Plio–Quaternary tectonic regime changes in the transition zone between Alborz and Kopeh Dagh mountain ranges (NE Iran)

Abstract This paper is concerned with the changes in the Plio–Quaternary stress states in NE Iran and provides evidence for regional active deformation across a wide transition zone between the Alborz and Kopeh Dagh mountains. The Arabia–Eurasia convergence is involved in the strike–slip faulting along NE-trending left-lateral faults in the eastern Alborz as well as NW-trending right-lateral faults in the western Kopeh Dagh. The inversion result of fault kinematic data (slip-vector measurement of fault plane and focal mechanisms) strongly indicates an active transpressional tectonic regime in the study area. We provide evidence for drastic temporal changes in the stress state during the Plio–Quaternary based on the inversions of fault kinematic data. A regional transpressional tectonic regime with a mean N036 ± 20°E trending horizontal σ 1 is representative for the modern stress state. The older state reveals compressional to transpressional tectonic regimes comprising a N135 ± 20°E trending horizontal σ 1 . The modern transpressional tectonic regime characterized by a regional NE-trending σ 1 is consistent with the inversion of earthquake focal mechanisms that yield a N47°E trending σ 1 for the present day stress state. The kinematic results are significantly homogeneous and mechanically compatible with the active geological structures, in the way that reverse and strike–slip faulting are coherent with a single σ 1 direction.