Structural Geology | Geology Optional for UPSC PDF Download

Introduction

• Structural geology involves examining the geometric arrangement of planes, lines, and deformed surfaces in rocks. These structures are the result of forces acting on or within the Earth's crust. Essentially, 'structure' refers to features created by the deformation of rocks, which is caused by various endogenic processes. Deformation is often a part of the regional metamorphic cycle, leading to a series of mineral and structural changes during metamorphism.
• Each rock has different mechanical properties that depend on the conditions under which deformation occurs. The complexity arises from the wide range of chemical and mineralogical compositions that rocks can have, as well as the diverse chemical environments in which deformation takes place. A crucial factor in this process is the role of pore fluid pressure in altering the mechanical properties of rocks. Therefore, studying the interactions between deformation and metamorphic processes is essential for understanding the origin of structures in deformed rocks. Deformation and metamorphism are closely related phenomena.

Materials of the Earth

A structural geologist typically approaches their work in two phases:

• Observing the degree of deformation and measuring various geometric parameters; and
• Interpreting the deformation path.

The attitude of these structures reflects the relationship between the deforming forces and the rock mass. In this unit, we will explore how rocks are deformed at different levels and the significance of these deformations. To understand deformation, it is crucial to discuss the behavior of various parameters such as temperature, pressure, and strain rate.

A three-dimensional analysis of geological structures, along with other geological investigations, is vital for assessing the stability of many civil engineering operations.

Objectives

After studying this unit, you should be able to

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

Deformation

• Deformation refers to the process that leads to the formation of various geological structures by altering the size, shape, and sometimes the orientation of a rigid body.
• This process is crucial in the field of structural geology, where the relationship between stress and strain is studied to understand how rocks respond to applied forces.
• The forces acting on the rocks in the Earth's crust are mainly gravitational, along with stresses generated by the movement of rocks at depth. These forces create a set of stresses that induce strain in the rocks.
• When a force (F) is applied to a unit surface area, it can be divided into normal stress and shear stress. Normal stress acts perpendicular to the surface, while shear stress acts parallel to it. In three dimensions, shear stress can be further resolved into two components that are perpendicular to each other.
• Normal stress is always perpendicular to the plane, while shear stress is parallel to the plane. In two dimensions, shear stress can be resolved into normal and shear components, resulting in three different stresses.
• When the principal stresses are uniform, the condition is referred to as hydrostatic stress, which is similar to the stress state of a fluid. At greater depths, rocks experience lithostatic stresses due to the weight of overlying rocks.
• If the principal stresses are not uniform, the system is called a stress system, which leads to distortion in the body. The response of a deforming body to a stress system falls into four categories:
• Rigid Body Translation: Movement of a body without any change in shape.
• Rigid Body Rotation: Movement of a body with change in shape, where lines before deformation rotate around a single point.
• Distortion: Involves change in shape of the body.
• Dilation: Causes positive or negative change in volume with no change in shape.

Strain

• Strain can be classified as homogeneous if it occurs uniformly throughout a body and heterogeneous if it varies within the body.
• In rocks, strain is generally heterogeneous, but in smaller domains, it may be homogeneous.
• During homogeneous strain, straight lines remain parallel, and a circle is transformed into a strain ellipse.
• In contrast, during heterogeneous strain, lines become curved, and parallel lines become non-parallel.
• Pure shear
•  In pure shear, strain occurs without any change in the axes during progressive deformation. 
• Shear strain
•  Shear strain involves rotational strain, where the body rotates in a clockwise manner during progressive deformation. 

Strain Ellipsoid

Strain in a deformed body is assessed by comparing the size and shape of the strain ellipsoid with that of the initial sphere. The strain ellipsoid represents the current state of deformation, with axes X, Y, and Z indicating the maximum, intermediate, and principal strain directions, respectively.

There are two key components of homogeneous strain: pure shear and simple shear.

• Pure shear, also known as irrotational deformation or irrotational strain, involves a translational deformation history.
• Simple shear, on the other hand, refers to a uniform volume rotational homogeneous deformation.

Behavior of Material Under Stress

Elastic Strain:

• In this initial stage, the strain is very small and increases progressively with applied stress.
• If the stress is removed during this phase, the rock returns to its original state, hence the term elastic, temporary, or recoverable strain.
• This type of strain is associated with the propagation of seismic waves.

Plastic Strain:

• Once the stress exceeds a certain limit, the strain becomes permanent and irreversible, leading to the deformation of the body.

Rupture:

• As stress continues to increase, the body ultimately fails by rupture.
• Several factors come into play during this stage, including the nature of deformation, the physical conditions at the time of rupture, and the orientation of the material.

It is important to note that laboratory tests often do not provide an accurate representation of how similar rocks behave under deformation caused by orogenic compressive stress. This is because laboratory results tend to yield exaggerated values. During natural deformation, factors such as the presence and composition of materials, temperature, and pressure play a significant role in influencing behavior.

Brittle and Ductile Deformation

Brittle Deformation: Brittle deformation occurs when rock is subjected to deviatoric elastic deformation, leading to failure. In this process, the deforming rock loses cohesion due to the formation of fractures, resulting in a loss of continuity. This type of deformation is characteristic of the surface or upper structural levels of the Earth's crust.

Ductile Deformation: Ductile deformation, on the other hand, is typical of the middle to lower structural levels in the crust. It produces heterogeneous strain, as evidenced by the development of folds. Unlike brittle deformation, ductile deformation is marked by a significant absence of faults and fractures.

