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_Journal of Structural Geology 32 (2010) 1685–1700_ _Contents lists available at ScienceDirect_ _Journal of Structural Geology_ _journal homepage: www.elsevier.com locate jsg_ _High shear strain behaviour of synthetic muscovite fault gouges under hydrothermal conditions_ _Esther W.E. Van Diggelen*, Johannes H.P. De Bresser, Colin J. Peach, Christopher J. Spiers_ _Faculty of Geosciences, Utrecht University, P.O. Box 80.021, 3508 TA Utrecht, Netherlands_ _article info_ _Article history: Received 3 April 2009 Received in revised form 24 July 2009 Accepted 25 August 2009 Available online 16 September 2009_ _Keywords: Frictional behaviour Muscovite Simulated fault gouge Rotary shear experiments Hydrothermal conditions_ _abstract_ _Major continental fault zones typically contain phyllosilicates and have long been recognised as zones of persistent weakness. To establish whether the presence of micas can explain this weakness, we studied the frictional behaviour of simulated muscovite fault gouge by performing rotary shear experiments in the temperature range 20–700°C, under constant effective normal stresses of 20–100 MPa, a fixed fluid pressure of 100 MPa and at sliding velocities of 0.03–3.7 mm s−1, reaching shear strains up to 100. Cataclasis causes substantial grain size reduction up to 600°C. With increasing strain, both pervasive and localized cataclasis and related compaction result in strain hardening until steady state is reached. This reflects the progressive development of a continuous network of fine-grained, hardening bands. Coarse grained relict lenses between these bands show folded and kinked muscovite grains indicative of ductile mechanisms. Samples deformed at 700°C show evidence for chemical alteration and partial melting._ _Since our data suggest that muscovite gouge strengthens with depth and strain, it is questionable whether its presence can contribute to the long-term weakness of major crustal fault zones, unless a substantial decrease in strength occurs at shear strain rates lower than attained in our study._ _1. Introduction_ _Major continental fault zones have long been recognised by field geologists as zones of highly localized deformation and hence of persistent weakness (Balfour et al., 2005; Holdsworth et al., 2001; Imber et al., 1997; Jefferies et al., 2006b; Rutter et al., 2001; White et al., 1986; Zoback et al., 2007; Zoback et al., 1987). The implication is that the quartz-, feldspar-, and phyllosilicate-rich fault rocks often found within such faults must be weaker than the surrounding country rock. Laboratory rock friction experiments have confirmed that typical continental fault rocks are indeed weak compared to intact quartzo-feldspathic host rocks (Bos et al., 2000; Dieterich, 1978; Hickman, 1991; Holdsworth, 2004; Lachenbruch and Sass, 1980; Logan and Rauenzahn, 1987; Morrow et al., 2000; Morrow et al., 1992; Niemeijer and Spiers, 2005, 2006; Shimamoto and Logan, 1981; Takahashi et al., 2007)._ _Lab data for long-term fault strength are usually expressed in terms of Byerlee’s Rule for fault friction (friction coefficient m ? 0.6–0.9) giving way to plastic flow at depths of 10–15 km, thus producing the classical ‘‘Christmas Tree’’ strength profile (Bos and Spiers, 2002; Byerlee, 1978; Goetze and Evans, 1979; Holdsworth et al., 2001). Such profiles fit well with the depth distribution of seismicity on major faults (Scholz, 2002; Sibson, 1983). However, laboratory-based strength profiles are not fully consistent with other geophysical observations on major mature fault zones, like the San Andreas fault zone. In the case of the San Andreas fault, the lack of a positive heat flow anomaly and high angles (w70°) measured between the in-situ principal stress s1 and the fault surface (Townend and Zoback, 2004; Zoback et al., 2007), imply a low mean resolved shear stress on the fault of around 10–20 MPa (Lachenbruch and Sass, 1980; Zoback et al., 1987) and a coefficient of friction of only w0.2. This is far less than measured for (simulated) fault rocks in laboratory experiments (Blanpied et al., 1995; Bos and Spiers, 2000; Byerlee, 1978; Carpenter et al., 2009; Moore and Lockner, 2004; Morrow et al., 2000; Morrow et al., 1992; Nakatani and Scholz, 2004; Niemeijer and Spiers, 2005; Tembe et al., 2006a; Tembe et al., 2006b)._ _A widely proposed explanation for the inferred long-term weakness of major crustal faults is that high pore fluid pressure reduces the effective shear stress required for slip. Fluids released during compaction and or dehydration reactions at depth might locally increase fluid pressures to approach lithostatic values (Byerlee, 1990; Collettini and Barchi, 