Unit 8

UNIT 8 STRUCTURAL GEOLOGY

Structure 8.1 Introduction

Objectives

8.2 Deformation 8.2.1 Behaviour of Mnterials Under Stress 8.2.2 Brittle nnd Ductile Deformation 8.2.3 Factors Controlling Behaviour of Materials 8.2.4 Mechanisms of Rock Deformation 8.2.5 Fundamental Structures 8.2.6 Structural Levels

8.3 Measurement of Structural Elements

8.4 Folds 8.4.1 Fold Morphology and Nomenclature 8.4.2 Fold Classification 8.4.3 Significance of Study of Folds

8.5 Faults 8.5.1 Elements of a Fault Plane 8.5.2 Movement along Faults 8.5.3 Classification of Faults 8.5.4 Complex Fault Types 8.5.5 Recognition of Faults in Field 8.5.6 Active and InactiveFnults I

8.5.7 Seismic Faulting 8.5.8 Effects of Faulting on Disrupted Beds 8.5.9 Significance of Study of Faults

8,6 Joints 8.6.1 Geometric Classification 8.6.2 Geuetic Classification 8.6.3 Quantitntive Description of Fractures/Joiuts 8.6.4 Significance of Joints

I 8.7 Foliation and Lineation 8.7.1 Cleavage 8.7.2 Lineation

8.8 Significance of Structural Geology

8.9 Summary 8.10 Key Words

8.1 1 Further Readings r; 8.12 Answers to SAQs I

8.1 INTRODUCTION

Structural geology is the study of geometrical arrangement of planes, lines, deformed surfaces hl rocks. These structures are produced by the action of forces acting on or within the earth's crust. In other words, the term 'structure' is used to indicate features created by deformation of rocks. These deformations are caused by various endogenic processes. Deformation commonly forms a part of the progress of a regional metamorphic cycle. As a result, a succession of mineral and structural transformation may take place during metamorphism. Each rock possess different mechanical properties and these properties depend on conditions under which deformation takes place. The phenomenon is complicated by the great range of chemical and mineralogical compositions that rocks may have and the large range in physical'and chemical environments in which deformation occurs. In this, the role played by pore fluid pressure in modifying mechanical properties of rocks is very important. Thus, study of interactions between deformation and metamorphic processes is fundamental to the understanding origin of structures in deformed rocks. Deformation of rocks and metamorphism are thus, closely related phenomena.

Materials of the Earth A structural geologist cmies out his work in two phases :

a) Observation of degree of deformation and measurement of various geometric parameters; and

b) Interpretation of the deformation path.

It is seen that attitude of these structures indicates the interrelationship between the deforming forces and pre-existing rock mass.

In this unit, we shall study how rocks are deformed at different structural levels and find out their significance. To understand deformation, it is necessary to discuss behaviour of different parameters, like temperature, pressure, strain-rate etc.

Three dimensional analysis of geological structures, along with other geological investigations, is an essential component in deciding stability of many civil engineering operations.

Objectives

After studying this unit, you should be able to

define and identify various deformation structures in tlle field, - analyse and interpret their deformation path, and - appreciate influence of these structures on the stability of rnajorlrninor man made structures.

8.2 DEFORMATION

Deformation is the process responsible for the development of observed geological struclures. In other words, it is the process that changes size or shape of a rockmass. Often, some rigid body rotation may also be involved in deformation. Study of stress-strain relationship is very imporlant in structural geology. I The forces acting on rocks of earth's crust are predominantly in the form of gravitational forces 'and are also created due to movement of rocks at depth, which give rise to a set of stresses and constitute strain in rocks.

A force (F) exerted on a unit surface area can be resolved into a normal stress and a shear stress (Figure 8.1 (a)) in which normal stress acts perpendicular to lhe surface and shear stress acts parallel to the surface. In three dimensions, shear stress can be further resolved inlo two components which are normal to one another (Figure 8.1 (b)).

Normal stress o is perpendicular to the plane, and shear stress z is parallel to the plane developed by force F acting on the plane in two dimensions and in three dimensions, shear stress 2 is resolved into 21 and z, at right angles yielding three stresses, resulting from force F.

a) Normal Stress and Shear Stress b) Shear Stress in Three Dimensions in Two Dimensions

Figure 8.1

When principal stress is unifonn, the stress is called hydrostatic stress. In this state, a change in volume of deformed rock is visualised. The condition is similar to the stress state of a fluid. Stresses in rocks at depth are created due to overburden or weight of overlying rocks,

called as lithostatic stresses. If the principal stresses are not uniform, the system is called deviatoric stress system Ulat leads to distortion in a body. Strain is the response of a deforming body Lo a stress system. It falls into four categories.

a) Rigid body translation is the movement of a body without any change in shape (Figure 8.2 (a)). The lines drawn before deformation will have same orientation after deformation.

b) Rigid body rotation is the movement of a body wilhout any change in shape The lines drawn before deformation will rotate around a single point (Figure 8.2 (b)).

c) Distortion involves change in shape of the body (Figure 8.2 (c)).

d) Dilation causes positive or negative ch'anges in volume with no change in shape (Figure 8.2 (d)).

(a) Rigid Body Trnnslation (b) Rigid Body Rotntion

(c) Distortion, a Chnnge in Sl~ape (d) Dilation, a Chmge in Volume

Figure 8.2 : The Nature of Strnin Compoucnts

Strain is homoge~ieous, if it is conslant throughout a body and helerogeneous, if deformation varies throughout a body. However, it can be said thal strairi in rocks is generally heterogeneous but in smaller domains strain may be homogeneous. In homogeneous strain, straight lines remain slr;iight, parallel lines remain parallel and a circle is deformed into a strain ellipse (Figure 8.3). In case of heterogeneous strain lines become curved and parallel lines become non-parallel (Figure 8.4).

Structural Geology

i ( a ) Undeformed ( b ) Deformed I

state state I

I Figurn L3 : Homogeneouq Strain

, I

Materials 6P the Emth

When strain is itrotational with no change in x and y during progressive deformation, it is known as Pure shear.

When strain is rotational and x and 2 rotate in a clockwise manner during progressive deformation, it is known as simple shear.

( a 1 Undeformed ( b ) Deformed state state

Figure 8.4 : Heterogeneous Strain

Strain Ellipsoid

The fmite strain TI a deformed body is defined by comparing the size and shape of the ellipsoid with the size and shape of initial sphere. This ellipsoid is called as strain ellipsoid. X, Y and Z are the maximum, intermediate and miniurn principal strain axes, respectively. The homogeneous strain has two major components, pure shear and simple shear. Pure shear is also called as irrotational deformation or irrotational strain. In this, thc deformation history is translational (Figure 8.5 (a)). Simple shear is a uniform volume rotational homogeneous deformation (Figure 8.5 (b)).

( a ) I r ro tat ional strain

( b 1 Rota t iona l s t r a i n

Figure 8.5 : Pure Shear and Simple Shenr '

8.2.1 Behaviour of Material Under Stress When a rock is subjected to stress, it deforms seldomly like an isotropic body or commonly like an anisotropic body. For an anisotropic body, elastic properties are different along different directions, so that a nonlinear stress-strain curve may be observed.

a) Elastic strain : In this stage strain is very small, with progressive increase in stress. If stress is removed, the rock returns to its unstrained state called as elastic, temporary or recoverable strain. This kind of strain is associated with the propagation of seismic waves.

b) Plastic strain : Beyond the elas tic limit, strain is permanent and irreversible (figure 8.6 (a)) resulting in deformation of the body.

c) Rupture : With increase in stress, the body fails by rupture (Figure 8.6 (b)). In this stage several factors are to be considered, i.e, the nature of deformation, the physical condition at the time of rupture, orientation of ruplure, etc.

I

t plastic t

14 Ngnre 8.6 : Typical Stress-strain Curves

Note that laboratory tests are not adequate criteria to visualise behaviour of similar rocks during deformation caused by orogenic compressive stress, as laboratory results yield exaggerated values. ~ u r h ~ natural deformation, factors like presence of iluids, composition .

of material, temperature, pressure, elc. are influential.

8.2.2 Brittle and Ductile Deformation

When a rockmass is subjected to deviatoric skess, often elastic deformalion leads to failure and the deforming rock looses cohesion by the fornlatioil of fractures across which the continuity is lost. This is brittle deformation characteristic of surface or upper structural level. On the other hand, ductile deformation is typical of middle lo lower structural level in the earlll's crust that produces permanent homogencous and heterogeneous strain as evident from development of folds and significant absence of faults and fractures.

8.2.3 Factors Controlling Behaviour of Materials a) Confining Pressure : Rocks at depth are constantly under lithostatic or confining

pressure (the pressure is said to be hydrostatic) which is a function of density and Ulickness of malerial. It is seen on tlle basis of experimental results that with increasing confining pressure, the effective strenglh and ductility of the deforming rock show an increase.(Figure 8.7).

5..

Strain ( % )

Figure 8.7 : Effect of L~creaving Confining Pressure on the Stress-shin Curves for Ute Experimental 1)ebrma~on of Marble (atYer Gdggs, 1967)

b) Temperature : An increase in temperature may change a rock from brittle to ductile stage of deformation (Figure 8.8).

