The Characteristics And Geological Background Of Thrust-Nappe Structures In Taoxi District,South Of Anhui Province

The junction region of the Zhejiang,Jiangxi and Anhui Provinces is located in th e eastern segment of the southeast Yangtze Block.There distributes multi-stage Meso zoic thrust-nappe structures,relecting that there had been underwent multi-stage comp ressional deformations.However,the deformation characteristics,assemblage patterns,formation time and geological background are not clear.It is helpful to improve the geological research level of the study area and provide important evidence for the study of geological background and tectonic evolution history.Through detailed on-site investigation,it is found that two period of thrust-nappe structures developed in Taoxi and adjacent areas in southern Anhui.The first period of thrust-nappe structures is constituted by three large faults which are Shaolian-Hengguan-Wucheng,Linxi(Tunxi South)-Sencun and Tunxi North-Xiongcun Thrust-nappe Structure from south to north.All of these thrust-nappe structures are nearly parallel,forming multiple tectonic schist which progressive thrust towards to NW.In this period of thrust-nappe structures,the Neoproterozoic Changqian Formation thrust towards NW over the Middle Jurassic Hongqin Formation,forming a imbricate fan structure that constitutes the hedge structure pattern with the western Zhanggongshan thrust-nappe structure system.The deformation degree of the rock is uneven and formed metamorphic bands with different degrees of deformation.The second period of thrust-nappe structures extends generally in the NE direction which composed of Baishikeng-Yuanfang-Shanglingkou and Shimen-Dalingjiao-Dafu thrust nappe structure.It shows that the Neoproterozoic Jingtan Formation overthrust towards NW over the changqian Formation of the first stage,and cut the first period of thrust-nappe structures,which partially covered directly above the Middle Jurassic Hongqin Formation and was deformed from the SE direction.NW gradually weakens.The first period of thrust-nappe structure in undeformed porphyry veins yields a U-Pb age at 141 Ma on zircon grains,limiting the first period of thrust-nappe formation to occur before 141 Ma,equal to the late Jurassic.During this period,NW-thrusting nappe structures probably result of the low-angle subduction of the paleo-Pacific plate in NNE direction under the South China continentAccording to the relationship between faults,the second period of thrust-nappe structure was superimposed on the first stage and from the perspective of the unconformity of sedimentary strata,the Neoproterozoic Jingtan Formation directly covers the Late Cretaceous strata.Therefore,the age of second period of thrust-nappe structures should be at the end of Late Cretaceous or between late Cretaceous and at the turn of the Mesozoic and Cenozoic.During this period,the thrust-nappe pushes towards NE and thrusts from SE to NW direction,which reflects that the regional stress field is NW-SE extruding and is the result of subduction of NW to South China plate from Pacific plate.