Geometric classification of folds

Fold classified geometrically

1. GEOMETRIC CLASSIFICATION OF FOLDS Submitted by , Ashikraj RS 2 nd sem Msc Geology Dept of geology University of kerala

2. FOLDS Folds are wave like structures that result from deformation of bedding , foliation ,or other originally planar surfaces in rocks . Such as sedimentary beds become bent or curved as a result of plastic (i.e. permanent) and ductile deformation .

3. F old The Chevron folds, North Cornwall, United Kingdom Source : http://www.mineralmuseum.eu/10-amazing-geological-folds-you-should-see/

4. Scale Microscopic Mesoscopic Megascopic Pumpelly’s rule Mimic structure and large-scale structure formed at the same time Deformation Brittle (near surface) ductile( lower crust) Source : structural geology textbook by Robert D ( page no-273 ) Pumpelly’s rule relating large and small scales

5. Pictorial representation of fold Source:http :// sanuja.com /blog/geometry-of-folds

6. Hinge  - zone of maximum curvature Axial plane – the plane defined by connecting fold hinge lines in multiple folded surface. Inflection points - points of zero curvature, where the sense of curvature changes from a convex to a concave line. They usually are aligned on either limb of a fold. Crest Trough Limb Hinge zone – greatest curvature zone Amplitude Wave length

7. Vergence - has been used to indicate the direction in which an  antiform or  synform is inclined or overturned Plunge – when the fold angle is inclined to the horizontal Plunge Source: https://www.britannica.com/science/plunge

8. Anatomy of Folds First-Order Folds – The largest fold Second-Order Folds – Smaller folds on the flanks of first-order folds Enveloping Surface – Enables the relation of small to large scale folds Source : structural geology textbook by Robert D (page no-274) Vergence of small and large folds Slip lines in a fold

9. GEOMETRIC CLASSIFICATION OF FOLD Based on …… Sense of curvature Direction of younging relative to sense of fold closure the symmetry of fold the nature of hinge line the plunge of fold axis the inter limb angle the thickness of the bed the orientation of axial plane the shape of hinge Miscellaneous fold

10. Kinds of folds based on the sense of curvature Antiform – A fold that is concave downward. Synform – A folds that is concave upward. Neutral fold – A fold that closes sidwise . Dome – An antiform where bedding dips away from the central point. Basin – A synform where bedding dips towards a central point.

11. Kinds of Folds v) Source : structural geology textbook by Robert D (page no-275)

12. Based on the direction of younging relative to sense of fold closure Anticline – is a type of fold that is an arch-like shape and has its oldest beds at its core Syncline – A folds that is concave towards younger rocks in its center Synformal anticline – fold that closes downwards, center of the structure are older rather than younger Antiformal syncline – fold that closes upwards but the younging is towards the center. Anticlinorium and synclinorium - Major anticline or a syncline on whose limbs are found minor folds, both anti- and synclines, generally asymmatrical and of opposite senses on the two limbs.

13. ANTICLINORIUM AND SYNCLINORIUM Source:http ://structuralgeology.50webs.com/pagea22.htm

14. Based on the symmetry of fold Symmetrical fold is one in which the axial plane is vertical. “M” type fold.   Asymmetrical fold is one in which the axial plane is inclined . “S” type or “Z” type fold. Source:https :// medium.com /@rohitgurjar009/folds-in-rocks-and-its-types-9f38a5a53def

15. Based on the nature of hinge line Folds with straight hinge lines are known as cylindrical folds while the folds with curved hinge lines are known as non-cylindrical folds . The cylindricity varies depend on the stress experienced by the fold Sheath folds are strongly noncylindrical and closed at one end. Source : structural geology textbook by Robert D (page no-277)

16. Source:http :// sanuja.com /blog/geometry-of-folds

17. Based on the plunge of the fold axis Horizontal fold : A fold whose axis is horizontal. Vertical fold : a fold whose axis is vertical. Inclined fold/plunging fold : fold in which axis is plunged. Source:http :// www.orefind.com /blog/ orefind_blog /2016/01/21/a-recipe-for-shrinking-a-mineral-resource-by-90- Source:http :// www.geographynotes.com /geology-2/structural-geology/folds-definition-parts-and-types-structural-geology-geology/1313

18. MICHAEL FLEUTY'S CLASSIFICATION BASED ON INTER LIMB ANGLE HINGE AREA Sub horizontal fold : plunge between 0-10 Gently plunging fold : plunge b/w 10 -30 Moderately plunging fold : plunge b/w 30 -60 Steeply plunging fold : plunge b/w 60 -80 Sub vertical fold: plunge b/w 80 -90

