Contractional salt-tectonic system in the south Dezful embayment, Zagros

Abstract

The Dezful embayment in Zagros has traditionally been interpreted as a typical fold belt with multiple detachment levels, including a lower detachment at the basement-cover interface and an upper detachment along the Miocene Gachsaran halite-bearing evaporites. This paper constrains diapiric processes in the Gachsaran detachment coeval with regional shortening that resulted in complex and hitherto unstudied salt structures and flanking synclinal minibasins. In this paper, well-tied seismic interpretation is integrated with field and remote-sensing mapping, cross-section construction, and stepwise tectono-sedimentary restoration. The results indicate that after the deposition of Mishan–lower Aghajari pre-kinematic overburden sediments, detachment folding initiated above the Gachsaran evaporites during late Tortonian times as a result of regional shortening. The salt-cored anticlines later actively pierced their roof to subsequently rise as passive diapirs and evolve into salt walls. Synclinal minibasins developed due to salt evacuation into flanking walls during the deposition of the Lahbari Mb. in Messinian–Pliocene. Remarkable sedimentary thickening and lateral shifts in minibasin depocenter towards the salt walls, accompanied by overturned collars and hook halokinetic sequences, are diagnostics of near-surface, syndepositional passive growth of diapirs during this stage. From Pliocene to recent, the major episode of regional shortening has squeezed these precursor salt walls to form thrust-welds. In conclusion, the well-exposed structures of south Dezful show an early shortening stage, then a transition to a combination of shortening and passive diapirism, and final thrusting along diapirs. This evolution model may be more common in other salt-influenced fold-thrust belts than is currently recognized.