Biomarkers Thermal Maturity Parameters in Bitumen as a Proxy for Seismic vs. Aseismic Mirror-like Fault Surfaces

Miriana Chinello, Andrea Schito , Stephen Alan Bowden, Elena Spagnuolo, Stefano Aretusini, Telemaco Tesei, Giulio Di Toro

Mirror-like Surfaces (MSs) are ultra-polished fault surfaces that experimental studies have shown can form during seismic and aseismic deformation (slip rates ≈1 m/s vs. <0.0001 m/s). To determine which MSs form during seismic (rapid temperature increase) or aseismic (negligible temperature increase) slip along the fault, we measured sterane and hopane thermal maturity parameters in bituminous MSs and their underlying slip zone. Samples with MSs were taken from faults hosted in bituminous dolostones from the Central Apennines, Italy, with displacements ranging from millimeters to meters. To support interpretation of the naturally formed MSs, thermal maturity was also assessed for experimental MSs obtained at seismic (1 to 3 m/s) and sub-seismic (0.0001 m/s) slip rates, and for powders statically heated at 400 °C for 2 seconds up to 12 hours. Higher thermal maturity in MSs with respect to the associated slip zone were measured in most of the natural MSs and in the “seismic” experimental MSs. For static-heating experiments, heating at less than 30 seconds was sufficient to change concentrations of biomarkers, while heating for more than 60 seconds was needed to increase sterane 20S and hopane 22S thermal maturity parameters. We show that sterane 20S and hopane 22S parameters have different friction- and burial-induced thermal maturation pathways. We conclude that, with respect to “aseismic” MSs, rapid temperature increase during earthquakes can locally alter biomarkers and increase thermal maturity in “seismic” fault Mirror-like Surfaces.