The Initial Timing,Extension Rates Of The N-S Trends Normal Faults In Tibetan Plateau And Its Deep Process And Dynamics

The N-S trends normal faults,widespread through the whole Tibetan Plateau and Himalayas,form the remarkable north-south garben landscape.it is generally agreed that these normal faults are caused by the east-west extension after the Tibetan Plateau reached to its maximum elevation due to gravitational collapse.Therefore,constraints on the timing of these N-S normal faults are of important scientific significance However,the initial timing,the spatial and temporal distribution of the exhumation and extension rates of these N-S normal faults and its influence factor are still remain unclear.Besides,the relationship between the N-S normal-faults and its related late Cenozoic magmatism,low-velocity-high-conductive layer beneath the Tibetan Plateau are still lacking detailed discussion.Regarding the issue above,our study performed systematic research on the field geological survey,low-temperature thermochronology,thermal modelling on cuona and Yalashangbo dome in the eastern,Xurucuo and Tangra Yumco in the central and Yare in the western Tibetan Plateau.Meanwhile,we systematically compared the initial timing results and exhumation rates,magnetotelluric data,faults-related ultrapotassic,potassic and the adakitic rocks ages and present-day GPS measurements to maintain more information on the geodynamics of the N-S normal faults.Thermochronological and modelling results reveal two phases of fast exhumation exist in the N-S normal faults.The age of the first rapid exhumation events during 20-10 Ma is basically similar with the faults-related magmatism.We suggest that this fast exhumation event can be related to the lithospheric structure and deep process beneath the Tibetan Plateau.Combined with the spatial relationship between the high-conductivity-low-resistivity and the N-S normal faults revealed by the magnetotelluric horizontal slices at different depth,our work concluded that the this stage of rapid exhumation may represent the initial activity of the N-S normal faults which was controlled by the tearing of the subducted Indian plate.The second fast exhumation events after 10 Ma,with no or little magmatism recorded.This indicates that this period of faults activity is limited to the upper crust.The exhumation and extension rates calculated by the Age2edot from our thermochronological data and present-day GPS data both showed that the extension rates and extension amount became greater to the east,which indicated that the activity of the N-S normal faults after 10 Ma was dominated by the middle-lower crustal flow.Combined with the data above and our new understanding,our study builds the evolution history of the N-S normal faults:1.During 20-10 Ma,the subducted Indian plate teared and triggered the initial activity of the N-S normal faults.The upwelling asthenosphere induced massive melts in the middle and lower crust.These melts distributed under the normal faults and aroused the faults-related magmatism.After 10 Ma,the flat subduction of the Indian plate cut off the material and energy change between the shallow and deep lithosphere of the Tibetan Plateau.Thus,this stage of fast exhumation event was not relevant with magmatism.The lower crustal flow dominated the evolution of the N-S normal faults.