Role of Detachment Surface in Local Thickening and Shear Folding,Shurab Syncline, Southeast Qom

Lower-red, Qom and Upper-red Formations with Pliocene conglomerates were all folded in Shurab syncline containing northwest-southeast axial trace in southeast Qom city. In the southeastern part of the northern limb of the Shurab syncline, gypsum and marl elastic units of the Qom Formation have played a detachment surface role against the most competent layers of the Upper- red Formation due to high competency contrast. Rock units of the Qom Formation were thickened and locally folded with about north-northeast orientation in the southeastern part of the northern limb of the Shurab syncline. The thickening and folding in this part of the Shurab syncline is interpreted as the shearing process which has been produced in a shear zone initiated between the detachment surface and the Qom fault segment in the northern part of the Shurab syncline that was activated by right-lateral strike-slip displacement of the Qom fault in post Miocene.