Structural Geology

By: Abdul Jabbar

What is Structural Geology?

• Structural geology is the study of the three-dimensional distribution of rock units with respect to their deformational histories.

• The primary goal of structural geology is to use measurements of present-day rock geometries to uncover information about the history of deformation (strain) in the rocks, and ultimately, to understand the stress field that resulted in the observed strain and geometries.

• This understanding of the dynamics of the stress field can be linked to important events in the regional geologic past.

Economical Importance of Structural Geology

• The study of geologic structures has been of prime importance in economic geology.

• Folded and faulted rock strata commonly form traps for the accumulation and concentration of fluids such as petroleum and natural gas.

• Veins of minerals containing various metals commonly occupy faults and fractures in structurally complex areas.

• Deposits of gold, silver, copper, lead, zinc, and other metals, are commonly located in structurally complex areas.

• Structural geology is a critical part of engineering geology, which is concerned with the physical and mechanical properties of natural rocks.

Structural fabrics and defects• Folds

• Joints

• Faults

• Foliations

• These are internal weaknesses of rocks which may affect the stability of human engineered structures.

Deformation of Rocks

• Within the Earth rocks are continually being subjected to forces that tend to bend them, twist them, or fracture them. When rocks bend, twist or fracture we say that they deform (change shape or size).

• Deformation common at plate margins.

• Deformation concepts…– Force– Stress– Strain

Stress

• The forces that cause deformation of rock are referred to as stresses (Force/unit area).

• Differential Stress – Unequal in different directions.

• A uniform stress is a stress wherein the forces act equally from all directions.

• 3 major types of differential stress– Compressional stress – Tensional stress– Shear stress

Compressional Stress

• Push Together stress.

• Shortens and thickens crust.

• which squeezes rock.

Tensional Stress

• “Pull-apart” stress.

• Thins and stretches crust.

• Associated with rifting

Shear Stress

• Slippage of one rock mass past another.

• In shallow crust, shear is often accommodated by bedding planes.

Strain• Changes in the shape or size of a rock body

caused by stress.

• Strain occurs when stresses exceed rock strength.

• Strained rocks deform by folding, flowing, or fracturing

How Rocks Deforms

• Elastic deformation – The rock returns to original size and shape when stress removed.

• When the (strength) of a rock is surpassed, it either flows (ductile deformation) or fractures (brittle deformation).

• Brittle behavior occurs in the shallow crust; ductile in the deeper crust.

We can divide materials into two classes.

• Brittle materials have a small or large region of elastic behaviour but only a small region of ductile behaviour before they fracture.

• Ductile materials have a small region of elastic behaviour and a large region of ductile behaviour before they fracture.

How a material behaves will depend on several factors

• Temperature - At high temperature molecules and their bonds can stretch and move, thus materials will behave in more ductile manner. At low Temperature, materials are brittle.

• Confining Pressure - At high confining pressure materials are less likely to fracture because the pressure of the surroundings tends to hinder the formation of fractures. At low confining stress, material will be brittle and tend to fracture sooner.

• Strain rate -- At high strain rates material tends to fracture. At low strain rates more time is available for individual atoms to move and therefore ductile behaviour is favoured.

• Composition -- Some minerals, like quartz, olivine, and feldspars are very brittle. Others, like clay minerals, micas, and calcite are more ductile This is due to the chemical bond types that hold them together. Thus, the mineralogical composition of the rock will be a factor in determining the deformational behaviour of the rock. Another aspect is presence or absence of water. Water appears to weaken the chemical bonds and forms films around mineral grains along which slippage can take place. Thus wet rock tends to behave in ductile manner, while dry rocks tend to behave in brittle manner.

Evidence of Former Deformation

• Evidence of deformation that has occurred in the past is very evident in crustal rocks.

• For example, sedimentary strata and lava flows generally follow the law of original horizontality. Thus, when we see such strata inclined instead of horizontal, evidence of an episode of deformation.

• In order to uniquely define the orientation of a planar feature we first need to define two terms –– Strike (trend)– Dip (inclination)

Mapping Geologic Structures

• Strike(trend)The compass direction of the line produced by the intersection of an inclined rock layer or fault with a horizontal plane.– Generally expressed as an angle relative to north.

• N37°E• N12°W

• Dip (inclination)The angle of inclination of the surface of a rock unit or fault measured

from a horizontal plane.– Includes both an angle of inclination and a direction toward which the

rock is inclined.• 82°SE• 17°SW

Mapping Geologic Structures

• In recording strike and dip measurements on a geologic map, a symbol is used that has a long line oriented parallel to the compass direction of the strike.

• A short tick mark is placed in the centres of the line on the side to which the inclined plane dips, and the angle of dip is recorded next to the strike and dip symbol as shown above.

• For beds with a 900 dip (vertical) the short line crosses the strike line.

• For beds with no dip (horizontal) a circle with a cross inside is used as shown below..

Joint

• Any fracture, without any movement is called as joint.

• When rock are under stress, and are at shallow depth then they may show brittle behavior and may get cracked.

• Often rocks are cracked at their elastic limit, which may vary respect to their material properties.

• Joints can be classified into three groups depending on their geometrical relationship with the country rock:

• Strike joints – Joints which run parallel to the direction of strike of country rocks are called "strike joints“.

