Author: Gaia Travan (CNRS Lille, France)
What makes the salt a unique geological material is its low viscosity value: at low temperature and considering geological times, salt moves as a Newtonian fluid. To have an idea of the viscosity contrast between salt and brittle rocks as carbonates, the difference in viscosity can be higher than between milk and peanut butter! As you may guess, having a layer of brittle sediments superimposed on a viscous layer gives origin to spectacular structures in long times, but let’s see how these have been studied through the centuries.
Like in almost every field of research, the beginning of the studies on salt tectonics (mid 18th century) gave birth to a variety of hypothesis. The salt structures, so complex and enigmatic, have been interpreted in the most disparate way, even as residual island! Funny for the knowledge we have now, quite understandable for that period:
It’s in the ‘30s, same period of the strong development of seismic reflection, that the studies on salt tectonics started to take a joint direction. I didn’t find a documented history on the beginning of what will be defined ‘the fluid era’, so I’ll invent one. If you want strictly the truth, skip some lines! One evening, Professor Diapir went home tired after a day spent interpreting seismic profiles, and while pouring oil on his salad he accidentally made the oil fall in his glass of water. Needless to say that, conditioned by the long hours spent on the halite seismic images he immediately realised the similarity of the 2 immiscible liquids with the salt and sediments behaviour.
Coming back to the objective facts, since 1933 and for almost half a century the salt and the overlying brittle cover have been modelled as fluids of different viscosity and density. A small irregularity in the surface separating the 2 fluids was considered sufficient to give origin to a salt structure, that was thereby able to grow because of density contrast.
At the end of the ‘80s, the idea of the 2 pressurised fluids has been questioned by a new concept: while the ‘pressurised fluid model’ fits well for the evaporitic layer, modelling the above sediments as fluids is too far from the real nature of the material. From that moment on, the above sediments have been modelled as brittle material, and the relative strength of the two layers acquired importance in the models.
After more than 30 years this concept is still valid, as we can see from the choice of materials for the analogue modelling of salt tectonics: a viscous layer is generally made of silicone, or sporadically of honey, molasses, cane sugar syrup etc, while layers of sand of specific grain size models the brittle sediments.
The same principles are applied also in the case of numerical modelling, where silicone and sand are substituted informatically by specific rheological parameters.
What I would like us to keep in mind from this brief history of the salt tectonics studies, is that at every new step a large part of the scientific community had the impression to have reached the truth. The presence of researchers questioning the old models had been the precious engine of progress, and will be questioning the actual models that we produce small or big steps towards the deeper comprehension of the salt tectonics mechanisms.
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