uplift belt. The hydrocarbon source rocks are chiefly Lower Cambrian and Lower Silurian.The hydrocarbon accumulation has a characteristic of two-step, one step is Caledonian epoch, another step is from Indosinian to Early Yanshan epoch. However, LateYanshanian and Himalayan epoch deformation reformed many of oil-gas reservoirs into paleo-reservoirs. It is hardly to find reservoirs in the margin of the uplift belt, while it is possible to discover oil-gas reservoirs in the place where is far from uplift and the preservation conditions are favorabale.