4. 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
5. 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 troughâ‹‹
to trough.
2 Ă— amplitude (a)
wavelength ( )â‹‹
6. 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
Interlimb
angle
From this angle folds are described with terms such as open or tight, look in the links
section for all the definitions.
8. 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.
9. 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.
11. 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
12. âś•
This single fold layer can be divided by an imaginary surface called the axial plane.
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Hinge line
E
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 line
E
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 line
E
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 line
E
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.