Atrak is a bed rock river which begins in the mountains of north-eastern Iran, and flows 669 kilometers westward draining into the Caspian Sea in Turkmenistan. The Atrak River is located in the north east Iran (Figure 1). The Atrak river is located in the
Figure 1. Physiographic-tectonic zoning map of Iran’s sedimentary basins, modified from  . The study area is shown in the black rectangle.
Kopetdagh and it is divided into 56 sub-basins (Figure 2). This area belongs to Kopetdagh geologic province   . Dominant structural trend in Kopetdagh province (Figure 3) is NW-SE. From tectonics view, it contains the Kopetdagh hinterland or
Figure 2. The fifty six sub-basins of the Atrak River basin, modified from  .
Figure 3. Two views from the Atrak streams (toward west) that show transverse topographic asymmetry in sub-basins No. 22 (left) and No. 17 (right).
Kopetdagh fold and thrust belt that formed in passive margin of Eurasian plate until late Triassic and then marine sedimentation on Kopetdagh proforeland basin has continued to Eocene. Kopetdagh hinterland has uplifted related to Karakorum foreland basin in northeast along Eshghabad fault.
The western part of studied area has located in South Caspian foreland basin. Dominant structural trend in South Caspian foreland basin province (Figure 3) is NW-SE. From tectonics view, it contains the northern foreland basin of West-Central Alborz and lesser Caucasus hinterland in the south margin of Eurasian plate since late Eocene. Although, median part of South Caspian and Black sea basin has uplifted by collision between Eurasian and Cimmerian plates  .
Based on previous work on the salt and mud diapirism  -  and neotectonic regime in Iran  -  , Zagros in south Iran is the most active zone  -  . Then, Alborz  -  and Central Iran  -  have been situated in the next orders.
2. Materials and Methods
The calculation of transverse topographic symmetry factor (T) is suitable for rapid assessment of active tectonics. Therefore, transverse topographic symmetry factors have calculated for the streams in the Atrak River. Based on digital elevation model of Atrak River, there are 56 sub-basins. The study area is located between longitudes 55˚E - 59˚E and latitudes 37˚N - 38˚N in the Northern Khorasan province, in the north east part of Iran.
The study area is mainly composed of limestone (Tiregan Formation), marl (Sarcheshmeh Formation), shale and marl (Sanganeh Formation), sandstone and shale (Aitamir Formation) and Quaternary alluvial deposits The East Alborz or Binalod Mountains are parallel to the Kopetdagh hinterland on the north east Iran. The Atrak valley, which separates the East Alborz and Kopetdagh, forms a major structural boundary between Central Iran and Eurasia, known as the Paleo-Tethyian suture zone, which formed before the Alpine-Himalayan orogenies since late Triassic.
Transverse Topographic Symmetry Factor (T)
The transverse topographic symmetry factor (T) was calculated as follows:
which Da is the space from the midline of the drainage basin to the midline of the active belt and Dd is the space from the midline to the basin limit  . In a completely symmetric basin T = 0 and as asymmetry increases T approaches to value of 1.0  . Two views from the Atrak streams that used for calculation of T index in study area have presented in Figure 3.
Finally, we can consider class 1 for T > 0.4, class 2 for T between 0.2 and 0.4 and class 3 for T < 0.2 and so, sub-basins No. 9, 11, 12, 15, 16, 22, 26, 27, 33, 36, 37, 38, 44, 45 and 54 shows higher activity (Table 1 and Figure 4).
3. Results and Discussion
There are three level of active tectonics. Low relative tectonic activities in sub-basins No. 1, 2, 3, 4, 10, 23, 25, 41 and 51, moderate relative tectonic activities in sub-basins No. 5, 6, 7, 8, 13, 14, 17, 18, 19, 20, 21, 24, 28, 29, 30, 31, 32, 34, 35, 39, 40, 42, 43, 46, 47, 48, 49, 50, 52, 53, 55 and 56 and finally high relative tectonic activities in sub-basin No. 9, 11, 12, 15, 16, 22, 26, 27, 33, 36, 37, 38, 44, 45 and 54 (Figure 4). On the other hand,
Figure 4. The classification map for T index.
Table 1. Values of T index for sub-basins.
T classification: 1 (High Activity): >0.4, 2 (Moderate Activity): 0.2 - 0.4, 3 (Low Activity): <0.2.
although based on  northern side of sub-basin No. 46 or the Qharajeh area is inactive and its southern side is active, but T factor value is 0.25.
Highest value of T index (0.81) for streams of Atrak river is related to sub-basins No. 11 and 36 and lowest value (0.03) is related to sub-basin No. 20 it seems that folds and thrusts parallel to this sub-basins caused more tilting. Calculating this geomorphic index for the study area shows that, average values of T factor are between 0.11 for sub- basin No. 3 and 0.75 for sub-basin No. 36 which the latter case shows high seismicity (Figure 5).
It is shows that some sub-basins on the western parts of study area that river has got west-east trending, are more active. But, some sub-basins on the eastern parts of study area that river has got northwest-southeast trending, are less active. It means that migration direction of the most streams are toward north and south and the major faults and folds in the western parts of study area caused river tilting be perpendicular to structural trend.
Also, there are no logic relationship between the values of T index and seismic frequency. It has shown in Figure 5.
The T factor variations along the most important river on the southeastern margin of
Figure 5. The seismic records of study area on sub-basin map (compare it with Figure 4).
Caspian Sea were analyzed for the first time in this research. The calculation of transverse topographic symmetry factor (T) is suitable for rapid assessment of active tectonics. The transverse topographic symmetry factor has calculated in the Atrak River.
The study area was divided into 56 sub-basins and then transverse topographic symmetry factors were calculated for each one. Based on values of this index, there are low, moderate and high relative tectonic activities levels.
Low relative tectonic activities level has been found in sub-basins No. 1, 2, 3, 4, 10, 23, 25, 41 and 51, moderate relative tectonic activities level has been found in sub- basins No. 5, 6, 7, 8, 13, 14, 17, 18, 19, 20, 21, 24, 28, 29, 30, 31, 32, 34, 35, 39, 40, 42, 43, 46, 47, 48, 49, 50, 52, 53, 55 and 56, and high relative tectonic activities level has been found in sub-basins No. 9, 11, 12, 15, 16, 22, 26, 27, 33, 36, 37, 38, 44, 45 and 54. It is shows that some sub-basins on the western parts of study area that river has got west- east trending, are more active. But, some sub-basins on the eastern parts of study area that river has got northwest-southeast trending, are less active.
This work has funded by department of geology, Islamic Azad University, Science and Research branch, Tehran, Iran. Also, special thanks to vice-president for research in Science and Research branch, Tehran.
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