Cambrian Synsedimentary Fault System And Its Formation Mechanism In Southern Ordos Basin

Because of the complex tectonic superposition,the early Paleozoic strata in southern Ordos basin were severely deformed and exposed sparsely,and it also makes the study about deep structure of early Paleozoic is still very weak.In order to reveal the deep structure in Cambrian around the southern basin,and discuss its relationship with the evolution of Erlangping back-arc basin,this paper focuses on the characteristics of structure and deposition based on the detailed field survey in outcrops region combined the interpretation of seismic profiles in coverage region.The geological data and seismic data indicate that there is a set of NE-trending extensional fault system developed in the Cambrian around the southern Ordos basin,and formed some small-scale EW-trending newly-generated faults in the local area.These faults composed a graben-horst assembly with high dips and different inclinations,controlled a uplift along Qishan-Zhengning-Fuxian area in the southwest.Faults have obvious characteristics about synsedimentary and multiple activity.Combined with sedimentary facies and formation thickness variation obtained from drilling,we consider the active time of this fault system should from early Cambrian to the XuZhuang period of middle Cambrian.The fault activity from early Cambrian also controlled the development of the rift in the southern basin.Tongchuan and Lingtai rifts were formed respectively on both sides of the Qishan-Zhengning-Fuxian platform.They extended along the NE-trending to the interior of the basin and controlled the deep-water deposition.Two rifts converged after passing through Fuxian area with its size and sedimentary thickness decreasing gradually.Erlangping back-arc basin was formed at 524-500Ma along the south Ordos basin due to the back-arc extension,which is coupled with the development of the extensional fault system in terms of activity time and stress.That means the formation of this fault system was controlled by the extension of Erlangping back-arc basin.Researches show that there were a lot of NE-trending pre-existing basement faults in southern Ordos basin.Subjected the SN-trending extensional stress of the back-arc basin during the early Cambrian to the XuZhuang period of middle Cambrian,these pre-existing basement faults were under an oblique extensional environment,that resulted the inherited activities of pre-existing faults and the form of the small-scale EW-trending newly-generated faults in the local area simultaneously.In addition,the fault displacement in the fault system increases gradually to the south,we speculate the location of present Weihe graben may develop the innermost rift at that extensional event.