Materials of the Earth

Strain ( '1. I

Figure 8.8': Effect of Increasing Temperature on the Stmss-strain Curves for the Exlrc~imenl;d Deformation of Marble at 5 kbars Confining Pressure (after Griggs, 1967)

c) Pore-fluid Pressure : The presence of fluid may have great influence on mechanical properties and mineralogy of rocks. Mechanical properties of rocks are altered, when

1) pore-fluid pressure approaches the magnitude of the confining pressure, the rock deforms in a brittle manner.

2) high fluid pressure reduces the strength of rock. This is best explained in Figure 8.9. Effect of pore fluid pressure on the stress-strain curves for the experimental deformation of wet and dry natural quartz at different temperatures, 15 kbars confining pressure and a strain rate of 0.8 x loe5 sec.

950 k (wet I

2 6 8 Strain ( "A 1

Figure 8.9 : Efted of Pore Fluid Pressure on Natural Quartz (After Griggs, 1967)

Experimental results have shown that under the influence of high fluid pressure, minerals, like quartz, undergo ductile deformation at lower temperature and pressure condition than otherwise (Figure 8.10). I

S t r a i n ( % ) Strain ( % 1 a) Dry nnd Wet Quartz b) Dry and Wet Olivi~~e

Figure 8.10 : EfTect of l'ore-lluid Pressul.~ on t l ~ c Stress-strain Curves for the Experimc~~W Dcfurmatior~

d) Strain Hate :and Time : Rocks, when subjected Lo a uniform stress field for a long tinlc (monlhs, years), show a slrairi thal may be explained in lllree sleps (Figure 8.1 1). Thesc are

1) Primary creep : rocks behavc in an clastoviscous manner,

2) Secondary creep : rocks exhibil viscous flow, and

3) Tertiary creep : rocks lead to failure.

Primary Failure creep Secondary Tertiary

I creep I creep I I

Accelerated viscous

C

I F

Time

1 Figure 8.11 : Striin-time C ~ ~ r v c for Lor~g Time Pcriud (Crccp)

It can be said that at low tcmperature and pressure, strain rale is fasl, il decreases wiLh increase in lemperature and pressure conditions.

8.2.4 Mechanisms of Rock Deformation Mechanism of rock deformation is controlled by mm~y factors including mineral composition, texture of rock, lemperature, pressure, pore fluid pressure, etc. However, a granite, wilh an equigranular texture, is stronger than a sandstone wid] an argillaceous (fine grained, clayey) cemenl or a carbonate cement irrespective of mineral composition. Similarly, a schistose melamorpliic rock is weaker U~an a rock with a granular texture. 11e mechanism of rock deformation is best ascertained by studying microfabric study under microscope. It is seen that a variety of micro-mechanisms including cataclasis, pressure solution, creep and dislocation glide operate during thc bulk creep of rocks.

Cataclasis is the process of brittle failure dong grain boundaries and across individual grains or crystals. n ~ i s is commonly associated with high strain rate and low temperature typical of upper structural level, i.e. near surface.

An important mechanism of deformation is pressure solution thal involves transport of material along grain boundaries. It is assumed that a fluid film exists along grain boundaries through which transport takes place.

At high temperature and low stress level, deformation is usually achieved by Islocation, creep and glide. In both brittle and ductile types of deformation, strain is induced by the glide of dislocations.

8.2.5 Fundamental Structures Rocks in field exhibit following structures. They are defmed here and described in appropriate sections.

1) Fractures : Fracture is a discontinuity along which the cohesion of the material is lost. Fractures are characteristic of competent rocks and generally develop at upper structural level. Two major types of fractures are recognised :

a) Faults : Faults are fractures that accommodate appreciable displacement parallel to the discontinuity surface (Section 8.5).

b) Soinb : Joints are discontinuities along which therc is negligible or no movement parallel to the plane of the fracture (Section 8.6).

2) Folds : A fold is a structure that forms, when an original planar surface becomes curved as a result of deformation. Folds are best displayed by inco~npctent beds or very plastic rocks (Section 8.4).

3) Foliation and Lineation : These are planar and linear structures commonly developed in deformed metamorphosed rocks (Section 8.7).

Figure 8.12 : Theoreticd Cross-section of a Folded Portion of the Earth's Crust Showing the Deformation Style at the Different Structural Levels (After Mattauer, 1%7)

8.2.6 Structural Levels Various rocks juxtaposed in nature do not have the same mechanical properties and Ulus they do not show the same behaviour under identical conditions of temperature and pressure; and also with vertical depth. Three stnrctural levels are recognised based on these behaviours (Figure 8.12).

a) Upper ~tructurd Level (USL) i.e., shallow depth where most of the rocks are brittle and exhibit shear ftacturing. Only very plastic rocks (i.e., shales) show ductile deformation.

b) Middle Structural Level WSL), where ductile deformation is widespread. However, a marked variation in style of folding may be displayed by hard (comp'tent) and soft (plastic, incompetent) rocks. The layers of incompetent rocks exhibit disorganized folding compared to parallel folding in competent rocks.

c) Lower Structural Level (LSL) is marked by development of cleavages accoqpanied by metamorphism; and then further lower levels by flow and melting of rocks.

Quantitative analysis of h e structures is an essential colnponant in the preliminary geological investigations carried out as a part of feasibility study in many civil engineering operations. m e data in the form of these structures are gathered in the field and presented in maps and sections. The infornution collecled sliould be sufficient to derive the lunematic movements and dynamics, responsible for the development of these structures. The observed structure may vary in size from microscopic to nlacroscopic with the magnitude of several kilometers. However, as the terminology used in descriptive and non-genetic, there is hardly any difference in the terminology and description of small and large sc'ale structures.

Rocks usually show a planar structure along which splitting is easy. This is represented in sedimentary rocks by bedding planes, stratification etc. A bed is a tabular body of rocks with upper and lower planar bounding surfaces. A bed is distinguished from the one above or the one below by differences in physical or chemical characters and general appearance. Similarly, metamorphic rocks may show schistosity and gneissose banding. In igneous rocks, planar structures, like flow struclures, are rarcly recorded. Sedimentary rocks show horizontal disposition in general. These, if subjected to a deviatoric stress, may become inclined or vertical. Attitude of these beds may be measured using Clinometer, Brunlon comp;lss, Prisrnalic Con~pass by considering Uleir strike 'and dip.

a) Strike : The strike of a bed is its direction measured along a horizontal line on the bedding plane. Dip of a bed is zero degrees measured along strike direclion.

b) Dip : The dip of a bed is the angle that Ule bed makes with the horizontal plane, measured in a verlical plane. The inclination is called lrue dip, if the line of section is normal to strike direction (i.c., along dip direction). The apparent dip is the inclination measured in a vertical pl,ule that nlakes an angle to dip direction.

. ..

a) Lnclined Beds, Strike : North-south (NS); Dip SO%mt, b) Vertical Beds c) Horizontal Beds Figure 8.13 : Attitude of Beds

It can be seen from Figure 8.13 (a) that beds are inclined, i.e. 0 = 60' and dipping towards eas

and strike is N-S. In Figure 8.13 (c), beds arc horizontal, i.e. dip is zero so strike I with strike oriented North - South (N-S). In Figure 8.13 (b), beds are vertical so that dip is 90.

measurements are not possible.

Remember ha t dip of a bed and slope of the ground are two different features. Slope of the ground is an expression of topographic surface, on the other hand dip is the inclination of a bed.

Planar slructures like bedding, fault, joints, schistosily, fold axial plane, etc. are measured in the following manner.

1) Find out the strike line (i.e. the horizontal line on the planar surlace) by locating the direction of zero dip on the plane (Figure 8.14). Measure its dircctioil from North and note it in your field book, e.g. N 240'.

2) Find the direction of maximum dip on plane or simply place the edge of compass normal to the strike line (Figure 8.14). Mark this as tlie dip direction of the plane and measure the angle e.g. 60°/1500 i.e. dip amount 60' and direction 150" from North, respectively.

C) Clinometer Compass : The dial of thc compass has an outer arc in which geographic directions from 0 to 360 are marked out. There is a pendulum haqging

Materials of the Earth From the centre of the compass dial. The clinometer also has a straight edge normal to the plane of its dial. There is an inner arc in the dial showing 90' position on either side of the zero mark. The outer arc is used to find strike and dip direction of a bed; and the inner arc, with the help of pendulum, is used to find amount of dip.

( a Measurement of dip amount

b Measurement of direction

Figure 8.14 : Attitude Measurement using Clinonleter Compass

Some of the commonly used syrhbols to plot structural features on geological maps are shown in Figure, 8.15.

Bedding Fault

\ Stri ko and dip of beds \ V ~ r t i c a l beds

Horizontal bods \ Str ike and dip of

\. Qverturrt~d beds Foliation

Strike and dip of foliation with plunge of l ineation

Vert ical f o l i a t i o n

Horizontal f o l i a t i on

F r a c t u res

Fault orientation

Infer red f au l t

Fault showing dip

Vert ical f a u l t

Fault with plunge of s t r a t i ons Horizontal fau l t

Fault showing re lat i ve move men t

\t SIri ke and d ip of joints @ Vort ica l tineati on

\ v e r t i c a l jo ints + Horizonta l joints

\ Hori zon ta l l ineat ion . \ Minor anticline. w i t h plunge I

\ of a x k

\ Minor p lunging synctine I

Dome I

Figure 815 : Some Common Symbols on Geologic Map

SAQ $. 9) Whi~t is ~ ~ ~ O P B P A & ~ ~ O B ~ ? h) Dislinlguish bctween

i ) brithlc and ductlBe cdcd-osl-srdion

ii) pure slncx simple shc;nr

iii) isouopic; arlti misotropic botiy

c) List Ihnnd;aamlental sstructu~cs displ;iyed by toa:ks in flcld.

d) Wlial is rnllcri~lt by alLiRntlc of CIS '!

e) Distinguish belwecn Cruc clip ruad appiarcnt dip.

