A new quantitative method for the characterization of weak cataclastic fault rocks (e.g. fault breccia, gouges) is presented. Applied to find correlations with geomechanical laboratory tests, the thin section and X-ray diffraction analyses used for the method are on a similar scale. The method is shown to be appropriate for the characterization of a large range of different types of weak cataclastic fault rocks (kakirites) and involves appraising their mineralogical composition and microstructural characteristics. The correlation of the geological and geomechanical properties of kakirites based on this approach has shown promising results. The method could provide, in the future, the required geological input data for a mathematical modelling of the geomechanical behaviour of cataclastic fault zones.