Salt tectonics

Salt tectonics, or halokinesis, or halotectonics, is concerned with the geometries and processes associated with the presence of significant thicknesses of

Salt structures (excluding undeformed layers of salt) have been found in more than 120 sedimentary basins around the world.^[2]

Passive salt structures

Structures may form during continued sedimentary loading, without any external tectonic influence, due to gravitational instability. Pure

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.^[citation needed]

Active salt structures

Active

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.^[citation needed]

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.^{[citation needed]}

Salt detached fault systems

When one or more salt layers are present during extensional tectonics, a characteristic set of structures is 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.^[6]

In areas of

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 modeling the migration of hydrocarbons. Salt welds may also develop in the vertical direction by putting the sides of a former diapir in contact.^[8]

Allochthonous salt structures

Salt that pierces to the surface, either on land or beneath the sea, tends to spread laterally away and such salt is said to be "allochthonous". Salt glaciers are formed on land where this happens in an arid environment, such as in the Zagros Mountains. Offshore tongues of salt are generated that may join together with others from neighbouring piercements to form canopies.^{[citation needed]}

Effects on sedimentary systems

On passive margins where salt is present, such as the Gulf of Mexico, salt tectonics largely control the evolution of deep-water sedimentary systems; for example submarine channels, as modern and ancient case studies show.^[9]

Economic importance

A significant proportion of the world's

• Plasticity (physics) – Non-reversible deformation of a solid material in response to applied forces

References