8.4 FOLDS

Folds are r~ldulations produced in stratified rocks. In simple terms, a fold is a curvature in a geologic surface or in a set of stacked geological surfaces. Folding results when the original planar surface is curved due b forces acting parallel to the bedding plane. Fold is one of the most spectacular of the earth's structures. Size of a fold varies from a few centimetres to even a few tens of kilometres.

8.4.1 Fold Morphology and Nomenclature

In order to understand and describe the nature of folding in rocks, it is necessary to get acquainted with various terms used to explain the form and size of fold.

Hinge is a point of maximum curvaturq on the folded surface, more commonly, hinge is a zone, called as hinge zone. A line, joining all hinge points, is called a hinge line. Limbs are the sides of a fold and each fold has two limbs. In other words, curved limbs of opposing convexity joins at inflexion point (Figure 8.16 (a) and (b)),

A fold axis is a slraighl line that generates the form of a fold (Figure 8.16 (c)). It may be horizontal, inclined or vertical. Plunge is an inclination of fold axis measured with respect to a reference horizontal plane (Figure 8.16 (d)).

Axial plane is an imaginary plane which divides the fold into two parts, symmetrical or asymmetrical (Figure 8.16 (e)). Orientation of a fold surface is measured by measuring its axial plane. When a fold is made of several surfaces, it is possible to define a common surface by joining successive hinge lines (Figure 8.16 (f)), such a curviplanar surface is known as axial surface.

Inter limb angle is the internal angle between the limbs of a folded surface (Figure 8.16 (g)). It is a measure of fold tightness.

Fold size is described by its amplitude and wavelength, when the fold is seen in profile.

Wavelength of a fold is the distance between the hinges on either side of the fold (Figure 8.17 (a)). In many cases, the entire fold is not seen in profile. In such cases, distance between two inflexion points is measured (Figure 8.17 (b)), known as half wavelength.

Hinge line I Hinqe zone

Fold axis Plunge of fold axis

Axial plane Axial

Figure 816 r Fdd Morphology

Amplitude or 'height' of a fold which is half the perpendicular distance from hinge to inflexion points (Figure 8.17 (a) and (b)).

Wavelength ( A ) I

(a) Amplitude of a Fold (b) Wavelength (A) of a Vold

Figure 8.17

When the axial plane is inclined, Ule points of highest elevation and lowest elevalion in a fold may not coincide with the hinge points. The point of highest elevation is called as crest and the line joining the points of highest elevation is called a crestal line. Similarly, the lowest point in the folded surface is called a trough point and the line joining tbe points of lowest elevation is called a trough line (Figure 8.18).

Crest

hinge ALL Trough

Figure 8.18 : Crest and Trough Points ofan Indmed Fold

An 'anticline' is a fold in which limbs are opposite inclined and are convex in the direction of Structural Geology

the youngest bed (Figure 8.19 (a)). Limbs dip away from each other.

A 'syncline' is a fold in which limbs are together inclined and are convex in the direction of the oldest bed (Figure 8.19 (b)). Limbs dip towards each other.

Youn est be]*

Oldest beds

( a Anticline ( b 1 Syncline

Figure 8.19

8.4.2 Fold ~lassification Folds are classified either on the basis of different geometric parameters (Geometric classification) or based on origin (Genetic classification). Geometric Classification This classification is purely descriptive and does not discuss the origin of the fold. Three methods are followed.

a) Classification Baqed on the Attitude of Axial Plane

Four major types are recognised on the basis of dip of axial plane, when the fold is seen in profile (Figure 8.20 (a)).

1) Symmetrical fold : Axial plane is vertical and it bisect5 the fold into two symmetrical halves. Also called upright Ibld.

2) Asylnmetrical fold : Axial plane is inclined and thus, it divides the fold into two asymmetrical parts, also called inclined fold. A reclined fold is a kind of asymmetrical fold where fold limbs and axial plane, both dip in same direction with same amount.

3) Overturned fold : Axial plane is inclined and dip direction of both limbs of a fold remains same. One limb is gently inclined and the other is steeply inclined.

4) Recumbent fold : Axial plane is horizontal.

b) Classification Based on Inter Limb Angle

The inter limb angle is measured by drawing tangents to the fold curves at the inflexion point. Five types are recognised (Figure 8.20 (b)).

?',

,, ,' 1) Gentle fold : Inter limb angle, > 120' ,

2) Open fold : Inter limb angle, 70' - 120'

3) Closed fold : Inter limb angle, 30' - 70'

4) Tight fold : Inter limb angle, 10' - 30'

5 ) Isoclinal fold : Inter limb angle, < 10'

c) Classification Based on Folds Seen in Profile

'Profile' is the shape of a fold seen in a plane, perpendicular to fold axis. Two major types are recognised (Figure 8.20 (c)). ;

1) Parallel folds : Form of a parallel fold, when seen in profile, is circular or elliptical and thiclcriess of the layer measured perpendicular to fold tangents

Materials or the Earth remain constant. The fold is also called as concentric fold. Strictly speaking, concentric folds are those where adjacent fold surfaces are arcs of a circle with a common centre.

/

2) Similar folds : In this type, thickness of a fold, remains uniform when measured parallel to axial plane. In other words, in similar folds, hinge zone is thickened and limbs are thinned.

I 1

1 , ~ymmet ri c a l 2 . Asymmetrical 3 . Overturned 4 . Recumbent ( a I Attitude of ax ia l plane

, axial trace

Parallel Similar

( c l Shape of folded layer in I profile

Isoclinal ( b ) Interlimb angle

Figure 8.20 : Geometrical Classification of Folds

Genetic Classification Two major types are recognised depending upon the manner in which folding takes place. 1) Flexure Folds

The process of development of a flexure fold is similar to bending a sheet due to compressive forces acting parallel to the layers. Thus, convex side is subjected to

I tension, whereas the concave side is subjected to compression (Figure 8.21). In bemeen, there is a surface of no strain (neutral surface). Here, most important process is sliding of beds past one another (Figure 8.21).

~ e u t ra I surface

(a) Outer Surface is Subjected b Tension (b) outer Sutface is Thinned and Inner Surface is n i c k e d While Inner Surface is Subjected b Cornprrssloa Separated by a Surface of DO Strnlo

Figure U21: Mwhpnism dFloxllre Folding

2) Shear-slip folds I Structural Geology ,

The folds result from minute displacement along closely spaced fractures, i.e., each fractures is a tiny fault (Figure 8.22 (a)). Movement along these fractures gives rise to a shear-slip fold (Figure 8.22 (b), (c) & (d)).

Close spaced fracture

beds i

[ a l (. b ( c ) I d )

Figure 8.22 : Stages in ilre Developn~ent of Shear Slip Folding

8.4.3 Significance of Study of Folds Study of folds is very important from civil eiigincering, petroleum exploration, mining and hydrogeology poinl of view. Figure 8.23 sliows cross-section of a typical erosional valley developed along an anticlinal axis. As the rock is bent in an arched fold, it is certain to be heavily jointed. Here, the dam construction may prove to be costly as il is necessary to make the foundation material water-ligh t.

I Figure 8.23 : Schematic Section of an Erosional Valley Along Antlclinnl Fold Axis

! Synclines are most important in engineering slruclures because of their capacity to convey and f

I accumulate fluids. If such a sile is selected for tunnel, then it will be water bearing (Figure : 8.24).

syncl ine [ c )

(a) Schematic Sediw Showing a (b) FreeCures Converge Down (c) Fractures Diverge Down In a 'l'unnel Route Through Folded of 8n Anticline Synche Beds. Dashed L i w Indicate Geometry d F r a h

Materials &the ~ar& Furthermore, fracturing is very severe at the hinge zone. In case of an anticlinal fold, fractures will converge inside the tunnel and wedge shape blocks will be developed in the tunnel: while in case of a syncline, fracture will diverge inside the tunnel and thus, problems llke roof

Figure 8.15 : Schematic Diagram Showing Fnvournblc Anticlinal Site for Accumulation of Natural 84 and Ges

Anticlinal structures are considered as most favourable from the point of view of accumulation of oil and gas (Figure 8.25). Oil and gas from source rocks migrate and accumulate in such anticlinal traps.

a ) How will you tlislira~?uisla an ;mticliax 1P.nnm a sylic.linc '!

8.5 FAULTS

A fault Is a rupture or fracture whose two surfaces are displaced against each other along a direction parallel to the plane of the fault. Normally, faults are characteristic of upper structural level and may develop on a scale that varies from a few centimeters Lo thousands of kilometers. San Andreas fault system is a good example.

Fault Zone and Shear Zone

A fault zone is a tabular zone of uncertain width that contains many parallel or complexly intersecting faults (Figure 8.26 (b)).

