CAUSES OF FOLDING

CAUSES OF FOLDING

• Folding can occur for either tectonic or non-tectonic reasons. The rocks fold as a result of various pressures from within the Earth.
• This is what we mean by "tectonic causes." Non-tectonic folding occurs when rocks bend or twist as a result of surface processes.
• However, it should be noted that geologists are primarily concerned with tectonic bending.
• Any or all of the following might cause the plates to fold:
• Any or all of the following might induce tectonic bending:
• Tangential compression causes folding.

Tangential compression folding

• Lateral compression is assumed to be the major source of crustal rocks being flung into various folds, depending on the types of rocks involved in the process as well as the direction and intensity of the compression impacting those rocks.
• Most of the time, this major force is assumed to work at right angles to the direction of folds.

When tangential stresses are present, folding can occur in three ways:

1. flexural folding
2. flowage folding
3.  shear folding

1. Flexural Folding

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• This is a folding process in which competent or stronger rocks are thrown into folds as a result of lateral compression when they slide against each other. This is also known as flexural-slip folding, since the layers slide or slip along the bedding lines of the layers.

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• In flexural folding, the amount of slip (and, by extension, the shape of the fold) depends on a number of factors, such as: I. layer thickness and contact type; the thicker the layers, the greater the slip; also, cohesion-less contacts make slips easier and bigger; and II. distance from the hinge point; the farther away from the hinge point, the bigger the movement, to the point where it might not be noticeable at the hinge.
• Rock type: siltstones, sandstones, and limestones bend and fold more easily than soft clays and shales.

2. Flowage Folding

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• This is the primary method by which rocks that aren't strong enough to keep their shape, such as clays, shales, gypsum, rock salt, and so on, fold.
• When the layers are crushed, their materials behave almost like a fluid or plastic mass.
• They bend and deform at varying speeds, resulting in uneven distortion. When this happens, the thickness of the fold does not remain constant.

3. Shear Folding

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• Shearing forces, rather than simple compression, are frequently what cause folding.
• It is believed that in this type of process, the rock first develops a large number of microscopic fractures close together.
• The produced blocks then travel in separate directions, and the rocks are either folded or twisted in the end.
• When the microscopic fractures close up again due to recrystallization, the bent form becomes more visible.

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