Salt deformation

A subsurface salt layer or a salt diapir that has not extruded the surface is considered as an underground salt structure. Buoyancy, gravitational differential loading and tectonic stress are the three main types of force that can drive salt flow. However, salt flow can be restricted by strength of overlaying sediments and boundary friction within the salt layer.^[1]

In the critical depth of 1.2-1.3 km, the density of rock salt and surrounding rocks are roughly the same. At greater burial depth, density inverse and rock salt become less dense than the overburden rocks, which leads to positive buoyancy of the rock salt and causes the rise of salt.^[6] As temperature increases with depth, salt is heated and expands, this also leads to increase in buoyancy of rock salt.^[1]

However, when the overburden is thick enough, the salt will not be able to pierce the overburden by buoyancy.^[6]

Gravitational differential loading is produced by a combination of gravitational forces acting on the overburden rocks and the underlying salt layer.^[1] The effect of gravitational loading on salt flow can simply be expressed by the concept of hydraulic head:

${\displaystyle h=z+{\frac {P}{\rho _{s}g}}}$

Where h is the hydraulic head, z is the elevation head that counts from a datum to the top of the salt layer, P is pressure exerted on the salt layer by overburden, ${\displaystyle \rho _{s}}$ is the density of the salt and g is the gravity acceleration. Pressure head are expressed as P over ${\displaystyle \rho _{s}g}$. As P is also equal to ${\displaystyle \rho _{o}t}$ , where ${\displaystyle \rho _{o}}$ is the density of the overburden and t is its thickness.

Therefore, the equation can be rewritten as:

${\displaystyle h=z+{\frac {\rho _{o}}{\rho _{s}}}t}$

Note that the pressure head P is then expressed as ${\displaystyle {\frac {\rho _{o}}{\rho _{s}}}t}$.

Assuming the ratio of density of the overburden rock to that of the salt layer remains unchanged in the following three cases:

Case	Thickness of overburden, t	Elevation head, z	Hydraulic head	Explanation	Image
1	Constant, ${\displaystyle t_{1}=t_{2}}$	${\displaystyle z_{1}=z_{2}}$	${\displaystyle h_{1}=h_{2}}$	When the thickness and the elevation head is constant, although the thickness of the salt layer is not uniform, there is no salt flow as a result of zero hydraulic gradient.
2	Constant, ${\displaystyle t_{1}=t_{2}}$	${\displaystyle z_{1}>z_{2}}$	${\displaystyle h_{1}>h_{2}}$	The thickness of overburden still remain constant however there is elevation head gradient, leading to h1 > h2 and causing salt flow from da direction of higher to lower elevation head gradient.
3	${\displaystyle t_{1}>t_{2}}$	${\displaystyle z_{1}=z_{2}}$	${\displaystyle h_{1}>h_{2}}$	Although the surface of the salt layer has uniform elevation, the difference in thickness of overburden rock produce pressure head gradient. Thus, there will be a gradient in the hydraulic head that drives the salt to flow.

Tensional stress affects salt structure deformation by (1) forming fractures in the overlying rocks, thinning of overburden and reduction of strength of overburden, (2) developing a graben in overburden that favors gravitational differential loading.^[16] Most salt diapirs in the world were initiated during regional extension, implying that salt diapirism is primarily activated by tensional stress.^[1][10]

Tensional stress leads to thin-skinned extension, which stretches the overburden but not the salt layer in the base.^[17] Deformation of salt structures from thin-skinned extension can be divided into three stages.^[18] However, it is important to note that diapir does not need to go through all these stages, depending on the amount and rate of extension, density of overburden, etc.^[1]

1) In the initial stage, regional extension thins and weaken the overburden, salt begins to rise and fill up the space created from thinning. When regional extension stops, the rise of diapirs will also stop. This stage is called reactive diapirism, as it is reacting to the extension.^[1]

2) As the thinning and weakening continues, the deformation proceeds to the second stage, in which the overlying rock becomes weak enough for salt to pierce and to be pushed up. The phenomenon only occurs when overburden is denser than salt, probably after reaching the critical depth. This phase is termed as active diapirism, the salt still continues to rise even after regional extension stops.^[1]

3) In the third stage, the diapir pierces through the overlying rock and is exposed at the surface. This phase is called passive diapirism.^[1]

