Paleozoic tectonic evolution in central Alborz (Iran): Continuous extension from Gondwanan active margin to Cimmerian passive margin

The Alborz Mountains of northern Iran are well known for inversion tectonics and active mountain building. The Paleozoic, pre-Cimmerian faults and basins played a key role in the Cimmerian and late Alpine-Himalayan orogenic phases. However, the Paleozoic structural geology and tectonic evolution of the Alborz Mountains have received comparatively little attention, despite their excellent preservation and extensive exposures. The aim of this paper is to elucidate the Paleozoic structural-tectonic framework and the transition of geodynamic regimes. To this end, we present the evolution of the Paleozoic stress field from paleostress analysis in central Alborz, supported by stratigraphic and petrologic studies. To obtain the highest possible temporal resolution and to exclude younger or potentially reactivated faults, each stress tensor represents a population of newly formed syn-depositional faults from a single formation.

The calculated paleostress tensors reveal a north–south extensional stress regime throughout the Paleozoic during successive geodynamic regimes, from the Gondwana active margin to the Cimmerian post-rift passive margin. The extension was accommodated by margin-parallel normal faults, including the major basin-bounding faults that were repeatedly reactivated during later phases of inversion and extension. The results indicate the absence of vertical axis rotations from later deformation phases, which is consistent with models for late Neogene internal deformation and oroclinal bending in the Alborz Mountains that place central Alborz at the hinge point between the rotating eastern and western branches.