Quantitative evaluation of neo-tectonic and morpho-tectonic activities at the north and northwest of Damghan (Eastern Alborz)

1- Introduction: Geomorphic investigation is a powerful tool for studies of active tectonics in active regions. Active faults and folds commonly have topography that can assist in identifying different geomorphic or structural segments along a structure, and estimating the potentially most active segments. In tectonically active regions, drainage networks and topographic features can be used as a way to quantitatively characterize the interaction between tectonics and surface process providing a basis for modeling landscape evolution. The study area (north and northwest of Damghan) is located in northern Iran and in the eastern part of the Alborz range. The Alborz mountains form a sinuous range over 1,200 km long, separating the high plateau of central Iran from the South Caspian basin, which lies near sea level. The active tectonics of this region appears to accommodate N–S regional shortening by partitioning onto separate thrust and left-lateral strike-slip faults. In this study, neotectonic activity of the north and northwest of Damghan, as a part of the Alborz tectonic-sedimentary unit including Astaneh fault, Damghan fault, North Damghan fault and Attari fault and as the main area destructed during the Qumes earthquake (856 A.D.), has been studied. The 856 A.D. Qumis earthquake (M 7.9) is the most destructive earthquake to have occurred in Iran, killing more than 200,000 people and destroying the cities of Damghan and the old Parthian capital of Shahr-i Qumis (Hecatompylos). 2- Methodology: Morphotectonic indices are useful in evaluating the tectonic activity of regions. Morphotectonic analysis using geomorphic indices has been developed as a basic reconnaissance tool in order to identify areas experiencing rapid tectonic deformation or estimate relative variations of tectonic activity in a specific area. In this research, neotectonic activity evaluation of the region was done mainly based on three important morphotectonic indices: Stream Length-gradient, Mountain-front Sinuosity, and Drainage Density. Using digital topographic data, digital elevation model (DEM), aerial photographs and geological maps, these indices were computed for different parts of the study area. 3- Results and Discussion: The results obtained were shown as digital maps provided using ArcGIS (10.7) software. Maps showing the spatial variation of different studied indices across the area were provided and interpreted in this study. As a result, it is concluded that parts of the northern Damghan area mainly with similar strike to the Damghan fault, North Damghan fault and Astaneh fault show more tectonic activities in comparison to the adjacent areas. In addition, the high correlation between the trend of anomalies and the strikes of faults of the area demonstrates the influence of faulting in forming these geomorphologic features. A comparison between the resulted maps and the general structural map of the area indicates that the approximate trends of abrupt changes in drainage density values, acceptably correlate with the general trends of the major faults and large-scale folds of the area. 4- Conclusions: The results obtained indicate that the analysis of morphometric indices, as well as the quantitative analysis of stream patterns, can be very useful for evaluation of the relative intensity of neotectonic activity of regions especially when the other techniques could not be employed for recognition and assessment of the active structures.