Frictional strength and healing behavior of phyllosilicate-rich faults

We study the mechanisms of frictional strength recovery for tectonic faults with particular focus on fault gouge that contains phyllosilicate minerals. We report laboratory and microstructural work from fault rocks associated with a regional, low-angle normal fault in Central Italy. Experiments were conducted in a biaxial deformation apparatus at room temperature and humidity, nominally dry, under constant normal stresses of 20 and 50 MPa, and at a sliding velocity of 10 mu m/s. Our results for nominally dry conditions show good agreement with previous work conducted under controlled pore fluid pressure. The phyllosilicate contents of our samples, which include clay, talc and chlorite range from 0 to 52 weight %. We study both intact rock samples, sheared in their in situ geometry, and powders made from the same rocks to address the role of fabric in fault healing. We measured frictional healing, Delta mu, using slide-hold-slide tests with hold periods ranging from 3 to 3000 s. Phyllosilicate-free materials show friction values of mu approximate to 0.6 and healing rates that are larger in powdered samples, beta approximate to 0.006 (Delta mu per decade in time, s) compared to intact wafers of fault rock, beta approximate to 0.004. For phyllosilicate-bearing materials, healing rates are low, beta < 0.002, and independent of fabric, phyllosilicate content and normal stress. We observe that frictional strength decreases systematically with increasing phyllosilicate content. Intact, phyllosilicate-bearing fault rock is consistently weaker than its powdered equivalent (0.2 < mu < 0.3 versus 0.4 < mu < 0.5, respectively). We compare our data to results from experiments conducted on a wide range of materials and conditions. Deformation microstructures show localized slipping along sub-parallel shear planes. We suggest that low values of frictional strength and near zero healing rates will combine to exacerbate the weakness of phyllosilicate-bearing faults and promote stable, aseismic creep.