A shear zone is a general term for a relatively narrow zone of large Shear strain bounded on both sides by relatively undeformed rocks (Figure 8.26 (c)).

Description of a fault zone or a shear zone should include following information.

a) The attitude (sLrike and dip) of beds.

b) Width of the zone.

C) Whether deformation structures are brittle or ductile within the zone.

d) Nature of change from underformed to deformed rock and relative movement across the zone.

[ a 1 Fault plane ( b l Fault zone ( c l Shear zone

Figure 8.26 : Conceptual Diagram Showing Presence of a a) Fault plan< b) Fault Zwe , and c) Shear Zone

8.5.1 Elements of a Fault Plane Attitude of the fault plane is measured by its strike and dip. The strike of a fault is the trend on the plane of the fault and the dip is the angle between the horizontal surface and the fault

64 plane when measured in vertical plane (Figure 8.27 (a)). Hade is an angle between the fault

and vertical plane that strikes parallel to Ule fault plane. In case of non-vertical faults, the block or wall below the fault is known as footwall and the block or wall above the fault is the hanging wall. The term slip is used to indicate relative movement. Displacement parallel or along the strike of the fault is called 'strike slip'. In this, dip slip component is zero. When the displacement is along dip direction of the fault, it is called 'dip slip' and here strike slip component is zero. If movement is oblique, Ulen the fault possesss both strike and dip slip components (Figure 8.27 (b)) and net slip, i.e, actual displacement is mentioned,

Down thrown sido

ss = Strike slip

( a ) ds = Dip slip Ns = Net slio

Figure 8.27 : Nomcndnture of n Fault

In case of inclined faults, the dip slip nlovement may further be resolved into a hoi-izontal component and a vertical component called as Heave and Throw respectively (Figure 8.28 a).

Figurc 8.28 : Throw (T) and IIenve (H) of an Indiucd Fault: ds : Dip Slip

The relationship between these two is given by

t,m 8 = ----- lhrow (Figure 8.28 (b)) heave

sin 8 = ' throw true displacemelll

(Figure 8.28 (b)

8.5.2 Movement along Faults Movements dong faults may be translational or rotational.

a) Translatqonal : Trllnslational movements are those in which all straight lines on opposite sides of the fault r e W parallel before and after displacement (Figure 8.29).

Fault plane

( a ) Before faulting

( b ) After faulting

Movacni dong Fault

Materials o~ t h e Earih b) Rotational : In case of rotational movements, the straight Iines on the opposite walls of the fault do not remain parallel to each other after displacement (Figure 8.30).

Fault plane Rotation A

l a ) Before fault ing I b 1 After f au l t i ng

FlgliFe 8.30 : Rotational Movemeut Along Fault

Fault movements are difficult to recognise in field a~ it is seldom possible to match exact points on both sides of the fault.

8.5.3 Classification of Faults Similar to folds, faults can also be classified both geonlelrically, i.e., by describing fault wit11 the help of certain geometric paranleters and genetically, i.e.: by considering the orientation of stresses that caused faulting.

a) Geometrical Classification

In this classification, tlle attitude of the fault is conipared with the attitude ofthe beds. Three major types are recognised.

1) Strike fault : 111 this type. strike of the fault is parallel to the strike of the beds. The dip of tl~e fault n~ny vary from the clip of the beds (Figure 8.31 (a)). A bedding fault is a kind of strike fault. where tlle fault is essentially parallel to tile attitude of beds (Figure 8.31 (h)).

Fault lane F a u l t plane

Figurr R3l : Strike Fault

2) Dip fault : The strike of the fault is oriented nnrn~al to tlle strike of the beds uid lies parallel to dip direc~ion of beds (Figure 8.32 (a) (b)).

Fault plane

Fau l t p lane - -

FSgure 832 : Dip Fa&

3) Oblique fault : ,h case of an oblique fault. orientation of the fault plane is oblique struct~ral Geology

to the attitude of beds (Figure 8.33).

Fault plane -

Figure 8.33 : Oblique or Diagonal Fnult

b) Genetic Classification

In this classification, the sense of relative displacement along fault and orientation of the fault with respect to the principal stress axes and axes of juxtaposed beds across h e fault, are important factors. Three major lypes are recognised.

I 1) Normal or Gravity Fault : A Nomul fault is one. where hanging wall moves

down in relation of foot wall (Figure 8.34 (a)). Herr 0,. the nmin~um principal stress axis, is vertical and relates lo gravitational load. Thus, the fault is also called gravity fault (Figure 8.34 (b)). The conjugate system of normal faults intersects parallel to 0 2 and normally, dips more than 45" (Figwe 8.34 (c)).

r

Figure 8.34 : N o m d or G m v i t ~ Fault

2) Thrust or Reverse Fault : In case of reverse fault hanging wall moves up against footwall (Figure 8.35). Here ol is horizontal and 03 is vertical (Figure 8.35 (b)). As the least stress is vertical, thrusts normally form at upper structural level where the lithostatic pressure is less. Thrust faults are usually low angle and extends several tens of kilonleters at times.

( a ) Plan view Figure &.G : Thmsl or Reverne Faun

Materials ofthe Eanth 3) Strike Slip Fault : Displacement is seen along the strike of the fault (Figure 8.36 (a)). It may be dextral or sinistral. A strike fault has a sinistral movement only if the opposite block moves to the left (i.e., left-lateral) (Figure 8.36 (d)). The movement is dextral if the opposite block moves to the right (i.e. tight- lateral) as observed by the viewer standing on one side of the fault (Figure 8.36 (e)). Here, both al and as are horizontal and a2, i.e., intermediate stcess is vertical or near vertical (Figure 8.36 (b) and (c)).

Figure 8.36 : Strike Slp Fault

8.5.4 Complex Fault Types a) Horst and Graben : Horst (meaning upthrow) is a special type of reverse fault, where

the area is uplifted due to the presence of two thrust faults. Graben (meatking Trench) is a structural depression formed due to the presence of two normal faults (Figure 8.37). Major graben features, that are traceable for long distances, are called as rifts, e.g., Godavari rift and Mahanadi rift that existed in Mesozoic times, now ofcourse they are covered with younger sediments.

( a I Horst ( b 1 Graben Figure 8.37 : Howt m d Grnbcn

b) Step Fault : When the area is affected by a series of normal faults so that the slip is always seen on one direction, lhe resultant structure creates a typical step-like appearance. Such faults are called step faults (Figure 8.38).

Figure 8.38 : Step Fault ,

8.5.5 Recognition of Faults in Field Faults may easily be recognised in stratified, i.e. layered rocks, by actual slip or displacement

t of beds. However, in most cases presence of a fault is interpreted on the basis of certain field 68 evidences.

fault fault

f n 01 0

CT c . - ul

Profi le

Mean sea level

U pper structural l e v e l L U S L )

L o w e r structura level ( L S L )

Struclud Geology

Mgun, 8.39 : Conceptunl Diagram Sl~owillg Bel~mviour of Rocks in n Iihult Zone nt Sorfnce tu~d Bclow Surfnce nt Difl'cre~~l Struchual Levels

Some of the features in rocks ba t help in recognition of faults and shew zones are given below :

1) Fault scarp : Presencc oT a long slraighl edge topographic scarp in an olhenvise uniform terrain would indicate presence of a fault.

2) Slickensides : These are striated polishcd surfaces wih fine grooves caused by polishing aclion or due lo the friction along thc opposite walls of a hult during aclual displacement.

3) Gouge and fault breccia : Many faults are recognised by the presence of pulverised rock similar to clay. This zone is softer Ulan adjacent rocks and produces elongated lopographic depressions. Fault breccia is a non cohesive fragmented rock of varying size characterislic of surface or shallow depth (Figure 8.39).

4) Cataclasites : Cataclasites arc cohesive fraginented rocks, wherc fine grained matrix predominates over visible fragments. These arc lypical of upper structural level 'uld tbus show effects of briltlc deformation (Figure 8.39).

5 ) Mineralisation : The irregular spaces left between Ule crushed and fragmented rocb in a faull zone are often filled with secoi~dary silica or calcite. Occasionally, economically import,mt mincrals and ores are found in such fault zones,

6) Mylonites : At great depth or lower structural level, rocks undergo ductile deformation, forming mylonites. Thcse are fine grained recrystallized rocks with a distinct change in fabric compared to adjacent rocks. Often, due to partial melting, a glassy rock (pseudotachylile) is formcd (Figure 8.39).

7) Depending upon the type oT raulting and lhe nalure of rock beds, that are affecled, repetition or omission of be& on the dowiithrown side.

8.5.6 Active and Inactive Faults Faults capable of slippage during a short span of time are known as aclive P~ults. These slippages are episodic and are known Tor their recurrence. The inlervals of non slippage vary from a few years to hundreds or lhousands of years.

To check whether a fault is active or inactive, it is necessary to elucidate history of past events from field evidences. Often neotectonic movement may be visualised along the faull. On the other hand, a fault that has remained undisturbed for thousands of years mdy be cdled as inactive or passive (Figure 8.40).

Materials of the Earth

( a ) Inactive fau l t

/----TI fault

( b ) Active faul t represented by a fault scarp on the surface

Figure 8.40 : Inactive and Active Faults

How to Recognise Active Faults in the Field ?

