The Regional Tectonic Characteristic And Their Evolution Of Zhenba-Ziyang, Southern Shaanxi

Zhenba - Ziyang area in sorth Shaanxi was an essential spot for Qinling orogenic belt study, which was located at superposition of the northwest margins of Yangtze block and obduction zone of arcuate structure of Daba mountains of south Qinling, and overlapped among three tectonic unions of Qinling orogenic belt, Mianlve tectonic belt and Yangtze block from east to west,and experienced a long-term and multistage tectonic evolution,eventually, formed the sophisticated structure of the material constitutes.Based on the new research findings and progress of the south Qinling orogen belt and the northwest margins of Yangtze block in general,especially the achievements of stratigraphy,petrology,structural geology and isotopic chronological data et al.,and analysis of each stratum of study area,together with coordination of the area in morphologic geometry,structural assemblage,movement study and regional structural background,the structural feature of Zhenba - Ziyang area in sorth Shaanxi was analyzed systematically,and its tectonic evolution was also analyzed in the paper.After concluding the research achievements of predecessors, field observations and indoor treatment systematically,some pieces of conclusions and recognitions about structural feature and tectonic evolution in Zhenba - Ziyang area were obtained as follows;1. The structural characteristics in Zhenba - Ziyang area in sorth Shaanxi were mainly featured as follows:â‘ The representations of tectonic faults were a large-scale angle thrust and a assemblage of fan-imbricate thrust in structure system;â‘¡The axial plane of folds in the thrust-nappes tectonic system mainly displayed east-inclined and its limbs were homoclinal and were reverse closed.â‘¢The structure superimposition characteristic was obvious that the folds and faults displayed for diagonal thrust pushes and right-lateral-slip.And there was a conspicuous superimposition compound characteristic between N-S direction and W-E direction in the Micang Stratum plot.â‘£The pattern of thrust-nappe tectonic expansion in this area was forward propagation from east to west.2. Experienced multistage and multi-mechanism tectonic evolution, the complex structure material composition appearance at present of Zhenba - Ziyang area in sorth Shaanxi had formed. Before Indo-Chinese epoch, the area mainly performed for extensional function and structural environment, and the structure in Chu-River Stratum plot on the eastward of Mianlve-belt and north Daba Mountain area was characterized by large-scale plow-like horst-graben tectonics which consisted of a series of normal faults in the same tendency, companying with synchronous deposited faults. Entered Indo-Chinese epoch, the whole north margin of Yangtze tectonic plate subducted downward to the Qinling tectonic plate from south to north, and the latter thrust upward to the former and formed structural reverse. Meanwhile, the tectonic background transformed from normal fault state under extentional condition to thrust state under compressional condition, and epicontinental collision orogenesis occurred.In Yanshan orogeny, the whole thrust system of Daba mountain obducted toward southwest in a large-scale and formed intracontinental orogenic belt, in which graded progress the major characteristics of present tectonic framework in the area had formed.The whole thrust system became reactive in Himalaya orogeny, which enabled preceding thrust to advanced toward Sichuan Basin continually.