Deformation And Vorticity Analysis Of Faku Ductile Shear Zone,Liaoning,China

From late Paleozoic to early Mesozoic the north margin of North China Plate had experienced a series of geological processes,such as the subduction of the Palaeo-Asian Ocean plate and the collision orogeny and the post-collisional extension.In that geotectonic background Faku ductile shear zone(FKS)occurred in the middle-lower crust at north Liaoning,China.The intrusive rocks of middle Permain and early-late Triassic were mylonitized into SL tectonites and L tectonites.Microstructure and quartz c-axis analysis of tectonites indicates that FKS showed top-to-the-north ductile shearing during middle Permian,and top-to-the-south ductile shearing from early Triassic to middle Triassic,and top-to-the-southeast ductile shearing during late Triassic.The stress is very important to mineral deformation and distribution.The micas concentratly distributed in mylonite foliation of SL tectonics,and of which rheology behavior was similar to lamina flow.However,In L tectonics mineral grains formed into tube structures,and the rock rheology behavior was similar to pipe flow.The temperature is very important to minerals ductile deformation.Quartz is in the ductile field in middle-lower crust(about 550-650?),of which major dynamic recrystallization mechanisms are subgrain rotation(SGR)and grain boundary migration(GBM),and its deformation depend on cylinder <a> glide.While feldspar is in the transformation stage from brittle field to ductile field,and its major dynamic recrystallization mechanism is bulging(BLG).In middle crust(about 400-550?)quartz is in the ductile field,While feldspar is in the brittle field.The major dynamic recrystallization mechanisms of quartz are BLG and SGR,and its deformation is depended on prismatic <a> glide.Through the kinematic vorticity(Wk)analysis we can know that FKS is characterized by thinning in thickness,and the shear zone was in a changing stress environment.From middle Permian to middle Triassic Wk is more than 0.85,and late Triassic Wk is about 0.75.This suggests FKS experienced another different geological process during late Triassic.We also get different Wk values in different part of the shear zone at the same stage,this is the result of some local geological structures in the shear zone,for example Wk is 0.70-0.766 near the dome structure,and Wk is more than 0.66 or less than 0.90 in L tectonics.Zircon LA-ICPMS U-Pb dating reveals that the age of Gaoligou intrusions is 264.7±3.6Ma(MSWD = 1.3),Shijianfang intrusions is 253.9±4.3Ma(MSWD = 3.3)and244.0±3.0Ma(MSWD = 1.9),Lishugou intrusions is 242.1±2.0Ma(MSWD=1.2).Combining the evidence from microstructure,quartz c-axis analysis of tectonites,kinematic vorticity analysis with Zircon LA-ICPMS U-Pb dating,We can get the following conclusions: In middle Permain the subduction of the Paleo-Asia Oceanic plate leaded to Faku ductile shear zone in middle-lower crust.NNE-direction shear movement was marked by lineations in Wulongshan intrusions and Gaoligou intrusions.In early-middle Triassic the Paleo-Asia Oceanic plate broke away and leaded to post orogenic extension,which was began at 253 Ma and continued to middle Triassic at least.Lineations in Shijianfang intrusions and Xiaofangshen intrusions indicated that the sense of shear movement then and the past were aligned.The age 253 Ma could stand for the end of the Paleo-Asia Oceanic plate evolution at the north margin of the North China Plate.In late Triassic,the ductile shear zone had experienced another geological evolution process.