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58 E. Tondi et al. / Journal of Structural Geology 37 (2012) 53e64

Fig. 7. (a) Extensional jog and (b) contractional jog along a zone of bands and a well-developed strike-slip fault, respectively.

compactive shear banding at an extremely low overburden pres- The evolving growth process of compactive shear bands in the
sure (Baud et al., 2004; Cilona et al., Submitted for publication). porous carbonate grainstones of Cala San Nicola at Favignana is
Compactive shear banding occurs only in rocks and sediments marked by distinctive values of their dimensional parameters
characterized by porosity values greater than about 14% (Aydin and (Fig.12a). Single bands are typically 30e100 cm long, 4e6 mm thick
Johnson, 1978; Antonellini and Aydin, 1994; Aydin et al., 2006; and accommodate 2e4 mm of displacement. Since all data were
Micarelli et al., 2006; Tondi et al., 2006a, b; Fossen et al., 2007; gathered from tectonic structures crosscutting the same lithofacies
Tondi, 2007; Rath et al., 2011; Rustichelli et al., 2012). Pore (III of Fig. 2), the observed deviations can also be due to the effect of
collapse, in fact, is an essential mechanism for their formation (e.g. other factors such as cementation and pressure solution (Tondi,
Baud et al., 2009; Vajdova et al., 2010). In this regard, the fact that 2007), which occurred during pore collapse through grain sliding
some compactive shear bands go through, and displace, other and rotation.
compactive shear bands (see Fig. 5d and Fig. 7a) poses an intriguing The zones of bands show large variations in their dimensions,
question. A possible solution is that compactive shear bands and accommodate an amount of displacement proportional to the
intersection likely occurs in the incipient phase of their formation number of individual bands they include (Fig. 9). As displacement
(stage I in Fig. 12a), when porosity is not reduced below a certain accumulated in the range of 20e30 cm, shear band development
value (i.e. 14%). Subsequently, both sets can simultaneously gave way to spatial localization and the formation of faults with
continue to accommodate shear and compaction by evolving into discrete slip surfaces. These two populations of structures differ
zones of compactive shear bands and, eventually, well-developed in dimensional parameters because they show breaks at about
faults (stage II and III respectively, Fig. 12a). This process occurs 10 cm for both thickness and displacement and at a value of
because of the intrinsic properties of the carbonate grainstones (i.e. about 8 m for length (Fig. 10). That data do not exactly fit the
the high-porosity value, grain size and grain contact characteris- power-laws (R 2 comprised between 0.3 and 0.8) could be
tics), which could easily accommodate volumetric (compaction explained by the fact that some measurements refer to jog zones
and/or dilatation) strain. Indeed, an increased band thickness is and linked faults. In fact, jogs are generally characterized by
observed at the intersecting zones, where some bands along one set greater values of thickness for given lengths and amounts of
diverge from the in-plane position and become parallel to the other displacement, whereas linked faults show greater length-
set of bands (see Fig. 12b). This implies that, at some point, the displacement ratio similar to that reported by Cartwright et al.
shear displacement across the approaching band had reduced to (1995).
zero before the sense of slip flip-flopped. The scenario proposed The displacement-distance graphs of Fig. 8 are qualitatively
above for the development of conjugate sets of compactive shear similar to those of faults in compact (tight) rocks, because they both
bands represents an important difference between faulting of develop along-strike displacement profiles with a central
porous and compact (tight) rocks. In the last type of rocks, in fact, maximum (e.g. Cowie and Scholz, 1992; Fossen et al., 2007).
two conjugate faults cannot accommodate shear simultaneously However, some differences between zones of bands and faults are
after the initial intersection. In this regard, interested readers may evident. For example, the displacement varies along the zones of
compare the conceptual diagram here with those proposed by bands in relation to the number of single bands locally present. In
Flodin and Aydin, (2004 and Figure 14 therein) and Davatzes and contrast, the number of bands around faults does not increase
Aydin (2003 and Figure 15 therein) for dilatant, pseudo-conjugate proportionally with the displacement. Moreover, in Fig. 10a and b,
faults formed by shearing of pre-existing joints, splaying, and the lower values of the D exponent for the faults suggest that these
sequential shearing of the splays. structures, because of the discrete slip surfaces they contain, are