Introduction
One of the tools for identifying landforms is the use of morphotectonic indicators, the use of these indicators is useful for studying areas that have experienced rapid uplift or tectonic transformation. Longitudinal profile of the river is one of the basic components in the river system and as a key topographical index it shows useful information of lithological, tectonic and erosion changes. The river system is a complex reaction process system in which various variables can play a role in its control, and any change in one of these variables causes the complexity of changes and adjustments in the river system. In geomorphological studies, the longitudinal profile of a river is used as a measure to detect tectonic uplift and changes in rock units, the longitudinal profile of rivers that are in equilibrium is concave, but various factors, including structural factors, change the longitudinal profile of the river . Abnormality in the longitudinal profile of the river, if it is caused by tectonic factors or lithological changes, can cause sudden changes in the slope and is associated with the creation of a kinck point, which can be seen as a waterfall and a rapid. Alborz is located on the southern margin of the Caspian and in the north of Iran. Considering that there are important rivers in the target area that reflect the tectonic conditions of the area and so far the mentioned methods have not been used for this study, the purpose of this study is to investigate the longitudinal profile of the river using MATLAB and GIS software in the area in order to Determining the level of tectonic activity of the main faults in the region.
Methodology
In this study, using Arc GIS software and a digital elevation model of 30 meters using the Strahler method, watersheds were divided and extracted. Then, by using topographic maps on a scale of 1:25000 and geological maps on a scale of 1:100000 and aerial photos on a scale of 1:20000 and through the Arc GIS software, different application layers including waterways, basins, faults, lithology and lines height, was prepared and finally, in order to carry out the present research in the area of Western Alborz and Azerbaijan, the studied area was divided into 50 basins and the value of the normal slope index was calculated for all parts of the main and secondary waterways and divided into 5 categories, then with Drawing the longitudinal profile of the longest river of each basin in MATLAB software, the numerical value of this index and the concavity index were calculated separately for each basin. Also, the Knickpoint extraction of a fault was carried out due to the sudden changes in the slope of the river in collision with the main faults, in order to investigate the tectonic activity of the region. Finally, the geological units and main structures of the region were examined and analyzed with the results of the longitudinal profile of the river and field observations.
Results and Discussion
The results of a quantitative study of changes in the longitudinal profile of the river and Knickpoint extraction in 50 drainage basins indicate high tectonic activity in the studied area, especially in Sangavard faults, other faults in the south, southeast and southwest of the region. . The index of normal slope (Ksn) and concavity () which was calculated using the longitudinal profile and in MATLAB software; This means that the high values of these indicators show good coordination with the known faults of the region, including the Firouzabad-Majder fault in the studied area. Sangavard fault, based on the numbers obtained from the normal slope index (Ksn) and concavity (), is the most influential fault in increasing the amount of these indicators in the studied area.
In this research, Knickpoint zones were extracted using a new method and their relationship with the structures of the region was studied. Knickpoints are widely spread in the studied area. The correspondence of sharp and steep knickpoints near the base of the mountain with active faults in the area shows that most of them were created by tectonic activity. The effect of lithological features on the frequency and size of knickpoints can be seen in some places, but they play a secondary role after tectonic activity. In this method of extracting fault lines, the location of fault lines corresponding to Sangavard faults, Kiwi fault, Sheikh-Janlu fault, Qalban-Qiyeh fault and faults located in the south, southeast and southwest of the studied area overlap. They are significant with the index of normal slope and concavity. The conformity of the location of Knickpoints in the field observations with the results of softening also confirms the accuracy of the methods used.
The correspondence of the location of Knickpoints in the field observations with Knickpoints extracted by the method mentioned in the previous sections shows the accuracy of the method used. The tectonic evidences obtained from field observations also prove the recent tectonic activity of structures and faults in the region. The existence of landforms such as high alluvial , the creation of triangular facets, the tilting of layers, the creation of narrow valleys, and the existence of V-shaped valleys are in addition to the results of the morphometric indices. Geostructural evidences such as faulting and folding in Neogene and Quaternary units, displacement of Neogene units due to the activity of the aforementioned faults, thrusting of old units on young units, is a strong proof of the high activity of the structures related to the faults in the case.
Conclusion
High tectonic activity in the study area, especially in Sangavard, Niki, Andalibi, Sheikh Janlu, and Qalpan Qepeh faults and other southern faults of the region using quantitative analysis of the river profile and extraction of Knickpoint in MATLAB software and high agreement of the results of quantitative profile studies River with field evidence and seismic history as an acceptable method to identify areas with high tectonic quality.