Fractal distribution of particle size in carbonate cataclastic rocks from the core of a regional strike-slip fault zone

We present particle size data from 31 samples of carbonate cataclastic rocks collected across the 26 m thick fault core of the Mattinata Fault in the foreland of the Southern Apennines, Italy. Particle size distributions of incoherent samples were determined by a sieving-and-weighting technique. The number of weight-equivalent spherical particles by size is well fitted by a power-law function on a log–log space. Fractal dimensions (D) of particle size distributions are in the 2.091–2.932 range and cluster around the value of 2.5. High D-values pertain to gouge in shear bands reworking the bulk cataclastic rocks of the fault core. Low D-values characterise immature cataclastic breccias. Intermediate D-values are typical of the bulk fault core. Analysis of the ratio between corresponding equivalent particle numbers from differently evolved cataclastic rocks indicates that the development of particle size distributions with D>2.6–2.7 occurred by a preferential relative increase of fine particles rather than a selective decrement of coarse particles. This preferentially occurred in shear bands where intense comminution enhanced by slip localisation progressed by rolling of coarse particles whose consequent smoothing produced a large number of fine particles. Our data suggest that during the progression of cataclasis, the fragmentation mode changed from the Allègre et al.'s [Nature 297 (1982) 47] “pillar of strength” mechanism in the early evolutionary stages, to the Sammis et al.'s [Pure and Applied Geophysics 125 (1987) 777] “constrained comminution” mechanism in the subsequent stages of cataclasis. Eventually, localised shear bands developed mainly by abrasion of coarse particles.