Folds: Meaning, Parts and Causes | Rocks | Geology

In this article we will discuss about:- 1. Meaning of Fold 2. Parts of a Fold 3. Examples 4. Causes 5. Engineering Considerations.

Meaning of Fold: 

Folds may be defined as undulations or bends or curvatures developed in the rocks of the crust as a result of stresses to which these rocks have been subjected from time to time in the past history of the Earth.

The folds may develop in any type of rock and may be of any shape and geometry ranging from simple up arched bends or downward curvatures to completely overturned flexures. The ultimate shape and extent of a fold depends upon a number of factors like the nature, magnitude and the direction of and duration for which these forces act upon the rocks and also the nature of the rocks being affected.

The process of development of folds in the rocks is called Folding. It is a very slow geological process and indicates an effort of the rocks in a particular environment to adjust themselves to the changing force fields operating on, within or around them. In general, Inkling is a ductile type of deformation experienced by the rocks compared to the brittle deformation where the rocks actually get broken and displaced when stressed.

Parts of a Fold:

A number of terms are used to describe the morphology or shape of a fold.

Some of the most common terms are explained below:

(i) Limbs:

These are the sides or flanks of a fold. An individual fold will have a minimum of two limbs but when the folds occur in groups, as they very often do, a middle limb will be common to two adjacent folds.

(ii) Hinge, Axial Surface and Axial Plane:

In a folded layer, a point can be found where curvature is maximum and one limb ends and the other limb starts from that point. This is the hinge point. When rocks occur in a sequence and their all hinge points are joined together, they make a line, called the hinge line. When the hinge line is traced throughout the depth of a folded sequence (i.e. a group of rocks lying one above another and having suffered the folding together), a surface is obtained which may be planar or non-planar.

It is referred to as axial surface, if it is non-planar (i.e. is irregular) and as axial plane, if it is of a planar nature. In other words, an axial plane is that imaginary plane that passes through all the points of maximum curvature in a folded sequence. It may be vertical, inclined or horizontal in nature. A fold is sometimes called a planar fold if the axial surface is planar in nature; otherwise it is a non-planar fold.

(iii) Axis of a Fold:

It is simply defined as a line drawn parallel to the hinge line of a fold. A more precise definition of an axis of a fold would be the line representing the intersection of the axial plane of a fold with any bed of the fold. When a folded sequence is made up of a number of layers, it will have a number of intersections with the axial plane, or, in other words, equal number of axes. But as all these axes are essentially parallel, one of them will be taken to represent the general trend and hence described as axis of the fold. Like the axial plane, the axis of the fold may be horizontal, inclined or vertical.

(iv) Plunge of a Fold:

The axis of a fold as defined above may be horizontal, inclined or even vertical in its geometrical position with respect to other parts of the fold. The angle of inclination of the fold axis with the horizontal as measured in a vertical plane is termed the plunge of the fold.

In other words, axis is a line and plunge is the angle which this line makes with a horizontal plane. A fold having a horizontal axis will obviously have a zero plunge. The plunge is measured just like dip of a bedding plane, both in terms of direction of plunge and degree of plunge.

(v) Crest and Trough:

Most folds are variations of two general forms; uparched and downarched bends. The line running through the highest points in an uparched fold defines its crest. A corresponding line running through the lowest point in a downarched fold makes its trough. The crest and trough may or may not coincide with the axis of the fold.

Examples of Folds:

FOLDS form an important feature of many mountain systems of the world. These mountains are sometimes referred as folded mountains. In India, the Himalayas show folding of all types on a large scale. Many major anticlines, synclines, overfolds and thrusts have been recognised and mapped at numerous places in different regions of the Himalayas extending from Kashmir in the North West to Assam in the south east. Many typical examples of folds may be found from the stratigraphy of the country.

The Liddar Valley Anticline in Kashmir Himalayas is often represented as a classic example in that almost complete sequence of Paleozoic Era (not so well preserved in any other part of the country) is fully developed in this anticline. In fact, at places, denudation has exposed some of the outcrops of this anticline in such a way that they form the most valuable places for recovering fossils of the Paleozoic times.

Among the Peninsular Mountains, only the Vindhyan and the Satpura ranges show folding in a prominent manner.

Causes of Folding:

Folding may be either due to tectonic causes or due to non-tectonic causes. By tectonic causes is understood folding taking place as a response of the rocks to various forces originating from within the body of the Earth. The non-tectonic folding is bending or warping of rocks due very conspicuously to superficial processes. It must be mentioned, however, that it is the tectonic folding, which is of main concern in all geological investigations.

The tectonic folding may be due to any one or more of the following mechanisms:

(a) Folding due to Tangential Compression:

Lateral Compression is believed to be the main cause for throwing the rocks of the crust into different types of folds depending upon the types of rocks involved in the process and also the direction and magnitude of the compression affecting those rocks. In general, this primary force is believed to act at right angles to the trend of folds.

Under the influence of the tangential stresses, folding may develop in any of the three ways:

(i) Flexural folding,

(ii) Flowage folding and

(iii) Shear folding.

(i) Flexural Folding:

It is that process of folding in which the competent or stronger rocks are thrown into folds due to their sliding against each other under the influence of lateral compression. This is also distinguished as flexural-slip-folding in which the slip or movement of the strata involved takes place parallel to the bedding planes of the layers.

