The methods of dynamic analysis for inferring principal stress axes from quartz, calcite, and dolomite petrofabrics (Carter and Raleigh, 1969) have been well developed during the last four decades through the availability of experimental equipment for rock deformation under controlled temperatures and pressures (Griggs, 1936) and the modern universal microscope stage (Emmons, 1943). Magnitudes of the stresses cannot be predicted from the methods they describe, however. Although Jamison and Spang (1976) proposed a method to infer differential stress from twinned carbonate aggregates, strain is in reality the only parameter that can be properly measured in deformed rocks.
Well-known procedures for measuring finite strain in racks can be applied only to highly deformed rocks and require speculation about the undeformed state of the rock fabric (Ramsay, 1967). Groshong (1972) has recently developed a least-squares fit strain-gage technique which can be used to calculate the orientations and magnitudes of principal strain axes for small strains in deformed rocks containing mechanically twinned minerals. Groshong (1974) experimentally checked his method using twinned calcite aggregates, but he suggested that it could be applied to other minerals.