19 Structural Geology

Meenal Mishra

Structure

27.0 Objectives

 

27.1 Concept Map

27.2 Introduction

27.3 Relationship between Structural Geology and Tectonics

27.4 Basic Concepts in Structural Geology

27.5 Dip and Strike

27.6 Clinometer Compass

27.7 Measuring Strike and Dip of an Inclined Bed

 

27.8 Societal benefits of Studying Structural Geology

27.9 Rock Structures

27.9.1 Planar Structures

27.9.2 Linear Structures

27.9.3 Folds and Faults

27.10 Forms of Igneous Rocks

27.11 Summary

 

 

27.0 Objectives

 

  1. Discuss the concepts in structural geology
  2. Study relationship between structural geology and tectonics
  3. Examine the concept of dip and strike
  4. Describe societal benefits of studying structural geology
  5. List linear and planar rock structures
  6. Get introduced to the elements of folds and faults
  7. Discuss criteria of recognition of folds and faults in the field
  8. Use of clinometer compass in measuring dip and strike
  9. Classify the forms of igneous rocks

27.2 Introduction

 

Structural geology is the branch of geology that deals with the recognition, representation, and genetic interpretation of rock structures. It also involves the study of the forces which give rise to these structures. The term ‘Structural’ is derived from Latin word ‘Struere’ which means ‘to build’.

 

Now let us define what is structural geology?

 

Structural geology is the study of the architecture of rocks insofar as it has resulted from deformation (Billings, 1990).

 

Structural geology deals with the geometry, distribution and formation of structures (Fossen, 2010). Geologic structures or rock structures incorporate symmetry and geometric configuration of rocks present in the Earth’s crust on all scales. Geologic structures result from the deformation caused by the tectonic forces present in Earth, i.e., they are endogenic. The term tectonics is derived from Greek word ‘Tektos’ meaning ‘builder’. Tectonics is the study of the forces and motion that result in rock deformation and structure.

 

 

 

27.3 Relationship between Structural Geology and Tectonics

 

Let us get acquainted with the relation between structural geology and tectonics. Structural geology is mainly concerned with the rock geometry whereas, tectonics deals with the forces and movements responsible for the generation of the rock or geologic structures. Therefore both structural geology and tectonics are responsible for building up the Earth’s lithosphere. We can say that tectonics is quite closely connected to the underlying processes that cause geologic structures to form. These structures provide information about the forces acting within the Earth. The objective of structural geology is to determine and explain the architecture of rocks as observed in the field. The field observations are supported by laboratory investigations to attain this objective. Geologic structures range in size from microscopic scale – to hundreds of kilometres.

 

27.4 Basic Concepts in Structural Geology

 

Let us get familiarised with some of the basic concepts of structural geology before we study about various geologic or rock structures, like folds, faults, etc.

 

The study of structural geology is mainly accomplished with the study of geological structures which are developed due to deformation. They are also known as deformational structures or secondary structures. Primary structures form as the result of the processes connected with the deposition of sediments. They are also known as depositional structures, such as bedding plane.

 

Structural geologists play a significant role in the identification of the geologic structures, their geometry and orientation, the time and sequence of their deposition. They also assess the physical conditions responsible for the development of these structures.

 

Let us read about the common terminologies used in structural geology.

 

Outcrop: They are exposure of rocks on the surface of Earth (Figure 1). If the rocks are not exposed on the surface of Earth, they are referred to as ‘incrop’. The outcrops are generally visible along valley walls, river/nala section, mountains, road/ railway cuttings and tunnels. Hence an outcrop denotes area on surface of the Earth, over which a rock mass is exposed. The line of intersection of the limiting surface of rock mass with surface of ground marks its’ limit. The study of geological structures is generally carried by the study of outcrops.

Layering: It is developed in rocks due to the deposition of sediments, rock materials or minerals. The deposition of sediments one over another in a basin results in layered or sedimentary rocks. Bed is defined as an individual layer or strata of a sedimentary rock (Figure 2). Each bed is separated by the adjacent bed by a plane called bedding plane. If the individual layers are less than one cm thick they are called lamination. The process of deposition of sediments in a layer by layer fashion is known as

stratification.

