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Salt deformation

Pure rock salt from from Pakistan's Khewra Salt Mine.

Salt deformation involves change of geometries of salt structures, either of salt surface structure and underground salt structure as a result of different forcing that controls the kinematics of salt flow.

Salt structures are made of rock salts that is composed of pure halite(NaCl) crystal when strictly speaking. However, most halite in nature appear as impure form, therefore rock salt usually refers to all rocks that composed mainly of halite, but sometimes also as a mixture with other evaporites such as gypsum and anhydrite[1]. In salt tectonics, the term "salt" is used to represent rock salt[1]. Due to the unique physical and chemical properties of rock salt such as its low density, high thermal conductivity and high solubility in water, it deforms distinctively in underground and subaerial environment when compare with other rocks, forming a variety of salt structures as a result of flow of rock salt, including rock salt itself and the surrounding rocks that deform accordingly.

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Physical properties of rock salt

Density and buoyancy

Rock salt has an effective porosity of nearly 50% in the surface, while the effective porosity decreases to less than 10% at a depth of 10 m[2][3]. When the burial depth reached about 45 m, the pore spaces are completely filled[2][3]. After rock salt losses its porosity, it becomes almost incompressible and remains a constant density of 2.2 g/cm3 until it reaches a depth of 6-8 km, where rock salt are metamorphosed into greenschist. In such burial depth, the density of rock salt is slightly decreased as a result of thermal expansion[4]. However, as the burial depth increases, shale and most other sedimentary rocks decrease in porosity and increase in density progressively without terminating in shallow depth. In the first 1000m of burial depth, rock salt has a higher density compared with other rocks such as shale[4]. When the buried material reaches a critical depth of 1.2-1.3km, the density of rock salt and other rocks are roughly the same, where neutral buoyancy is reached. Starting from 1.3-1.5km below the surface, the density of other rocks exceeds that of rock salt, density inversion takes place, meaning that salt has positive buoyancy when buried under other rocks at around 1.3 km[4]. In this depth, salt rises and intrude into the overburden, forming a diapir.

Thermal conductivity

Rock salt is characterized by its high thermal conductivity. For example, at 43°C, it has a thermal conductivity of 5.13W, while shale only has a thermal conductivity of 1.76 W at the same temperature[4].

The volume of rock salt can be largely affected by thermal gradient. When rock salt is buried underground at 5km at a thermal gradient of 30°C/km, its volume expands by 2% due to thermal expansion, while pressurization only causes volume reduction of 0.5%[5]. Therefore, the larger the burial depth of rock salt, the lower the density of it, which in turns favors the positive buoyancy induced by density inversion.

Heat can also lead to the internal flow of rock salt. When the burial depth of rock salt is over 2.9 km at a thermal gradient of 30°C/km with viscosity below 1016 Pa.s, a flow of rock salt by thermal conduction occurs. However thermal conduction is not the dominant mechanism of salt flow in the sedimentary basin, which is completely different from the flow of magma. Salt flows at the surface if it is sufficiently wet, for instance, the flow of salt glaciers.

Viscosity

Viscosity is a measure of the resistance of fluids to flow that can be represented by the ratio of shear stress to shear strain[4]. High viscosity means a high resistance to flow and vice versa. Experimental results show that rock salt has a higher viscosity compare with bittern and rhyolite lava, however, the viscosity is lower than that of mud rock, shale, and mantle[4]. Besides, the viscosity of rock salt is closely related to the water content. The more the water content in rock salt, the lower the viscosity of it.

When salt glaciers fed from diapir is exposed at the surface and is infiltrated by meteoric water, the viscosity of rock salt is reduced. Consequently, the flow rate of salt glaciers is much faster than that of salt tongue spreading and salt diapir rise[6].

In general, fine-grained wet salt flows as a Newtonian fluid, except for coarse-grained salt. Otherwise, it will spread due to gravitational force as it extrudes to the surface[4].  

Strength

Rock salt has lower strength than other rocks, when stress is applied, it behaves like a fluid, while other rocks of higher strength are brittle under such condition[7]. When comparing the tensional and compressional strength of wet salt and dry salt with other typical rocks at a strain rate of 10-14s-1, such as shale and quartzite, both wet and dry salt shows lower strength than the other rocks[5]. Wet salt is even weaker than dry salt: when the water content of rock salt exceeds 0.01%, the rock salt behaves as week crystalline fluid[8]. Therefore, wet salt deforms more easily compare with dry salt.

Salt structures

Underground salt structure

Subaerial salt structure

Salt dynamics

Underground salt structure

The following three types of force can drive salt flow: buoyancy, gravitational differential loading and tectonic stress.

Buoyancy

In the critical depth of 1.2-1.3km, 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 cause the rise of salt. However when the overburden is thick enough, the salt will not be able to pierce the overburden by buoyancy[6].

