Subaerial salt extrusions in Iran as analogues of ice sheets, streams and glaciers

Ice (H20) and salt (halite, NaCl) share many physical properties and resemble each other in hand specimens and subaerial gravity-driven flows. However, while most significant bodies of ice accumulate in cold highlands and gravity-spread where and soon after they form, most significant bodies of salt accumulate in tropical marine basins and have to be buried by > 1 km of other rocks before they flow. Buried salt is driven by differential loading into various categories of piercing structures known as diapirs. Many diapirs extrude onto the surface as sheets of allochthonous (out of place) salt. Thousands of sheets of allochthonous salt have been interpreted in over 35 basins worldwide in the last 25 years, mainly in the toes of passive continental margins and in orogenic belts where some are > 103 km2 in area. Most former salt sheets are now submarine or subsurface but several active examples are beautifully exposed in Iran. These were compared to ice glaciers soon after they were introduced to western science, a comparison that has been neglected since. Here we update this analogy and use modern understanding of flowing ice and salt to examine the similarities and differences that might be mutually beneficial to both fields of study as well as to extraterrestrial scientists.The profiles, internal structures and fabrics in flowing bodies of ice and salt are sensitive gauges of the histories of their budgets of supply and loss. However, whereas snow merely compacts where it accumulates, salt sheets are fed from below by already deformed salt. When salt diapirs first emerge on land they extrude domes that mature to the profiles of viscous fountains that often feed glacier-like flows known as namakiers. After locally exhausting their deep source layers, salt fountains spread to the profiles of viscous droplets normal for ice caps.Ice typically deforms at > 80% (usually > 90%) of its absolute melting temperature while most salt deforms at < 50% of its homologous temperature; as a result, grain shape fabrics in salt are clearer and have longer strain memories than in ice. Foliations in deformed salt map streamlines aid in the understanding of how internal folds develop. Salt sheets seldom erode their channels like flowing ice and internal debris accumulates on their tops rather than their bases. An ice sheet floats on water but as salt is twice the density of ice; rain that falls onto the top surface of namakiers tends to stay there. Both glaciers and namakiers surge but the association between surges and changes in boundary conditions are much clearer for namakiers than glaciers. Because the rate of delivery of land ice to the oceans is such an important control on sea level, we end by considering how the implications of surging salt converge on recent glaciological findings about changes in boundary conditions other than their bases.