Faults: Meaning, Causes and Effects | Rocks | Geology (2024)

In this article we will discuss about:- 1. Meaning of Faults 2. Causes of Faulting 3. Effects 4. Recognition 5. Engineering Considerations 6. Examples.

Meaning of Faults:

Under the influence of stresses developing from within the Earth, the rock masses adjust themselves either by bending, when they lie deep below the surface (in zone of flowage) or by fracturing with or without any accompanying displacement, in the upper depths (the zone of fracturing). Fracturing is favoured when the stresses are shearing in nature and the rocks are brittle in character. It normally occurs when the shearing strength of the rocks is overcome by the operating shearing stresses.

Those fractures along which there has been relative movement of the blocks past each other are termed as FAULTS. The entire process of development of fractures and displacement of the blocks against each other is termed as FAULTING.

The key words in this definition are- fracture and movement. The exact significance of these key words must be clearly understood at this stage- Fault is always a crack or surface of rupture or a simple fracture surface or a zone having numerous closely spaced fractures that has to be present in the rock; it may be pre-existing or may develop instantaneously just before the movement of the parts on the opposite side of the fracture takes place. There can be no fault if there is no fracture surface or zone.

Similarly, unless there is evidence of clear movement of the blocks created on either side of the fracture, the fracture will not be considered as a fault: it may be a simple fracture or a crack or a joint or a fissure. Evidence of some relative movement of the blocks against each other is a must for qualifying that fracture as a fault.

Faulting is a major tectonic process of great geological importance. The geological history of the Earth bears innumerable events recorded in the script of folding, faulting and jointing that make it most interesting and challenging for correct reading and interpretation.

Thus, faulting on local scale may produce faults with relative displacement as little as a fraction of centimeter, whereas faulting on large scale, often at regional scales may involve displacements along them measured in meters, tens of meters, hundreds of meters and even hundreds of kilometers. Further, the displacement of blocks created in the rock due to faulting may take place in any direction: parallel to the fault surface; in an inclined manner or even rotational.

A highly simplified description of development of faults is explained through Fig. 7.1. A sequence of three layers of rocks occurring somewhere within the earth comes under the influence of stresses (A), which produces a fracture ff₁ dividing the original layers into two distinct parts – a₁ and a₂.

Under the influence of the same stresses or others developing subsequently, the block a₂ is moved down-slope; (B). The result is shown in Fig. 7.1C. This fracture, ff₁, along which there has been a relative displacement of the two blocks, a₁ and a₂ is a FAULT. There are some other fractures also in the blocks such as SF, but they are NOT faults, because there has been no movement along them.

Causes of Faulting:

Faults are generally caused under the influence of stresses acting upon the rocks of the crust of the earth from within. Any rock on or below the crust may withstand all the operating stresses up to a limit, which depends upon its cohesive strength and internal friction. But when that limit is crossed by the operating stresses, the rock yields by fracturing or breaking along certain directions.

Immediately (or even gradually) after the development of these fractures, the blocks created along the fractures suffer sudden (or gradual) displacement along those fractures under the influence of the same (or different) stresses that caused the fracturing of the rocks at the first place. The displacement may take place essentially along the fracture surface or in different directions and for different distances depending upon the magnitude of the operating stresses thus giving rise to different types of faults.

In a highly oversimplified situation, the type of fault likely to form is related to a stress field operating in a given area. Thus, talking in terms of the three principal stresses, normal faults would form when σ₁, the maximum stress is vertical. Similarly, strike-slip faults form when σ₂, the intermediate stress is vertical and reverse or thrust faults form in situations where in the stress field, the minimum stress direction that is σ₃ comes to occupy the vertical direction. (Fig. 7.24) In all these idealized situations, it is assumed that the rocks are isotropic in character and the Mohr-Coulomb Law of rock failure holds good in those cases.

As to the source of these forces that are responsible for causing faults of great dimensions in the crust, in many cases even for several hundred kilometers, it may be said that this has been one of the most baffling problem in Geology. Many theories were put forward and supported by many and opposed by others.

According to one view, the shrinkage experienced by the crust of the earth due to its gradual cooling from an originally hot molten state is primarily responsible for the tension developing in the crust. Another view is that hot molten material existing below the surface of the earth is in a state of movement governed principally by convection currents. These currents are created due to the flow of heat from the lower, hotter part of the earth and partly from radioactive sources.