 

The bedding planes are found in sedimentary rocks. Layering is developed in volcanic rocks (igneous rocks) (Figure 2a) because of flow of lava. It is also referred as primary foliation because these are developed simultaneously with formation of the rock. The term secondary foliation is used for layers found in metamorphic rocks (Figure 2b). The layers in metamorphic rocks develop as a result of the development of new minerals and reorientation of mineral particles from pre-existing rocks during the process of metamorphism. Slaty cleavage, schistosity, gneissosity, etc. fall in the category of secondary foliation.

Figure 2 (a) Primary foliation developed in igneous rocks, and (b) Secondary foliation developed in metamorphic rocks

 

 

Intrusion: Igneous bodies can be extrusive or intrusive. The intrusive rocks are those igneous rocks which penetrate or intrude in a pre-existing host rock. They might have consolidated either at great depth or at shallow depth. Many times the layering seen in the intrusive igneous rocks which can be observed in parallelism with the surrounding country rocks (also called host rock). But sometimes igneous bodies cut across the beds of the country rock and because of their late entry into the system along some joints or fractures. We will discuss various types of intrusions based on their geometry, later in this module.

 

Deformation: This term applied for a process in which the original rock is modified. The original rock body is usually undeformed and can be modified by folding, jointing, faulting or under the effect of gravity. Let us discuss the three types of deformation that are caused mainly by the horizontal movements of the lithospheric plates relative to one another. These forces are:

 

Tensional forces stretch and pull rock masses apart, Compressive forces squeeze and shorten rock masses,

Shearing forces push two parts of rock masses in opposite directions.

If a rock mass is subjected to directed force for a short period of time, it usually undergoes through three stages of deformation:

 

Initially the deformation is elastic and on withdrawal of stress the body returns to its original form. The limiting stress called elastic limit.

The deformation is plastic when the stress exceeds the elastic limit; that is the body only partially returns to its original shape even if the stress is withdrawn.

When  there  is  a  stress  continuously increases,  one  or  more  fracture  develops  and  the  body eventually fails by rupture.

 

Ductile and brittle are two commonly used terms by the structural geologist. Brittle deformation relates to the fracturing of the rock. Ductile deformation indicates bending, stretching, folding, thinning of rocks, and realigning of grains.

 

 

27.5 Dip and Strike

 

Initially the sedimentary rocks are deposited on flat or gently inclined surfaces. Post depositional deformation is caused by the movements due to tectonic forces acting on rock body or mass which causes the tilting of the beds.

 

Dip: It is defined as inclination of the bedding plane with respect to horizontal. It is measured in a vertical plane lying at right angles to the strike of the bedding.

 

Strike: It is the geographical direction of a line produced by intersection between inclined layers and a horizontal plane (parallel to surface to Earth).

 

Direction of dip: It is the geographical direction, along which a bed has maximum slope.

 

Amount of dip: It is the angle which varies from 0o to 90o, according to the inclination of the bed.

As discussed above the disposition of the beds (Figure 3) can be:

Horizontal: angle of dip=0o (no dip)

Inclined or tilted: angle of dip varying between 0o-90o

Vertical: angle of dip=90o

 

Dip is the vector quantity as it has both direction and amount. Amount of dip denotes the angle of the bed inclination with respect to the horizontal. Strike is a scalar quantity because it has only direction and no magnitude. Strike of the bed is independent of its amount of dip.

True and apparent dip: Dip can be of two types (Figure 4). Let us read about them.

True dip: The maximum amount of inclination or slope of bed along a line perpendicular to the strike is the maximum slope with respect to the horizon, it is called true dip.

Apparent dip: The dip of the bed measured in any direction other than that of true dip is called apparent dip. The amount of apparent dip is always less than amount of true dip.

 

 

Relation between dip and strike: The direction of dip and strike of any inclined or tilted bed must lie at right angles to each other. Thus true dip is in the direction perpendicular to strike. While mentioning the attitude of any inclined bed, dip amount and dip and strike direction should be mentioned.

Importance of Strike and Dip: Let us discuss the importance of strike and dip in structural geology.

  • (a) Determination of the younger bed or formation: In geological formations the older rocks deposit at the base which is superimposed by the younger rocks. Hence, in a tilted rock sequence when we move in the direction of dip then relatively beds of younger age will be encountered and vice-versa.
  • (b) Classification of geological structures: Dip and strike data provides useful information in the classification of rock or geologic structures.

