Carbonate-clay mixing in cataclasite during fault activity

Natural faults produce granular wear material, known as gouge or cataclasite, as a function of shear and grinding along the slipping surfaces. The characteristics of fault gouge have been studied extensively in the field, laboratory, and numerical simulations in order to gain a better understanding of fault mechanics (e.g., Marone and Scholz, 1989). However, observations of natural fault gouges in active fault zones can still provide precious information about fault activity and mechanical processes acting during fault evolution. Here, we report detailed microstructural observations (optical and electronic microscopy) on natural fault rocks from the scarp of an active fault in carbonate rocks: the Tre Monti fault, in the Lazio-Abruzzi Apennines. This area is one of the most seismic regions in the Mediterranean area (e.g., L’Aquila Earthquake, Mw 6.3, 2009). We revealed, for the first time in this area, the occurrence of very comminute localization zones enriched with exotic material mostly composed of clays of the smectite group, minor biotite/muscovite, quartz, feldspar and other minerals. Clay minerals completely enwrap carbonate particles (<10 μm) and thick clay rich zones show fluid-like structures, carrying small carbonate particles in them. Previous studies in this area considered fault cataclasites to be composed only of carbonate wear material, smeared from pure limestones exposed in the footwall of the faults (Agosta and Kirschner, 2003). Chemical analysis confirmed that allogenic material derives from smearing and infiltration from clay-rich sedimentary sequences (Orbulina Marls Fm. and Flysch deposits.), within the fault zone. Moreover geophysical and geological studies revealed that Orbulina Marls and Flysch deposits occur in the hangingwall of the Tre Monti fault, buried beneath Plio-Pleistocene continental deposits (Cavinato et al., 2002), this evidence confirms our observations. Using field and microstructural data is possible to reconstruct the long-term evolution of a fault. Lithological juxtaposition during time, along a fault plane, can change the mechanical properties and fault strength by mixing of different lithologies progressively involved during fault activity. This mixing could control different deformation mechanism and earthquake potential, both in terms of nucleation and propagation. Further experimental studies will be performed to characterized frictional properties of natural mixture of clay rich fault gouges.