Gravitational differential loading

Understanding the flow of salt using the concept of hydraulic flow. The top figure shows the topography of uniform thickness of overburden and constant elevation head, the middle figure shows a uniform thickness of overburden but with a difference in elevation head. The bottom figure illustrates constant elevation head with a varying thickness of the overburden. Modified from Hudec & Jackson, 2007[1]
Evolutionary diagram showing diapir development during thin-skinned extension. Modified from Vendeville & Jackson, 1992[9]

Gravitational differential loading is caused by the difference in the topography of the overburden rock. It 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:

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, is the density of the salt and g is the gravity acceleration. Pressure head are expressed as P over . As P is also equal to , where is the density of the overburden and t is its thickness.

Therefore, the equation can be rewritten as:

Note that the pressure head P is then expressed as .

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
1 Constant, Consider the case of a constant thickness of overburden and same elevation head, although the thickness of the salt layer is not uniform, there is no salt flow as a result of zero hydraulic gradient.
2 Constant, For the second case, the thickness of overburden still remain constant however the there is elevation head gradient, leading to h1 > h2 and causing salt flow from da directionof higher to lower elevation head gradient.
3 For the third scenario, 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.

Tectonic stress

Tensional stress

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

Thin-skinned extension

Thin-skinned extension stretches the overburden but not the salt layer in the base[6]. Deformation of salt structure from thin-skinned extension can be divided into three stages[11]. 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 first stage, regional extension thin and weaken the overburden, salt begins to rise and fill up the space created from thinning. When regional extension stops, the rise of diapir will also stop. This stage is called reactive diapirism, as it is reacting to the extension.

2) As the thinning and weakening continue, it proceeds to the second stage, which the overlying rock is week enough for salt to pierce in and pushes 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 regional extension stops.

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

Compressional stress
With preexisting diapir structure, rock salt moves upward and is cut off from the source layer by compression. Additional sediments are deposited on top at the same time.

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

Shear stress

Shear stress does not affect much on the salt layer, but salt will still flow if the compressional stress and tensional stress are induced from the shear, and results in similar salt deformation behavior in the stressed zone[1]. 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
arrows indicating right lateral
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[10], but will rise if there is sufficient salt supply to push up the overburden[1]
arrows indicating left lateral
3 No preexisting Localized compression No diapir is formed after slip
arrows indicating right lateral
4 No preexisting Localized extension Trigger reactive diapirism after slip
before and after slip (4)

The strength of overlying sediments

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

Boundary friction within the salt layer

Boundary friction along the top and the bottom of the salt layer restrict the ability of salt to flow[1]. 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 resisting 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 increase towards to fluid boundary, the boundary layer is thinner.

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[1], which means if doubling the thickness of the salt layer, the volumetric flux will speed up the flow by 8 times. Power-law flow has a relatively smaller effect on slowing down salt flow[1].

Kuh-e-Namak, also known as Dashti salt dome or Jashak salt dome) in the Zagros Mountains, Iran. Rock salt is extruded and flow at the surface, giving a white color as shown in this satellite image.
Deformation and recrystallization in different part of a salt glacier, arrows indicate the direction of salt flow.

Subaerial salt structure

When salt extrudes and flows at the surface, it becomes salt glaciers (also known as salt fountain). 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 seasonally[12][13].

Uplift of salt glaciers

When an underground salt diapir rises and extrudes 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[14][15] and Iran salt glaciers[16][17].

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[18].

Deformation by precipitation

Different part of a salt glacier deforms at different mechanisms[12]. A microstructural study shows that as salt flow from the summit of the salt fountain to the distal part, pressure solution become 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[12]. In other words, it has been suggested that the infiltration of rainwater into rock salt will cause deformation in grain-size level.

Plastic flow of salt glacier during rain season and individual storm event and shrinkage after drying of the glacier were observed in Kuh-e-namak, Iran, suggesting seasonal movements in salt glaciers in response to weather condition[13]. However, another study in the Kuqa fold-thrust belt tried to test the seasonal responsiveness of glacier movement regarding rainfall did not observe the correlation between salt deformation and precipitation, stated their result may be attributed to limited satellite and ground observation data[19].

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

Deformation by temperature change

Other then crystallization and hydration, thermal expansion is one of the most frequently mentioned mechanisms in salt weathering[20][21]. Rock salt expands when heated[13][22]. It is known that most salt weathering occur in regions with arid climate[20][23]. Due to the high thermal conductivity of salt glacier, heat can be transmitted hundreds of meter through dry salt in a few minutes[13].

See also  

Rock salt (Halite)