What is relevant here about the convection currents is that they exert dragging effect at places below the crust where they diverge causing tension and compressive effect at other places where the currents happen to converge. This gives rise to stresses of different kinds that are ultimately responsible for throwing the rocks above into folds and faults of great variety.

Gravity or normal faults are believed to be caused under the influence of horizontal tension whereas thrust faults are the result of compressive forces that may throw the rocks into severe type of folding before actual development of faults.

The thrusts associated with folding often develop when:

(a) A fracture is formed parallel to the axial plane of a fold where the shearing strength of the beds is overcome by the shearing stresses responsible for the development of the fold;

(b) One of the limbs (generally the upper limb in a recumbent fold) is displaced by the same shearing forces that are associated with the folding to a considerable distance.

Plate Tectonics:

The concept of plate tectonics is the latest attempt in describing the origin of all the forms of crustal deformations, including folds and faults. It has satisfactorily explained the causes leading to crustal deformation by establishing linkage of these deformations to the almost imperceptible movement of the huge crustal blocks called the tectonic plates, over the top layer of the mantle.

Effects of Faulting:

Faulting is essentially a process of rupturing and displacement along the plane of rupture.

Its effect may involve:

i. Changes in the elevation of the ground,

ii. Omission of some strata where they are normally expected,

iii. Repetition of some strata in a given direction against the normal order of superposition, and,

iv. Displacements and shifts in the continuity of the same rocks in certain regions.

In faults of some magnitude, it needs lot of fieldwork involving extensive mapping on the exposed outcrops and also geophysical measurements for establishing contacts of different types of rocks. It is only from the study of geological maps that the existence of faults at the first place and their effects on the rocks may get established with some certainty.

Further, the features produced due to faulting on the ground are subject to modifications by the subaerial processes of weathering and erosion with the passage of time. Hence what we describe today as the effects of faults may be, in fact, the effects of faults as modified by erosion and weathering.

We shall describe below, only in bare outline, the effects of some types of faults on outcrops and on topography.

1. On Outcrops:

Faults invariably change the original position of the outcrops traversed by them. These changes depend primarily on the type of the fault, the attitude of the fault, and the nature and attitude of the disrupted rock. Thus, effects produced by strike-slip fault shall differ markedly from those produced on the same rock by a dip-slip fault and so on.

The effects described below are those that would be seen when the displacement has been followed by extensive erosion to the extent of levelling off (assumed level of erosion-ALE) of the up thrown side:

(A) Strike Faults:

Strike faults are those, which are developed parallel to the strike of the outcrops. These faults produce, besides other changes, two pronounced effects on the outcrops- repetition and omission of strata.

Repetition of the strata occurs when the downthrow is against the direction of the dip of the bed in which faulting has taken place.

Omission of the strata takes place in a strike fault when the downthrow is parallel to the direction of the dip of the faulted bed.

(B) Dip Faults:

In dip faults which occur parallel to the dip of the outcrop, the most prominent effect observed after faulting and erosion of the upthrown block is a horizontal shift between the two parts of the outcrop.

(C) Oblique Faults:

These faults cause an offset in the sequence, which is associated with either a gap or an overlap depending upon the downthrow direction.

Thus:

(i) Oblique faults with downthrow to the left side result in an offset with an overlap;

(ii) Oblique faults with downthrow to the right side result in an offset with a gap.

Effects on Folded Strata:

The effects of faults on different types of folded sequence are broadly the same as in plainly dipping strata. But with the changes in the attitude of the faults or that of rocks, quite complicated results may be seen. In fig. 7.22 effects of a dip-fault on a folded sequence comprising simple anticlines (A) and synclines (S) has been shown.

In Fig. 7.22(a), a single layer bent as anticline (A) and syncline (S) is disrupted by a normal fault, the front portion going down with respect to the rear portion that stands elevated. This is shown in Fig. 7.22(b). It can be clearly seen that the anticlinal part on surface appears quite restricted compared to synclinal part. However, after prolonged erosion on the upstanding block, as represented in Fig. 7.22(c), the anticlinal limbs stand separated much more than the synclinal limbs which now stand closer.

Students are advised to draw different types of strata in different attitudes and also in folded forms in block diagrams and then see effects of different types of faults on the displaced and eroded blocks.

