Morphometry of drainage basins and stream networks as an indicator of active fold growth (Gorm anticline, Fars Province, Iran)

Abstract

The Zagros Folded belt, as one of the most tectonically active areas in the world, contains NW–SE trending whaleback anticlines that are growing vertically and laterally. Morphometric indices are widely used in tectonic geomorphological research because of their low cost and relative ease of application. Drainage basins and their networks adapt to tectonically induced changes in surface slope, and hence their qualitative and quantitative analysis can reveal aspects of active anticlinal growth. This study evaluates the morphometric properties of drainage basins and their networks developed on the fold limbs, using this quantitative data to detect lateral propagation of the Gorm anticline in the Zagros Simply Folded Belt (ZSFB). The Gorm anticline was considered to be suitable for this approach because of the variability of drainage basins and their networks along the axis of the anticline and also transversely across its forelimb and backlimb. The morphometric properties of stream networks including drainage density (Dd), drainage frequency (Df), ratio of 1st-order streams to the total number of streams of all orders (N1/N), length of 1st-order streams (L1), spacing ratio (R)of main streams on flanks, and drainage patterns were analyzed in three zones of the anticline (southeastern (SE), central, and northwestern (NW)). Also, the morphometric indices of drainage basins such as basin area (Ba), shape (Bs), asymmetry factor (AF), hypsometric integral (HI), a new proposed crescentness index (CI), and sinuosity of anticline divide (SAD) were evaluated. Results show that an increase in N1/N ratio and L1 from the center towards both the southeastern and northwestern fold noses suggests lateral growth of the anticline towards the southeast and northwest. The SE and NW zones are characterized by the smaller, elongated and crescent-shaped basins, lower elevations, lower dip slopes, higher hypsometric integral, and lower sinuosity of anticline ridge. These results collectively suggest that the southeastern and northwestern fold noses are younger, less eroded and laterally growing. The formation of crescent-shaped basins with asymmetric forked drainages developed in the SE and NW zones are considered to be useful indicators of lateral growth of the Gorm anticline. Although higher values of Ba, SAD, Df and S indexes in the central part of the fold can be attributed to the older relative age of this area, the activity of the Nezamabad Fault and the exposure of some weak rocks also play some roles in explaining the increase the values of parameters. Overall, the N1/N, L1, CI, SAD indexes, as well as the fan-shaped and asymmetric forked drainage pattern are the most useful tectonic geomorphological criteria to detect lateral propagation of anticlines.