1. Fold FOLD CLASSIFICATION
The bases of fold classification are as follows:-
1. Morphological Classification:- based on fold shape and style.
2. Geometric Classification:-
a) based on the orientation of the fold elements such as axial plane and fold axis.
b) based on the interlimb angle.
c) based on the attitude of the fold limbs.
d) based on the dip isogons pattern.
3. Genetic Classification:- based on the mechanics of origin and internal kinematics.

2. Fold Morphological Classification
a) Parallel fold / Concentric fold / Competent fold
b) Similar fold / Incompetent fold
c) Box fold
d) Fan fold
e) Chevron / Accordian / Acute / Zig-zag / Angular fold
f) Dome
g) Basin
h) Disharmonic fold
i) Kink band

3. Fold Nomenclature of Folds
a) Parallel fold: A fold in which each bed maintains the same thickness (assuming it was initially of uniform thickness) throughout all parts of the fold, i.e., constant orthogonal thickness.
Form of fold must change upward and downward. Anticlines become sharper with depth, but broader and more open upward. Conversely, the synclines become broader with depth, but sharper upward.

4. Fold Nomenclature of Folds
b) Similar fold / Incompetent fold: A type of folding in which each successively lower bed shows the same geometrical form as the bed above. The thickness of the bed measured parallel to the axial plane (i.e., axial plane thickness is constant. The lithologic units do not retain their original thickness, thicken hinge and thinned limbs.

5. Fold Nomenclature of Folds
c) Box fold: A flat-topped broad anticline with steep flanks and have two hinges. The fold resembles a box in profile.

6. Fold Nomenclature of Folds
d) Fan fold: A fan fold is one in which both limbs are overturned. In the anticlinal fan fold, the two limbs dip toward each other; in synclinal fan fold the two limbs dip away from each other.

7. Fold Nomenclature of Folds
e) Chevron fold: A chevron fold is one in which the hinges are sharp and angular with straight limbs.

8. Fold Nomenclature of Folds
e) Chevron fold: A chevron fold is one in which the hinges are sharp and angular with straight limbs.

9. Chevron folds are a large scale version of kink fold
Chevron fold, southeast coast of Scotland
Image:
Chevron folds are a large scale version of kink fold

10. Fold Nomenclature of Folds
f) Dome: An anticlinal uplift in which the beds dipping in all directions, more or less equally from a point. Dome has no distinct trend.
g) Basin: A synclinal depression with the strata dipping inward.

11. Fold Nomenclature of Folds
f) Dome: An anticlinal uplift in which the beds dipping in all directions, more or less equally from a point. Dome has no distinct trend.
g) Basin: A synclinal depression with the strata dipping inward.

12. Fold Nomenclature of Folds
f) Dome: An anticlinal uplift in which the beds dipping in all directions, more or less equally from a point. Dome has no distinct trend.
g) Basin: A synclinal depression with the strata dipping inward.

13. Fold Nomenclature of Folds
h) Disharmonic fold: A fold in which pronounced changes in shape occur from layer to layer.
i) Kink band: Kink bands are narrow bands, usually only a few inches or few feet wide, in which the beds assume a dip that is steeper or gentler than in the adjacent beds..

14. Fold
Kink Folds
Kink folds are mesoscopic to microscopic folds, <1m, which are found in finely laminated, strongly anisotropic rocks like shale and slate.
The fold itself has sharp hinges and straight limbs.

15. Fold Geometric Classification 2. Geometric Classification:-
a) based on the orientation of the fold elements such as axial plane and fold axis.
b) based on the interlimb angle.
c) based on the attitude of the fold limbs.
d) based on the dip isogons pattern.

16. } } Fold Geometric Classification
a) based on the orientation of the fold elements such as axial plane and hinge.
Angle 00
1 – 100
10 – 300
30 – 600
60 – 800
900
Term
Horizontal
Sub-horizontal
Gentle
Moderate
Steep
Sub-vertical
Vertical
Dip of Axial Plane
Plunge of Hinge }
Recumbent fold
Horizontal fold
Gently inclined fold
Moderately inclined fold
Steeply inclined fold
Gently plunging fold
Moderately plunging fold
Steeply plunging fold }
Upright fold
Vertical fold

17. Fold Recumbent fold Geometric Classification
a) based on the orientation of the fold elements such as axial plane and hinge.
Recumbent fold

18. Fold Geometric Classification
a) based on the orientation of the fold elements such as axial plane and hinge.

19. Fold Geometric Classification
a) based on the orientation of the fold elements such as axial plane and hinge.

20. Fold Geometric Classification
a) based on the orientation of the fold elements such as axial plane and hinge.

21. Fold Geometric Classification
a) based on the orientation of the fold elements such as axial plane and hinge.
Plot the following fold elements in
Rickard’s triangular diagram and
name each fold.
Hinge surface Hinge line
(dip) p-plunge, r-pitch
a p = 60
b r = 80
c p = 60
d r = 85
e p = 20

22. Fold Geometric Classification b) based on dihedral / interlimb angle.
Description of Fold
Fold Profile
180 – 1200
120 – 700
70 – 300
30 – 20
2 – 00
Gentle fold
Open fold
Close fold
Tight fold
Isoclinal fold

23. Fold Geometric Classification c) based on attitude of the fold limbs.
1) Isoclinal Fold: Both limbs have the same amount of dip in the same direction, i.e., limbs are parallel and axial plane may be inclined or vertical.
2) Overturned Fold / Over Fold: Both the limbs dip in the same direction usually at different angles where one limb is rotated through more than 90 degrees. The beds which are rotated more than 90 degrees are called overturned beds.

