Switching deformation mode and mechanisms during natural faulting of Carrara marble

A study of meso- and microstructural features of an exhumed high angle normal fault observed in Carrara marble of the Alpi Apuane NW Tuscany (Italy) is here presented. The studied fault was formed at shallow crustal depth and related with the recent deformation history of the Alpi Apuane metamorphic core (Molli et al. 2015, Molli et al., 2016). On the basis of meso- and microscale deformation features different stages of fault zone evolution were recognized. Stage 1 was associated with extensional and shear veins now observable in both hangingwall and footwall blocks as part of the deformation zone developed at decameter-scale. Geochemical data indicate vein-development in a locally closed system where a “stationary” fluid phase migrates over meter-scale distances (Molli et al., 2011). At stage 2, a localization of the deformation took place. During this second stage, crystal-plastic deformation affected a millimeter-thick zone along the main slip surfaces, whereas cataclastic deformation produced a decimeter to meter thick fault core with brecciation and far-derived fluid channelling leading to significant geochemical alteration of the fault rocks with respect to the protolith (Molli et al. 2011). Deformation was then localized within ultracataclasite of the fault core where diffusive-mass transfer dominates the microstructural development (stage 3). The fault history ends with stage 4, associated with a vertical shortening and development of slickolites at the contact between the main slip surface and fault core and microveins plus stylolites within the fault core. The analysis of the studied fault within Carrara marble reveals therefore a switching in the deformation mode in terms of distributed vs localized and a changing in deformation mechanisms; these features may be associated, following the most recent experimental work (e.g. Smith et al., 2012), with a “fossil” seismic cycle.