Keywords: 
• Neo-tethyan Ocean /
• Zagros /
• Agha Jari Formation /
• Sedimentary Environment /
• Regression

Abstract: [Objective] The collision of the Arabian-Eurasian plates led to the formation of Zagros orogenic belt and the related foreland basin. The gradual filling of the Zagros foreland basin resulted in the retreat of the Neo-Tethys Ocean in the Zagros region, which causes the transition from the marine to terrestrial environments. This process provides the possible chance to explore the demise of the Neo-tethys Ocean in the Middle East. [Methods] In this study, we conducted detailed stratigraphy, sedimentology, sandstone petrolography, and detrital zircon U-Pb age on the Miocene sedimentary strata in the Lurestan and Khuzestan regions of the Zagros Mountains of southern Iran, which provides new constraints on the filling of the Zagros foreland basin and the regression process of the Neo-Tethys Ocean in the Zagros area. [Results] The Agha Jari Formation in the Lurestan region is characterized by large-scale, medium-to-thick layered sandstones interbedded with mudstones. Sedimentary environment analysis suggests that it has been formed in a fluvial environment. The sandstone petrography shows a lot of metamorphic clasts. In combination of the detrital zircon U-Pb age peak of Jurassic ~170 Ma, it is constrained the the Agha Jari Formation in the Lurestan region has mainly derived from the Sanandaj-Sirjan Zone to the north. The Agha Jari Formation in the Khuzestan area is featured as a large set of thick mudstone beds, thick mudstone interbedded with sandstone, thick sheeted sandstone interbedded with mudstone and sand-mudstone interbedded. The sedimentary environment analysis shows that it has been formed in a transitional deltaic environment. The sandstone modal composition yields the clasts of the sedimentary rocks occurred. The detrital zircon ages show a lot of Mesozoic ages, comparable with the Zagros Fold-and-Thrust Belt. So it is concluded that they have been mainly recycled from the sedimentary strata in Zagros Fold-and-Thrust Belt. Based on the underlying Gachsaran-Mishan formations with composing of shallow marine limestone and evaporite, the Agha Jari Formation represents the regional youngest marine sedimentation, recording the transition from the marine to the terrestrial environments. The calcareous nannofossil analysis from the underlying Mishan Formation yield the youngest age of the late Miocene. In consideration of the other biostratigraphic studies, the age of the Agha Jari Formation is constrained by no earlier than the Miocene. Therefore, in according to the sedimentary and biostratigraphic studies on the Agha Jari Formation in the Khuzestan region, the demise of the Neo-tethyan Ocean in the Zagros region has been constrained by no earlier than the Miocene and been shown the trend from the northwest to the southeast. [Conclusions] Combined with the analysis of regional paleogeography and provenance analysis on the Miocene strata in the Zagros orogenic belt, the gradual uplift of the Zagros orogenic belt has occurred after the Arabia-Eurasia collision. The uplifted Zagros orogenic belt has been eroded so as to provide the clastic materials for the Zagros foreland basin. As a result, the regression of the Neo-Tethys Ocean in the Zagros area is generally controlled by the continual filling in the foreland basin. This paleogeography of northwest-southeast regression of the Neo-tethyan Ocean has been existed no later than the early Miocene.