Frictional behavior and microstructures of calcite fault gouges deformed under extreme conditions of normal stress and sliding velocity

In recent years several experimental studies have been performed using rotary-shear apparatus to investigate the frictional behavior of gouge materials at seismic slip rates. However, because of technical difficulties confining gouge layers, a majority of these experiments were conducted at normal stresses <2-3MPa, making extrapolation to natural conditions challenging. Here, we present results from an experimental study on calcite gouges (<250μm grain size) deformed in a purpose-built sample holder and using a rotary-shear apparatus at INGV, Rome. Ring-shaped (25/45mm int./ext. diameter), 2.8mm-thick layers of gouge were deformed up to 34MPa normal stress, at slip rates of 10μm/s - 3m/s, in both room-dry and water-present conditions. A peak slip rate of 3m/s was achieved after 0.5s, and total displacements were 1-3m. CO₂ emissions were monitored using a mass spectrometer connected to a capillary tube positioned approximately 1cm from the gouge sample holder. Samples were preserved in ultra-low viscosity resin for optical and Field-Emission SEM observations. At slip rates >0.3m/s frictional strength, μ, increases to a peak value of 0.6-1.0 followed by a rapid decay to a lower steady-state value, μ_ss, before finally undergoing dynamic strength recovery during decelerating slip. CO₂ starts to be liberated almost instantaneously (within 500μm of slip) during acceleration and reaches a peak value during steady-state sliding of up to 10,000ppm, before decreasing to 380-420ppm within a few seconds following the experiment. μ_ss decreases with increasing slip rate, but in contrast to previous experiments we find that extremely low values of μ_ss<0.2 are only achieved at a slip rate of 1m/s for normal stresses >22MPa. An unexpected result is that steady-state shear stress at slip rates >1m/s does not increase monotonically (either linearly or not, e.g. in the presence of lubrication) with normal stress, but begins to decrease above a normal stress of 15-20MPa, a behavior that may be described as "hyper-lubrication". Gouge layers deformed at slip rates >0.5m/s and normal stresses >11MPa are cut by mirror-like slip surfaces underlain by fine-grained (<1μm) decomposition zones up to 500μm thick. The slip surfaces are lined by interlocking, polygonal grains between 100nm and 2μm in size. Grain boundaries are straight or slightly curved and contacts often make triple-junctions with large interfacial angles. In cross-section, the decomposition zones contain elongate grains up to 20μm long that collectively define a strong shape-preferred orientation inclined in the shear direction. We interpret these and other microstructures to indicate plastic deformation and annealing of the calcite gouges immediately beneath and on the slip-surfaces, particularly during the deceleration phase of the experiments as slip velocity decreases but temperatures may locally remain high.