Explaining the tectonic-erosive position of Eastern Alborz heights located in the north of Damghan playa during Quaternary

Damghan area has been considered by earth scientists due to the existence of numerous active faults. These faults with slow and progressive movements have led to changes in the shape of landforms in the area. Even during the history, the activity of these faults led to major earthquakes such as Qumis earthquake in 856 A.D. This earthquake led to the complete destruction of Qumis province in present-day Damghan. Researchers in the area, including Hollingsworth et al 2010., have concluded that major faults in the area, such as the Astaneh and North Damghan faults, have ruptured during the Qumis earthquake. The changes in landforms and high potential of faults in creating seismic events indicate their continuous activity during Quaternary up to now. To study these changes, we need those features that are sensitive to tectonic and erosive events. Rivers are one of these features that react quickly to active tectonic and erosion changes. The main purpose of this study is investigation of the changes in landforms using anomalies in stream orders. Because there is a hypothesis that claims tectonic and erosive changes lead to irregularity in Strahler orders of streams. In this study we use isobase and differential map method to investigate this hypothesis.

Sensitivity of drainage network to tectonic processes and geological contradictions leads to changes and irregularities in river orders. Isobase maps are extracted from information about the spatial classification of rivers and their elevation information in the environment. In fact, these maps show the relationship between the pattern of waterways and the topography of landscapes. Streams with the same Strahler order are formed by the same geological events, and most likely the same order is the same age. We can draw iso base maps for different stream orders. Early orders (1-3) are mostly more sensitive to tectonic events. These early orders due to flow along seams and fault gaps are representative of neotectonic. Rugged structures in the lithology of the region can limit the evolution and development of the early orders, and therefore in these areas the values of the isobase map increase. But isobase map of evolved orders can represent longer periods, such as the Quaternary or Pleistocene. Because evolved orders are formed over longer periods and are able to show older tectonic and erosive events. Differential maps are obtained from the difference in height between the maps of the evolved orders and the present day topography map.

Examination of the isobase map of the primary orders show that most of the values related to the map can be seen in the west and north of the region. These areas are based on mountainous structures. The high values of the isobase map in these areas indicate that the northern and western parts, affected by neotectonic activities and the development of fault gap and seams, have more primary orders of rivers. Because most of these primary orders are formed and developed along these levels of weakness. Resistant lithology of the area has also prevented the development of these orders. The highest value of them is 3830-2312 meters. The differential map prepared from the study area shows a wide range of positive values indicating uplift and negative values indicating subsidence. By examining these values, we can understand the features related to tectonics and geomorphology of various landforms in the region. The highest values of 1191 to 800 meters are related to the high mountain structures of Eastern Alborz in the north and west of the region. The decrease in the value of the differential map indicates the effect of tectonic subsidence on the morphological landforms of the region. The Astaneh pull apart basin in the western part of the region shows the amount of subsidence between -4 to -1 meters. Also, the southern playa of Damghan in the form of a concave foreland basin affected by active faults in the region shows negative values, which indicates the existence of a submerged basin with high sedimentation potential.

Studies conducted on the area show that the two factors including active tectonic and lithology have a great impact on the morphological structures and landforms of the area. The results of isobase map and the differential map show and confirm this issue. The methods used in this study to investigate the tectonic activity of the faults in the area indicate their activity during the Quaternary. These faults have different mechanisms of thrust and streak slip which in some areas also have transtensional and transpressional position; they have created a collection of uplifted or concave landforms in Damghan area. The spatial relationship between different values from north to south in the prepared maps indicates the creation of an equivalent states between topographic growth and its destruction in lower altitudes. Based on the studies, we confirm that the preparation of isobase and differential maps can be a suitable method for studying and examining morphological structures. Also studies related to differential map show anomalies in drainage networks and uplift processes associated with active Quaternary landforms can be quantified by these maps. Using baseline and differential maps is one of the appropriate methods to study the Plio-Quaternary tectonics in an area. This study confirms the work of previous researchers based on the available information and documents related to the study area.