Mirror-like fault surfaces in bituminous dolostones (Central Apennines, Italy)

Mirror-like surfaces (MSs) are ultra-polished fault surfaces that reflect visible light thanks to their low surface roughness (nm-scale). These ultra-polished surfaces are often found in seismogenic fault zones cutting limestones and dolostones (e.g., Siman-Tov et al., 2013; Fondriest et al., 2013; Ohl et al., 2020). Both natural and experimentally-produced fault-related MSs were described in spatial association with ultrafine matrix (grain size <10µm), nanograins (<100nm in size), amorphous carbon, decomposition products of calcite/dolomite (i.e., portlandite, periclase) and larger in size but “truncated” clasts (Verberne et al., 2019). However, the mechanism of formation of MSs is still a matter of debate. Indeed, experimental evidence shows that MSs can develop both under seismic (slip rate ≈1 m/s; Fondriest et al., 2013; Siman-Tov et al., 2013; Pozzi et al., 2018; Ohl et al., 2020), and aseismic (slip rate ≈0.1-10 µm/s; Verberne et al., 2013; Tesei et al., 2017) deformation conditions, involving various physical-chemical processes operating over a broad range of P-T conditions, strain and strain rates.

To understand how MSs form and their role in the seismic cycle, 10 samples were collected and analysed from normal faults cutting bituminous dolostones (Central Apennines, Italy). The MSs samples were from faults with increasing cumulated slip (from < 1 mm to few meters) and different resolved stress.

Ultra-high resolution scanning electron microstructural investigations of the MSs and the associated slip zones, show that the mirrors consist of exposed surfaces of ultra-flat dolostone grains and dolomite nanoparticles cemented by a <1-2 μm thick matrix of smeared bitumen. Cataclastic flow and pressure solution aided by the presence of bitumen are the main deformation mechanisms, probably associated with aseismic creep and fault healing/sealing during the seismic cycle.

Surface microroughness measurements (White Light Profilometry) reveal that (1) the RMS microroughness is < 500 nm over a lateral distance < 1 mm and (2) both the profile and the areal RMS show a weak inverse correlation with increasing displacement. Power Spectral Density (PSD) analysis shows that only in the sample with a displacement less than 1 mm is there a dependence of roughness on slip direction (that is, striae are observed).

Finally, Gas Chromatography-Mass Spectrometry analysis of bitumen from a fault MS which accommodated 86 cm of slip displacement has less quantities of larger molecular weight biomarkers and enrichment in lower molecular weight homologues relative to unfaulted rock. A difference that can be explained by frictional heating during seismic slip causing the destruction of higher molecular weight homologues.

This multidisciplinary study, by investigating the mechanism of formation of MSs, show that these ultra-polished features record the main phases of the seismic cycle, including coseismic slip (changes in the biomarkers structure), aseismic creep (viscous flow of bitumen) and inter-seismic fault sealing/healing (pressure-solution and cold sintering).