Godratolah Farhoudi - Department of Geology, Islamic Azad University, Shiraz Branch, Shiraz, Iran
Jafar Rahnamarad - Department of Geology, Zahedan Branch, Islamic Azad University, Zahedan, Iran
Reza Derakhshani - Department of Geology, Faculty of Basic Science, Shahid Bahonar University of Kerman, Kerman, Iran

Image processing of satellite photographs combined with fieldwork and extensive use of aerial photographs show that the whole Helmand block is a compressional depression produced by the activities of strike slip faults. These faults have generated bending in the East Iranian mountains and formed them to the present sigmoidal appearance. The deepest area is in the Sistan, where the East Iranian mountains have been mostly curved and the Helmand River generates several seasonal lakes. This area has been the base level for the Helmand River, probably since the Upper Cretaceous. Through the extreme pressures the mountains have been broken and partly totally removed by subsequent erosion. There are some natural deeper patches, inside the Sistan depression, called Chahnimeh, where the structural grains are more suitable for the generation of these phenomena. The very strong winds over this area have no significant effect on the origin of the depressions, as some authors believe. The Chahnimeh depressions have not been caused by wind, since they are usually covered by water, which is the only water resource used for drinking and agriculture. The water in the depressions is drained by underlying faults, since it is not salty.

Aerial photograph, Chahnimeh depressions, Morphotectonic, Satellite photograph, Sistan