Structural analysis and the late Cenozoic tectonic evolution of the SE Alborz Mountains in northern Iran: insights into the Arabia-Eurasia collision

Structural analyses in the SE Alborz Mountains show that deformation style changes in different parts of the area. In the southwestern parts, presence of a thick succession of low competent middle-upper Eocene to Pliocene layers that include marl and evaporitic rocks have facilitated the occurrence of open to gentle upright folds. However, to northeast, where a thinner succession of low competent layer is observed, high-angle reverse faults has occurred. The low competent units seem to have deposited in a shallow-water marine basin formed as a result of a regional uplift in response to onset of reverse kinematics on the Bashm Fault during the late Eocene time. The onset of this reverse kinematics is contributed by the collision between the Arabian and Eurasian Plates. As a consequence, our data mainly agrees with the studies proposing the late Eocene-early Oligocene as the time of commencement of the Arabia-Eurasia collision. Additionally, since the SE Alborz Mountains seem to have experienced a prolonged extensional setting during the middle Eocene, the above-mentioned reverse kinematics may indicate inversion of a middle Eocene basin. Our data also revealed that the late Miocene-late Pliocene was a critical time in the study area, when regional surface uplift took place and substantial clastic materials were shed upon the already folded rock units. A significant episode of deformation may have occurred at this time.