Paleostress analysis around the Lar Dam (Central Alborz), to recognize the structures involved in water escape


1 Graduate Student of Petrology, School of Geology, College of Science, University of Tehran, Tehran, Iran

2 Assistant Professor, School of Geology, College of Science, University of Tehran, Tehran, Iran

3 Associate Professor, School of Geology, College of Science, University of Tehran, Tehran, Iran

4 Head of GIS group, Geological Survey of Iran


Lar Dam is located 85 km to the north-east of Tehran. It supplies a fraction of agricultural water in Mazandaran and the drinking water in Tehran. This dam was built in 1980 and 2 water escape ways around it have been explored; one of them is under the right shoulder
of the dam and the other is located north of the dam toward the Haraz road (Ab-e-Ask region). For this reason, new researches are designed for shifting it to west of the same zone (Gozal Darreh). But our investigations show that structural factor (Faulting) is effective for water escape and this agent has expanded to the whole of this region and it has covered new places as well.
Geologically, the studied area is located on calcic formations related to the second and third geological periods (Jurassic and Cretaceous). These formations have a W-NW trend parallel to the general trend in the Central Alborz. In the studied region, folds, thrusts and reverse faults have the same trend is due to continuous pressure from the Arabian plate on the Iranian plate.
Originally, the aim of our research was to obtain the direction of effective stresses that are responsible for forming and expanding the structural factor for water escape. So we considered some structures such as faults, joints and fractures which are useful for reconstructing the tectonic events (the diverse directions of stress with respect to relative time) in this region. "Inversion Method" is the base of tectonic software and is designed with respect to some of these structures. The important point which is also the target of using the slickenside is to exploit the data and return step by step to reach the initial stress conditions.
In order to obtain the direction of the main stress axis in the time of affecting stress using the inversion analysis, a considerable amount of structural data was compiled. All slickenside was categorized into 12 groups, according to Yamaji software (2005) that is known to MIM (Multiple Inversion Method). Four final parameters form software are  which are the results of solving reduced stress tensor. , the shape of stress field, is a quantitative parameter. So the examination of the trend of changes in the shape of stress field will be reconstructible.
Field work resulting in the structural data and the computer analysis performed afterwards shows intensive changes in the rate of stress field shape all around the dam. The path of these changes of the rate of the stress field shape shows the existence of a new fault beneath the dam which is probably a natural channel for water escape. These abrupt changes are from a prolate stress field shape (Φ=5/0-1) to an oblate stress field shape (Φ=0-5/0) with a linear WNW trend. This path is parallel to the linear trend of the pitch of the sink holes located at the basement of the dam. The first channel of the water escape is beneath of the right shoulder. Based upon detailed studies, the proposed position for constructing a new dam, at the western end of the Lar Dam (Gozal Darreh), is still located on the continuation of the new fault we discovered. In conclusion water would escape through this fault anyway.