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Abstract
Phyllosilicate-bearing faults are characterized by an anastomosing foliation with intervening hard clasts and are believed to be long-term weak structures. Here, I present results of sliding experiments on gouges of 80 wt% quartz and 20 wt% muscovite, sheared under hydrothermal conditions at constant velocity. The results show that significant strengthening occurs over a narrow range of sliding velocities (0.03–1 μm/s). At the lowest velocity investigated, weakness is achieved after a considerable sliding distance of over 20 mm with friction reaching a value of 0.3. Microstructural observations and the application of existing models point to the operation of frictional-viscous flow (FVF), through the serial operation of frictional sliding over a weak foliation and pressure solution of intervening clasts, resulting in low frictional strength and pronounced velocity-strengthening. At higher velocities, grain size reduction becomes dominant in a localized zone, which results in disruption of the foliation and the cessation of the FVF mechanism. In natural settings, earthquakes originating elsewhere on the fault would be rapidly arrested when encountering a foliated part of the fault deforming via FVF. Furthermore, pulses of elevated slip velocity would lead to grain size reduction which would destroy the foliation and cause a long-term strengthening of the fault.
Highlights
Understanding the sliding strength of fault rocks over a wide range of loading conditions is important to better evaluate seismic hazard
The presence of phyllosilicates leads to a lowering of the frictional strength of composite gouges, with the strength of the composite gouge approaching that of the phyllosilicate end member when the phyllosilicate content is 80 wt% or more[11,12]
The operation of frictional-viscous flow (FVF) was included in the model of Den Hartog & Spiers[17], based on experimental results under hydrothermal conditions on phyllosilicate-rich (65 wt%) quartz-gouge mixtures, but the dramatic weakening observed in the analogue experiments was not observed, probably due to the high sliding velocities employed (1–100 μm/s)
Summary
There have been many experimental studies of frictional strength of both natural and analogue phyllosilicate-bearing fault gouges[11]. The deformation mechanism was termed frictional-viscous flow (FVF) and involves the frictional sliding between the soluble (halite) grains and the anastomosing foliation with dissolution, diffusion and precipitation of the intervening soluble grains (Fig. 1), operating at near-zero porosity and producing a microstructure resembling a mylonite without the operation of dislocation creep[13,14,16] This model and the experimental results are characterized by a normal stress-dependent frictional strength that is strongly rate-dependent (Fig. 1). The operation of FVF was included in the model of Den Hartog & Spiers[17], based on experimental results under hydrothermal conditions on phyllosilicate-rich (65 wt%) quartz-gouge mixtures, but the dramatic weakening observed in the analogue experiments was not observed, probably due to the high sliding velocities employed (1–100 μm/s). I discuss the implications of these results on long-term fault strength and earthquake rupture propagation into creeping segments of large scale fault zones
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