Age of folding in NW Lurestan arc across the Khanaqin fault by magnetostratigraphy on Aghajari growth strata, Zagros folded belt

Abstract

The timing of deformation and the age of sediments in foreland basins are key information for reconstructing the tectonic history of orogenic belts. This study, utilized magnetostratigraphic techniques to determine the ages of ∼ 2.5 km-thick strata within the Zagros folded foreland basin in the flank of the NW-SE-trending Emam Hassan anticline, located above the N–S-trending Khanaqin blind basement fault. This fault separates the Lurestan arc in Iran from the Kirkuk embayment in Iraqi Kurdistan. These ages were combined with a detailed structural study to establish the timing of folding and integrate it into the regional framework of the Zagros folded belt. Magnetostratigraphic results indicate a Serravallian age at ∼ 13.4 Ma for the stratigraphic boundary between the Gachsaran Formation and the pre-growth Aghajari Formation, whereas the base of the Aghajari growth strata is Tortonian age (early Late Miocene) at ∼ 10 Ma. The mean sediment accumulation rate for the Aghajari Formation is ∼ 44 cm/kyr, with short peaks reaching up to 90 cm/kyr. Linear extrapolation of these rates proposes a younger age than ∼ 6.5 Ma (Messinian) for the base of the unconformable conglomerates of the Bakhtyari Formation. The Emam Hassan folding and related Gachsaran diapirism began ∼ 10 Ma and continued for at least 3.5 Myr until ∼ 6.5 Ma, coevally with the deposition of ∼ 1.3 km of the upper Aghajari Formation. Regional synthesis reveals that multi-detachment folding within the Lurestan arc propagates in-sequence towards the foreland. Although not the central focus of this study, our results, combined with the regional structural understanding, illustrate the potential 3-D structure at the depth of the seismogenic Khanaqin Fault, the source of the 2017 Mw 7.3 Sarpol-e Zahāb Earthquake.