Late Cenozoic Tectonic-sedimenatary,Uplifting And Denudational Process Of The Yuncheng Basin And Northern Gushan Mountain

Under the influence of the uplift of the Tibet Plateau and the westward subduction of the Pacific Ocean plate,the geomorphological pattern of China has significant change,and step-like geomorphology of high in the west and low in the east gradually emerged.In the western North China,a series rift basin has formed around the Ordos Block including the Yinchuan-Jilantai Basin in the west,Hetao Basin in the North,Weihe Basin in the south and Shanxi Graben System in the east.A shared characteristics among these basins is the formation of extremely thickness of Cenozoic strata in very short time interval.The thickness of Cenozoic strata in Yinchuan Basin yields 7000 m,Hetao Basin yields 14800 m,Weihe Basin yields 8000 m and Shanxi Granben system yields 6000 m.In the eastern Ordos,Shanxi Graben system display distinct characteristics that it consistes of s series of NNE trending oblique rift basin including Datong,Xinding,Taiyuan,Linfen and Yuncheng from north to south.At the same time,with the uplift of Taihang Moutain,the eastern North Chian Experienced experienced long-term extension,broad extensional rift and depression basins formed and began to accept sediments.Especially since the integration of the Yellow River,extensively denudation process of the western part of North China bagan,and accumulation of vast amount of sediments formed a huge alluvial plain in the eastern part of North China.Study and limitation on the uplift-denudation-transportdeposition process between the western and eastern North China is of great significance for understanding the late Cenozoic surface processes in North China.Yuncheng basin is located in the south of Shanxi graben system,and the thickness of Cenozoic strata in the deepest part is more than 5000 m.The Gushan Mountain on the north side of Yuncheng basin is rather attracting because of its relative elevation of the more than 700 m above the surface.Consideing the depth of Cenozoic strata of 300 m on the E'mei Platform and emplacement depth of Gushan rock body obtained in this paper at 2.1-3.3 km,the uplift height of Gushan Mountain is at least 3.1-4.3 km.Gushan Mountain is a completely exposed granodiorite at present,indicating that the Precambrian,Paleozoic and Mesozoic rocks surrounding the Gushan Mountain have been completely eroded.Except that huge amount of material that deposited in the Yuncheng basin itself,most of them must be transported by the Yellow River to the North China Plain.When did the Yuncheng basin begin to develop? When did the rapid uplift of Gushan Mountain occur? When did the significant erosion process take place? Therefore,the study of the late Cenozoic tectonosedimentary and the denudation process of Gushan Mountain on the north side of Yuncheng basin can be used to discuss the performance of the Tibet Plateau tectonic domain and Pacific tectonic domain in the middle of North China block,and the formation and development of Shanxi graben system.It is of great significance to understand the uplift-denudation-transport-deposition process in the eastern and western North China during the late Cenozoic.The author participated in the 1:50000 geological mapping work of Shangguo Sheet(I49E005012)and Yunchengxian Sheet(I49E006012)of Chna Geological Survey and carried our systematical study on the strata and tectonics of Yuncheng Basin and Northern E'mei platform during the doctoral work.On this basis,we carried the strata sequence division work on the SG-1 borehole in the Yuncheng Basin.Then detailed sedimentary facies analysis and high-resolution magnetostratigraphy study has been done and the sedimentary evolution history of the late Cenozoic basin was discussed.Furthermore,through multi environmental sensitive proxies,the effects of tectonics and climate on the sedimentary process were analyzed.The evolution process of geomorphology and drainage systemin the northern margin of the basin has also been studied by means of detrital zircon U-Pb dating method.On the other hands,we have studies the uplift and denudation history of Gushan Mountain by means of low temperature thermochronology,and also studied the erosion rate of Gushan Mountain cosmic nuclide chronology.The main conclusions are as follows:(1)The magnetostratigraphy of SG-1 revealed that the Yuncheng basin began to form since 9.1 Ma,which is precisely the time when the Qinghai-Tibet movement occurred and suggest that the formation of Yuncheng basin,even Shanxi graben system is closely related to the uplift and