Studies of calcite twin lamellae in an experimental thick-beam fold, a multilithologic, layered thin-beam buckle, and in several multilithologic faulted and drape-folded specimens provide a test of the use of these lamellae to map the strain and stress fields developed under known, yet complex, boundary conditions. Even though the number of grains available for sampling per domain of homogeneous deformation is small (8 to 100), we find that in 24 of 28 domains studied the derived orientations and relative magnitudes of the principal strain axes agree to + 30 with the corresponding inferred principal stress axes, with those inferred from accompanying microfractures and faults, and with those expected from the bending and the boundary conditions. The absolute differences in strain between those inferred from the twin lamellae and those calculated from bedding-thickness changes or from the surface strains recovered from distorted grid-lines averages ±0.01. The poorest correlations are for domains where the sample size is < 20. In addition, the twin-lamellae strain analyses permit better definitions of superposed deformation and anticlastic bending than do the corresponding stress analyses. For strains up to about 0.15, calcite twin lamellae are remarkably good indicators of the strain and stress histories of deformations.
Stresses and Deflections of Thick, Curved Plates
