Early Paleozoic Tectonic Evolution Of The Ordos Basin And Geodynamics

Based on the plenty of seismic data,drilling well data,field data as well as geochemical data,we analysed the evulotion of the Central Paleo-Uplift in Ordos Basin,the characterisitcs of the structural,sedimentary and paleo-environment in the Ordos Basin during the Early Paleozoic,and the geodynamics via the structural analysis technique,mountain-basin coupling analysis technique,and dynamic-systematic – comprehensive analysis technique,respectively.We drawed some conclusions as shown below:the Central Paleo-Uplift was formed in the Early Paleozoic,and its evolution can be divided into five stages: The first one was the Early Cambrian,the shape of the Central Paleo-Uplift developed under the influence of the pre-structural framework;The second stage was form the Middle Cambrian to the Late Cambrian,the uplift of the Central Paleo-Uplift was weak,so the Middle and Upper Cambrian developed.However,under the influenced the Huaiyuan Movement at the end of the Late Cambrian,the Central Paleo-Uplift uplifted with small sacall;The third one was the Early Ordovician,affected by the Huaiyuan Movement,the Central Paleo-Uplift uplifted with a middle scall,the there was no Lower Ordovician formed;The fourth stage was the Middle Ordovician,the Central Paleo-Uplift descended gradurally,until into the sea water;The last stage was the Late Ordovician,influenced by the Caledonian movement,the Central Paleo-Uplift uplifted dramatically,the stratums on the Central Paleo-Uplift was corroded,the final shape of the Central Paleo-Uplift was decided.Abundant faults were developed in the Lower Palaeozoic,which can be divided into four types according to the trending,they are NE trending,NW trending,NS trending and EW trending,respectively.Meanwhile,the faults can also be ranked four grade basded on their scall,which are second,third,fourth and fifth grade,respectively.And the former two controlled the latter two.There are three types of structural style formed in the Lower Palaeozoic in the southern and western margin of the Ordos Basin,which are extend style,extruding style and inverted style,respectively.Furthermore,the diferent style distributed in the different stratums,this kind of phenomenon correspond with the basin evolution.We belived that the initiation of Huaiyuan Movement in the Ordos region was from the end of the Late Cambrian,and the whole process of the Huaiyuan Movement canbe divided into two episodes,the first one was at the end of the Late Cambrian,the second one was occurred at the end of the Early Ordovician.The sedimentary features was obviously controlled by the tectonic characteristics in Ordos Basin.And the evolution of the Central Paleo-Uplift affected some special sedimentary formation.We also analysed the Early Paleozoic paleo-climate and paleo-weathering of western and southern margin of the Ordos Basin through Sr/Cu and CIA index.The results shown that the paleo-climate was wet and warm during the most of period,but feateured by two special stages,which were the Late Cambrian to the Early Ordovician,and Middle Ordovician to Late Ordovician,respectively.During these two stage,the paleo-climate changed from wet and warm to dry and hot.We considered that these two special stage corresponded to the Huaiyuan Movement and Caledonian movement,respectively.The geodynamics of the Ordos baisn variaed with time and region.From the Cambrian to the Middle Ordovician,the geodynamics of the southern margin and middle and south parts of the western margin was affected by the Qinling Ocean,the tectonics environment was the passive continental margin environment.But the north part of the western margin was cotronlled by the North Qilian Ocean during this stage,the tectonics environment was the arc environment.During the Late Ordovician,the trench-arc-basin system was formed in the both margin,the tectonic environment was the active continental environment.