Mohsen Ehteshami-Moinabadi - Department of Geology, Faculty of Basic Science, Tarbiat Modares University-Tehran

Valasht Lake is a long standing landslide dam in the Alborz Mountains, north of Iran, which has occurred where a relatively large earth slide blocked the Valasht Valley. Geomorphology, geology, structural geology and climate conditions for the landslide-damming area were described and several other paleolandslides were identified. Landslide dimensions were measured using a high resolution triangulated irregular network (TIN) for the area. Based on the field observations and geologic information this landslide has occurred in marl and marly limestone rocks of Cretaceous, and in the limb of a syncline on a low angle Quaternary thrust fault. Based on historical data, the lake may be considered as an earthquake-triggered landslide that blocked the valley, after a devastative earthquake (M~7.7) in 958 AD, however it requires radiocarbon data. The potential hazard of dam failure threatens over 10000 people living downstream. The lake occupation by new landslides from adjacent slopes and direct rupture due to faulting on Valasht Thrust are the main sources of hazard. Occurred in the hanging wall of an active thrust fault, the case is a good example of structural control of active faulting on the slope movements.