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DEEPCHAND V ROLL NO: 07 M.Sc. GEOLOGY DEPARTMENT OF GEOLOGY UNIVERSITY OF KERALA [email protected] DEFORMATION

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DEEPCHAND VROLL NO: 07M.Sc. GEOLOGYDEPARTMENT OF GEOLOGYUNIVERSITY OF KERALA [email protected]

DEFORMATION

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• Introduction• Stress

• Principle stress• Hydrostatic and Deviatoric stress

• Strain• Homogenous and Inhomogeneous strain• Pure shear and simple shear• Volume change during deformation• Progressive deformation and finite strain

• Elastic and plastic Materials • Deformation

• Brittle Deformation• Ductile Deformation

• Mechanisms of rock deformation • Rheology• Conclusion• Reference

CONTENTS

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• The word “Structure” means any thing that build or constructed or produced by deformation.

• “Deformation” is the process responsible for the formation of structures

or

The process which changes the shape or form of a rock body.

• Stress and Strain deals with the way in which materials react with force.

• Rock deformation depends on physical properties

INTRODUCTION

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• Deformation - physical changes produced in the material due to applied force

• Force acting on a rock produce a set of stresses.

• This stress can change the dimension of the rock

• This change may be change in shape, volume or both and constitute the strain.

DeformationHow structures are formed

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Figure1: Effect of stress on a cube change in shape and volumeSource: “Foundations of Structural Geology” by R G PARK, pp 55

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• A stress is a pair of equal and opposite forces acting on unit area of a body

stress=force/area

• Normal stress and shear stress

A force F acting on unit area of a surface can be resolved into a normal stress perpendicular to the surface and a shear stress acting parallel to the surface

Stress

Figure2: A normal stress perpendicular to the plane and a shear stress parallel to the plane produced by opposite forces F acting on a plane Source: “Foundations of Structural Geology” by R G PARK, pp 56

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• Principle stress

The three mutually perpendicular planes on which the shear stress is zero is called principle stress planes and the normal stress across them are called principle stress axes

• Hydrostatic and Deviatoric stresses

where the principal stresses are equal ,the state of stress is called hydrostatic; corresponds to the state of a fluid.

A stress component in the system which consists of unequal principal stress the state of stress is called Deviatoric stress

Stress

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• Geometrical expression of amount of deformation caused by the action of a system of stresses on a body.

• Dilation =volume change

Distortion= shape change

• Homogenous and Inhomogeneous strain

the amount of strain in all parts of a body is equal Homogenous strain

strain in different parts of the body unequal Heterogeneous strain

Strain

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Figure3: The nature of strain, dilatation, distortion and rotationSource: “Foundations of Structural Geology” by R G PARK, pp 63

Figure4: domains of homogenous (H) and Inhomogeneous strain(I) Source: “Foundations of Structural Geology” by R G PARK, pp 64

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• Pure shear and simple shear (distortion and rotation)

if the orientations of principle strain axes X, Y, Z have not changed during deformation the strain may be described as irrotational and process is known as Pure shear

if the orientation of principle strain axes are changed during deformation the strain may be described as rotational and the process is known as simple shear

Strain

Figure5: pure shear and simple shear.A.Irrotational strain B. Rotational strainSource: “Foundations of Structural Geology” by R G PARK, pp 66

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• Volume change during deformation• Change in volume means change in shape also

• The volume change or dilation

Where

V volume of deformed state

V₀ volume of unreformed state

Strain

∆ = ( V-V₀)/V₀

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• Progressive deformation and finite strain

• A strained body represent total strain produced in the body up to that time of measurement

• This is produced by adding a series of strain increments as the body takes stages of different shapes and positons with respect to the applied stress.

Strain

Figure6:Progressive deformation. Finite strain is achieved by adding successive strain increments to the initial unstrained shapeSource: “Foundations of Structural Geology” by R G PARK, pp 68

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• This process from initial to final positon is termed as progressive deformation.

• And final strain at the time of measurement is known as Finite strain.

• At any point during progressive deformation it is possible to examine both finite strain(up to that time) and infinitesimal strain at that point in time.

• The finite-strain ellipse divided in to two sectors- Elongation and Contraction. Separated by no longitudinal strain (zero extension)

Strain

Figure7:Changing field of elongation and contraction during progressive deformation field of elongated lines (boudinage)And contracted lines (fold) separated by lines of zero extension (radius of undeformed circle) Source: “Foundations of Structural Geology” by R G PARK, pp 69

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StrainFigure8:The infinitesimal strain ellipsoid. currently expanding and contracting sectors Source: “Foundations of Structural Geology” by R G PARK, pp 69

By super imposing two ellipse we can get four zones zone 1 continued elongationzone 2 elongation followed by contractionzone 3 contraction followed by elongationzone 4 continued contraction

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• Distribution of these zones is depend upon the strain history whether the starin is irrotaional (pure shear ) or rotational (simple shear )

• Observation of folded and boudinage layers has great value in the investigation of progressive deformation

Figure9. Super imposition of A and B for Pure shear will produce three zones1. Continued elongation 2. Contraction followed by

Elongation3.Continued contraction Source: “Foundations of Structural Geology” by R G PARK, pp 69

Figure 10.Super imposition of A and B for simple shear will produce four asymmetrical zones1. Continued elongation 2. Contraction followed by Elongation3. Elongation followed by contraction4. Continued contraction Source: “Foundations of Structural Geology” by R G PARK, pp 69

