Accommodation of late Cenozoic oblique shortening in the Alborz range, northern Iran

Abstract

The Alborz range, northern Iran, deforms by strain partitioning of oblique shortening onto range-parallel left-lateral strike-slip and thrust faults. Deformation is due to the north–south Arabia–Eurasia convergence, and westward motion of the adjacent South Caspian relative to Iran. Roughly north–south shortening occurs on thrusts that dip inwards from the range margins. Precambrian basement is not exposed because of detachment along upper Proterozoic evaporites and the thickness of overlying strata (10 km). Other detachments occur within Phanerozoic sediments. Active left-lateral strike-slip faults trend ENE in the east of the range, WNW in the west. The eastern Mosha Fault has a left-lateral offset of 30–35 km. Shortening across the range is 30 km (25–30%) at the longitude of Tehran. This is hard to reconcile with the crustal thickness of 35 km, which is similar to adjacent basins. Percentage shortening across the Alborz is similar to the adjacent Kopet Dagh, but because the latter is twice as wide, finite shortening is roughly half. The difference may be accommodated by subduction of the adjacent South Caspian Basin northwards under the middle Caspian. South Caspian sediments began folding in the Pliocene; the present kinematics of the Alborz may have begun at this time, including a reversal from range-parallel right-lateral to left-lateral strike-slip faulting, but the regional stratigraphy indicates uplift as early as the mid Tertiary.