2. On Topography:

One of the main effects of the faults on topography is that they very often result in the development of distinct types of steep slopes which are aptly called fault scarps. Three types of fault associated scarps are often recognized- fault scarps, fault-line scarps and composite-fault scarps.

In fault scarps, the relief is developed due to downward slip along the fault surface. In the fault-line scarps, however, the slope relief is produced due to process of unequal erosion along the fault line with the passage of time. When a given slope is believed to be the result of both of these processes, the scarp is of a composite type.

Besides fault scarps, faulting is also responsible for development of Block Mountains like horsts and deep elongated valleys called the grabens and the rift valleys. Faults are also known to cause deflection in the course of streams. Similarly, in certain regions, a number of springs may come into being along a fault line that happens to cut across an aquifer. These aligned springs may often prove to be an important evidence of faulting in the region.

Recognition of Faults:

Recognition of faults on the ground very often demands a thorough and systematic geological and very often geophysical study of the area, often to considerable depth. The evidence of faulting lies in their effects on the faulted rocks in particular and on the topography of the area in general. It is only on the basis of geological maps prepared in the area and recording of such evidence that the nature and type of fault as also its extent can be established.

Very common and characteristic field evidence of faulting is summarized as follows:

(a) Presence of Slickensides etc.:

An exposed or covered surface may be suspected of being a fault surface if it is polished, and carries grooves and striations. Similarly, presence of brecciated zone and/or sheared, mylonized material at the base of a slope (fault scarp) indicate the possibility of a fault.

(b) Abnormal Behaviour of Strata:

In any region made up of stratified rocks, a normal order of superposition is to be expected under normal conditions. But when the region is suspected to have undergone tectonic deformation, many abnormalities in the sequence may be observed.

The following abnormalities may be indicative of faulting:

(i) Abrupt Termination:

A group of beds or some veins or dykes may abruptly terminate along a surface in a given region. This may generally be due to breaking of-the strata into blocks and movement of the disrupted blocks away from each other. Sometimes the two displaced parts are easily visible and at other times their previous continuity can be established after some field work. The evidence is then conclusive.

(ii) Repetition and Omission:

When in the field the same layer or rock is encountered more than once in a certain section, that is, it is repeated in space, faulting is indicated. Similarly, omission of certain beds in some directions as proved by thorough study of stratigraphy of the region is also indicative of faulting.

(iii) Offset etc.:

Disruption of the beds due to faulting generally results in their displacement, which may be determined in terms of slip, separation, offset and gap etc. When any one or more such features of displacement are observed, faulting is indicated.

(c) Physiographic Evidence:

Some physiographic features may serve as indirect evidence of faults.

Among them, the most important are:

(i) Aligned Springs:

In some regions, a number of springs may occur along the base of a slope almost in the same line. The existence of a fault along the spring line is strongly suggested.

(ii) Offset Streams:

Sometimes streams may show an interruption or offset or break in their profile at some places. The offset in the course of a stream is among the possible effects of faulting and should be taken as an evidence for the same.

Engineering Considerations of Faults:

Faults are important for a civil engineer in that these mark the sites where dislocation of the ground has occurred in the past and where such dislocations cannot be entirely ruled out in future. It is the dislocation aspect, which may introduce considerable complications in the site for a proposed engineering project. This may necessitate thorough investigations for the stability of the intended project.

The engineer has to consider faults and faulting from three angles:

Firstly:

What have been the effects of faulting on the rocks of the region in general and that on the proposed site in particular? Especially, how far the rocks of the site would be suitable from strength point of view as foundations or abutments or roof as the case may be.

Secondly:

If the effect of the past faulting is such that the rocks have not been rendered practically useless, or in other words, structures can still be designed and built on them with or without some treatment, then, will these proposed structures be safe in future? Is there no probability of faulting again at the same site during the expected life span of the proposed civil engineering project?

Thirdly:

What factor of safety is essential to be adopted in the design and construction of the proposed structure if the site is faulted and there is no alternative available to it?

As regards the first consideration, that is, the influence of faults on the rocks, it has already been made clear that faults cause very much shearing and crushing of the rocks located along or near the fault surfaces and zones. These rocks become weak and unstable on the one hand and porous and permeable on the other hand.

