Limitations of Conjugate Fractures and Scope of Reactivated Faults in Palaeostress Analysis

This article reviews the conditions that must be satisfied for application of the Anderson's law to decipher the orientations of the principal stresses from conjugate pairs of shear fractures/faults. It shows that the reactivated faults and fractures are more powerful stress indicators than the Andersonian fractures, because the former can reveal not only the orientations of the principal stresses, but also the relative magnitudes of the principal stresses in terms of the shape of the stress ellipsoid. A few commonly used graphical and numerical methods for palaeostress analysis from the reactivated faults are evaluated critically. Finally, the significance of relative magnitudes of the principal stresses is demonstrated with an example of the striated faults cutting through the Amritpur granite in the Kumaun Lesser Himalaya.