Strain measurements from phyllosilicate preferred orientation - a precautionary note

An X-ray texture goniometer has been used to measure the preferred orientation of muscovite and chlorite basal planes in four slate specimens of somewhat complex microstructure. Strains were calculated from these preferred orientations using the March method. It is shown that strain values so determined can be unreliable both in orientation and magnitude. Examination of the microstructure of foliated rocks is essential before using this technique of strain measurements. Particular care should be taken with domainal microstructure, and with rocks showing evidence of operation of diffusive processes during deformation. In comparing the results of different methods of strain determination, it is important to distinguish results referring to tectonic strain alone from those that measure the total strain which includes any pretectonic deformation. X-ray goniometry measures the total strain.