Mechanics of Folds

Different types of rocks are composed of different types of material, and are the result of different types of rock formations. Therefore, all rocks do not respond in the same way to the same forces. Depending on their reaction to folding, the rocks are classified into two groups: (i) competent (stiff) rocks, and (ii) incompetent (ductile) rocks. Competent rocks are the resistant rocks, which bend and crack without much flowage, while incompetent rocks are the weaker rocks, which deforms due to plastic flow. There are three types of mechanisms involved in the process of folding; bending, buckling, and flowing.

• Bending of Rocks: Bending of rocks occurs when the deforming force is applied across (at a high angle to) rock layers. For example, basement uplift along a fault, magma intrusion or salt diapirs all produce bends (folds) in the overlying sedimentary rocks. Bending often produces gentle or broad folds, especially in continental interiors (cratons) situated far from plate boundaries but subjected to some vertical stresses.
• Buckling of Rocks: Buckling occurs when the deforming force is applied parallel to rock layers. This is usually caused by horizontal compressional tectonic forces and results in layer-parallel shortening of rocks and thickening (relief) of the rock body perpendicular to stress direction. Geologists have worked out mathematical relationships between the wavelength of a buckle fold and the thickness of the stiff layer embedded in a ductile rock mass. As a general rule, in a given stress field and ductility contrast between the layers, thicker stiff beds will have longer fold wavelengths and thinner stiff beds will have shorter wavelengths.
• The flow of Rocks/ Mass displacement: The incompetent rocks behave like a thick paste and therefore, they can not transmit pressure easily. The incompetent rock material is removed from the path of stress caused by the competent strata. The ”flow folds’ that are formed in the incompetent rocks are accompanied by rock flowage. In the flow folds the rock material flows into the crest and trough thereby causing thinning of limbs and thickening of hinges of the fold. Many minor folds frequently develop in the incompetent strata. The concentration of the incompetent rock material in the crest and trough of the folding is known as the ”flow of rocks” or ”mass displacement”.

The folds that develop as a result of the above mechanisms are (i) flexure folds, (ii) flow folds, and (iii) shear folds.

(1). Flexure Folds:

The true folds that are formed by the compression of competent rock beds are called the ”flexure folds”. These folds are frequently later and their outlines are relatively simple.

Flexural Slip and Flexural Flow: Bending and buckling produce flexural folds in which (as stated before) viscosity contrast between competent (stiff) and incompetent (ductile) rock layers plays an important role in the folding process. In flexural folds, competent layers do not change their thicknesses and incompetent layers are marked by cleavage sets nearly parallel to fold axial surface. Flexural folds are the most common folds in sedimentary basins.

Flexural folds are subdivided into:

1. Flexural-slip folds: In flexural-slip folds, there are displacements along bedding surfaces, much like the bending of a telephone directory book. These slips are greatest along the fold limbs and approach zero along the fold hinge. Flexural slip typically produces parallel or concentric folds in which the attitude and thickness of layers remain the same throughout the folded sequence.
2. Flexural Flow Folds: In flexural-flow folds, rock material in incompetent layers flows from fold limbs toward fold hinges, and therefore appreciable thickness changes occur in the rock layer. Obviously, flexural-flow requires more ductility contrast between layers than flexural slip. Flexural flow produces similar folds in the weak layers.

(2). Passive Folds:

Passive folding can be considered as fake folding in which layers become folded by means of mesoscopic or microscopic axial plane cleavages without really being flexed or bent. The strata do not actually bend, but rather a grain scale movement occurs. Passive folding takes place in a mechanically isotropic rock mass and on a grain scale rather than a layer scale. Passive folds are subdivided into two types; (i) Passive Slip/ Shear folds, (ii) Passive Flow folding.

1. Passive Slip Folding/ Shear Folding: In passive-slip folds there is minor but discrete displacement across (perpendicular to) rock layers and more conveniently along cleavage planes; they are also called shear folds. These folds are produced in brittle rocks by the movement of minute closely spaced fractures called the ”shear planes”. These shear planes develop across beds and thin slices of the rock move in relation to each other thereby forming a fold.
2. Passive Flow Folding: In passive-flow folds, there is across-layering material flow in a ductile environment and in the direction of folding. The ductile rocks concentrate in the crests and troughs of the fold. The material flows as a result of high pressure and stress, which can not be accommodated by the incompetent rock/ductile rock, thereby flowing in the area of low pressure (crest and trough).

(3). Flow Folds:

The incompetent rocks behave like a thick paste and therefore, they can not transmit pressure easily. The ”flow folds’ that are formed in the incompetent rocks are accompanied by rock flowage. In the flow folds the rock material flows into the crest and trough thereby causing thinning and thickening of beds. Many minor folds frequently develop in the incompetent strata.

A flow fold may be a ”passive flow fold” as well as a ”flexural flow fold”, as mentioned in the above two headings.

1. Passive Flow Folds: In passive-flow folds, there is across-layering material flow in a ductile environment and in the direction of folding. Passive flow occurs when there are two different types of strata, one is competent/brittle, and the other is incompetent/ductile. The pressure and force exerted on the competent layer make it bend. This pressure is also exerted on the ”ductile layer (incompetent), which also bend, as well as flows toward the area of low stress/pressure. The area of low pressure and stress is generally the crest and trough. The flow of ductile rock material to the crest and trough is known as the “passive flow folding”.
2. Flexural Flow Folds: In flexural-flow folds, rock material in incompetent layers flows from fold limbs toward fold hinges, and therefore appreciable thickness changes occur in the rock layer. Obviously, flexural-flow requires more ductility contrast between layers than flexural slip. Flexural flow produces similar folds in the weak layers.

Read related posts:

1. Dip and Strike (geological structure)
2. Folds (elements of folds, types of folds,)