Frictional properties and slip stability of active faults within carbonate-evaporite sequences: The role of dolomite and anhydrite

Seismological observations show that many destructive earthquakes nucleate within, or propagate through, thick sequences of carbonates and evaporites. For example, along the Apennines range (Italy) carbonate and evaporite sequences are present at hypocentral depths for recent major earthquakes (5.0<M_w<6.3). However, little is known about the frictional properties of these rocks, which are dominated by dolomite and anhydrite and differ from typical quartzofeldspathic rocks. Here, we report on laboratory experiments designed to illuminate friction constitutive properties and associated microstructures of anhydrite and dolomite fault rocks. We performed frictional sliding experiments on powdered samples for a range of normal stresses (σ_n) from 10 to 150MPa and sliding velocities from 1 to 300μm/s. Experiments employed the double direct shear configuration and were conducted both dry, at room temperature, and water saturated at 75°C under true triaxial stresses. At 25°C, sliding friction is ~0.65 for all samples and conditions studied. At 75°C, samples develop cohesive strength, and sliding friction ranges from 0.4 to 0.5 for σ'_n from 10 to 35MPa. Velocity step tests were used to assess rate/state frictional behavior. At 25°C the slip behavior is velocity strengthening/neutral for all samples. Microstructural observations indicate localized deformation on R₁ Riedel and incipient Y-shear planes with marked grain-size reduction. At 75°C and larger strain we find that rate/state friction behavior correlates with velocity-dependent layer dilation and compaction and this is associated with the development of through-going Y-shear planes. We conclude that elevated temperature, strain and the presence of pore fluids enhance shear localization into continuous Y-shear planes, which could promote frictional instability at seismogenic depth within dolomite-anhydrite sequences.