Deformation Characteristics And Geological Significance Of The Weiyuanpu-Yehe Ductile Shear Zone

The formation time,initiation mode and formation environment of the northern segment of the Tan Lu fault zone have been widely studied.There are different understandings of the formation time in Cretaceous and Jurassic,and the initiation mode also has divergences of extensional or strike slip shearing.In order to better understand the formation process of the northern segment of the Tan Lu fault,the ductile shear zone found in the Weiyuanpu-Yehe area near the Yilan-Yitong fault is studied.Through field geological survey and thin section observation under indoor microscope,the whole Weiyuanpu-Yehe ductile shear zone is 45°NE along both sides of the graben.Only ductile shear zone is developed in the granitic pluton.The occurrence of mylonitic foliation is steeply dipping and the mineral tensile lineation is gently dipping.The field outcrop and microscopic microstructure indicate that the shear zone is a kinematic phenomenon of left lateral strike slip.The deformation temperature of mylonite in the Weiyuanpu-Yehe ductile shear zone should be about 400-500? based on the comprehensive analysis of the microscopic deformation characteristics of minerals,EBSD quartz c-axis composition and the open angle of the extremely dense diagram.It is a middle temperature metamorphic mylonite.The differential stress of the shear zone is 20 ? 40 MPa calculated by Mtex toolbox of Matlab.It is inferred that the exposed mylonite was formed in the central crust,and no mylonite formed in lower crust was found.Zircon U-Pb dating was carried out by using the deformed rock mass and the undeformed intrusive dyke in the shear zone to limit the left strike-slip activity of the ductile shear zone to occur at 172?163 Ma(late Middle Jurassic),and the apatite U-Pb dating of the deformed rocks indicates that the shear strike slip occurred at 164-162 Ma,It means that the Tanlu fault extended to Northeast China in the late Middle Jurassic to form the Yilan-Yitong fault as the western branch of the northern segment of the Tanlu fault.Combined with the tectonic background,it is inferred that it is the response of the subduction of the paleo Pacific plate and the southward movement of the Siberian plate,which also reveals the start time of the Pacific Rim tectonic domain and the Yanshan movement.