Figure 4 True dip, apparent dip and relation between dip and strike direction. PQRS is a horizontal plane. Beds are all dipping 60o towards east and they strike north-south

 

 

27.6 Clinometer Compass

 

We have read that many rocks show planar structures in the form of bedding plane or foliation in any outcrop. They are not always horizontal, may show inclination of varying amounts. We have discussed that strike and dip describe the attitude of a rock layer in an outcrop. Let us recall that strike is the compass direction of a line formed by the intersection of a rock layer’s surface with horizontal surface. Whereas the dip is measured at right angles to the strike and it is simply the amount of tilting or in other words the angle at which the rock layer is inclined from the horizontal. The amount of dip of a bedding plane varies. In order to understand the attitude and orientation of the rock structures, you are required to measure their dip and strike on the outcrops during the fieldwork. The measurement is done with the help of Clinometer Compass (Figure 5).

 

Clinometer Compass consists of a compass and clinometer. The compass has a circular dial which is anticlockwise graduated with 360 divisions in the outer most circle for reading the direction. This circle is called azimuthal circle; where 0 or 360 represents for North, 90 for East, 180 for South and 270 for West. It has a magnetic needle which rests in north- south direction of the Earth, when set free. The desired direction is read on the dial with help of N (north) marked end of this magnetic needle. One of the inner circles on the dial is also marked with directions such as E, NE, NNE, W etc. in supplement to the outermost circle to read the direction directly.

The inclination of a line with respect to horizontal is read with the help of clinometer fitted in compass (Figure 5). It has a frame with open space within the clinometer frame which allows seeing one of the inner circles in the dial marked with 0 to 90o graduation. The reading of inclination (or dip amount) which may be variable between horizontal (0o) to vertical (90o) is measured with the help of a mark in the middle of the frame opening. The bridge fitted outside the dial helps in taking reading of the inclination of bed. The bridge is movable which can rotate upto 90o angles from a plane parallel to the dial up to a plane perpendicular to the dial. The bridge line is always parallel to the North-South line of the dial. The locking device is present outside the dial ring which on pressing locks the magnetic needle as well as clinometer so that the reading may be noted even after removing the compass from outcrop.

 

27.7 Measuring Strike and Dip of an Inclined Bed

 

Let us discuss following procedure to measure the strike and dip of bed:

 

  1. To measure the strike direction, place the compass on bedding plane in so that the bridge of the compass touches the bedding plane completely.
  2. Then rotate the compass so as to ensure that the bridge becomes horizontal and one end of the bridge still touches the bedding plane. Now let the magnetic needle move freely. Let the needle come to rest and then read both ends of the azimuthal circle which represent strike.
  3. In order to measure the amount of dip of the bed, draw a line on the bed perpendicular to strike and keep the bridge on the bedding plane along this line in such a way that the dial plane is vertical. The reading in clinometer gives the amount of dip.
  4. In order to measure the direction of the dip, place the bridge along the line drawn on the bedding plane so that the dial face the sky. Then rotate the compass to horizontal so that the bridge and N-S line of the dial both remain parallel to the line. The crown is often marked as N. Take care the crown in the dial is towards the dip direction of the bed.
  5. On rotation of clinometer to the horizontal, the N marked end of the magnetic needle gives the dip direction. You can check your reading. The true dip and strike directions are always perpendicular to each other.

27.8 Societal benefits of Studying Structural Geology

 

The structural geologist has significant role to perform for the benefit of society. Let us discuss.

  1. The crust of the Earth is structurally very complicated. It has undergone many modifications in the geological time scale. The study of structural geology enables our understanding in unravelling the mysteries of Earth crust.
  2. It is important for planning mining activities.
  3. Exploration, mapping and exploitation of Earth resources depend on careful interpretations of structural geologists.
  4. Artificial groundwater recharge projects and rainwater harvesting techniques require study of structural features of the geological terrain.
  5. The selection of site of engineering projects depends on its structural setting. The sites are even changed or rejected due to adverse structural settings.
  6. It is said ‘civilisations exist with geological consent’. Ancient civilisations existed along the rivers valleys, which usually flow along major geological structures.

