Evolution And Structural Characteristics Of Zijingguan Fault Zone: An Example From Northern Part Of Taihang Mountains

Zijingguan Fault Zone is the main structure in Taihang Mountains that are located in the central part of 20 ~30 NE-striking Daxing'anling-Taihan-Wuling tectonic belt. It is composed of two diagonalized faults that are Wulonggou-Shanghuangqi Fault located in the west and Zijingguan-Lishan Fault in the east. Since the fault zone had been activated in the later Paleozoic, it controlled the evolution of magmatism and metallization belt in Taihang Mountains. Our field workspace is in the northern part of Taihang Mountains, which is the diagonal area of two main faults. Detailed structural information and specimens of tectonic rock had been collected in the outcrop sections along the fault zone, and deformation feature and superposition about the tectonic rocks were analyzed, and paleotectonic stress field was also analyzed by stereograms illustrating of joints in the following work.Zijingguan Fault Zone is characterized with cataclasite series of tectonic rocks and joints alongside the deformational zone, and sometimes with uniclinal flexure or folds. The intensity of deformation increases from the sides to the center of the fault in the section, which appears from joints to cataclastic rocks. However, there are joints without any tectonites at the end of the two main faults. The NNE-striking linear structure resulted from the main fault is sharp-cut in the TM image. The cataclastic rocks appeared in the fault zone include structural lenticles, fault breccias, protocataclasite and cataclasite. The microscopical cataclastic deformation mechanisms involved are microcracking, frictional sliding and pressure-solution, but there might have had ductile deformation relating to dislocation sliding in feldspar and quartz of the plutons. Three stages of deformation can be distinguished from the cataclastic superposition of matrices that infilled the microcracks in cataclastic rocks, and two phases of paleotectonic stress field from the typical conjugated shear joints by stereograms illustrating. Those two phases of conjugated shear joints are differ from each other in spatial distribution, which is a reflection of the spatial evolution of the fault zone.It can beinferredfrom the information about macrostructure, superimposed deformation of tectonites, paleotectonic stress field of joints and other related documents that there had been three stages of significant activity since Zijingguan Fault Zone was activated. The first stage was about from middle Juiassic to early Cretaceous, when the fault zone activated left-laterally under NW-SE striking transpression. The fault controlled the magmatism and metallization relevant to the contact zone, and deformed successively and produced cataclastic rocks and two sets of joints; The second stage was from late Cretaceous to Eocene. The fault zone deformed right-laterally under NE-SW striking transtention, and new joints zone was produced at the end of the two main faults because of local stress concentration. The fault produced inchoative cataclastic rocks, and two sets of typical joints in plutons; The third stage of deformation was relatively faint and only a little of microcracks was produced and infilled with veins in the deformed rocks, which was during the Himalayan Movement.