THE DUCTILE SHEAR ZONE ON THE NORTHERN MARGIN OF THE CENTRAL TIANSHAN MOUNTAINS ,XINJIANG

A gigantic ductile shear zone has been found on the northern margin of the Central Tianshan Mountains,Xinjiang.It generally extends in a WNW direction as long as more than 500 km with a width of about 3 km.Studies of various kinds of deformation structures ,microstructures and magnetic fabrics indicate that it has undergone strong ductile shear .Finite strain measurements show that the shear was an inhomogeneous simple one with a strain strength of 1.09,a Flinn factor of 0.9,a deformation slope close to 45° and a compression volume up to 59%.The horizontal（strike-slip）displacement of the shear zone is 2849 m and the vertical subduction （dip-slip） displacement of 3455 m with a total displacement of 4482 m.The ductile shear zone was formed in the Late Silurian-terminal Devonian and the shear persisted about 60 Ma.The deformation occurred at 312℃ and 433 bars,in a stress field with σ 1=351°∠11°and σ 3=93°∠29°,at a strain rate of 2.4×10 -5 /sec and with an average strain of 4.5.The origin of this ductile shear zone is related to the collision of the Junggar Paleo-ocean and the Central Tianshan massif.