Geological and geophysical evidence suggests that some crustal faults are weak compared to laboratory measurements of frictional strength. Explanations for fault weakness include the presence of weak minerals, high fluid pressures within the fault core and dynamic processes such as normal stress reduction, acoustic fluidization or extreme weakening at high slip velocity. Dynamic weakening mechanisms can explain some observations; however, creep and aseismic slip are thought to occur on weak faults, and quasi-static weakening mechanisms are required to initiate frictional slip on mis-oriented faults, at high angles to the tectonic stress field. Moreover, the maintenance of high fluid pressures requires specialized conditions and weak mineral phases are not present in sufficient abundance to satisfy weak fault models, so weak faults remain largely unexplained. Here we provide laboratory evidence for a brittle, frictional weakening mechanism based on common fault zone fabrics. We report on the frictional strength of intact fault rocks sheared in their in situ geometry. Samples with well-developed foliation are extremely weak compared to their powdered equivalents. Micro- and nano-structural studies show that frictional sliding occurs along very fine-grained foliations composed of phyllosilicates (talc and smectite). When the same rocks are powdered, frictional strength is high, consistent with cataclastic processes. Our data show that fault weakness can occur in cases where weak mineral phases constitute only a small percentage of the total fault rock and that low friction results from slip on a network of weak phyllosilicate-rich surfaces that define the rock fabric. The widespread documentation of foliated fault rocks along mature faults in different tectonic settings and from many different protoliths suggests that this mechanism could be a viable explanation for fault weakening in the brittle crust.

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[1] Previous rotary shear experiments, performed on a halite-muscovite fault gouge analogue system have shown that the presence of phyllosilicates, under conditions favoring the operation of cataclasis and pressure solution in the matrix phase, can have major effects on the frictional behavior of gouges. While 100% halite and 100% muscovite samples exhibit rate-independent frictional/brittle behavior, the strength of mixtures containing 10-30% muscovite is both normal stress and sliding velocitydependent. At high sliding velocities (>1 mm s À1 ), such mixtures show unusually marked velocity weakening, along with the development of a structureless, cataclastic microstructure. In the present paper, a micromechanical model is developed in an attempt to explain this behavior. The model assumes a granular flow process involving competition between intergranular dilatation and compaction by pressure solution. The predictions of the model agree favorably with the experimental results. Extension of the model to quartz-mica systems implies that the presence of phyllosilicates plus the operation of pressure solution can strongly promote (unstable) velocity-weakening behavior at rapid slip rates on natural faults, under midcrustal conditions. Static stress drop predictions based on the model agree reasonably well with estimates from seismic observations. Our results may help explain the discrepancy between laboratory-derived rate-and-state friction parameter values, obtained for dry, low-strain and/or single-phase rock systems, and the values for natural fault rocks inferred from seismological data.Citation: Niemeijer, A. R., and C. J. Spiers (2007), A microphysical model for strong velocity weakening in phyllosilicate-bearing fault gouges,

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Despite the fact that phyllosilicates are ubiquitous in mature fault and shear zones, little is known about the strength of phyllosilicate-bearing fault rocks under brittle-ductile transitional conditions where cataclasis and solution-transfer processes are active. In this study we explored steady-state strength behaviour of a simulated fault rock, consisting of muscovite and halite, using brine as pore fluid. Samples were deformed in a rotary shear apparatus under conditions where cataclasis and solution transfer are known to dominate the deformation behaviour of the halite. It was found that the steady-state strength of these mixtures is dependent on normal stress and sliding velocity. At low velocities (<0.5 µm s−1) the strength increases with velocity and normal stress, and a strong foliation develops. Comparison with previous microphysical models shows that this is a result of the serial operation of pressure solution in the halite grains accommodating frictional sliding over the phyllosilicate foliation. At high velocities (>1 µm s−1), velocity-weakening frictional behaviour occurs along with the development of a structureless cataclastic microstructure. Revision of previous models for the low-velocity behaviour results in a physically realistic description that fits our data well. This is extended to include the possibility of plastic flow in the phyllosilicates and applied to predict steady-state strength profiles for continental fault zones containing foliated quartz-mica fault rocks. The results predict a significant reduction of strength at mid-crustal depths and may have important implications for crustal dynamics and seismogenesis.

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