Frictional Behaviour of Carbonates: Defining the Seismogenic Zone in Dolomite

Understanding the rheology of carbonates is crucial, as destructive earthquakes frequently occur in tectonically active carbonate regions (e.g., the Corinth Rift Zone, the Italian Apennines, and the Sichuan Basin, China), leading to fatalities and severe economic impacts. Seismic to aseismic deformation can be understood in terms of the frictional response, which is based on the underlying deformation mechanisms. To grasp the seismic potential of faults in carbonates, we studied the conditions under which dolomite fault gouge is frictionally unstable, based on the rate and state friction law (RSF) and complemented the mechanical results with microstructural observations to understand the active deformation mechanisms.
We will present results from experiments conducted using the hydrothermal rotary shear apparatus on simulated fault gouge of dolostone. These experiments were carried out at sub-seismic slip velocities ranging from nm/s to hundreds of µm/s, temperatures from room temperature to 600 °C, and at a constant effective normal stress and fluid pressure of 50 MPa. Our mechanical results show a strongly temperature dependent steady-state sliding strength during initial sliding, with Byerlee friction values of ~0.7 at low (< 300 °C) and the highest (600 °C) temperature, but values of ~0.4 at intermediate temperatures (300 – 500 °C). Additionally, the friction values show a strong dependence on both velocity and temperature, where cooler temperatures (< 300 °C) are mostly velocity strengthening and therefore conditionally stable, while higher investigated temperatures (400 – 600 °C) result in mostly velocity weakening, potentially unstable outcomes. An exception to this is found at the highest velocities (> 100 µm/s) at 400 – 500 °C and the slowest velocities (< 0.03 µm/s) at 600 °C. Further, we observed oscillatory and stick-slip behaviour at negative and near-zero RSF (a-b) values with amplitudes decreasing with increasing friction stability values. Microstructural observations on deformed samples revealed that brittle deformation mechanisms are active across all investigated temperatures. However, microstructural analysis using SEM-XRD, Raman, and FTIR on samples deformed at temperatures >300 °C provides evidence of dolomite decomposition into calcite and brucite. This dissolution-precipitation creep is enhanced with increasing temperature.
These results suggest a transition from stable to unstable behaviour at ~300 °C which continues until the highest temperature of 600 °C with frequently accompanied stick-slips, translating to a seismogenic depth range of 10 – 20 km, assuming a continental geothermal gradient of 30 °C/km.