Salt tectonics
Salt tectonics is concerned with the geometries and processes associated with the presence of significant thicknesses of evaporites containing rock salt within a stratigraphic sequence of rocks. This is due both to the low density of salt, which does not increase with burial, and its low strength.
Structures may form during continued sedimentary loading, without any external tectonic influence, due to gravitational instability. Pure halite has a density of 2160 kg/m3. When initially deposited, sediments generally have a lower density of 2000 kg/m³, but with loading and compaction their density increases to 2500 kg/m³, which is greater than that of salt. Once the overlying layers have become denser, the weak salt layer will tend to deform into a characteristic series of ridges and depressions, due to a form of Rayleigh–Taylor instability. Further sedimentation will be concentrated in the depressions and the salt will continue to move away from them into the ridges. At a late stage, diapirs tend to initiate at the junctions between ridges, their growth fed by movement of salt along the ridge system, continuing until the salt supply is exhausted. During the later stages of this process the top of the diapir remains at or near the surface, with further burial being matched by diapir rise, and is sometimes referred to as downbuilding. The Schacht Asse II and Gorleben salt domes in Germany are an example of a purely passive salt structure.
Such structures do not always form when a salt layer is buried beneath a sedimentary overburden. This can be due to a relatively high strength overburden or to the presence of sedimentary layers interbedded within the salt unit that increase both its density and strength.
Active tectonics will increase the likelihood of salt structures developing. In the case of extensional tectonics, faulting will both reduce the strength of the overburden and thin it. In an area affected by thrust tectonics, buckling of the overburden layer will allow the salt to rise into the cores of anticlines, as seen in salt domes in the Zagros Mountains.
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