For a civil engineer it is important to know, how a geologist recognises active faults in field.

1) Recent deposits, unconsolidated alluvium, soil, etc., are useful in recognition of recent crustal movements because of their young age and widespread occurrence. If a fault has displaced these recent deposits, it clearly indicates that the shift is younger than the host material. Passive fault, on the other hand, may be covered by the younger undisturbed deposits. Thus, uninterrupted alluvium beds may be considered as free from recent crustal fault movenlents.

2) Often physiographic criteria may be used to ascertain whether a fault is active or inactive, such as

a) escarpments in alluvium or recent sediments. These are not to be confused with the fault scarps which may be a direct evidence of active faults (Figure 8.40), and

b) straight stream channels or stream offsets. It may be remembered that such evidences are indirect have to be necessarily supported by unambiguous evidences

8.5.7 Seismic Faulting

Seismicity is associated with movement along fault planes for a time span varying from a few seconds lo tens of seconds. Shearing normally takes place along faultslshear zones by ductile processes that do not give rise to seismic events, i.e. earthquakes, It is believed that ductile deformation is achieved by high confining pressures and high temperatures. Seismic faulting can only occur at a depth more than 5 kilometers if hydrostatic pressure is comparable with total confining pressure.

Recurrence of a Fault/Earthquake Recurrence

It is often observed that a particular prominent fracture or rupture zone in different stratigraphic horizons, may be represented by a fault, silica veins, a fracture zone, cataclasites, mylonites etc. at different levels. Some splays of brittle fracture or a fracture zone may be reactivated due to recurrence along older fault zonelplane, which may be explained by considering Figure 8.41.

The idealised stress-displacement relationships for initial brittle failure followed by , subsequent shear sliding on a faultfiacture plane is shown in Figure 8.41 (b). Initial failure

I S occurs at the critical or peak stress (3,) that the rock can sustain. This is followed by a sharp

% 2 decrease in the ability of the rock to sustain the differential stress. However, if a confining

I pressure exists such that here is a positive stress acting normal to the fracture plane, the rock I mass is capable of sustaining the differential stress, the magnitude of which is decided by the

frictional resistance to sliding pn the fault/fracture plane. mils differential stress (SJ, is called . 70 as residual stress.

With an increase in differential stress lo a level SF; (Figure 8.41 (b)), subsequent displacement takes place in a successic~n of jerks (1,2,3, elc. Figure 8.41 (b)) at whlch an increment of slip takes placc and the diflerential stress again falls to S i . In natural condition increments of fault movement may be separated by Lens, hl~ndred or thousands of years. In the passive periods (i.e. between 1 and 2, Figure 8.41 (b)), the faultlfracture plane may develop a smdl dcgree of cohesion duc Lo percolating iluids. This change in slrcss from S, to S i brought out by re-shear (recurrence or reactivation) causillg seismic activity. Magnitudes and periodicity of subsequent earthquakes arc however, unpredictable.

~t must be reincnlbered Ulat faults are initiated only once and may reactivate many times at different stratigraphic levels.

Fracture zone

Fault zone

(USL I

I Shear I zone

( L S L I

Surface ~ x p r ~ s s i o n P--i

l

Structural Gcology

I ( a ) Recurrence of a faulting Displacement ( d I

( b I Earthquake recurrence

Figure 8.41 : Rcshenr; n) Concoptunl Block D i n g m Showing Rcslrenr nt Different Structural and Stmtigmphic Lcvds in the Fonn of n Shear Zonc (LSL) Fault Znoe (USL) and n Fracture ZOIIC (Sudncc), b) Sttws-strnin Diagram for. Reshear, S,, : Criticnl or Pcnk Stress, Sr : Residual Strcss,

SR' :Stress Rcquircd for Rcshenr. It ia Lcss Ulm SII

8.5.8 Effects of Faulting on Disrupted Beds We have seen that faulls are interpreled on the basis of certain slruclures in fields (Section 8.5.5). However, h e most important point of discussion is the effecl of faulting on disrupted beds whicl~ may be visualised in terms of relative movements and apparent movements. The apparent rnove~nel~l is a function of several variables, and depends on the net slip, strike and dip of beds, slrike and dip of fault and the attilude of the surface on which observations are madc. 'Ihe relative movement is controlled largely by net slip.

a) Horizontal beds : Normal or reverse faulting of horizontal beds juxtapose older and younger beds on either side of the fault against each other. Throw is equal to the change in altitude of beds.

b) Inclined beds : The effects are complicated by the attitude of Ule fault and also by the dip of beds. Figure 8.42 is an example of a strike fault with essentially dip slip component. The hanging wall has moved down relative to the foot wall (Figure 8.42 (a)). It is seen from Figure 8.42 (b) that same bed is repeated on the other side due to faulting. Figure 8.43 (a) indicates a strike fault where tlre hanging wall has moved up against the footwall. The net slip is equal to apparent movement as evident from the front face of the block in Figure 8.43. After the removal of upper portion of the right block due to erosion, the bed shown in solid black does not repeat out at the surface.

Materials oE the Earth

Footwal l -

Figure 8.42 : Effects of n Strike Fault on inclined Beds. Note that Downtlimwn Direction i s Opposite to Dip Direction of Beds. Beds are Repented, a) Before Erosion, b) After Erosion

Hanging wall

Figure 8.43 : Effects of astrike Fault on Lnelioed Beds. Downthrown Direction m d Dip Direction of Beds is Same. Beds are Onlittcd due to Removal of Uppcr Portion of Hanging Wall; a) Before Erosion, b) Alter Erosiol~

Figures 8.44 and 8.45 illustrate the presence of dip faults. In Fi y r e 8.44 (a), the net slip is equal to dip slip. Figure 8.44 (b) shows a relation after the left hand block has been eroded upto the level of right hand block. Here, apparent moverhent is indicative of displacement

I

parallel to the strike of the fault.

Hanging wall

Rgure 8.44 : Effects of aDip Wuli on Inclined Beds. Note that Apparenl Movement is Along the Strike of Ulc Fault due to Removal oEUppcr Portion of Footwall a) Before Erosion, b) After Erosion

In Figure 8.45 (a), the net slip is equal to strike slip; and the relative movement is indicative of displacement along the strike of the fault. Figure 8.45 (b) displays a relation after erosion. Here, the apparent movement on the front view of the block gives false idea of reverse faulting.

S t n ~ e t u d Geology

Figure 8.45 : Effects of n Dip Wult on Inclined Bcds. Displacement is I'romincntly Along the Strike Direction of a Fault. Note lltnt Apparent Movenlent Indicates Preseuce of n Revcrsc Bnult n) Before Erosion, b) After Erosion

8.5.9 Significance of Study of Faults Study of faults is often critical from civil engineering point of view. Faults may be recognised in the field or inay be deeply buried so that there is no surface expression. If during excavation, for engineering works, the floor is traversed by gouge and brecciated fragmentrury material, it is, oi'ten,'advisable to abandon the sile or atleast seek expert advice for lrcatment of such conditions.

Another important point in the discussion, is whether the fault is active or illactive (section 8.5.6). In such cases, it is advisable to study history of the area of investigation. An exposed fault may be permeable and may be troublesome in reservoirs. Minor seepage may cause along fault.

Presence of fault gouge may cause serious settling problems. This is the impermeable material which may hinder or stop movement of groundwater to cause unevcn settlement. Similarly, in cold climatic terrains, problems of squeezing are common along tlie faults.

IS an active fault is encountered during geological investigation;, extra carc must be taken to make the designs of engineering structure earlhquake resistant.

saQ 3 ;I) Di~;lil1g;l~is11 hceween a fault zorre iiJld it d~citl xrme.

h) B7~pli~iai hlle tcmm '11;hllging wall' and 'foot wall'.

;) Dci'ine strike and dip faults.

d) What is nlloant by nomul and rcvcrse fault ?

c) Draw a diagam showing horst and graben.

8.6 JOINTS

Joints are discontinuities or ruplures along which a negligible or no visible movement is observed. n e s e may be formed due to normal or shear stresses acling on a rockmass, The cause of the stress may be in the form of contraction during cooling of rocks, compression, unusual uplift, subsidence, earth tremors, etc. Joint occurs at the outcrop scale in all rocks and thus, constitutes the most abundant structure in the earth's crust.

Materials of Ule E a ~ A joint set is a group of uniformly oriented joints of common origin. Joint sets inlersecl lo form a joint system. Normally, the geometry of a joint system, i.e, the size, spacing and orientation of joints, vary across contacts between rocks of different lithology.

Attitude of joints is measured by their strike and dip. The strike is the direction of horizontal line on the surface of the joints and dip is the inclination of the joint plane. Joints may be classified geometrically or genetically.

8.6.1 Geometric Classification Similar to faults, i~ this classification, attitude of joints is compared with the attitude of bcds. Three types of joints are recognised as shown in Figure 8.46.

a) Strike joints are those tllat strike parallel to the bedding plane. Inclinatioll of the strike joints may differ from the dip of the bedding plane. Bedding joints, in Ule true sense, are parallel to bedding plane.

b) Dip joints arc those that strike normal to the strike of the bedding plane. In other words, dip joints are oriented in the dip direction of the bedding plane.

C) Oblique or diagonal joints strike oblique to the strike of bed.

