Cenozoic Structure And Exhumation Mechanism Of The Metamorphic Complexes In Southeast Tibet

The collision and post-collision processes between India and Eurasia plates since55 Ma have played an important role in the formation and evolution of the lithospheric structure in southeast Tibet and southeast Asia.Sundaland block extruded southeast in a mass displacement during the post-collision process.Bounded by the Ailao Shan-Red River tectonic belt as the east side and the Gaoligong-Sagaing shear zone as the west side,the fan-shaped area exposed many metamorphic complexes.In order to deeply analyze how the lithosphere adjust its structure during the lateral escape process of the Sundaland block,this paper mainly focuses on the Diancang Shan,Ailao Shan and Yao Shan complexes along the Ailao Shan-Red River tectonic belt,Yulong complex in its adjacent area,and the Ximeng complex within the Sundaland block,and carry out macrostructure,microstructure analyses,thermochronology,and geochronology,to reveal the composition,structural framework,deformation characteristics,kinematics and exhumation mechanism of these complexes.Through the structural analyses of these complexes,the dome structures are widely developed in Southeast Tibet.The long axes of the domes are parallel to the regional lineation and the hinges of small scale A folds,which is different from the traditional strike-slip shear zone.The north-middle to south segments of the Ailao Shan complex and the middle-south segment of the Yao Shan complex constitute broken monoclinal structures and the northeast limbs were cut by the Holocene faults,respectively.Full dome structure could be observed in the northwestmost end of the Ailao Shan complex,middle-north segment of the Yao Shan complex,and full parts of the Diancang Shan,Ximeng and Yulong complexes.The formation of dome structures is associated with progressive shearing,during which rocks are characterized by rheological stratification: the rocks in the core part are highly metamorphosed,high-temperature deformed,and locally superposed by late deformation at low temperature,the mantle rocks are characterized by low-grade metamorphic rocks and low temperature deformation.In most cases,tectonic discontinuity contacts exist between the core and mantle parts.The rocks above and below the contacts have same structural elements,i.e.,foliation,lineation and kinematics,but have significantly different metamorphic grades.The tectonic-thermal evolutions of these dome structures in southeast Tibet reveal that they experienced inconsistent cooling and exhumation paths.The initial time of cooling and exhumation are mostly concentrated in Oligocene,but there is no uniform trigger time.Most of the complexes have two stages of cooling and exhumation processes.The early stage of Diancang Shan complex is slow cooling(11-21 ?/Myr)and late stage is rapid(45-50 ?/Myr).Ailao Shan is characterized by early rapid(44.80 to 714.29 ?/Myr)and late slow cooling(10.00-17.79 ?/Myr).Ximeng complex is characterized by early rapid cooling(112.6-648.1 ?/Myr),then later slow cooling(7.5-16.9 ?/Myr).The complexes,located along the east boundary of Sundaland block(Diancang Shan,Ailao Shan and Yao Shan)or near it(Yulong),or located in the west boundary(Gaoligong),or located within the block(Ximeng),all have developed ductile shear deformation,exhumation and tectono-magmatism since Oligocene,showing that the deformation and flow of the middle-lower crust are not only confined to the edges of the block,but also have a significant performance within the Sundaland block during lateral extrusion.The formation of boundary shear zones may be the result of high strain localization during progressive shearing.The structural and microstructural analyses show that the formation and exhumation of the dome structures in southeast Tibet are related to middle-lower crustal flow,caused by the extrusion of the Sundaland block since Oligocene,which is accompanied by shearing,doming and exhumation.