Salt tectonics

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Salt tectonics or halokinesis describes the mechanical mobilization of salt rock , the resulting structural changes (deformation) in the overburden of these salt rocks and thus the formation of so-called salt structures . Svante Arrhenius and Richard Lachmann around 1911 and Donald Clinton Barton around 1930 laid the foundations for the theoretical description of halokinesis . The German geologists Ferdinand Trusheim and Rudolf Meinhold did not use the terms "halokinesis" or "salt tectonics" until the end of the 1950s embossed. Originally, halokinesis referred to the independent movement of salt rock, while salt tectonics (or halotectonics ) describes the movement of salt rock that was triggered by external tectonic tensions.

Geological and physical basics

Salt rock consists of evaporite minerals , especially the mineral halite ( NaCl , "rock salt"). The vast majority of the salt rocks on earth were created by precipitating evaporites from strongly evaporated sea water, depositing them on the sea floor and subsequently covering them with further layers of sediment (a specific scenario for this is described in the article salt dome).

Salt rock has the special rheological property of ductile deformation even at relatively low pressures and temperatures, such as those already found at a depth of several 100 m, and can therefore be regarded as flowable over geological time periods. This distinguishes salt rock from most other sedimentary rocks, e.g. B. sandstone , claystone or limestone , which break brittle at low pressure and temperature . One speaks here of a competence contrast between the salt rocks (visco-elastic deformable, i.e. incompetent) and the other sedimentary rocks (brittle-plastic breaking, i.e. competent). The deformation processes in the minerals of the salt rocks occur primarily through dislocation creep and solution -precipitation creep . The latter predominates if there are fluid inclusions to a certain percentage in the crystal lattice at the grain boundaries of the salt minerals (> 0.05% by weight) and if the deformation rate is relatively low.

A second important property of salt rock is its incompressibility. In contrast to other sediments, evaporites do not compact with increasing cover, so that their average density (around 2.2 g / cm³ for halite ) remains almost unchanged with depth. From a certain thickness of the overburden (approx. 650 to 2000 m) a density inversion occurs, i.e. This means that the sediments of the overburden are now denser than those of the salt (“Rayleigh-Taylor instability”). The resulting buoyancy contributes significantly to the formation of salt structures.

Triggers, Processes, and Secondary Effects

Since salt rock can be regarded as flowable in geological time periods, concepts of fluid mechanics are applied to the deformation processes in salt tectonics. According to this, the flowable salt rock moves along a hydraulic pressure gradient. Such a pressure gradient can be caused by lateral density or thickness variations in the overburden, by tectonic faults in the overburden or by inclinations in the salt layer.

Active diapirism

The classic, partly outdated model of the formation of a salt structure is based on a viscous rheology of the salt rock and the overburden and describes salt tectonics as a purely gravitationally driven phenomenon. As soon as the density inversion occurs, the ascent of the salt begins with the formation of a wide bulge (salt cushion). At a certain height, the salt rock breaks through the overburden (English piercing ) and forms a diapir . Since this salt rock actively penetrates the overburden, we speak in this process of active diapirs (Engl. Active diapirism ).

Since the 1980s, rock-physical measurements have shown that most sedimentary rocks (with the exception of most evaporites) deform in a brittle-plastic manner and therefore have a certain strength. The mechanical stresses that are applied by the ascent of the salt rock are generally not high enough to overcome this strength. Because of this, "active diapirism" is only considered possible in modern literature when the salt structure has already reached a certain height, namely when the height of the salt structure has reached approximately 2/3 the thickness of the overlying overburden.

Reactive diapirism

This process assumes that external triggers are required for the formation of salt structures. External triggers include tectonic processes, i.e. extension or compression. The overburden is stretched and thinned by extension. As a result, the salt rock can intrude into the fault zones that are forming and ultimately form diapirs again. This process is reactive diapirs (Engl. Reactive diapirism ) called because the salt rock only reacts to external influences.

In the case of tectonic triggers, a distinction is also made between “ thin-skinned ” and “ thick-skinned ” mechanisms. Due to the easy deformability of the salt rock, tectonic faults at the base of the salt layer are not always transferred directly to the overburden, but are decoupled. The faults in the base are offset laterally, a process known as "thin-skinned extension / compression". Only if the salt layer is relatively thin, the offset at the fault is very large or the deformation rate is very high, a fault can be transmitted directly into the overburden (“thick-skinned extension / compression”).

The sediments of the overburden layer are compressed, unfolded and pushed over by compression (e.g. in the Jura Mountains northwest of the Alps or in the Zagros Mountains , Iran). The salt rock is thereby pressed into the core of the fold and can even be squeezed through the crest of the fold.

Passive diapirism

Once the salt rock has penetrated to the surface, the ascent continues, while further sediments are deposited in the neighboring marginal depressions. This process is referred to as passive diapirism (English passive diapirism or downbuidling ) and was postulated by Barton in the 1930s as an essential formation process for the salt structures in the northern Gulf of Mexico.

