Paleo-tethyan Rift Tectonics And Cenozoic Structural Deformation In The Middle-northern Segment Of The Ailaoshan Tectonic Belt

The Ailaoshan tectonic belt of Southeast Tibet,located at the place where the eastern Paleo-tethyan belt and the eastern Indian-Eurasian plate collision zone are superimposed,has experienced the geological processes of both the late Paleozoicearly Mesozoic Paleo-tethyan ocean evolution and the Cenozoic Indian-Eurasian plate continental collision and post-collision stages.The Ailaoshan tectonic belt comprises,from west to east,the western Ailaoshan magmatic belt,the Ailaoshan low-grade and high-grade metamorphic belts.The Ailaoshan suture zone occurs along the western margin of the low-grade metamorphic belt.The high-grade metamorphic belt represents part of the Ailao Shan-Red River shear zone.The area bounded by the Ailaoshan suture zone to the west and the Red River Fault to the eastisreferred as the Ailaoshan micro-block.Based on zircon U-Pb geochronology and Hf isotopic characteristics,combined with geological observations,we propose that the Ailaoshan micro-block was derived from the western margin of the Yangtze block,and is comparable to the Zhongzan and Nam Co micro-blocks.The presence of late Permian mafic rocks with rift-related geochemical characteristics within the Ailaoshan micro-block,together with granitoids derived from partial melting of ancient/juvenile crust,indicates the presence of an Ailaoshan rift.This possible rift may correspond to the Ganzi–Litang ocean to the northwest and the Jinping–Song Da rift to the southeast.The spatial–temporal evolutionary relationshipindicates that westward subduction of the Jinshajiang–Ailaoshan–Song Ma oceanic lithosphere triggeredthe separation of the Zhongzan,Ailaoshan,and Nam Co micro-blocks from the western passive continental marginof the Yangtze block through the opening of the Ganzi–Litang–Ailaoshan–Jinping–Song Da ocean/rift.This ocean/rift may represent a subsidiary branch of the Paleo-Tethyan Ocean along the western margin of the Yangtze block.Based on macro-and micro-structural observations and fabric analysis of quartz,the Ailaoshan high-grade metamorphic belt can be divided into,from northeast to southwest,S,L,S-L and S tectonites zones.These rocks experienced metamorphic and deformation condition changed regularly from medium-high temperature of amphibolites facies to medium temperature of upper greenschist facies,and to low temperature of lower greenschist facies,suggesting different depth within the crust.Vorticity analyisis indicates the increase in simple shear component of deformation from the margin toward the middle of the high-grade metamorphic belt.Meanwhile,the shear zone was dominated by general shear but changed to a simple shear dominated flow during the last stage of deformation.Geometry and kinematics studies indicate that middle-northern segment of the Ailaoshan high-grade metamorphic belt constitutes a reversed antiform with hinge trends NW-SE,axial plane dips NE and pitch to NW.In the most northwestern part,it constitutes a full antiform with foliation mainly dips W and shear senses indicate a left-lateral(top-to-the-NW)shear on the southwestern limb,and dips NE and shear senses indicate a right-lateral(top-to-the-SE)shear on the northeastern limb.In the middle-northern part,it constitutes a monoclinal structure with foliation dips NE and shear senses indicate a left-lateral(top-to-the-NW)shear.This could be attributed to the reversion of the southwestern limb and the northeastern limb was cut by the Red River Fault.Continuous compressional effects of Indian-Eurasian collision and the extrusion of middle-upper crust related to lower crust of Indochina led to the uplift of the Ailaoshan high-grade metamorphic belt in the form of an antiform and reversed subsequently.The late normal dextral strike-slip brittle faulting of the Red River Fault has transformed the Ailaoshan high-grade metamorphic belt.