Compressional stress thickens and strengthens the overlying rock, this resists the rock salt from piercing up and slows down the formation of a diapir, except when an anticline formed from compression force is seriously eroded to great depth. In the case where there are preexisting salt diapir structures that are mechanically weaker, the diapirs are reactivated during regional compression, rock salt then moves upward and is cut off from the source layer. For another case of no preexisting rock salt diapirs, salt mainly acts as a lubricant to form décollement.^[1]

Shear stress does not much affect the salt layer, but salt will still flow if compressional stress and tensional stress are induced from the shear, and results in similar salt deformation behavior in the stressed zone. Deformation of salt structure can be classified into four types:^[1]

Types	Diapir	Force	Deformation of salt structure
1	Preexisting	Localized compression	Salt displace to the top and is cut off from the source layer
2	Preexisting	Localized extension	Widening of diapir, diapir can fall if the rate of salt supply is not enough to support the weight of the overlying rock,[16] but will rise if there is sufficient salt supply to push up the overburden[1]
3	No preexisting	Localized compression	No diapir is formed after slip
4	No preexisting	Localized extension	Trigger reactive diapirism after slip

As the burial depth increase, the strength of sedimentary rocks increases with the rising pressure.^[6] Therefore, most thick overburden is more difficult to pierce by underlying salt and deform accordingly. Overlying sediments that have a thickness of few hundred meters rarely deform if external forces such as compression and extension do not exist.^[1]

Boundary friction along the top and the bottom of the salt layer restricts the ability of salt to flow. When salt shear passes the boundary between the salt layer and the surrounding rigid rocks, a drag force opposite to the direction of flow exists in the shear zone and resists salt flow. The thickness of this boundary shear zone can affect the flow rate of the salt layer. If the flow has a constant dynamic viscosity, which means it is Newtonian viscous, the boundary layer is thicker. For salt flow that is power-law viscous, such that it decreases in dynamic viscosity as the rate of shear increases towards the fluid boundary, the boundary layer is thinner.^[1]

Newtonian flow has a great impact on the flow rate of salt, which the volumetric flux is proportional to the thickness of the salt layer to the power of three, which means if the thickness of the salt layer is doubled, the volumetric flux will speed up the flow by eight times. Power-law flow has a relatively smaller effect on slowing down salt flow.^[1]

Surface salt structures are formed when underground salt diapirs pierce through the overlying rock.^[8]

When salt extrudes and flows at the surface, it becomes a salt glacier (also known as a salt fountain).^[8] Unlike underground salt structures, when rock salt is uncovered, it is exposed to rainwater, wind and heat from the sun that could lead to rapid deformation of salt structure within a short time, which can be daily to seasonal.^[13][7]

When an underground salt diapir rises and extrudes at the surface, it pushes up the overlaying rock and results in an uplift movement of the salt glacier together with the overlying rock. Uplift movements in the rate of mm/yr are observed in various locations such as Mount Sedom in Israel^[19][20] and the salt glaciers in Iran.^[21][22]

Salt diapirs that are exposed at the surface rise faster than the diapirs that remain in the subsurface, as the strength of overlying sediments is decreased.^[23]

Different parts of a salt glacier deform with different mechanisms. A microstructural study shows that as salt flows from the summit of the salt fountain to the distal part, pressure solution becomes the dominant process as a result of infiltrated rainwater and decreased grain size, instead of subgrain rotation recrystallization and grain boundary migration which are dominant in the top and the middle part of the salt fountain. In other words, it has been suggested that the infiltration of rainwater into rock salt will cause deformation at the grain-size level.^[13]

Plastic flow of salt glaciers during rainy seasons and individual storm events and shrinkage after drying of the glacier were observed in Jashak salt dome (also known as Dashti salt dome or Kuh-e-Namak), Iran, suggesting seasonal movements in salt glaciers in response to weather condition.^[7] However, another study in the Kuqa fold-thrust belt tried to test the seasonal responsiveness of glacier movement regarding rainfall, but did not observe the correlation between salt deformation and precipitation, and stated their result may be attributed to limited satellite and ground observation data.^[24]

Further investigations, by using remote sensing technique and especially performing field observations are needed to confirm the relationship.

Other than crystallization and hydration, thermal expansion is one of the most frequently mentioned mechanisms in salt weathering.^[25][26] Rock salt expands when heated.^[7][27] It is known that most salt weathering occur in regions with arid climates.^[25][28] Due to the high thermal conductivity of salt glaciers, heat can be transmitted hundreds of meters through dry salt in a few minutes.^[7]

• Diapirism
• Rock salt (halite)
• Salt surface structures
• Salt glacier
• Salt tectonics