1. CAUSES OF FOLDING
Introduction: Folds may be classified as tectonic or non-tectonic in origin. Those of tectonic origin result more or less directly from forces operating within the outer shell of the earth. Those of nontectonic origin are largely the result of movements under the influence of gravity near the surface of the earth, although the ultimate cause in many cases is tectonic.

2. There are two processes that produced folds.
CAUSES OF FOLDING
There are two processes that produced folds.
a) Tectonic Processes
b) Non-tectonic Processes
a) Tectonic Processes
b) Non-tectonic Processes
1. Horizontal compression
2. Intrusion of magma
3. Intrusion of salt
4. Vertical forces of unspecified origin
5. Plate movements
1. Those formed near the surface under the influence of gravity
2. Those related to chemical processes
3. Those related to glaciation
4. Differential compaction of sediments
5. Contemporaneous deformation

3. CAUSES OF FOLDING
a) Tectonic Processes
1. Horizontal compression: By horizontal compression we mean a compressive force acting parallel to the surface of the earth. In other words, the greatest principal stress axis is parallel to sea level. In most cases the active force, analogous to a moving piston, operates on one side of the folded belt, whereas a resisting force is induced by a stationary block on the other side.
Laboratory experiments have been performed to produced folds by compression. The most exhaustive study was made approximately 110 years ago by Bailey Willis, who sought to understand more thoroughly the folds of the Appalachian Province.

4. CAUSES OF FOLDING
a) Tectonic Processes
The experiments were performed in a pressure box, 39 3/8 inches long, 5 inches wide and 20 inches deep. A moveable piston at one end.
(after Willis 1893)

5. CAUSES OF FOLDING
a) Tectonic Processes
(after Willis 1893)
Alternating layers, representing sedimentary rocks were laid in the box and the piston was then slowly moved inward to produce folds. This experiment demonstrates the effectiveness of horizontal compression to produce folds and the primary force acted at right angles to the trend of the folds.

6. CAUSES OF FOLDING
a) Tectonic Processes
2. Intrusion of Magma: The intrusion of magma may cause folding. The clearest example of this is offered by laccoliths. A laccolith is an intrusive body that has domed up the strata into which it has been inserted. Not only are the overlying sediments bowed up into a dome, but the roof may be fractured and faulted. Such structural features, however, will be rather limited size and distribution. During major orogenic periods considerable deformation may accompany the intrusion of magma, but in many such cases the movement of the magma is the result of orogeny and not the cause.

7. CAUSES OF FOLDING
a) Tectonic Processes
3. Intrusion of Salt Domes: Uplift and faulting caused by the emplacement of salt domes. These are small structural features seldom more than a few miles in diameter.

8. 3. Intrusion of Salt Domes:
CAUSES OF FOLDING
a) Tectonic Processes
3. Intrusion of Salt Domes:

9. CAUSES OF FOLDING
a) Tectonic Processes
4. Vertical Forces of Unspecified Origin: Vertical forces are genetically related to folding in many ways. Some folds are the direct result of vertical movements, others are indirect product.
Large open folds that are tens or hundreds of miles across with limbs that dip less than a degree or at most only a few degrees, are produced by vertical movements. Broad anticlines, such as those in the Southern and Middle Rockies, may be the result of vertical movements rather than the product of horizontal compression.
Some Russian geologists believe that the folds in the Caucasus Mountains are due to vertical movements.

10. a) Tectonic Processes 4. Vertical Forces : Folding by vertical uplift.
CAUSES OF FOLDING
a) Tectonic Processes
4. Vertical Forces :
Folding by vertical uplift.
A, B & C are successive stages.

11. CAUSES OF FOLDING
a) Tectonic Processes
5. Plate Movements: In recent years the theory of plate tectonics has been developed to explain folding, sea-floor spreading, continental drift and thrusting. Sediments that have accumulated in front of the moving plate would be crumpled to give rise large scale folds.

13. a) Tectonic Processes 5. Plate Movements:
CAUSES OF FOLDING
a) Tectonic Processes
5. Plate Movements:
The animation above of a subduction zone, where an oceanic plate is converging with a continental plate, attempts in cartoon fashion to show the deformation.

14. As the continents collide, the sediments are squeezed, folded, and deformed.
This process results in a new mountain range composed of deformed and metamorphosed sedimentary rocks and fragments of volcanic rocks.

15. Active Subduction Zone
E U R A S I A P L A T E
6 cm/yr
B U R M A
P L A T E
2 cm/yr
I N D I A P L A T E
Andaman Sea
Active Subduction Zone

16. CAUSES OF FOLDING
a) Tectonic Processes
5. Plate Movements:

17. b) Non-Tectonic Processes
CAUSES OF FOLDING
b) Non-Tectonic Processes
The non-tectonic processes are those processes that are not directly related to movements within the outer shell of the earth. In many cases the term surficial processes would be equally good because the deformation is the result of the movements near the surface of the earth under the influence of gravity. Even in these instances, however, the deformation is usually an indirect result of movements within the earth.
The structural features discussed in the non-tectonic processes may be classified under the following major categories:-
1. Those formed near the surface under the influence of gravity
2. Those related to chemical processes
3. Those related to glaciation
4. Differential compaction of sediments
5. Contemporaneous deformation

18. b) Non-Tectonic Processes
CAUSES OF FOLDING
b) Non-Tectonic Processes
1. Structural Features formed near the surface under the Influence of Gravity:
Hillside Creep: In many areas, especially those that have not been glaciated, it is not uncommon to find incompetent rocks, such as shale, dipping into the hillside. Hillside creep occur due to the influence of gravity.

19. b) Non-Tectonic Processes
CAUSES OF FOLDING
b) Non-Tectonic Processes
2. Chemical Process: Solution of a chemically vulnerable formation may produce large structural features. Calcium sulphate is most commonly precipitated from evaporating water as anhydrite.
Anhydrite + H2O => Gypsum (volume increase 40%)
Water is subsequently added to convert the anhydrite into gypsum, and the increase in volume is approximately 40%. If the beds are flat-lying and if all the expansion takes place upward, the beds thicken, but no folds develop. If, however, much of the expansion is horizontal, compressive forces are setup and folding ensues,. The resulting folds are small, with a height of only a fraction of an inch, or, at the most a few feet.

20. b) Non-Tectonic Processes
CAUSES OF FOLDING
b) Non-Tectonic Processes
3. Glaciation: Glaciation, pushing against the steep slope of a cuesta, may throw the strata, if they are poorly consolidated, into folds. Moreover, ice may override weak sediments and cause drag folds.
Glacial ICE
Glacial ICE

21. b) Non-Tectonic Processes
CAUSES OF FOLDING
b) Non-Tectonic Processes
4. Differential Compaction of Sediments: Some folds, anticlines and synclines are due to downward movements of rock masses directly under the influence of gravity. Although anticlines that result from the differential compaction of sediments over buried ridges have comparatively low dips, they form important traps for accumulation of petroleum.
Ridge left by erosion
Ridge covered by sediments but before compaction
Ridge after compaction shows folding in sediments

22. Fold b) Non-Tectonic Processes
CAUSES OF FOLDING
b) Non-Tectonic Processes
5. Contemporaneous Deformation: Contemporaneous deformation takes place as sediments are being deposited. Small folds and faults may form in soft sediments due to sliding down gentle slopes. The hinges of the folds and the strike of the thrust will be at right angles to the direction of movements.
Competent bed
Incompetent bed
Competent bed