Factors Controlling Behaviour of Materials

• Confining Pressure: At greater depths, rocks are subjected to lithostatic or confining pressure, which is a hydrostatic pressure based on density and depth. Experimental evidence indicates that as confining pressure increases, the effective ductility of the deforming rock also increases.
• Temperature: An increase in temperature can transition a rock from a brittle to a ductile stage of deformation.
• Pore-fluid Pressure:The presence of fluid significantly affects the mechanical properties and mineralogy of rocks. The mechanical properties of rocks are altered when:
  • Pore-fluid pressure approaches the magnitude of confining pressure, leading to brittle deformation.
  • High fluid pressure reduces the strength of the rock. Experimental results have shown that under high fluid pressure, minerals like quartz can undergo ductile deformation at lower temperature and pressure conditions.
• Time:Rocks subjected to a uniform stress field over an extended period exhibit creep behaviour, which can be explained in three stages:
  • Primary creep: Rocks deform in a clastoviscous manner.
  • Secondary creep: Rocks deform through viscous flow.
  • Tertiary creep: Rocks undergo accelerated viscous deformation leading to failure.

Mechanisms of Rock Deformation

The process of rock deformation is influenced by various factors such as mineral composition, rock texture, pressure, and pore fluid pressure. For instance, granite, despite its mineral content, is stronger than sandstone, argillaceous rocks (which are fine-grained and clayey), or carbonate cement due to its texture. Additionally, rocks with a granular texture are generally weaker.

To understand the mechanism of rock deformation, microfabric studies under a microscope are crucial. During the bulk creep of rocks, several micro-mechanisms come into play, including cataclasis, pressure solution, creep, and dislocation glide.

Cataclasis

• Cataclasis refers to the process of brittle failure that occurs along grain boundaries and within individual grains or crystals.
• This mechanism is typically associated with high rates of deformation and is common in the upper structural levels, near the Earth's surface.

Pressure Solution

• Pressure solution involves the transport of material along grain boundaries.
• This process is facilitated by the presence of a fluid film along the grain boundaries, which enables the transport of material.

Creep and Dislocation Glide

• At elevated temperatures and low stress levels, deformation is primarily achieved through creep and glide mechanisms.
• In both brittle and ductile types of deformation, strain is induced by the glide of dislocations within the material.

Fundamental Structures

The rocks present in the field exhibit the following structures, which will be described in detail in the appropriate sections.

• Fractures:. fracture is a type of discontinuity where the cohesion of the rock is lost. Fractures are typical of competent rocks and usually develop at higher structural levels. There are two main types of fractures:
• Faults: Faults are fractures that allow significant displacement along the surface of the discontinuity.
• Joints: Joints are fractures where there is little or no movement parallel to the plane of the fracture.
• Folds:. fold is a structure that occurs when an originally flat surface becomes curved due to deformation. Folds are most clearly seen in beds or very plastic rocks.
• Foliation and Lineation: These are planar and linear structures that are commonly found in deformed metamorphosed rocks.

Structural Levels

Different rocks in nature have varying mechanical properties and do not behave the same way under identical conditions of temperature, pressure, and depth. Based on these differing behaviors, three structural levels are recognized:

• Upper Level: At shallow depths, most rocks are brittle and exhibit shear deformation. Only very plastic rocks, such as shales, show ductile behavior at this level.
• Middle Structural Level: This level is characterized by widespread ductile deformation. However, there is a noticeable variation in the style of folding between hard and soft (plastic, incompetent) rocks. Incompetent rocks display disorganized folding, while competent rocks exhibit parallel folding.
• Lower Structural Level: This level is marked by the development of cleavages through metamorphism. At even greater depths, rocks may undergo flow and melting.

Quantitative Analysis of Structures

The quantitative analysis of geological structures is a crucial component of preliminary geological investigations, often conducted as part of feasibility studies for various civil engineering projects. During these studies, data in the form of observed structures are collected in the field and subsequently presented in maps and reports. Observational Data

• The collected data is essential for deriving the movement dynamics responsible for the formation of these structures. Structures can vary significantly in size, ranging from microscopic features to those measuring several kilometers in magnitude.
• Despite this size variation, the terminology used to describe these structures is generally descriptive and non-genetic, meaning it does not imply a specific origin or process. Therefore, there is little distinction in the terminology and description of small and large structures.

Planar Structures in Rocks

• Rocks often exhibit a planar structure that facilitates easy splitting. In sedimentary rocks, this is represented by features such as bedding planes and stratification. A bed is a tabular rock body with upper and lower planar boundaries. Beds are distinguished from one another by differences in physical or chemical characteristics and overall appearance.
• Metamorphic rocks may display features like schistosity and gneissose banding, while igneous rocks may show planar structures such as flow patterns.
• Sedimentary rocks typically show a horizontal disposition. However, if subjected to deviatoric stress, they can become inclined or vertical. The attitude of these beds can be measured using Brunton compasses by considering strike and dip angles.

