Deformation Characteristics And Tectonic Significance Of Sanmen Ductile Shear Zone In The Area Of Northern Guangxi

The Sanmen ductile shear zone,located in northern Guangxi,is 40 km long and 7 km wide.It is composed of a 5-km-wide high-strain main zone and marginal weak-strain zones.The ductile shear zone strikes at NNE and dips toward NWW(255°-340°)at 33°-85°.The early stage of the ductile shearing was kinematically characterized by left-lateral slip and thrusting while the late stage by right-lateral slip and extension.The deformed strata within the ductile shear zone include Hetong Formation of the Danzhou Group and the Neoproterozoic mafic-ultramafic intrusions.Macroscopic ductile deformation structures such as dense penetrative foliation,stretching lineation,and A-type folds are developed in the ductile shear zone.Micros-structures such as grain-size reduction,quartz-grain undulose extinction,dynamic recrystallization and S-C fabrics are common.Magnetic fabric analysis of the mylonite within the Sanmen ductile shear zone shows that 18 magnetic fabric measurement stations(out of 28)show flattening strain type,accounting for 64.3%of the total;2 stations show plane strain type,accounting for 7.1%of the total,and 8 stations show constrictional strain type,accounting for 28.6%of the total.Based on P values,the ductile shear zone is divided into the high-strain zone(P?1.10)and the weak-strain zone(1.10>P?1.05).On cross-sections,the strain is higher in the center of the ductile shear zone than its southeast and northwest margins.The strain pattern shows that the shear zone in the south has higher strain than the middle and the north section of the shear zone.Kinematic vorticity calculation shows that,among 15 sampling stations within the shear zone,11 stations have W_k value higher than 0.71,accounting for 73%of the total.The result shows that the ductile shear zone is dominated by simple shear.LA-ICP-MS U-Pb geochronological analysis of hydrothermal zircon grains collected from the shear zone yielded a 441.0±2.0 Ma age,suggesting that the Sanmen ductile shearing took place during the collision collage process between the Yangtze and Cathaysian blocks in the Early Paleozoic Caledonian orogeny.Kinematically,the collage is presumed to be a thrusting process with the Yangtze block thrusted southeastward over the Cathaysian block.