Slip behavior of simulated gouge‐bearing faults under conditions favoring pressure solution
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Abstract:
Geophysical observations as well as deformation experiments indicate that under hydrothermal conditions, crustal faults can be significantly weakened with respect to conventional brittle‐plastic strength envelopes. Pressure solution has long been proposed as a mechanism leading to fault weakness. However, pressure solution has also been proposed as contributing to interseismic fault healing, and the competition between the weakening and healing effects of pressure solution is unclear. To investigate this issue, we have conducted rotary shear experiments on synthetic faults containing granular halite (NaCl) gouge using NaCl‐saturated mixtures of water and methanol as pore fluid. The NaCl‐water‐methanol system was chosen as a rock analogue because pressure solution is known to be important in this system at ambient conditions. We explored the influence of varying pore fluid composition (hence pressure solution rate), gouge grain size, and wall rock surface roughness, as well as normal stress and sliding velocity on slip behavior. All experiments were done under drained conditions. An acoustic emission detection system allowed detection of brittle events in the gouge. The results show no evidence for steady state pressure solution‐controlled fault slip. Frictional, rate‐insensitive behavior was observed, whereas the microstructures and compaction behavior clearly demonstrated that pressure solution was active in the gouge. Our data show that fluid‐assisted healing effects dominated over weakening, causing fault strength to be controlled mainly by brittle‐frictional processes. Existing models describing pressure solution‐controlled fault creep may not be applicable to a porous gouge undergoing compaction as well as slip.Keywords:
Fault gouge
Brittleness
Pressure solution
Overburden pressure
Halite
Cataclastic rock
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 velocity‐dependent. At high sliding velocities (>1 μ m s −1 ), 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.
Cataclastic rock
Fault gouge
Halite
Muscovite
Pressure solution
Brittleness
Overburden pressure
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Abstract Carbonate faults commonly contain small amounts of phyllosilicate in their slip zones, due to pressure solution and/or clay smear. To assess the effect of phyllosilicate content on earthquake propagation in carbonate faults, friction experiments were performed at 1.3 m/s on end‐members and mixtures of calcite, illite‐smectite, and smectite gouge. Experiments were performed at 9 MPa normal load, under room humidity and water‐saturated conditions. All dry gouges show initial friction values ( μ i ) of 0.51–0.58, followed by slip hardening to peak values of 0.61–0.76. Slip weakening then ensues, with friction decreasing to steady state values ( μ ss ) of 0.19–0.33 within 0.17–0.58 m of slip. Contrastingly, wet gouges containing 10–50 wt % phyllosilicate exhibit μ i values between 0.07 and 0.52 followed by negligible or no slip hardening; rather, steady state sliding ( μ ss ≪ 0.2) is attained almost immediately. Microstructurally, dry gouges show intense cataclasis and wear within localized principal slip zones, plus evidence for thermal decomposition of calcite. Wet gouges exhibit distributed deformation, less intense cataclasis, and no evidence of thermal decomposition. It is proposed that in wet gouges, slip is distributed across a network of weak phyllosilicate formed during axial loading compaction prior to shear. This explains the (1) subdued cataclasis and associated lack of slip hardening, (2) distributed nature of deformation, and (3) lack of evidence for thermal decomposition, due to low friction and lack of slip localization. These findings imply that just 10% phyllosilicate in the slip zone of fluid‐saturated carbonate faults can (1) dramatically change their frictional behavior, facilitating rupture propagation to the surface, and (2) significantly lower frictional heating, preventing development of microscale seismic markers.
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Fault gouge
Hardening (computing)
Pressure solution
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Halite
Cataclastic rock
Pressure solution
Fault gouge
Mylonite
Muscovite
Brittleness
Overburden pressure
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Halite
Fault gouge
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Cataclastic rock
Overburden pressure
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A seismically active low-angle normal fault is recognized at depth in the Northern Apennines, Italy, where recent exhumation has also exposed ancient examples at the surface, notably the Zuccale fault on Elba. Field-based and microstructural studies of the Zuccale fault reveal that an initial phase of pervasive cataclasis increased fault zone permeability, promoting influx of CO 2 -rich hydrous fluids. This triggered low-grade alteration and the onset of stress-induced dissolution–precipitation processes (e.g. pressure solution) as the dominant grain-scale deformation process in the pre-existing cataclasites leading to shear localization and the formation of a narrow foliated fault core dominated by fine-grained hydrous mineral phases. These rocks exhibit ductile deformation textures very similar to those formed during pressure-solution-accommodated ‘frictional–viscous’ creep in experimental fault rock analogues. The presence of multiple hydrofracture sets also points to the local attainment of fluid overpressures following development of the foliated fault core, which significantly enhanced the sealing capacity of the fault zone. A slip model for low-angle normal faults in the Apennines is proposed in which aseismic frictional–viscous creep occurs on a weak, slow-moving (slip rate <1 mm a −1 ) fault, interspersed with small seismic ruptures caused by cyclic hydrofracturing events. Our findings are potentially applicable to other examples of low-angle normal faults in many tectonic settings.
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Fault gouge
Pressure solution
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Two recent experimental studies on the frictional behavior of synthetic gouge-bearing faults under the operation of pressure solution are compared. One is triaxial shear experiments on quartz gouge at high pressure-temperature hydrothermal conditions (Kanagawa et al., 2000), and the other is rotary shear experiments on halite gouge at atmospheric pressure and room temperature in the presence of methanol-water mixtures (Bos et al., 2000). In spite of quite different experimental settings and conditions, the results of these two series of experiments are strikingly similar; both cataclasis and pressure solution being active during the experiments, gouge strength rate-controlled by cataclasis, two different frictional behaviors of slip hardening and softening, slip hardening associated with gouge compaction, distributed deformation and wall-rock failure, slip softening associated with localized slip along the gouge-wall-rock interface, and the transition from slip-hardening to slip-softening behavior according to decreasing rate of pressure solution. Although there is a difference in velocity dependence of strength between quartz and halite gouges, these similarities clearly demonstrate the important effects of pressure solution on the frictional behavior of gouge-bearing faults.
Fault gouge
Cataclastic rock
Halite
Hardening (computing)
Pressure solution
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Cataclastic rock
Fault gouge
Pressure solution
Overburden pressure
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Cataclastic rock
Fault gouge
Pressure solution
Deformation bands
Microscale chemistry
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