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(a) Measurement of dip amount

(b) Measurement of dip direction

Some of the common methods to plot structural features on geological maps include:

(a) Bedding and dip of beds

(b) Strike and dip of horizontal beds

(c) Strike and dip of foliation with plunge of lineation

(d) Vertical foliation

(e) Horizontal foliation

(f) Fractures

(g) Dip of joints

(h) Horizontal joints

(i) Fault orientation

(j) Inverted fault

(k) Fault showing dip

(l) Vertical fault

(m) Fault with plunge of strata

(n) Horizontal fault

(o) Fault showing relative movement

(p) Horizontal lineation

(q) Minor lineation with plunge

(r) Minor lineation plunging

Folds

Folds are formations found in layered rocks. Essentially, a fold is when a geological surface or a set of stacked geological surfaces becomes curved. This occurs when the original flat surface is bent due to forces acting parallel to the bedding plane. Folds are among the most impressive structures on Earth. Their size can range from just a few centimeters to several tens of meters.

Understanding Fold Morphology and Terminology

To grasp and describe how rocks are folded, it's important to familiarize oneself with the various terms that describe the shape and size of a fold.

• Hinge: The hinge is the point of maximum elevation on the folded surface. More commonly, it refers to a zone known as the hinge zone. A line that connects all the hinge points is called the hinge line.
• Limbs: Limbs are the sides of a fold, and each fold has two limbs. In other words, limbs with opposing convexities meet at the inflection point.
• Fold Axis: The fold axis is a line that defines the shape of a fold. This axis can be horizontal, inclined, or vertical.
• Plunge: Plunge refers to the inclination of the fold axis in relation to a horizontal reference plane.
• Axial Plane: The axial plane is an imaginary plane that divides the fold into two parts, which can be either symmetrical or asymmetrical. The orientation of a fold surface is determined by its axial plane. When a fold consists of multiple surfaces, a common surface can be defined by connecting successive hinge lines; this is known as the axial surface.
• Inter Limb Angle: The inter limb angle is the internal angle between the limbs of a folded surface. It serves as a measure of how tight the fold is.
• Fold Size: Fold size is determined by its amplitude and wavelength when viewed in profile.
• Wavelength: Wavelength is the distance between hinges on either side of the fold. When the entire fold is not visible in profile, the distance between two inflection points is measured, known as half wavelength.
• Amplitude: Amplitude, or the "height" of a fold, is half the perpendicular distance from the hinge to the inflection points.

When the axial plane is tilted, the highest and lowest points in a fold may not align with the hinge points. The highest point is called the crest, and the line connecting the highest points is known as the crest line. Conversely, the lowest point in the folded surface is called the trough point, and the line connecting the lowest points is called the trough line.

Types of Folds

Anticline: An anticline is a type of fold where the limbs are inclined in opposite directions, with convexity facing towards the youngest bed. In this case, the limbs dip away from each other.

Syncline:. syncline is a fold where the limbs are inclined towards each other, with convexity facing towards the oldest bed. In this type of fold, the limbs dip towards each other.

Folds

Folds are categorised based on their geometric characteristics or their origin.

Geometric Classification

This classification is descriptive and does not consider the origin of the fold.

a) Based on the Attitude of Axial Plane

• Symmetrical Fold: The axial plane is vertical, dividing the fold into two symmetrical halves.
• Asymmetrical Fold: The axial plane is tilted, creating two asymmetrical parts.
• Reclined Fold:. type of asymmetrical fold where both fold limbs and the axial plane dip in the same direction.
• Overturned Fold: The axial plane is inclined, and both limbs of the fold dip in the same direction, but one limb is steeply inclined.
• Recumbent Fold: The axial plane is horizontal.

b) Based on Inter Limb Angle

• Gentle Fold: Inter limb angle is large.
• Open Fold: Inter limb angle is moderate.
• Closed Fold: Inter limb angle is small.
• Tight Fold: Inter limb angle is very small.
• Isoclinal Fold: Inter limb angle is very small, and limbs are parallel.

c) Based on Folds Seen in Profile

• Parallel Folds: The fold appears circular or elliptical in profile, and the thickness of the layer remains constant.
• Similar Folds: The thickness of the fold remains uniform when measured parallel to the axial plane.

Genetic Classification

Flexure Folds: These folds develop by bending a sheet due to compressive forces acting parallel to the layers. The convex side is subjected to tension, while the concave side is subjected to compression.

Shear-Slip Folds: These folds result from minute displacements along closely spaced fractures, each of which is a tiny fault. Movement along these fractures gives rise to shear-slip folds.

Importance of Studying Folds

The study of folds is crucial for various fields such as civil engineering, petroleum exploration, mining, and hydrogeology.

Civil Engineering: Folds can significantly impact construction projects. For instance, when a fold is present, the rock is typically heavily jointed at the bend. This jointing can make dam construction more expensive because the foundation material needs to be water-tight.

Petroleum Exploration: Synclines, which are a type of fold, are important in engineering because they have the capacity to convey and accumulate fluids. This makes them potential sites for water, oil, and gas accumulation.

Mining: Folds can create natural traps for minerals, making them important sites for mining activities.

Hydrogeology: The study of folds is also important in hydrogeology because they can influence the movement and accumulation of groundwater. Synclines, in particular, are significant because they are water-bearing formations.

Fracturing and Tunnel Construction: The degree of fracturing at the hinge zone of a fold can impact tunnel construction. In the case of an anticline (a type of fold where the rock layers bend upwards), fractures converge inside the tunnel, leading to the development of wedge-shaped blocks. In contrast, in a syncline, fractures diverge inside the tunnel, which can create problems for the roof of the tunnel.

Accumulation of Oil and Gas: Folds, particularly synclines, are considered favorable sites for the accumulation of oil and gas. Source rocks release oil and gas, which then migrate and accumulate in these geological traps.

