Structural Geology: Folds
Ø 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.
Development of folds
Ø 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.
Ø 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 effected.
Ø The process of development of folds in the rocks is called Folding.
Ø 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.
Ø Folding 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.
1 PARTS OF A FOLD
Ø 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.
Ø In a folded layer, a p oint can be found where curvature is maximum and one limb ends and the other li mb starts from that point. This is the hinge poin t.
Ø When rocks occur in a sequence and their all hinge points are joined together, they make a line, called the hinge line.
Ø When the hinge line is traced throughout the depth of a folded sequence a surface is obtained which may be planar or non-planar. It is referred to as axial surface
Ø Axial plane is the im aginary plane that passes through all the points of maximum curvature inclined or horizontal in nature.
Ø A fold surface is plan ar in nature; otherwise it in a folded sequence.
Ø It may be vertical, is sometimes called a planar fold if the axial is a n on-planar fold.
Axis of a fold
Ø It is simply defined a s a line drawn parallel to the hinge line of a fold .
Ø A more precise defin ition of an axis of a fold would be the line repr esenting the intersection of the ax ial plane of a fold with any bed of the fold.
Plunge of a fold
Ø The angle of inclination of the fold axis with the horizontal as meas ured in a vertical plane is term ed the plunge of the fold.
Crest and Trough.
Ø Most folds are variations of two general forms; uparched and downarched bends. The line running thr ough the highest points in an uparched fold defi nes its crest.
Ø A corresponding lin e running through the lowest point in a down arched fold makes its trough. The crest and trough may or may not coincide wit h the axis of the fold.
2.CLASSIFICATION OF FOLDS Anticlines are defined as tho se folds in which
the strata are uparch ed, that is, these become CONVEX UPWARDS ;
the geologically olde r rocks occupy a position in the interior of the fold, oldest being positioned at th e core of the fold and the youngest forming the outermost flank,
the limbs dip away from each other at the crest in the simplest cases.
the strata are downar ched, that is, these become CONVEX DOWNWARDS;
the geologically younger rocks occupy a position in the core of th e fold and the older rocks form the outer flanks, provided the normal order of superposition is not d isturbed,
in the simplest cases in synclines, the limbs dip towards a common c enter.
Position of Axial Plane
Depending upon the nature a nd direction of the stresses the axial plane in a r esulting fold may acquire any position in space, that is, it may be vertical, inclined or eve n horizontal. Following main types are re cognized on the basis of position of the axial plane in the resulting fold:
Ø These are also called normal or upright folds. In such a fold, the axia l plane is essentially vertical.
Ø The limbs are equal i n length and dip equally in opposite directions.
it may be an anticline or syncline and when classified, may be described as symmetrical anticline / syncline as the case may be.
All those folds, anticlines or synclines, in which the limbs are unequ al in length and these dip uneq ually on ether side from the hinge line are termed as asymmetrical folds.
Ø These are folds with inclined axial planes in which both the limbs are dipping essentially in the same general direction.
Ø The amount of dip of the two limbs may or may not be the same.
Ø Overfolding indicates very severe degree of folding.
Ø One of the two limbs (the reversed limb) comes to occupy the present position after having suffered a rotation through more than 90 degrees.
The other limb is known as the normal limb.
Ø In certain cases, both the limbs of a fold may get overturned because of very high lateral compression.
Ø It may be originally either an anticline or a syncline but the extreme compression from opposite sides results in bringing the limbs so close to each other that the usual dip conditions may get reversed -anticlinal limbs dip towards each other and the synclinal limbs dip away from each other.
Ø Such a type of fold is commonly referred to as a fan fold
Ø In such folds, the anticlinal tops are said to have opened up into a broad, fan-shaped outline due to intense compression in the lower region.
Ø These are group of folds in which all the axial planes are essentially parallel, meaning. that all the component limbs are dipping at equal amounts.
Ø They may be made up of series of anticlines and synclines
Ø These may be described as extreme types of overturned folds in which the axial plane acquires an almost horizontal attitude.
Ø In such folds, one limb comes to lie exactly under the other limb so that a drill hole dug at the surface in the upper limb passes through the lower limb also.
Ø The lower limb is often called the inverted limb or the reversed limb.
