Salt tectonics
Encyclopedia
Salt tectonics is concerned with the geometries and processes associated with the presence of significant thicknesses of evaporite
s 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.
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, diapir
s 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 dome
s 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.
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 anticline
s, as seen in salt dome
s in the Zagros Mountains
.
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.
, 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.
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.
s.
Salt welds may also develop in the vertical direction by putting the sides of a former diapir in contact.
reserves are found in structures related to salt tectonics, including many in the Middle East
, the South Atlantic passive margin
s (Brazil
, Gabon
and Angola
) and the Gulf of Mexico
.
Evaporite
Evaporite is a name for a water-soluble mineral sediment that result from concentration and crystallization by evaporation from an aqueous solution. There are two types of evaporate deposits, marine which can also be described as ocean deposits, and non-marine which are found in standing bodies of...
s containing rock salt within a stratigraphic
Stratigraphy
Stratigraphy, a branch of geology, studies rock layers and layering . It is primarily used in the study of sedimentary and layered volcanic rocks....
sequence of rocks. This is due both to the low density
Density
The mass density or density of a material is defined as its mass per unit volume. The symbol most often used for density is ρ . In some cases , density is also defined as its weight per unit volume; although, this quantity is more properly called specific weight...
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 haliteHalite
Halite , commonly known as rock salt, is the mineral form of sodium chloride . Halite forms isometric crystals. The mineral is typically colorless or white, but may also be light blue, dark blue, purple, pink, red, orange, yellow or gray depending on the amount and type of impurities...
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, diapir
Diapir
A diapir is a type of intrusion in which a more mobile and ductily-deformable material is forced into brittle overlying rocks. Depending on the tectonic environment, diapirs can range from idealized mushroom-shaped Rayleigh-Taylor instability-type structures in regions with low tectonic stress...
s 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
Schacht Asse II
The Asse II pit is a former salt mine used as a deep geological repository for radioactive waste in the mountain range of Asse in district Wolfenbüttel in Lower Saxony, Germany.-History:...
and Gorleben
Gorleben
Gorleben is a small municipality in the Gartow region of the Lüchow-Dannenberg district in the far north-east of Lower Saxony, Germany, a region also known as the Wendland....
salt dome
Salt dome
A salt dome is a type of structural dome formed when a thick bed of evaporite minerals found at depth intrudes vertically into surrounding rock strata, forming a diapir....
s 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 salt structures
Active tectonicsTectonics
Tectonics is a field of study within geology concerned generally with the structures within the lithosphere of the Earth and particularly with the forces and movements that have operated in a region to create these structures.Tectonics is concerned with the orogenies and tectonic development of...
will increase the likelihood of salt structures developing. In the case of extensional tectonics
Extensional tectonics
Extensional tectonics is concerned with the structures formed, and the tectonic processes associated with, the stretching of the crust or lithosphere.-Deformation styles:...
, faulting will both reduce the strength of the overburden and thin it. In an area affected by thrust tectonics
Thrust tectonics
Thrust tectonics or contractional tectonics is concerned with the structures formed, and the tectonic processes associated with, the shortening and thickening of the crust or lithosphere.-Deformation styles:...
, buckling of the overburden layer will allow the salt to rise into the cores of anticline
Anticline
In structural geology, an anticline is a fold that is convex up and has its oldest beds at its core. The term is not to be confused with antiform, which is a purely descriptive term for any fold that is convex up. Therefore if age relationships In structural geology, an anticline is a fold that is...
s, as seen in salt dome
Salt dome
A salt dome is a type of structural dome formed when a thick bed of evaporite minerals found at depth intrudes vertically into surrounding rock strata, forming a diapir....
s in the Zagros Mountains
Zagros Mountains
The Zagros Mountains are the largest mountain range in Iran and Iraq. With a total length of 1,500 km , from northwestern Iran, and roughly correlating with Iran's western border, the Zagros range spans the whole length of the western and southwestern Iranian plateau and ends at the Strait 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.Salt detached fault systems
When one or more salt layers are present during extensional tectonicsExtensional tectonics
Extensional tectonics is concerned with the structures formed, and the tectonic processes associated with, the stretching of the crust or lithosphere.-Deformation styles:...
, 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.
In areas of thrust tectonics
Thrust tectonics
Thrust tectonics or contractional tectonics is concerned with the structures formed, and the tectonic processes associated with, the shortening and thickening of the crust or lithosphere.-Deformation styles:...
salt layers act as preferred detachment planes. In the Zagros fold and thrust belt
Zagros fold and thrust belt
The Zagros fold and thrust belt is a ~1800 km long zone of deformed crustal rocks, formed in the foreland of the collision between the Arabian Plate and the Eurasian Plate...
, variations in the thickness and therefore effectiveness of the late Proterozoic
Proterozoic
The Proterozoic is a geological eon representing a period before the first abundant complex life on Earth. The name Proterozoic comes from the Greek "earlier life"...
Hormuz salt are thought to have had a fundamental control on the overall topography.
Salt 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 modeling the migration of hydrocarbonHydrocarbon
In organic chemistry, a hydrocarbon is an organic compound consisting entirely of hydrogen and carbon. Hydrocarbons from which one hydrogen atom has been removed are functional groups, called hydrocarbyls....
s.
Salt welds may also develop in the vertical direction by putting the sides of a former diapir in contact.
Economic importance
A significant proportion of the world’s hydrocarbonHydrocarbon
In organic chemistry, a hydrocarbon is an organic compound consisting entirely of hydrogen and carbon. Hydrocarbons from which one hydrogen atom has been removed are functional groups, called hydrocarbyls....
reserves are found in structures related to salt tectonics, including many in the Middle East
Middle East
The Middle East is a region that encompasses Western Asia and Northern Africa. It is often used as a synonym for Near East, in opposition to Far East...
, the South Atlantic passive margin
Passive margin
A passive margin is the transition between oceanic and continental crust which is not an active plate margin. It is constructed by sedimentation above an ancient rift, now marked by transitional crust. Continental rifting creates new ocean basins. Eventually the continental rift forms a mid-oceanic...
s (Brazil
Brazil
Brazil , officially the Federative Republic of Brazil , is the largest country in South America. It is the world's fifth largest country, both by geographical area and by population with over 192 million people...
, Gabon
Gabon
Gabon , officially the Gabonese Republic is a state in west central Africa sharing borders with Equatorial Guinea to the northwest, Cameroon to the north, and with the Republic of the Congo curving around the east and south. The Gulf of Guinea, an arm of the Atlantic Ocean is to the west...
and Angola
Angola
Angola, officially the Republic of Angola , is a country in south-central Africa bordered by Namibia on the south, the Democratic Republic of the Congo on the north, and Zambia on the east; its west coast is on the Atlantic Ocean with Luanda as its capital city...
) and the Gulf of Mexico
Gulf of Mexico
The Gulf of Mexico is a partially landlocked ocean basin largely surrounded by the North American continent and the island of Cuba. It is bounded on the northeast, north and northwest by the Gulf Coast of the United States, on the southwest and south by Mexico, and on the southeast by Cuba. In...
.