
Abstract
The study area situated within the western part of the Zagros Fold-Thrust Belt, is recognized as one of the most complex structures within the high folded zone, particularly at the transitional point between the high folded and low folded zones. The current study focuses on the Bamo anticline, and it is characterized by three distinct segments (Northern, Middle, and Southern) based on the structural complexity and variation of its major trends. The Northern segment aligns with the main Zagros trend, which runs northwest-southeast. However, the Middle and Southern segments exhibit a nearly north-south trend, which differs from the main trend of the Zagros belt, and not similar with the surrounding structures. This unique configuration adds to the complexity and geological significance of the Bamo anticline in the region. The backlimb (eastern limb) of Middle and Southern segments is absent because the eastern limb of these segments has been affected by the Pshta thrust fault. This fault has thrust over the Pshta anticline and the Azgla anticline to the eastern limb of the Middle and Southern segments, respectively.
Detailed field geological and structural maps have been constructed for the study area. Additionally, five balanced cross-sections and their accompanying restored sections were created based on field observations and available seismic sections. In each segment, one detailed cross-section was constructed to represent the geological and structural features. Additionally, two more cross-sections were constructed to capture the transitions between segments, with one located between the Northern and Middle segments, and the other between the Middle and Southern segments. The shortening percentages for these cross-sections were determined, indicating an increase in shortening towards the southern part of the Bamo anticline as deformation, including folding and faulting, intensifies in that direction.
According to the sections and interpretation, it seems that an early compressional/transpressional tectonic phase occurred during the Late Cretaceous period, particularly at the lower part of the Tanjero Formation. During that period, the Khanaqin basement fault seemed to have undergone reactivation within a transpression tectonic phase, leading to the formation of upward reverse faults that appear similar to a possible positive flower structure. The Middle Tertiary and Quaternary compression led to the formation of the main frontal thrust-related folds. This thrust-related folding affected the entire area, with structural geometries also being influenced by previously formed transpressional structures. During this stage, transpressional faulting occurred, involving the upper stratigraphic successions and resulting in the formation of the positive flower structure.
This study examines the state of active tectonics along the Bamo anticline through quantitative analyses of the evolved landscape using geomorphic indices. For that reason, six indices have been chosen for the analysis, such as stream length-gradient index (SL), drainage basin asymmetry (AF), hypsometric integral and hypsometric curve (HI & HC), ratio of valley-floor width to valley height (VF), basin shape (BS), and mountain front sinuosity (Smf). Each index’s results were categorized into three classes, and the results from the first five indices, excluding Smf, were integrated to get the index of relative active tectonics (IRAT). This index was then compared with the results of Smf to assess the relative active tectonics (RAT) along the anticline. The results of the IRAT, classified into four classes from very high to low tectonic activity, reveal that no area is classified as class 1 (very high activity). However, 38% and 56% of the region are categorized as classes 2 (high activity) and 3 (moderate activity), respectively. Furthermore, the remaining 6% of the research area exhibits class 4 (low activity). The Smf values for the Northern, Middle, and Southern segments of the anticline are 1.12, 1.18, and 1.27, respectively. Consequently, based on the Smf data, all mountain fronts are classified as class 2 (moderate tectonic activity).
The current study also computed tectonic activity by combining six thematic layers of selected factors using the analytical hierarchy process technique: earthquake activity, fault density, folding intensity, elevation, slope, and lithology. The resulting tectonic activity mapping was classified into four classes: very high (class 1), high (class 2), moderate (class 3), and low (class 4). The analysis revealed that 7.8 km2 (2.04%) of the study area was classified as very high (class 1) tectonic activity, approximately 93.5 km2 (24.47%) of the study area is classified as high (class 2) tectonic activity, nearly 185.49 km2 (48.54%) belongs to moderate (class 3) tectonic activity, and the remaining area, approximately 95.36 km2 (24.95%), belongs to low (class 4) tectonic activity.