Brief notes on the Classification of faults

There are two type of classification of faults:

1. Geometric classification.

2. Genetic classification.

1. Geometric classification:

This classification is strictly based on the attitude of the faults. There are five bases of geometric classification, which are as

(i) The rake of the net slip,

(ii) The attitude of the fault relative to the attitude of the adjacent rocks,

(iii) The patterns of faults,

(iv) The angle at which the fault dips, and

(v) The apparent movement on the fault.

(i) Rake of the net-slip:

On this basis folds are classified as

(a) Strike slip fault:

Where the net slip is parallel to the fault and rake of the net slip is equal to zero.

(b) Dip slip fault:

Here the net slip is equal to the dip-slip. Rake of the net-slip is therefore 90°.

(c) Diagonal-slip fault:

Where there is both a strike-slip and dip-slip component and rake of the net slip is more than ‘0°’ but less than ’90°’.

(ii) Attitude of the fault:

Six-types of faults have been recog­nized on this basis, which are

(a) Strike-fault:

Where strike of the fault is parallel to the strike of the rock-beds forming the country.

(b) Dip-fault:

Where the strike of the fault is parallel to the dip of the country rocks.

(c) Diagonal fault:

It is also known as oblique fault, which strikes diagonally to the strike of the adjacent rocks.

(d) Bedding fault:

In this case the fault plane is parallel to me bedding planes of the adjacent rocks.

(e) Longitudinal fault:

Here the fault strikes parallel to the strike of the regional structure.

(f) Transverse fault:

It strikes perpendicularly or diagonally to strike the regional structure.

(iii) Fault Pattern:

On this basis the following types of faults nave been recognised:

(a) Parallel faults:

It consists of a series of faults having the dip and strike.

(b) Step-faults:

If in a series of parallel faults the successive blocks are down-thrown more and more towards a particular direc­tion, the resulting structure will be a step-fault.

(c) Arcuate fault:

These are also known as peripheral faults which have circular or arc-like out-crop on a level surface.

(d) Radial faults:

Here a number of faults belonging to the same system, radiate out from a point.

(e) Enechelon faults:

There are relatively short faults which overlap each other.

(iv) On the basis of dip value:

Two important types of faults have been recognised on this basis. They are as

(a) High-angel fault:

Where dip amount is more than 45°.

(b) Low-angle fault:

These faults dip less than 45°.

(v) Apparent-movement:

On this basis faults can be classified info:

(a) Normal faults:

Which are inclined faults in which the hanging-wall side appears to have moved relatively downwards in comparison to the adjoining foot-wall side?

(b) Reverse faults:

In this case the foot-wall side appears to have been shifted downwards in comparison to the hanging walls.

Besides the above types, there is an important type of fault, known as the pivot or scissor fault or hinge-fault. In this case one block appears to have rotated about a point on the fault plane such that for part of its length the fault, is normal with a decreasing throw and for the remainder of its length is a reverse fault with an increas­ing throw. The position of no displacement being the point around which rotation appears to have taken place.

2. Genetic classification:

It is well known that along the shear fractures the displacement is parallel to the walls and there is no movement perpendicular to the fracture. It is assumed that the displacement is caused by some stresses. Three types of principal stresses have been assumed.

Of the three principal stresses two are horizontal and the third one is vertical and due to gravity alone. Three sets of conditions in which all the stresses compressional may arise accordingly three sets of faults originate. Besides, this classification considers whether the forces are compressional, tensional, shearing or torsional.

Three main types of faults have been recognized basing on the orientation of the three principal stresses.

(a) Normal faults. In this case,

(1) Maximum stress -Vertical (M₁)

(2) Mean stress -Horizontal (M₂)

(3) Minimum stress -Horizontal (M₃)

Here the hanging wall has moved relatively downward, it is also known as ‘gravity fault’ as well as ‘tensional faults’. In this category, we find Horst and Graben, Dip-slip fault, Antithetic and Synthetic faults, parallel, bedding, and step-faults, etc.

(b) Strike-slip fault. In this case,

(1) Maximum stress -Horizontal (M₁)

(2) Minimum stress -Horizontal (M₃)

(3) Mean stress -Vertical (M₂)

Here the displacement remains essentially parallel to the strike of the fault. These are also known as transcurrent, transform, wrench as well as tear faults.

This category includes two types of faults; in one case, the strike of the fault is transverse to the strike of the country rock but displacement is along the strike of the fault plane, where it is know n as tear fault; in the other case the strike of the fault plane is parallel to the strike of the adjacent rocks (but the net slip is along the strike of the fault plane), and this is known as rift-fault.

(c) Thrust fault. Here,

(1) Maximum stress -Horizontal (M₁)

(2) Mean stress -Horizontal (M₂)

(3) Minimum stress -Vertical (M₃)

In this case, the hanging wall moves relatively over the foot- wall. It includes the following types of faults:

(a) Reverse fault:

Where the dip of the fault is more than 45°.

(b) Thrust foul:

Where the fault has a dip less than 45°.

Thrusts are again subdivided into two types:

(i) Over thrust:

In which the initial dip is 10° or less and the net-slip is measured in terms of miles.

(ii) Under thrust:

In this case, the foot-wall side actually moved and pushed itself underneath the hanging-wall side.

(c) Nappes.

(d) Imbricate or Schuppen structures, etc.

Schuppen structure:

When several thrust planes develop in Parallel sets, a series of high angle reverse-faults may also develop between pairs of thrust planes, giving rise to what is known as imbricate or schuppen structure’.

Recognition of faults in the field and their effects on outcrops. To recognise faults in the field, a number of criteria are used. The faults may be directly seen in the field, particularly in artificial exposures such as river-cuttings, road cuttings, etc. But in majority of cases, faults are recognised by stratigraphic and physiographic evidences as

1. Discontinuity of structures.

2. Repetition or omission of strata.

3. Silicification and mineralisation.

4. Features characteristic of fault-planes.

5. Sudden change in the sedimentary facies.

6. Physiographic evidences, etc.

1. Discontinuity of structure:

If there is an abrupt termina­tion of any geologic structure, a fault can be expected near about the point of termination. Sudden termination of dykes and veins, etc. also suggests the existence of a fault.

2. Repetition and omission strata:

Sometimes a bed may suddenly terminate but occurs again somewhat off from the place where it terminated besides sometimes rock beds forming the country are found to be repeated and/or omitted (but maintaining the same order) indicates the occurrence of a fault.

3. Silicification and mineralization:

Faults are often the avenues for moving solutions. The solution may replace the country rock with fine-grained quartz, causing silicification and sometimes also they form mineral deposits at that site. It points to the occur­rence of a fault in that area.

4. Features characteristic of fault plane:

These features are produced due to friction of the blocks on either side of the fault plane and include features like slickenside, grooves, drag or fluccan (i.e., pulverized clayey matter), breccia (commorly known as crush breccia), mylonites, horses and slices etc.

5. Difference in sedimentary facies:

Different sedimentary facies of rocks of the same age may be brought into juxtaposition by large horizontal displacements, is suggestive of faulting.

6. Physiographic evidences:

The ‘effects of faulting on outcrops’ constitute the physiographic evidences for faulting. They include features like:

(i) Offset ridges, (ii) Fault scrap, (iii) Piedmont scrap, (iv) Triangular facets, (v) Offset streams, (vi) Springs following a straight course (sometimes springs aligned along the fault planes also), (vii) Lineament suggest the presence of faults in the field (viii) Alluvial fan, (ix) Monocline, etc.