Effect of strong and weak inclusions on the frictional behaviour of fault gouges

Faults are heterogeneous at all scales. Crustal faults extend for tens or hundreds of kilometers across which they intersect many different lithologies. In the fault core, meter-scale blocks are embedded within a shear zone mélange and, at the grain-scale, fault segments comprise patches of weak and strong minerals. This structural heterogeneity may be  associated with an heterogeneous stress distribution on-fault and has therefore been invoked to explain observations of different slip behaviours occuring simultaneously and at the same location on individual faults.Conceptually, the structural and mechanical heterogeneity along fault is often described in terms of rheological asperities that can either be competent, have a velocity-weakening frictional behavior and tend to slip unstably or be less competent, have a velocity-strengthening frictional behaviour and slip stably. To understand the spectrum of slow, intermediate, and fast slips behaviors observed in nature, the laboratory studies have investigated the effect of rheological asperities on fault stability and slip behaviour. Laboratory experiments have been performed using mixtures of gouge materials with different frictional properties mixed homogeneously in various proportions or using spatially heterogeneous gouges with predefined layering perpendicular or parallel to the shear direction. In gouge samples prepared using talc-calcite mixtures, a 20% fraction of talc – the weak phase – was shown to drastically change the frictional properties of calcite gouge. However, recent results for vertically segmented gouges prepared from claystone and sandstone showed that fault friction and its rate-dependence are not simply controlled by the weakest lithology nor by a homogeneous mixture of the juxtaposing lithologies.We performed friction experiments at room temperature in a servo-controlled biaxial apparatus using homogeneous and heterogeneous gouge samples prepared from calcite and talc minerals in various proportions by weight. Calcite and talc were chosen for their well-known antagonist frictional behaviour, as the strong velocity-weakening lithology and weak velocity-strengthening lithology, respectively. Heterogeneous gouge samples consist of a cylindrical inclusion of talc/calcite embedded within calcite/talc. For each experiment, two identical layers of gouge were placed in between three grooved sliding blocks, in a double-direct shear configuration, and a constant normal stress of 40 MPa was applied. Samples were sheared at a sliding velocity of 10 µm.s-1 until a steady state was reached. Velocity-stepping and slide-hold-slide sequences were then performed, under fully dry conditions. Overall, we observe that the coefficient of friction decreases with increasing talc content. However, depending on the geometry of the slip interface – a weak inclusion in a stronger and continuous lithology or the opposite – the decrease in strength associated with the presence of phyllosilicate minerals varies non linearly. Velocity weakening during shearing is reduced in the case of a strong calcite inclusion embedded in a weaker talc continuum. Our results show that the rheological heterogeneity associated with the presence of weak or strong inclusions exerts a second order control on the frictional behaviour of our simulated gouges.