19. Fleuty 5 Fold classification Source:https :// www.geoexpro.com /articles/2013/07/folds-and-folding-part- i

20. Based on the orientation of axial plane Upright fold : Fold with vertical axial plane. Inclined fold : fold with inclined axial plane. Reclined fold : Folds have fold axis plunging down the dip of the axial surface. Overturned fold :A fold in which the AS inclined also both the limbs dip in the same direction is called an overturned fold . Recumbent fold : horizontal axis and AS is horizontal almost. Open fold : limb is away from one another Tight fold : limb is almost near to each other

21. Types of folds based on axial surface Source : structural geology textbook by Robert D (page no-278)

22. Based on inter limb angle Inter limb angle is a indicator of tightness of fold Gentle fold - 180 -120 open fold – 120 - 70 Closed fold –70 - 30 Tight fold – 30 - 0 Isoclinal fold – 5 0- Isoclinal fold : folds both limbs have the same angle(~zero) and are parallel to each other . Many recumbent fold are isoclinal .

23. Source:http :// www.files.ethz.ch / structuralgeology /JPB/files/English/8folds.pdf

24. Based on the thickness of bed Parallel Folds – Maintain constant layer thickness Concentric Folds – Parallel folds in which the folded surfaces define circular arcs Ptygmatic Folds – Have a lobate shape and look like intestines Similar Folds – Maintain their shape throughout the section and do not die out vertically

25. Disharmonic Folds – Shape or wavelength changes from one layer to another Supratenuous Folds – The synclines are thinner at the crust and thicker at the trough

26. Source : structural geology textbook by Robert D (page no-280)

27. Based on the shape of the hinge Round Hinged / broad hinge fold : A fold with broad hinge zone compared to limb Chevron and Kink Folds – Have straight limbs and sharp angular hinges Arrowhead Fold- A fold with a sharp hinge and distinctly curved limbs (flame fold) Cuspate fold : sharp hinge on one closure and rounded closure on oppositivelly directed closure

28. Source : http://www.mineralmuseum.eu/10-amazing-geological-folds-you-should-see/ Flame fold Cuspate fold

29. Based on the number of Hinge Single hinge fold Multiple hinged fold

30. Based on the geometrical relation among neighbouring structure Periodic Fold : train of fold with more or less same geometry b/w alternate points of inflection. Non periodic Fold : fold which are not periodic

31. Polyclinal folds : belong to groups of folds with sub-parallel hinge lines but non-parallel axial surfaces. Disharmonic Folds : Folds in which the amplitude, wavelength and style change along discontinuous axial surfaces from one layer to another are disharmonic. http://geologylearn.blogspot.in/2015/08/geometric-description-of-folds.html

32. Source:http :// www.files.ethz.ch / structuralgeology /JPB/files/English/8folds.pdf

33. en échelon : folds are stepped and consistently overlapping; they define an en échelon fold. Source:http :// www.files.ethz.ch / structuralgeology /JPB/files/English/8folds.pdf

34. Fan fold : in anticlinal fold the limbs dip towards each other with reference to the axial plane Source:http :// www.yourarticlelibrary.com /geology/folds-meaning-classification-and-system-geology/91637

35. Box Fold : crest is broad and flat, 2 hinges are present on either side of flat crest and trough Conjugate fold : A double hinged fold with sharp hinges http://sanuja.com/blog/geometry-of-folds

36. Miscellaneous fold Kinks fold : are folds with straight, planar limbs (there is no inflexion point) and angular hinges (the hinge zone is reduced to a point). Source:http :// www.files.ethz.ch / structuralgeology /JPB/files/English/8folds.pdf

37. Parasitic folds Hinge zones and limbs of large folds often display folds of smaller wavelength and amplitude: larger and smaller folds are together polyharmonic . The small folds are called parasitic or subsidiary folds with respect to the larger ones. The largest folds are termed first-order folds, the next largest are called second-order folds and so forth.

38. Parasitic folds Source:http :// www.files.ethz.ch / structuralgeology /JPB/files/English/8folds.pdf

39. REFERENCE Hatcher ,Robert D., Jr (1995), Structural geology: principle concept and problems , second edition , Prentice-Hall Englewood Cliffs, NJ 07632, P. 272- 298 Ramsay , J.G, and Huber, M.I., 1983, The Techniques of modern structural geology,V.2: Folds and Fractures: London, Academic press, p.347-361 Davis, G.H, and Reynolds, J.H., 1984,Structural geology of rocks and regions: New York, John Willey & Sons, p. 372-423 http://www.files.ethz.ch/structuralgeology/JPB/files/English/8folds.pdf