Deformation mechanisms and slip localization in seismically active carbonate faults: A case study on natural carbonate fault gouges from one of the most seismically active regions in Europe

Earthquakes are one of the most devastating and unpredictable natural disasters. Geographic regions prone to high magnitude seismic events, like the Mediterranean, are often densely populated suffering human casualties and costly infrastructural damage. Research on rocks that host earthquakes and the fault products affiliated with them show remarkable advances during the past few years with respect to the nucleation and reactivation mechanisms. Fault structures and materials show a strong material dependence and especially carbonate rocks gained attention because of their non-melting behavior during seismic slip, when compared to silicate rocks. The resulting carbonate fault gouges can reach extremely small grains sizes of about 100 nm, demanding high spatial resolution techniques to investigate the involved slip nucleation processes. Our work focusses currently on natural samples from two seismically active locations in Greece to study the deformation processes inside the fault gouge. We combine state-of-the-art analytical techniques for micro- and nanostructural analysis, encompassing electron backscatter diffraction (EBSD), Raman spectroscopy, focused ion beam scanning electron microscopes (FIB-SEM), and automated crystal orientation measurement transmission electron microscopy (ACOM-TEM). The results show strong slip localization within a layer of approx. 20 μm constituting the principal slip surface and forming a fault mirror. Slip localization takes place on top of a previously recrystallized fault gouge. Instead of an expected gradual grain size reduction by comminution, grain size changes abruptly. The energy dissipation during seismic slip produces an amorphous slip surface coating and cavity infiltration of the amorphous material deeper into the fault gouge volume. Nanoclay precipitates inside the amorphous phase co-coat the slip surface and impregnate grain boundaries inside the fault gouge. These results imply that mechanical amorphisation of calcite and the reprecipitation of nanoclay play a central role for fault rock deformation and fault plane reactivation. Especially the reprecipitation of clay minerals is important for fault reactivation because they exhibit a weak slip behavior. The results further imply that powder lubrication by nanograin coating, where we see no evidence for, is not a necessary scenario to explain fault weakening in carbonate rocks.