ABSTRACT
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 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. This variation along the most important river on the northeastern part of Iran was analyzed for the first time in this research.