The Analysis of Transverse Topographic Symmetry Factor ( T Index ) in the Chekene-Mazavand , North East Iran

T index has been calculated in the Chekene-Mazavand region in the NE Iran. This index shows lateral tilting may be related to active tectonics. There are many streams and transverse topographic symmetry factor has been calculated for all streams of Kashafroud river. Firstly, based on digital elevation model, the study area was divided into 28 sub-basins. Then transverse topographic symmetry factor was calculated for each one. The values are between 0.14 and 0.57 and so, there are low, moderate and high levels of relative tectonic activities. The areas with low relative tectonic activities have found in sub-basins No. 14, 15 and 24, moderate relative tectonic activities have found in sub-basins No. 2, 5, 6, 7, 9, 10, 13, 16, 18, 19, 21, 22, 23, 25, 27 and 28 and high relative tectonic activities have found in sub-basins No. 1, 3, 4, 8, 11, 12, 17, 20 and 26. The results show that the plains have got low to moderate level of tectonic activity and faulted areas have got high tectonic activity. This matter is compatible with its tectonic setting on the Kopet Dagh belt in north east Iran.


Introduction
The Chekene-Mazavand area is located in the Kopet Dagh physiographic province, NE Iran (Figure 1).Dominant structural trend in Kopet Dagh province is NW-SE.From tectonics view, it contains the Kopet Dagh hinterland or Kopet Dagh fold and thrust belt that formed in passive margin of Eurasian plate until late Triassic and then marine sedimentation on Kopet Dagh proforeland basin has continued to Eocene.Kopet Dagh hinterland has uplifted related to Karakorum foreland basin in north east along Eshghabad fault [1]- [3].
Based on previous work on the salt and mud diapirism [4]- [15] and neotectonic regime in Iran [16]- [21], Zagros in south Iran is the most active zone [22]- [44].Then, Alborz [45]- [84] and Central Iran [85]- [100] have been situated in the next orders.The analysis of T index that shows lateral tilting is the main goal in this paper.

Materials and Methods
The transverse topographic symmetry factor shows lateral tilting that it may be related to active tectonics.There are many streams and transverse topographic symmetry factor has been calculated for all streams of Kashafroud river (Figure 2 and Figure 3).Kashafroud river is the main river in the Chekene-Mazavand area (Figure 4).Based on digital elevation model of Chekene-Mazavand area, there are 28 sub-basins.The study area is located between longitudes E58˚, 30'-61˚ and latitudes N35˚, 30'-37˚ in the Khorasan province, in the NE Iran.

Transverse Topographic Symmetry Factor (T)
T index or transverse topographic symmetry factor has been calculated as T = Da/Dd and it can be between 0 in the symmetric basin and 1 in the asymmetry basin.
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 [101].In this research, we were considered 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-basin No. 1, 3, 4, 8, 11, 12, 17, 20 and 26 shows higher activity (Table 1).

Results and Discussion
Our results show that Mashhad plain have got serious land subsidence which it was documented by field observations (Figures 5-7).Also, there are three levels of active tectonics.Low relative tectonic activities in sub-basins No.

Conclusions
In this research, the transverse topographic symmetry factor has been calculated in the Chekene-Mazavand area, north east Iran.Based on digital elevation model, the study area was divided into 28 sub-basins.Then transverse topographic symmetry factor was calculated for each one.The values of this index are between 0.14 and 0.57, there are low, moderate and high levels of relative tectonic activities.The most parts of study area have shown moderate relative tectonic activities.

Figure 2 .
Figure 2.An oblique view on Landsat satellite image that shows some streams of the Kashafroud river.

Figure 3 .
Figure 3. Land subsidence value around the middle part of Kashafroud river plain.

Figure 4 .
Figure 4.The sub-basins on Landsat satellite image of the Chekene-Mazavand area.

Figure 5 .
Figure 5.A view from cracks due to land subsidence in sub-basin No. 6.

Figure 6 .
Figure 6.A close view from a crack in Figure 5.

Figure 7 .
Figure 7.A view from a large crack which is related to land subsidence in sub-basin No.7.
and high relative tectonic activities level has been found in sub-basins No. 1, 3, 4, 8, 11, 12, 17, 20 and 26.The latter cases are compatible with faulted parts of study area.

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