Basin Evolution And Geodynamics Of Northwestern Upper Yangtze Block During Late Paleozoic To Mesozoic

The Mt.Micangshan locates on the north margin of Upper Yangtze Block south to Qinling orogen.It is a part of the foreland fold-and-thrust belt of Qinling.Thus,its sedimentary and deformational evolution is inferred to be closely related to the tectonic evolution of the Qinling orogen.The Late Triassic collision between North China Block(NCB) and South China Block(SCB) and consequent uplift of Qinling which stands for the final collision was widely accepted by most geologists. Nevertheless,few works have focused on the history before the final collision along the whole northern margin of SCB.Thus,we took research work focusing on the Late Permian-Early Triassic sedimentary and deformation evolution to discover the pre-final-collision history of the Qinling orogen.Some new evidences and understanding are concluded as below:1.The three formations of Lower Triassic in northern Sichuan basin are Feixianguan Fm.,Tongjiezi Fm.,and Jialingjiang Fm.,respectively from the bottom up and are thought to be conformable contact with each other in the previous literature.Our research work find that there exits an unconformity within the Lower Triassic characterized by weathering crusts,conglomerates, breccias,sandstones etc.2.The unconformity can been found in the seismic profiles south to Micangshan characterized by truncation and onlap phenomena near the bottom of the 2^nd See.of Jialingjiang Fm.In our research scale,the unconformity in the seismic profiles and in the field can be considered as contemporaneous.Thus,we infer that the Early Triassic tectonic event resulted in the formation of the unconformity.3.By analyzing the spatio-temporal configuration of the Kaijiang-Liangping rift—Hanhan-Micangshan uplift—Foping dome trinity and Chengkou-Exi rift—Shennongjia-Huangling terrane—Wudangshan terrane trinity,we proposed that the Kaijiang-Liangping and Chengkou-Exi rifts on the northern margin of SCB which strike approximately orthogonal to the Qinling orogen are related to the early collision by South Qinling Block and SCB.In other words,they are twin impactogens(collional rifts).4.On the basis of the research work mentioned above,with a synthetic consideration of the whole tectonic evolution of Qinling,we proposed the tectonic evolution modal of Micangshan area from P₂-T.5.Integrating evidences from sedimentary,seismic profiles,AFT data,and paleomagnetic data,we put forward that the Mt.Micangshan was uplifted in the Early Cretaceous.The uplift of the mountain itself took place before that of the basin south to Micangshan which was thought to happen in the Late Cretaceous.Furthermore,the apparent uplift rate of the Mt.Micangshan is greater than that of the basin area south to it.