Structural segmentation of a mountain front fault evolved as an oblique thrust system: the North Tehran Fault case study

Mountain front faults form the boundary between mountains and adjacent plains. These faults can propagate toward the plains and escalate the risk of seismic hazard for near cities. The North Tehran Fault (NTF) is a mountain front fault bordering the Central Alborz with Tehran and Karaj plains. Structural and morphotectonic data from interpreted aerial photographs, satellite images, airborne geomagnetic data as well as field surveying have been used for detailed segmentation and evolution of the North Tehran Fault. This resulted in identification of the fault segments as the Niknamdeh, Darband, Darakeh-Garmdarreh, and Karaj from east to west. Active kinematics of these segments includes both thrusting and left-lateral components; but the dominant component is different among the segments. The Niknamdeh segment is connected to the Mosha Fault with a hard linkage, while its connection with the Darband segment is a widespread deformation zone. The connection zone between the Darband and Darakeh-Garmdarreh segments has the highest density of minor faults along the North Tehran Fault. The boundary of the Darakeh-Garmdarreh and Karaj segments is controlled by the F-3 transverse fault that has offset the NTF for ~3 km right-laterally. The NTF has inverted from normal to dextral oblique fault in Miocene. The fault kinematics has changed from dextral to sinistral in Pliocene-Quaternary. Further regional oblique convergence resulted in minor fault reactivation such as relay ramp breaching faults, propagation of several footwall branches and hanging-wall bypasses geometrical change of alluvial fans, and transfer of deformation front southwardly to the Tehran and Karaj plains. The findings of this paper are also applicable to other active oblique converging mountain fronts, inverted mountain front faults and the transition of deformation from these structures to the foreland basin.