Faults

A fault is a crack or break in the Earth's crust where the two sides have moved past each other, along a direction that is parallel to the fault plane. Faults are typically found in the upper levels of the Earth’s structure and can vary in size from a few centimeters to thousands of kilometers.

Fault Zone and Shear Zone

Fault Zone:. fault zone is a tabular area of uncertain width that contains multiple parallel or complexly intersecting faults.

Shear Zone:. shear zone refers to a relatively narrow area of significant shear strain, which is bounded on both sides by relatively undeformed rocks.

When describing a fault zone or shear zone, the following information should be included:

• The attitude (orientation) and dip of the beds.
• The width of the zone.
• Whether the structures within the zone are brittle or ductile.
• The nature of the change in the deformed rock and the relative movement across the zone.

Elements of a Fault Plane

• The attitude of a fault plane is described by its strike and dip.
• Strike refers to the trend on the plane of the fault, while dip is the angle between the horizontal surface and the fault plane when measured in a vertical plane.
• In the case of non-vertical faults, the block or wall below the fault is called the footwall, and the block or wall above the fault is the hanging wall.
• Displacement refers to the relative movement along the fault. There are different types of displacement:
• Strike Slip. Displacement parallel to the strike of the fault. In this case, the dip slip component is zero.
• Dip Slip. Displacement along the dip direction of the fault, with the strike slip component being zero.
• Oblique Slip. Displacement that involves both strike and dip slip components.
• In inclined faults, dip slip can be further resolved into a heave component (horizontal) and a throw component (vertical).
• The relationship between throw (T) and heave (H) is given by the formula:
• H = T × sin(θ), where θ is the angle of dip.

Movement along Faults

Translational Movements: In translational movements, all lines on opposite sides of the fault remain parallel before and after the displacement.

Rotational Movements: In rotational movements, the straight lines on opposite walls of the fault do not remain parallel to each other after the displacement.

Field Identification: Fault movements can be challenging to identify in the field because it is often difficult to match exact points on both sides of the fault.

Classification of Faults

• Strike Fault: In this type, the strike of the fault is parallel to the strike of the beds. The dip of the fault is from the clip of the beds. A bedding fault is a kind of strike fault where the fault is essentially parallel to the attitude of the beds.
• Dip Fault: In a dip fault, the strike of the fault is oriented parallel to the dip of the beds.
• Oblique Fault: An oblique fault is characterized by the orientation of the fault plane being oblique to the attitude of the beds.

Displacement and Stress Orientation

In this classification, the sense of relative displacement along the fault and the orientation of the fault with respect to the principal stress axes are crucial. The axes of juxtaposition across the fault are also important factors.

Normal or Gravity Fault:

• In a normal fault, the hanging wall moves down in relation to the footwall.
• The principal stress axis is vertical and relates to gravitational load, which is why it is also called a gravity fault.
• The conjugate of normal faults intersects parallel to and dips more than the normal faults.

Thrust or Reverse Fault:

• In a reverse fault, the hanging wall moves up against the footwall.
• The principal stress axis is vertical, and the least stress is vertical as well.
• Reverse faults typically form at upper structural levels where the pressure is less.
• Thrust faults are usually low-angle faults and can be several tens of degrees at times.

Strike Slip Fault:

• In a strike-slip fault, displacement is seen along the strike of the fault.
• Strike-slip faults can be classified as dextral (right-lateral) or sinistral (left-lateral) based on the movement of the opposite block.
• A sinistral movement occurs when the opposite block moves to the left, while a dextral movement occurs when the opposite block moves to the right.
• In strike-slip faults, both the strike and dip are horizontal, and the fault is vertical or near vertical.

3. Complex Types

Horst and Graben:. horst is a specific kind of reverse fault where the land is uplifted between two thrust faults, creating a raised block of land. On the other hand, a graben is a structural depression formed by two normal faults, where the land sinks between the faults. Major graben features that extend over long distances are known as rifts. Examples include the Godavari rift and ancient rifts from the Mesozoic era, which are now covered by younger sediments.

Step Fault:. step fault occurs when an area is impacted by a series of normal faults that cause slipping in one direction, resulting in a distinctive step-like appearance in the landscape.

Identifying Faults in the Field

Faults in stratified or layered rocks are relatively easy to spot due to visible slip or displacement of the beds. However, in many cases, the presence of a fault is inferred from specific field evidence.

Here are some features that help in recognizing faults in rocks:

• Fault Scarps:. long edge topographic scarp in uniform terrain suggests the presence of a fault.
• Striated Surfaces: These grooves are caused by polishing or friction along opposite walls of a fault during displacement.
• Gouge and Fault Breccia: Faults are often identified by the presence of pulverized rock, similar to clay, which is softer than adjacent rocks and creates depressions. Fault breccia, a non-cohesive fragmented rock of varying sizes, is also an indicator.
• Cataclasites: These are cohesive rocks where a fine-grained matrix predominates over visible fragments. Cataclasites show signs of deformation and are found at upper structural levels.
• Mineralization: Irregular spaces left by crushing and fragmentation in a fault zone are often filled with silica or calcite. Occasionally, economically valuable ores are found in such fault zones.
• Ductile Deformation: At great depths or lower structural levels, rocks undergo ductile deformation, resulting in fine-recrystallized rocks with a distinct change in fabric compared to adjacent rocks. In some cases, melting occurs, producing a glassy rock known as pseudotachylite.

