Fault breccia present along strike-slip faults was studied by mean of an integrated field and laboratory investigation. In particular, samples of sheared pre-existing, syn-sedimentary gravitative breccia were detailed studied in order to decipher the deformation mechanisms associated to strike-slip faulting. The sheared gravitative breccia is made up of matrix-supported, monomictic, unsorted intraformational fragments embedded within a reddish hematite-rich, clayish matrix. Microsparry calcite cement is also present around the limestone fragments. In contrast, tectonic breccia consists of comminuted limestone fragments embedded in a calcite-rich matrix also including small amounts of reddish matrix. Particle shape analysis of representative images of the study hand specimens shows a variability of the box-counting dimension (D0-value) between the two different types of breccias, which can be explained in terms of peculiar strain localization within a threefold aggregate that characterizes the gravitative breccia, which is made up of calcite fragments, clayish matrix and very minute oxides.