Ngum 8.46 : Geometrical Classification olJoints. SJ : Strike Joint, BJ : Bedding Joiut, DL : Dip Joint, OJ : Oblique or Dingonal Joint, B : Bed

8.6.2 Genetic Classification Joints, on the basis of their origin, are classified into different categories

a) Joints Due to Erosional Unloading in Isotropic Rocks : This is best seen in massive rocks when exposed on surface. It is believed that deeply buried rocks are constantly under great hydrostatic pressure created due to the weight of great thickless of overburden. Once the overburden is removed due to erosion, pressure exerted on deeper rocks reduces continuously. As a result the massive rocks get expancled normal to the direction of pressure release. Normally, the joints develop parallel to tl~e ground surface. These joints are known as sheet joints or topographic joints.

Sheet or topographic joints are close spaced. It is seen that perpendicular distance between two successive joints varies from a few millimeters to centimeters and are traceable for a distance of few meters. Thus, these joints may act as inflow zones or passage ways for groundwater movement. This is a very commonly encountered type of joints. They aid weathering of rocks.

b) Contraction Jolnts : Polygonal jouils often result due to contraction observed in a cooling igneous rock (Figure 8.47).

BasicaIly, these initiate as tension cracks formed due to shrinkage of cooling rocks. During the process of cooling, the tensional forces acting towards a number of cenlres are set in layers (Figure 8.47 (a)). 'These forces tend to open a series of joints with a hexagonal pattern. With further increase of crystallization towards the centre of cooling mass, the joints develop in depth (Figure 8.47 (b)). In other words, during cooling, a uniform tensional stress may be developed in the plane of contraction, thus, giving rise to hexagonal columnar joints. These are seen in igneous rocks and also in some sediments.

Plan v i e w Shrinkage ,-, direction Structural Geology

FTgure 8.47 : Wonnation of Cooling Joints, Polygonal in Shnpe in n Cwliog lgncous Rock

When igneous dykes are intruded in the rocks, both the walls of the dyke remain relatively cooler compared to internal hot zone of the dyke mass. Coolil~g joints thus initiate normal to these walls, forming a set of horizontal joints in addition to the normal vertical sets of joints. As a result brick like blocks, that can be easily excavated, develop in a dyke. Such dyke material is used as road metal in many places.

c) Tectonic Joints : Tectonic joints are those fraclures which owe their origin Lo some kind of tectonic activity taking place in the eruth's crust.

Flexure Folds are sften accompanied by fractures. An analysis of the pattern of these fractures, i.e. their orientation, frequency, spacing etc., may give some clue to the forces that gave rise to folding.

Extension Joints normally develop in the direction of maximum compressional slress and are seen perpendicular to the fold axes (Figure 8.48).

Joints that form parallel to the axial plane of the folds are known as longitudinal joints (Figure 8.48). These are tension fractures or release joints similar to those formed in the direction of maximum strain.

Cross joints or oblique joints are those that develop oblique to fold axis. These normally form conjugate arrays and thus, called shear joints (Figure 8.48).

RJ EJ : Extewioo Joints, RJ : Releme Joints, SJ : Shear Joints-

Rgurc! 8.48 : Tectonic Joints (called FrPdures) Associated with edding

Materials of the Earth d) Superfacial Movements : In many cases, joints are created due lo superfacial movements, e.g., glacier crevasses in thick soil/weatl~ered rocks.

e) Distinction between Fractures and Joints : A fracture is defined as a discontinuity of tectonic origin with in the upper and middle structural level (refer section 8.2.6), while a joint is adiscontinuity of mechanical origin (includes Section 8.2.6 a, b and d) typical of surface or upper structural level.

In Figure 8.49, A, B, C, D, E, Fare a sequence of beds lava flows. All beds are affected by fracturing and the fracture is traceable for an elevation of 400 m. On the other hand, bed A shows presence of vertical contraction joints, while beds B and D exhibit sheet joints. In other words, nature of jointing may vary from bed to bed.

Fracture J

Rgure 8.49 : Difference Between a Fracture and A Joht

Distinction between fractures and joints can be made in horizontal undisturbed beds and lava flows, when these are seen invertical sections. However, such distinction is very difficult in disrupted or folded beds. It is seen from Figure 8.49 that if more than one bedllava flow is affected by similar kind of brittle deformation then the failure is called as a fracture. Jointing, on the other hand, is restricted to a particular bedflava flow, e.g. in Figure 8.49, the lowermost bedllava flow shows development of columnar joints (untraction joints) while sheet jointing is typical of upper bed/flow. In other words, the lower bedAava flow is indicative of presence of cooli~lg joints and the upper bedAava flow shows presence of erosional unloading joints.

8.6.3 Quantitative Description of FracturesIJoints Rock masses are seldom free from fractures which divide the rock into blocks of differenl sizes, shapes and discontinuities. Rock mass characterisation or quantitative description of rock mass for design and coristruction of excavations in gcologic media is probably most

. crucial and deinanding element in applied geology. In this, the requisite condition is the ability of the rock to be stable under modified conditions. Thus, quantification of rock mass characteristics helps in distinguishing weak, moderately strong and strong rocks. Another imporlant factor taken cognisance in the design of man-made structures is the scale of structural features. They can be

a) Microscopic : Features can be seen only with the help of microscope.

b) Mesoscopic : Features on the scale of excavation.

c) Macroscopic : Features on the scale of engineering site or region. Broadly speaking, the term rockmass includes the rock fabric and jointslfractures. Traditionally the data on joints have included the geometric characteristics altitude, spacing, extent, aperture, roughness and some cormnents on infilling nuterials and wall conditions. Joints are then grouped into families based on similarity of attitudes. Figure 8.50 shows a typical intricate jointifracture pattern seen in field.

a) Joint orientation and dip : Joint orientation and dip are important in controlling the configuratio11 of landform assemblages. Failure by slidingor toppling is more likely to occur on a joint plane which dips towards thc slope. Possible situations are referred in Table 8.1

" Joint orientation and dip are, often, quite variable. It is seen that major or master joints extend over considerable distance. This weakplane causes rapid erosion. Landforms, those develop along them, normally show negative relief.

Table 8.1 : Classification for Joint Orientation (after, Selby, 1980) I

Potential Strenph Condition

Nature of Joints

i

Very favourable

Fair I Favourable

Tensile (Waviness)

Joints dip > 30" into the slope

Shear (Smooth)

Joints are weakly developed

Joints dip from horizontal to 30" into the slope

Horizontal to 10' di , Nearly vertical (80-800) in hard rocks

Joints are not interlocked

Horizontal to 10" out of slope

Unfavourable

Joint spacing : Joint spacing is the perpendicular distance between two adjacent joints (Figure 8.50) and can be measured directly with the help of a tape (Table 8.2)

1 %?vowable

Table 8.2 : Suggested Classification for Joint Spacing (after, USAG, 1977)

Joints dip out of surface, master joints 10-30°, Random joints 10-70'

1 Description I Spacing 1

Joints dip out of surface. Master joints 10-20' Random joints 10-30"

Joints dip out of surface Master joints 30-80' Random ioints > 70"'

I Extremely Wide 1 > 2 m I

Joints dip out of surface. Master joints 20°, Random iou~ts > 30'

1 Wide 1 20 cm to 60 cm I I Moderately wide 16cmto20cm I 1 Narrow ( 6mm to6cm I 1 very narrow ) < 6 m m

It is seen that when joint spacing is narrow, the rock is broken into small pieces. Joint spacing may also be expressed in terms of area intensity index in which the area of joint surface per unit volume of rock and the average size of the unbroken block are

A : M u r e , B r Spacing, C r #enisteuce D I Block She, E I Aperture FLIYng, F I Roughness (a) Quantitative Description ~PDiseoatinllitiei

Structural Geology

Materials of the Earth 'Plumose structure

,

Trace of main joint face

(b) Schematic Diagnm Showing Ideal Structures Along a Joint Face

Figure 8.50

) Joint surface morphology : Qualitatively joint surfaces may be designated as smooth, rough or wave. Rough and wavy surfaces provide greater strength to the rock. This is thus, an indication of resistance offered by the rock during applied load.

Some joint faces are characterised by plumose markings (Figure 8.50 (b)), a structure distinguished by featherlike surface patterns. They are commonly seen on joint surfaces of hard rocks. Plumose markings are physically composed of a series of tiny ridges and troughs, a microtopographic relief on the main joint face.

The presence of plumose markings indicates a brittle deformation achieved by a rapid, near instantaneous, snapping apart of the rock in an explosive way. Thus, this feature, if encountered, should he given due importance in rock mass characterisation.

d) Joint aperture and infill material : Aperture is the perpendicular distance between both walls of a joint (Figure 8.50 (a)). It is an important parameter that decides the movement of water inside thefrock and thus, governs secondary permeability values.

Aperture is described by terms given in Table 8.3.

Table 8.3 : Description of Aperture

1 10-100 cm 1 Extremely wide 1 Open ~oints 1 ,

Aperture

> l m

Description

Cavernous

:,i0.5mm-2.5mm 'Open

0.25 rnm - 0.5 mm Partially open Closed Joints

0.1mm-0.25mm Tight

< 0.1 mrn Very Tight .- - Often, fractures may be filled by calcite, silica, fault gouge, silt, etc. that may enhance . ' impermeable nature of rock and may also increase strength, i.e., presence of clay can decrease joint stiffness and also shear strength. When filled fractures are encountered following points are noted down.

