DEEPCHAND V ROLL NO: 07 M.Sc. GEOLOGY DEPARTMENT OF GEOLOGY UNIVERSITY OF KERALA DEFORMATION

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

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

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. Deformation  How structures are formed

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

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

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

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

Figure3: The nature of strain, dilatation, distortion and rotation Source: “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

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 strain Source: “Foundations of Structural Geology” by R G PARK, pp 66

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

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 shape Source: “Foundations of Structural Geology” by R G PARK, pp 68

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

Strain Figure8: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 elongation zone 2 elongation followed by contraction zone 3 contraction followed by elongation zone 4 continued contraction

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 zones 1.Continued elongation 2.Contraction followed by Elongation 3.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 zones 1.Continued elongation 2.Contraction followed by Elongation 3.Elongation followed by contraction 4.Continued contraction Source: “Foundations of Structural Geology” by R G PARK, pp 69

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

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 rock Source

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 deformation Source:

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 deformation Source qrius.si.edu

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

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

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.

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

Figure 13 Microfaric at crystal scale. A.Deformation bands in naturally deformed quartz B.Knik boundaries in naturally deformed biotite C.Flattened texture in deformed calcite C1 undeformed C2 shortened by 50% Source: “Foundations of Structural Geology” by R G PARK,

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

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.

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

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

1.TWISS R.J.,MOORES E.M. (1938), Structural geology, W.H. Freeman and Company, New York, First Edition, pp Park, R.G.(1997) Foundations of Structural Geology, Chapman & Hall, Second Edition, pp 35-37, , REFERENCE

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