Progressive unconformities within an evolving foreland fold-thrust belt, Zagros Mountains

A major angular unconformity between the Bakhtyari conglomerates and the un derlying Agha Jari Formation has long been interpreted as indicating that o rogeny in the Zagros Simply Folded Zone took place in Plio-Pleistocene time s. This study uses field evidence Of Unconformities between older units in conjunction with geological maps and cross sections to argue that the front of the Zagros Simply Folded Zone has propagated in time and space. These u nconformities indicate that deformation started as early as end Eocene in t he northeast of the Simply Folded Zone and propagated progressively to the southwest. where unconformable contacts are only seen between younger units . As shortening continued, the southwest migration of the deformation front drove the foreland basin in front of it to its present position along the Persian Gulfand Mesopotamia. The climax or orogeny took place at end Plioce ne time,when the most extensive unconformity in the Zagros Simply Folded Zo ne developed between the (upper) Bakhtyari Formation and older units. Activ e seismicity and documented present uplift imply that the Simply Folded Zon e is still propagating southwestward.