Folds

The folds are plastic deformations of rocks that affect multiple layers. They are caused by compressive stresses that fail to break the rocks (if they did, we would speak of failures).

Geometric elements of the folds

In order to classify and determine the origin of a fold, it is necessary to first describe the main parts of a fold:

• Hinge: It is the imaginary line that joins the points of maximum curvature of the fold. It is the line that joins the two flanks.
• Flanks: They are each of the lateral areas of the fold, located on both sides of the hinge.
• Axial plane: imaginary plane formed by the union of all the hinge lines of all the strata that make up the fold. Divide the fold into two parts, leaving a flank  on each side. If the axial plane is inclined, it is said to be verging or inclined towards that side.
• Fold axis: It is the imaginary line formed by the intersection of the axial plane with a horizontal plane. The angle between the fold axis and the hinge indicates the depth of the fold.
• Direction of the fold: It is the angle that the axis of the fold forms with the north.
• Core of the fold: It is the central, internal and most compressed part of the fold.

By Socsic (Own work) [CC BY-SA 3.0 or GFDL], via Wikimedia Commons

Types of folds

The folds can be classified according to several criteria:

Types of folds according to the arrangement of the layers

Anticline. In anticlines, the oldest layers are located in the core of the fold and the most modern are on the outside. We can easily recognize them because it is A-shaped (like anticline).

Syncline. Fold in which the most modern materials are at the core of the fold. They are V-shaped.

By Syncline_and_anticline.svg: User:Velosqueakderivative work: Fulvio314 [CC BY-SA 3.0], via Wikimedia Commons

Monocline or knee folds . Folds that have only one side . They are not anticlines or synclines, just an inflection in the strata.

By Kilom691 (Own work) [CC BY-SA 3.0], via Wikimedia Commons

Types of folds according to their symmetry

Symmetrical pleats. In symmetric folds, the axial plane is vertical, but so the two flanks are symmetrical.

Asymmetric pleats. The two flanks have different slopes.

Physical Geology by Steven Earle used under a CC-BY 4.0 international license.

Types of folds according to their axial plane

Straight folds. The axial plane is vertical.

Slanted folds. The axial plane of the fold forms an angle of less than 45º with the vertical.

Lying folds. The axial plane the fold is almost horizontal.

By No machine-readable author provided. Pastranec assumed (based on copyright claims). [GFDL, CC-BY-SA-3.0 or CC BY-SA 2.5-2.0-1.0], via Wikimedia Commons

Types of folds according to the thickness of the layers

Isopaque or concentric folds. The layers do not vary in thickness throughout the entire fold. These folds have been produced by bending stresses.

Anisopaque or similar folds. The thickness of the layers is greater in the area of ​​the hinge than in the flanks. They are caused by compression.

Images: Gallery of folds.

Fold associations

The folds are not isolated structures, but are usually found associated with other folds. The following fold associations are distinguished:

• Isoclinal series. The folds that intervene in the series have their axial parallel planes.
• Aniclinoria. The folds together form the structure of a large anticline, in which the axial planes of each fold converge inwards.
• Synchronories. The folds together form the structure of a great syncline, in which the axial planes of each fold converge outwards.

Any plane, such as the axial plane of a fold (or a stratum, or a fault plane), can be oriented in space by two measures:

• Direction. It is the geographical orientation, the angle formed by a horizontal line of the surface of a plane with respect to geographical north.
• Dip. It is the angle with respect to the horizontal, which forms the plane that we want to measure with respect to the horizontal plane. It is always less than 90º.

Interactive activity: Direction and dip.