Active and Inactive Faults

• Active Faults: These are faults that can slip in a short period of time. The slippages on these faults are episodic and are known to recur. The period of non-slippage can range from a few years to hundreds or thousands of years.
• Inactive Faults:. fault that has remained undisturbed for thousands of years is considered inactive or passive.

To determine whether a fault is active or inactive, it is necessary to investigate the history of past events from field evidence. Movement along the fault may indicate activity, while a lack of movement for a long period suggests inactivity.

Recognizing Active Faults in the Field

• Recent deposits such as unconsolidated alluvium and soil are helpful in identifying recent movements because of their young age and widespread occurrence. If a fault has displaced these recent deposits, it indicates that the shift is younger than the host material. In contrast, a passive fault may be covered by younger undisturbed deposits.
• Physiographic criteria such as escarpments in alluvium or recent sediments can also be used to determine whether a fault is active or inactive. However, these should not be confused with fault scarps, which are direct evidence of active faults.
• Indirect Evidence: Straight channels or stream offsets can be indirect evidence of fault activity, but they need to be supported by clear evidence.

Seismic Faulting

• Seismicity refers to the occurrence of seismic events like earthquakes, which are associated with movement along fault planes. This movement can last from a few seconds to tens of seconds.
• Normally, shearing occurs along zones through ductile processes that do not trigger seismic events like earthquakes. Ductile deformation is believed to happen under conditions of high confining pressure and high temperatures.
• Seismic faulting is thought to occur only at depths greater than 5 kilometers, where hydrostatic pressure is comparable to total confining pressure.

Recurrence of Faulting

• It is often observed that a prominent fracture or rupture zone in different stratigraphic horizons may be represented by various features such as a fault, silica veins, a fracture zone, cataclasites, etc. at different levels.
• Some splays of brittle fracture or a fracture zone may be reactivated due to recurrence along an older fault.
• The idealized stress-displacement relationships for initial brittle failure followed by subsequent shear sliding on a plane are depicted in the figures.
• Initial failure occurs at the critical or peak stress that the rock can sustain, followed by a sharp decrease in its ability to sustain differential stress.
• If a confining pressure exists, such that a positive stress acts normal to the fracture plane, the rock mass can sustain differential stress, known as residual stress.
• As differential stress increases, subsequent displacement occurs in jerks, with increments of slip and stress falling to lower levels.
• During passive periods between seismic events, the fracture plane may develop cohesion due to percolating fluids, leading to re-shear or reactivation of seismic activity.
• The magnitudes and periodicity of subsequent earthquakes are unpredictable.

Key Points

• Faults are initiated only once but may reactivate multiple times at different levels.
• The process of seismic faulting and recurrence involves complex interactions of stress, strain, and fluid dynamics within the Earth's crust.

Effects of Faulting on Disrupted Beds

Horizontal Beds:

• Normal or Reverse Faulting: When horizontal beds are subjected to normal or reverse faulting, older and younger beds are juxtaposed against each other on either side of the fault.
• Throw: The throw in this case is equal to the change in altitude of the beds.

Inclined Beds:

• Complications: The effects of faulting on inclined beds are complicated by the attitude of the fault and the dip of the beds.
• Strike Fault with Dip Slip Component: In a strike fault with a dip slip component, the hanging wall moves down relative to the footwall. This can result in the same bed being repeated on the other side of the fault due to faulting.
• Strike Fault with Hanging Wall Up: In some cases, the hanging wall may move up against the footwall. The net slip in such cases is equal to the apparent movement. After erosion, the bed may not repeat at the surface, creating a misleading impression of reverse faulting.

Dip Faults:

• Net Slip and Dip Slip: In some instances, the net slip is equal to the dip slip. This means that the displacement is parallel to the dip of the fault.
• Erosion and Apparent Movement: Erosion of certain blocks can affect the apparent movement, making it indicative of displacement parallel to the strike of the fault.
• Strike Slip and Apparent Movement: In cases where the net slip is equal to the strike slip, the relative movement is indicative of displacement along the strike of the fault. After erosion, the apparent movement may give a false impression of reverse faulting.

Significance of Study of Faults

Study of faults is often critical from civil engineering point of view.

Study of Faults in Civil Engineering

Importance of Fault Study

• The study of faults is crucial in civil engineering.
• Faults can be visible in the field or deeply buried without any surface indication.
• If faults are encountered during excavation and the floor is disrupted by gouge and material, it is advisable to abandon the site or seek expert advice.

Active vs. Inactive Faults

• Determining whether a fault is active or inactive is important (refer to section 8.5.6).
• Researching the history of the investigation area can provide insights into the fault's activity.

Exposed Faults and Reservoirs

• An exposed fault can be problematic in reservoirs.
• Minor seepage along the fault may lead to issues.

Fault Gouge and Settlement

• The presence of fault gouge can cause significant settlement.
• Fault gouge is an impermeable material that can obstruct or halt groundwater movement, leading to settlement.

Cold Climate Concerns

• In cold climatic regions, squeezing problems along faults are common.

Encountering Active Faults

• If an active fault is discovered during geological exploration, design adjustments for engineering structures may be necessary to ensure resilience.

Joints

Joints are cracks in rocks where there is little or no visible movement. They can form due to various stresses, such as cooling, compression, uplift, subsidence, or earthquakes. Joints are present in all types of rocks and are the most common geological feature in the Earth's crust.

