Shear-sense determination on striated faults from e twin lamellae in calcite

In limestones, most fault planes are coated with calcite. The deformation of these calcite crystals is generally achieved by twinning on the e planes (01 overline12). The aim of this paper is to propose a rapid determination of the displacement sense on the fault planes by the study of the geometry of associated twinning. Calcite coatings have been sampled from faults for which the displacement sense is known. Thin sections perpendicular to the fault plane and parallel to the fault striation have been examined with an ordinary polarizing microscope. It is shown that in some crystals, the twin geometry leads unequivocally to a determination of the displacement sense. This is supported theoretically by the use of the right dihedron method. A figure summarizes the data for crystals which are twinned on one, two or three sets of twin planes.