1. Looking at different types of folds on seismic data
Introduction to folds
Looking at different types of folds on seismic data

2. These folds are asymmetrical.
One limb is longer than the other.

3. This fold is symmetrical.
The limbs are the same length.

4. Folds can be gentle, symmetrical and rounded.
1.5 km N
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Fold axial plane
TWT (msec)
TWT (msec)
Folds can be gentle, symmetrical and rounded.

5. Or inclined, angular and asymmetric.
Look at this horizon
Fold axial plane
Or inclined, angular and asymmetric.

6. 1.5 km N ⇃ ↾ ⇃ ↾
There may be other deformation features associated with a fold.
Can you spot the faults?
What sort of faults are they?

7. S
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Reverse faults often occur with folds as both are caused by compressive forces.
But not all the faults are reverse faults.
This normal fault was re-activated.

8. When is an antiform an anticline?
(…and when is a synform a syncline?)

9. antiform
anticline
anticline
antiform
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antiform
syncline
synform
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syncline
Remember this?
To be an anticline or a syncline the relative age of the rocks must be known.
In this series of folds the rocks have not been inverted, therefore the youngest overlie the oldest rocks.

10. ⤚ ⤙ ⤙ ⤚ anticline youngest youngest oldest oldest syncline
Look at the anticline:
From the centre of the structure the age of the rocks decreases towards the outside:
the oldest rocks are at the core.
Now look at the syncline:
From the centre of the structure the age of the rocks increases towards the outside:
the youngest rocks are at the core.

11. Overview
The individual properties of a rock and the physical setting (e.g. temperature and confining pressure) determine how it will react under stress.
Some layers may be more competent than others and so deform in different ways, i.e. buckling, flexural slip or shearing (see links for more details).
There are many causes of stress in rocks that can cause folds:
Plate movements e.g. orogenies
Igneous intrusions e.g. granite batholiths
Salt diapirs (where large quantities of low density salt rise towards the surface causing deformation).
The time over which these stresses are applied can also affect the deformation e.g. low strain rates over extended time periods produces plastic deformation.
Folds vary in size from microscopic to many km. Only the large scale folds can be seen on seismic data.