Abstract: Data of telluric electromagnetic sounding illustrate that there is a sliding plane with highly electric conductivity developed at the depth of 3～5km in the southern margin of the north China plate,In accordance with the directional structures of mylonites,the occurrence of major fold axis plane and sense of tectonic windows and napping emplacement in the Dabie uplifting area,it is suggested a large-scale of southward detachment should have developed along the upper crust of southern margin of the north China plate since the late Hercynian period.Till Yanshan period, this movement was blocked by severe uplifting of the Dabieshan,resulted in the development of northward Dabieshan overthrust system.