| The mechanism of intracontinental orogen is one of the popular scientific issues in international tectonic geology in recent years.Intense intracontinental orogenesis occurred in the Xuefeng orogen of the South China.The establishment of a tectonic model using the Xuefeng intracontinental orogen has not only great significance for revealing the mechanism of the intracontinental orogen,but also for the constraint and evaluation of denudation history of the ore deposits in the region.In this paper,the central Hunan retrovergent belt,eastern part of the Xuefeng orogen,is taken as the key research area,and the field structural analysis,microscopic structural observation and deep structural interpretation are carried out.Based on the synthetically analysis of paleotectonic stress field and tectonic deformation sequence,this paper focuses on the tectonic evolution and structural transition process of the Xuefeng orogen by means of tectono-thermalchronology and structural physical analogueA detachmental fault with depth of about 10 km and propagation folds,in the central Hunan retrovergent belt,the eastern part of the Xuefeng orogen,are identified by comprehensive geophysical data.Since the Yanshanian,the eastern margin of the Xuefeng orogen has recorded the five stages of tectonic stress field by means of estimation of paleo-stress field.They are the NW-SE compression,the NWW-SEE to extension and NNE-SSW compression,which belongs to the strike-slip tectonic regime,nearly east-west compressive tectonic regime,NNW SSE to extensional tectonic regime,NEE-SWW compression and NNW-SSE extension,which belongs to the strike-slip tectonic regime.By structural analysis,we suggest the central Hunan retrovergent belt experienced three stages of deformation during Yanshanian.They are:(1)stage D31,characterized by the top-to-NW thrusting.(2)stage D32,featured by conjugate strike-slip faults.(3)stage D33,featured as the eastward brittle thrusting and dome group.According to the result of U-Pb isotope dating of the youngest zircon from T3-J1 sandstone samples at Lianyuan,is 176 Ma,the lower limit of the Yanshanian tectonic event in the eastern Xuefeng orogen was constrained.The results of zircon and apatite fission tracks from the central Hunan retrovergent tectonic belt show that since the late cretaceous,the average exhumation rate ranges from 0.06-0.07 km/Ma,and the exhumation thickness ranges from 6.5-6.6 km,of which is twice in the western in thick-thin tectonic belt.A comprehensive analysis of thermal evolution history reveals a rapid cooling event in the east side of xuefeng Mountain at about 90 Ma.However,in stage D33,the tectonic stress field converted to the west-east trending compression stress leads to the large-scale exhumation of the Central Hunan retrovergent tectonic belt,which is the result of eastward thrusting and gradual propagation of the Xuefeng orogen,e.g.exposure of the Dachengshan,Longshan and Zhuposhan dome.The time of tectonic transition and finalizing is regarded at about 90 Ma.The tectonic transition of the Xuefeng orogen towards to the east is related to the obstruction of the Sichuan basin rigid craton.By sandbox analogue experiment and comprehensive analysis of the above research results,we conclude that the essential reason for the propagation towards to the retrovergent tectonic belt is the orogenic taper angle increases up to the critical state.Since the Cenozoic,rapid uplift of about 60-30 Ma and 10Ma is basically consistent in the Xuefeng orogen,which may be related to the India-Eurasia plate collision and the expansion of the South China Sea plate. |
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