15.5: Geologic Structures: Folds

Geologic Structures: Folds

Folds are geologic structures created by ductile (plastic) deformation Links to an external site. of the Earth’s crust. To understand how folds are generated, take a piece of paper and hold it up with a hand on each end. Apply compressional forces (push the ends towards each other). You have just created a fold (bent rock layers). Depending on how your paper moved, you created one of the main fold types!

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Simple Folds

Simple folds have distinct regions or anatomy. For example, a plane can be drawn through the fold axis (or hinge) Links to an external site. of a fold and is called the axial plane Links to an external site. of the fold. The sloping beds on either side of an axial plane are limbs Links to an external site. (Figure 15.8). An anticline or syncline is described as symmetrical if the angles between each of limbs and the axial plane are generally similar, and asymmetrical if they are not.

Anticlines

An anticline Links to an external site. has layers which dip away from the hinge of the fold, and in cross-section resembles a capital letter A, rainbow, or arch (Table 15.2). In addition to the distinct cross-sectional shape, rocks along the hinge tend to be older, particularly after erosion. This can be determined by using the Principle of Links to an external site.Superposition Links to an external site.. Examine Figure 15.9. Mentally flatten the cross-section, so that layers O, S and D are flat and horizontal again. Where does layer O go? It is located at the bottom, which according to superposition, means it is the older layer. In this instance we also have geologic ages (Devonian, Silurian, and Ordovician), but these periods are only helpful if we know their order or can look up a geologic time scale.

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Synclines

A syncline Links to an external site. has layers which dip towards the hinge of the fold, and in cross-section represents a capital letter U, smile, or sink (Table 15.2). In addition to the distinct cross-sectional shape, rocks along the hinge tend to be younger, even after erosion (Figure 15.10). Much like above, this can again be determined by using the Principle of Superposition Links to an external site..

It is important to note that anticlines do not always represent mountains or high areas and synclines do not always represent basins or low areas. They are simply folded rock layers, and do not necessarily indicate topographic high and low points. Notice in both simple folds, the cross-section looks much different from the map view. In map view, rather than seeing folds, you encounter beds that, after erosion, will be organized as a parallel striped pattern (Figures 15.9, right and 15.10, right). To help determine the simple fold type, it will be helpful to interpret the strike and dip of the beds, age of the rock along the hinge, or any general symbology present.

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Table 15.2: The simple folds​. (CC-BY 4.0, Chloe Branciforte)

Simple Folds	Cross-sectional shape	Direction of dip of layers	Relative age of beds along axis
Anticline	A-shape or rainbow	Away from axis	Older
Syncline	U or V-shape or smile	Towards the axis	Younger

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Plunging Folds

A plunging fold, which is essentially a tilted fold that creates a V-shaped pattern on the surface (Figure 15.11). In a plunging anticline Links to an external site., the oldest strata can be found at the center of the V, and the V points in the direction of the plunge of the fold axis. In a plunging syncline, the youngest strata are found at the center of the V, and the V points in the opposite direction of the plunge of the fold axis.

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Monoclines

A monocline Links to an external site. is a simple fold structure, with only one limb folded, and the other remains horizontal. This is similar to the shape of a carpet draped over a stair step (Figure 15.12).

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Complex Folds

The complex folds include domes and basins, which are basically the circular (or elliptical) equivalent of the simple folds. Both domes and basins can be recognized by their bulls-eye patterned map-view (Figure 15.13 and Table 15.3).

A dome Links to an external site. has a cross-section similar to an anticline. In a dome, the oldest rocks are exposed at the center, and rocks dip away from this central point. A basin Links to an external site. has a cross-section, like a syncline. In a basin, the youngest rocks are in the center, and the rocks dip inward towards the center.

Table 15.3: The complex folds​. (CC-BY 4.0, Chloe Branciforte)

Complex Folds	Cross-sectional shape	Direction of dip layers	Relative age of beds at center
Dome	A-shape or rainbow (all sides)	Away from center	Older
Basin	U or V-shape or smile (all sides)	Towards the center	Younger