Qualitative analysis of three-dimensional strain using twins in calcite and dolomite

Abstract

Turner's (1953) technique of locating the compression ( C) and tension ( T) axes from the known orientations of C-axis and twin pole ([022̄1] in dolomite and [011̄2] in calcite) for each grain yields the orientation of the unique stress system when a great majority of the grains in the rock shows only singlet twins. However, since it assumes the highest possible value for the coefficient of resolved shear stress s 0 (= 0.5), application of the technique to rocks in which a large number of grains show doublet and triplet twins results in great dispersion of the C- and T-axes. In such cases the unique stress system can be established by preparing separate C-axes fabric diagrams for the untwinned grains, grains with singlet twins, grains with doublet twins and grains with triplet twins; measurement of the orientations of the twin planes becomes unnecessary. Two examples are presented. In the light of this work, the significance of the appearance of “conjugate twin planes” is discussed.