The Controlling Factors Of Glacial Erosion And Its Influence On Landscapes In The Tibetan Plateau

During the glacial-interglacial periods in the Quaternary,there has been a change in the relative dominance of glacial and fluvial processes at high altitudes,whereas erosion processes and ability differ between the two agents,which lead to different influence on landscapes.Therefore,knowing the relative efficiency of fluvial and glacial erosion rate is a key problem for the understanding of mountain landscape evolution.Nevertheless,how does glacial erosion sculpt landforms,whether glacial erosion is more effective than fluvial erosion,and what influence of glacial erosion plays on mountain topography still remain subjects of debate.Glacial erosion directly affects earth surface,and conditions of climate and geology are diversity at different regions,which result in differences of glacial erosion and landscapes,thus knowing what controls glacial erosion and their interactions is not only very important for understanding the physical mechanisms of glacial erosion and glacial landforms evolution,but also the fundamental for exploring the relationships between climate,topography and glacial erosion.The Tibetan Plateau has been an important glaciated zone in the mid-to low-latitudes during Quaternary,and various climate and tectonics in different regions result in diversity of glacier types and glaciation duration between different mountains,which both make the Tibetan Plateau become an appropriate study area for discussing the scientific problems above.Firstly,in order to investigate the relative efficiency of glacial and fluvial erosion,we select two different mountain ranges with different climate and uplift rates–the Qionglai Shan(monsoonal temperate glaciation)and Lenglongling(subcontinental glaciation),and then use a 1-D model to construct presumptive fluvial longitudinal profiles if glaciation had not occurred,and compare these with the actual glacial profiles to obtain relative incision rates of fluvial and glacial erosion.Based on the relative incision rate of glaciers and rivers,we further compare topography between fluvial and glacial basins under similar drainage areas,and acquire the different influence of glaciers and rivers on valley relief.Secondly,for understanding the controlling factors of glacial erosion in the Tibetan Plateau,we choose nine mountains which differs in climate,lithology,glacier type and glaciation duration as study areas,and extract glacial valleys and cross profiles to fit the power law model(y=ax~b,b could indicate the glacial erosion)in each mountain,then quantitative data of glacial erosion could be obtained,afterwards we make quantitatively and qualitatively analysis between glacial erosion and its influential factors to explore the controlling factor of each mountain and the Tibetan Plateau.Finally,on the basis of the distribution of glacial erosion along the valley longitudinal profile,we get the divide of glacial erosion changes,and then make contrast analysis between glacial topography and the divide to discuss how glacial erosion influent the elevation of the Tibetan Plateau.According to our work,we get the following conclusions:(1)The relative relationship between glacial and fluvial incision contrast between Lenglongling and Qionglai Shan—glacial incision rate is smaller than fluvial incision rate in Lenglongling,while larger than incision rate in the Qionglai Shan,which could be attributed to the controlling effect of tectonics on fluvial incision and the controlling effect of glacier types on glacial incision.Topography comparisons between glacial and fluvial basins under similar drainage areas demonstrate that in Lenglongling glacial erosion leads to valley widening,slope gradient increase,and valley relief increase,which may be induced by dominant widening process of glacial erosion and the cold based glaciers on the ridges that protect the ridges from intense denudation;but in the Qionglai Shan glacial erosion results in no obvious topography differences between glacial and fluvial basins,which may reflect slope process responses to threshold hillslope limiting the height above valley bottoms.Therefore,we think the relative efficiency of glacial and fluvial erosion is not permanent,tectonics and glacier types may result in different relationship between glacial and fluvial erosion.And compared with fluvial erosion,whether glacial erosion could lead to relief increase may be affected by threshold hillslope.(2)In the Tibetan Plateau of mid-to low-latitude,average glacier size(or ice flux)controls glacial erosion,except Nianbaoyuze and eastern Himalaya(where temperature plays a controlling effect),which is different from the controlling factor of glacial erosion in the mid-to high-latitudes(temperate has best positive relationship with glacial erosion).The different controlling factors between mid-to low-latitude and mid-to high-latitudes may be attributed to the obvious difference of climate.The temperature is generally low in the mid-to high-latitudes,and glaciers here are mostly cold based,a little increase of temperature will change the basal thermal condition and sliding velocity,then lead to increase of glacial erosion;while the temperature in the mid-to low-latitudes is much higher,the melt water under glaciers could ensure enough basal sliding,and temperature increase could just result in more ablation.Furthermore,temperature correlate positively and negatively with glacial erosion in mid-to high-latitudes and in several mountains of mid-to low-latitudes,which may reflects the influence of temperature on basal thermal regime and glacier size(or ice flux).In addition,the slope also has positive and negative relationship with glacial erosion in the eastern Kunlun and Qionglai Shan,individually,which may present two contrast effects on glacial erosion—positive relationship indicates the influence on shear stress,and negative relationship indicate the influence on mean glacier size(or ice flux).(3)Longitudinal distribution of glacial erosion in mountains of the Tibetan Plateau shows that glacial erosion is most intense at the ELA,and decreases upwards and downwards,which may result in the maximum area of area-altitude distribution and mean elevation lie below the ELA,the mean slope decreases from above ELA to near ELA,and the peak elevations have good positive relationship with LGM ELA,and then limit the mean elevation and peak elevation.Therefore,glacial erosion plays a buzz saw effect on the 9 mountains with different glacier types,the possible processes is that the most intense glacial erosion in the ELA leads to the concentration of topography near the ELA,and thus limit the mean elevation.In addition,the difference of glacial erosion between the ELA and above ELA also cause the increase of the relief and slope between ELA and the mountain peak,when the slope reaches threshold,slope processes work and limit the peak elevation.