A joint set is a group of joints that are oriented in the same direction and have a common origin. Joint sets can form a joint system. The geometry, size, spacing, and orientation of joints can vary at the boundaries between rocks of different types.

The attitude of joints is measured by their strike and dip. The strike is the direction of a horizontal line on the joint surface, while the dip is the angle of inclination of the joint plane. Joints can be classified based on their geometry or their origin.

There are three main types of joints:

Strike joints. These joints are oriented perpendicular to the bedding plane. The strike of these joints may differ from the dip of the bedding plane.

Bedding joints. These joints are oriented parallel to the bedding plane.

Dip joints. These joints are oriented parallel to the dip direction of the bedding plane.

Oblique or diagonal joints. These joints are oriented at an angle to the strike of the bedding plane.

Genetic Classification of Joints

Joints Due to Erosional Unloading in Isotropic Rocks:

• When massive rocks are exposed on the surface, they undergo a process called erosional unloading. Deeply buried rocks are under significant hydrostatic pressure due to the weight of the overlying material. When this overburden is removed through erosion, the pressure on the deeper rocks decreases.
• As a result, these massive rocks adjust to the new pressure conditions, typically developing joints that are parallel to the ground surface.
• These joints, known as sheet joints or topographic joints, are closely spaced, with the distance between successive joints ranging from a few millimeters to centimeters. They can be traced for several meters and serve as pathways for groundwater movement.
• Sheet joints are commonly encountered and play a significant role in the weathering of rocks.

 Contraction and Polygonal Joints in Cooling Igneous Rock: 

• Contraction in cooling igneous rock can lead to the formation of polygonal joints. Initially, as the rock cools and shrinks, tension cracks develop due to the shrinkage.
• These tension forces create a series of joints with a hexagonal pattern.
• As cooling continues and crystallization progresses towards the center of the cooling mass, these joints further develop, resulting in the characteristic hexagonal columnar joints seen in igneous rocks and some sedimentary rocks.

Shrinkage Direction

Joints in Polygonal Rock

When igneous dykes intrude into rocks, the walls of the dyke remain relatively cooler than the hot interior of the dyke. This temperature difference causes joints to form perpendicular to the walls of the dyke, resulting in both horizontal and vertical sets of joints. As a consequence, brick-like blocks that are easy to excavate develop within the dyke material. This type of dyke material is often used as road metal in various locations.

Tectonic Joints: Tectonic joints are formed due to some kind of tectonic activity occurring in the Earth's crust.

Flexure: Folds are accompanied by fractures. Analyzing the pattern of these fractures, including their orientation, frequency, and spacing, can provide insights into the forces that caused the folding.

Extension Joints: These joints typically develop in the direction of maximum compression and are oriented perpendicular to the fold axes.

Longitudinal Joints: These joints form parallel to the plane of the folds. They are tension fractures or release joints, similar to those formed in the direction of maximum strain.

Cross Joints or Oblique Joints: These joints are oriented obliquely to the fold axis and are usually part of conjugate arrays. They are commonly referred to as shear joints.

Superficial Movements: In some cases, joints are created due to superficial movements, such as glacier crevasses in rocks.

Distinction between Fractures and Joints:. fracture is a discontinuity of tectonic origin within the upper and middle structural levels, while a joint is a discontinuity of mechanical origin, typical of surface or upper levels. Fractures are associated with structural disturbances, whereas joints are formed due to mechanical processes.

Examples of Jointing: In a sequence of lava flow beds, different types of joints can be observed. For instance, bed A may exhibit vertical contraction joints, while beds B and D show sheet joints. The nature of jointing can vary from bed to bed, and the distinction between fractures and joints is more apparent in horizontal, undisturbed beds.

Jointing vs. Fractures: Jointing is restricted to specific flows, while fractures affect multiple flows. For example, the lowermost bed may develop columnar joints, while the upper flow shows erosional joints. The lower flow is indicative of joint presence, while the upper flow displays erosional jointing.

Quantitative Description of Rock Mass

Rock masses are typically not free from fractures, which divide the rock into blocks of varying sizes, shapes, and discontinuities. The characterization or quantitative description of rock mass is crucial for the design of excavations in this medium. The key requirement is the rock's ability to remain stable under modified conditions. Therefore, quantifying rock mass characteristics helps in distinguishing between weak, moderately strong, and strong rocks.

Another important factor in the design of man-made structures is the scale of structural features, which can be classified as:

• Microscopic: Features that can only be seen with a microscope.
• Mesoscopic: Features visible on the scale of excavation.
• Macroscopic: Features observable on the scale of an engineering site or region.

Joint Characteristics

The term "joint characteristics" broadly includes rock fabric and various geometric characteristics of joints, such as altitude, spacing, extent, aperture, roughness, and wall conditions. Joints are grouped into families based on the similarity of their attitudes.

Joint Orientation and Dip

• Joint orientation and dip are crucial in controlling landform assemblages. For instance, failure by toppling is more likely to occur on a joint plane that dips toward the slope.
• Major or master joints can extend over considerable distances, leading to rapid erosion and the development of landforms with negative relief along them.

Joint Spacing

• Joint spacing, measured as the perpendicular distance between two adjacent joints, can be classified as extremely wide, moderately wide, or narrow.
• When joint spacing is narrow, the rock is broken into small pieces. Joint spacing can also be expressed in terms of area intensity index, which considers the area of joint surface per unit volume of rock and the average size of the unbroken block.

