Complex frictional behaviour of clay-bearing carbonate faults revealed by integrated field and experimental investigation

Seismicity in the Northern Apennines of Italy nucleates in and propagates through a complex carbonate multilayer sequence, comprising limestones with regular marl interbeds. Observations from the Gubbio fault (1984, Ms = 5.2) indicate that earthquake displacement is localized within narrow principal slip zones, <1.5 mm wide, characterized by cataclasites, gouges and calcite veins, and containing up to 50% phyllosilicate. Due to their predominantly velocity-strengthening nature, phyllosilicates are often considered to promote aseismic behaviour during the inter-seismic and post-seismic (afterslip) periods, and may act as barriers to rupture propagation in the upper crust. To assess the effect of clay content on the frictional behaviour and microstructural evolution of carbonate faults during earthquake propagation, we performed high-velocity friction (HVF) experiments, using a rotary-shear apparatus, on end-member gouges of calcite (Cal), montmorillonite (Mont) and mixed-layer illite-smectite (I/S), and on 50:50 and 80:20 mixtures of Cal+Mont and Cal+I/S. Experiments were conducted at room temperature, 1.3 m/s slip rate and 9 MPa normal load. Each sample was run under dry and water-saturated conditions, and experiments terminated both at peak friction (μ_p) and steady-state friction (μ_ss) for comparison of microstructures at contrasting strength stages. Results for the dry gouges are in-line with previous HVF experiments: all samples attain a peak in friction at the onset of slip, ranging from 0.59 (Mont) to 0.72 (Cal), followed by a dramatic decrease in strength within the first ~0.5 m, after which friction maintains a constant μ_ss value, ranging from 0.15 (Cal) to 0.22 (Mont). Frictional behaviour of the wet gouges is very different: clay-bearing samples typically do not exhibit a peak in friction; steady-state behaviour is attained immediately at the onset of sliding with friction coefficient values as low as 0.05. In summary, when dry, calcite controls the strength of the mixtures. However, under saturated conditions, and with the presence of just 20 wt.% clay, the strength of the mixtures is controlled by the clay and steady-state weakening occurs immediately at the onset of sliding. Our results suggest that in the presence of fluids, as evidenced at Gubbio and elsewhere in the Apennines fault system (e.g. Colfiorito and L'Aquila), very small amounts of phyllosilicate can produce significant fault lubrication and facilitate earthquake propagation in carbonate-hosted faults.