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

We report ring shear experiments on simulated calcite fault gouges performed at fixed temperatures (T) within the range from 20 degrees C to 600 degrees C. The experiments were performed wet, using pore fluid pressures (P-f) of 10 <= P-f <= 60 MPa. One series of experiments employed a constant effective normal stress (sigma(eff)(n)) of 50 MPa, while in a second series, sigma(eff)(n) was sequentially stepped from 30 to 100 MPa. In all experiments, sliding velocity (v) was stepped in the range from 0.03 to 100 mu m/s. The results showed stable, velocity-strengthening behavior at 20 degrees C, but velocity weakening at 100 degrees C to 550 degrees C (for all v steps to < 3 mu m/s), which was frequently accompanied by stick slip. At 600 degrees 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 degrees C giving way to a pervasive, shear plane-parallel grain shape fabric at 600 degrees C. Using existing flow equations for dense calcite polycrystals, we show that dislocation and/or diffusion creep of 10-30 mu m-sized bulk gouge grains likely played a role in experiments performed at T >= 400 degrees 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 degrees C exhibited essentially brittle/frictional mechanical behavior (stick slip), the corresponding microstructures resembled that of a mylonite.