Introduction

Folds are geological structures formed by the bending or curving of the Earth's crust due to compressive stress after rock layers have been deposited. These structures rarely remain intact over time, as they are continuously reshaped by tectonic activity and erosion. Rivers, in particular, play a significant role in altering the roughness and form of these folds. Given the critical importance of understanding fold patterns in various geological systems, it is essential to analyze and describe the elements of fold styles as a core component of structural geology studies. Understanding the mechanisms behind folding, the conditions that perpetuate geomorphological changes, and how erosion impacts these structures highlights the significance of this research.

In the Zagros region, the relationship between erosion and tectonics remains unclear. Therefore, this study aims to explore this relationship by examining the morphometrics of anticlines and identifying indicators of river activity's impact on the erosion of folded structures in Kermanshah province. The folded Zagros zone in Kermanshah province was selected for this study due to its characteristic northwest-southeast orientation. In this direction, anticlines and synclines feature north-west and south-east plunges, often exposing the Asmari limestone formation.

Methodology

To achieve the research objectives, the study began by mapping the axes and extents of the anticlines in the study area using 1:100,000 geological maps. The boundaries of the waterway catchment areas on both sides of the anticline ridges were delineated using the Arc Hydro add-on in the ArcGIS environment. Each catchment area was then named and numbered for every anticline.

The slope of each catchment area was determined based on geological signs from the 1:100,000 maps and included in the descriptive table of each catchment layer. Following this, the morphometric parameters of the catchment areas were extracted and analyzed. The identified anticlines and their respective parameters were entered into Excel and SPSS for further statistical analysis.

Results and Discussion

1. Relationship Between Anticline Area and Erosion: Measurements of the anticlines in Kermanshah province revealed a total area of 6435 square kilometers for 20 anticlines. The smallest anticline covered 21 km², while the largest spanned 2868 km². The analysis indicated that larger anticlines have correspondingly larger catchment basins, suggesting that river erosion occurs in parallel. A paired T-test on the significance of the anticline areas (x̄ = 321 km²) and their eroded areas by catchment basins (x̄ = 297 km²) showed no significant difference, implying that the anticlines have uniformly reacted to erosion. The statistical analysis also revealed that there is no significant relationship between the total surface area of the anticlines and their drainage basins, indicating that the ratio of these areas can reflect differences in erosion levels. By calculating the percentage of the basin area relative to the total anticline area, a ratio was obtained showing the proportion of anticlines protected from erosion.

2. Drainage Patterns on Anticlines: The study identified 4847 catchment areas across 20 Zagros anticlines in Kermanshah. Out of this total, 2302 catchment areas are located on the western banks of anticlines, 2338 on the eastern banks, and 207 related to closes. Rivers have carved out 13443 kilometers of flow paths, with significant differences in river length between the western, eastern, and close ruz. The total length of rivers is 4556 km on the western banks, 3679 km on the eastern banks, and about 3679 km for closes. Statistical tests showed no significant difference between the lengths of ruz on either side of the ridges but highlighted a clear difference between ruz and closes. Closes have caused more significant destruction compared to ruz.

3. Drainage Indicators: Three main characteristics were used to investigate the drainage situation on anticlines: drainage density, river frequency, and drainage texture. Drainage density is calculated from the length of the river divided by the catchment area, providing an index of the degree of cutting in the anticlines. The study found that the average length of streams in the western ruz, eastern ruz, and closes is 2.9 km. This significant difference between closes and ruz can serve as a decision index for distinguishing them in satellite images. A statistical test of variance on the average index of stream length for the western, eastern, and close ruz confirmed no significant difference between ruz on both sides of ridges but a notable difference between ruz and closes. The index of the average length of the catchment area, considered in this research, can better indicate the slowness and depth of the rivers compared to the usual indicators of drainage density and texture.

Conclusion

1. All anticlines generally react to erosion in a similar manner, indicating that if the structural formation plays a role in erosion, it is not significant at the scale of the entire anticline. This uniformity suggests that structural influences on erosion are consistent across different anticlines.

2. There is no significant relationship between the total surface area of anticlines and the area of their drainage basins. This lack of correlation means that the ratio of these areas can illustrate the variation in erosion levels among different anticlines. Calculating the percentage of the basin area relative to the total anticline area provides a ratio indicating how much of the anticline is protected from erosion.

3. The study found no significant difference between the lengths of ruz on both sides of ridges. However, there is a clear distinction between ruz and closes, with closes causing significantly more destruction. This difference underscores the impact of river activity on the erosion of folded structures.

4. The index of the average length of the catchment area is a more effective measure of river activity's impact on erosion compared to traditional indicators like drainage density and texture. This index can help identify the slowness and depth of rivers more accurately, providing valuable insights for understanding erosion processes.