Grain size distribution and thickness of breccia and gouge zones from thin (!1 m) strike-slip fault cores in limestone Andrea Billi * Dipartimento di Scienze Geologiche, Universita ` ‘Roma Tre’, Largo S. L. Murialdo 1, 00146 Rome, Italy Received 14 April 2004; received in revised form 9 May 2005; accepted 12 May 2005 Available online 15 July 2005 Abstract 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-valuez2.2, the thickness of the corresponding gouge zone is only about 3% of the fault core thickness, whereas for a gouge D-valuez3.0, 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. q 2005 Elsevier Ltd. All rights reserved. Keywords: Breccia; Gouge; Fault core; Fault rock; Grain size distribution; Strike-slip fault 1. Introduction Fault zones consist of volumes of intensely fractured rocks (i.e. the damage zone) associated with numerous closely spaced fault surfaces (Ben-Zion and Sammis, 2003; Kim et al., 2004). The thickness of a fault zone may vary from sub-millimetres to a kilometre or more (Scholz, 1987; Sibson, 2003). Cataclastic fault zones usually include a fault core (Chester and Logan, 1986; Chester et al., 1993; Caine et al., 1996; Evans et al., 1997), which is where pre-existing rock fabrics (e.g. sedimentary and tectonic structures) are erased by the generation of fault rocks (Billi et al., 2003a; Graham et al., 2003). The development and physical attributes of fault cores and fault core rocks have been extensively studied in natural faults (Wibberley and Shimamoto, 2003), in laboratory experiments (Frye and Marone, 2002) and in numerical simulations (Abe and Mair, 2005). It is often documented that grain comminution and hence wear increase with increased fault displacement (Engelder, 1974; Mandl et al., 1977; Aydin, 1978; Hooke and Iverson, 1995; Amitrano and Schmittbuhl, 2002). It is also documented that the grain size distribution of fault rocks usually follows power-laws (Blenkinsop, 1991; Storti et al., 2003) and that the relative fractal dimension (D) increases with the number of fracturing events and hence with fault displacement (Blenkinsop, 1991; Monzawa and Otsuki, 2003). Specific D-values have been correlated with different micromechanical models of grain fragmentation, namely the ‘pillar of strength’ model of Alle `gre et al. (1982) for DZ1.97, the ‘constrained comminution’ model of Sammis et al. (1987) for DZ2.58 and the ‘plane of fragility’ model of Turcotte (1986) for DZ2.84. Several data from laboratory simulations of faults (Beeler et al., 1996) show power-law grain size distributions of fault rocks with Dz2.6, thus supporting Sammis et al.’s model of constrained comminution. Sammis et al. (1986) inferred a steady-state self-similar process of cataclasis leading progressively to the development of fault gouge. However, data from natural fault rocks show a large variability of D-values, thus suggesting that the mechanisms of grain Journal of Structural Geology 27 (2005) 1823–1837 www.elsevier.com/locate/jsg 0191-8141/$ - see front matter q 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.jsg.2005.05.013 * Tel.: C39 0654888016; fax: C39 0654888201. E-mail address: billi@uniroma3.it.