Load differences

Another external influence is sedimentary differences in load ( differential loading ). If sediments are unevenly deposited over a salt rock layer, the salt rock flows from areas with a high sediment load to areas with a low sediment load. The redistribution has a self-reinforcing effect, as additional sediments can be accumulated in areas from which the salt migrates. Sedimentary differences in load occur e.g. B. by progressive deltas, but also by vertical offsets of the base of the salt deposit.

Raft tectonics

In sedimentary basins with a sloping basement, e.g. B. passive continental margins or foreland basins, overburden sediments slide above the soft salt rock layer down the slope. This operation is symbolically as Floßtektonik (engl. Raft tectonics ) and can take place over distances of more than 100 km. Typically, in the upper area of ​​the slope, expansion structures arise in the overburden, i.e. trenches and half-trenches, accompanied by reactive diapirs or so-called roll-overs . The latter are structures that are created by synkinematic sedimentation on the hanging block of a fault , while at the same time the overburden slides down the slope on a decollment of salt rock. In the lower area of ​​the slope, the overburden layer is compressed. This creates thrusts there, accompanied by salt anticlines and salt covers. Raft tectonics occurs mainly on passive continental margins , as there is a sufficient slope of the salt base there, e.g. B. in the Lower Congo Basin and Kwanza Basin off the coast of Angola , in the Gulf of Mexico , in the Nova Scotia Basin or in the Nile Delta .

literature

  • MR Hudec, MPA Jackson: Terra infirma: Understanding salt tectonics. In: Earth Science Reviews. Volume 82, 2007, pp. 1–28, doi: 10.1016 / j.earscirev.2007.01.001
  • John K. Warren: Evaporites: Sediments, Resources and Hydrocarbons. Springer, Berlin / Heidelberg / New York 2006, ISBN 3-540-26011-0 , chapter Salt tectonics. Pp. 375-452.

Individual evidence

  1. ↑ DC Barton: Mechanics of formation of salt domes with special reference to Gulf Coast salt domes of Texas and Louisiana. In: AAPG Bulletin. Volume 17, No. 9, 1933, pp. 1025-1083.
  2. ↑ F. Trusheim: About Halokinesis and its significance for the structural development of Northern Germany. In: Journal of the German Geological Society. Volume 109, 1957, pp. 111-158, (abstract)
  3. ↑ R. Meinhold: Comments on the question of the salt rise. In: Freiberg research books. Volume C22, 1956, pp. 65-77.
  4. ^ R. Meinhold: Salt movement and tectonics in Northern Germany. In: Reports of the Geological Society in the German Democratic Republic for the entire field of geological sciences. Volume 4, No. 2/3, 1959, pp. 157-168.
  5. Jump up ↑ JK Warren: Evaporites: Sediments, Resources and Hydrocarbons. Springer, Berlin / Heidelberg / New York 2006, ISBN 3-540-26011-0 , chapter Salt tectonics. Pp. 375-452.
  6. ^ MR Hudec, MP Jackson: Terra infirma: understanding salt tectonics. In: Earth Science Reviews. Volume 82, No. 1, 2007, pp. 1-28.
  7. ↑ JL Urai, CJ Spiers, HJ Zwart, GS Lister: Weakening of rock salt by water during long-term creep. In: Nature. (London) Volume 324, No. 6097, 1986, pp. 554-557.
  8. ^ MPA Jackson, CJ Talbot: External shapes, strain rates, and dynamics of salt structures. In: Geological Society of America Bulletin. Volume 97, No. 3, 1986, pp. 305-323.
  9. ^ MR Hudec, MP Jackson: Terra infirma: understanding salt tectonics. In: Earth Science Reviews. Volume 82, No. 1, 2007, pp. 1-28.
  10. ↑ F. Trusheim: About Halokinesis and its significance for the structural development of Northern Germany. In: Journal of the German Geological Society. Volume 109, 1957, pp. 111-158 (abstract)
  11. ^ D. Sannemann: About salt dome families in NW Germany. In: Erdoel Z. Volume 79, 1963, pp. 499-506.
  12. ^ BC Vendeville, MPA Jackson: The rise of diapirs during thin-skinned extension. In: Marine and Petroleum Geology. Volume 9, No. 4, 1992, pp. 331-354.
  13. ^ MPA Jackson, BC Vendeville: Regional extension as a geologic trigger for diapirism. In: Geological Society of America Bulletin. Volume 94, No. 1, 1994, pp. 57-73, doi : 10.1130 / 0016-7606 (1994) 106 <0057: REAAGT> 2.3.CO; 2
  14. ↑ DC Barton: Mechanics of formation of salt domes with special reference to Gulf Coast salt domes of Texas and Louisiana. In: AAPG Bulletin. Volume 17, No. 9, 1933, pp. 1025-1083.
  15. ^ MR Hudec, MP Jackson: Terra infirma: understanding salt tectonics. In: Earth Science Reviews. Volume 82, No. 1, 2007, pp. 1-28.
  16. ↑ J.-P. Brun, TP-O. Mauduit: Rollovers in salt tectonics: The inadequacy of the listric fault model. In: Tectonophysics. Volume 457, No. 1-2, 2008, pp. 1-11, doi: 10.1016 / j.tecto.2007.11.038