Deformation of calcite results in polysynthetic twinning on the e plane. Twinning distorts the original grain shape by simple shear of an amount proportional to the total thickness of twinned material. Each twin set is viewed as an internal strain gage in a deformed aggregate of calcite. Each twin set provides one equation relating the strain in a specific orientation to the strain in the aggregate as a whole. The deviatoric strain tensor that characterizes the aggregate is calculated by the simultaneous solution of at least five equations involving the measured shear strains and twin-set orientations as knowns, and the components of strain in the aggregate as unknowns. More than five equations are solved by least squares to provide an estimate of the precision of the strain components. A preliminary field test of the strain-gage calculation technique suggests that the accuracy is within a factor of two.
Used on a computer-simulated aggregate of deformed calcite, the strain-gage calculation technique produces the exact magnitudes and orientations of the principal strains. Used on the same simulated aggregate, the dynamic analysis technique of Turner and the earlier strain calculation technique of Conel both estimate the relative magnitudes and orientations of the principal strains.