Investigation of T Factor Variations along Atrak River to the Caspian Sea

The study area has located on Atrak river basin in the Iran-Turkmenistan border zone that is the greatest river on the southeastern margin of Caspian Sea. The study area was divided into 56 sub-basins and then T factors were calculated for all streams of the Atrak river. T factor is a geomorphic index for tilting identification by active tectonics. Our results show that, 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. Also, most part of the study area has got moderate tectonic activity that it is compatible with its tectonic setting on the Kopetdagh belt in Iran. It is shows that some sub-basins on the western parts of study area that river has got westeast 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. This variation along the most important river on the northeastern part of Iran was analyzed for the first time in this research.


Introduction
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 [1].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 [1] [2].Dominant structural trend in Kopetdagh province (Figure 3) is NW-SE.From tectonics view, it contains the Kopetdagh hinterland or  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 [3].

Materials and Methods
The calculation of transverse topographic symmetry factor (T) is suitable for rapid as-

Transverse Topographic Symmetry Factor (T)
The transverse topographic symmetry factor (T) was calculated as follows: T Da Dd = 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 [100].In a completely symmetric basin T = 0 and as asymmetry increases T approaches to value of 1.0 [101].
Two views from the Atrak streams that used for calculation of T index in study area have presented in Figure 3.

Results and Discussion
There are three level of active tectonics.Low relative tectonic activities in sub-basins No.  4).On the other hand,   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.

Conclusions
The T factor variations along the most important river on the southeastern margin of Low relative tectonic activities level has been found in sub-basins No.

Figure 3 .
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).

Figure 4 .
Figure 4.The classification map for T index.

Figure 5 .
Figure 5.The seismic records of study area on sub-basin map (compare it with Figure 4).

Table 1 .
Values of T index for sub-basins.