1) Mineralogy of filled material

C ;,

2) Particle size.

1 -1Ocm

> 1 cm

2.5 mm - 10 rnm

Very wide

Wide

Moderately wide Gapped Joints

k

d *

3) Permeability 4) Consolidatio~l ratio 5) Wall roughness 6) Width of fracture

e) Joint persistence : Persistellce is the lrace length of a fracture observed in an exposure or aerial photograph (Figure 8.50 (a)). In this, it is necessary to trace the kind of termination of the fracture which may be beyond the exposure (x), with is an exposure (r), or against other discontinuity (d ). This helps in differentiating the master (systematic) and minor (non-systematic) fractures.

Termination Index (TI ) for a domain may be calculated as follows

where

T, = Termination Index, N = Number of fractures

Cr = Fractures ending in rockmass compared to total number of fractures (x + r + d)

f) Weathering Index : Rocks are frequently weathered and altered by hydroll~ermal processes. However, the degree of weathering is not uniform in the exposures. Thus, fist describe the illtensity of weathering of the rockmass as a whole (that is, intensely wealhered, moderately weathered, fresh surface, etc.), followed by the description of the rock blocks consisting of walls of individual fractures.

g) Surface roughness : 'Roughness' of rracture walls can be explained by 'waviness' and 'unevenness'. Wavincss is influencia1 in deciding the initial direction of shear displacement relative to the mean fracture plane while unevclmess changes the shear strength. Waviness can also be explained by wavele~lgth and amplitude.

The surface roughness of fracture walls is described by terms like polished, slickensided, smoolh, rough, defined ridges, small steps and rough face.

The abovemei~tioned geo~nelric parameters may be measured accurately in Ule field and be recorded carefully in the fracture survey data sheet (Table 8.4). 'Ihe collected facts are very important in predicting bebaviour of the foundation material and also crucial in deciding stability of man made structures. '

Table 8.4 : Fracture Survey Data Sheet

Location :

Day/Date/Time :

Size of an outcrop :

Sheet No.

No. Remarks (Weathering index, block size, number of joint sets, etc.)

W = Wavelength A = AmpliNde

Strike Direction

Dip amount

Direction

Infilling Material (if any)

.

Persistence Penne- ability

Aperture Roughness

W

.

A

Materials oC the EsrUl 8.6.4 Significance of Joints Joints are important in a number of ways. Their presence appreciably affects the slrength of a I

rock and they must be studied very carefully in surface excavations and also in construction of tunnels and dams. Figure 8.51 shows a parabola interrelationship between fractures and

I

stability of a tunnel, Though, the influence of fractures has many components, only four components are discussed here (1,2,3,4, Figure 8.51).

High fracture density/ Intersecting fractures / more surface area in f l o w ' o f water

for weathering

I

High stress / alteration of rock

s t r l l c t u r ~

(1) High Fracture Dcmity (he Close Spaced Fractures) (2) Continuous Flow of Water Along Intersected Fractures (3) Prescsceof Wqter in the FFaauren Reduces B e (4) High Streosw may Alter the Rock Structure

Noknal Stress

Figure 851 : Conceptual Diagram Showing Effecb d lour Types of bacture S y s h on Tunnelling C o d t i w

1) High fracture density creates more surface area for weathering that reduces mechanical strength of rocks. Rocks often show variable weathering index even in an area of a few square meters. In such cases, it is necessary to measure attitude and persistence of indigidualfractures and assess qualitatively-weathering index of rocks along with other parameters.

2) Presence of high intersecting index of fractures may increase permeability that could alter mechanical and hydrogeological properties of rocks. In this condition, it is necessary to find hydrogeological properties of rocks and role played by intersecting fractures in modifying these properties should be established.

3) Presence of substantial amount of water in fractures may reduce the normal stress and could alter ,the rate of erosion.

4) It is seen that rock mass is capable to sustain the differential stress, called as residual stress (SR, Figure 8.41 (b)). However, high stresses could altei'the rock structure (section 8.5.8). Therefore, it is always desirable to find out seismic history of the area of investigation.

Table 8.5 : The Effect of Fractures in Tunneling

Strike normal to tunnel axis

I

Often, stability of slopes depends on the ratio of excavation dimensions and joint spacing. - Commonly, fractures dipping into the slope initiate movement of water and thus, promote

80 . increase in porewater pressure in the potential slip zone. - -

I

Strike parallel to tunnel axis

Drive along dip

Gentle dips irres ect~ve of stryke

Gentle dip

20" - 45O

Favourable

Steep dip

45" - 90"

Very favourable

Drlve agalnst dip

Gentle dip

20° - 45"

Unfavourble

Steep Dip

45" - 90'

Satisfactory

Gentle dip

20" - 45O

Satisfactory

0" - 20"

Unfavourable

Steep dip

45" - 45O

Very unfavourable

I

SAQ 4 a) P!is&J:ngois&n between R joint ulrl a fxackure.

b) Gi~:e classifir;;etion of' tectonlis joirats.

c.) Craw rsna alraotcd c3iagr.m sPlowing different pxirsaneters of joi~mts/frackures that arc sneasuacd ip field.

d) Wklat is joht aperture ?

e) WlmQ is surface roughness ? .

8.7 FOLIATION AND LINEATION

The deformation structures which develop in rocks at a deeper structural level are differetlt from the structures developed at shallow structural level in the crust. Tlais change in behaviour in rocks is possibly due to a combined effect of higher temperature and lithostalic pressure prevailing at deeper struclural level. As a result, sets of new planar and linear structures associated with strong intricate folding and ductile shear zones develop in these rocks. They are foliations and lineations.

Foliation is a common term used for planar slructures including cleavage, flow banding in igneous rocks, pebble alignment in conglomerate, elc. Tl~ese can be classified into

a) Primary, if their origin is due to primary igneous and sedimentary processes, and

b) Secondary, if they develop by processes of deformation and metamorphism, Lineation is the parallel alignment of elongate, linear elements in a rock. For example rods, mullions, boudings, pebbles, etc.

Three major types of foliation are recognised, (a) cleavage, (b) schistosity, and (c) gneissose banding. Only cleavage and its significance is discussed here.

8.7.1 Cleavage Cleavage is a common term used for tectonic foliation which develop in rocks during deformation. It refers to close spaced, aligned, planar to curviplruiar discontinuities lhal are normally associated with folds and oriented parallel to subparallel to the 'axial surraces of folds. Cleavage consists of two fundamental structures, namely,

a) Cleavage bands, defined by preferred orientation of mica rich layers, and

b) Microlithons, Intervening quartz - feldspar rich thicker layers. Significance of Cleavage

a) Relative orientation of cleavage and bedding is often useful to predict the general locatioil and direction of the fold closures (Figure 8.52 (a)). ?he same relationship also depicts whether the bed is overtumed or nol. The thumb rule is-if bedding and folialion dip in opposite direction, the bedding must be upright and if they dip in same direction and also if bedding is sleeper than cleavage then bedding is overturned (Figure 8.52 @)) while bedding is upright if it dips shallower than cleavage.

In Figure 8.52, (a) shows geometric relation between cleavage and folding, (b) shows recognition of upright and overturned beds, (c) shows recognition of shear zone, where S is a continuous coarse foliation charackrised by preferred orientation of micas and elongated quartz and C is a set of shear fractures that develop parallel or at an angle to the shear zone walls, and (d) and (e) shows relation between foliation and dam axis. In the most favourable condition dam axis is parallel to foliation as shown in (d) md in (e) seepage of water from reservoir is most likely the cause.

Structulal Geology

Materials of the Earth a x i a 1 trace

cleavages Weo k, incompetent

rock Hard,competent

I B>C

I C > B

Overturned normal, upright l imb limb

1 seepage 7 I

foliation planes f o ~ idt ion planes

( d 1 Favourablu condition ( e ) Unfa vou r a b I e condition:

Figure 8.52 : Signilicancc of ~leavagel~olintion

b) Cleavage is useful to recognise ductile shear zones (Figure 8.52 (c)). In this, two foliations are recognised, i.e. S-C fabric. The S - foliation is a continuous coarse foliation characterised by prefetred orientation of micas and elongated quartz. The C - foliatiori is a set of shear bands that develop subparallel to the shear zone walls. Normally the S - foliations are oblique to C - foliation.

c) Relation between foliation and dam : If foliation is parallel to the dam axis, the condition is acceptable. However, if it is across, there is a fear of seepage along foliation and such a dam alignment is regarded as unfavourable.

8.7.2 Lineation

Lineation is a set of penetrative linear structures produced in a rock as a result of deformation. Many types of lineations are icnown to occur, for example, intersection of colour banding on a cleavage surface produces a typical stripping lineation.

Following are the major types of lineation

a) crenulation Lineation, . *

b) directional orientation of elongated mineral grains, c) intersection lineation,,

d) mineral lineation produced due to platy, acciculat or fibrous minerals, and e) slickensides, striation;

82 The different groups of lineations are well illustrated in Figure 8.53.

Structural CeologY

r 1

( a ) Stripping lineation I b ), Crenulation ( c ) Intersection lineation

I .{.d ) Mineral \i neation ( e ) ~ l o n ~ a t e d [ s t r o t c h i n ~ ( f ) Striation lineation lineation

L J

Figure 8.53 : Types of Lineations

The study of lineations helps to explain the structural geometry of the area as they generally develop parallel to fold axes.

