Compaction creep of simulated anhydrite fault gouge by pressure solution: theory v. experiments and implications for fault sealing

The sealing and healing behaviour of faults filled with anhydrite gouge, by processes such as pressure solution, is of interest in relation both to the integrity of faults cutting geological storage systems sealed by anhydrite caprocks and to seismic events that may nucleate in anhydrite-bearing sequences, such as those present in the seismogenic zone beneath the Apennines. We have developed a detailed series of kinetic models for pressure solution in anhydrite fault gouge, allowing for dissolution, diffusion and precipitation control, to estimate the time scale on which such sealing and healing effects occur. We compare the models obtained with previously reported experimental data on compaction creep rates in simulated anhydrite fault gouge, tested under wet, upper crustal conditions. The results confirm earlier indications that compaction under these conditions likely occurs by diffusion-controlled pressure solution. Applying our most rigorous model for diffusion-controlled pressure solution, constrained by the fit to the experimental data, we infer that anhydrite fault sealing will occur in a few decades at most, which is rapid compared with both CO2 storage time scales and with the recurrence interval for seismicity in the Apennines.