Fold Geometry

# EMSC 3002

## Fold Geometry

- Louis Moresi (convenor)
  - **Romain Beucher** (lecturer)
  - Chengxin Jiang (lecturer)
  - Stephen Cox (curriculum advisor)

Australian National University

_**NB:** the course materials provided by the authors are open source under a creative commons licence. 
We acknowledge the contribution of the community in providing other materials and we endeavour to 
provide the correct attribution and citation. Please contact louis.moresi@anu.edu.au for updates and 
corrections._

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## Resources

1. **Fossen, H, 2011.** *Structural Geology.* Cambridge University Press, 2nd Edition **Chapter 11**
1. **McClay, K.R. 1991.** *The Mapping of Geological Structures.* John Wiley & Sons.  **Chapter 3**
1. **Park, R.G., 1995.** *Foundations of Structural Geology.* Blackie & Sons Ltd. **Chapter 2**
1. **Davis, G.H. and Reynolds, S.J., 1996.** *Structural Geology of Rocks and Regions.* 2nd Edition, John Wiley & Sons. **Chapter 7**

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## Intended learning outcomes

Students will be able to:

- Describe and name basic elements of fold geometry
- Use classifications to describe fold styles

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## Folds, Why are they important?

<div>

Hydrocarbon industry:

- Folds and associated structures can form traps.
- Understanding folds periodicity and geometry is key for the selection of drilling sites.

Mineral Industry:

- Understanding folds structures is important in assessing the extent of ore reserves and the
viability of mines.

</div>

![Anticlinal Oil Trap](Module-v-Ductile-Deformation/Figures-Fold-Geometry/figures/Anticlinal_Oil_trap.png)

</div>
</div>

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## Basic Elements of Fold Morphology

The conditions of deformation usually vary both in space and time, differences in rock type,
temperature, pressure and strain-rate translates in a wide range of fold geometry or **fold styles**.

Correct interpretation of the variations in fold style is important to reconstruct the
geological history of an area.

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## Basic Elements of Fold Morphology

### Observation of folds in section

![Fold](Module-v-Ductile-Deformation/Figures-Fold-Geometry/figures/Fold_section.svg)

**Folds** are usually best studied in section perpendicular to the layering, or
perpendicular to what is defined as the **axial surface**.

Folds are made up of a **hinge** that connects to **limbs**.

The **hinge** may be abrupt but usually the curvature is gradual and define a 
**hinge zone**.

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## Basic Elements of Fold Morphology

Accurate description of folded structures requires using a specific vocabulary.

Key to compare natural structures across different contexts.

![Fold](Module-v-Ductile-Deformation/Figures-Fold-Geometry/figures/Fossen_Fig11.1.jpg)

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## Fold styles
### Tightness and Angularity (Interlimb angle)

<div>

![Fold](Module-v-Ductile-Deformation/Figures-Fold-Geometry/figures/Interlimb_angle.svg)

</div>

![Fold](Module-v-Ductile-Deformation/Figures-Fold-Geometry/photos/Gentle_fold.jpg) 
</div>
<div>

![Fold](Module-v-Ductile-Deformation/Figures-Fold-Geometry/photos/Close_folds.png)

</div>
</div>
</div>

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## Fold styles
### Wavelength and Amplitude

<div>

The Shape of folds can be compared to mathematical functions.

We use terms such as **amplitude** and **wavelength**.

Folds do not necessarily follow the regularity of simple mathematical function.
Nevertheless, simple harmonic analysis can be applied to describe fold shape.

</div>

![Fold](Module-v-Ductile-Deformation/Figures-Fold-Geometry/figures/Fold_amplitude_wavelength.svg)

</div>
</div>

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## Fold styles
### Cylindrical, non-cylindrical

<div>

Folds may be cylindrical so that the fold axis or hinges plunges in one direction.

Especially on the large scale the fold may also be non-cylindrical with a fold
axis that plunges in either direction.

</div>

![Fold](Module-v-Ductile-Deformation/Figures-Fold-Geometry/figures/Cylindrical_vs_non-cylindrical_folds.svg)

</div>
</div>

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## Basic Fold shapes

<div>

![Fold Shapes](Module-v-Ductile-Deformation/Figures-Fold-Geometry/figures/SynformAntiform.svg)

</div>

![Fold Shapes](Module-v-Ductile-Deformation/Figures-Fold-Geometry/figures/SynclineAnticline.svg)

</div>
</div>

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## Anticline

![Anticline](Module-v-Ductile-Deformation/Figures-Fold-Geometry/figures/Anticline.jpg)

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## Syncline

![Syncline](Module-v-Ductile-Deformation/Figures-Fold-Geometry/figures/Syncline.jpg)