It has been established that in flexural folding, the amount of slip (and hence the ultimate type of fold) depends on a number of factors such as:

I. Thickness of the layers and nature of the contact; thicker the layers, greater is the slip; further, cohesion less contacts favour easy and greater slips;

II. Distance from the hinge point; greater the distance from the hinge points, larger is the displacement, so much so that it may be negligible at the hinge point;

III. Type of the rocks involved; siltstones, sandstones and limestones are more prone to flexure slip folding compared to soft clays and shales.

(ii) Flowage Folding:

It is the principal process of folding in incompetent or weaker, plastic types of rocks such as clays, shales, gypsum and rock salt etc. During the compression, the material of the involved layers behaves almost as a viscous or plastic mass and gets buckled up and deformed at varying rates suffering unequal distortion. In such cases the thickness of the resulting fold does not remain uniform.

(iii) Shear Folding:

In many cases, folding is attributed to shearing stresses rather than simple compression. It is assumed that in such a process, numerous closely spaced fractures develop in the rock at the first stage of the process. This is followed by displacement of the blocks so developed by different amounts so that ultimately the rocks take up folded or bent configuration. The folded outline becomes more conspicuous when the minor fractures get sealed up due to subsequent recrystallization.

The ULTIMATE CAUSE of most of the folds is now attributed to the movement of tectonic plates. It is believed by many that where two tectonic plates converge, the margins of the plates are buckled and warped, that is, are thrown into folds. The same process of folding may take place in the margins of the upper plate where two tectonic plates approach each other and instead of simply converging, one plate subducts, i.e. goes down under the other plate.

The converging plates may be:

i. Two continental plates;

ii. A continental and an oceanic plate;

iii. A continental plate and an island arc.

Plate tectonics offers sufficiently convincing examples from the present and the past distribution of folded mountain belts on the globe. Hence, development of folding through plate movements is accepted almost universally as a valid cause.

(b) Folding due to Intrusions:

Intrusion of magma or even rock salt bodies from beneath has been found to be the cause of uparching of the overlying strata. In magmatic intrusions, highly viscous magma may be forced up very gradually and with considerable force so that the overlying sedimentary host rocks are bodily lifted up to provide space for the rising magma. In extreme cases, the magma may even rupture the overlying strata to flow out as lava.

(c) Folding due to Differential Compression:

Strata that are being compacted under load in a basin of sedimentation develop, with passage of time, downward bending especially in the zones of maximum loading. If the strata in question are not homogeneous, the bending may not be uniform in character and results in warping or folding of different types.

Such folds are, however, totally dependent on the load from above and are attributed to superficial causes. These are, therefore, non-tectonic folds. Similarly, downslope buckling of the layers of rocks underlying slowly failing or creeping slopes may be grouped under non- tectonic folding.

Engineering Considerations for Folds:

Folds developed in the areas of work are important for a civil engineer in that these make more complicated. If these structures are not thoroughly investigated and properly interpreted, any civil engineering project standing on or driven through the folded rocks may prove not only uneconomical in the ultimate analysis but also, unsafe as well.

Due consideration is, therefore, always to be given to the presence of folds in deciding about the designing and construction of such structures as driving of traffic and hydropower tunnels, selection of sites for dams and reservoirs and in fixing the alignments of roads, bridges and highways.

Presently, we may summarize the general effects of folds on major civil engineering projects:

(i) Change in Attitude:

Folding of any type would cause a change in the attitude (dip and strike) of the same strata in the aerial extent and also in depth. Hence same layers may be repeated along an alignment or one or more different layers may be unexpectedly encountered. If it happens so and the unexpectedly repeated or encountered layers are of undesirable nature, the project costs may be affected as also the time schedule and safety of the project.

(ii) Shattering of Rocks:

We should remember that folding is the response of the rocks to the stresses induced during the process. These stresses are often strong enough to break or shatter the rocks, especially in the axial zones, which are the places of maximum concentration of these forces. Hence, in folded rocks, axial regions are likely to be the areas containing fractured zones.

This effect is of utmost importance because shattered rocks become:

I. Weak in strength parameters of all types;

II. Porous and pervious in character;

Such areas of the folded rocks cannot be trusted as roofs and floors in tunnels or foundation sites for dams. Axial regions in the folded rocks should be thoroughly studied and if possible, should be avoided for other better alignments or sites as the case may be. If it is not possible to avoid them, these areas must be subjected to suitable processes of rock treatment for developing in them desired qualities of strength and imperviousness.

(iii) Strained Nature:

All the stresses that have acted on the rocks during their folding are generally absorbed by these rocks by undergoing strain. In essence, the folded rocks are considerably strained, the magnitude of strain varying from point to point in the folded sequence. Now, as and when there is an effort by nature or by the engineer to disturb this adjustment of the rocks to the stresses, the rock may respond by release of some strain energy.

This is what often happens when tunnels are excavated through the folded regions. Enough stored strain energy is released as soon as (or soon after) the excavations are made and huge blocks of rocks start caving in or falling with great force called the rock bursts. This often involves fatal accidents besides causing considerable delay in the progress of the work. A proper planning of the work in folded areas is, therefore, of utmost importance to avoid these possible hazards in construction work.