What is fold in structural geology

Geological structures

Geological structures are basically all three-dimensional properties of a rock. Often they are the result of powerful, tectonic forces that occur in the earth's crust. These forces can break or fold rocks. In the process, faults form or a mountain range is bulging. The repeated action or the combination of different forces creates very complex situations of structures for which a lot of experience is required to develop a coherent picture.

Structural geology deals with the study of the processes described and the effects on structures and rocks through the action of forces. The investigation ranges from the microscopic level to a global scale.

The structures can for example linear or planar be trained and can be relevant for construction projects or deposit science.

Note:
This page is about geological structures in terms of structural geology. Not to be confused with the structures of a rock, which, for example, describe the size and shape of individual components. Texture, in turn, means the spatial arrangement of the components.


Formation of structures

Primary structures, or a primary structure, i.e. the relationship between different structures, is formed during the formation of a rock. During the deformation of the rock, the primary structure can be affected by forces (e.g. plate tectonics) secondary structures form.

It should be noted that there are also borderline cases for which there is no uniform assignment. Here it is important to deal with suitable literature in detail. In addition, practical experience is essential here and can help you. If you do not have this yourself, it will certainly help to contact a lecturer.

Structure types

Disruptions

  • High and deep floe
  • Postponement
  • deportation
  • Thrust
  • Horizontal / page shift or page shift
  • Inclined displacement / postponement


There are three types of disruption related to the type of displacement:

  1. Deportation (normal fault): with downward movement of the Hagendblock and the resulting extension.
  2. Thrust fault: when the hagend block moves upwards over the prone block and the resulting doubling of the rock layers.
  3. Blade displacement (strike-slip fault): when the two blocks move horizontally in opposite directions and the resulting friction when sliding past each other.
(Created by A. Mazon 2018)

wrinkles

  • Saddle / anticline
  • Mulde / Syncline
  • upright fold
  • submerged fold
  • asymmetrical fold
  • overturned fold
  • Vergence (tilting direction)
  1. Synclinale, created when the sequence is bent downward so that the youngest rock layers lie in the center of the structure.
  2. Saddle (anticline), arises when the rock layers are bent upwards so that the oldest rock layers lie in the core of the structure.

(Created by A. Mazon 2018)

Domes and basins

An example here is the rising of salt as a diapire, since salt can "flow" and has a lower density than the surrounding rock. (one speaks of holokinesis)

Clefts

Deformation structure

Foliations

Corridors

Horst and Graben

Metamorphic core complexes

scale

The geological structures described can be designed on different scales. Here you can see the rough division of scales as used in structural geology. Even within structural geology, the division of size ranges is not always exactly the same. When writing a report or a thesis, it is therefore particularly important to refer to the nomenclature according to which the terms are used and then to stick to this consistently.

Note:
The classification of structures according to their size differs from subject to subject. For example, in mineralogy, macroscopic is understood to mean a different size range than in structural geology.

The following classification of the scale is used in structural geology:

  • megascopic (megascopic)
    107 - 106 m
    (Photo: Test)

  • macroscopic
    >106 - 103 m
    e.g. mountains
    (Photo: L. Sidorenko)

  • mesoscopic
    103 - 10-0 m (km-cm)
    e.g. several folds (digestion scale)
    (Photo: C. Trepmann)

  • mesoscopic
    100 - 10-2 m (m -cm)
    lower limit e.g. internal management
    (Photo: C. Trepmann)

  • microscopic
    10-2 - 10-6 m (cm - µm)
    several small folds (still a digestion scale) up to structures that can only be seen well under the microscope, e.g. in thin sections
    (Photo: C. Trepmann)

  • submicroscopic
    10-6 - 10-9 m (µm - nm)
    e.g. internal structures in minerals
    (Photo: C. Trepmann)

Recording of geological structures

From the kinematic analysis of structures, conclusions can be drawn about the underlying tectonic movements. Then, based on the sequence of movements, the orientation of recent and historical regimes of forces and tension can be reconstructed.

Before starting the description, the following questions should be answered:

  • How much time do you have for the exposure? (5min, 30min, a whole day?)
  • Why is the exposure relevant? (Rock type, storage conditions, structures)
  • How detailed should the execution be? (only rough overview, precise description)


The recording of geological structures is basically similar to the general description of exploration. Here, too, you go from big to small. The drawing in the Field book, as well as the creation Geological photographs. In both the drawing and the photographs, one should always make sure to include a suitable scale (ruler, compass, field book, hammer, fellow students, etc.). After having obtained an initial overview, an overview sketch should be drawn up in which Later further details can be entered which one has observed from a smaller distance. Here you can later find the sampling points (see Geological sampling), if any were taken.

In all descriptions it is very important to note that in most cases an exposure is only a two-dimensional section of a three-dimensional structure. This can lead to illusions such as apparent Come to mind to lead. In order to determine how the existing rock units are stored, they can be measured with the aid of the structural compass (Swipe and drop). The more measurements you take, e.g. on a fold or fault, the more precisely you can describe its morphology later. For a clear description of where which measured value was taken, you should also create a detailed sketch of the structure in which the measured values ​​are entered. Individual details that are only available in certain places of the structure (e.g. internal folding, magma mixing / mingling, dikes, chilled margins, casts and flow marks, important fossils, etc.) can be recorded again with a separate sketch and their position in the Overview sketch can be entered.

The next thing to try is to determine which different rock units are present. For this one uses the common methods for Addressing rocks in the area and Rock description.


Courses

Geological maps and profiles


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Categories

Terrain methods
Terrain preparation
Orientation in the field
Working in the field
Evaluation of terrain data

Pages of the category work in the field

Keeping a field book
Outline record, outline sketch and profile recordings
Prepare an outline sketch
Addressing rocks in the area
Mineral determination in the field
Geological sampling
Geological photographs
Geological profile section
Measurements in the field
Soils and loose sediments
Paleontological excavation
security


credentials

Grotzinger J. (2017): Press / Siever Allgemeine Geologie.– 7th edition, 769 p .; Heidelberg (Springer Spectrum).

Coe A.L., Argles T. W., Rothery D., A., Spicer R.A. (2010): Geological field techniques. –323 p .; Department of Earth and Environmental Sciences, The Open University, Walton Hall, Milton Keynes, UK.

(2009): Geology as a place of learning - Bavarian State Ministry for Environment and Health (StMUG), State Institute for School Quality and Educational Research (ISB), Munich.

https://commons.wvc.edu/rdawes/Basics/structures.html#folds


Further information and literature

Grotzinger J. (2017): Press / Siever Allgemeine Geologie.– 7th edition, 769 p .; Heidelberg (Springer Spectrum).

A.L., Argles T. W., Rothery D., A., Spicer R.A. (2010): Geological field techniques. –323 p .; Department of Earth and Environmental Sciences, The Open University, Walton Hall, Milton Keynes, UK.


Author: inside

This article was created by:
Andrea Mazon, Lukas Sidorenko, Felicitas Kaplar