Shear zone

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A shear zone is a significant area of tectonic discontinuity in the earth's crust and upper mantle . Their creation can be traced back to a non-homogeneous deformation process , the energy of which is concentrated on flat or slightly curved fault surfaces. In between (crusts) areas remain relatively unaffected by larger deformations. Due to shear movements of the surrounding more rigid medium, a rotational, non-coaxial component can be induced in shear zones. Since these discontinuities usually run through different depths, they create a wide range of different rocks . Shear zones occur on the earth's surface as fractional tectonic faults .

General introduction

Schematic representation of the change in deformation at a shear zone with increasing crust depth. Above: only fractional deformation (corresponds to a warpage ). Middle: plastic and fragile deformation. Bottom: only plastic deformation (corresponds to a ductile shear zone). Deformation field and shear stress distribution are shown schematically.

A shear zone is an area of ​​very strong deformation (with a high rate of deformation ) surrounded by rocks with much less finite deformation . Their length to width ratio is greater than 5: 1.

Shear zones form a wide continuum of geological structures. They range from brittle shear zones ( faults ) through brittle-ductile shear zones and ductile-brittle shear zones to purely ductile shear zones . In brittle shear zones, the deformation is concentrated on a narrow fracture surface between adjacent rock blocks, whereas the deformation in ductile shear zones extends over a wider area and varies continuously in strength between the undeformed blocks. Between these two end members of the continuum mediate the intermediate stages of the brittle-ductile and ductile-brittle shear zones, which represent mixed forms of the two end members.

This structural continuum reflects the various deformation mechanisms in the earth's crust - from brittle fracture deformation at or near the surface to ductile, viscous flow with increasing depth. When the brittle-ductile transition zone is reached , ductile deformation mechanisms set in for the first time. The transition does not take place abruptly, but is distributed over a broader depth range in which brittle fracture and ductile flow occur together. The main reason for this lies in the structure of rocks in the crustal area, which are usually composed of several different types of mineral with different deformation behavior. For example, the ductile behavior of quartz sets in much earlier (ie at a lower temperature) than that of feldspars . Differences in lithology, grain size and given structure consequently determine a different rheological behavior. But purely physical factors also influence the brittle-ductile transition:

According to Scholz's model, a crust made up of quartz and feldspars (with a geothermal gradient typical of southern California) sets in ductile deformation mechanisms at a depth of around 11 kilometers and 300 ° C. The transition zone then extends down to a depth of around 16 kilometers, the prevailing temperature there is around 360 ° C. Below 16 kilometers, only purely ductile deformations occur.

The seismogenic zone , ie the depth range in which ordinary earthquakes occur, remains limited to the brittle area, the so-called schizosphere . After crossing the transition zone, the plastosphere follows . The seismogenic layer is characterized by real cataclasites . It usually begins at a depth of 4 to 5 kilometers below an upper stability transition . Hardly any sources of tremors can be made out above it. The seismogenic layer then extends to a depth of 11 kilometers. Large earthquakes can, however, break right up to the earth's surface and into the transition zone, sometimes even into the plastosphere.

Characteristic rocks

The deformation processes taking place in the shear zones are responsible for the formation of different structures and mineral compositions. These reflect the prevailing pressure and temperature conditions (pT path) during the deformation and also document the respective sense of movement, the flow behavior and the specific chronological sequence of the deformations. Shear zones are therefore of great importance for understanding the geological history of terrans .

Typically, the following types of rocks are encountered in shear zones as the depth increases:

Both fault lines and cataclasites are caused by abrasion on brittle, earthquake-generating faults.

The first mylonites appear when the ductile deformation behavior begins in the transition zone. They are by adhesive (wear processes Engl. Adhesive wear ) emerged. Pseudotachylites can also still develop in the transition zone, but disappear when green slate facial conditions are reached, so that ultimately only mylonites are found. Striped gneisses are high-grade mylonites from the lowest depths of ductile shear zones.

Direction of movement and sense of movement in shear zones

The sense of movement in shear zones (right or left) can be determined on the basis of macroscopic and countless microscopic structures. The main indicators are armor (welts, grooves and mineral growth) and also elongation and mineral linear. They show the direction of movement. The sense of movement can then be determined by means of the offset on structures such as layering or corridors . The bending over of planar structures (spreading) such as layering or foliation in the direction of the shear zone is also a reliable indicator of movement.

