Study On The Tectonic Geomorphology And Active Timing Of The Yadong Rift,Southern Tibet

A series of?NS-trending rifts represents the most significant and recent deformation within the Tibetan Plateau.The Yadong-Gulu rift?YGR?is the largest one among them,with intensive tectonic activity.In this thesis,we focus on the Yadong rift,the southern section of YGR,and systematically conduct detailed geomorphic,kinematic and geochronologic studies,to understand and constrain their activity,kinematic role,regional seismic hazard,onset timing,and dynamic background in the tectonic evolution of the Tibetan Plateau.Field investigation shows that the boundary fault of the Yadong rift dips angle of50°-70°.Our main focus is to determine the throw rates along the Yadong fault bounding the eastern margin of the Yadong rift at the Late Quaternary timescale.Based on field and Unmaned Aerial Vehicle?UAV?surveys,Riegel VZ1000terrestrial Li DAR 3D scanner and kinematic GPS,we accurately measured the vertical offsets of alluvial and glacial deposits at four sites in the Yadong Rift.Combined with the surface abandonment age,dated by cosmogenic ¹⁰Be surface-exposure dating,U-series dating and Optically Stimulated Luminescence?OSL?dating,we determined uniform throw rate of 0.9±0.3 mm/yr and an E-W extension rate of 0.8±0.3 mm/yr in the Yadong rift since?20 ka.These rates are consistent with those along the central section of YGR where earthquakes occur frequently.Given the many relics of prehistoric large earthquakes in the central and southern grabens of the Yadong rift?landslide,surface rupture?,a high seismic hazard exists in the Yadong rift.In terms of geochronology research,X-Ray Diffraction?XRD?analysis and K-Ar dating were conducted to constrain the timing of fault gouges in the boundary fault of the Yadong rift.The results show that the formation timing of the fault gouges is 9±1 Ma.Zircon LA-ICP-MS U-Pb geochronology reveals an emplacement age of leucogranite of 15-22 Ma,which also represents the activity timing of the Southern Tibetan detachment system?STDS?.Combined with other limited ages from the Yadong rift and with the structural relationship that the Yadong fault cuts?and offsets by?70 km?the STDS in the field,we suggest that the Yadong rift initiated at?13 Ma,and was strongly active at 9±1 Ma,when the Yadong rift started to open up develop at a large-scale.Systematic analyses and comparison with other rifts in southern Tibet and in the Qiangtang terrane?northern Tibet?are also conducted in the thesis.The rifts in southern Tiber are much larger than those in the Qiangtang terrane,and most of the rifts in the interior of the Tibetan plateau are bounded by high-angle normal faults.The rates along the rifts in southern Tibet are much higher?by one order of magnitude?than those in the Qiangtang terrane.However,the limited geochronology data showed that the rifts in southern-central Tibet were formed around the same time.We conclude that the evolution of these rifts has undergone two stages:the first stage at10-15 Ma,during which gravity collapse caused by the maximum height of the Tibetan Plateau is considered as the main mechanism;the second stage at 4-9 Ma,when the tearing of the subducted Indian plate is responsible the upwelling mantle creating E-W shear force that promoted the further extension of the rifts in southern Tibet.