northeastward expansion of the Tibetan Plateau.In another way,the remote effect of the uplift of the Tibetan Plateau has reached the central North China Craton at least at 9.1 Ma.The sedimentation rate or sedimentary facies of the basin changed significantly at 3.6 Ma,1.2 Ma and 0.2 Ma and was consistent with the occurrence of the QinghaiTibet movement(stage A),Kunhuang movement and Gonghe movement,indicating that the uplift and NE extension of the Qinghai-Tibet Plateau have always controlled the development and evolution of the basin and implying the formation of Yuncheng Basin,even the Shanxi graben system and the graben systems around the Ordos Block have close genetic relationship with the uplift and NE extension of Tibetan Plateau.(2)The northern part of the late Cenozoic basin is dominated by fluvial deposits,and the changes of tectonic activity and erosion base level played a leading role in the evolution of the sedimentary environment of the basin.The environmental sensitive proxies(grain size,chromaticity and magnetic susceptibility)of SG-1 indicate that climate also had an important influence on the deposition of Yuncheng basin.U-Pb chronology of detrital zircons indicates that the sediments in the northern Yuncheng basin mainly come from the eastern block of the North China Craton.Due to the continuous extension,the Fenhe River was formed at about 3.6Ma,and the related deposits were revealed in the ND-1 well in the middle of the Emei platform.The Fenhe River channel appeared in the eastern part of the Emei platform since 0.72 Ma,and the Fenhe River completely withdrew from the Yuncheng basin at about 0.20 Ma.(3)The uplift and denudation process of Gushan Mounain is an important aspect which restricts the formation and sedimentary evolution of Yuncheng basin.Based on the Al-in-hornblende Barometry,this paper calculate the emplacement depth at 2.1-3.3 km.The Gushan mountain has a altitude of 1411 m at present and is about 700 m from the surface of Emei platform.Considering the Cenozoic strata depth of Emei platform of ?300 m,the minimum height of uplift and denudation of Gushan granodiorite body is about 1000 m.Coupled with the emplacement depth,the range of crustal uplift in Yuncheng area in Meso-Cenozoic may be as high as 3.1-4.3 km.(4)The AFT and AHe data reveal two rapid uplift and denudation events of Gushan Mountain at 120-90 Ma and 50-30 Ma.We believe that Gushan Mountain has uplifted close to present heigjt at about 30 Ma.The results of provenance analysis show that Gushan granodiorite body may have been exposed to the surface before 8.7 Ma.There are a large number of detrital zircon ages similar to granodiorite of Gushan Mountain at the depth of 629.5 m(?8.7 Ma)in SG-1 borehole,and a sedimentary layer rich in granodiorite detritus from Gushan Moutain is developed in ND-1 well at the depth of 143.2 m(?3.6 Ma),indicating that the surroudding materials have been eroded completely and the granodiorite has been directly exposed at least8.7 Ma.Considering that this time is close to the formation time of the basin,we speculate that the Gushan granodiorite has been completely exposed before the formation of the Yuncheng basin.(5)The late Cenozoic sedimentary process in the Yuncheng basin,and uplift and denudation process of Gushan Moutain clearly reflect that the formation of the Yuncheng basin on the eastern margin of the Ordos basin is closely related to the uplift and eastward expansion of the Tietan Plateau.The initiation and development of the basin has been restricted by the tectonic movement of the Tibetan Plateau.The Gushan granodiorite is exposed at 700 m above the surface,which on the one hand indicates that the crustal uplift in Yuncheng area of Meso-Cenozoic may be as high as 3100-4300 m.On the other hand,it also implies the strong denudation brought by running water.The surrounding rock during the emplacement of Gushan granodiorite was basically eroded during 30-8.7 Ma.Except for accumulation in the Yuncheng basin,a large amount of sediments were transported and deposited into the North China Palin and formed huge Yellow River alluvial fan.(6)The results of cosmic nuclide dating on samples from the top to the bottom of Gushan Mountain show that Gushan Mountain expericed a strong erosion process since 26.5-39.5 ka with high erosion rates(16.3-19.6 mm/ka)which were close to that of the areas around the Tibetan Plateau.This may be due to the decrease of the regional erosion base level that caused by the integration of the Yellow River in the late Pleistocene,and the regional geomorphology was finally shaped during this period.