Joint Surface Morphology

• Joint surfaces can be qualitatively classified as smooth, rough, or wavy. Rough and wavy surfaces provide greater strength to the rock, indicating higher resistance during applied loads.
• Some joint faces exhibit plumose markings, characterized by featherlike surface patterns. These markings, composed of tiny ridges and troughs, indicate brittle deformation achieved through rapid snapping apart of the rock, often in an explosive manner.

Joint Aperture and Material

• Aperture, the perpendicular distance between both walls of a joint, is crucial in determining water movement and secondary permeability values. It can be described using various terms such as open, cavernous, very wide, and extremely wide.
• Fractures may be filled with materials like calcite, silica, fault gouge, or silt, which can enhance the impermeable nature of rock and increase strength. However, the presence of clay can decrease joint stiffness and shear strength. When filled fractures are encountered, factors such as mineralogy of the filled material, particle size, permeability, wall roughness, and width of the fracture are noted.

Joint Persistence

• Joint persistence refers to the length of a fracture observed in an exposure or aerial photograph. It is essential to trace the termination of the fracture, which may be beyond the exposure, against other discontinuities. This helps in differentiating between master (systematic) and minor (non-systematic) fractures.

Termination Index (TI)

TI for a domain is calculated by comparing the number of fractures that end within the domain to the total number of fractures.

TI = N / Fractures

Weathering Index

• Rocks undergo weathering and alteration, but the degree of weathering is not uniform across all rocks.
• Initially, describe the overall weathering condition of the rock, such as whether it is intensely, moderately weathered, or has a fresh surface.
• After that, provide details about the weathering of individual rock blocks, focusing on the walls of specific fractures.

Surface Roughness

• Surface roughness refers to the waviness and unevenness of fracture walls.
• Waviness is important for determining the initial direction of shear displacement relative to the mean fracture plane, while unevenness affects shear strength.

Surface roughness is described using terms like polished, slickensided, rough, ridges, small steps, and rough face.

Fracture Survey Data

• The parameters related to fracture walls should be accurately measured in the field and recorded carefully in the fracture survey data sheet.
• These collected facts are crucial for predicting the behavior of foundation materials and are essential for ensuring the stability of man-made structures.

Fracture Survey Data Sheet

• Location of an outcrop: Specify the location where the fracture survey is conducted.
• No.. Number assigned to the fracture for identification purposes.
• Amount. Describe the quantity or extent of the fracture.
• Direction. Indicate the orientation or direction of the fracture.
• Persistence. Mention how long the fracture extends or its persistence.
• Sheet No.. Reference number for the data sheet.
• Remarks. Include additional observations such as weathering index, block size, number of joint sets, etc.

Joints

Joints in rocks are crucial for various reasons:

• They significantly impact the stability of the rock.
• It's essential to study them carefully in surface excavations and during the construction of tunnels and dams.

High Fracture Density

High fracture density refers to a situation where there are many fractures in a rock mass close together. This condition is important because it greatly increases the surface area available for weathering processes. Weathering is the gradual breaking down of rocks due to various environmental factors, and it weakens the mechanical strength of the rocks.

Variable Weathering Index

Rocks can exhibit different levels of weathering, known as the weathering index, even within a small area of a few square meters. When this happens, it becomes necessary to measure the attitude (orientation) and persistence (continuity) of fractures in the rock. Additionally, assessing the weathering index qualitatively, along with other factors, is crucial for understanding the rock's condition.

Intersecting Fractures

When there is a high number of intersecting fractures in a rock mass, it can lead to increased permeability. Permeability is the ability of the rock to allow fluids, such as water, to pass through it. This change in permeability can significantly alter the mechanical and hydrogeological properties of the rocks. In such cases, it is important to establish the hydrogeological properties of the rocks and understand how fractures are influencing these properties.

Water in Fractures

The presence of a substantial amount of water in fractures can have a significant impact on the normal stress acting on the rock. Normal stress is the force applied perpendicular to the surface of the rock. When normal stress is reduced due to the presence of water, it can alter the rate of erosion of the rock. Erosion is the process by which rock and soil are worn away by natural forces, such as wind, water, and ice.

Residual Stress and Differential Stress

The rock mass has the ability to sustain a condition called residual stress, which is a type of stress that remains in the rock after external forces have been removed. However, when high stresses are present, they can disrupt the rock structure. Therefore, it is important to investigate the seismic history of the area being studied, as this can provide information about past stress conditions and their impact on the rock mass.

Table: The Effect of Fractures in Tunneling

Importance of Joints in Rock Stability and Engineering

Joints in rocks play a crucial role in determining the stability and mechanical properties of the rock mass. Their density, orientation, and spacing significantly influence how rocks behave under various conditions. For instance, rocks with a high density of closely spaced joints are more prone to weathering and have reduced mechanical strength. On the other hand, the presence of water in these joints can alter stress conditions and affect erosion rates.

Table 8.5 illustrates how different types of fractures impact tunneling. Fractures that are strike normal to the tunnel axis are considered very favorable, while those that are strike parallel to the tunnel axis can be unfavorable. Gentle dips are generally satisfactory for tunneling, whereas steep dips can pose challenges. Understanding these factors is essential for engineers and geologists when assessing rock stability for construction projects such as tunnels, dams, and slopes.

