Cenozoic Tectonic Characteristics And Evolution Of The Daxiangling And Its Adjacent Areas In The Southwestern Margin Of The Sichuan Basin

The deformation pattern of the lithosphere of the Tibetan Plateau is one of the focal issues on the study of uplift and expansion mechanism of the plateau.At present,the eastern boundary of the Tibetan Plateau has expanded eastward from orogenic belt into the basin.Therefore,detailed research on surface tectonic deformation is still needed.Aiming at this scientific issue,this paper focused on the basin-mountain tectonic transition area in eastern Tibetan Plateau,and selected Daxiangling tectonic belt located in the hinterland of Lonemenshan-Chuandian area,southwestern margin of Sichuan Basin,as our research region.By using the tectonic deformation analysis,paleotectonic stress field analysis and the theory of anisotropy of magnetic susceptibility,we conducted detailed research on the structural character,evolution process and formation mechanism of Daxiangling tectonic belt since Cenozoic era.Main conclusions are as follows:(1)The Daxiangling tectonic belt is bounded by the Shimian fault,the Jinping fault,the Sanhe-Fengyi fault and the Dahe-Qinglong fault from the south to the north.It can be divided into the south(Shimian),the middle(Jiuxiang)and the north(Yingjing).Three main structural belts and one outer edge deformation belt(Tianquan-Ya’an).The main fault zone is a NW trending,high angle brittle ductile thrust fault with a certain sinistral strike-slip component,conjugating with NE-NNE dextral strike-slip compressional fault in the tectonic belt.Tectonic sheet is mainly manifested as wide and gentle fold structure.In this paper,a large number of fault slip vectors and other data were collected in the main fault zone.Inversion analysis of the tectonic paleostress field was analyzed as well.The results show that the main stress field in the study area is a compressional tectonic stress field.The direction of the maximum compressional stress in early stage is NW-SW trending,and in later stage nearly E-W trending.(2)The rock magnetism results of Jiuxiang basin show that magnetite and hematite are the main magnetic carrier minerals.The results of magnetic fabric exprement show that the initial deformation magnetic fabric and pencil magnetic fabric are mainly developed in the basin.Most magnetic lineation suggest NW direction,correlating well with the tectonic lines.The magnetic fabric in the study area is mainly affected by the tectonic deformation,which indicates that the basin mainly experienced the NE-SW and nearly E-W compressional tectonics.The occurrence of magnetic lineaments in qinglong fault zone is consistent with that of field scratches,indicating that it is a sinistral strike-slip compressive thrust fault.(3)The Daxiangling tectonic belt can be regarded as a transition zone of the Tibetan Plateau and the Sichuan Basin,being the forward zone of the eastward expansion of the plateau.The deep lithosphere thickened and there existed weak layer in middle-lower crust moving towards southeast,resulting in the compressional tectonic movement of the upper crust along pre-existing or newborn faults,forming a series of basement-involved strike-slip compressive faults,leading the research area uplift and deform.Above all,this paper holds that the Daxiangling tectonic belt is a result of the combined and superimposed tectonics,which formed on the basis of the early tectonic movement firstly,and then influenced by the eastward expansion of the Tibetan Plateau since Cenozoic era,under the effect of Longmenshan fault and Xianshuihe fault together.