Strength, stability, and microstructure of simulated calcite faults sheared under laboratory conditions spanning the brittle-plastic transition

Abstract

Destructive earthquakes are commonplace in tectonically-active carbonate-bearing terrains, often leading to severe economic damage and major loss of life (e.g. the Apennines, Italy). Efforts to improve seismic risk assessment in such terrains require a quantitative understanding of the slip behaviour of faults in carbonate rocks. In this thesis I report the results of an experimental investigation of the mechanisms controlling slip of simulated fault rocks composed of calcite (CaCO3). Shear (friction) tests were carried out at conditions relevant to earthquake nucleation in the upper ~20 km of Earth’s crust, alongside correlative micro-/ nanostructural study of recovered sheared samples. Experiments conducted at an effective normal stress (sneff) of 50 MPa, and sliding velocities (v) of 0.1 to 10 µm/s, showed a transition from stable, velocity (v-) strengthening to potentially unstable, v-weakening behaviour at temperatures above 80° to 100°C. Corresponding values of the apparent coefficient of friction at steady-state (µ = shear stress τ/sneff), determined at v = 1 µm/s, measured ~0.6 to 0.8. Experiments conducted up to 550°C continued to display v-weakening, until a transition to v-strengthening behaviour occurred at ~600°C, accompanied by a maximum in shear strength (µ ≈ 0.7-1.0). Microstructures of gouges recovered from experiments conducted at 20° to 200°C consistently showed a pattern of strain localization into boundary-parallel and inclined, nanocrystalline shear bands, characterized by a crystallographic preferred orientation. When internally split, these shear bands display striated, highly-reflective (shiny) patches composed of ~100 nm-wide fibres that are ductile at room conditions. At 400° to 600°C, localized slip in a single boundary shear occurred, as well as more distributed shear involving grain size sensitive (GSS - diffusion creep) and/ or grain size insensitive (GSI - dislocation creep) deformation processes. A mechanism of dilatant, athermal granular flow, operating in competition with thermally-activated compaction creep is proposed to explain the observed transitions in frictional velocity dependence. Specifically, the transition from v-strengthening to v-weakening behaviour at 80° to 100°C is interpreted to occur due to accelerated intergranular diffusive mass transfer at elevated temperatures, while at 550° to 600°C, localized, v-weakening slip involving balanced dilatant flow and creep-controlled compaction gives way to pervasive, v-strengthening viscous/ plastic shear controlled by stable GSS and/ or GSI shear deformation. The mechanical data and microstructures reported in this thesis have important implications for faulting and seismicity in limestone terrains. The seismogenic zone is expected to span a range from ~2 to 4 km to ~18 to 24 km depth. Although the depth limit of particularly the lower stability transition may depend on slip rate, temperature, and effective normal stress in a complex way, these depth estimates are in good agreement with observations of seismicity in tectonically-active limestone terrains. Further, the sheared gouge micro- and nanostructures reported in this thesis point to the importance of nanoscale fault slip processes in limestone at upper crustal conditions, and to the potential seismogenic nature of calcite mylonites that are apparently formed by pure plastic flow.