عنوان مقاله [English]
The study area is located in the western and central Alborz and includes the Taleghan- Alamut and the part of Astaneh and Manjil Basins. There are important fault zones in the area including the Manjil, Jirandeh, Rudbar, Taleghan, Deylaman and Angol. Usually, deformations of the surface of the earth occur slowly about more than a thousand years. Then, in order to detect the activities that cause deformation, we need to look at landforms that have kept the changes over the years. Geomorphic indices are useful for tectonic studies, and these indicators are used for the rapid evaluation of tectonic activity in a particular region. Morphometry and morphology are defined as quantitative measurements and descriptions of shapes and specimens of the landscapes. Morphotectonic is an invaluable instrument in the study of active tectonic. Morphometry is described as quantitative measurements of morphology shapes. Simply, morphologies are described in terms of height and gradient. The study of the tectonic geomorphology of the drainage area can provide important and valuable information about the tectonic conditions of the area and its recent activity. Quantitative measurements allow for the objective comparison of different morphologies and the calculation of less-understood variables and make identification of specific features, including the level of tectonic activity of an area possible. For example, survey of rivers, cutting, and diversion of the river provides important information on the development and evolution of folds and faults in the region. On the other hand, digging the bedrock river basin is mainly due to the tectonic activities that cause the change in the shape and topography of the rivers. The determination of anomalies resulted from active tectonics in the rivers using geomorphological indicators is very useful and can reveal active structures in the region. so, studying the drainage and river diversion pattern provides important information about the structural development of the area. Since detailed studies have not been conducted to investigate the relationship between active tectonic and geomorphology in the study area, a morphometric study to identify the effect of active tectonic on tectonic evolution of drainage basins and rivers is necessary. For this purpose, the active tectonics of Western and Central Alborz in the Shahrud, Taleghanrud and Alamutrud basins were evaluated by measuring seven geomorphologic indices of Hierarchical anomaly (a), Bifurcation (R), Hypsometric integral and curve (Hi), Relative relief (Bh), Drainage density (Dd), Form factor (Ff) and Stream-Length gradient (SL).
In order to determine the tectonic activity in the drainage area scale using morphometric indices for the study area, drainage area and streams were first used a digital elevation model (DEM) of 30 meters in Arc GIS software Extraction and after the required adjustments, 27 basins were determined. Finally, morphometric indices have been measured on the basins. Finally, the total tectonic activity level was determined based on the Relative active tectonics index (Iat) of the area and the study area was classified in terms of tectonic activity. In the next step, geological maps with 1: 100000 scale of geological survey and mineral exploration of Iran and satellite images were used to determine the main structures and geological units of the area from the field data.
Results and Discussion
In this section, the results of calculating the indicators with the structures of the region were analyzed and for each index the zoning map of tectonic activity in the study area was plotted. 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. 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.
In some areas, observed that there are several classes of tectonic activity during a fault, so the activity in several parts of the fault is different, and may also there was areas with a medium and low activity level in the along of an active fault. Finally, the main structures and geological units of the region were adapted to the results of measuring the morphometric indices and field observations and analyzed. Field studies document the accuracy of the results. The results of this research indicate that the most basins of the study area show the high and very high recent tectonic activity levels which Field evidences and existing earthquakes also confirm this. According to the values of morphometric indices, the zoning map of the tectonic activities of the study area was prepared. A survey of this map shows that about 35 percent (1748 square kilometers) of the study area has very high tectonic activity and about 53 percent (2684 square kilometers) represents relatively very high tectonic activity. Various geomorphic landforms were observed in the study area where fault scarp, deformed alluvial fan deposits, unpaired terraces, triangular facets, V-shape valleys and deep gorges incised near mountain fronts exist. In the study area the most activity is related to the Sefidkhani, Taleghan, Alamutrud-Shahrud, Khashchal, Angol faults, eastern segment of Manjil fault zone and also the faults in the east of the region. Also, the basins associated with Jirandeh, Rudbar, Bolukan and Kharkhon faults show high tectonic activity.