• Dip joints – Joints which run parallel to the direction of dip of country rocks are called "dip joints“.

• Oblique joints – Joints which run oblique to the dip and strike directions of the country rocks are called "oblique joints".

Folds• Any bent or curved in a rock strata as a result of

permanent deformation due to tectonic forces, is called as FOLD.

• They occur singly as isolated folds and in extensive fold trains of different sizes, on a variety of scales.

• A set of folds distributed on a regional scale constitutes a fold belt, a common feature of orogenic zones.

• Folds are commonly formed by shortening of existing layers.

Faults• Fault is a planar fracture or discontinuity in a

volume of rock, across which there has been significant displacement along the fractures as a result of earth movement.

• Energy release associated with rapid movement on active faults is the cause of most earthquakes.

• These earth quake may cause tremendous loss of life and property.

Faults

Faults

• Faults occur when brittle rocks

fracture and there is an offset along

the fracture.

• When the offset is small, the

displacement can be easily measured,

but sometimes the displacement is so

large that it is difficult to measure.

Fault Terminology

• A fault line is the surface trace of a fault, i-e the line of intersection between the fault plane.

• A clearly seen line is formed by the intersection of faulted surfaces and can be observed even on satellite image.

• Hanging wall: fault block above the fault plane is called as hanging wall.

• Foot wall: fault block below the fault plane is called as foot wall.

• Fault blocks classified as

Footwall (rock mass

below the fault)

Hanging wall

(rock mass

above the fault)

• Three dominant types

– Normal fault

– Reverse Fault

– Thrust (a low angle reverse fault)

– Strike Slip Fault

Normal fault – Hanging wall moves down relative to the footwall.

– Accommodate lengthening or extension of the crust.

– Exhibit a variety of scales

Normal Fault

• Larger scale normal faults are associated with fault-block mountains (Basin and Range of Nevada).

• Normal fault bounded valleys are called graben

• Normal fault bounded ridges are called horsts. • Basin area has a series of horsts and grabens.

Reverse faults

– Hanging wall block moves up relative

to the footwall block

– Reverse faults have dips greater than 45o

– Accommodate shortening of the crust

– Strong compressional forces

Thrust fault• A special case of reverse fault.

– Hanging wall block moves up relative to the footwall block

– Thrust faults are characterized by a low dip angle (less then 45o).

– Accommodate shortening of the crust

– Strong compressional forces

Strike-Slip Faults• Dominant displacement is horizontal

and parallel to the strike of the fault

• Types of strike-slip faults

– Right-lateral – as you face the fault, the block on the opposite side of the fault moves to the right

– Left-lateral – as you face the fault, the block on the opposite side of the fault moves to the left

Types of Strike Slip Fault

• Fault Splays: Fault is segments into many small faults.

Sometimes, A big fault initiate many small other fault known as fault splays or implication.

Sierra Nevada mountains

Criteria to identify the faults

Fault scarp

• A fault scarp is the topographic expression of faulting attributed to the displacement of the land surface by movement along faults.

• During faulting, one block may rise and appear as a raised ridge and shows steep bedding.

• Slickenside:In geology, a slickenside is a smoothly polished surface

caused by frictional movement between rocks along the two sides of a fault. This surface is normally striated in the direction of

movement. The surface feels smoother when the hand is moved in

the same direction that the eroded side of the fault moved.

Mineralization

• Friction along blocks of faults may cause dynamic metamorphism, fracturing and brecciation etc

Stream alignment:

Offset streams are found along strike slip fault . If a stream is changing its path then it shows the presence of faults.

Raised ridges along coastal areas

Valleys:Valleys are of great importance because it is said that

90 % of the valleys are being formed along the faults e.gKaghan valley has alignment with Kunhar river and these streams are found along strike slip fault

Kaghan Valley .

Hot water streams:

Hot water streams highly suggests the presence of fault .

Waterfalls:

Water fall also suggests the presence of oblique faults.

Surface Geomorphology

IMPORTANCE OF STRUCTURAL

GEOLOGY IN CIVIL ENGINEERING

What Structural Geologists Should do in Studying Structures?

Map the geometry of structures accurately in the field and construct an accurate geologic map.

Measure the orientation of small structures in the field to know the shapes and relative position of larger structures

Study the sequence of development and superposition of different kinds of structures to determine the sequence condition of deformation.

Try to apply rock-mechanics data to relate structures to stresses that present in the Earth at the times of deformation.

Try to compare structures in one area with those else-where that may have formed by similar-mechanism.

Utilize the geophysical data and other geology disciplines. Geophysical data such as gravity, magnetic, and seismic

IMPORTANCE OF STRUCTURAL GEOLOGY AND ITS RELATIONSHIP TO OTHER FIELDS

• Engineering: Problems such as construction of bridges, dams, power plants, highways, and airports, and beneath buildings problems

• Environmental: Problems such as land use, planning, earth quake hazard, volcanic hazard, waste isolation and disposal, control of the distribution of ground water

• Petroleum and mining geology: Understanding the geometric techniques, projection of faults geologic contacts, larger trends of regional processes that control the concentration of mineral and hydrocarbons

Questions????????