Particle size distributions in natural carbonate fault rocks: insights for non-self-similar cataclasis

Particle size distributions of cataclastic rocks influence the mechanical and fluid flow behaviour of fault zones. Available data from natural cataclastic rocks are still controversial and do not fully support a self-similar evolution for the cataclastic process, a concept derived from laboratory experiments and micromechanical modelling. Our analyses of particle size in carbonate fault rocks show power law distributions with fractal dimensions spanning a broad range. This confirms that the idea of a persistent fragmentation mechanism for describing the entire evolution of natural cataclastic fault cores in carbonate rocks is inadequate. Conversely, we propose that the fragmentation mechanism progressively changes with the intensity of comminution. Slip localisation within narrow shear bands is favoured when a favourable cataclastic fabric with fractal dimensions D similar to 2.6-2.7 is achieved in the fault zone. Intense comminution in the narrow shear zones produces the preferential formation of small diameter particles resulting in particle size distributions characterised by D values approaching or exceeding 3. The non-self-similar evolution of natural cataclastic rocks has an important impact on the frictional and permeability properties of fault zones. (C) 2002 Elsevier Science B.V. All rights reserved.