Zoning Tectonic Activities of Banarovan Using Morphotectonic Indices

Document Type : Research Article

Authors

• Fariba Hemmati
• Davoud Mokhtari
• Shahram Roostaei
• Behzad Zamani Gharechamani

Tabriz University

10.22067/geo.v6i2.59240

Abstract

1. Introduction
There is almost no region in the world that has not undergone tectonic changes during the recent millennia (Keller & Pinter, 2001). For this reason, evaluation and investigation of active tectonic processes and their effects, for example earthquakes, are of vital importance for many human activities including the design and construction of cities, power plants, and dams as it helps minimize their risks and damages (Soleymani, 1999). The Benaravan fault, located on the east of a mountain, is part of the Mianeh-Ardebil fault. The 20 km long Benarvan fault is one of the most significant ones in the region. It is located on the southern skirt of Azerbaijan’s Mount Bozgush and extends in the southwest-northeastern direction. Its route features special attributes concerning domain instabilities and geomorphological phenomena (Monroe & Wicander, 2001). Tectonic and geomorphic activities, relatively low-resistance lithology (Miocene era degenerative sediments), intense faulting of the region and being located in the very high risk region of earthquake (Benarvan fault and adjacent faults), and the high level of subterranean waters result in the possibility of natural risks such as earthquake and other types of mass movement. The significance of such a research is due to the location of numerous residential areas across the Benavran fault. So far, there have been many studies on the behavior of faults based on morphotectonic indicators in Iran and the world, the results of which indicate the potential of these indicators in analyzing the behavior of faults.
2. Material and Methods
In this research, the basin was divided into sub-basins initially in order to ease calculations and outcome comparisons. Then, using quantitative indicators, the researchers investigated the effects of tectonic activities on the valleys and riverbeds. The bulk of the data needed for indicators was acquired from 1:25000 scale topographic maps, especially digital and aerial maps (Keller & Pinter, 2002). The morphological indicators used for the region are the following: hypsometric curves and integrals, asymmetry index of the drainage area (asymmetry factor), symmetry factor of the transverse topography, stream length gradient index (SL), valley width to height ratio (VF), the V ratio, and drainage basin shape ratio (BS). After calculating the geomorphic indicators of the studied area, the researchers estimated the tectonic activities using the IAT index. The IAT index is obtained from the mean value of different geomorphic indicator classes.
3. Results and Discussion
In this research, the waterways and the basin border were marked using a 1:25000 and 1:100000 scale geologic maps, respectively.
3.1. The hypsometric curve and integral
Eight hypsometric curves were produced for the studied basin. According to the curves for sub-basins, sub-basins 1 and 2 had an under-curve area of above 50%, being relatively convex. This indicated the young age of the basin. Moreover, the convex form of the dimensionless curve indicated the prevailing of neo-tectonic activities over eroding ones.
3.2. Calculating the asymmetry index of the basin.
The asymmetry index was higher in basins 2, 5 and 6, and lower than 50 in 1, 3, 4, 7 and 8, which points out their deviation. The T index indicated the semi-asymmetric form of the majority of sub-basins that fall into class 2 regarding their tectonic activities. The river gradient index was one of the significant indicators for differentiating between active and non-active tectonic areas. The mean stream length gradient index or SL for the basins varied from 482 for basin 2 to 71 for basin 6. The VF index showed that the bulk of the basin is within the active area; indicating that tectonic activities have not given the stream sufficient opportunity for erosion. The basin shape index for each case indicated that basins 2 and 7 are active and elongated, while the rest are inactive and fall into class 3.
3.3. Relative evaluation of regional tectonic activates according to geomorphic indicators (IAT)
In this study, eight morphotectonic indicators were calculated for each of the 8 basins, dividing them into 3 geomorphic levels. Then, the values for geomorphic index (S/n) were measured discretely for each basin; being classified into 4 levels, i.e. the index of relative active tectonics (IAT) of the study’s expanse. The classification for geomorphic indicators proposed by Hamdoni, Irigaray, Fernandez, Chancon & Keller (2008) classified these indicators based on the quantitative value into 4 classes. The class quantities of all indicators were combined, and their mean values were presented as the IAT index, indicating the tectonic activity. According to the results obtained from the IAT index, there is very strong tectonic activity in basin 2, with strong, moderate and weak tectonic activation witnessed in the remaining basins. The results of evaluation showed that the basins’ tectonic activities range from very strong to strong, moderate and weak.
4. Conclusion
Quantitative measurement allows geomorphologists to compare different landforms in a factual, rational manner, subsequently calculating morphologic indices. Regarding the hypsometric curve and integral for sub-basins 1 and 2, the under-curve area was above 50%, indicating its young age. Also, the convexity of dimensionless curves indicated the prevalance of neo-tectonic activities over eroding ones. For sub-basins 3 to 8, the under-curve area was below 50%, resulting in a more concave hypsometric curve that indicated eroding activities in these sub-basins. Concavity in the dimensionless curve indicated the dominance of eroding activities over neo-tectonic ones.
4.1. The asymmetry index of the drainage basin
In basins 2, 5 and 6, this index was above 50, while for basins 1, 3, 4, 7 and 8, it was below 50. This indicated the deviation of basins. The topographic symmetry index indicated that the majority of basins are semi-symmetric, falling into class 2 regarding their tectonic activities. In the studied region, the SL index for the main waterways of 8 sub-basins was measured. The mean value of the gradient index or SL for the basins varied from 482 for basin 2 to 71 for basin 6. According to the valley width to height index, the bulk of the basin is within the active region. This indicated that tectonic activities have not given the regional stream sufficient opportunity for erosion. The V ration index, called the valley morphology index, was obtained via comparing the transverse area of the actual valley to the area of a hypothetical semicircle with a radius equal to the valley depth. In sub-basin 7, it indicated the existence of a valley with larger width and less depth, the type which incurs higher erosion.
The basin shape index indicated that basins 2 and 8 are active and elongated, while the rest are inactive and fall into class 3. The IAT index classified the tectonic activities of the region in the three classes of very strong, strong and moderate. According to this index, there is no basin with weak activities in the region. According to the segmentation map, the highest activity occurred in sub-basin 2, and the least in sub-basins 3, 4 and 7. The study and evaluation of different geomorphic indicators of the region show that it is young and active regarding neo-tectonic activities, yet this activity is not equal in all sectors. In sub-basin 2, young activities were the highest. This could be attributed to the stronger activity of the major and minor faults, resulting in the rise of the area.

Keywords

• Benarvan fault
• Neo-tectonic evaluation
• Morphological indicators

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