Structural evolution of the Lachinag synformal anticline at the southwestern termination of the Dochah Fault, west of Qom Province, northern Central Iran

The Lachinag synformal anticline is located to the northwest of the Mil anticline and the western end of the Dochah Fault in northern central Iran. This synform includes the terminal members of the Qom Formation (members E and G) and the Upper Red and Pliocene conglomerates. Additionally, with an axial surface plunge from the northwest to the southeast, this fold trends toward the southeast, resulting in its asymmetric geometry. In this synform, the deformable marl and gypsum layers of the E and G members of the Qom Formation have contact with the competent conglomerate and sandstone layers from the Upper Red Formation. The juxtaposition of these layers and the occurrence of deformation phases resulted in the migration of ductile layers as well as a significant increase in the thickness of the marl and gypsum deposits of the Qom Formation (particularly in the E member). Due to the migration of these layers towards low pressure areas and their substantial thickening at the hinge of the Lachinag synformal anticline, the layers as well as the limbs of this fold were overturned. Finally, a synformal box fold was formed. Structural investigations have revealed that this fold initially formed as a result of the right-lateral strike-slip shear parallel to the Dochah Fault. This process occurred during the post-Miocene under the influence of counterclockwise left-lateral strike-slip shear forces around a pole axis at approximately 135 degrees from the north. Consequently, the fold acquired a synformal geometry. The left-lateral shear force may be due to the clockwise rotation of the South Caspian basin and the application of left-lateral shear forces on the northern parts of Central Iran, similar to what has been observed in the Kushk-e Nosrat Fault.