关键词: 走滑断层, 剪切构造, 挤压阶区

Abstract: Field investigation and airphoto interpretation have revealed that shear structures were developed in two compressional stepovers at Menggu on the northern segment of Xiaojiang fault zone and Dayingpan on the eastern branch of the fault zone. The shear structures appear on the earth surface mainly as fault troughes,fault scarps,surface fissures,beaded ponds and depressions. The inner shear structures in Menggu stepover extend about 5km,while those in Dayingpan stepover for about 3km. The results of high pressure and high temperature triaxial compression experiment,finite element analysis of fault dynamics of en echelon fault system and the deformation modeling of en echelon faults,have also indicated that the inner shear structures in compressional stepover are the same as those in the extensional stepover. They are newest active structures that are formed in stepover due to the action of local shear stress during the long term activity of the strike slip fault zone. The innner shear structures are intermittently outcropped on the earth surface. However,they are geomorphically less distinct than the shear faults controlling the boundary of the stepover. The inner shear structures in stepover display obvious recent activity. Historical and modern earthquakes often produced surface fractures along these structures. The surface fractures produced by the Dongchuan M7³/4 earthquake of 1733 were distributed just along the inner shear structures of Menggu and Dayingpan stepovers.

Key words: Strike slip fault, Shear structure, Compressional stepover