The roles of quartz and water in controlling unstable slip in phyllosilicate-rich megathrust fault gouges

Abstract

A recent microphysical model for the steady-state frictional behaviour of wet illite/quartz gouges in subduction megathrust settings predicts that velocity-weakening in the seismogenic zone results from a competition between shear-induced dilatation and compaction involving water-assisted, thermally activated deformation (pressure solution) of quartz clasts. While this model is supported by experimental data, proof that quartz and water are a requirement for velocity-weakening is lacking. Here, we report on shearing experiments on water-saturated (near-)pure illite and dry 65/35 illite/quartz gouges, deformed at P-T conditions near those in situ at seismogenic depths along subduction megathrusts. We used low sliding velocities relevant to earthquake nucleation and slow slip events (1 to 100 μm/s). Previous experiments on wet illite/quartz gouges under the same conditions documented three regimes of slip stability, with velocity-strengthening at 150°C to 250°C and 400°C to 500°C, and velocity-weakening at 250°C to 400°C. In the present study, wet illite gouge exhibited similar three-regime behaviour, but with velocity-neutral rather than velocity-weakening behaviour at the intermediate temperatures. Dry illite/quartz gouge exhibited near velocity-neutral behaviour at all temperatures investigated. These results confirm that water-assisted, thermally activated quartz deformation is a key process in the velocity-weakening behaviour at intermediate temperatures in wet illite/quartz gouges and support the existing microphysical model. The implication of this model is that seismogenesis occurs under conditions where creep by thermally activated quartz deformation is fast enough to moderate 'brittle' dilatation to remain at subcritical porosity values but too slow to allow ductile shear of clasts.

Chemo‐mechanical behavior of clay‐rich fault gouges affected by CO2‐brine‐rock interactions

“…Typically competent minerals show friction coefficients of ∼0.6–0.85 and phyllosilicates ∼0.2–0.5, and our modeled polymineralic fault gouges fit in that trend (see Fig. B–C). However, such a trend is not visible for ( a – b ), as the latter depends on many factors, including mineralogy (Fig.…”

“…Intact natural materials and individual minerals were crushed using a mortar and pestle, and sieved to a grain size of <35 μm. For selected compositions (Table ), powders of quartz (99.9% pure; Beaujean quartz sand), kaolinite (100% pure; Merkx®), muscovite (90% muscovite plus ∼10% quartz impurities for which no correction was made; commercially obtained), calcite (Iceland spar), dolomite (97% dolomite plus ∼3% labradorite; Forno–Apuan Alps), and illite‐rich shale (Rochester shale, 65% illite plus 35% quartz impurities) were mixed by weight and thoroughly stirred to ensure a homogeneous mixture. Siderite, Ca‐nontronite, and paragonite were difficult to obtain; analogue minerals – respectively dolomite, Ca‐montmorillonite, and muscovite – were used instead.…”

“…, c Giorgetti et al. , d Moore and Lockner, e den Hartog et al ., f Bakker, g Van Diggelen et al. , h den Hartog et al.…”

“…, c Giorgetti et al. , d Moore and Lockner, e den Hartog et al ., f Bakker, g Van Diggelen et al ., h den Hartog et al ., i Lu and He, j Verberne et al ., k Carpenter et al ., l Pluymakers et al. , m Scuderi et al …”

Frictional behavior of talc‐calcite mixtures

“…Adding talc, they observed an increase in ( a − b ) in correspondence to the steeper portion of the friction degradation curve (Figure b). As observed also in other experimental studies on phyllosilicate‐rich gouges [ den Hartog et al ., ], an increase in the amount of quartz shifts the ( a − b ) parameter toward more negative values, promoting velocity‐weakening behavior. However, the velocity dependence does not increase monotonically with increasing phyllosilicate content.…”