بررسی تغییرات مورفومتری در سطوح میزبان و روند رخنمون گنبدهای نمکی سری هرمز

Abstract
Salt domes of Hormoz Formation has outcropped in large areas in south and southwest of Iran, Persian Gulf, and Eastern Saudi Arabia, with their extension delimited by major faults such as Kazeroon, Zagros, Minab, and Oman-Ural axis. Today, these domes have found various industrial applications such as radioactive waste disposal, gas storage, and tourism. In the meantime, these contribute to such problems as increased salinity of water and soil resources. Even though researchers have referred to the faults and density difference between salt and overlying sediments as the reasons behind the outcrop of the salt domes of Hormoz Series, the domes are significantly different in terms of morphometric indices and outcrop location. The present research is aimed at identification of the effects of tectonics on the form of the salt domes by calculating some morphometric indices (roundness, elevation, area) and analyzing them in Zagros and Persian Gulf zones while detecting the trend along which the domes have outcropped.
Materials and method
In this research, 1:250,000 geological maps, satellite images and digital elevation model (DEM) were used to identify locations of salt domes and digitize the areas where the domes outcropped followed by the calculation of morphometric indices (roundness, elevation, and area) using ArcGIS Software. Subsequently, through elevation-based classification of the outcrop area on the basis of elevation and physiographic similarities and separation of salt domes based on host fold and local outcrop surface (onshore or offshore), the morphometric indices were analyzed in each area. Moreover, trend diagram was utilized to identify trend of the salt domes.
Results
The results indicate that, the farther one moves from Persian Gulf region toward higher latitudes and higher onshore areas, the lower is the roundness of the salt domes while their elevation increases, so that the salt domes exhibit larger area and further roundness yet lower elevations in expansion zone of Zagros folds and synclines. This is while, the domes within the trust zone, contraction zone of folds, and high lands show higher elevation yet smaller area and lower roundness. Investigations based on the trend diagram and the researchers’ work show that, the domes outcropped at levels higher than Jahrum and Fasa and High-Zagros Faults follow the trend of the major faults, i.e. a northwest-southeast trend. However, the domes outcropped at levels below these faults tend to follow basement faults and form along a northeast-southwest trend.
All of the outcropped salt domes of Hormoz Formation within Oman, Persian Gulf, and Zagros zones have been developed along basement and major faults. However, the faults are not visible in Persian Gulf zone because of the presence of water bodies, and also the basement faults in Zagros-Fars and Hormozgan areas are commonly buried because of erosion of the respective anticlines. These have made it somewhat difficult to explore the relationship between the faults and salt domes. For the bare salt domes outcropped within the contraction and high zone of Zagros, it is easy to detect their connection to the faults. An investigation on the roles of height and volume of overburden sediments along with the role played by the faults show that, in general, the higher the elevation of the outcropped zone, the lower is the number of outcropped salt domes. In the Persian Gulf zone, sea dynamics effect and burial of the salt domes have contributed to lower number of outcropped domes. Increased volume of overburden sediments on top of Hormoz Formation have hindered the outcrop of salt domes within high lands and contraction zones in Zagros. However, the large fracture generated by major faults set the stage for outcrop of the domes onto the surface. Regarding roundness and area of salt domes, it can be stipulated that, the salt domes could better develop and grow within Zagros-Fars and Hormozgan zones because of the expansion of folds, where those exhibit not only more extensive area, but also further roundness. In high Zagros zone, however, the anticlines and areas where folds are in contraction mode, lack of adequate space for dome development has resulted in a situation where the domes have been mostly developed longitudinally. This has ended up with elongation and smaller area of the outcrops, as compared to those of Fars and Hormozgan zones as well as plain areas.