Salt tectonics

Salt tectonics is concerned with the geometries and processes associated with the presence of significant thicknesses of rock salt within a sequence of rocks. This is due both to the low density of salt, which does not increase with burial, and its low strength.

Passive salt structures

Structures may form during continued sedimentary loading, without any external tectonic influence, due to gravitational instability. Pure halite has a density of 2160 kg m^-3. When initially deposited, sediments generally have a lower density than this, but with loading and compaction their density increases to be greater than 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 Gorleben salt dome in Germany is 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 within the salt that increase both its density and strength.

Active salt structures

Active tectonics will increase the likelihood of salt structures developing. In the case of

If the pressure within the salt body becomes sufficiently high it may be able to push through its overburden, this is known as "forceful" diapirism. Many salt diapirs may contain elements of both active and passive salt movement. An active salt structure may pierce its overburden and from then on continue to develop as a purely passive salt diapir.

Reactive salt structures

In those cases where salt layers do not have the conditions necessary to develop passive salt structures, the salt may still move into relatively low pressure areas around developing folds and faults. Such structures are described as "reactive".

alt detached fault systems

When one or more salt layers are present during extensional tectonics, a characteristic set of structures are formed. Extensional faults propagate up from the middle part of the crust until they encounter the salt layer. The weakness of the salt prevents the fault from propagating through. However, continuing displacement on the fault offsets the base of the salt and causes bending of the overburden layer. Eventually the stresses caused by this bending will be sufficient to fault the overburden. The types of structures developed depend on the initial salt thickness. In the case of a very thick salt layer there is no direct spatial relationship between the faulting beneath the salt and that in the overburden, such a system is said to be "unlinked". For intermediate salt thicknesses, the overburden faults are spatially related to the deeper faults, but offset from them, normally into the footwall; these are known as "soft-linked" systems. When the salt layer becomes thin enough, the fault that develops in the overburden is closely aligned with that beneath the salt, and forms a continuous fault surface after only a relatively small displacement, forming a "hard-linked" fault [ Stewart, S.A. 2007. Salt tectonics in the North Sea Basin: a structural style template for seismic interpreters. Special Publication of the Geological Society, London, 272, 361-396 ] .

In areas of thrust tectonics salt layers act as preferred detachment planes. In the Zagros fold and thrust belt, variations in the thickness and therefore effectiveness of the late Proterozoic Hormuz salt are thought to have had a fundamental control on the overall topography [Bahroudi, H. and Koyi, H.A. 2003. Effect of spatial distribution of Hormuz salt on deformation style in the Zagros fold and thrust belt: an analogue modelling approach. Journal of the Geological Society, 160, 719-733. ] .

alt weld

When a salt layer becomes too thin to be an effective detachment layer, due to salt movement, dissolution or removal by faulting, the overburden and the underlying sub-salt basement become effectively "welded" together. This may cause the development of new faults in the cover sequence and is an important consideration when modelling the migration of hydrocarbons.

Economic importance

A significant proportion of the world’s hydrocarbon reserves are found in structures related to salt tectonics, including many in the Middle East, the South Atlantic passive margins (Brazil, Gabon and Angola) and the Gulf of Mexico.

References

External links

* [http://www.endlagerung.de/generator.aspx/templateId=renderPage/lang=en/id=12482.html Gorleben salt dome]

* [http://oceanexplorer.noaa.gov/explorations/02mexico/background/brinepool/brinepool.html NOAA site on brine pools]

ee also

* Salt dome