Late Mesozoic Thrust Fault Movement In Nanling-xuancheng Area Of The Middle-Lower Yangtze Region

The middle-lower Yangtze reigon was under obvious extensional background in the late Mesozoic,formed a series of faulted basins and volcanic basins,accompanied with a lot of magmations and metallogenic events.However,the discovery of the Maoshan nappe structure indicates that there was an obvious compressional event in the late Mesozoic,and the specific time of the compressional event is still unclear.In the southern extension of Maoshan nappe structure,the Jiulianshan anticline outcropped clear late Mesozoic thrust faults.Based on detailed field observations and large-scale profile measurements,with the zircon dating at the key location of the thrust fault,in order to limit the time of the thrust fault movement,and provide evidence for the study of regional evolution history and ore deposits.A fold-thrust deformation belt was formed in Jiulianshan and its surrounding area,the strata involved were mainly Silurian-Triassic.The fold-thrust deformation zone generally strike NE,which is mainly characterized by linear closed folds,low-angle thrust faults,and imbricate thrust faults with low angle and high angle thrust faults intersection.Qiaotouyang pluton is located at the core of Tongling uplift,its dating result is about 141 Ma.Previous dating results of Shatanjiao and Fenghuangshan pluton were about 144-141 Ma.These three plutons were arranged in NE direction and distributed at the core of synclinal,indicating that emplacement occurred after the formation of folds,and the main body of fold-thrust belt was formed before 144 Ma.Late Mesozoic thrust fault movement outcropped in Jiulianshan and its surrounding area.The main fault outcropped in Liqiao and Yangxian area,which shows that Devonian on SE side overlying Cretaceous on NW side,in which fault fracture zones of several meters wide are developed,mainly composed of mudstones on the top of Wutong Formation,indicating stratum inversion.The SE tending folds and two conjugate shear joints are developed in the upper Devonian sandstones.Cleavages developed in the footwall pyroclastic rocks.There are two fault fracture zones of several meters wide below the Devonian at SE side and near the contact position with Cretaceous at NW side.There are tear faults in the pyroclastic rocks which is perpendicular to the thrust faults.The dating result of the pluton in liqiao is about 132 Ma,the dating result of the pyroclastic rocks in Yangxian and north of Xuancheng are about 137 Ma and 134 Ma.The upper Cretaceous are mostly developed nearly horizontally in the study area,indicated that they were not affected by the thrust faults.The late Mesozoic thrust fault movement in Jiulianshan and surrounding area occurred after 132 Ma and before the late Cretaceous.The dating result of the dikes in the fracture zone in north of Xuancheng are about 132 Ma.Combined with the dating result of the footwall pyroclastic rocks,it shows that the thrust movement in north of Xuancheng occurred at about 132 Ma.