تاثیر بلندای قدیمی هندیجان بر هندسه ساختاری و تکامل تکتونیکی تاقدیس مدفون تنگو (جنوب غرب ایران)

IntroductionThe reactivation of the Paleohighs, including the Hendijan basement fault, has created a series of tectonic buildings in the northwest of the Persian Gulf and Abadan plain. Their structural trends and formation mechanisms are different from the folds formed in the main phase of the Zagros folding in the Neogon. In this study, based on geological maps, satellite imagery, seismic profiles and well data, geometric analysis of the Tangu anticline in the southern part of the Dezful Embayment was carried out in relation to the Hendijan Paleohigh performance. Then the effective factors on the geometry of this anticline are identified.Materials and MethodsTo investigate the geometry of the Tengu anticline in southwest Iran, the seismic profile perpendicular to the axis of this anticline is analyzed. The quality of seismic profiles allow the identification and interpretation of structures down to the Fahlian formation (figure 2). This profile shows an upright fault with a slope to the east which due to its function, incision and upwards moving and as a result the thrust component in the eastern part of the anticline is visible. Thinning on the fault zone of the Fahlian formation (Cretaceous) to the middle Aghajari formation (Upper Pliocene) has occurred, which shows the activity and uplift of the Hendijan fault during the Cretaceous to Tertiary times. Also in this seismic profile, thickening in the Aghajari formation is evident. This case shows two different fold patterns in depth and surface, in which the Fahlian to Asmari formations, we see an asymmetric fold with a western limb with more slope than the eastern limb. While in the upper part of this section, the curvature of the layers has produced a gentle fold. It seems that the difference in the two patterns of the folds’ geometry indicate two distinct compression and shortening phases that will be discussed.Results and discussionThe calculation of the geometric values of the Tangu anticline, including the L parameter, ratio of the size (R), bluntness of fold (b(, the Fourier coefficients, and their position in the corresponding diagram, indicate that the Tangu anticline is an asymmetric fold, and has a sinewave to chevron shape. Also, the interlimbs angle, bluntness of fold (b) and the variations in the thickness of the limbs relative to the hinge area based on the isogone line, represent this anticline of the subangular type. They suggest 1A class for it and as the seismic section shows, the thickness of the fold limbs is more than the hinge area. Due to the aspect ratio of the Tangu anticline which is 0.75 and the position of the Tangu anticline in 1A class, and also its position on the Hendijan paleohigh, this anticline is a forced fold type. Its formation is directly related to the movement and uplift along the Hendijan basement fault. The northeast trend of Hendijan basement fault is one of the pre-existing faults that were inherited from the Pan-African structure in the Arabian plate. As an effect of the reactivation of this oblique fault, the trend of the main Zagros structures represent dextral displacements. Due to the effects of this fault’s lineament on the change in the pattern of the isopach maps in the Permian Triassic, it can be concluded that, according to the normal movement of the Hendijan-Izeh fault in Permian Triassic, this fault probably acted as a deformation transformer between the normal faults in the northeast of the Arabian plate. At the end of the middle Cretaceous and the onset of the upper Cretaceous, a significant change has occurred in the tectonic regime from the passive continental margin to the active margin. This was related to the oceanic crust subduction and the placement of ophiolites in the northeastern margin of the Arabic plate. The reactivation of the basement faults has led to the formation of paleohighs and depressions between them in the Dezful Embayment and Persian Gulf. The presence of these paleohighs has caused severe changes in facies and sedimentary disconformity in the form of interruption or erosion in the carbonate formations during this period time (Hajikazemi et al. 2010; Mehrabi and Rahimpour-Bonab 2014; Farzipour-Saein, et al. 2009). ConclusionIn the northwest of the Persian Gulf, the role of Hendijan oblique basement fault in controlling the stratigraphy of the region, has played a major role in the formation of sedimentary disconformity at the upper boundary of Sarvak formation and the absence of stratigraphy in the Ilam and Gurpi formations. Our results show that the rough topography caused by the activity of this basement fault is the main factor of thickness reduction in the Cretaceous to the Tertiary sediments. The gentle folding in the Tertiary seismic reflectors indicate that the Hendijan height has experienced another growth phase during the Tertiary period. The upper Aghajari onlapping syn-tectonic sediments on the northwestern and southeastern flanks of the Hendijan heights correspond to the reactivation of the Hendijan paleohigh in the Early Tertiary. This indicates the main phase of the folding after the Cretaceous. Also, the activity reflected in the Aghajari syn-tectonic sediments suggest the Pliocene age as the start of the main folding phase in the Tangu anticline.