24. Fold Geometric Classification c) based on attitude of the fold limbs.
3) Homocline: A general term for a series of rock strata having the same dip; i.e., one limb of a fold, a tilted fault block, or an isocline.

25. Fold Geometric Classification c) based on attitude of the fold limbs.
4) Monocline: A step like bend in otherwise horizontal or gently dipping beds, i.e., a local steepening of dip in an area which is characterized otherwise by very low dips.

26. Fold Geometric Classification c) based on attitude of the fold limbs.
5) Structural Terrace: An area where beds which generally have a regional dip are locally horizontal.

27. Fold Geometric Classification d) based on dip isogons pattern.
DIP ISOGONS: A line of equal dip / slope of the successive folded surface drawn in the plane of fold profile.

28. Fold Geometric Classification d) based on dip isogons pattern.
Based on the pattern of dip isogons the shape of the fold is divided into three classes.
1. Class I Folds with convergent dip isogons.
2. Class II Folds with parallel dip isogons.
3. Class III Folds with divergent dip isogons.

29. Fold
Geometric Classification
d) based on dip isogons pattern.
1. Class I Folds with convergent dip isogons.
Folds have dip isogons that converge toward the core of the fold. Curvature of the inner fold arc always exceeds that of the outer arc.

30. Fold
Geometric Classification
d) based on dip isogons pattern.
1. Class I Folds with convergent dip isogons.
Folds of Class I can be subdivided into three sub-classes depending on the strength of the convergence of the dip isogons.
i. Sub-Class IA Strongly convergent
ii. Sub-Class IA Moderately convergent
iii. Sub-Class IA Weakly convergent

31. Fold i. Sub-Class IA Folds with Strongly convergent dip isogons
Geometric Classification
d) based on dip isogons pattern.
i. Sub-Class IA Folds with Strongly convergent dip isogons
In these folds the orthogonal thickness of the beds always exceeds that at the hinge of the fold.

32. Fold ii. Sub-Class IA Folds with moderately convergent dip isogons
Geometric Classification
d) based on dip isogons pattern.
ii. Sub-Class IA Folds with moderately convergent dip isogons
This is well known geometrical model in which the layer keeps a constant orthogonal thickness throughout the fold.

33. Fold iii. Sub-Class IC Folds with weakly convergent dip isogons
Geometric Classification
d) based on dip isogons pattern.
iii. Sub-Class IC Folds with weakly convergent dip isogons
Folds that fit into this sub-class differ from IA and IB in that the orthogonal thickness of the folded layer on the flank of the fold is always less than that at the fold hinge.

34. Fold
Geometric Classification
d) based on dip isogons pattern.
2. Class II Folds with parallel dip isogons.
In these type of folds, the curvature of the inner and outer arcs are equal. The distances between the folded boundaries of the layer in a direction parallel to the axial surface are equal. Thickness changes from limb to hinge, i.e., thicken hinge and thinned limbs.

35. Fold
Geometric Classification
d) based on dip isogons pattern.
3. Class III Folds with divergent dip isogons.
Folds of these class have curvature of the inner arc is always less than that of the outer arc. They also have thinner limbs and thicker hinge zones

36. Fold Genetic Classification
The basis of this classification is the mechanics of origin and internal kinematics.
KINEMATICS: The study of the relation between the forces and the resultant movement is kinematics which is a branch of dynamics.
Folds of this class are as follows:-
1. Diapiric fold
2. Drag fold
3. Flexural fold
4. Flow fold
5. Passive fold
6. Quasi flexural fold
7. Rheid fold
8. Shear / slip fold
9. Supratenuous fold
10. Ptygmatic fold

37. Fold Genetic Classification
1. Diapiric Fold: A piercement structure in which a fold formed in a highly movile material breaks through, or pierces, less mobile layers. Salt diapir and clay diapir are the examples of diapiric structure.

38. Fold Genetic Classification
2. Drag Fold: Generally a small scale asymmetric fold formed as a result of a localized couple caused by relative movement of layers in a layer flexural fold, or along a fault.

39. Fold Genetic Classification
3. Flexural Fold: Where flow or slip is restricted by layer boundaries, the layering exercises an active control on the deformation, and the resulting folds represent a true bending of layers.

40. Fold Genetic Classification
4. Flow Fold: A fold formed by rock flowage in which the rocks behave as fluids.

41. Fold Genetic Classification
5. Passive Fold: Where flow or slip crosses the layer boundaries, the layering exercises little or no control on deformation (the layering is passive) and layer boundaries serve as markers, parts of which are displaced relative to other parts to produce an apparent bending..

42. Fold Competent layer Incompetent layer Competent layer
Genetic Classification
6. Quasi-Flexural Fold: A gradational class of folding, particularly characteristic in rocks of moderate to high ductility, in which individual layers within a folded sequence are flexed in response to passive behavior in the associated rocks.
Competent layer
Incompetent layer
Competent layer

43. Fold Genetic Classification
7. Rheid Fold: Flow folding in which the rocks have remained solid or crystalline, but have deformed as fluids because the duration of the loading was much longer than the relevant deformation time constant.

44. Fold Genetic Classification
8. Shear Fold / Slip Fold: A fold formed as a result of minute displacement of beds along closely spaced fractures or cleavage planes.

45. Fold Genetic Classification
9. Supratenuous Fold: A fold in which the beds thicken toward the syncline because the basin subsided during sedimentation.

46. Fold Genetic Classification
10. Ptygmatic Fold: Curious meandrine contorations of quartzo-felspatic veins typically seen in gneisses especially in injection complexes and transfused rocks are known as ptygmatic structures or ptygmatic folds.