Shear (geology)
In geology, a shear describes a fault that runs parallel to the layers or that intersects the layers at a very shallow angle. Shearings can be extensional (stretching) or contracting (shortening). Basically, a shear can be described as a mostly horizontal displacement of rock stacks along a surface.
requirement
A prerequisite for shearing is usually an incompetent, i.e. easily deformable, base (often a water-saturated clay layer or other deformable rock such as evaporite ) on which the upper part of a layer package is offset from the lower part. This easily deformable layer is also known as the shear surface . Often, a shear occurs gravitationally (triggered by gravity) in slightly inclined rock layers.
deformation
Shearing usually occurs together with folds and thrusts . If the shearings are large, one also speaks of shear blankets . However, as a rule these have been moved much less than a thrust cover and are therefore mostly still on their original base. The transitions between the two forms are fluid.
In special cases, the offset of shearings is several tens of kilometers, for example in metamorphic core complexes . The shear area is then often referred to with the English / French term decollement (parallel to the layer) or detachment (slightly inclined to the layering).
The deformation style of the lower ( lying and not moving) and upper ( hanging and moving) rock package can be different.
A shear can occur with large-scale tectonic expansion of an area, but also with large-scale tectonic narrowing.
literature
- Lexicon of Geosciences. Volume 1: A to Edi. Spectrum / Academic Publishing House, Heidelberg 2000, ISBN 3-8274-0299-9 .
- Jean-Pierre Burg: Extension Systems. Lecture ETH Zurich, p. 54 f ( files.ethz.ch PDF; 2.13 MB).
- Gerhard H. Eisbacher: Introduction to Tectonics . 1st edition. Ferdinand Enke Verlag, Stuttgart 1991, ISBN 3-432-99251-3 .