Structural Features of Yeba Tectonite Group in Jiama(Gyama)-Qulong Area of Tibet

Based on field geological survey and oriented thin section observation,the authors studied deformation,kinematics and dynamics of the Yeba tectonite group,and reached the following conclusions:① the previously named "Yeba group" is actually a tectonite group and should be called "Yeba tectonite group",which consists of four lithologic sections and can be divided into two volcano-sedimentary cycles;② macroscopically,from north to south,Yeba tectonite group shows an inclined anticlinorium and an inclined synclinorium whose axis trends northward;③ Yeba tectonite group experienced two-stages of ductile deformation and multi-stage brittle-ductile deformation,and the C lithologic section experienced three-stages of foliation replacement and one stage of kink deformation at the superficial level,lying near the main napping plane of Jiama(Gyama)-Kajunguo thrust-gliding nappe tectonic system;④ in Yeba tectonite group,kinematic direction of two-stages of ductile deformation is on the opposite:in the first stage,the upper layer shearing was from south to the north;in the second stage,the upper layer shearing was from north to south.The second ductile deformation and later multi-stage ductile-brittle deformation had multi-stage progressive deformation characteristics,and after southward nappe-fold lifting to the superficial level,Yeba tectonite group also experienced multi-stage brittle-ductile deformation,especially at the position near the main napping plane of Jiama(Gyama)-Kajunguo thrust-gliding nappe tectonic system;⑤ the first stage ductile deformation of Yeba tectonite group occurred around 94-85 Ma,belonging to Late Cretaceous Turonian-Coniacian period and related to back-arc rift stretching that resulted from northward subduction of the Brahmaputra oceanic crust;the second stage ductile deformation and the later main brittle-ductile deformation occurred around 50 Ma in Eocene,related to Indo-Asia collision,and the post collision tectonism resulted in the superposition of multi-stage brittle-ductile deformation.