27.9 Rock Structures

 

Rock structures can be broadly divided into:

 

Planar structures;

 

Linear structures;

 

Folds and faults

 

27.9.1 Planar Structures

 

Planar structures are those structures which are found as a plane or as a surface in the rocks. Planar structures go hand in hand with linear structures. Let us discuss major planar structures:

 

  1. Bedding plane: They are the planes that bound a sedimentary bed. A bedding plane separates the two beds in a sedimentary rock (Figure 1 and 3).
  2. Metamorphic foliation: It is a planar structure which is formed during deformation and metamorphism of a pre-existing rock (Figure 2b). They are characteristic feature found in the metamorphic rocks such as slate, schist and gneiss.
  3. Igneous foliation: They are observed with igneous rocks and are also known as primary foliation (Figure 2a).
  4. Fault plane: Fault plane is a fracture along which the rock blocks have been displaced (Figure 6).

Figure 6 (a) Displacement has taken place along the fault plane. Direction of movement is marked by arrows in opposite directions, and (b) Field photograph of slickensides

 

 

  1. Bedding plane: They are the planes that bound a sedimentary bed. A bedding plane separates the two beds in a sedimentary rock (Figure 1 and 3).
  2. Metamorphic foliation: It is a planar structure which is formed during deformation and metamorphism of a pre-existing rock (Figure 2b). They are characteristic feature found in the metamorphic rocks such as slate, schist and gneiss.
  3. Igneous foliation: They are observed with igneous rocks and are also known as primary foliation (Figure 2a).
  4. Fault plane: Fault plane is a fracture along which the rock blocks have been displaced (Figure 6).
  5. Joints: They are irregular or regular planar separated portion found in the rocks (Figure 7). Joints are the fracture surfaces along which movement is negligible and/or not observable. They are among the most common of all geological features. The systematic study of joints in an area can unravel the timing and sequence of its formation. The cooling of lava or magma can produce joints in the rocks (Figure 7). Fracture and joints are among the most important geological structures considered in hydrology, engineering, mining projects.

 

 

27.9.2 Linear Structures

 

Let us discuss about the linear structures. As the name suggests these structures is line like features observed in the rocks.

  1. Mineral lineation: If the longer dimensions of the minerals are aligned in a particular direction, it gives rise to mineral lineation (Figure 7).
  1. Crenulation lineation: They form when any planar surface such as foliation is affected by folding on small or microscopic scales so that hinge lines of these folds are aligned in a particular direction (Figure 8).
  1. Slickensides: They are lines like features developed on the fault plane because of friction generated between two faulted blocks and usually show polished surface (Figure 6b).
  2. Boudinage, pinch and swell and roddings: When the competent rock layers stretch and deforms into segments they form boudins. Individual boudins are commonly much longer in one dimension than other two. Roddings describes elongated mineral aggregates.

 

27.9.3 Folds and Faults

 

Now we will discuss in detail about folds and faults.

 

(A) Folds

 

Folds are combination of planar and linear structures. You can imagine folds in rocks are like folds in clothing. Similarly when the layers of rocks suffers gradual compression by the tectonic forces in the crust, they are pushed into folds. The layers of rocks can be crumpled or buckled into folds. Folding  can be defined as the bending of rock strata due to compressional forces acting tangentially or horizontally towards a common point or plane from the opposite sides. We can define fold as the wave-like undulation wherein the bending or arching of the rock layers takes place due to forces of the Earth. Folding is a common form of deformation displayed in the layered rocks. Folds are best displayed by stratified formations, viz., sedimentary or volcanic rocks or metamorphic rocks. Folds vary in size from centimetre on outcrop scale to hundreds of kilometers on the regional scale.

 

Importance of folding: The study of folds in geological studies is important because of following reasons:

Folding exposes the deep seated rocks on the surface of the Earth.

 

It increases the mineral deposits because of repetition of layers due to folding in a limited area. It facilitates development of site for deposition of mineral bearing solution.

Folds serve as good host for oil and natural gas.

Folding causes beautiful landscapes to develop which may enhance geotourism.

Wavelength of fold can be defined as the minimum distance between its two successive points of same phase. Alternatively it can also be explained as the distance between two alternating inflection points (Figure 9).

Crest is the highest point in the profile section of the fold (Figure 10a). Trough is the lowest point in the profile section of a fold (Figure 10b).

Crestal line is highest located line in the fold. It can be obtained by joining the crestal points in a folded layer (Figure 10a).

