Deformation and recrystallization mechanisms in actively extruding salt fountain: Microstructural evidence for a switch in deformation mechanisms with increased availability of meteoric water and decreased grain size (Qum Kuh, central Iran)

Microstructural study of rocksalt samples from an active salt fountain (Qum Kuh, central Iran) enabled to identify the relative contribution of different deformation mechanisms on extrusive salt flow. The microstructural study combined reflected and transmitted light microscopy of gamma-irradiated thin sections, textural analysis of digitized microstructures and Electron Back Scattered Diffraction (EBSD). Deformation microstructures record the strongly variable deformation conditions of salt flow in the diapiric system from the diapiric stem towards the distal part of the mature viscous fountain. High-stress deformation conditions typical for diapiric stems are recorded in the small subgrains within the porphyroclasts of all documented samples. Recovery and recrystallization due to divergent and decelerating flow associated with differential stress drop in the salt extrusion above the diapiric orifice is reflected by abundant growth band microstructures. This study reveals also evidence for penetration of rainwater into the salt mass and documents the switch from the dominant dislocation creep into dominant solution-precipitation creep from the upper part to the distal part of the fountain. This deformation mechanism switch is provided by influx of meteoric water and grain size decrease likely controlled by subgrain rotation and grain-boundary migration recrystallization.