Abstract: An E W trending gigantic ductile shear zone was found during regional geological survey of the 1∶200 000 Qüxü Sheet. According to its geometric form and microfabrics, this shear zone has the nature of a normal shear zone. It is the product of collision of the Indian plate and Eurasian continent during the late Eocene （40～42 Ma B.P.）. The tectono magmatism in the study area resulted in the décollement in different levels of the earth’s crust and formation of a ductile shear zone. This gives relatively reasonable explanations of crustal thickening, tectonic deformation and geophysically indistinct multi layered structure.