Assessment of active tectonics by using morphometric indices in Sepidrud basin, Western Alborz

Tectonic geomorphology is a valuable knowledge in the study of active tectonics that can determine the effect of active tectonics on the river. Morphometric studies are defined as quantitative measurements of the shape and landscape of the earth. With the advent of tectonic science of geomorphology, Active tectonic processes can affect the shape and performance of the river. The extraction of geomorphic indices using digital elevation map (DEM) in the GIS in the past two decades has been a fast and accurate method for drainage basin analysis, so that these indices are used for quick evaluation of recent tectonic activity in a particular region. The morphometric characteristics of the drainage system of many basins and sub basins in various parts of the world have been studied extensively. With the study of topographic landforms and the model of drainage systems by using geomorphic indices and the geological structure of each area, it is possible to evaluate the active tectonic performance and to determine the absence of active tectonic movements. The quantitative measurements provide conditions that allow them to identify the status of active tectonics areas. From the natural scenery, rivers are the first environmental forms that show a relatively rapid response to changes in the bedding or changes in the outflow of the bed. Regarding the proved reactions of rivers to the occurrence of normal changes, it is possible to analyze them by using geomorphic indices in the study of the effects of tectonic on the river's route to the results of them. Geomorphic indices are especially used for active tectonic studies. According to the tectonic position of the area and the location of the SefidRud River, the study of morphometric indices in determining the tectonic activities of area is useful. In this study, active tectonics of the western Alborz in Rudbar and Rasht basins with measuring morphometric indices has been surveyed. The study area is located in Western Alborz and Manjil, Jirandeh, Deylaman, Dorfak faults are some of the most important faults in this area. There are many inferred faults in different parts of the study area. General trend of study area is NE- SW. To assess tectonic activities in the area, seven morphometric indices of the Hierarchical anomaly (Δa), Bifurcation (R), Hypsometric integral and curve (Hi), Relative relief (Bh), Drainage density (Dd) and Form factor (Ff) and Stream gradient index (SL) have been assessed. First, by using Arc GIS and digital elevation maps (DEM) with a horizontal resolution of 30 m; basins and streams of the study area was extracted and 19 sub basins were formed. Then, using geological maps 1:100,000 of the study area, structural units including fold and faults were extracted. In the next step, each of the indices was calculated for 19 studied basins. Each index was divided into five classes in terms of tectonic activity. In fact, indices of the Hierarchical anomaly (Δa), Bifurcation (R), Hypsometric integral and curve (Hi), Relative relief (Bh), Drainage density (Dd) and Form factor (Ff) and Stream gradient index (SL) were classified according to their values. Class 1 (very high tectonic activity), class 2 (high activity), class 3 (intermediate), class 4 (low), class 5 (very low). According to the values of the calculated indices, to determine the total tectonic activity, the relative active tectonics index (Iat) was evaluated.  Then, for each index, the map of the tectonic activity level was plotted in the study area. Based on (Iat), the area was divided into 4 categories. Category 1 (very high tectonic activity), Category 2 (high tectonic activity), Category 3 (intermediate), Category 4 (low). Almost half of the total area of the basins is in the high and very high activity. Indices measured in each sub basin indicate that in the sub basins corresponding to Manjil, Jirandeh, Dorfak, Deylman, as well as areas with high fault density, the measured indices exhibit high anomaly. The above anomalies indicate the effect of the above faults on the region and high tectonic activity in the study area. In addition, Field observations show the existence of several generations of alluvial drainage, several knickpoints, triangular facets and narrow and tight valleys. According to the results of field observations and calculation of morphological indices, it can be concluded that the study area is active due to the performance of Manjil, Rudbar, Jirandeh, Deylaman faults.