Quantitative evaluation of morphometric parameters of drainage system in the forelimb and backlimb of the Asmari Anticline, Zagros, Iran

Quantification of drainage basins and their networks on fold limbs can lead to a better understanding of spatial relationship between active tectonics and drainage variations. This study aims to evaluate the effect of vertical fold growth on the morphometry of drainage basins and their related networks formed in the Asmari Anticline in the Zagros Fold-Thrust belt. This belt is one of the youngest continental collision zones hosting one of the largest petroleum provinces in the world so that giant oil fields are found in its anticlines. We selected the Asmari Anticline due to the variability of drainage basins and stream networks across the fold's forelimb and backlimb. The basin area (Ba), topographic slope (TS), hypsometric integral (HI), basin shape (Bs), drainage basin orientation (DBO), crescentness index (CI), sinuosity of the main drainage (Smd), spacing ratio (R), and fault density (FD) for 68 drainage basins were calculated. Also, the morphometric characteristics of drainage networks including drainage density (Dd), drainage density of 1st-order streams (Dd1), and drainage frequency (Fs) were analyzed for each limb. Results show that southwestern limb (forelimb) is characterized by high topographic and dip slope, large and relatively circular basins, with high values of CI, Smd and DBO, implying pronounced lateral and headward erosion. Conversely, the smaller and elongated basins, with higher values of hypsometric integral and spacing ratio in the northeastern limb, show lower erosion of backlimb. Higher Dd, Dd1, and Fs values in the southwestern limb (9.54, 6.39, and 39.92 respectively) than the northeastern limb (8.96, 5.69, and 28.33 respectively), suggest higher rates of forelimb erosion, especially where dendritic drainage pattern is developed. This study implies that the fold's divide migration towards NE during fold growth has played a role in the variations of the morphometric parameters in the southwestern and northeastern limbs of the Asmari Anticline. Also, faults and fractures have important effects on the mentioned morphometric parameters. Higher density of faults dominantly with NW-SE trend in the SW limb, especially where trellis drainage pattern is developed, has also facilitated the lateral erosion of the steeply dipping forelimb, resulting in the enlargement of basins with higher values of Ba, CI, DBO, S (spacing of adjacent basins outlets), and Smd.