عنوان مقاله [English]
Landforms and their response to environmental changes is one of the most interested topics among geomorphologists. One of the landforms that is most affected by tectonic and erosion processes is the rivers. Rivers respond to tectonic processes that increase the height of landscapes and erosion processes that try to reduce the height of landforms. This reaction can be well studied by analyzing the longitudinal profile of the rivers. One of the effective parameters in the study of tectonic and erosional status of regions is steepness and concavity. These parameters can be examined in the form of Stream Power Law (SPL). This function is related to incision power of streams.This relationship in the form of the linkage between slope and drainage area of the river in a logarithmic plot based on power regression, extracts the values of two parameters, the steepness and the concavity of the channel. In fact experimental studies by other researchers have shown that there is a direct relationship between rivers steepness and concavity with tectonic – erosive processes in the regions.
It generally accepted that steep landscape are associated with areas of high uplift rate and active tectonic. Rivers system are well adopted to tectonic processes to provide useful information about the rate of uplift in landforms. The steepness of rivers which depends on the declivity of channels is fraction of uplift rate. So we expect that if the amount of the steepness in the longitudinal profile of the river is low, the uplift rate is slight too and if the steepness is high, the uplift rate is intense too. Concavity index usually depends on bed material. But erosion efficiency has direct connection with incision power law and its steepness. But weakness of bed material especially alluvial can increase rate of erosion efficiency in channels. Erosion efficiency is the volume of sediment that is completely removed from the environment after erosion. Erosion efficiency is a function of sedimentary flux. This parameter can be directly related to the tectonic processes and characteristics of the bedrock. If tectonic processes lead to an increase in the height of the landforms, it can increase orographic precipitation in mountainous areas, and it can lead to increasing sedimentary flux, then erosion efficiency also increases. The main purpose of this study is to analysis the effect of active tectonic and erosion on equilibrium profile of the main rivers of the Damghan Mountain based on the Stream Power Law. These steepness and concavity parameters are influenced by set of lithological, geological, topographic and erosion factors. All of these factors are effective in location of knick points of rivers and are able to provide useful information about the geological and erosion status of the area
Material and Method: In order to investigate the power incision law, the DEM map in 30m resolution was used to extract the channels. For extracting the rivers, the D8 algorithm method was used to calculate the flow direction. In this method, the flow path of each pixel that fall on the lower pixel with a lower slope was calculated and the flow directions was determined. In this regard, we first need to create a DEM map with the least inconsistency. This method focuses on extracting central flows in valleys and reducing parallel flows. After extracting the channels, their slope-area logarithmic diagram were plotted. The regression line considered for the logarithmic plot is the power regression, which is the relation of the river incision power. In this regression, the slope of the regression line is concavity and the intercept of line is steepness. To obtain information about lithological features of the area that are effective in analyzing the concavity and steepness parameters, the geological map of Damghan and Shahrud was used. The study area is part of the mountain structure of Eastern Alborz and has several active faults. North Damghan Mountain is located on the southern side of eastern Alborz between 36。14'0.3" to 36。18' 82" and 55。00' 26" to 53。59' 56" in north of Iran plateau. There are different outcrops of lithostratigraphic formations from Precambrian to Quaternary in this area. Geologically speaking, the study area is composed of set of over thrust blocks and nappes. The thrust faults and nappes within piggy back style have pushed eastern Alborz stratigraphy sequences on each other. The folds in the region have a strong connection to thrust structures and nappes. These folds are of different types and sizes but most of them are inclined and recumbent because of widespread compressive component in eastern Alborz.
Result and Discussion: The three main rivers of the region, CheshmehAli, Astaneh and Tepal, were studied. All three rivers flow on the colluvium bed in the upstream and alluvial bed in the downstream. And all three affected by faults in some areas. Some such as CheshmehAli River in the southern part, has flowed into a fault valley. The activity of faults along the rivers, both in the resistance and alluvial parts has led to uplift of the rivers. These effects are seen in the high values of steepness index and low values of concavity index. The increase in the stream incision is seen in both the upper and lower section of the rivers due to the activity of faults in the region. But the steepness is higher in the upstream which is made of colluvium sediments. While in the downstream due to weakness of alluvial sediments the rate of erosion efficiency is higher. Therefore the change in the rate of steepness, concavity and erosion efficiency, in addition to active tectonic, is strongly affected by the bedrock of channels. Each rivers that is most faulted also has higher values of the steepness index. CheshmehAli River, part of which is located completely in the faulted valley, has the highest rate of steepness compared to other rivers. The Astaneh River has been affected by the Astaneh fault in several parts, and the fault has led to the uplift of the river by cutting off the Quaternary sediments. The high values of steepness parameter in this river confirm existence of active tectonic. The Tepal River in its upper part shows high values of steepness parameter, but in the downstream part where the river flows on agricultural lands, the rate of erosion efficiency has increased and in contrast the rate of steepness parameter has decreased. This is due to human activities that have caused the rate of erosion to exceed the rate of tectonic processes. Therefore, human activities are able to transform the relationships between internal and external processes that are effective in changing landforms.
Conclusion: The results show that reaching the equilibrium profile in each river depends on a set of factors include erosion, tectonic and lithology. Fault in the channel path leads to an increase in the height and slope of the river channel and erosion accurse in response to this change. Tectonic processes increase the incision capacity of rivers as a result of increasing the slope of the channel, which increase the volume of sediments produced in the river. Of course, like that Tepal River, we must consider the role of human activities in increasing the rate of erosion efficiency.
Key Words: Eastern Alborz, Damghan, Active Tectonic, Morphotectonic, SPL Model.