| During the Cenozoic Indian-Eurasian collision and post-collision process,the Chongshan tectonic belt plays an important role in accommodating the tectonic extrusion of the Indochinal block and the formation of the tectonic framework of the southeastern Tibet plateau.The shearing and mechanisms of exhumation of the Chongshan tectonic belt have been the key to understand the tectonic evolution of the southeastern Tibet plateau.Based on the southern segment of the Chongshan tectonic belt(the five profiles of Wayao-Yunxian segment)as the main object of study in this paper,based on the regional structural studies and analysis,we carry out the study of rock formation unit,macro-and micro-structural deformation,geometry,kinematics,dynamics,deformational temperature,shearing deformational dating,etc,using the deformational rock fabric,zircon U-Pb geochronology,etc.The southern segment of the Chongshan tectonic belt extends through Wayao,Changning to Fengqing from northwest to southeast,and lineation also trends NW-SE.In the Fengqing-Xiaowan-Yaojie segment,it transforms to trend nearly E-W and pinchouts in the southeastern Yunxian-Yaojie-Lancang River bridge,forming into arcuate tectonic belt combined with Wuliangshan antiform.The Chongshan tectonic belt mainly constitutes of highly deformed high-grade metamorphic belt in the middle,weakly deformed low-grade metamorphic belt on both sides,and highly deformed unmetamorphic belt in the southern margin.The penetrative development of foliation and stretching lineation in high-grade metamorphic belt forms into S-L tectonic rock;the development of foliation in low-grade metamorphic belt forms into S tectonic rock.Foliation mainly dips NE,but partial foliation dips SW and constitutes local fold.On the macro,it may constitute a compound antiform with nearly erect axial plane dips NE with hinge and lineation trending NW-SE.The eastern part of high-grade metamorphic belt is dominated by right-lateral shear sense,while its west is dominated by left-lateral shear sense.The general kinematics shows that the upper limb of low-grade metamorphic rocks was moving towards the SE,while the lower core of high-grade metamorphic rocks was moving towards the NW.The near E-W extended segment of the Chongshan tectonic belt constitutes a more complete antiformal dome with the hinge and stretching lineation all pitch to SEE,and rightlateral shear sense indicates the upper part was moving towards the SEE.Based on macro-and micro-deformational characteristics and fabric analysis of quartz,it reveals the amphibolite facies environment of high temperature(550°C-650°C)ductile-deformation in the core of the Chongshan tectonic belt,and brittleductile deformation of medium-low temperature in the limb of the Chongshan tectonic belt,with the overall superposition of medium-low temperature brittle-ductile deformation or low temperature brittle deformation.The geochronology study of syn-shearing granitic dikes and quartz diorite dikes in the Chongshan tectonic belt,shows that the dating of zircon U-Pb crystallization is30?25.6 Ma,and the dating of post-shearing undeformed quartz diorite dikes is25.01 Ma.Therefore,Cenozoic ductile shear deformation of the southern segment of the Chongshan tectonic belt started about 32 Ma and continued to 24 Ma.Connecting with the previous literature,this paper suggests that there are mainly three stages of tectonic deformation along the Chongshan tectonic belt since Cenozoic period.During Oligocene to the early stage of Miocene(32?24Ma),it suffered from ductile shearing and the middle to lower crustal scale antiformal doming of the southern segment of the Chongshan tectonic belt;during the early-middle stage of Miocene(20?14Ma),the ductile shear zone and the antiformal dome structure was gradually uplifted to the shallow structural level and suffered from the superposition of brittle-ductile or brittle shearing deformation;since Pliocene(5Ma-),it suffered from the superposition and reconstruction of brittle deformation of regional NE-SW compressing and thrusting. |
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