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 carbonate multilayer sequence, comprising limestones with regular marl interbeds. Observations from the Gubbio fault (1984, Ms = 5.2) show 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- 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 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 do not exhibit a peak in friction; steady-state behaviour is attained immediately with values of µ_ss as low as 0.05.

When dry, calcite controls the strength of the mixtures. However, under saturated conditions, and with 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. This suggests that in the presence of fluids, as evidenced at Gubbio, very small amounts of phyllosilicate can produce significant fault lubrication and facilitate earthquake propagation in carbonate–hosted faults.