The geomorphology of the mature, fluviokarst-dominated Mesijune salt extrusion (Zagros Mountains, Iran). Insights into the evolution of salt fountains

The Fars Arc of the Zagros Mountains hosts around one hundred salt extrusions at different morpho-chronological stages (e.g., domes, salt fountains, salt droplets), being the best natural laboratory worldwide for the study of salt karst in active diapirs. A number of publications document the geomorphology and hydrology of salt fountains at a juvenile stage dominated by polygonal karst (holokarst) with internal drainage. However, studies on mature salt fountains and the associated long-term evolution remain lacking. This work characterises from the morpho-hydrological perspective the mature Mesijune salt fountain. This is the largest salt fountain in the region (137 km 2 , ca. 11,650 Mm 3 ) showing: (1) a multidirectional salt glacier (i.e., namakier) around the summit dome; (2) a well-developed network of throughgoing radial valleys; (3) prevalence of external drainage (ca. 65%) and fluvial landscapes; (4) extensive caprock covers (ca. 58%) that record protracted periods of dissolutional weathering; and (5) abundant and diverse slope movements, including DSGSD. The application of an ergodic concept allows inferring the changes associated with the transition in salt fountains from a juvenile to a mature stage, mainly involving the replacement of polygonal karst areas into fluviokarst and fluvial landscapes. This major transformation implies the reduction in the number and density of sinkholes, and the increase in their clustering and the proportion of sinkholes acting as ponors. Landsliding plays a progressively greater morphogenetic role favoured by rapid fluvial erosion, and mechanical erosion prevails over dissolutional erosion. Mesijune can be ranked as a geosite of superlative value, representing a paradigmatic example of a mature salt fountain dominated by fluvial and fluviokarst landscapes. • Mesijune, the largest salt fountain in the Zagros Mountains • Morpho-hydrological characterisation of a mature caprock-dominated salt fountain • Replacement of polygonal karst by fluviokarst and fluvial landscapes • Sinkhole patterns in a mature salt extrusion • Abundant and diverse slope movements on rock salt, including DSGSD