Morphometric analysis of alluvial fans around anticlines in the Fars Zagros (Case study: the Chah-Nahr, Dasht-Kenar, and Sherkat Anticlines)

Introduction
Alluvial fans represent prominent depositional landforms that have developed across extensive areas of Iran, particularly in arid and semi-arid regions. These geomorphological features form at the transition zone where fluvial streams descend from steep mountainous terrains to low-gradient plains, leading to a rapid decrease in sediment transport capacity and initiating deposition. The widespread development of alluvial fans dates back to the Quaternary period, and these structures have often served as suitable substrates for human activities and settlement expansion. From a process-based perspective, alluvial fans are primarily categorized into two main types: those formed by debris flows and those resulting from alluvial processes or sheet floods. The piedmont alluvial fans in arid and semi-arid regions are direct reflections of the complex interplay between tectonic, erosional, climatic, and sedimentary processes. Within the Zagros Folded Belt, and specifically in the Fars Zagros, the evolution of anticlines within the framework of active Alpine-Himalayan tectonics has created conditions for the activity of marginal alluvial fans, which act as sensitive geomorphic indicators of tectonic movements. The primary objective of this research is to conduct a comparative assessment of the morphometric indices of alluvial fans marginal to three active anticlines in the northeastern part of Lar (Chah- Nahr, Dasht-Kenar, and Sherkat anticlines) and to analyze the controlling role of tectonics in their geometry, dimensions, and evolution. To achieve this, by simultaneously examining the morphometric characteristics of the alluvial fans and their upstream drainage basins, an attempt has been made to elucidate the relationship between quantitative indices and the relative intensity of tectonic activity. Iran, as part of the Alpine-Himalayan orogenic belt, is situated within one of the most tectonically active regions globally. Among these, the Zagros Mountains hold a unique position due to their prominent uplift, shortening, and extensive seismicity, making them a focal point for tectonic studies. The anticlines of the Fars Zagros, owing to their persistent tectonic activity, provide an ideal setting for studying the evolution of marginal alluvial fans. The process of anticline growth and uplift can directly influence the morphometry, morphology, and evolutionary trajectory of these alluvial fans. Numerous national and international studies have indicated that alluvial fans are among the most sensitive geomorphic features to tectonic activity, and changes in their morphology and morphometry are a direct manifestation of active tectonic movements. Research conducted on the Zagros have revelaed that tectonic activities have effectively controlled the morphometry of alluvial fans. For example, Bahrami (2013) evaluated the effects of active tectonics on the morphometry of alluvial fans aroud Danehkhoshk anticline in the Folded Zagros.
In this study, the geometric and morphometric properties of alluvial fans including area, topographic slope, width to length ratio, sweep angle as well as dip of anticline layers were calculated. Results showed that parameters such as sweep angle of alluvial fans has a positive relationship with dip anticline layers, and alluvial fans with higher values of sweep angle and width to length ratio reveals the tectonically more active segments of Danekhoshk anticline. Also, the weak relationship between the morphometric parameters of drainage basin (area and topographic slope) and parameters of alluvial fans is likely due to the complex karstic features of the drainage basins that make a complicated hydrologic system. The mentioned study has confirmed that, although climate can also affects aggradation of alluvial fans, tectonic activities effectively control the morphometric properties of alluvial fans.
In the comparative analysis among alluvial fans of the three studied anticlines, the ANOVA test results showed that the mean fan area and topographic slope differed significantly between the anticlines, while the differences in sweep angle, width-to-length ratio, and FCIR between anticlines were not statistically significant. On average, the alluvial fans surrounding the Sherkat anticline exhibited larger areas and higher FCIR values, which indicate a more prominent role of tectonic activities or specific structural conditions of this anticline in creating greater fan irregularity. Comparing the alluvial fans in the central and plunge zones revealed that in the central anticlinal regions, the average area, width-to-length ratio, topographic slope, and FCIR were higher, whereas the sweep angle was greater in the plunge areas. This pattern might suggest that in the central regions, due to greater uplift and steeper slopes, the system's energy increased, and leading to steeper and more geometrically irregular alluvial fans. Conversely, in the plunge zones, with a relative decrease in topographic slope, the alluvial fans experienced more lateral expansion, resulting in an increased sweep angle. The examination of coalesced and un-coalesced alluvial fans also showed that coalesced fans, on average, had larger areas, steeper slopes, and higher FCIR values compared to un-coalesced fans. This could be attributed to spatial constraints along the anticline margins, high rates of sediment deposition, and the simultaneous lateral progradation of alluvial fans in response to tectonic uplift. Results of drainage basin parameters indicated that higher values of the Basin Shape Index (Bs) and Hypsometric Integral (Hi) were predominantly observed in the basins of the Dasht-Kenar anticline, suggesting a relatively younger topography and a greater influence of recent tectonic activities. In contrast, higher Circularity Ratio values in some basins of the Chah-Nahr anticline indicate more erosion and a tendency towards more rounded shapes. Statistical correlations demonstrated a significant positive relationship between basin area and fan area, as well as between basin topographic slope and fan topographic slope, indicating a direct dependency of alluvial fan characteristics on the morphometric conditions of the upstream basin. Numerous studies (Alavi, 1994; Heydari, 2007; Alipoor et al., 2011, Jafari et al., 2023; Delchiaro et al., 2023) have shown that the Zagros belt is one of the most tectonically active fold belts in the world, and the lateral and vertical movements of folds in this region play a significant role in the evolution and morphometry of their landforms. A quantitative analysis of 125 alluvial fans around the Chah-Naاr, Dasht-Kenar, and Sherkat anticlines indicates that the tectonic activities of the anticlines, including their vertical and lateral growth and flank slopes, have a decisive role in the morphometry of the alluvial fans 
and their upstream drainage basins
Conclusion
Overall, the findings of this research indicate that the morphometric indices of alluvial fans, particularly the modified fan conformity index (FCIR), topographic slope, and area, are effective tools for assessing the relative degree of tectonic activities in active fold zones. The observed differences among the anticlines, as well as among different structural positions, demonstrate that the lateral growth and heterogeneous uplift of anticlines play a decisive role in controlling the geometry and evolution of marginal alluvial fans. Therefore, quantitative analysis of alluvial fans can be employed as a complementary approach in neotectonic studies and the assessment of the geodynamic evolution of arid and semi-arid regions.