Characteristics And Ages Of Fluvial Terraces At Northeastern Region Of The Pamir

The formation of fluvial terraces includes“drived by climate change”model,“drived by tectonic uplift”model,and“drived by the combination of climate change and tectonic uplift”model.These models emphasize the importance of climate change and/or tectonic uplift in the formation of fluvial terraces.With accumulation of natural case studies,physical modeling and numerical modeling,the model of“drived by the combination of climate change and tectonic uplift”has been widely accepted.However,because of the differences of climate conditions,tectonic deformation,and river scales in different regions,the development characteristics and the formation mechanisms of fluvial terraces are probably quite different.The Pamir,located in the northwestern end of the Himalayan-Tibetan orogenic belt,is one of the most strong-deformed regions in the India-Asia convergence zone.Meanwhile,because of the high-altitude area and low temperature,the Pamir is characterized by significant climate fluctuations during late Quaternary and strong glaciations.Within the Pamir,a series of broad,flat and well-preserved fluvial terraces are developed,providing an execellent chances to investigate the formation process of fluvial terraces.Many studies of fluvial terraces have been carried out,which are dominantly focused on their tectonic deformation and formation ages.For the purpose of investigating the formation process of fluvial terraces,however,many questions remains waiting for answers.These includes:1)The existing studies are focused on the tectonic deformation and formation ages of fluvial terraces.Their basic types,elevations relative to each other and the modern riverbed and sedimentary texture,as well as the incision process of the rivers remains poorly known.2)The existing ages of fluvial terraces are based on the Optically Stimulated Luminescence?OSL?dating of fine-grained quartzs.Considering the poor bleaching of the fine-grained quartzs,the dating results probably are significantly older than the ture age of fluvial terraces.3)Is the formation of fluvial terraces within the Pamir concordance with the model of“drived by the combination of climate change and tectonic uplift”?Do the river downcutting and terrace formation occur during the climate transition from glacial period to interglacial period.Is the downcutting space dominantly provided by tectonic uplift?4)Compared to the Pamir front,the Pamir interior is dominated by strong glaciation.Are the development characteristics and formation process of fluvial terraces within the Pamir interior different from those of fluvial terraces at the Pamir front?Through interprations of high-resolution satellite images,geologic and geomorphic mapping,and cosmogenic radionuclide ¹⁰Be dating,in this study,we acquire geomorphic framework and time sequence of major fluvial terraces of Akesayi and Wulu within the Pamir,and Mushi anticline and Wuheshalu syncline at the Pamir front.We then correlate these ages with the late-Quaternary climate transition periods,and evluate the contribution of tectonic uplit to the river incision.Finally,we discuss the formation models of fluvial terraces in the Pamir.The major outcomes are as follows:1)At Akesayi,fluvial terraces can be subdivided into three groups?T3,T2a-d,and T1?,most of which are strath terraces.Based on the cosmogenic nuclide ¹⁰Be dating methods,the exposure ages of T3,T2d,T2c and T2b are 9.4+1.2/-0.9?4.3±0.4?3.8±0.4,and 3.2+0.6/-0.4 ka,respectively.Combining with the riverbed-elevated heights of terrace surfaces,the incision rates of the river is varying from 1.7 to 10.3mm/a.The uplift rate of the Muji Fault is 0.1-0.3 mm/a,contributing less than 1/5 to the river incision.2)At Wulu,fluvial terraces can be subdivided into four groups?T4,T3a-d,T2a-b,and T1?,most of which are strath terraces as well.The exposure ages of T4,T3b,T3a,T2b,and T2a are 47.5+6.3/-6.4,18.8±1.3,18.1±1.2,17.35±2.72 and 15.31±1.74 ka,respectively.Combining with the riverbed-elevated heights of terraces surfaces,the incision rate is?0.8 mm/a at the river's western bank and?2.4 mm/a at the river's eastern bank.The uplift rate of the Muji Fault is?0.2 mm/a,contributing less than 1/4to the river incision.3)At Mushi anticline,fluvial terraces can be subdivided into four groups?T4,T3a-b,T2a-b,and T1a-b?,most of which are strath terraces.The exposure ages of T4,T3b,T3a,T2b,and T2a are 122.6+23.2/-18.6,43.9±3.1,28.1±2.1,15.6±1.9,and14.8+2.9/-2.4 ka,respectively.Combining with the riverbed-elevated heights of terraces surfaces,the incision rate varies from 0.4 to 2.2 mm/a.This rate is lower than the uplift rate of?2.3 mm/a of the Mushi anticline,indicating that the tectonic uplift provides enough incision space of the river.4)At Wuheshalu syncline,fluvial terraces can be subdivided into four groups?T4,T3a-b,T2a-b,and T1?,most of which are strath terraces.As a result of the significant disruption that is cased by the COVID-19 pandemic,the dating results have not been received.According to the exiting OSL dating results of the T3 terrace,the average incision rate of the river is?2.1 mm/a.Based on the height of fold scarp on the T2b terrace and its height above the modern riverbed,the contribution of tectonic uplift to the river incision is less than 1/3.5)All above,the Pamir region is characterized by three major groups of fluvial terraces,whose ages are 123-138ka,41-56ka,and 15-19ka,respectively.The ages of these three groups of fluvial terraces can be well correlated with both the global transition from glacial period to interglacial period and the transition from glacial advance to glacial retreat of the Pamir.The Holocene terraces at Akesayi,however,their ages cannot well correlated with the global climate change and the Pamir glaciation stages.This is probably either because the large uncertainties of dating results or because more complicated driving mechanisms of the Holocene terraces.Except the Mushi anticline,the river incision amount?or rate?is significantly larger than the tectonic uplift amount?or rate?at all other three sites?Wuheshalu,Akesayi,and Wulu?.These results indicate that the river incision space at these three sites is not contributed by tectonic uplift,but is probably contributed by the deposition/incision processes controlled by regional climate.