Investigating the Effects of Salt Diapirism on the Shahrud – Mashhad Railway in the East of Shahrud County

Salt diapirism is a process that the overlaying rocks are impregnated with upward moving evaporites, and produce highlands. Due to their importance in human life and activities, these phenomena are of interest to researchers. The studied area has been affected by salt diapirism. Landforms made by these activities have changed the topography of the area. The associated topographic features, in turn, play an important role in human activities such as Tehran – Mashhad railway. Due to the nature of salt diapirism and fast changes, this phenomenon can lead to sever problems for human societies. Ignorance of salt diapirism in civil projects such as road and railway constructions can produce irrecoverable costs. Due to the importance of salt diapirism in human life, these phenomena have been vastly studied.
Studies on diapirism have been carried out in Southern Iran (Arian & Nowruzpour, 2015; Talbot, 1979), Southern Spain (Gattirose, 2010), and the Netherlands (Harding & Haus, 2015). These studies in Southern Iran mostly dealt with the fundamentals and they did not investigate their applications.
Due to their even topographies and geographical settings, the areas affected by the salt diapirism have been used for railway network. However, the risks related to these areas have not been considered by the national Disaster Management Organization and the authorities.
The Tehran–Shahrud–Mashhad railway in the study area between Shahrud and Bakran stations could be affected by diapirism and the related risks. Through the present study, field observations and remote sensing approach were integrated to evaluate diapirism, and the results can be applied for preventing the railway-related hazards. The study area is located in Shahrud County, Semnan Province between the coordinates of N36˚27ˊ - N36˚31ˊ and E55˚32ˊ - E55˚38ˊ. Shahrud – Jilan railway and Meyami – Jilan road are the access ways to the study area.
Topographic and geological maps along with field observations of different salt diapirism morphologies were used in this study. In addition, digital elevation model (DEM) data of 30 m special resolution from shuttle radar topography mission (SRTM) were used to produce digital layers. Field and remote sensing data were investigated using the indexing natural hazards (INH) method. In this method, salt morphologies in the studied area were classified according to the distance from railway, the production of secondary phenomena, reoccurrence, predictability, and structural geology. According to field measurements and library research, they were then ranked from 1 to 3 in terms of the danger to the railway. Based on the gathered information, the processes and phenomena were weighed to understand the role of each one in producing hazards for the railway. All the maps were produced by ArcGIS. 3-1- Mechanisms of Salt Diapirism
Tectonic activism has been the most important mechanism in the formation of salt diapirs. Structural deformations in the studied area is under the control of tectonic activities between the Central Iran and Alborz structural zones. The encounter of these two structural zones has provided an appropriate setting for the accumulation of evaporates. The subsequent folding has resulted in the formation of Jilan Anticline and Jilan–Farashian fault. These deformations led to the impregnation of evaporates into the overlaying beds and formed salt domes. Landform formation due to upcoming salt is called salt tectonics. Additionally, buried salts in Pre-Miocene sediments changed their volume and pushed the overlaying strata upward and formed salt domes. Erosion had an important role in salt movements. Sediment overlaying the salts (gravel, sand, and marls) were removed by erosion and let these salts to emerge on surface. Salt dissolution in water had the greatest effect after the aforesaid events. Two forces oppose the salt diapirism which their removal by geological and anthropogenic processes can activate salt diapirism. The construction of the Miamey–Jilan road crossing the middle of the anticline had a crucial role in erosion and outcrop of salt strata. The emergence of salt strata in the center on the anticline shows that the thickness of salt reaches to more than 40 to 50 m in the uplifted area. After the structural processes, the dissolution of salt in water has the most important role in landform formation.
The dissolution potential of salt rocks is more than other rocks. However, the amount of dissolution depends on the temperature. The higher temperature is, the more dissolution there is. Considering that the temperature of the studied area in warm seasons reaches to 27˚ C, this can increase the rate of dissolution by increasing the water temperature.
3-2- Effects of Salt Diapirism on the Railway
One of the most effective landscapes of salt diapirism in the region is the Jilan Anticline, which was affected by the Meyami – Farashian fault. Perhaps, activism of Meyami in the South and Jilan–Farashian in the North had an important role in salt diapirism. Continuous pressure by these faults led to the instability of the northern flank of the anticline. Considering that the Tehran–Mashhad railway passes the North of the Anticline, it is possible that these tectonic activities result in the collapse of sinkholes and ground subsidence beneath the railway. Salt domes are other features affecting the railway by uplifting in the area. Salt karstification also can affect the area. Sinkholes are among the most typical features of salt karstification. As sinkholes are formed rapidly, the existence of the buried sinkholes under the railway is not entirely unexpected considering their trends. Steep-sided, deep sinkholes (karstic wells) are considered as possible threats for the railway because they are covered. Salt ponorscause underground dissolution by draining runoffs into the earth. The existence of more than 50 ponors at the neighboring of the railway indicate high potential risks. Salt polje have formed in the southern part of the studied region. They might be used as substrate for constructions because of the appropriate angle of slope. However, owing to the possible dissolution under the polje, they can produce high risks. Salt glaciers along both sides of the railway can be hydrated in wet seasons and produce an unstable substrate. Salt springs are other karstic features which play a key role in the transport of the salt from underground to the surface. They could be a source of water for animals, which increases the risk of running into the trains. Salt valleys are hydrologic morphologies that during the wet seasons are flooded by deluges and can destroy railway and its related structures.
Salt cones are also present in the area and their effectiveness depends on their shapes. Although the shape of salt cones indicates their activeness, they can be considered as stable morphologies due to salt alluvium accumulation. Salt cauliflowers are the features formed in the salt formation. Their accumulation alongside the railway can produce unstable substrates by receiving water. Salt polygons can make risks by watering/dewatering and the consequent expansion/shrinkage.
Due to the presence of salt rocks, gypsum, marls, and clays with intercalations of salt, the strata can be ascribed to the Pre-Miocene which has been folded lately. The exposure of salt rocks has produced the landform of domes, valleys, sinkholes, caves, ponors, rivers, springs, polje, polygons, and the alluvial fans affecting the Tehran–Mashhad railway. Through a quantitative analysis using the application of IHN, it is proved that sinkholes and wells with 1.37 and 1.5 coefficients, respectively, are among the most hazardous features, and their destructive effects on the railway can be visible as collapsing and sliding. Ponors and salt rivers with the coefficients of 1.62 and 1.75, respectively, are classified as high risk features by producing underground caves and surficial floods. Glaciers, polygons, and domes with the coefficient of 2, the Jilan Anticline and salt cones with the coefficient of 2.42, and valleys and springs with the coefficient of 2.14 have moderate risks. Salt polje with the coefficient of 2.57 has the lowest risk for the railway.