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## Dome

![Anticline](Module-v-Ductile-Deformation/Figures-Fold-Geometry/figures/Dome.jpg)

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## Basin

![Basin](Module-v-Ductile-Deformation/Figures-Fold-Geometry/figures/Basin.jpg)

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## Fold shape and stratigraphy

![Refolded Fold](Module-v-Ductile-Deformation/Figures-Fold-Geometry/figures/Refolded_fold.svg)

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## Classification of folds

![Parallel vs similar](Module-v-Ductile-Deformation/Figures-Fold-Geometry/figures/Parallel_vs_Similar_Folds.svg)

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## Fold Styles

<div>

A spectrum of hinge shapes exists, from the pointed hinges of kink-bands and
chevron folds to the rounded hinges of concentric folds.

Classification according to the hinge curvature is referred to as bluntness.
</div>

![Fold](Module-v-Ductile-Deformation/Figures-Fold-Geometry/figures/Fold_types.svg)

</div>
</div>

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## Fold styles
### Kink Folds

<div>

![Kink Folds](Module-v-Ductile-Deformation/Figures-Fold-Geometry/figures/KinkFolds.svg)

</div>
<div style="width:50%; float:right">

![Kink Folds](Module-v-Ductile-Deformation/Figures-Fold-Geometry/photos/kink_folds_pyrenees_jordi_carreras.jpg)

</div>
</div>

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## Fold styles
### Concentric Folds

<div>

![Concentric Folds](Module-v-Ductile-Deformation/Figures-Fold-Geometry/figures/cylindricalFolds.svg)

</div>

![Concentric Folds](Module-v-Ductile-Deformation/Figures-Fold-Geometry/photos/ConcentricFolds.jpeg)  
*Core of concentric anticline. Keele River, McKenzie Mountains, Northwest Territories, Canada. [64.170859,-125.598908]*

</div>
</div>

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## Fold styles
### Chevrons Folds

<div>

![Chevron Folds](Module-v-Ductile-Deformation/Figures-Fold-Geometry/figures/chevronFolds.svg)

</div>

![Chevron Folds](Module-v-Ductile-Deformation/Figures-Fold-Geometry/photos/Chevron_folds.jpg)

</div>
</div>

*Chevron folds with flat-lying axial planes, Millook Haven, North Cornwall, UK* source: ![GeologyIn.com](https://www.geologyin.com/2015/02/types-of-folds-with-photos.html)

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## Fold styles
### Box Folds

<div>

![Box Folds](Module-v-Ductile-Deformation/Figures-Fold-Geometry/figures/boxFolds.svg)

</div>

![Box Folds](Module-v-Ductile-Deformation/Figures-Fold-Geometry/photos/boxFolds_calico_mountains_Barstow_California.jpg)

</div>
</div>

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## Fold styles
### Ptygmatic Folds

<div>

![Ptygmatic Folds](Module-v-Ductile-Deformation/Figures-Fold-Geometry/figures/ptygmaticFolds.svg)

</div>

![Ptygmatic Folds](Module-v-Ductile-Deformation/Figures-Fold-Geometry/photos/Ptygmatic_folds.jpg)

</div>
</div>

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## Geometric Aspects of folds

<div>

![Fold](Module-v-Ductile-Deformation/Figures-Fold-Geometry/figures/Plunging_fold_3d.svg)

</div>

![Fold](Module-v-Ductile-Deformation/Figures-Fold-Geometry/photos/Sheep_Mountain_Anticline_Wyoming_bernhard_edmaier_photography.png)

</div>
</div>

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## Fold Attitude

![Fold](Module-v-Ductile-Deformation/Figures-Fold-Geometry/figures/PlungeUprightRecumbent.svg)

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## Fold Attitude

<div>

- Horizontal folds have horizontal fold axis.
- Vertical folds have vertical fold axis.
- Plunging folds have plunging fold axis.
- Reclined folds have a plunging fold axis AND younging pointing downward.

</div>

![Fold](Module-v-Ductile-Deformation/Figures-Fold-Geometry/figures/PlungeUprightRecumbent.svg)

</div>
</div>

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## Fold Attitude

![Fold](Module-v-Ductile-Deformation/Figures-Fold-Geometry/figures/Fold_Attitude.svg)

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## Fold Attitude

![Fold](Module-v-Ductile-Deformation/Figures-Fold-Geometry/figures/Fold_attitude_2.jpg)

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![Fold](Module-v-Ductile-Deformation/Figures-Fold-Geometry/photos/Caledonian_orogeny_fold_in_King_Oscar_Fjord.jpg)

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