Terminology of folds

Some basic information about fold description and geometry

Tectonic stress slowly applied to rocks will cause them to bend.

Flat rocks Folded rocks

Tectonic stress +

lots of time

Photographs reproduced with the permission of the British Geological Survey ©NERC.All rights Reserved.

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The features produced are called folds.

The rocks undergo permanent strain.

antiform

antiform

antiformsynform

synform

As geologists we need to be able to describe folds:

An upward fold is an antiform.

A downward fold is a synform.

This is a series of folds.

As folds are wave shaped (waveforms) we use the same terminology that is used in physics to identify the parts of a wave:

The very top of an antiform is the crest.

The very bottom of a synform is the trough.

crest

trough

Amplitude (a) is the maximum displacement from a central position in either direction.

Wavelength ( ) is the distance for one complete cycle e.g. from crest to crest or trough ⋋to trough.

2 × amplitude (a)

wavelength ( )⋋

The limbs are the flanks of the fold and are relatively planar.

The angle created by the intersection of the two limbs is called the interlimb angle and is used to describe a fold.

limb limb

Interlimbangle

From this angle folds are described with terms such as open or tight, look in the links section for all the definitions.

Looking in 2D

Some terms to describe the orientation of a fold

The hinge is the area of maximum curvature on the same layer.

limb limb

hinge

As a seismic profile only gives us a 2D view, we can only see the hinge as a point here.

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The hinge is really a line, see the 3D slide for more.

When there are many fold layers:

The hinge is marked on every layer.

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All the hinges are joined to create an imaginary surface (remember the hinge is a line).

This line is called the axial surface.

axial surface(seen end on)

The axial surface divides all the fold layers as symmetrically as possible.

Look in 3D

Because this is an E-W view, we do not know what this fold does to the south.

It could be flat, or it may tilt, a parallel seismic profile would tell us.

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Plot the position of the hinge on the southern fold.

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Hinge line

Joining these up gives us the hinge line.

E

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This single fold layer can be divided by an imaginary surface called the axial plane.

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Hinge lineE

It is a planar surface that cuts the interlimb angle and divides this single fold as symmetrically as possible.

Fold axial plane

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The intersection of the hinge with the axial plane is called the fold axis.

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Hinge lineE

It is a direction, here the fold axis is NW to SE (it can be measured as a bearing). Sometimes this direction is called the trend.

Fold axial plane

Fold axis

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Plot the hinge lines on both profiles for all layers.

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Hinge lineE

Join the hinge lines together to create a surface – fold axial surface.

Fold axial surface

When there are many fold layers a fold axial surface is needed to describe the orientation of the fold.

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Observe what happens when the fold axial plane for the topmost layer is added.

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Hinge lineE

The fold axial plane is for only one layer.The fold axial surface is for many layers, notice it is not planar.

Fold axial surface

Fold axial plane

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The orientation of the axial plane or axial surface is used to describe the attitude of the fold, using terms such as upright, inclined etc. Look in the links section for all the definitions.

E

This is an upright fold.

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If the hinge line (or fold axis) is not horizontal but dips, then the angle from the horizontal is measured as the plunge.

E

This fold has a plunge of 40°.

Angle of Plunge

Horizontal