SAQ 5 a) What arc tPlc milji~a' typcs trf foliation ?

b) Idow clt:;iv;lgc is useful in rrcognitio~1 of upright and overEurrbetl tiia~b of :I fold '?

c) What is lincalion ?

1) Wlut is the: impt~rt:balcc of study of lillratioil ?

8.8 SIGNIFICANCE OF STRUCTURAL GEOLOGY

Quantitative analysis of geological structures is an important component in many engineering activities. This study is crucial in deciding s@bility of various man-made structures, e.g. dams and reserviors, tunnels and shafts, rock and soil slopes, foundations, underground operations, etc. The first four catagories are more relevant to civil engineering works. Hence, only these are discussed below. a) Dams and Reservoirs

The relation between local geology and dam is vital in the design and selection of type of dam to be constructed. The geologic considerations of dam sites include both the surface and subsurface. 83

Materials of the Earih Following detailed information is required for site selection of dams and reservoirs :

1) Rock : Composition, thickness, texture, vertical and lateral variation, weathering index.

2) Structure : Folds, faults and joints.

3) Hydrogeology : Aquifer characteristics, porosity, pem~eability, surface run-off, elc.

4) Seismicity : Hjstory of earthquakes in the area for a couple of hundreds of years.

Figure 8.54 shows the relation between foundation rock characteristics and dam. Analysis of above factors particularly fractures is important in deciding many design details.

Wgure 8.54 : Relation between Foundation Materid and Dam

b) Tunnels and Shafts

Thk stability of tunnels and shafts depends on rock structure, rock stress, groundwater conditions and construction techniques. A detailed careful study of fundamental principles of structural geology provides invaluable guidelines for stability assessments (Figures 8.24,8.50 and 8.51).

C) Rock and Sol1 Slopes

Three basic mechanisms for failurc are recognised, viz, slide, flow and fall. The potential for failure in these modes can be identified using the principles mentioned in this unit. Thus, the need and scope for further detailed quantitative analysis can then be assessed (Figures 8.23,8.50 and 8.51).

d) Foundations

Rocks are believed to be excellent foundation materials, but near surface, rocks, in general, are deformed as evident from presence of fractures, joints, faults and folds. It is always desirable to establish the relationship between strength of the rock and proposed load (Figures 8.23,8.39,8.40, 8.41 and 8.52).

e) Regional Structures

Regional structures or the macroscopic structures. extended over kilometres, are often influential in creating the expression of the earth's surface, e.g. presence of a linear valley, large streams, etc. It is seen #at drainage pattem of streams is oflen (Figure 8.55) controlled by fractures and folds. In places, where the rocks are cut by parallel zones of faulting or steeply inclined systems of joints, the streams normally follow the easily eroded fracture zones and the tributaries are controlled by minor fractures (Figure 8.55, (a) and (b)). The typical pattern developed here, is known as trellis type river system. Similarly, presence of a dome or basin may be interpreted from the

84 presence of radial drainage pattern (Figure 8.55 (c)). Radial pattern may also be

recorded around the plunging axes of anticlines ar~d synclines. Figure 8.55 (d) indicates 3 Structural Geology

a large anticline plunging to the left. Here, the lithology is in the'form of alternate hard and sofl rocks.

( b ) Trellis

( c ) Radia t ing ( d l Annular

Figure 8.55 : Mnjor Types of Stmctun! Cot~trolled Drrai~~age Patterns

Struclural geology is the study of geometrical configuration of planes, lines in rocks and deformed surfaces. Three major structures are recognised, viz, fractures, folds and foliations. It is seen that these structcres normally develop at different structural levels under varied conditions like elevated temperature, pore-fluid pressure, confilling pressure, etc. During deformation, very onen the original characters of the rock ace obliterated. This is called metamorpl~ism.

Based on relative movement of becls along planer surfaces, fractures are classified into faults and joints. Study of these weak surfaces is oflen decisive as these are manifestations of major subsurface structures and defor~ned terrains. These may be influencial in shaping tile surface of the earth. Occasionally they provide a clue to causative faults in seismically active regions.

Folds are undulations produced in stralified rocks that are traceable for a distance of a few centimetres lo a few lens of kilometers. These are normally seen in incompetent beds and me typical of lowcr structural level. It is seen that mechanical and hydrogeological properties ace altered due to development of folds and associated syngenelic lectonic joints. As a result, the deformed rock mass gets divided into blocks of uneven shapes and sizes and thus, introduce the concept of a domain (an isotropic body of smallest dimension),

Materials of the Eadh - .. -.

8.10 KEY WORDS

Anticline and Syncline : Anticline is a fold with a corc ~Ssualigraphically older rocks, a synclinc is a fold with a core of stratigraphically younger rocks.

Deformation : Deformation is the proccss that changes size or shape of a rockmass.

Dip : The dip of a bed is the angle between the bedding and horizontal plane mcasurcd in a vertical plane.

Domain : An isotropic rock body of smidlesl dimension.

Fault : A fractured surPacc or zone in rock along which appreciable displacement has taken place.

Fold : A curvature in a geological surface.

Foliation : Closely spaced penetrative plana anisolropy, both primary and secondxty, in rocks.

Fracture : Fracture is a discontinuily/hrc& along which cohesion of the nlaterial is lost.

Horst and Graben : Horst is a raised block bounded by two parallel faults, in which, length parallel to strike, is greater than the width. Graben is a zone of subsidence betwcen two parallel faults in which, the length parallel to strike, is Inore U'lan thc widlh.

Lineation : Subparallel to parallel alignmcnl of elongate, linear elements in a rock body that arc penetrative at tlic outcrop scale and/or handspccimen scale of observation.

Pure shear : Irrotational strain.

Shear zone : A relatively narrow zone of large shear strain bounded 011 both sides by relatively mldefor~ned rc~ck. i

Simple shear : Uniform volume rotational strain.

Strain : Nonrigid body movement involving dilation and/or distortion.

Strike : ' h e strike of a bed is the direction o l a line formed by the intersection of bedding and horiz,onlal plane.

Wavelength and Amplitude : Wavelength of a fold is the distance between two succcssive hinges and amplitude is a distance bclween hinge and inflexion point when thc folded surface is seen in profile.

8.11 FURTHER READINGS -

I) Billing, M.P., 1994, Strucrural Geology, Prenticc-Hall, New Delhi, 4th edition, 2) Jaeger, J.C., and Cook, N.C., 1969, Alnflomentals of Rock Mechanics, London, Metheun.

8.12 ANSWERS TO SAOs SAQ 1

a) Deformation is the process that changes size or shape of a rock mass.

b) i) Refer sub-section 8.2.2.

ii) Pure shear is irrotational deformation and simple shear is a uniform volume , rotational homogeneous deformation (Figure 8.5).

iii) An isotropic body has uniform mechanical properties in all directions. An anisotropic body has different mechanical properties in differenl directions.

c) Refer sub-section 8.2.5. 86 d) Attitude of &ds is described by their strike and dip.

el

SAQ 2

4

b)

c)

SAQ 3

a)

b)

c)

dl

e) SAQ 4

a)

b)

c)

d)

e) SAQ 5

a)

b)

c )

d)

True dip is the inclination of a bed that is measured nornu1 to strike direction or along tlie direction oS maxilnunl inclination. Apparent dip is the inclination measuretl ill a vertical plime that makes an ruigle to truc dip direclion.

Anlicline is a fold in which liribs diverge downwards and older rocks occupy core or the concave side of the fold. Syncline is a fold which is concave upwar-ds with younger rocks in the core.

Ilel'er sub-section 8.4.1 and lygure 8.17.

Plunge is an inclinalio~l of the fold 'axis rncasured with respect to a reference horizo~ital plane (refer Figure 8.16 (d)).

.4 fault zone is a tabulu zone of uncertain width that contains many parallel or coinplcxly ultcrsecting nlinor faults. A shear zone is a narrow zone of large shear strain bounrled 011 both sides hy relalively undeformed rocks (refer Figure 8.26 (b) and (c) respcclivcly).

Rcfer sub-seclir?n 8.5.1 iuid Figuru 8.27.

Refer sub-section 8.5.3 and Figure 8.31 for strike fault, and sub-section 8.5.3 and Figure 8.32 for dip Tault.

Refer suh-sectictn 8.5.3 r~ild Figure 8.34 for norinal fault, and sub-seclion 8.5.3 and F'igure 8.35 lor reversc hull.

Refer Figurc b.37

Refer sub-section 8.6.2 anti F~gure 8.49.

Refer sub-scclion 8.6.2 and Figure 8.48.

Rcler Figure 8.50 (a).

Aperture is Ule perpliclicular distance between both walls of a joint.

Refer sub-scclion 8.6.3.

Three types of fr~liatians art: recognisd, (a) cleavage, (h) schistosity, and (c) gnciss(')sc hilndillg.

Refer sub-section 8.7.2 anti Figute 8.52 (b).

Linealion is a set of penetrative liliciu structures produced in a rock as a result of deformalion.

The study of lineations helps to explain the structural geometry of the area as these arc colnnlonly seen parallel to fold axes.