Trough line is obtained by joining the trough points of a folded layer (Figure 10b). This line is located lowest in the fold.

Culmination is the highest point located on the crestal line (Figure 11a). Depression is the lowest point located on the trough line (Figure 11b).

Figure 11(a) Culmination, and (b) Depression

 

 

Fold axis is an imaginary line which by moving parallel to itself generates the fold. The hinge line of a fold is considered equivalent to ‘fold axis’ if it is straight and the fold is cylindrical in nature (Figure 12).

Inflexion point is that point where the fold limb changes its attitude (Figure 12).

Inflexion line is obtained by joining the inflexion points of a folded layer. (Figure 12). Limb is a side between inflexion point and hinge (Figure 12).

(b) Figure 12 Parts of fold (a) Sketch, and (b) Field photograph of a fold

 

 

Axial surface/ plane is formed by joining of fold hinge lines of successive beds. When the axial surface forms a plane it is called axial plane is formed by the axial surface (Figure 12).

 

Amplitude of fold Amplitude of a fold is the length of perpendicular drawn from hinge point of the fold on the line joining the two inflection points of the fold (Figure 12).

Hinge point It is the point of maximum curvature on the profile section of a fold (Figure 12). The profile of the fold is a cross section or transverse section across the hinge line of the fold.

Hinge Zone: Sometimes the maximum curvature of the fold is not at a point but a set/group in a zone called hinge zone (Figure 12).

Hinge line: It is the locus of hinge points of a particular bedding plane. Hinge or hinge line of a fold is the line of maximum curvature in the folded bed (Figure 12).

 

  • (B) Faults

Now let us discuss about faults.

 

Fault can be defined as a fracture along which there is an observable amount of dislocation or displacement of the two rock blocks (Figure 13). Like folds, faults also occur in all sizes. The development of fault in rock takes place due to tectonic stresses such as tensional, tangential or compressional or in combination.

Importance of Faults: Faults play a significant role in geological studies and can pose challenge to geologists while mapping.

Faulting exposes the rocks from the deeper level to the Earth’s surface which provides knowledge of the subsurface geology.

Faults provide the excellent channel for the movement of mineralized solution or petroleum. They trap petroleum from migration and loss.

Faults may create beautiful landscapes which enhance geotourism. They may be the cause of origin of earthquakes.

Fault locations are very important prospects in mining and exploration.

Figure 13 Field photograph of fault

Parts of Fault: Let us analyse different parts of fault.

 

Fault surface/ fault plane: It is a surface or plane along which the dislocation takes place (Figure 14).

Fault zone: It is a zone of numerous small scale fractures constituting fault (Figure 14).

Fault line/ fault trace: It is the line formed by intersection of a fault with surface of the Earth or any given surface (Figure 15).

Dip of the fault plane: It is the angle of inclination of the fault plane with respect to horizontal. The true dip is measured on a vertical section perpendicular to strike (Figure 15).

Strike of the fault plane: It is the direction in which the fault plane cuts the horizontal ground and is strike of that plane (Figure 15).

Hade of the fault plane: It is the angle made by fault plane with respect to a vertical line (Figure 15).

Figure 15 Parts of the fault

 

 

HANGING WALL

 

Hanging wall: The rock mass resting above the inclined fault plane is known as hanging wall (Figure 15).

Foot wall: The rock mass resting below the inclined fault plane is known as foot wall (Figure 15).

Slip: It refers to any displacement parallel to fault plane in the given direction (Figure 15).

Net- Slip: It is the total amount of displacement measured parallel to fault plane (Figure 15).

Dip-slip: It is the movement parallel to dip direction of the fault plane (Figure 15).

Strike-slip: The movement parallel to strike of the fault plane (Figure 15).

Oblique-slip: It is the movement called oblique-slip, if both dip-slip and strike- slip movements are present. (Figure 15).

Heave: It is the horizontal component of the dip slip (Figure 15).

Throw: It is the vertical component of the dip slip (Figure 15).

 

 

Recognition of Faults in the Field: Let us discuss features that are characteristically associated with fault plane. The presence of these features in the field is indicative of faulting.

 

Visible displacements of veins, dikes, strata, etc. in the field are best evidence of faulting generally seen on the outcrop scale. For larger scale faulting geologists look for indirect evidences in the field.

