| River terrace,which could reflect tectonic activity,climate fluctuations,and base-level changes,records the environmental information in the process of river evolution.Upstream knickpoint propagation acts as an essential mechanism for channel erosion,communicating changes in base-level,tectonic,and climate changes across the landscape.Previously,researchers mainly studied terraces on a cross-section at “steady-state”.However,many landscapes in the field seem to be in a transient state.Here,we tend to explore the mechanism for knickpoint initiation and fluvial evolution in a transient setting,where the upper Yellow River cuts through Anyemaqen Shan and eroded into Zoige basin in northeastern Tibetan Plateau.This could act as a meaningful case for studying the impacts of climate and tectonic impacts on fluvial evolution in a transient state during the downcutting phase in a regional setting of continuous uplift.In this study,we report new data of channel profiles,terrace mapping,sedimentary analysis,and optically stimulated luminescence(OSL)dating of the Yellow River terraces in Zoige basin.The results are as following:1)In the lower reach of the Zoige Yellow River,the activations of the Kunlun fault are estimated to trigger several terraces formation,especially in the downstream area.We identified 5 levels of Yellow River terraces at downstream area and 3 levels of terraces at the upper reach.Terraces in the upper reach are continuous and flat while the terraces in the lower reach are scattered distribution.According to the sedimentary facies of the terraces,we divide three sedimentary units from the bottom to the top: U1 is the angular to sub-angular gravel layer developed at the base of part of the terraces,which is poorly sorted and slightly imbricated.We interpreted this unit as hyper-concentrated flow deposits.U2 is rounded to sub-rounded gravel layer developed in the lower part of the terraces;U3 is the sand layer developed at the top of the terrace,representing floodplain processes under low-energy fluvial conditions.2)Analysis of channel profiles and terrace correlation,reflecting that the channel is adjusted to match increase rock uplift in the area.Consequently,a series of terraces were formed in Zoige basin.Numbers of terrace step increase downstream in the basin.T3 were formed by climatic change and not disturbed by the tectonic knickpoint.Terrace formed at around 4.2 ka in the upper reach.But correlating to the continuous topographic gradient surface of these terraces,three terrace steps were formed in the down reach in the basin during the period from around 9.5 ka to 4.2 ka.The graded long profiles of the terrace levels T1 and T2 also indicate that their genesis was controlled by the tectonic knickpoint,for the shape of the terrace long profiles is parallel to the modern long profile.During the fluvial evolution the position of the knickpoints only migrated a few kilometers upstream(for modern profile is at260 km).Since the formation of the T2 level to the modern river bed(last c.9.5 ka)this knickpoint propagated upstream c.25 km.Since the formation of the T1 level to the modern river bed new knickpoint generated and propagated upstream c.5 km.3)The topper terrace,distributed continuous with a general gradient in the whole basin,was deposited during cold period and abandoned at the climatic transition from cold to warm state,at around 14.6 to 12.0 ka.Since then,one terrace formed and finally abandoned at around 4.2 ka in the upper reach.But correlating to the continuous topographic gradient surface of this terrace,three terrace steps were formed in the down reach in the basin.This indicates multi-phases of headward migration waves of fluvial knickpoint and terraces formation,caused by fault activity and tectonic uplift of the gorge at the outlet of the basin,rather than the integration of the Yellow River into Zoige basin.This shows a transient fluvial incision in response to the uplift rate increase,after the integration of the Yellow River and basin exhumation in NE Tibetan Plateau. |
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