Investigation of morphological-tectonic structure of Siah-kuh heights in Damghan area using steepness and concavity parameters

Identification of active tectonics in an area and its effects on the morphology of landforms is one of the topics that has always been of interest to geomorphologists. Waterways are among the features that flow a wide range of landforms. These features are sensitive to lithological or tectonic changes and react quickly to these changes. The riverbed has been affected by these anomalies and the effects of these anomalies can be studied in the longitudinal profile of the river. In other words, waterways are active tectonic markers that give us information about the characteristics of a landform.

The study area is located in the south of the Eastern Alborz Mountains and is known as the Siah-Kuh Heights. This ridge consists of a set of dolomitic and sandstone rocks of Sibzar and Padha Formation and Damghan fault passes along it. The main purpose of this study is to investigate the morphological and tectonic structure of Siah-Kuh using steepness and concavity parameters based on the anomalies of longitudinal profiles of the rivers based on the uplift and subsidence axes. In order to calculate the rate of steepness and concavity of longitudinal profiles, the stream power low formula is used based on the two main parameters of drainage area and slope of river. This formula is based on a logarithmic plot of the slope and drainage area for which the appropriate regression line is determined. In this regression relation, the slope of the line is the concavity parameter and the intercept is steepness parameters.

The extent of the Siah-Kuh heights among a series of young Quaternary sediments at a distance of 4 km from the main mountain front, the structure of the Siah-Kuh is similar to Foreberg forms. Damghan fault in this area has led to the intersection of Quaternary sedimentary strata related to the surrounding alluvial fans and by creating a transtensional position in the form of a set of trapped traps, the Siah-Kuh Foreberg has appeared as a restraining bend on the surface Which has led to the elevation of the central part and the relative subsidence of both sides of this structure. The eastern part of the Siah-Kuh Foreberg appears to have been covered by Neogene and Quaternary sediments. Its remnants have appeared in the form of deep gorges on the surface. One of the rivers flows on the main surface of Foreberg and the other flows in the drained or buried part of Foreberg. Both rivers have several knick points in their flow path, which are taken from the location of faults and lithological differences of the riverbed. The values of the steepness parameter for the main river and the gully show high values so that the value of this parameter is 121.4 for the main river and 119.96 for the gorges. In contrast, the depression parameter rate in both rivers shows very low and even negative values. (Main river: -0.18 and gorges river: -0.92). Since the sharpness parameter is directly related to the tectonic processes; It can be said that the values of this parameter in both rivers indicate the effect of active tectonics at the level of Foreberg.

The studies performed on the study area show that the two factors of active tectonics and lithology have a great impact on the morphological structure of Siah-Kuh Foreberg. Also, factors related to sedimentary flows originating from the uplands of the area have been effective in changing these landforms in the form of burial. Also, the results showed that the use of the method based on longitudinal profile anomalies of the river in the form of steepness and concavity parameters has an effective role in identifying erosion and subsidence axes related to tectonic situation of features in relation to their topographic changes. Slight difference in the amount of steepness parameter in gorges compared to the river on the main surface of Foreberg, especially in the first trends of both rivers, which show high values of steepness parameter with a small difference (1.44), Indicates the existence of an uplift axis in this area, which somewhat confirms the burial of a structure similar to the Siah-Kuh Foreberg under sediments.