Presence of slickensides or fault breccia (contains angular fragments) along the fault plane is indicative of fault in the field.

Dragging of strata is observed; there is some flexure or bending of beds present along a plane which may be a fault plane.

Presence of crushing, shearing and pulverization of rock indicates faulting.

 

Presence of mylonites (fine crushed rock materials) is indicative of fault zone. Silicification and mineralization is often associated with fault plane.

Sedimentary or metamorphic rock types which are usually found at distant places, if they are juxtaposed together, presence of a fault may be indicated.

Faulting often causes abrupt change in the topography, which can be observed in the form of cliff, triangular facets on mountains.

If a river or stream shows an abrupt change in its flow direction or it makes waterfall; it is indicative of presence a fault.

If the springs are found along a line, there may be presence of fault.

 

 

27.10 Forms of Igneous Rocks

 

Igneous rocks are formed due to cooling and consolidation of molten rock material. Magma is the molten rock material occurring below the Earth’s surface. But when it comes out on the surface of the Earth, it is called lava. The magma from the deep seated magma chamber ascends and due to buoyancy or any tectonic activity it intrudes into country rock (host rock) or it may come out at surface and solidify. Extrusive igneous rocks are formed when the magma reaches the Earth’s surface. Intrusive igneous rocks form when magma solidifies beneath the surface of the Earth. Based on the place of consolidation of magma, the igneous rocks are grouped as:

Plutonic rocks are formed when consolidation of magma occurs under deep seated conditions.

 

Hypabyssal rocks are formed when magma consolidates at shallow depth.

 

Volcanic rocks are formed when the lava consolidates at the Earth’s surface.

 

When the igneous body occur parallel to the bedding plane of the country rock, it is called a concordant body. Whereas when the igneous body cuts across the bedding plane of the country rock, it is called a discordant igneous body.

 

Let us discuss the various forms of intrusive or extrusive igneous rock bodies.

Sill is a concordant igneous body which lies parallel to the bedding plane of the country rock (Figure 16). They are sheet-like masses of igneous material which have spread parallel to the bedding planes. Sills may be horizontal, vertical or inclined. But it maintains parallelism with the bedding of the host rocks.

Dyke or Dike is discordant igneous body with more or less tabular shape, and it exhibits a cross cutting relationship with the country rock (Figure 16). Dykes are wall like masses of igneous rocks cutting across a country rock. The country rock may be igneous, sedimentary or metamorphic. Sometimes dykes are vertical, generally few feet thick.

 

Figure 16 Sill is concordant and dyke is discordant body

 

Laccolith is more or less concordant intrusive dome-like mass of igneous rock which arches upward and has a more or less flat floor (Figure 17). They are generally formed at shallow depths and are  exposed at Earth’s surface only after prolonged and continued erosion. The thickness of laccolith varies from hundreds of metres to a few thousand metres.

Figure 20 Phacolith

 

Chonolith is a irregular igneous intrusion whose form can not be classified as a laccolith, dike, sill or any other recognized body.

Volcanic Neck is a pipe-like discordant igneous body in which lava has been consolidated and looks like elevated monolith mass (Figure 22).

 

Figure 23 Block diagram of some intrusive bodies

 

Batholith is a large emplacement of plutonic rock that forms from cooling and solidification of magma deep in the Earth’s crust (Figure 24). Batholiths are huge plutonic igneous rock masses covering hundreds or thousands of sq. km and occupy the core of the mountain. They are formed at depth below but are visible only when after the removal of overlying rock.

Figure 24 Batholith

 

27.11 Summary

 

Structural geology is study of architecture of the solid Earth. In this lecture we learnt about: Definition of structural geology

Concepts in structural geology and introduction to basic terminology Relation between structural geology and tectonics

Concept of dip and strike

Societal benefits of studying structural geology Rock structures like linear and planar

 

Introduction to elements of folds and faults and their recognition in field Use of clinometers compass in measuring dip and strike

 

Forms of igneous rocks

 

27.12 References

 

Billings, M.P. (1990) Structural Geology, Prentice Hall of India Pvt Ltd. ISBN-0-87692-059-8, p606.

Fossen, Haakon (2010) Structural Geology. Cambridge University Press, ISBN 978-0-521-51664-8, 463p.