Stress and Strain

Abstract

The chapter provides basic information about stress and different types of strain. Significance of orientation of layering with respect to stress orientation is described. Homogeneous and inhomogeneous strains are illustrated. Flinn diagram is described to represent shapes of three-dimensional strain ellipsoids on a two dimensional diagram. Different methods for determination of finite strain are based on certain assumptions that should be kept in mind while interpreting the results. The relevant precautions are briefly described such as: (i) Strain data obtained from one rock type is generally not representative of the regional or bulk strain ellipsoid. Strain values can also vary along a fold profile depending on the mechanism of folding. (ii) When a method is based on measurement of grain shapes, competence contrast between the grain and matrix should be minimum otherwise only the matrix will undergo deformation (if least competent) and the grain will not reveal the true strain ellipsoid. Similarly a deformed fossil will indicate strain suffered by the fossil, not by the entire rock. (iii) Reduction spots can be used as strain markers provided the reduction pre-dates the strain. (iv) Methods based on measurement of buckle shortening ignore initial layer parallel strain (homogeneous shortening) that has occurred prior to development of the fold. (v) Magnetic strain, obtained by anisotropy of magnetic susceptibility method, provides bulk strain, i.e. of the entire rock sample including the matrix. Hence petrofabric and magnetic strains can differ noticeably.