Seismic Versus Sub‐Seismic Experimental Mirror‐Like Fault Surfaces in Bituminous Dolostones

Abstract Mirror‐like Surfaces (MSs) are ultra‐polished surfaces frequently found in exhumed seismogenic fault zones in carbonates. They form over 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 on incohesive samples under water‐dampened and room‐humidity conditions, to determine the conditions favoring MSs formation. MSs developed under all tested conditions, regardless of water content. Mechanical data indicate that, compared to pure dolomite, the presence of bitumen lowers the friction coefficient and promotes slip‐neutral behavior at seismic slip rate. FEG‐FIB‐SEM microstructural investigations revealed that all experimental MSs consist of a 10s‐nm‐thick film of rounded nanoparticles ≈5–15 nm in diameter. Subtle microstructural differences emerged in the “room‐humidity” MSs. The “sub‐seismic” MSs display nanoparticles arranged in sub‐spherical aggregates measuring a few 10s nm. Differently, “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. Changes in porosity, grain size and foliation are observed in the firsts μm of the slip zone under all the “seismic” MSs. These results suggest that MSs represent a 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 natural faults, may help distinguish between “seismic” and “sub‐seismic” MSs, with implications for seismic hazard assessment.