Mechanical behavior and microstructure of simulated calcite fault gouge sheared at 20-600°C: Implications for natural faults in limestones

Abstract

We report ring shear experiments on simulated calcite fault gouges performed at fixed temperatures (T) within the range from 20°C to 600°C. The experiments were performed wet, using pore fluid pressures (Pf) of 10 ≤ Pf ≤ 60 MPa. One series of experiments employed a constant effective normal stress (σneff) of 50 MPa, while in a second series, σneff was sequentially stepped from 30 to 100 MPa. In all experiments, sliding velocity (v) was stepped in the range from 0.03 to 100 μm/s. The results showed stable, velocity-strengthening behavior at 20°C, but velocity weakening at 100°C to 550°C (for all v steps to <3 μm/s), which was frequently accompanied by stick slip. At 600°C, velocity strengthening occurred. Microstructural observations suggest increasing importance of ductile deformation with increasing temperature, as reflected by a localized shear band structure at 20°C giving way to a pervasive, shear plane-parallel grain shape fabric at 600°C. Using existing flow equations for dense calcite polycrystals, we show that dislocation and/or diffusion creep of 10-30 μm-sized bulk gouge grains likely played a role in experiments performed at T ≥ 400°C. We suggest that the observed velocity-weakening behavior can be explained by a slip mechanism involving dilatant granular flow in competition with creep-controlled compaction. Our results have important implications for the breadth of the seismogenic zone in limestone terrains and for the interpretation of natural fault rock microstructures. Specifically, while samples sheared at 400-550°C exhibited essentially brittle/frictional mechanical behavior (stick slip), the corresponding microstructures resembled that of a mylonite.