Origin of the mud volcanoes in the south east Caspian Basin, Iran

The Gorgan Plain (SE Caspian Basin) contains several mud volcanoes. The Gharenyaregh, Neftlijeh, Sofikam and Inche mud volcanoes are active and erupting mud and gas. Mud volcanoes occur in an area characterized by seismic activity. The Sofikam mud volcano consists of five craters that define a NE-directed (60 degrees) linear arrangement, which coincides with a subsurface anticline of the western Kopeh Dagh fold-thrust belt. The main craters of the Inche and Neftlijeh mud volcanoes are submerged. The Gharenyaregh mud volcano is an active mud volcano whose crater is nearly 500 m in diameter. Clasts collected from the Gharenyaregh mud volcano (GMV) mainly consist of glauconite-sandstone, limestone (+/- ammonite), marl, and fossiliferous limestone (lumachelle) with bioclasts ranging in size from millimeters to centimeters. X-ray diffraction (XRD) analyses revealed quartz, calcite, feldspar, chlorite, illite, glauconite, halite and gypsum minerals in the clasts and soft materials of the GMV. The clasts are comparable with rock fragments originating from the underlying Kopeh Dagh related formations and the uppermost Caspian basin sediments. Regression of the South Caspian Sea shoreline from east to west (across modern-day Golestan province), seismic activity and buried folds indicate that the south east Caspian basin mud volcanoes are located in a tectonically active area and generated from deeper parts of the Gorgan Plain. Some large, conical hills with a record of historical human activity, located between the Kopeh Dagh zone and the South Caspian Sea, seem to be inactive mud volcanoes.