Following general conclusions can be drawn:

(i) The faulted rocks will form weak foundations and abutments for dam, despite the fact that originally they might have been strong and impervious;

(ii) The shear and fault zones will serve as easy pathways for water and cause leakage when left untreated in dams and reservoir sites; in fact these may become source of great trouble when encountered along or across highway and tunnel alignments;

(iii) Once the fault zones, shear zones or fault surfaces become lubricated with water, they become potential areas for further slips and slides. They may create critical conditions if they happen to occur within the foundation or abutment zones of dams and reservoirs or in the roof and walls of the tunnels;

(iv) Faulting products like gouge and breccia create additional problems; the site has to be cleared of them or taken below to the sound bedrock.

Thus, solution of the problem of placing an engineering project in a region with suspected faulting may be analysed as under:

First:

The civil engineer has to ascertain for the presence of faults, their type and extent and also obtain thorough report from an engineering geologist of the effects caused by these faults on the rocks of the project area. The number, size and inclination of the shear zones should be given top consideration. Ideally, the proposed site for any major civil engineering project should be located as far away as possible from an active fault and never on active faults.

In many cases, faulting leaves the rocks only partially affected and there may not be much choice available for shifting the site of the project. The embankment of the Bhakra Dam in India showed occurrence of numerous shear zones in them; the site could not be changed because of other reasons; hence it was decided to treat the shear zones by extensive excavations of the sheared material and back filling with cement grouting. The embankments have withstood the test of the time. Similar treatments have been applied in many other projects as well in different countries.

Second:

The safety of a civil engineering structure built on or near a faulted rock can be ascertained only in a general way. The tectonic history of the area under consideration must be known thoroughly. Faults of any significance are always associated with earthquakes.

So, such a study would virtually mean obtaining information about frequency of the earthquakes as also their magnitude and effects that they have left from time to time on the rocks of the region. The exact position of the area of construction with respect to the seismic zoning of the country must be thoroughly established.

Third:

Even if the evidence collected from the study of the tectonic history of the area leads to the conclusion that no movement may be expected in the rocks of the area during the projected life span of the structure raised on them, some factor of safety must be introduced into the design of the structure, especially in the big projects in faulted areas, so that if the unexpected happens, there is minimum loss to the project.

In all big countries, maps of seismic classification are available. In most cases recommendations of the statutory authorities are available about introducing suitable factor of safety in major civil engineering projects of any public importance that are proposed to be constructed in areas of known seismic zones. These must be adhered to in letter and spirit.

Examples of Faults:

What has been said regarding folds is also true for faults? All the folded mountains of the world also show examples of various types of faults. Many basins and valleys (the tectonic valleys) are created by faults.

In Indian subcontinent, the Himalayas provide some classic examples of geological faulting. The Thrusts are most typical of these mountains.

A few examples are given below:

(a) The Kashmir Himalayas:

At least three thrusts are of wide development- the Murree thrust, the Punjal thrust and the Zanskar thrust. Of these, the Panjal thrust is regarded as the most severe- it is held responsible for the nappee zone of Kashmir which is assumed to have been thrust up along this nearly horizontal thrust.

(b) The Simla Thrust:

Conspicuous and well developed thrusts of the Simla Himalayas are- the Krol thrust which brings the KROL series against the lower Tertiary rocks of the area; the Giri thrust, which lies between the Krol series and the Simla slates and the Chail thrust, Juttogh thrust and the Shali thrust.

The Jutogh and the Chail thrust are responsible for bringing pre-Cambrian rocks of these names up and against the younger rocks of Carboniferous and Permian systems.

The various thrusts in the Simla Himalayas are believed to be due mostly to bodily displacement (from north to south west) of faulted recumbent folds.

(c) The Garhwal Himalayas:

The two major thrusts, the Krol thrust and the Garhwal thrust have resulted in superposition of two nappe zones in the region.

In the Tertiary Zone of the Outer Himalayas, a series of parallel faults showing essentially identical tectonic features have been studied. These faults are typically of the reverse types, which have developed in a folded sequence with the lower and older strata having been thrust up against the younger rocks.

Such a plane of contact between the older-tertiary is most conspicuous in the case of Siwalik rocks and has resulted everywhere:

(i) In reversing the normal order of superposition;

(ii) In producing throw of considerable magnitude, sometimes of the order of many meters.

This reverse-fault contact is termed in Indian Geology as Main Boundary Fault and extends throughout the extension of the outer Himalayas.