An experimental investigation into the role of phyllosilicate content on earthquake propagation during seismic slip in carbonate faults
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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.Keywords:
Cataclastic rock
Fault gouge
Hardening (computing)
Pressure solution
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 This paper describes the results of petrographical and meso‐ to microstructural observations of brittle fault rocks in cores obtained by drilling through the Nojima Fault at a drilling depth of 389.52 m. The zonation of deformation and alteration in the central zone of the fault is clearly seen in cores of granite from the hanging wall, in the following order: (i) host rock, which is characterized by some intragranular microcracks and in situ alteration of mafic minerals and feldspars; (ii) weakly deformed and altered rocks, which are characterized by transgranular cracks and the dissolution of mafic minerals, and by the precipitation of zeolites and iron hydroxide materials; (iii) random fabric fault breccia, which is characterized by fragmentation, by anastomosing networks of transgranular cracks, and by the precipitation of zeolites and iron hydroxide materials; and (iv) fault gouge, which is characterized by the precipitation of smectite and localized cataclastic flow. This zonation implies that the fault has been weakened gradually by fluid‐related fracturing over time. In the footwall, a gouge layer measuring only 15 mm thick is present just below the surface of the Nojima Fault. These observations are the basis for a model of fluid behavior along the Nojima Fault. The model invokes the percolation of meteoric fluids through cracks in the hanging wall fault zone during interseismic periods, resulting in chemical reactions in the fault gouge layer to form smectite. The low permeability clay‐rich gouge layer sealed the footwall. The fault gouge was brecciated during coseismic or postseismic periods, breaking the seal and allowing fluids to readily flow into the footwall, thus causing a slight alteration. Chemical reactions between fluids and the fault breccia and gouge generated new fault gouge, which resealed the footwall, resulting in a low fluid condition in the footwall during interseismic periods.
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Cataclastic rock
Breccia
Wall rock
Pressure solution
Brittleness
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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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Abstract Cataclastic rocks found in the Disaster Prevention Research Institute, Kyoto University (DPRI) 500 m drill core and outcrops along the Nojima Fault zone on Awaji Island, southwest Japan, were examined at mesoscopic and microscopic scales. The damaged zone of this fault in granitic rocks, observed on the southeast side of the fault, is 50–60 m wide and is composed of fractured host rocks and cataclastic rocks including cataclasite, fault breccia, and fault gouge. The fault breccia and gouge of small scales are scattered in the damaged zone. Fault core (zone of extremely concentrated shearing deformation along a fault) consists of fault gouge measuring several tens to approximately 150 mm in width, as recognized both in the drill core and at outcrops of the Nojima Fault along which surface ruptures formed during the 1995 Kobe earthquake. Fault breccia, measuring a few meters wide, has developed pervasively in the damaged zone, just next to the fault core. Pseudotachylyte has been found interlayered with fault gouge within the fault core only at outcrops at Hirabarashi, not in the DPRI 500 m core. Petrological studies and powder X‐ray diffraction analysis show that the pseudotachylyte and fault gouge are composed mainly of fine‐grained angular clasts of the host granitic rocks, suggesting the pseudotachylyte is of ‘crush origin’. Foliated cataclasite is characterized by the preferred orientation of elongated biotite clasts and granular aggregates of quartz and feldspar clasts, and by the development of cataclastic shear bands. Unlike cataclastically deformed quartz and feldspar in the cataclasite, biotite in the foliated cataclasite shows combinations of brittle and plastic deformation, such as biotite ‘fish’, cleavage steps, bending and kinking. These textures suggest that the foliated cataclasite formed at a deeper level than the cataclasite, fault breccia and gouge, possibly before the Quaternary period during which the Nojima Fault has moved as a dextral strike–slip fault with some reverse movement resulting in the uplifting of Awaji Island. Examination of fault rocks from surface outcrops can yield similar results to those obtained from drill cores with regard to the internal structures of a fault zone.
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Fault gouge
Breccia
Mylonite
Outcrop
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Halite
Cataclastic rock
Pressure solution
Fault gouge
Mylonite
Muscovite
Brittleness
Overburden pressure
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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.
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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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