Grain size distribution and thickness of breccia and gouge zones from thin (<1 m) strike-slip fault cores in limestone

Through field and laboratory analyses, the cross-sectional structure and the grain size distribution of 10 strike-slip fault cores less than 1 m thick were studied. The fault cores are exposed in Jurassic platform limestone within the Mattinata Fault zone located in the Adriatic–Apulian foreland of southern Italy. Each fault core consists of a breccia zone and a gouge zone, which differ in thickness and grain size distribution. Through the conventional sieving-and-weighting method, the grain size distribution of 20 samples of fault rocks was obtained. The distributions follow power-laws with fractal dimension (D) in the 2.00977–3.04008 range. Gouge D-values are proportional to the normalised thickness of the corresponding gouge zones. For a gouge D-value≈2.2, the thickness of the corresponding gouge zone is only about 3% of the fault core thickness, whereas for a gouge D-value≈3.0, the thickness of the corresponding gouge zone is almost 90% of the fault core thickness. Results from this study suggest that, with progressing fault displacement: (i) grain comminution in fault cores occurred mostly by early bulk fragmentation of grains and late grain abrasion; (ii) breccia zones were progressively incorporated into the adjacent gouge zones.