Assessment of active tectonics by using morphometric indices in Saadat abad basin, High Zagros

Many studies have been done in different parts of the world on basins and waterways using morphometric indices that indicating their capability to identify active areas. Morphotectonic analysis is a useful tool for identifying forms created on earth by tectonic processes. Tectonics Geomorphology is the knowledge of the study of shapes and forms created on the ground by tectonic mechanisms. The geometry of the rivers network can be described qualitatively and quantitatively in a number of ways. The use of morphometric indices to evaluate tectonic activity was started by (Horton, 1945) and followed by other researchers such as (Stahrler, 1952) and (Bull & McFadden, 1977) and has continued to this day. The study area is Saadat Abad basin that is located in south of Iran and in Fars province and it is one of the sub-basins of Bakhtegan-Maharloo catchment. The study area is located in the high Zagros range. There are several faults in the region that including the Sivand River, Sivand, Musaikhani, and Avenjan faults. Since the relationship between active tectonics and geomorphology in Saadat Abad basin has not been studied so far, therefore, it is necessary to investigate the morphometry in order to identify the effect of active tectonics on the tectonic evolution of drainage basins and rivers in this area. In this study, Active tectonic in Saadat Abad basin using five geomorphological indices of the Drainage Asymmetry Factor (Af), Relative relief (Bh), Form factor (Ff), Hypsometric integral and curve (Hi), and Stream gradient index (SL) have been assessed and Finally a final index called Recent relative active tectonic index (IAT) was calculated.

Using the Digital Elevation Model (DEM), the characteristics and indicators of the river can be accurately extracted and analyzed the drainage basin. In this study, in order to determine active tectonics using morphometric indices in the study area, at first, using the digital elevation map (DEM) with a horizontal resolution of 30 m in geographic information systems (GIS) and by using one of the GIS software extensions called Arc Hydro, sub-basins and rivers of the study area were extracted and the study area was divided into 21 sub-basins. Then we calculated the index for each sub-basin separately. In the next step, Using geological maps with scale of 1: 100,000 and 1: 250000 Geological Survey of Iran in the study area, the geological units and major structures of the region, including faults were determined and for each index the zoning map of tectonic activity in the study area was plotted.

In this section, results of the index measurements with the main structures of the area were analyzed. Each index was divided into three categories in terms of tectonic activity: Class 1 (high relative tectonic activity), Class 2 (moderate tectonic activity) and Class 3 (low tectonic activity). In the last step, in order to determine the level of total tectonic activity, a relative active tectonic index (Iat) was calculated and the zoning map of this index was plotted. Drainage Asymmetry Factor (AF) shows tectonic tilt in drainage basin. Structural factors, such as rock layering, may play an important role in increasing the basin asymmetry index of the drainage basin. Relative relief index (Bh) indicates the relative height of the basin. This index is calculated from the difference between the highest and lowest points in the basin and the high rate of represents the high level of uplift that is because of active tectonics of area. According to the form factor index (Ff), the drainage basins are more elongated in geologically active areas. Therefore, basins with lower values of the form factor index are more active in terms of tectonics. The Hypsometric integral (Hi) describes the relative distribution of elevation in a drainage basin and the high rate of Relative relief index, represents the high level of uplift in the study area that is because of active tectonics. Stream gradient index (SL) shows the effect of environmental changes on river longitudinal profile.

According to the relative active tectonic index (Iat) and zoning map of this index which shows the total tectonic activity in the Saadat Abad Basin, Basins 1 and 5 due to the activity of the Sivand River fault, basin 6 due to the activity of the Avanjan fault and basin 8 due to the activity of the Musa-Khani fault, have a high recent relative tectonic activity in the area. Also basins 12 and 19, which are affected by faults activity that are located in the south of the region, have high tectonic activity.