Analyzing the drainage system anomaly of Zagros basins: Implications for active tectonics

- Drainage system anomaly of 15 basins in Zagros Folded belt, Iran was studied.
- New equations for calculation of stream's hierarchical anomaly number are presented.
- More elongated and tilted basins have higher stream's hierarchical anomaly.
- Tilting & shape of basins are controlled by anticlines trend and uplifting rate.

Morphometric analysis of hierarchical arrangement of drainage networks allows to evaluate the effects of external controls especially tectonics on basin development. In this study, a quantitative method for calculation of stream's hierarchical anomaly number is introduced. Morphometric parameters such as hierarchal anomaly index (∆a), percent of asymmetry factor (PAF), basin Shape (Bs), basin length to mean width ratio (Bl/Bmw), stream's bifurcation ratio (Rb), bifurcation index (R), drainage density (Dd), drainage frequency (Df) and anticline's hinge spacing (Hs) of 15 basins in Zagros Mountains were examined. Results show that the strong correlations exist between pairs ∆a-PAF (r = 0.844), ∆a-Bs (r = 0.732), ∆a-Bl/Bmw (r = 0.775), ∆a-R (r = 0.517), PAF-Bl/Bmw (r = 0.519), Bs-R (r = 0.659), Bl/Bmw-R (r = 0.703), Hs-∆a (r = − 0.708), Hs-PAF (r = − 0.529) and Hs-Bs (r = − 0.516). The variations in trend of anticlines control the shape of basins so that where anticlines hinges become closer to each other in the downstream direction, basin become narrower downward and hence the ∆a increases. The more uplifted northeastern anticlines cause the trunk river of the basins to migrate toward the younger anticlines in southwest and hence ∆a increases because the trunk river receives a lot of first order streams. Data reveal that the rate of ∆a is higher in elongated synclinal basins. Due to the decrease in the intensity of deformation from northeast toward southwest of Zagros, the hinge spacing of anticlines increases southwestwards. Data reveal that the variation in hinge spacing of anticlines strongly controls the basin's shape and tilting as well as the hierarchical anomaly of drainage system. Since the elongation and tilting of basins are associated with the variations in rates of folding, uplift and hinge spacing of anticlines, it can be concluded that the hierarchical anomaly of drainages in studied basins is controlled by the intensity of Zagros tectonic activities.