Investigation of active tectonics in Koohrang region with a special look at the behavior of Koohrang salt dome

Changes in land surface shape in active tectonic  zones result from interplay between tectonic forces and surface processes (Keller and Pinter, 1996; Keller et al., 1999). Salt Domes  are dome-shaped Geological structures whose central core is composed of salt. Their economic importance is due to the location of oil tanks in structures with salt domes and also the presence of sulfide sources of potash, iron and salt in them. Zagros region, in which Koohrang city is located, is a pressure zone where Koohrang fault is located in the southwest, Dehno fault is located in the east , Mohammadabad fault is located in the north of and Zardkuh fault is located in the south of salt dome. In the meantime, the salt dome has come to the surface with a lower density due to the movements of faults and stress from the Arabic plate to the Iranian plate. There are many linear springs in this area, which are faults and indicate the activity of faults in the area.

In this study, in order to evaluate the new construction activity of the region, the location of Koohrang salt dome was determined, using Baba Haidar 1: 100000 maps, Shahrekord 1: 250,000 map.  Using the data of these maps as basic data in ArcGis software (10.3), drainage network and digital elevation model (DEM) of the study area were used as basic elevation data. Using the data of these maps as basic data in ArcGis software (10.3), and digital elevation model (DEM) of the region and ArcHydro toolbar, 98 basins were considered for the study area. Then, according to the characteristics of the region, drainage basin asymmetry index (Af), hypostometric integral index (Hi),stream length gradient index (SL), mountain front sinusitis index (Smf), valley to depth-to-depth ratio index (Vf), index Relative activity (Iat) was calculated for the study area and the results of each index are plotted. 

In terms of basin asymmetry index, 36 basins are active in class 1 and in terms of tectonic activity, 38 basins are moderately active, 24 basins are in class 3 and have low tectonic activity. In terms of hypsometric integral index, 15 basins have new active tectonic activity (their height is higher than the average height of the study area, high areas with deep incisions), 17 basins are in class 2 and have medium activity and 66 basins have low tectonic activity. For the SL index, 19 basins in class 1 have high tectonic activity, 8 basins in class 2 have medium tectonic activity, 34 basins in class 3 have low tectonic activity and 37 of its basins have very little new tectonic activity, which are located near the main river. For the mountain frontal sinusitis index, 28 basins have high tectonic activity, 12 basins have medium tectonic activity, 7 basins have low tectonic activity and 51 basins have very low activity (data not obtained). The highest value of Smf index is 1.04 for basins 2, 13, 23 and the lowest value is 1.69 for basins that are in class 3 and yellow in the map . The lower the value of the index, the more active the tectonics of the region and the straight mountain fronts. In valley to depth-to-depth ratio index, a number of basins that cut off the mountain winding all include V-shaped and young valleys. In the general survey, 8 basins are very active, 16 basins are active, 54 basins have low tectonic activity and 20 basins have very low tectonic activity.

In this study, according to the measurement of stream length gradient index (SL) , drainage basin asymmetry index (Af), mountain sinusitis index (Smf), hypostometric integral index (Hi), valley width to depth ratio index (Vf), Relatively active tectonics index (IAT) has been used to study the relative active tectonics activity of the region. According to the zoning map obtained from the above indicators, the most activity in the study area is related to the basins that have active faults and high activity of the newest structure can be seen in the heights of Klonchin, Khadang Valley and Zardkooh. In this area, Koohrang salt dome and agricultural lands, Koohrang city and Dehno village have low to medium activity. The more we move from the Koohrang salt dome to the heights of Zardkooh, the more and more active the new tectonic activity of the region. Evidence from the diversion of streams, digging the bed of alluvial fans, V-shaped valleys and triangular shapes in the study area show high neotectonic activity This part (Zardkooh heights) of the study area.