Salt Tectonics © Cambridge University Press 2011 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). 3 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 © Cambridge University Press 2011 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 bullet- shaped 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 Areas where salt is withdrawn creates basins that then become filled with additional sediment (called mini basins) Once the floor of a minibasin sinks through the entire salt section, it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 Minibasins Localized depocenters formed above areas where salt is withdrawn.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. Crestal faulting Flank collapse Flank collapse Salt Salt 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. (a) (b) Model results of minibasin subsidence showing no turtle formation 26 in the absence of extension (a) and turtle formation triggeredcourtesy by 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. © Cambridge University Press 2011 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 © Cambridge University Press 2011 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 © Cambridge University Press 2011 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 thin- skinned 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). 50 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 .