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• Elastic Materials

• Materials deform by an amount proportional to the applied stress and , but when the stress is released , the material returns to its undeformed state

• The deformation of elastic material is said to be Recoverable

• Leaving no permanent change in the shape

• Plastic materials

• If the applied stress is smaller than the yield stress these materials do not show any permanent deformation

• If the applied stress exceeds the yield stress the material may show either brittle or ductile deformations -These kind of materials are referred to as plastic materials and they leave permanent changes in the shape

ELASTIC AND PLASTIC MATERIALS

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• Deformation in a rock depends upon

Orientation and intensity of force applied

Motion to which the rock are subjected

Physical conditions ; Temperature and pressure

Mechanical properties of rocks

Based on these factors deformation classified into two they are brittle and ductile deformation

Deformation

Figure 11 Schematic diagram showing formation of brittle and ductile deformation from undeformed rockSource www.columbia.edu

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• Ductile deformation

• At high temperature and pressure(below melting point) and at low intensity of applied forces or a very slow imposed deformation rock undergoes ductile deformation

• Produce permanent strain which exhibit smooth variation across the deformed sample or rock with out any marked discontinuity

Deformation

Figure 12 Rock showing Ductile deformationSource:www.columbia.edu

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• Brittle deformation

• At low temperature and pressure and at high intensity of applied forces or a rapid imposed deformation , rock undergoes Brittle deformation

• Where the elastic deformation leads to failure , the material lose cohesion by the development of fracture or fracture across the continuity of material is broken

• It involves fracturing of rocks. If two sides of fracture slide relative to each other along a fracture surface results a Fault

Deformation

Figure 13 Rock showing brittle deformationSource qrius.si.edu

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• Rock consists of aggregates of individual crystal grains and different mineral species

• The way in which a rock deform depends partly on the properties of individual crystals and partly on the texture of the whole rock.

E.g. Granite with interlocking crystalline structure stronger than a sandstone with weak carbonate cement

A granite without planar fracture stronger than other granite which cut by planar fracture

• When a crystal lattice is subjected to stress the atomic spacing is changing and this depends on amount of stress and inter atomic bonding force.

Mechanism of rock deformation

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• Low temperature deformation involves relative displacement of individual grains causing fracturing and mechanical granulation - cataclasis

• Permanent strain is produced by varies slip or micro fracture mechanisms.

• Gliding, mechanical twinning, intergranular displacement

• Accompanied by recrystallization

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• Fabric • Fabric of a rock body is the geometrical arrangement of all structural elements within the

body.

• only small scale structures are considered in fabric instead of large scale structures

(for analysis at grain scale level)

• A fabric consists of a number of planar or linear fabric elements

• By studying the microscopic fabric of deformed rocks, it is possible to reconstruct in detail how the final strained shape of that deformed rock was achieved by successive changes

• Deformation effects on individual grains are carried out using electron microscope.

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• Deformation can occur even in crystal scales. There are different types of crystal discontinuities - crystal defects.

• Planar defects• limited extent• Stacking faults

• Formed if there a very small displacement of crystal structure

• Sub grain boundaries • within grain region• separating regions of different lattice orientation• Visible when there is a small change in extinction angle.

• Deformation bands • narrow planar zones • contains material that is deformed from adjoin part of crystals

• Deformation lamella • deformation bands • same structure but different refractive index

FABRIC

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Figure 13 Microfaric at crystal scale.A. Deformation bands in naturally deformed

quartzB. Knik boundaries in naturally deformed biotiteC. Flattened texture in deformed calcite

C1 undeformed C2 shortened by 50%

Source: “Foundations of Structural Geology” by R G PARK,

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• Rheology is the science of deformation and flow of solid materials,

• A solid is made up of particles which are inter-related. They are rigid and resist a change of shape

• Fluid has no rigidity - particles can move freely

• To study about the deformation we need to know the relationship between stress and strain to make mathematical models of deformation

RHEOLOGY

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• With such models we can calculate how structures are developed

• The models are based on real behaviour of rock and rock deformation experiments

• The models of deformation in the earth may be both physical and mathematical.

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• The study of “Deformation” is very important to geological studies because these processes are responsible for the formation of structures on earth which have so many applications.

• A stress is a pair of equal and opposite forces acting on unit area of a body and produce strain which leads to deformation structures. And its different components and property can change the type of strain produced and also the deformation.

• A strained body represent total strain produced in the body up to that time of measurement by adding a series of strain increments. This process of initial to final position is termed as Progressive strain. It is possible to examine both finite and infinitesimal strain at any point during progressive deformation.

CONCLUSION

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• Based on stress applied and effect of deformation materials are classified in to Elastic and plastic materials

• Based on the variation in pressure , temperature and intensity of applied forces the deformation classified into Brittle and ductile deformation.

• The way in which a rock deform depends upon partly the properties of individual crystals and partly on the texture of whole rock.

• Fabric of a rock body is the geometrical arrangement of all structural elements within the body; small scale deformational structures are studied through fabric of a rock

• Rheology is the science of deformation and flow of solid materials. It use relationship between stress and strain to make mathematical models for study about deformation

CONCLUSION

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1. TWISS R.J.,MOORES E.M. (1938), Structural geology, W.H. Freeman and Company, New York, First Edition, pp 27-50.

2. Park, R.G.(1997) Foundations of Structural Geology, Chapman & Hall, Second Edition, pp 35-37,271-273,457-462.

REFERENCE

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Thank you for the attention…