عنوان مقاله [English]
نویسندگان [English]چکیده [English]
The recent studies about the geomorphology is more based on the systematic comparison of the features and deposits. It also focus on determination of the primary forms of features and identification of processes. Qualitative measurement includes examining the shape of the earth's surface. Quantitative measurements allow the comparison of different landforms and calculation of less-understood parameters and allow identification of specific features including the level of tectonic activity. Geomorphic evidence can be evaluated using quantitative and qualitative methods, which is the quantitatively assessment is known as morphometric method. The use of geomorphic indices is a common method for classification of the tectonic activity. In this research, assessment of the active tectonic structures on recent geomorphology of the Green Mountains have been studied using morphometric indices and also effects of the Nahavand fault zone on the drainage network of the Grin Mountain (south west of Nahavand) has been investigated.
In this research, structural maps, drainage network map and digital elevation model (DEM) of the study area were prepared using topographic map of the Hamadan, geological maps of the Nahavand and Malayer and Landsat 7 satellite images. Then, the stream length-gradient index (SL), asymmetry factor index (Af), transverse topographic symmetry index (T) and mountain front sinuosity index (Smf) were calculated for 47 basin in the study area. Finally, the results of these indices were compared by analyzing of the fractal dimension of the study area.
Results and discussion
The drainage pattern of the study area have two main NW-SE and NE-SW trends. The value of the SL index varies from 962 to 50 in the region with high and low tectonic activity, respectively. The value of this index is increased along the faults trend of the region and intersection of the drainages with active structures. It can be proposed that the tectonic movements, especially uplift, have increased the gradient of the drainages. According to the results of the Af index, 17 basins is classified in class 1 (high activity), 16 basins in class 2 (moderate activity) and 14 basins in class 3 (low activity). The existence of active fault system in these regions can be attributed to this asymmetry and tilting. The values calculated from the T index are classified in the classes 1 and 2, which indicate the asymmetry of basins, especially in the central part and perpendicular to the Green Mountains. In 19 basins, the values of this index are classified in class 1, in 24 basins in class 2 and just in 2 basins in class 3. The mountain front of the study area were divided into 10 sections along the Garin Mountain, in order to assessment of the Smf index. Then, from the north to south, this index was calculated for different sections. Measured values of the Smf index for most part of the study area show values close to 1, indicating that the northern part of the Green Mountains have a higher relative activity. In this study, all indices were combined, in order to classification of the study area based on Iat index. For each basin, the mean values of the geomorphic indices (S/n) were calculated and classified into four classes of index of relative tectonic activity (Iat). Considering the final zoning map of the relative tectonic activity in the study area, it can be proposed that the basins of the central and southern part of the region have more relative tectonic activity. Also, fault and drainage network layers were mapped in the Arc GIS software and the fractal dimension was calculated for the study area by using the Box Counting method (the most common method for fractal analysis). Regarding the surface geomorphology, the study area was divided into 6 squares with dimensions of 16 km, in order to applying the Box Counting method. In the fractal study, the fault system of each square was evaluated separately and the fractal dimension was calculated for each square. Fractal dimensions of the drainage networks were calculated using aerial photos and satellite images similar to the faults map. The maximum and minimum fractal values of faults are 1.87 and 1.41, respectively. Also, the maximum and minimum fractal values of drainages are 1.66 and 1.27, respectively.
According to the final zoning map of the relative tectonic activity in the study area, it can be proposed that the central and southern basins of the study area have more relative tectonic activity, where the faults also have a high density in these basins. The activity of fault systems in these basins have played an important role in high rates of relative tectonic activity. Increasing the fractal dimension of the faults indicates less maturation and the activity of the study area. The major and active structures in the Nahavand fault zone in the southern part and along the Garin Mountains caused the uplift and tectonic activity in this part of the study area.