The frictional strength and stability of spatially heterogeneous fault gouges

Along-fault lithological heterogeneity is observed in all fault zones that cross-cut compositional layering. Numerical modelling studies on fault rupture nucleation, propagation and arrest often assume that the fault mechanical behaviour is governed by either the rheologically weak phase or by a homogeneous gouge mixture of juxtaposing lithologies. However, the effects of spatial heterogeneity on fault gouge composition and hence its frictional behaviour are less known. In this study, we simulate a mixture of mechanically contrasting rheologies of claystones and sandstones in fault gouges by using lithologies available in the well-known Groningen gas field stratigraphy (Ten Boer and Slochteren members, respectively). Friction experiments were performed in a rotary shear configuration to accommodate the large displacements required to study mixing and clay smearing in faults with large offsets. A velocity stepping procedure was conducted to quantify the rate-dependence of friction and its evolution with displacement. A spatial heterogeneity was introduced by segmentation of the simulated gouge in claystone and sandstone patches. In contrast to previous studies, we show that Slochteren sandstone gouges can exhibit velocity-weakening behaviour related to strain-localization in a principal slip zone with reduced grain size. Our experiments on segmented gouges show displacement-dependent changes in the sliding friction and its rate-dependence. Clay smearing and shear localization on foliation planes cause weakening of the gouge and a shift from velocity-weakening to velocity-strengthening behaviour. Progressive shearing leads to juxtaposition of sandstone segments that are separated only by a thin clay smear. We propose that the associated increase in friction is caused by lithology mixing at the interfaces between the clay smear and the bulk Slochteren sandstone gouge, and by the disruption of continuous Y-shears. Progressive shearing does not lead to a decrease in the rate-sensitivity parameter (a-b). This observation suggests that shearing remains localized on phyllosilicate foliations, possibly accommodated by the increased width of the principal slip zone (PSZ) with displacement. Our results show that fault friction and its rate-dependence are not simply governed by the weakest lithology along a fault plane, nor that they can be simply represented by a homogeneous mixture of the juxtaposing lithologies. Detailed knowledge of the stratigraphic layering in combination with the fault offset is required to predict the macroscopic frictional behaviour of heterogeneous fault gouges.