Staggered Fiederspaltensysteme characteristic of ductile-brittle shear zones, and vestibular folds (engl. Sheath folds ) are equally macroscopic motion indicator.

The following structures can be cited among the microscopic indicators:

Width of shear zones and resulting lateral offset

The width of individual shear zones can vary from grain size to kilometers. Shear zones, which run through the entire crustal area, are up to 10 kilometers wide. The lateral offset that has occurred on them ranges from several tens of kilometers to over a hundred kilometers.

Brittle shear zones (faults) usually widen with depth. The same effect is also achieved by increasing the lateral offset.

Deformation softening and ductile behavior

The hallmark of shear zones is an increased rate of deformation, which, however, remains limited to a limited area in the rock. So the rock can react at all vividly in this area, a kind of must Deformationserweichung (Engl. Strain softening ) have occurred. The following processes can contribute to the softening of the rock:

  • Grain size reduction.
  • geometric softening.
  • reaction-related softening.
  • liquid-induced softening.

An increase in ductility should take place without fracture behavior in order to ensure continuous flow deformation. The following deformation mechanisms (on the grain size level) ensure this:

  • Diffusion creep (various types).
  • Dislocation creep (various types).
  • Syntectonically occurring recrystallizations.
  • Print solution processes.
  • Grain boundary displacements (superplasticity) and grain boundary area reductions.

Occurrences and examples of shear zones

The San Andreas Fault in California, a major right-hand shifting shear zone

Since shear zones can reach very deep, they are found in all metamorphic facies . Brittle shear zones (faults) are present everywhere in the upper crust. Ductile shear zones begin in the green slate area and are therefore bound to metamorphic terranes.

Shear zones occur in the following geotectonic situations:

  • Disturbances generated under expansion - more or less horizontal:
    • Shearings (e.g. on metamorphic core complexes)

Shear zones are neither tied to a rock type nor to a specific time period. They usually do not occur individually, but rather form fractal , interlinked networks, which in their training provide information about the prevailing sense of movement in a terrain.

Good examples of lateral displacement-type shear zones are the South Armorican Shear Zone as well as the North Armorican Shear Zone in Brittany and the North Anatolian Fault in Turkey . Shear zones of the Transform type are the Dead Sea fault in Israel , the San Andreas Fault in California and the Alpine Fault in New Zealand . An example of the ceiling type is the Moine Thrust in northwest Scotland . The median zone in Japan is a fossil subduction zone. Shearings of the core complex type are very common in southeastern California; B. the Whipple Mountain Detachment Fault . An example of huge interconnected shear zones is the Borborema shear zone in northeastern Brazil .

meaning

The importance of shear zones lies in their size. Usually these weak zones run through the entire crust area up to the Moho and can even reach down to the upper mantle. Shear zones can be in motion over very long periods of time and therefore often show several stages that overlap in time. Material can be transported up or down in shear zones. The most important reagent here is undoubtedly water , with which a wide variety of dissolved ions circulate through the weak zones. A significant consequence is the metasomatic change in the host rocks. Even the metasomatic enrichment of rocks in the Upper Mantle can ultimately be traced back to shear zones.

Shear zones can host economically valuable mineralization, the best example of this are the significant gold deposits of the Precambrian , which are mostly directly linked to shear zones (examples: gold mines in the Superior Craton , Canada and in the Yilgarn Craton in Western Australia ).

literature

  • Cornelis W. Passchier, Rudolph AJ Trouw: Microtectonics. Springer, Berlin et al. 1996, ISBN 3-540-58713-6 .
  • John G. Ramsay, Martin I. Huber: The Techniques of Modern Structural Geology. Volume 2: Folds and Fractures. Academic Press, London et al. 1987, ISBN 0-12-576902-4 .
  • Christopher H. Scholz: The mechanics of earthquakes and faulting. Cambridge University Press, Cambridge et al. 1990, ISBN 0-521-33443-8 .

Individual evidence

  1. ^ John G. Ramsay, Martin I. Huber: The Techniques of Modern Structural Geology. Volume 2: Folds and Fractures. Academic Press, London et al. 1987, ISBN 0-12-576902-4 .
  2. ^ Christopher H. Scholz: The mechanics of earthquakes and faulting. Cambridge University Press, Cambridge et al. 1990, ISBN 0-521-33443-8 .