Seismic vs. sub-seismic Experimental Mirror-like Fault Surfaces in Bituminous Dolostones

Mirror-like Surfaces (MSs) are ultra-polished surfaces frequently found in exhumed seismogenic fault zones in carbonates. They form under a wide range of pressure-temperature conditions and slip rates, from seismic (≈1 m/s) to sub-seismic (≈0.1-100 μm/s), reflecting distinct phases of the seismic cycle. Motivated by the abundance of MSs in bituminous dolostones from the Central Apennines (Italy), we performed rotary-shear experiments at seismic and sub-seismic slip rates, under water-dampened and room-humidity conditions, to determine the conditions favoring MSs formation. The resulting fault surfaces were analyzed using FEG-FIB-SEM to identify micro- and nano-structural indicators of slip rate. MSs developed under all tested conditions, regardless of water content. All experimental MSs consist of a 10s-nm-thick film of rounded nanoparticles (5-15 nm in diameter) overlying a 10-100 µm-thick Cataclastic Slip Zone (CSZ). Under room-humidity conditions, subtle differences emerged. The “seismic” MS is cut by micro-cracks due to thermal contraction or desiccation, and nanoparticles are arranged in flat, sub-rounded aggregates measuring 5-150 nm. Differently, in “sub-seismic” MSs, the nanoparticles are arranged in sub-spherical aggregates measuring few tens of nanometers. The CSZ displays micro- to nano-porosity in the first 2-3 µm beneath the “seismic” MS, while it is finer-grained and free of porosity in the “sub-seismic” slip zone. We conclude that MSs are the morphological convergence of physical and chemical processes acting across a broad range of deformation conditions. However, micro- to nano-scale structural features, if preserved in nature, may help distinguish between the “seismic” and “sub-seismic” origin of MSs.