Drainage Divide Migration In Response To Tectonic Activities

As stream power erosion model and numerical landform simulation has been adopted in geomorphological research,significant progress has been made on theory of drainage divide migration.Despite the fact that drainage divide migration analyses has open up a new window in tectonic geomorphological study,few natural examples have been given for how drainage divide migrate in response to tectonic activities.Considering the great tectonic geomorphological research concern at and around Tibetan Plateau,and the important status of the Tibetan Plateau in geoscience research.This thesis focus on study of drainage divide migration in response to tectonic activities around the Tibetan Plateau.Taking Longmen Shan,Altyn Tagh,and Qinling Mountains as natural examples,judging drainage divide stability by Gilbert metrics and χ-plot methods,this thesis discussed the relationship between drainage divide migration and landform evolution in different natural tectonic settings.The innovative insights gained from this thesis are as follows:(1)Based on geomorphic and drainage divide stability analyses,strike-slip faulting will drive river system reorganization,including adjustment of channel longitudinal profiles,disturbance of plan view channel distribution,drainage divide migration,and river capture events.Along the Beichuan-Yingxiu fault(BYF),northern part of the Longmen Shan on eastern margin of the Tibetan Plateau,we find that strike-slip faulting will regularly drive drainage divide migrating: in Guixi area,following continuous dextral strike-slip faulting along the BYF,a capture event will likely occur in the near future.Before this capture event,different sections of the drainage divide on opposite sides of the BYF are expected to migrate in opposite directions.In Leigu area,the Anchang River was beheaded by the Baicao River due to dextral strike-slip faulting along the BYF,the drainage divide is mostly stable but moving southwest where it is adjacent to the BYF.So the occurrence of drainage divide migration could initiate before and after a capture event,and drainage divide migration analyzing could be used to identify strike-slip fault activity and fault slip sense(dextral vs.sinistral).(2)At northern margin of the Tibetan Plateau,where defined by strike slip fault,an overall southward drainage divide migrating phenomenon was observed.The Tibetan Plateau was uplifted because of density and thickness discrepancy from inner and outer the Plateau,which dominated the drainage divide southward migrating.Besides,drainage divide moves northward near the fault bend area because of differential uplifting,as the Altyn Tagh Fault moving is related to the northeastward compression at the northern edge of the Tibetan Plateau and uplifting of the Altyn Tagh.Under the Altyn Tagh fault moving and the arid climate environment,the Qaidam drainage basin was limited as endorheic basin,rather than expanding to outside of the Plateau.(3)The uplifting of Qinling mountains are both affected by extensional tectonics and the northward expanding of the Tibetan Plateau.By analyzing drainage divide stability on different sections of the Qinling mountains,it is found that unstable drainage divide segments are mainly distributed at western the Taibai mountain where migrate southward,and drainage divide are generally stable at eastern of the Taibai mountain.Drainage divide migration rate are calculated from the erosion coefficients of the Qinling mountains and the channel steepness coefficient.The calculated result revealed obvious spatial variations of drainage divide migration rate among the Qinling mountains,the high drainage divide migration rate are concentrated at southwest margin of the Weihe basin.The drainage divide migration rates reach between 19.80-143.18 m/Ma at the area west of the Taibai mountain,and are mostly lower than 5 m/Ma at east of the Taibai mountain,so the drainage divide east of the Taibai mountain remain basically stable.In comparation to low-termperature thermal dating and geophysical resources around the Qinling mountains,it can be conducted that the east-west difference in the drainage divide stability and migration rate at the Qinling mountains is the geomorphic response to the northeastward expansion of the Tibetan Plateau and the followed continuous westward expansion of the Weihe graben,the western Weihe basin is still involving in early developing stage of basin growing.(4)By geomorphic characteristics and formation process analyzing,we argue that the Leigu basin at Longmen Shan region,Taibai basin and Fengxian basin at Qinling region are formed through the positive feedback of drainage divide migration and river piracy.The forming mechanism emphasize the key role of river process in landform evolution.