Structural Geology (1)Folds

FOLDS

Folds are the undulations in the rocks.  They are best displayed in stratified formation.

Folds May originate in many ways

1.     Horizontal Compression

2.     Horizontal Couple

3.     Convection Currents

4.     Intrusion of viscous magma

5.     Differential compaction of sediments.

6.     Chemical changes

7.     Contemporaneous deformation

8.     Ice push

Parts of the Folds –

1.     Limbs – Sides of the  fold

2.     Core – Central part of the fold

3.     Crest – Highest point of the fold

4.     Trough – Lowest point of the fold

5.     Crestal Line – The line passing through highest point of fold

6.     Trough line – The line passing through lowest pt. of fold

7.     Crestal plane – The plane passing through crest lines

8.     Trough planes- The plane passing through trough lines

9.     Amplitudes – The maxi displacement of beds in one direction

10.Wavelength – Distance between two successive crest or two successive          

11.Axis of fold – It is the line on a folded bedding plane along which the  

12. direction of dip changes is called axis of fold or Hinge line or Axial line

13.Axial plane- It is the plane containing axes of all  bedding plane.

Types of Folds

01-                                                                   Anticline Fold –

·        Convex upward

·        Limbs dip away from each other

·        Older beds occurs at core

02-                                                                   Syncline Fold –

·         Concave upward

·        Limbs dip towards each other

·        Younger beds occurs at core

03-                                                                   Isoclinals Fold-

·        Due to strong lateral compressive

·        forces acting from one side

·        Both limb dips in the same direction

 

04-                                                                   Symmetrical fold

·        Axial plane is vertical

·        Limbs dip in opposite dir

·        The amt of dip is same

·        Distance of limb from axial plane is equal.

05-                                                                   Asymmetrical Fold

·        Axial plane is inclined

·        Limbs dip in opposite dir with different amt of dip

·        One limb is steeper than the other

·        Distance of the limb from axial plane is not same.

06-                                                                   Recumbent Fold-

·        Axial plane is horizontal

·        One of the limb is turned through 180^o approx.

·        Both limbs Horizontal

·        The beds in over turned limb are much thicker than normal limb

·        Many sec. fold in normal are called digitations

The curve linear portion joining the normal limb to overturned limb is called  arch bent.

07-                                                                   Chevron Fold

·        Hinges are sharp & angular

08-                                                                   Box Fold

·        Crest and trough are b road & f lat

09-                                                                   Mono cline Fold

·         

·        Beds are relatively that but appear to haven been bent locally to exhibit higher dip localized change in their attitude.

                                                                                                                                               

10-                                                                   Fan fold – Both Limbs are over turned\

11-                                                                   Closed or tight fold – Thin at crest or trough