The Shear Deformation Characteristics And Activity Time Of The South Tibetan Detachment System In Chumba Yumco

The deformation characteristics and mechanism of ductile shear zones and the age of activity are fundamental and central to the study of orogenic belts.The Himalayan orogenic belt,located in the Southern Tibetan Plateau,has a series of large-scale E-W-trending tectonic zones,of which the South Tibetan Detachment System(STDS)is not only an important ductile shear zone but also an important tectonic boundary.The STDS together with the Main Central Thrust(MCT),controls the exhumation of the Greater Himalayan crystalline sequence(GHS).The deformation mechanism and activity time of the STDS demonstrate the evolutionary history and geodynamic processes of the Himalayan orogenic belt.The STDS in Chumba yumco of southern Tibet is located in the south of the Yadong-Gullu Rift and exposes a suite of rock assemblages including ductile deformation facies of granitic mylonite and carbonate mylonite,and brittle deformation facies of fault breccia and fault gouge.In this study,a detailed macro-and microstructural analysis and tectonic chronology of the STDS were carried out in the Chumba yumco in southern Tibet.The results will shed light on the processes and mechanisms of the STDS deformation and the deep processes of the orogenic belt,and promote our understanding of the deformation characteristics and activity history of the STDS in response to evolutionary processes in the Himalayan orogenic belt.The main innovative insights were listed as followings:(1)The rock assemblages of the STDS in Chumba yumco in southern Tibet,from bottom to top,are mylonitic gneiss,granitic mylonite,carbonate mylonite,fault breccia,and fault gouge.The ductile shear zone composed of mylonite and mylonitic rocks is about >500 m thick,and the brittle fault zone is about 3-4 m thick.This part of the STDS has complete rock assemblages.The STDS foliation is 248°(N)?19°,stretching lineation plunge 324°dip 14°,and various shear sense signals indicate a top-to-the-NNW movement.(2)The feldspar of the mylonitic gneiss in the footwall of the STDS shows dynamic recrystallization.The quartz in granitic mylonite underwent strong ductile deformation.The dynamic recrystallization mechanism of quartz gradually changes from grain boundary migration(GBM),sub-grain rotation(SGR)to bulging recrystallization(BLG),indicating that the deformation went through a process of high temperature(>500-650oC),medium(400-500oC)to low temperature(300-400oC).Microstructural characteristics of ductile deformation in the STDS revealed the exhumation processes of the Himalayan orogenic belt.(3)The carbonate mylonite in the hanging wall of the STDS is over 50 m thick and has various asymmetric pudding structures(e.g.,?-type and ?-type),S-C fabric,and different types of shear folds within it.The pudding varies in size from millimeters to 8?10 m and is dominated by feldspathic material and leucogranite,with minor mafic rocks(mainly hornblendite).Fractures are visible in the feldspathic pudding,some of which cut through the carbonate mylonite matrix,indicating the deformation characteristics of different minerals(feldspar,quartz,calcite,etc.)in different temperature environments.Three different forms of calcite recrystallization are also identified,suggesting that the carbonate mylonite underwent successive deformation processes at different temperatures during an exhumation process.(4)Zircon U-Pb ages of leucogranite in granitic mylonite and carbonate mylonite of the STDS reveal an early ductile shear activity age of 22?17 Ma,representing intrusion and strong shearing of the leucogranitic magma at depth in the crust(medium-high temperature).Later,from 16 Ma,the leucogranite was intruded into the shallow carbonate rocks and continued to shear at low temperatures(medium-low).The authigenic illite K-Ar age of the fault gouge at the top of the STDS is ?14 Ma,indicating that the STDS was active in a low-temperature environment between 22?14.(5)Combined with previous research,our results reveal that the Greater Himalayan sequence was subjected to continuous exhumation between 22 and 14 Ma,forming the main framework of the Himalayan orogenic belt.Later,the north-south-trending rift began to activate and cut through the STDS.