Salt surface structures

Salt glacier

Salt tectonics

References

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  2. ^ a b Enrique Casas, Tim K. Lowenstein (1989). "Diagenesis of Saline Pan Halite: Comparison of Petrographic Features of Modern, Quaternary and Permian Halites". SEPM Journal of Sedimentary Research. Vol. 59. doi:10.1306/212f905c-2b24-11d7-8648000102c1865d. ISSN 1527-1404. {{cite journal}}: |volume= has extra text (help)
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  4. ^ a b c d e f g K., Warren, John (2006). Evaporites Sediments, Resources and Hydrocarbons. Springer. ISBN 9783540323440. OCLC 780961839.{{cite book}}: CS1 maint: multiple names: authors list (link)
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  6. ^ a b c Jackson, M. (1994-01-01). "Structural Dynamics of Salt Systems". Annual Review of Earth and Planetary Sciences. 22 (1): 93–117. doi:10.1146/annurev.earth.22.1.93. ISSN 0084-6597.
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  8. ^ Urai, Janos L.; Spiers, Christopher J.; Zwart, Hendrik J.; Lister, Gordon S. (1986-12). "Weakening of rock salt by water during long-term creep". Nature. 324 (6097): 554–557. doi:10.1038/324554a0. ISSN 0028-0836. {{cite journal}}: Check date values in: |date= (help)
  9. ^ Vendeville, B.C.; Jackson, M.P.A. (1992-08). "The rise of diapirs during thin-skinned extension". Marine and Petroleum Geology. 9 (4): 331–354. doi:10.1016/0264-8172(92)90047-i. ISSN 0264-8172. {{cite journal}}: Check date values in: |date= (help)
  10. ^ a b ), 雷刚林, (1970- (2014). 塔里木盆地库车坳陷盐相关构造特征及变形机理. Shi you gong ye chu ban she. ISBN 9787518305391. OCLC 917887528. {{cite book}}: |last= has numeric name (help)CS1 maint: multiple names: authors list (link) CS1 maint: numeric names: authors list (link)
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  12. ^ a b c Desbois, Guillaume; Závada, Prokop; Schléder, Zsolt; Urai, Janos L. (2010-04). "Deformation and recrystallization mechanisms in actively extruding salt fountain: Microstructural evidence for a switch in deformation mechanisms with increased availability of meteoric water and decreased grain size (Qum Kuh, central Iran)". Journal of Structural Geology. 32 (4): 580–594. doi:10.1016/j.jsg.2010.03.005. ISSN 0191-8141. {{cite journal}}: Check date values in: |date= (help)
  13. ^ a b c d Talbot, Christopher J.; Rogers, Eric A. (1980-04-25). "Seasonal Movements in a Salt Glacier in Iran". Science. 208 (4442): 395–397. doi:10.1126/science.208.4442.395. ISSN 0036-8075. PMID 17843617.
  14. ^ Weinberger, R.; Lyakhovsky, V.; Baer, G.; Begin, Z. B. (2006-05). "Mechanical modeling and InSAR measurements of Mount Sedom uplift, Dead Sea basin: Implications for effective viscosity of rock salt". Geochemistry, Geophysics, Geosystems. 7 (5): n/a–n/a. doi:10.1029/2005gc001185. ISSN 1525-2027. {{cite journal}}: Check date values in: |date= (help)
  15. ^ Weinberger, R.; Begin, Z.B.; Waldmann, N.; Gardosh, M.; Baer, G.; Frumkin, A.; Wdowinski, S. (2006), "Quaternary rise of the Sedom diapir, Dead Sea basin", Special Paper 401: New Frontiers in Dead Sea Paleoenvironmental Research, Geological Society of America, pp. 33–51, ISBN 0813724015, retrieved 2018-11-14
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  17. ^ Aftabi, Pedram; Roustaie, Mahasa; Alsop, G.Ian; Talbot, Christopher J. (2010-01). "InSAR mapping and modelling of an active Iranian salt extrusion". Journal of the Geological Society. 167 (1): 155–170. doi:10.1144/0016-76492008-165. ISSN 0016-7649. {{cite journal}}: Check date values in: |date= (help)
  18. ^ Weinberg, Roberto Ferrez (1993-12). "The upward transport of inclusions in Newtonian and power-law salt diapirs". Tectonophysics. 228 (3–4): 141–150. doi:10.1016/0040-1951(93)90337-j. ISSN 0040-1951. {{cite journal}}: Check date values in: |date= (help)
  19. ^ Colón, Cindy; Webb, A. Alexander G.; Lasserre, Cécile; Doin, Marie-Pierre; Renard, François; Lohman, Rowena; Li, Jianghai; Baudoin, Patrick F. (2016-09). "The variety of subaerial active salt deformations in the Kuqa fold-thrust belt (China) constrained by InSAR". Earth and Planetary Science Letters. 450: 83–95. doi:10.1016/j.epsl.2016.06.009. ISSN 0012-821X. {{cite journal}}: Check date values in: |date= (help)
  20. ^ a b COOKE, R. U.; SMALLEY, I. J. (1968-12). "Salt Weathering in Deserts". Nature. 220 (5173): 1226–1227. doi:10.1038/2201226a0. ISSN 0028-0836. {{cite journal}}: Check date values in: |date= (help)
  21. ^ Bryant, Robert (2010-03). "Deserts and Desert Environments - By Julie Laity". Geographical Journal. 176 (1): 119–119. doi:10.1111/j.1475-4959.2009.00347_6.x. ISSN 0016-7398. {{cite journal}}: Check date values in: |date= (help)
  22. ^ Rubin, Thor; Johnston, H. L.; Altman, Howard W. (1961-01). "THERMAL EXPANSION OF ROCK SALT1". The Journal of Physical Chemistry. 65 (1): 65–68. doi:10.1021/j100819a021. ISSN 0022-3654. {{cite journal}}: Check date values in: |date= (help)
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