عنوان مقاله [English]
The Tabas zone in the central Iran hosts a series of active landforms, which developed during the late Quaternary. Evolution of these landforms has been controlled by many active faulting and folding systems that have developed since Quaternary. Iran forms a relatively compact zone of active continental deformation resulted from the northward collision of Arabian plate with Eurasia plate during late Cenozoic times, which continues to the present‐day at a rate of 25 mm/yrs based on GPS data. The evidence of active tectonic in different parts of Iran has been studied by many researchers. The arid climate, low rates of erosion, and minimal vegetation cover across the majority of the country result in excellent preservation and exposure of surface deformation produced by active faults. Geomorphic indices are useful tools in the evaluation of active tectonics because they can provide rapid insight concerning specific areas within a region which is undergoing adjustment to relatively rapid and even slow rates of active tectonics. Alluvial fans are one of the most important landforms that indicate active tectonics and active faults. When a river reaches from high gradient mountains to low gradient plain, aggradations is occurred and alluvial fans are created. At least, five factors influence fan processes including catchment bedrock lithology, catchment shape, neighboring environments, climate, and tectonics. Active tectonics has a very important role in deformations of the alluvial fans. Without continued tectonics, fans and other quaternary landforms might become minor or short-lived features. In this study, we try to investigate the impacts of the active faults in quaternary landforms and morphometry of alluvial fans and morphology in the Tabas playa.
Method and materials
An integrated multidisciplinary approach was adopted to better constrain the morphotectonic evolution of Tabas region and to reconstruct its Late-Quaternary landscape evolution. The procedure is based on the obtained qualitative and quantitative data. The quantitative data includes satellite image interpretation and digital elevation models, alluvial fan and folds morphometry, channel displacement and rate of sediment uplift. The analysis of longitudinal and cross profile and gradient variable are used to interpret the active fault effects on folding and alluvial fans. Topography maps (1:25000), Sentinel 2 and Quickbird satellite image with 10 and less than 3 m spatial resolution, geology maps (1:100000) and digital elevation models (10m pixel resolution) were employed in this study. For such interpretation, we have employed ArcGIS, ENVI and Freehand software.
Results and discussion
Tabas area is characterized by active faults and folds. The strike-slip faults are the dominant structures in the study area. However, the reverse faults are the most important for the landscape, because they form canyons and affect the drainage pattern. The study of satellite imagery and geological maps of the area, especially in the west part of the Shotori Mountains, reveals large and numerous alluvial fans, sometimes created to the desert along the lower sections, developed and expanded by the sedimentation of seasonal and permanent rivers. These alluvial fans are joined together in most parts of southern Tabas. According to the criteria for the differentiation of alluvial fans from each other, three types of old, active and midway are identified at the confluence of the Tabas playa and Shotori Mountains.
There are several new folds in quaternary sediments between Shotori Mountains and Tabas Playa. One of them is Tabas (or Sardar) fold in north part of Tabs city. This fold can be seen among the old alluvial fans hardly excavated by Tabas and Sardar Rivers. Sardar River makes a deep gorgeous canyon through this fold. Other folds are Fahlanj and Fosh that take place in south part of Tabas city and the latter located in north east of Tabas.
The faults are important tectonic elements in the creation of various early landforms such as horst, graben, thrust, etc. in the study area that are along the Nayband fault system with a N-S direction. In the east part of the village of Estahpak, an active strike-slip fault series displaces sandstone and conglomerate sediments of Miocene up to 130 meters. Estahpak fault and its lateral branches in some cases are displaced by the Quaternary sedimentary layers, more than330 meters. In Fosha fold, there is a strike slip displacement of Quaternary sediments, about 350 meters. Stream and rivers are so sensitive to tectonic so that they quickly response to tectonic activities. Estahpak strike-slip fault result in displacement of streams up to 390 meters.
According to seismic data from the US Geological Survey, from 1973 to 2016, about 60 earthquakes ranged from 4.5 to 7.4 in magnitudes has been occurred in the region. Meanwhile, the average depth of earthquakes is at an approximate depth of 31 km, as the earthquake centers between depths of 58 - 2 km. The earthquake centers are mainly concentrated in the beginning of the mountain front and immediately after the occurrence of roughness. In 1979, the Tabas earthquake created a fracture with a length of 80 Km along the Tabas fault and killed more than 25000 people.
The tectonic landforms of the Tabas region are consisted of the faults, folds, segmented alluvial fans and diverted gullies. In addition, elevated anticlines indicate young tectonic movements and their planform outlines are affected by faults. From the morphology and the outcrops, we conclude that the Tabas region contains many active strike-slip faults, arranged presumably en-echelon. Secondary features are reverse faults and folds. The age of the associated basins shows that this tectonic system has been active since late Quaternary. Recent faulting activity is inferred from earthquake activity and the morphology changes with young fault scarps. The findings show that Tabas, Shotori, Baharestan, Feyzabad and Darya faults were active in quaternary epoch. The position and forms of alluvial fans and river channel and folds are affected by the activities of these faults. These faults have had either lateral or vertical displacements. Findings also show that the alluvial fan formed their longitudinal and lateral profiles are affected by Tabas, Darya and Shotori faults. Investigations show that there is no statistical correlation between the variables affecting the alluvial fans. Generally, tectonic activities disrupt natural evolution of alluvial fans. Each fault has a different effect on alluvial fan evolution. Shotori and Tabas fault caused the displacement of the main channel and the formation of river terraces. Therefore, it has also increased slope of this part. Other faults have caused uplift of fan deposits and changed the intersection point and reconstruction of new alluvial fans in the lower part of this point.