Folds are one of the most common structures affecting geological materials. They are structures that are formed as a consequence of the deformation of geological materials, without breaking those materials.
Folds are geological structures that respond to plastic deformation, and are formed by the action of compressive tectonic stresses that occur in the earth’s crust. Therefore, geological folds are formed in materials that respond to plastic or deformable behavior, such as sedimentary rocks (shale, siltstone, and sandstone).
Parts of a fold
The flanks or limbs are the lateral planes that make up the fold, at least two flanks are needed to form a fold, they can also be interpreted as the stratification surface.
The axis or hinge is the line that corresponds to the maximum curvature of the fold and is formed by the intersection between the flanks of the fold, in the different layers or stratification surfaces.
The axial plane is that plane that is formed by the union between the lines of the axes, of each consecutive fold layer (in the same fold), depending on the type of fold, the axial plane will have a different dip angle.
Classification and types of folds
Although there are various ways to classify folds, the most common and used are related to the shape and orientation of the main elements that make up the folds, that is, the axial plane, the axis, and the angle between flanks (interlimbal angle). ).
| Classification and types of folds | |
|---|---|
| Classification according to its shape | Anticline |
| Syncline | |
| Classification according to the dip of the axial plane | Symmetrical |
| Inclined | |
| Invested | |
| lying down | |
| Classification according to the fold axis | cylindrical |
| conical | |
| Classification according to the angle between the limbs | Weakly folded, interlimbal angle greater than 120° |
| Open fold, interlimbal angle 70° to 120° | |
| Closed fold, interlimbal angle 30° to 70° | |
| Narrow fold, interlimbal angle 10° to 30° | |
| Isoclinal fold, interlimbal angle = 0° |
Classification according to the shape of the fold
The classification according to the shape of the fold It is the most used, due to its simplicity, these types of folds are based on the concavity or convexity that the fold presents. The folds are anticline and syncline.
The anticline It is characterized by being convex towards the top of the fold, and because the younger materials that make up the fold make up the top of the fold, while the older materials make up the core of the fold.
If it is not known which materials are younger or older, it is better to name this structure as “antiform”
The syncline It is characterized by being concave towards the upper part of the fold, and because the younger materials make up the core of the fold, while the older materials make up the lower part of the fold.
If it is not known which materials are younger or older, it is better to name this structure as “unformed”
Classification of folds according to the dip of the axial plane
The classification of folds according to the dip of the axial plane, takes into account the dip angle of the axial plane between 0° and 90°.
The folds are: symmetrical, inclined, inverted, lying down.
The symmetrical folds, are those where the angle formed by the axial plane with the flank, are equal on both sides, in these folds the axial plane is vertical.
The inclined folds, in this case the axial plane of the fold is inclined, being different from 0° and 90°, in such a way that one of the flanks of the fold has a greater angle of dip with respect to the other, and the interlimbal angle that the axial plane forms with the flank are different.
The inverted fold It has similar characteristics to inclined folds, but in this case, the dip angle of one of the flanks is inverted.
A lying fold It is formed when the axial plane is totally horizontal, that is to say that the angle of dip is equal to 0°.
classification of folds according to the immersion of the fold axis
The classification of folds according to the immersion of the fold axis, are based on the angle of inclination (pitch) or immersion that the axis of the folds presents.
The folds are cylindrical and conical.
In the cylindrical folds, the axis is horizontal, that is to say that its inclination is equal to 0°
In the conical folds, the axis is inclined, therefore it forms an angle with respect to the horizontal.
Visualization of folds in geological maps and sections
In a geological map, the folds are usually recognized by the symmetrical repetition of the materials with respect to a central axis, which is the intersection of the axial plane with the topographic surface.
In this symmetrical repetition of materials, we must not take into account the outcrop surface of the materials; since, as has been commented, the outcrop surface and the apparent surface thickness of the materials depend, apart from the real thickness, the dip of the materials and the topographic surface
Recognition of conical and cylindrical folds on a geological map is fairly straightforward.
In flat topographical surfaces where outcropping materials form a cylindrical fold (horizontal fold axis), the outcrop surfaces of the materials present a subparallel direction on the two flanks of a fold.
While in conical folds (the fold axis presents immersion), the outcrop surface of the materials tends to converge, drawing the closure of the folded structure on the geological map.
In topographical surfaces with relief, the outcrop geometry becomes very complicated, although they generally follow guidelines similar to those described for areas with flat topography.
How are geologic folds formed?
For geological folds to form, it is necessary that the stresses that occur in the earth’s crust affect rocks with a ductile or semi-ductile behavior, that is, they are easily deformable.
For a rock to be easily deformable, it must be rich in clay minerals. These rocks are mainly shale, siltstone and mudstone, and sometimes sandstone with a high percentage of clay.
Then it is the compressive stresses that affect these rocks that will cause the geological folds to form.
Therefore, it is common for these geological structures to form near subduction zones and near active geological faults, which are the places where compressive stresses act on these rocks.