The Components and Structures of the Zhangxian-Chechang Fault Zone in the North Margin of West Qinling and Its Deformation History
西秦岭北缘断裂带漳县—车厂断层的结构及构造演化

The north margin fault zone of West Qinling is one of the tectonic boundary faults and earthquake faults in the northeast margin of Tibetan plateau. Its structural characteristics and deformation history can provide important constraints on the Mesozoic-Cenozoic tectonic processes of West Qinling and the remote tectonic responses to the Indian-Europe plate collisions. Based upon the tectonic observation and analysis of Zhangxian-Chechang fault zone which was uncovered by engineering construction and is located in the central segment of the north margin faults of West Qinling, the authors identified three phases of deformation events with different properties, scales, and kinematics. The first event is characterized by steep NNE-dipping extensional normal faulting. According to the relationship between Cretaceous red bed basin and the fault, it is suggested that this normal faulting event documented the Cretaceous regional crustal extension and development of the red bed basin. The fault gouge with a lot of grinding gravels and well-developed faulting foliations indicates its long duration, probably lasting to the Late Oligocene, i.e., 29 Ma when Linxia basin began developing. The second event is characterized by south-dipping inverse faulting, which led to the folding and thrusting of the Cretaceous red bed strata, possibly indicating the transition from crustal extension to contraction. This tectonic transition probably suggests that the India-Europe plate collision might have been spread to West Qinling, and strong thrusting should have occurred at 3.6 Ma when Jishishan and Wuquanshan conglomerates occurred. The third event is characterized by sinistral strike-slipping along the nearly vertical NNW-striking fault plane, which began in Late Quaternary and has continued up till now. Zhangxian-Chechang fault as one of the north margin faults of West Qinling is a typical brittle fault, which failed to document the large-scale ductile shearing in Indochina orogeny. Therefore it is held that the north margin fault zone of West Qinling, which has been generally believed to be the tectonic boundary fault between the Central Qinling and North Qinling or Central Qilian Block, is a newly generated fault from the Cretaceous instead of the boundary fault in the Indochina orogeny.