# Structural fold

Folds are geologic structures that develop due to deformation of rocks. Layered strata are compressed and shortened, resulting in wave-like formations, consisting of crest and troughs.

## How they form

Horizontal stress causes shortening and bending of rock. Credit: Thompson Graham, Turk Jonathan, 1998, Introduction to Physical Geology.

Folds develop from deformation of rock in response to tectonic stress. This type of deformation is considered plastic, since the rock is not broken but instead it is deformed and bent into a new permanent shape. There are number of factors which determine how a rock reacts to stress, these include: material properties, pressure, temperature, and duration of stress.[1] Ideally for folds to form the temperature and pressure should be high, and duration of stress must be applied over a long period of time rather than quickly. Folds are commonly related to compressive forces from convergent boundaries, which shorten horizontally and force the rock to bend. Folds can occur in series as well as singularly. In the presence of faulting, stresses can also be generated which create fault-bend fold features.

## Fold attributes

Folds can be characterized by their angle, scale, symmetry, attitude of hinge line and attitude of axial plane.

### Scale

Folds vary in scale from huge bends in orogenic belts to mountain side features to microscopic folds. Similarly to other waves, an amplitude and wavelength can be calculated for the fold.

### Parts of folds

Fold parts. Credit: Moores Eldrige, Twiss Robert J, Structural Geology 2nd Edition.
• Limbs- Sides of the fold.
• Inflection Line- Line where surface changes sense of curvature. Transition line from anticline to syncline.
• Hinge Line- Line in the surface where the curvature is greatest.
• Axial Plane- Surface joining all hinge lines corresponding to a set of folds.
• Interlimb Angle – Angle between tangents to the fold surface drawn through the inflection lines.

### Characteristics

Symmetry- If the shape of fold is a mirror image and of equal length on both sides of the hinge it is classified as symmetric, otherwise it is an asymmetric fold.

Tightness – Degree of folding is directly related to interlimb angle. Further categorized into acute or obtuse.

Interlimb angle "i" is used to describe tightness of fold.Credit: Moores Eldrige, Twiss Robert J, Structural Geology 2nd Edition.

Acute

• Gentle 180 > i > 120
• Open 120 > i >70
• Close 70 > i > 30
• Tight 30 > i > 0
• Isoclinal i= 0

Obtuse

• Fan 0 > i > -70
• Involute -70 > i > -180

Attitude- Attitude in folds is measured by the strike and dip of axial plane, as well as trend and plunge of hinge line. Axial plane ranges from upright, to inclined, to recumbent or horizontal. Hinge line ranges from horizontal, to plunging, to vertical.

Fold attitudes. Credit: Moores Eldrige, Twiss Robert J, Structural Geology 2nd Edition.

## Types of folds

• Anticline

Folds in which the strata arch upwards. Older rocks are located in the center of the fold, limbs dip away from center.

• Syncline

Folds in which the strata arch downwards. Younger rocks are located in the center of the fold , limbs dip towards center

• Chevron

Symmetrical multi-layer folds typically seen in schists and other rocks with platy minerals. Tightness ranges from gentle to close.

• Kink

Asymmetrical multi-layer folds , developed in schists and finely lamitated rocks.

• Dome

Antiformal doubly plunging fold. Oldest rock are found in the center of the dome.

• Basin

Synformal doubly plunging fold. Youngest rock are found in the center of the basin.

## Seismic exploration

For seismic interpreters, analyzing folds is key to comprehending the deformation processes that took place in the area of interest. For exploration, folds are valuable economically because they are proven hydrocarbon traps. In anticlines, oil migrates upward towards the crest within a reservoir rock and becomes trapped underneath an impermeable cap rock, thus becoming a hydrocarbon trap. In seismic, such a trap would appear at the top of the fold structure and it is characterized by a high negative amplitude formed by the impedance contrast between cap rock and the oil saturated rock underneath.

Anticline acting as hydrocarbon trap. Credit: GeologyIn [1]
Anticline, displaying prospect. Credit: Seismic Atlas of SE Asian Basins [2]

## References

1. Thompson Graham, Turk Jonathan, 1998, Introduction to Physical Geology 2nd Edition. https://books.google.com/books/about/Introduction_to_Physical_Geology.html?id=4tsSAQAAIAAJ
2. Moores Eldrige, Twiss Robert J, Structural Geology 2nd Edition, 1992, https://books.google.com/books?id=14fn03iJ2r8C&dq=structural+geology+Twiss+and+Moores+second+edition&source=gbs_navlinks_s