Salt
Tectonics
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Where and why do salt deposits occur?
Primarily in rift basins and along passive margins
Shallow marine settings in enclosed basins usually in areas
undergoing active extension
Within rift basins typified by half grabens and rotated blocks.
Can be related to changes in sea level when spill points
emerge such at the Mediterranean (the Messinian salinity
crisis) when the entire Med dried up.
Locations of salt basins around the world (Jackson and Talbot, 1991).
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Internally drained extensional basins
Rheology of Salt
A weak rock, with about the strength of concrete at the surface.
Salt is much weaker than any other sedimentary rock.
Salt deforms as a viscous material that flows plastically at short
timescales, even on the scale of months.
Salt basically behaves like a pressurized fluid.
Deformation is not driven by the salt itself. Salt responds (usually
passively) to outside forces such as gravity, loads due to
sedimentation and in some cases, erosion.
The density of salt relative to other sediments changes with
depth. Salt is nearly incompressible, thus as other sediments
become more dense by compaction, a density inversion
occurs, usually at around 1.0 – 1.5 km
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Loading of salt by deposition of sediments creates an
increased “fluid” pressure that causes salt to move into areas
of lower pressure. Salt structures include dome or bulletshaped features called diapirs and many other types of
features (walls, sheets/nappes, etc).
Areas where salt is withdrawn creates basins that then
become filled with additional sediment (called mini basins)
Once the floor of a minibasin sinks through the entire salt
section, it stops moving and forms a feature called a salt weld
(denoted by pairs of dots across the contact).
Cross section of entire Gulf of Mexico (GoM) How large is this salt province?
Minibasins
Localized depocenters formed above areas
where salt is withdrawn.
These record salt removal by the growth strata
preserved in them.
They typically stop growing when the floor of
the minibasin touches down on subsalt strata.
This contact is termed a weld and denoted by
two dots across it.
Animation of minibasin and weld
Mini Basin animation
In the first part of animation, the structure behaves like a
simple mini basin. This get more complicated after that and
We will discuss this later in the section under turtles.
Seismic reflection profile of a minibasin, GoM
How and where does subsidence change?
Seismic reflection profile of a minibasin, GoM
How and where does subsidence change?
Expulsion rollovers
Expulsion rollovers – giant squeegees
Expulsion Rollovers
Structures that form where a thick deposit of sediment
pushes salt forward, sort of like a gigantic squeegee.
They are recognized by the offlapping nature of the growth
strata that prograde from the landward side of the overall
basin.
Small salt pillows are sometimes formed where
sedimentation moves basinward at rates faster than the salt
flows. These pillows are not allochthonous per se.
The floor of an expulsion rollover is a weld.
Expulsion rollovers – giant squeegees
Turtles
Turtle structures, minibasins that growth outward
with increasing salt withdrawal. Note weld at base.
Basically the top of the wedge-shaped layer of salt starts
to be removed more rapidly towards its thicker section,
causing collapse of the flank of the basin.
Salt
Crestal
faulting
Flank collapse
Flank collapse
Initial depocenter
Weld
Example of a turtle structure from the Precaspian Basin, with an inverted
central depocenter above a salt weld and flanked by younger depocenters
and adjacent diapirs. Note the crestal faulting and erosion.
Salt
(a)
(b)
Model results of minibasin subsidence showing no turtle formation 26
in the absence of extension (a) and turtle formation triggered
courtesyby
of B. Vendeville
extension (b) (courtesy of B. Vendeville).
Mini Basin animation
Diapirs
Salt Diapirs
Cylindrical columns that form by upward movement of salt
Come in a variety of shapes depending on how they develop.
Bullets, mushrooms, columns, etc.
They can either be “passive” and formed between minibasins
or “active” where they actively move upward, deforming the
strata above them. A number of cases in the Middle East are
Basically large, nearly circular mountains of nearly purre
halite.
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Diapirs
An example of a diapir in the process of having its stem pinched off.
Diapirs in the East Texas basin. Note the variety of shapes related to
the stage to which they have evolved
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An isolated diapir in the North Sea
Diapirs that have reached the surface of the Earth in the Zagros of Iran
Strain in the center of diapirs is very complex, owing to the very high
shear strains the salt has undergone. Features in the center of diapirs
Are often termed “curtain” folds; these are equivalent to sheath folds
Formed in large shear zones in contractile (thrust) settings.
Strain in the center of diapirs is very complex, owing to the very high
shear strains the salt has undergone. Features in the center of diapirs
Are often termed “curtain” folds; these are equivalent to sheath folds
Formed in large shear zones in contractile (thrust) settings.
Top of a salt diapir, note the dome-like nature of the rocks surrounding it
Salt Glaciers
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Paradox Analogs: Active Salt Diapirs and Glaciers in the Zagros Mtns of Iran
Buckle Fold
Salt in Thrust Belts
Diapir & Glacier
Buckle Fold
Salt in Thrust Belts
When thrust belts form in salt provinces, the structures
that form are distinctly different than in typical thinskinned settings.
The primary different is the geometry of the entire thrust
belt itself. Critical wedges are usually tapered much
more thinly (less material is built up relative to a given
width of a particular belt).
The structures in these belts often display opposite
vergent (i.e. box folds versus fault-related folds.
Critical wedge taper theory, in which a weak detachment (e.g., salt)
results in a narrower taper angle, a wider zone of deformation, and no
preferred vergence of faults (Jaumé and Lillie, 1988).
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Cartoons showing difference in structural style between foldbelts with (a) and
without (b) salt (Jackson and Talbot, 1991). Those with salt have folding
dominating over faulting, symmetrical structures with a regular wavelength and no
preferred vergence, and high-angle reverse faults instead of low-angle thrusts.
Map of northeastern Mexico showing the width of the Sierra Madre foldbelt
increasing and the taper angle decreasing in the Monterrey salient due to the
presence of salt (Marrett and Aranda-García, 2001).
Cross sections through the Sierra Madre foldbelt in (a) the Monterrey
salient, where there is a salt detachment; and (b) to the south, where
salt is absent (Marrett and Aranda-García, 2001).
Future material on Rafts, Rojos
Allochthonous Salt sheets