Analysis Of Mesozoic Tectonic Environment In The Middle Part Of Helan Mountain

The western margin of ordos basin is located in the adjoining part of East-west tectonic in China. Its unique tectonic position and Mesozoic Era frequent activity has aroused extensive attention of many researchers.The conclusion of the large scale united east-west trend and south-north belt tectonic pattern of its early tectonic research has left deep impressions in people's minds.Neverthless, with the development of the research, more and more evidence proves that: The western margin of ordos is not absolutely the simple tectonic pattern. It is necessary to do further analysis and discussions on the western margin of Ordos Mesozoic New Era tectonic environmental differences based on more materials and further studies.Helan Mountain, situated on the western margin of Ordos basin, is characterized by clear Mesozoic New tectonic activities. Further concrete analysis and discussions are not only important for the research of the western margin in Ordos, but also helpful for the prospecting of the region.Based on the analysis of Ordos western margin east-west trend thrusting pattern and the dissecting of several typical geological phenomenon (Ruqigou basalt, late Triassic period-early middle Jurassic period sedimentation feature, Xiaosongshan Mountain etc.) ,the article summarizes and discusses the Mesozoic Period tectonic environment of the middle range of Helan Mountain with reference to the regional characteristic. Moreover it also summarizes the Mesozoic New Period tectonic evolution.(1) The article is based on the regional background analysis and the contemporary researches of the western margin of Ordos. After the analysis and demonstration of several crucial tectonic questions (such as whether the western margin of Ordos block front basin and its united east-west pattern exist or not, horizontal tectonic belt features, south-north division features and the existence of south-north movement etc.), we can draws a conclusion : The western margin of the basin has apparent south-north differences and the tectonic environment and evolution of each segment is apparently different. The south-north united tectonic environment and tectonic pattern does not exist. The east-west trend thrusting Nappe structure of Majiatan and Hengshanpu can not represent the whole western margin of Ordos. The attempt to solve the regional tectonic problem with simple tectonic pattern is against the geological theory, especially in the Helan Mountain region.(2) After the geochemical data analysis of Ruqigou basalt in Helan Mountain, the article illustrates that it belongs to continental rift basalt which appeared in the late-Triassic and early middle Jurassic. Meanwhile the article states that the region is located in the rift environment of an extension background and Rudigou is located in the central part of the rift. (3) The article illustrates that the western ridge of Helan Mountain used to be on a slope and topography slowly declines from west to east after analysis and demonstrations of the distribution under XiaoSongshan bedding fault. Through comprehensive analysis of the no-strong deformation and bedding fault features, the article states that the Xiaosongshan bedding fault used to be gravity slip detachment structure and it represents the extension from the east to the west. Based on the above and analysis and demonstrations of isotope age and track datting, the article illustrates that the ? characteristic of the east of the western ridge of Helan mountain being higher than the west is the result of its late tectonic reverse.(4) Through comparison between Rujigou region Mesozoic Era late Triassic period-early and middle Jurassic period deposit and that of Ordos Basin and the analysis of the late Triassic period extension and early -middle Jurassic Period deposit povenevance it sets forth that during the period, Rujigou region was part of a large deposit basin which was closer to the northern povenevance and farther from the western povenevance.Combining the swell of Helan Mountain and the date of the depression of Yinchuan graben. It states that there was no clear division between the Rujigou region and the inner part of the basin in Mesozoic Era, and it is part of the Ordos basin.(5) It can be concludes that, in Mesozoic Era ,Helan Mountain belongs to east-west region extension bedding and Rujigou used to be the center of the continental rift. Its northern part was confined by Zhengyiguan fault which is the east-west direction Zhengyiguan fault become the northern border of Ordos basin. Its western region is likely a slope extending to Alashan Block.(6) Bassed on all above, the article sets for the that in middle period of Mesozoic Era the middle part of Helan Mountain has experienced five tectonic evolution stages: the east-west stretch in late Triassic -middle Jurassic period, the thrust nappe and tectonic reverse in late Jurassic period, regional sinking in early Cretaceous period, regional swell in late Cretaceous period and frequently and difference pushing in Cenozoic Era.