Iran country has composed from the several physiographic-tectonic provinces. Each physiographic-tectonic province has a same structural trend and dominant mechanism of deformation, because these have related to tectonic settings, magmatic histories, stratigraphic variations and deformational models of crust. Mud volcanoes have been fund in the some parts of Iran. These parts are very important from the occurrence of petroleum point of view. The most mud volcanoes have been occurred along convergent plate margins where fluid-rich sediment is accumulated in deep-sea trenches at high rates. Such deposits then enter the subduction factory, where liquids and volatiles are released due to increasing compaction stress and temperature.

The mud diapirism is very common in convergent tectonic settings that are marked by compressive forces. As for the first, tectonic activity is an additional trigger to the buoyant driving mechanism. Fluid for mud volcanism is supplied from various sources, including meteoric and volcanic waters, pore water expulsion, hot springs, mineral dehydration reactions, and gas hydrate destabilization [1] . The questions are why and how mud-volca- noes form provokes a lively discussion. Many researchers have worked in different areas and postulated usually a variety of geologic; tectonic; geochemical and hydrogeological reasons.

The high sediment accumulation rate at passive continental margins and the lateral tectonic compression at active continental margins can be consider as the main reasons for space distribution of mud diapirs that formed by density inversion, faulting, fluid migration. In addition, it is obvious that plastic clay layers must be presented for mud volcanoes to form. In some regions, submarine mud volcanoes form in areas characterized by a spatial combination of rapid sediment accumulation and lateral tectonic compression [2] .

Characteristics of mud diapirs in the Gorganregion in the northern part of Iran are the main aim in this research. This area is located on convergent tectonic setting between Cimmerian and oceanic part of Eurasian plate (Figure 1).

Previous research on Iran’s tectonic setting [4] -[6] , salt diapirism [7] -[16] , Seismotectonics [17] -[20] have shown that the Zagros belt is the most active zone in Iran [21] -[34] . Alborz belt in north Iran [35] -[66] and Central Iran [67] -[81] have been situated in the next orders. The study area is located on the south Caspian foreland basin.

Dominant structural trend in South Caspian foreland basin province is NW-SE. From tectonics view, it contains the northern foreland basin of West-Central Alborz and lesser Caucasus hinterland in the south margin of Eurasian plate since late Eocene. Although, median part of South Caspian and Black sea basin has uplifted by collision between Eurasian and Cimmerian plates. There are several Mud volcanoes along the coastal part of Gorganarea. The biggest one is Garniarig-Tapeh Mud volcano (Figure 2).

Based on our investigation, the Garniarig-Tapehmud volcano have got a shield shape with near to 25 meters height and near to 650 meters diameter and it has been formed on the flat quaternary plain in coastal region (Figure 3) of Caspian Sea. Also, there are several Salses or small mud craters which they have filled by water (Figure 4 & Figure 5). These features have been appeared during dormant periods of mud volcanoes.

In central part of the Garniarig-Tapehmud volcano, radial streams have formed in a big pound that it has been filled by viscous clay (Figure 3). Also, there are several mud volcanoes along coastal region Caspian Sea that one of them is Naft-Lije that fund at north of Gomish-Tapehcity (Figure 6).

These mud volcanoes have been triggered by buoyant driving mechanisms, because there are not find geological structures such as folds and faults. This mechanism can be related to neotectonic regime of the south Caspian basin. It means that oceanic lithosphere beneath of the Caspian Sea has been sloped to the south, however it has been covered by thick sediments.

This mud diapirs has been rise from quaternary deposits and probably mud diapirism has been started since Early Pliocene, because it has covered quaternary plain. There are not the index structures in the Gorgan region. It means that mud diapirs have not been triggered by tectonic forces, in contrast of mud diapirism in Makran in the south part of Iran [82] . Also, there are several dispersed mud diapirs, but all of them are the smaller than Garniarig-Tapehdiapir.

Tectonic geomorphology and origin of the exposed mud volcanoes on the Gorgan area have determinated. Mud diapirism in this zone has not been controlled by only tectonic forces, however there are an old

suture zone between Cimmerian and Eurasian plates.

It means that buoyant driving mechanism is predominant compared with tectonic forces in the coastal parts of the Caspian Sea. Therefore, formation of the new mud volcanoes and more development of the older mud diapirs are not expected for short times. Because mud diapirism has been developed before regional shortening on the old convergent zone between Cimmerian and Eurasian plates. Also, there are several salses that have been appeared during dormant periods of mud volcanoes activity.

This work has funded by the Department of geology, Islamic Azad University, Science and Research branch, Tehran, Iran. Also, Special thanks to Amir Chizari, M.Sc. from Tehran Municipality for preparation of Figure 4 & Figure 5.