Characteristics and evolution of thrust nappe structure in the Xiangshan-Tianjingshan area, central-western Ningxia

The thrust nappe structure in the Xiangshan-Tianjingshan area, Ningxia, consists of the Tianjingshan northern piedmont fault zone (main detachment fault) and Xiangshan-Tianjingshan thrust sheet (which may further divided into nine second-order thrust sheets). The front of the thrust sheet is composed of a series of nearly parallel listric, imbricate thrust nappe faults and its tectonic deformation is strong, while tectonic deformation inside the thrust sheet weak. The thrust nappe as a whole underwent Yanshanian and Himalayan compression and nappe-thrusting. In the Yanshanian period the principal compressional stress σ1 was oriented in a 44° direction and the nappes was thrust northeastward with a total displacement of 26.6 km. In the Himalayan period, the principal compressional stress σ1 was oriented in a 75° direction and the nappes was thrust from SWW to NEE. Since the late Pleistocene, there also occurred left-lateral strike-slip motion with a total displacement of more than 4 km.