Field, microstructural and phase characterization of mirror-like fault surfaces in bituminous dolostones (central Apennines, Italy)

Mirror-like surfaces (MSs) are easily recognizable in the field since they reflect natural 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 better constrain the formation mechanism of MSs and their role in the seismic cycle, field, and high-resolution microstructural investigations, combined with thermal maturity analyses, were conducted on MSs cutting Triassic bituminous dolostones from the Italian Central Apennines. This region is one of the most seismically active areas in the Mediterranean (e.g., L’Aquila 2009, Mw 6.3 earthquake), with mainshocks and aftershocks propagating along extensional faults, cutting km-thick sequences of carbonates. The studied faults are hosted in the footwall of the younger-on-older Monte Camicia thrust, related to the Pliocene to Holocene in age Apenninic compressional to extensional tectonics and exhumed from < 4 km depth. The MSs samples were collected from faults with evidence of increasing cumulated slip (from few mm to few meters) and different attitudes (variable resolved stresses) to evaluate i) whether the thermal maturity of organic matter on fault surfaces preserved a trace of frictional heating and ii) to estimate the role of variable mechanical work in their formation.

The microstructures of the MSs and the associated slip zones display a polyphasic deformation history; smeared bitumen along the slip surfaces is spatially associated with (i) discrete ultracataclastic slip zones containing fragments of older bitumen-rich slip zones and calcite-rich vein-precipitated matrix and, (ii) lower strain cataclastic layers with evidence of pressure-solution in the dolostone clasts and viscous shear in the bitumen. Such different deformation styles of bitumen-rich materials might be an evidence of high strain rate coseismic embrittlement and long-term aseismic creep during the seismic cycle.

Micro-Raman analyses on the MSs and their wall rocks have been aimed at quantifying the thermal maturity of the organic matter on slip surfaces that can reveal thermal pulses associated to frictional heating during seismic slip. This multidisciplinary study, though finalized to a deep understanding of their formation mechanism, may lead to recognize microstructural or mineralogical/geochemical features specifically associated to earthquake ruptures in natural faults with a potential impact on seismic hazard studies.