Factors Affecting Slope Stability

The stability of slopes is influenced by the ratio of excavation dimensions to joint spacing. When fractures dip into the slope, they can initiate water movement, leading to increased pressure in potential slip zones, which compromises stability. Understanding these dynamics is crucial for ensuring the safety and integrity of slopes in various engineering and geological contexts.

Foliation and Lineation

Foliation and Lineation are deformation structures that form in rocks at deeper structural levels due to a combination of higher temperature and lithostatic pressure. At these depths, new planar and linear structures develop, associated with intricate folding and ductile shear zones.

Foliation refers to any planar structure in rocks, such as cleavage, flow banding in igneous rocks, or pebble alignment in conglomerates. Foliation can be classified into:

• Primary Foliation: This type of foliation originates from igneous and sedimentary processes.
• Secondary Foliation: This type develops through processes of deformation and metamorphism.

Lineation involves the parallel alignment of elongated, linear elements within a rock, such as rods, mullions, boudins, or pebbles.

Three major types of foliation include:

• Cleavage:. type of foliation that is significant in understanding the deformation history of rocks.
• Schistosity: Characterized by the parallel alignment of platy minerals, giving the rock a shiny appearance.
• Gneissose Banding: Involves the alternating light and dark bands in a rock, resulting from the segregation of minerals.

Cleavage

Cleavage refers to a type of tectonic foliation that forms in rocks during deformation. It is characterized by closely spaced, aligned, planar to discontinuous features that are typically associated with folds. Cleavage is oriented parallel to or subparallel to the axial planes of folds. There are two main structures that make up cleavage:

• Cleavage bands: These are defined by the preferred orientation of mica-rich layers within the rock.
• Intervening quartz-feldspar rich layers: These are thicker layers of rock that contain more quartz and feldspar, situated between the cleavage bands.

Cleavage

Relation between Cleavage and Bedding

• The relative orientation of cleavage and bedding is often used to predict the general orientation and direction of fold closures.
• This relationship helps determine whether a bed is upright or overturned.
• A general rule is:
• If bedding and cleavage dip in opposite directions, the bedding is upright.
• If they dip in the same direction, and if bedding dips shallower than cleavage, then bedding is upright.

Recognition of Ductile Shear Zones

• Cleavage is also useful for identifying ductile shear zones.
• In such zones, two types of foliation are typically present:
• S foliation: This is a continuous coarse foliation characterized by the preferred orientation of micas and elongated quartz grains.
• C foliation: This consists of shear bands that develop subparallel to the shear zone walls.
• Typically, the S foliation is oblique to the C foliation.

Relation between Foliation and Dam Axis

• The relationship between foliation and dam axis is crucial for dam safety.
• If foliation is parallel to the dam axis, the condition is considered acceptable.
• However, if foliation is across the dam axis, there is a risk of seepage along the foliation plane, making such a dam alignment undesirable.

Lineation

Lineation refers to a series of linear features that penetrate through rock as a result of deformation. There are various types of lineations, such as the typical stripping lineation formed by the intersection of banding on a cleavage surface.

Major Types of Lineation

• Directional Orientation: Lineation resulting from the directional orientation of elongated mineral grains.
• Intersection Lineation: Formed by the intersection of different structural elements.
• Mineral Lineation: Produced by platy or fibrous minerals.
• Slickensides and Striation: Features related to the sliding and striating of rock surfaces.

The study of lineations is crucial for understanding the structural geometry of rocks, as they typically develop parallel to fold axes.

Importance of Structural Geology

Quantitative analysis of geological structures plays a crucial role in various engineering activities. This analysis is essential for the planning and construction of man-made structures such as dams, reservoirs, tunnels, shafts, rock and soil slopes, foundations, and underground operations.

Dams and Reservoirs

• The relationship between local geology and dam sites is crucial for determining the type and design of dam to be constructed. Both surface and subsurface geological considerations are important in this context.
• The following detailed information is necessary for the site selection of dams and reservoirs:
• Rock: Composition, texture, vertical and lateral variation, and weathering index.
• Structure: Presence of folds, faults, and joints.
• Hydrogeology: Characteristics of aquifers, porosity, and surface run-off.
• Seismicity: History of earthquakes in the area over the past few hundred years.

Tunnels and Shafts

• The stability of tunnels and shafts is influenced by various factors such as rock structure, rock stress, groundwater conditions, and construction techniques.
• A thorough understanding of the fundamental principles of structural geology is essential for assessing stability.
• Rock and Slopes
• There are three main mechanisms for rock failure: slide, flow, and fall.
• Identifying the potential for failure using these mechanisms is crucial for assessing the need and scope for detailed quantitative analysis.
• Foundations
• Rocks are generally considered excellent foundation materials.
• However, near-surface rocks are often deformed, as indicated by the presence of fractures, joints, faults, and folds.
• It is important to establish the relationship between the strength of the rock and the proposed load when considering foundations.

Regional Structures

• Regional structures, also known as macroscopic structures, play a significant role in shaping the Earth's surface.
• The presence of features such as linear valleys and large streams is often influenced by these structures.
• The drainage patterns of streams can be controlled by fractures and folds, leading to the development of trellis-type river systems.
• The presence of a dome or basin can be inferred from radial drainage patterns, which may also occur around the plunging axes of anticlines and synclines.
• For example, a large anticline plunging to the left may be indicated by a radial drainage pattern, with alternating hard and soft rocks.
• Radiating
• Trellis
• Annular

The study of regional structures involves examining the geometric configuration of planes, lines, and deformed surfaces in rocks.