Active tectonic assessment related to the Ghalajeh anticline using hierarchical anomalies of stream networks, west Iran

The vertical and horizontal movements of the Earth crust have caused extensive changes in surface phenomena, in the active tectonic regions. In order to measure some of these changes, morphometry analyses have been used to evaluation of the amount of tectonic activity. These analyses are useful tools for analyzing existing feature at the ground level and provide a proper understanding of the condition of drainage network, changes of mountain front and uplifting. The anticlines of the Zagros fold and thrust belt indicate some evidences of tectonic activity. The Ghalajeh anticlines is one of the folded structures in the Zagros fold and thrust belt. So, this study has focused on assessment of tectonic activity along the northeastern and southwestern of limbs of this anticline, using quantitative study and analytical hierarchy analysis of stream patterns. These analysis as an indicator of geomorphic systems can help us to understand the behavior of tectonically drainage basins.

In this study, Google Earth images, digital elevation model (DEM) and GIS software have been used for extraction of drainage network pattern and divide of the area around the Ghalajeh anticline into seven basins. Then, the streams of each basin were ordered and the order and length of each stream were measured. The indexes of hierarchical anomaly (Δa), drainage density (Dd), drainage frequency (Df), bifurcation ratio (R), bifurcation index (Rb), direct bifurcation ratio (Rbd), crescentness index (CI), precent of basin asymmetry (PAF), basin shape (BS), basin length to mean width ratio (Bl/Bmw), mean length of first order streams (LN1), Hinge spacing (Hs), Hypsometry integral (Hi), for each basins was calculated separately and the obtained results were analyzed in the Spss software.

The Zagros fold and thrust belt is the result of convergence between the Arabian plate and Iranian micro-continent due to closure of the Neotethys Ocean and subsequent continent continent collision. This belt is composed of long and asymmetrical anticlines with NW–SE trend and most of these folded structures indicate asymmetric geometry (the steepest limbs are on the southwestern limbs). The rate of tectonic activity is not same in all parts of the Zagros fold and thrust belt, and the uplift and shortening rate on each anticline is different. These mentioned changes caused differences in the amount of morphometric indexes as the indexes related to the drainage network. The study of drainage network pattern of the study area exhibits three types of drainage pattern along the Ghalajeh anticline. The first is the dendritic drainage pattern which are usually observed in wide or circular basins (basins1, 4, 6, 7).The second cluster is a parallel drainage pattern that is usually seen in long and narrow basins, such as the basins 5, 2. The third category of these drainage patterns is a parallel - trellis pattern that is integrated from the parallel drainage pattern and trellis (basin 4). The surface outcrop of the southwest limb of the anticline are the Gachsaran formation and the quaternary deposits and the Asmari-Shahbazan and Aghajari formations and quaternary deposits cover the surface outcrop of the northeast limb. Because of, existence of the evaporative rocks (salt and gyps) in the southwest limb, erosional basins (basins 6 and 7) have formed (sensitive to erosion and weathering) along this limb. But in the northeastern limb, most of the rock units are made of thick layer of limestone and resistant to erosion. Due to the deep drilling and erosion in the southwest of the limb, the number of stream order 1 decreases and the index values LN1 are reduced and consequently the index of DF is decreased. The minimum and maximum value of Hat index is 185 (basin 1) and 527 (basin 5) respectively. Result show an increase in Hat values suggest increase in the number of low order streams (order1) which is connected to the upper order (4 or 5). The maximum and minimum calculated values for Bs and Bl /Bmw indexes are in the basin 5 in southwest limb and basin 3 on the northeast limb respectively, showing the straight relationship between these two indexes. Also, calculated values for Hi index indicate medium tectonic activity and low tectonic activity of basins. The basin 4 contains the highest number of R and Rb indexes while the basin 6 contains the minimum value of the R, the basin 2 shows the lowest numeric value for the Rb index.

The results show that there is a strong positive correlation between Bs-Bl/Bmw indices as a result of a direct linear relationship with coefficient 0.93. Also, positive correlation between the R - Rb indices, resulting in a direct linear relationship with the 0.93 coefficient. There is a negative correlation between the R-DF indices and a strong negative correlation between the Δa-Hs indexes. It can be concluded that the southwestern limb of the Ghalajeh anticline is more active than the northeastern limb, according to the index values obtained for each basin and correlation between these indexes. The basin number 6 and 7 in the southwestern limb and basin number 1 in the north northeastern limb are affected by the intensive erosional processes resulting from tectonic activity.