Active tectonics evaluation from morphometric indices for the Dinevar Basin in northern Zagros, western Iran

Active tectonics evaluation from morphometric indices for the Dinevar Basin in northern Zagros, western Iran

Introduction
Active tectonic assessment has significant applications in environmental planning, natural hazard assessment, and landuse management (Pedrera et al., 2009). Active tectonic zones are constantly at risk of devastating earthquakes that seriously threaten human life and properties. Quantitative evaluation of tectonic activities can provide more accurate information about the state of tectonic activity. In this regard, the analysis of morphometric parameters of drainage networks—as the quantitative description of drainage networks (Strahler, 1964)—has shown particular usefulness in identifying tectonic activities (Gao et al., 2016). The morphometric analysis involves the calculation of linear, shape, relief, and slope parameters of the drainage networks (Nag and Chakraborty, 2003). Quantitative assessments of drainage networks identify tectonic and/or erosional transformations responsible for the evolution of landscapes (Segura et al., 2007). In tectonically active regions, drainage networks exhibit the relationship between surface processes and structural deformations (Burbank and Anderson, 2001), and thus their morphometric parameters assist in identifying active tectonic zones (Chen et al., 2003).
Study area
The Dinevar basin with an area of 2214 km2 is extended from 47 03 E to 47 50 E and 34 25 N to 34 50 N in the west of Kermanshah Province, western Iran (Fig. 1). The basin is a seven-order basin with a dendritic pattern and involves 13 sub-basins with areas ranging from 17 to 571 km2. Geomorphologically, the Dinevar basin constitutes of three units, fluvial plains, hills, and highlands. The highlands of Dalakhani, Bisotun, and Hajar are mainly composed of carbonate formations and characterized by steep slopes and high relief.
Materials and methods
Geomorphological characteristics of the Dinevar basin were investigated during a field study in 2020. The boundaries of the Dinevar basin and its sub-basins were extracted from the DEM in GIS environment. Stream ordering was determined using the Strahler ordering system (Strahler, 1964), and then geometric features of the basin (area, environment and length of the basin), characteristics of the drainage network (number and length of streams) and topographic features (minimum, medium and maximum elevations) of the Dinevar basin and its sub-basins were calculated. Subsequently, values of the morphometric parameters including six linear, six areal, and three relief parameters were calculated (Table 2). The main parameters were determined using the Principal Component Analysis (PCA) method. Based on the values of these parameters and the Relative Active Tectonic Index (RATI) method, the active tectonics of the Dinevar basin and its sub-basins were calculated and classified. Finally, by taking the geological, lithological and geomorphological conditions of the basin into account, the active tectonic and its spatial differences in the Denver basin and its sub-basins were determined.
Results and discussion
The circularity ratio (Rc) is expressed as the ratio of the area of the basin to the area of a circle having the same perimeter as that of the basin (Strahler, 1964). Rc is mainly controlled by geology and structure, relief, slope, climate, stream frequency and length within the basin area (Rai et al., 2018). High Rc values, approaching 1, indicate that the basin shape is circular. The Rc values of (0.4> Rc), (0.4 Rc) represent high, medium and low tectonic activity, respectively (Shukla et al., 2014). The Rc value for the Dinevar basin is 0.17 and fluctuates between 0.06 and 0.33 for the sub-basins. Based on the Rc values, the Dinevar basin and all its sub-basins indicate high tectonic activity. This is mainly due to the tectonic uplift of the Dinevar basin which reinforces the erosion processes and thereby prevents the formation of circular basins.
The relief ratio (Rr) determines the overall slope and the degree of erosion of the basin (Sanaullah et al., 2018) and reflects the lithological conditions. Based on the Rr values, drainage basins are divided into three classes with high tectonic activity (Rh <0.1), medium tectonic activity (Rh> 0.1 <0.05) and low tectonic activity (Rh <0.05) (Shukla et al., 2014). The Rh value in the Dinevar basin and sub-basins 1, 2, 5, 6 and 8 is less than 0.05 and indicates low technical activity. The Rh value for sub-zones 7, 9 and 12 are more than 0.1 indicating high tectonic activity in these basins. The elongated shape of the basins is one of the important factors responsible for the low values of Rh parameter in the Dinevar basin and most sub-basins. The Ruggedness number (Rn) for the Dinevar basin 4.48 and varies between 2.97 and 5.32 for its sub-basins (Table 3).
The ordering of the sub-basins follows a reverse trend, where sub-basins with the lower values for each parameter have an order of 5. The RATI value for the Dinevar basin is 2.35 and varies between 2.05 and 4.35 for its sub-basins (Table 6). Based on the RATI values, the sub-categories are classified into 5 categories: class 1 with very high tectonic activity (2.05 - 2.51), class 2 with high tectonic activity (2.51 – 2.97), class 3 with medium tectonic activity (2.97 - 3.43), class 4 with low tectonic activity (3.43 – 3.89) and class 5 with very low tectonic activity (3.89-4.35). Accordingly, sub-basins 2, 5, 7 and 8 fall into class one with highest tectonic activity, while sub-basin 3 and 4 place in class five with lowest tectonic activity.
Conclusion
In this study, we used linear, areal, and relief morphometric parameters to evaluate the active tectonics of the Dinevar basin and its 13 sub-basins located in the NW Zagros Mountains. Based on the results of the RATI method, the Dinevar basin places in a class with very high tectonic activity. The tectonic activity of the sub-basins located close to Sahneh and Mianrahan faults, as well as the sub-basins developed in the Bisotun massif, is higher than the other sub-basins. Sub-basins with very high, high and moderate tectonic activity cover about 96% of the area of the Dinevar basin, indicating an increase in tectonic activity in the entire basin. In general, the active tectonic of the Dinevar basin can be inspected in two aspects. First, the active tectonic and tectonic uplift of the Dinevar basin result from the shortening of the high Zagros zone due to the pressure from the Arabian plate. Evidence such as sub-basins being at the young stage of the erosion cycle, young drainage networks, high relief of the sub-basins, high drainage density, narrow and deep stream beds and the lack of hierarchical order in the drainage network all are associated with the tectonic activity and tectonic uplift of the entire Dinevar basin due to shortening of the high Zagros zone. Second, the existing faults in the basin are active and cause changes in the flow path of the streams, displacement and disconnection of the streams, and drainage network anomalies. Therefore, as revealed by our results, the NW Zagros is tectonically very active and the occurrence of devastating earthquakes in this region is very likely, it is thus necessary to pay more attention to this issue in the urban and regional planning.

Keywords: Active tectonic, drainage network, morphometric indices, RATI, Zagros.