The Study Of Sedimentary Characteristic And Tectonic Setting In Eastern OinlingOrogenic Belt

The Qinling belt belongs to the eastern part of the Kunlun-Qinling giant orogenic belt, and is also the suture zone of the Sino-Korean and Yangtze plates. The research on the Qinling orogenic belt would reveal the relationship between the Dabie and Kunlun-Qilian orogenic belts, and is also of great significance to the study of Chinese even East Asian continental crust’s formation and evolution. The dissertation is focused on the Devonian sedimentary basin whose sedimentary environment and tectonic setting is controversial. The study is mainly carried out based on integrated studies of Paleontology and Stratigraphy, Sedimentology, Structural Geology, Geochemistry and Geochronology. Finally the results and new understandings are as follows.1) According to the field investigation, the contact relationship between the Devonian and its underlying Caledonian folded basement is angular unconformity. Because of the characteristic deformation between the above two formations, the fold orogeny occurs in the South Qinling belt, which indicates that the Caledonian movement develops in the whole Qinling belt besides North Qinling.2) The angular unconformity, deformation difference, strata hiatus and Devonian sedimentary sequence show that the Devonian basin is a new continental extensional basin on the Caledonian folded basement growing in the Late Paleozoic but not the successor of Early Paleozoic Qinling ocean basin.3) The sediment in the Devonian basin may derive from the recycled orogenic belt and are likely the passive continental margin with moderate to advanced maturity sediments. The geochemical analysis indicates that the sources of the sediments are mainly quartzite rocks with a few felsic igneous rocks. The study of the heavy minerals and detrital zircon geochronology implies that the Qinling ocean located between the Sino-Korean and Yangtze plates closed before the Devonian whose sources are just right the above two plates.4) The sedimentary environment of the Devonian basin in eastern Qinling is the normal shallow sea platform to shelf facies based on the comprehensive analysis of the sedimentary structures, paleocurrent, rock assemblages and sedimentary facies. From south to north, west to east, it is a process of overlap transgressive sequence during the Early to Late Devonian. The Devonian Liuling group formed in the coastal and shallow sea environment but not the deep-water flysch deposition according to some previous research.5) After the detailed study of the Devonian strata, Mianyuzui-Hetaoyuan and Fengzhen-Shanyang faults’ deformation features in eastern Qinling, the author raises the following views. The Nianyuzui-Hetaoyuan fault experienced two stages tectonic movements which was the extrusion thrust shearing from north to south at the early stage with sinistral strike-slip ductile shearing at the late stage. The Fengzhen-Shanyang fault was firstly a thrust fault from north to south, while during the Late Yanshan period, it was a normal faulting with south inverted to north pour, which controls the Shanyang basin formation. The Fengzhen-Shanyang fault has the left-lateral slip features on the local scale. The N-S compressional folding dominates the deformation of the Devonian formation in the East Qinling belt.6) The rocks from the Qinling group, Wuguan group and Silurian strata yield the metamorphic events’ age of the Early Caledonian (553Ma,489 Ma and 475Ma), the Late Caledonian (418-415Ma and 400-401Ma) and the Variscan (372Ma,335Ma and 315Ma) by SHRIMP and LA-ICP-MS zircon U-Pb dating. Combined with the previous work, the eastern Qinling belt has experienced multicycle orogeny, among which the Caledonian and Indosinian are especially important, followed by the Variscan.7) Based on the above study, it is observed that the Qinling orogenic belt has experienced the following processes. The Qinling ocean closed and Sino-Korean and Yangtze plates collided together in the late Early Paleozoic (430-405Ma). The continental crust was subducted and strike-slip extrusion orogeny in the Variscan (386-261 Ma). Paleotethys shallow sea basin has deposited during late Paleozoic to early Mesozoic (D1-T2). The Indosinian continental crust subduction orogeny appeared in the early Mesozoic (236-183Ma).8) After collection and analysis of previous research results, the author found out the issues to be solved and did detailed work. Finally, the new understandings above are obtained and a 400 km length of geological section is compiled which extend across the whole East Qinling region.