An evolutionary model of fracturing and cataclasis in platform carbonate fault zones.

The evolution of fracturing and cataclasis influences the mechanical and permeability properties of fault zones. Fault zones can behave as strong or weak mechanical discontinuities and as conduits or barriers to fluid flow mainly depending on the nature and amount of cataclastic rocks that developed in their cores. Our structural investigations in fault zones affecting carbonate platform rocks in the Apennines, Italy, allow us to propose an evolutionary model for the progressive development of cataclastic fault core rocks from damage zones, and for the progression of cataclasis in fault cores. Analyses of particle size in the cataclastic rocks show power law distributions with fractal dimensions spanning over a broad range. This suggests that the idea of a persistent fragmentation mechanism (self-similar cataclasis) for describing the entire evolution of natural cataclastic fault cores in carbonate rocks is inadequate. Self-similar evolution for the cataclastic process is a concept mainly derived from laboratory experiments and micromechanical modelling. 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 ~ 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 progression pathway of fracturing in damage zones and the non self-similar evolution of cataclasis in fault rocks have an important impact on the frictional and permeability properties of fault zones.