Stratigraphic Records Of The Dynamic Uplift And Structural-sedimentary Evolution Of The Emeishan Large Igneous Province

The Emeishan basalts are the only large igneous province recognized by the international academic in our country,much and fruitful work has done to analyze the eruption environment and formation mechanism of Emeishan large igneous province(ELIP)from petrology,geochemistry and isotopic chronology.However,some key problems are still not clear up.Based on the detailed field investigation and laboratory analysis,sedimentary profiles in research area have been studied and compared on the theory of sedimentology,provenance analysis,isotope dating and biostratigraphy.The new results and progressions as follows:(1)The sedimentary environment in the Maokou stage was not uniform shallow-water carbonate platform facies but rather sedimentary facies with a north-south linear alignment and west-east differential distribution controlled by syn-depositional normal faulting along Changhai fault and Xiaojiang fault.The deep-water depositions and event sediments of trench facies and slope facies developed to the west of the Changhai fault,and abundant siliceous nodules and silicalites developed in the middle-upper limestone stratigraphy of the Maokou Formation on the eastern side of the Xiaojiang fault,whereas bioclastic limestone of carbonate platform facies occurred in the Central Region.The syn-depositional tectonic activities and sedimentary facies differentiation are the result of dynamic uplift induced by deep mantle activity.Based on biostratigraphy,the initial response time to dynamic uplift was Neoschwagerina simplex zone,which is 10 Ma before the main eruption of the Emeishan large igneous.(2)The provenance analysis of conglomerate samples in the Pingchuan section and Daqiao section indicates the Pingchuan conglomerates formed before the Emeishan basalts eruption,and small-scale underwater magmatic activity had begun in the middle and late Maokou stage on the western side of the Chenghai fault(zircon U-Pb ages,267±3Ma);whereas the Daqiao conglomerates formed during the Emeishan basalts eruption and was the product of proximal erosion and accumulation controlled by syn-eruptional intense normal faulting of Xiaojiang fault,the provenance horizon contained from the double basement of Yangtze plate to limestoneand basalts in Permian.Therefore,the eruption time and environment of Emeishan basalts are also controlled by the activities of the Changhai fault and Xiaojiang fault,the eruption environment changed progressively from marine in the west to terrestrial facies in the east.(3)Based on the thickness and biostratigraphic comparison of Maokou limestone sections through out the whole ELIP,we can find thinner but complete fusulinid zones in the Maokou Formation were well preserved in some areas of the Central Region because small-scale subsidence occurred along deep fractures and secondary faults,although extensive surface denudation taken place after the deposition of Maokou formation in the whole Central Region.As a consequence,the thinning of the Maokou limestone was the product of the different initial depositional thickness caused by the underwater uplift and post-depositional surface uplift and erosion,but the post-depositional uplift was much smaller than the km scale.Based on these geological evidences,we summarized the model of the Permian tectonic-sedimentary evolution in the ELIP.