"Frictional processes" in carbonate-bearing rocks at seismic deformation conditions

Abstract:

Moderate to large earthquakes often rupture and propagate along faults in carbonate-bearing rocks (dolostones, limestones, marbles, etc.). Compared to silicate-bearing rocks, which melt, weaken and wear when sheared at seismic slip rates (ca. 1 m/s), carbonate-bearing rocks do not melt, the minimum friction coefficient can be much lower (down to 5% of static friction) and the wear rate is negligible at seismic slip rates. In cohesive carbonate-bearing rocks, experiments simulating seismic deformation conditions and stopped at slip initiation (< 5 mm) suggest that initial frictional decay down to 50% of static friction is associated with CO₂ emission plus formation of nanograins and amorphous carbon. Experiments stopped at larger slips (< 5 mm) show that the slipping zone consists of a foam-like sintered surface overlying a microporous fabric made of calcite and lime (plus periclase in the case of dolomite) nanograins. In non-cohesive rocks (e.g. gouges) a similar evolution is observed after an initial period of strengthening and strain localization. Experiments with pressurized H₂O show that the contribution of thermal and thermochemical pressurization is negligible in cohesive and questionable in non-cohesive carbonate-bearing rocks. We propose that initial frictional weakening is due to the formation of patches of amorphous carbon (solid lubricant) at asperity contacts. With progressive slip and bulk temperature increase, nanograins accommodate large strain rates (ca. 10⁴ s^-1) by grain boundary sliding as suggested by several authors. The presence of a microporous fabric boosts pore-controlled diffusive process propelled by CO₂ gas exhaust due to decarbonation. Enhanced pore-controlled diffusive processes allow (1) efficient mass transfer during grain boundary sliding and (2) sintering of the nanograins into a foam-like slip surface at the end of the experiment.