Folds

Topic: Folds its geometry, nomenclature and their field study

1. UNIVERSITY OF AZAD JAMMU AND KASHMIR MUZAFFARABAD T he Institute of Geology Topic: Folds its geometry, nomenclature and their field study Prepared by : Abdur Rauf Roll no: 125 presented To: Dr. jahanzeb khan SB And whole class

2. Topics Folds Geometry of folds Classification of folds Field study of folds

3. 1) Fold The bending of rock strata of plastic nature due to Compressional forces acting horizontally toward a common point or plane from opposite directions is known as fold. Or Folds may be defined as undulation, bending or curvatures in rocks of the crust’s as a result of stresses. Folds are one of the most common geological structures found in rocks. a set of horizontal layers are subjected to compressive forces, they bend either upward or downward.

4. 2) Geometry of a fold

5. Limbs Limbs or a flank of the fold is sloping side from the crest to the trough. b) Hinge points   The point of maximum curvature of a folded layer is located in the center of the hinge zone and is called the  hinge point . c) Hinge line Hinge points are connected in three dimensions by a  hinge line . Or The line which connect all the hinge pints is called hinge line.

6. d) Axial plane The axial plane is the surface connecting all the hinge. It may be a simple plane or a curved surface. It may be horizontal or vertical. e) Axis of the fold It is a line parallel to the hinge line of a fold or it may be defined as a line of interaction of a axial plane of a fold and with any bed of a fold. f) Inflection point The point where the convex change into concave.

7. g) Inflection line The line joining the inflection points is called inflection line. h) Crest and trough The line running through the highest points in a uparch fold is called crust, and if the line running through lowest point in a downarch fold is called trough. i) Wave length of a fold The distance between two adjacent crest or trough is known as wave length. Interlimb angle The interlimb angle is define as the angle between the planes tangential to the fold surface at the inflection line.

8. 3) Classification of folds Anticline. The word anticline is a Greek word which means “opposite inclined”. it may be define as a fold that is convex upward; it may also be defined as a fold that has older rocks in the center and the younger are at limbs. In this type of fold, the limbs are dip away from each other. An anticline expose during road cut at new jercy USA

9. Syncline The word syncline is a Greek word which means “together inclined”. It may be define as a fold that is convex downward. In this kind of folds the two limbs dip toward each other. So its means that the younger rocks are at the center and older are at limbs. This photo showing syncline

10. Symmetrical fold That type of fold in which the axial surface is essentially vertical. The term upright is also used. In symmetrical folds no limb is steeper then other. This photo showing symmetrical fold

11. Hormonic and disharmonic fold The type of folds in which the geometry of different layers matches with each other is called hormonic fold. In this type, folding is not uniform throughout the stratigraphic column. Folding changes upwards or downwards. Conversely, when the geometry of different layers of a fold cant match each other is called disharmonic fold. This photo shows hormonic folding This photo shows Dis hormonic folding

12. Cylindrical and non cylindrical folds. If the surface of a fold can be translated with the help of a strait line, the fold will called cylindrical. And if the surface of a fold can’t be translated with the help of strait line, the fold will known as non cylindrical fold. The geometry of non cylindrical folds is like egg tray. This photo shows cylindrical folding Non cylindrical folding

13. Monocline fold That fold in which only one limb is dipping is termed as monoclonal folds. Overturned fold In the overturned fold the axial plane is inclined and both limbs dip in the same direction, usually at different angles. The overturned, inverted, or reversed limb is the one that has been rotated more than 90. This photo showing overturned fold. This photo showing monoclonal folding

14. Recumbent fold A recumbent fold is one in which the axial plane is essentially horizontal. These types of folds are produce when the deformation is intense. And one fold lies on the top of next fold so that the axial plane become sub horizontal. Such kind of fold can be found in alps mountains. This photo shows recumbent folding.

15. Isoclinal fold An isoclinal fold, from the Greek word meaning “equally inclined”, refers to folds in which the tow limbs dip at equal angles in the same direction. Chevron fold chevron fold is one in which the hinges are sharp and angular. Box fold A box fold is one in which the crest is broad and flat. Two hinges are present, one on either side of the flat crest.

16. This photo shows chevron folding This photo shows box folding This photo shows isoclinal folding

17. 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 the synclinal fan fold, the two limbs dip away from each other. This diagram shows fan folding

18. Drag fold It forms when a competent bed slides past an incompetent bed. Such minor folds may form on the limbs of larger folds because of the slipping of beds past each other, or they may develop beneath overthrust blocks. Such structural features may form during sedimentation, when a sheet of sediment slides over a weaker bed. This photo shows Drag folding

19. Dome and basin A dome is an anticlinal uplift that has no distinct trend. And a basin is synclinal depression that has no distinct trend. Anticlinorium and synclinorium A major anticline that is composed of many smaller folds is called anticlinorium. The term is restricted to large folds that are least several miles across. Similarly, a synclinorium is a large syncline composed of many smaller folds it should be at least several miles across.

20. This photo shows dome This photo shows anticlinorium This photo shows synclinorium Sorbas basin in south-east Spain. 

21. Geosyncline Literally it means an earth syncline but should not be used for large syncline. It is a large depression, hundred of miles long and ten of miles wides, in which many thousands or ten of thousands of feet of sediments accumulate.

22. On the basis of interlimb angle the folds are classified as following. Interlimb angle Description of fold 180d – 120d Gentle 120d – 70d Open 70d – 30d close 30d – 0d Tight 0d Isoclinal -angle Mushroom

23. Here we are using the following classification Interlimb angle Folder description Od - 10d Isoclinal fold 30d- 70d Close fold 70d- 120 Open fold 120d – 180d Gentle fold 10d – 30d Tight fold

24. 4) The field study of folds Direct observation Fold may be recognized in many ways. The easiest and most satisfactory method is to observe the fold, but this can be done in comparatively few regions. Wherever small folds are observed in single outcrops, it is desirable to record their attitude. To do this we must measure the attitude of the hinges and the attitude of the axial plane. The pattern show The plane should also be recorded by minor folds as it is of great use in conjunction with plunge of fold in deducing large structures.

25. Plunge of axis The plunge is measured by means of clinometer attached to the compass. Attitude of axial plane A note book or sheet of paper may be held parallel to the axial plane of a fold in order to measure its attitude. N 40 degree The fold is anticline 40 degree west The fold is syncline east west horizontally 25 d AP Axial plane, striking north & dipping 25d west

26. Attitude of axis and axil plane The symbol of axial plane and hinge will be combined to form the symbols shown bellow. 35d N A fold plunging 35d east & a vertical axial plane striking east 51d 60d An anticline plunging 51d NNW, with the axial plane striking N and dipping 60d W.

27. Inferred fold Fold large than an outcrop are based on inference moreover the part of the fold that was above the present surface the earth has been removed by erosion. One or more of the following pieces of information are commonly used to deduced fold. Deference in attitude of some planer features at different localities. aerial map pattern Subsurface exploration by drilling and tunneling Subsurface studies by geophysical methods.

28. Any question! ستاسو د ټولوډیره ډیره مننه