Ø Other parts of a recumbent fold are sometimes named as follows:
the arch, which is zone of curvature corresponding to crest and trough in the upright folds;
the shell, which is the outer zone made up mostly of sedimentary formations;
the core, which is the innermost part of the fold and maybe made mostly of crystalline igneous or metamorphic rocks;
the root or the root zone, which is the basal part of the fold and may or may not be easily traceable; once traced it can throw light whether the fold was originally an anticline or syncline that has suffered further inversion.
In certain cases a pair of folds that are apparently related to each other may have mutually inclined axial planes.
Such folds are described as conjugate folds.
The individual folds themselves may be anticlinal or synchnal or their modifications.
It may be described as a special type of fold with exceptionally flattened top and steeply inclined limbs almost forming three sides of a rectangle.
Ø In both the anticlinorium and synclinorium, presence of large number of secondary folds, faults and fracture systems is a characteristic feature.
Ø Similar folding but signifying still larger bending and uplifting of strata on sub-continental scales is expressed by the terms GEANTICLINES AND GEOSYNCLINES respectively.
Ø Great importance is attached to the major depressions, the geosynclines, in the process of mountain building discussed elsewhere.
Ø The geosynclines are believed to serve as depositional fields or basins of sedimentation to which sediments derived by the erosion of the adjoining gentilities get accumulated and compacted.
Ø This material is then compressed and uplifted in the second stage of orogeny, to gradually take the shape of mountain systems.
3 CAUSES OF FOLDING
The Tectonic Folding may be due to any one or more of the following mechanisms:
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 effecting 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 either of the three ways: flexural folding, flowage folding and shear 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 o r movement of the strata involved takes place parallel to the bedding planes of the layers.
It has been establis hed that in flexural folding, the amount of slip (and hence the ultimate type of fold) depends on a number of factors such as:
thickness o f the layers and nature of the contact; thick er the layers, greater is the slip; further, cohesionless contacts favour easy and greater slips;
distance fr om the hinge point; greater the distance from the hinge points, larger is the
displacement, so much so that it may be negligible at the hin ge point;
type of the rocks involved; siltstones, sandstones and li mestones are more prone t o flexure slip folding compared to soft clays and shales.
Ø It is t he principal process of folding in incompetent or weaker, plasti c type of rocks such as clays, shales, gypsum an d rock salt etc.
Ø Duri ng the compression, the material of the involved layers behaves almost as a viscous or plastic mass and gets buckled up and d eformed at varying rates suffering unequal disto rtion.
Ø In such cases the thickness of the resulting fold does not remain unifor m.
Ø In many cases, folding is attributed to shearing stresses rather than simple compression.
Ø It is assumed that in such a process, numerous closely spaced fractures develop in the rock at the first stage of the process.
Ø This is followed by displacement of the blocks so developed by different amounts so that ultimately the rocks take up folded or bent configuration.
Ø The folded outline becomes more conspicuous when the minor fractures get sealed up due to subsequent recrystallisation.
Folding Due to lnsrusions
Ø Intrusion of magma or even rock salt bodies from beneath has been found to be the cause of uparching of the overlying strata.
Ø 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.
Ø In extreme cases, the magma may even rupture the overlying strata to flow out as lava
Folding Due to Differential Compression
Ø 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.
Ø If the strata in question is not homogeneous, the bending may not be uniform in character and results in warping or folding of different types.
Ø Such folds are, however, totally dependent on the load from above and are attributed to superficial causes.
Ø These are, therefore, non- tectonic folds.
Ø Folds developed in the areas of work are important for a civil engineer in that these make his work more complicated.
Ø 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.
Ø 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.
Change in Attitude
1 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.
2 Hence same layers may be repeated along an alignment or one or more different layers may be unexpectedly encountered.
3 If it happens so and the unexpectedly repeated or encountered layers are of undesirable nature, the project costs may be effected as also the time schedule and safety of the project.
Shattering of Rocks.
1 The 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.
2 hence, in folded rocks, axial regions are likely to be the areas containing fractured zones.
3 This effect is of utmost importance because shattered rocks become:
• weak in strength parameters of all types;
• porous and pervious in character;
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.
1 All the stresses that have acted on the rocks during their folding are generally absorbed by these rocks by undergoing strain.
2 In essence, the folded rocks are considerably strained, the magnitude of strain varying from point to point in the folded sequence.
3 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.
4 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.
5 This often involves fatal accidents besides causing considerable delay in the progress of the work.
6 A proper planning of the work in folded areas is, therefore, of utmost importance to avoid these possible hazards in construction work.