Study On Late Mesozoic Tectonic Deformation In The Middle Segment Of Daqing Mountain,Inner Mongolia

The Daqing Shan area of Inner Mongolia is situated in the middle segment of the E-W extending Yinshan-Yanshan intracontinental deformation belt in the northern margin of the North China Plate.The developments of the late Mesozoic large-scale thrust nappe structures and metamorphic core complex have been extensively investigated.The present study demonstrates that the tectonic patterns of the eastern and western segments of the Daqing Shan are significantly different.However,why the differences in tectonic deformation characteristics between the eastern and western segments formed are still uncertained.As for this problem,we select the key tectonic site of the middle segment of Daqing Shan which connecting the eastern and western parts of Daqing Shan as the research object.Through detailed structural analysis,we determine the Late Mesozoic tectonic deformation sequence and tectonic evolution process in the Louhua Shan area of Daqing Shan middle segment.The causes for the differences in tectonic deformation between the eastern and western segments are analyzed.At the same time,the dynamic source of the dynamic mechanism change is analyzed through the restoration of the tectonic stress field.The influence of the closure of the Mongolian-Okhotsk ocean and the Palaeo-pacific subduction on the geological evolution of the Late Mesozoic in the study area was discussed.Through the above research,we have achieved the following main achievement:(1)Determined the late Mesozoic four tectonic deformation events in Louhua Shan area of Daqing Shan middle segment.High-angle hedge structure with near-EW trend developed in the west of Louhua Shan.Limited tectonic deformation time is the late Jurassic period.The low-angle thrust nappe structure developed at the periphery of Louhua Shan truncates the high-angle hedge structure and defines the tectonic deformation time was the Late Jurassic-Early Cretaceous period.The NW dextral strike-slip structure developed in north of Louhua Shan transformed the low-angle thrust nappe structure.The deformation time of the dextral strike-sliding structure was defined as the end of the Early Cretaceous,and the displacement of the dextral strike-slip of the fault was estimated to be about 5-6km.The extensional tectonics that developed in the south of Louhua Shan and the Huhhot metamorphic core complex in the eastern part of Daqing Shan are the same extensional tectonics system.The deformation time is limited to the end of the Early Cretaceous.(2)Why the differences in tectonic deformation characteristics between the eastern and western segments of Daqing Shan are analyzed.There is a clear structural superposition relationship between the late Mesozoic tectonic deformation events in the middle segment of the Daqing Shan.The complex tectonic deformation indicates that the intracontinental deformation belt has a very complicated evolution.The eastern part of the Daqing Shan may have developed near E-W high angle thrust structures.However,due to the superposition of the low-angle thrust nappe structures and the metamorphic core complexes in the latter period,the possible near EW trend of high-angle thrusting structures is hard to find,resulting in structural differences in the eastern and western parts of Daqing Shan.(3)The late Mesozoic intracontinental deformation belt tectonic stress field is recovered in the middle part of Daqing Shan.And the early high-angle hedge structure is affected by the S-N direction extrusion stress,which may be related to the remote stress effect of the Mongolia-Okhotsk ocean closure,and the late large-scale low-angle thrust nappe structure may be controlled by the subduction of the Paleo-pacific plate.The dextral strike-slip motion is a structural turning point from compression to extension.The extension tectonics is related to the overall extension of eastern China.