Folds

Folds may be defined as undulations or bends or curvatures developed in the rocks of the earth crust as a result of stresses. 

Dip-

• It is the inclination of the bedding planes, with the horizontal Dip is always expressed in term of  degrees.

True dip-

• It is the maximum inclination of bedding planes with the horizontal.

Apparent dip-

• It refers to the inclination of the bedding planes, with the horizontal, in any other direction, other than the direction of the true dip, .

• The value of apparent dip is always less than the true dip.

Strike-

• It is direction, measured on a horizontal surface, of a line formed by the intersection of dipping bed with the horizontal plane.

• Strike is expressed in term of main four directions i.e., North, South, East or West.

• The strike is always at right angle to the dip of the beds.

Outcrop-

•  Outcrop id the exposed area  of the bed or rock on the surface of the earth.

Hinge point: 

• The point on the profile section of the fold which have maximum curvature is known as Hinge point .

Fold axis: 

• Fold axis is an imaginary line which when moved parallel to itself generates the fold. 

Crest: 

• It is that point which is located highest in the profile section of the fold. 

Trough:

• It is that point which is located lowest in the profile section of a fold. 

Axial plane:

• Axial plane is a dividing  imaginary plane, which divides the fold symmetrically in equal parts. 

Anticline:

• It is an arch like structure which is formed due to folding of the rock beds .

Syncline: 

• It is a trough like structure which is formed due to folding of the rock beds. 

There are lots of folds but we will not discussed all of them and will restrict ourselves to some of the main and important types-

1.Anticlinorium-

• It is a large anticline comprised of numerous smaller anticlines and synclines. 

2.Synclinorium-

•  A large syncline comprising of numerous smaller anticlines and synclines. 

3.Symmetrical and non symmetrical fold-

•  In a symmetrical fold, the axial plane divides it into two parts which are mirror images of each other but not in case of non symmetrical folds. 

4.Recumbent fold and inclined fold-

•  In this fold axial plane is more or less horizontal. The dip of axial plane varies between 0 to 10 degree and there is dip more than 10 in inclined fold. 

5.Chevron fold-

•  It is a special type of fold which has sharp hinge and straight limbs. 

6.An isoclinal fold -

• It has limbs that are essentially parallel to each other and thus approximately parallel to the axial plane.

Geosyncline: This is a very large depression or basin which is found in the oceans and can accumulate large thickness of sediments. 

7.Drag fold-

• As minor folds developed within the body of weaker rocks surrounded on both the sides by layers of stronger rocks. 

8.Overturned fold-

• It is a types of folds which have inclined axial planes and  both the limbs are dipping essentially in the same direction. The amount of dip of the these two limbs may or may not be the same. 

9.Homocline-

• It is the sequence wise arrangement of the strata dipping in the same general direction at a uniform angle, especially when such structure is established to be a limb of a major fold.

Tectonic Causes: 

• The tectonic reasons developed due to compressive forces may include Endogenitic - Orogenic and epirogenic forces. 

Non-tectonic causes:

The non-tectonic reasons includes-

• Hill side creep.

• Collapse or gravity sliding.

• Erosion of certain bed.

• Glacial ice push, solution etc.

Faults are fractures, along which the movement of one block, with respect to other block, has taken place. 

Fault plane -

• A plane along which the rupture has actually taken place and one block has moved with respect to the others is knows as a fault plane.

Hanging wall -

• The block which lies  above the fault plane is called the hanging wall. 

Foot wall -

• It is the block which lies below the fault plane.

Hade -

• It is the inclination of the fault plane with the vertical.

Throw -

• It is the vertical displacement between the hanging wall and the footwall. 

Heave -

• It is the horizontal displacement between the hanging wall and the footwall.

Fault line/ fault trace-

• The line formed by intersection of a fault with surface of the Earth or any given surface.

Strike of the fault plane-

• When the fault plane cuts the horizontal ground then the direction of that cut is called as strike of the Fault plane .

Dip of the fault plane-

• The angle of inclination of the fault plane with respect to horizontal is called dip of the fault. The true dip is measured on a vertical section perpendicular to the strike.

Slip-

• It is the actual displacement which  is parallel to fault plane in a given direction. 

Normal Fault -

• A normal fault is that fault in which the hanging wall has apparently moved down with respect to foot wall. 

Reverse Fault-

• In reverse fault, the hanging wall apparently moves upward relative to foot wall. 

Dip Fault-

• In this the strike of the fault plane is parallel to dip direction of the beds. In other words the strike of fault plane and bedding plane are mutually perpendicular to each other. 

Strike Fault-

• In this the strike of the fault plane and the bedding plane are parallel. 

Longitudinal Fault-

• In this the fault plane is parallel to the axial trace of the major folds. 

Some other terms related to Faults-

Nappe: 

• A portion of a region of intensely folded strata may be displaced to several kilometers because of overthrusting.

• The displaced block which lies above another rock unit is termed as ‘nappe.

A klippe (means Drag )

• It is the remnant portion which has been detached and isolated from the main nappe. 

Horst:

• A horst is an uplifted block which is bounded between two parallel sets of normal faults dipping in opposite directions. 

Graben:

• A graben is a downthrown block which is bounded between sets of two parallel but opposite dipping normal faults with same strike. 

• Faults are also caused due to Tectonic and non tectonic reasons which causes Tension ,Compression and Shearing of the Rocks. 

Joints-

• Joints are defined as divisional planes or fractures along which there has been no relative displacement.

Important One Liner 

What is a fault?

Answer: A fault is a break or fracture in the Earth's crust where rocks on either side have moved past each other.

How are faults classified?

Answer: Faults are classified as either dip-slip faults or strike-slip faults.

What is a dip-slip fault?

Answer: A dip-slip fault is a type of fault where the movement is vertical (up or down) along the fault plane.

What is a strike-slip fault?

Answer: A strike-slip fault is a type of fault where the movement is horizontal (side to side) along the fault plane.

What is a reverse fault?

Answer: A reverse fault is a type of dip-slip fault where the hanging wall moves upward relative to the footwall.

What is a normal fault?

Answer: A normal fault is a type of dip-slip fault where the hanging wall moves downward relative to the footwall.

What is a thrust fault?

Answer: A thrust fault is a type of reverse fault with a low angle of dip.

What is a strike-slip fault boundary?

Answer: A strike-slip fault boundary is a boundary where two tectonic plates are sliding past each other horizontally.

What is folding?

Answer: Folding is the bending of rock layers due to compressional forces.

What is an anticline?

Answer: An anticline is a type of fold where the rock layers are bent upward into an arch.

What is a syncline?

Answer: A syncline is a type of fold where the rock layers are bent downward into a trough.

What is a monocline?

Answer: A monocline is a step-like fold in otherwise horizontal sedimentary strata.

What is a dome?

Answer: A dome is a type of fold where the rock layers are bent upward into a circular or elliptical shape.

What is a basin?

Answer: A basin is a type of fold where the rock layers are bent downward into a circular or elliptical shape.