Geological Faults

A fault is a fracture along which there is visible offset of movement by shearing.

OR

A fault is a fracture along which there is a relative movement between the two opposite blocks.

Faults and shear zones range in length and magnitude of offset from small structures visible in hand specimen to long spectacular crustal breaks extending hundreds of kilometers.Some joint may undergo small movements but their movement is so small that they are often neglected and are called

microfaults.

CLASSIFICATION OF FAULTS

1    On the basis of slip

2    On the basis of separation

Classification on the basis of slip

Slip on a fault is the displacement vector between two points that were coincident before faulting. There are two major types of faults on the basis of slip;

1.     Strike slip faults

2.     Dip slip faults

Strike Slip faults

These are the faults in which we have the horizontal movement between the adjacent blocks in the direction parallel to the strike of the fault plane.

They are further divided into two types on the basis of sense of relative movement.

A.     Left handed strike slip faults:

             If the left block of the fault moves towards the observer then the fault would be left handed.

B.     Right handed strike slip faults

             If the right block of the fault moves towards the observer then the fault would be right handed.

2. Dip slip faults

           These are the faults in which we have the movement along the two block in the direction parallel to the dip of the fault.

           These are further divided into three major types;

A. Normal slip faults

These are the faults in which hanging wall moves downward relative to the footwall in the direction parallel to the dip of the fault plane.Normal faults form in response to tensional stresses.

B. Thrust slip and reverse slip faults

These are the faults in which hanging wall moves upward relative to the footwall in the direction parallel to the dip of the fault plane.

The difference between thrust slip fault and reverse slip fault is the dip angle of the fault plane. In reverse slip faults the fault plane dips at angle greater than 45° In thrust slip faults the fault plane dips at an angle less than 45°.

C.     Oblique slip faults:
These are the faults in which hanging wall moves obliquely relative to the footwall in the direction
somewhere between strike and dip of the faults plane.If strike slip component is dominant then left lateral
or right lateral are used are suffix.If the dip slip component is dominant then normal reverse or thrust will
be used as suffix and left lateral or right lateral would be used as prefix.

D.   Rotational Faults:

These are the faults which are rotational or scissor like.

Criteria for recognition of faults:

1 Geological Map Relations

Faults are relatively easy to recognize in continental regions of moderate to excellent exposure.Systematic Geological mapping has proved to be an extremely effective method for locating faults, particularly where the faults cuts a geological column of sedimentary or volcanic rocks, whose stratigraphy is well known.

2 Repetition and Omission of Strata

Faults can cause repetition in the strata and they can also omit the strata as shown in figure 9.4. Therefore the faults produce in consistent stratigraphy in an area.

3 Physical Features:

a. Slickensides:

These are the polished surfaces produced due to the frictional abrasion of the opposite fault blocks.

b.      Chatter Marks

These are small, asymmetrical, step like features formed on the fault surface. And these are oriented perpendicular to the striations.

c.      Groove Marks

Large faults may display surfaces which contain deeply furrowed slickensided grooves and known as groove marks. These are also the indicators of faults.

Fault Rocks

Fault Breccia:

The debris that are detached from the faulted blocks in response to grinding and milling are accumulated in form of a rock known as breccia.These breccias are usually in unconsolidated form and show low cohesion but can convert into a consolidated rock due the cementation by solutions.It is comprised of angular sediments of different sizes detached from either sides of the fault plane.

Fault Gouge:

If the rocks on either sides of the fault plane are weak and incompetent, then they respond in a plastic manner to faulting and are commonly converted to gouge.Gouge is a very fine grained, clayey crushed rock.In its dry state it feels like talcum p0wder having bigger grains and pieces of harder rocks.The zone of gouge can be thin but they can be mostly a meter wide.

Mylonite:

Mylonites are formed due to intense faulting, when the rocks involved in faulting are soft.The difference between the fault gouge and Mylonite is the grain size. The mylonite is much fine grained and much powdery as compared to the fault gouge.