2002; Faulkner and Rutter, 2001; Hickman et al., 1995; Miller et al., 1996; Miller and Olgaard, 1997; Sibson, 2004; Sleep, 1995). However, fluid pressures inside a fault can only increase if the zone has a low permeability caused by ubiquitous phyllosilicates or continuous wall rock cementation (Schleicher et al., 2008; Schleicher et al., 2009b; Zhang et al., 2001). Faulkner and Rutter (2001) showed that dehydration of phyllosilicates (assuming 50% phyllosilicate content at 15–20 km depth) is sufficient to maintain an elevated fluid pressure for w12 ky, but insufficient to weaken a large fault on geologic timescales. Moreover, in-situ measurements in fault zones show little evidence for continuously high fluid pressures (Wintsch et al., 1995). In particular, fluid pressure measurements in the SAFOD (San Andreas Fault Observatory at Depth) drill hole indicate near-hydrostatic rather than lithostatic fluid pressures at depths up to 3.5 km (Tembe et al., 2006b; Zoback et al., 2007). It thus seems unlikely that high fluid pressures alone can account for the apparent long-term weakness of large scale fault zones._ _Alternatively, the presence of relatively weak reaction product minerals, such as clays, micas, chlorite or talc may explain the inferred weakness of major faults (Arancibia and Morata, 2005; Jefferies et al., 2006a; Logan and Rauenzahn, 1987; O’Hara, 2007; Shea and Kronenberg, 1992; Wibberley, 1999; Wintsch et al., 1995). Experiments have shown that many such phyllosilicates are significantly weaker than quartz and feldspars (Ikari et al., 2007; Logan and Rauenzahn, 1987; Mariani et al., 2006; Moore and Lockner, 2004; Moore and Lockner, 2007; Moore et al., 1997; Morrow et al., 2000; Morrow et al., 1992; Shimamoto and Logan, 1981) and are stable up to 15 km depth. It follows that a continuous, through-going foliation of phyllosilicate minerals may strongly influence fault zone rheology on a crustal scale (Holdsworth et al., 2001; Imber et al., 1997; Ranalli, 1995; Rutter et al., 2001; Rutter et al., 1986). Reaction-softening of this type has frequently been proposed as an explanation for the weakness of the San Andreas fault zone (Evans and Chester, 1995; Moore and Rymer, 2007; Wintsch et al., 1995). Recently, core material retrieved during SAFOD phase 3 drilling has shown that the actively deforming strands of the fault contain a grayish-black, foliated, phyllosilicate-rich fault gouge. Visible clasts make up about 5% of the rock volume and consist of fragments of serpentinite, sandstone and siltstone protolith (SAFOD core atlas, 2007). These observations confirm that phyllosilicate foliation development and reaction weakening are key processes in determining fault zone rheology._ _Additionally, fluid-assisted deformation processes, such as pressure solution, have been put forward as a mechanism for weakening of fault rocks (Blanpied et al., 1995; Chester, 1995; Collettini and Holdsworth, 2004; Hickman et al., 1995; Jefferies et al., 2006b; Kanagawa, 2002; Kanagawa et al., 2000; Lehner and Bataille, 1984; Schleicher et al., 2009a; Wu et al., 1975). Experiments on wet quartz gouge under hydrothermal conditions (Kanagawa et al., 2000; Niemeijer et al., 2002), and on wet granular halite used as an analogue (Bos et al., 2000), show competition between pressure solution, compaction and cataclasis, leading to high frictional strength (m ? 0.8–0.9) at low slip rates. However, experiments on simulated gouges consisting of halite plus kaolinite, or of halite plus muscovite, show a reduction in m-values to 0.3–0.4 (Bos and Spiers, 2000, 2001; Niemeijer and Spiers, 2005). This is due to the development of a through-going phyllosilicate foliation on which frictional slip occurs with accommodation by pressure solution of the intervening halite clasts. Bos and Spiers (2002) and Niemeijer and Spiers (2005, 2007) developed a microphysical model explaining their experimental results and predicting upper crustal strength–depth profiles for foliated quartz-mica fault rock some 2–5 times weaker than obtained using Byerlee’s Rule. The low strength predicted is due to the serial effect of easy pressure solution of quartz clasts at low sliding rates and the low frictional strength (m ? 0.3) assumed for the enveloping phyllosilicate foliation._ _In the experimental study of Mariani et al. (2006), pure muscovite fault gouge shows strain rate insensitive strain hardening behaviour at shear strain rates faster than 1.4 × 10−5 s−1 at 700°C and at all strain rates tested (10−7 to 10−3 s−1) at temperatures of 400–600°C. This behaviour is primarily attributed to mutual misalignment of mica flakes with contributions from progressive porosity reduction and formation of oblique shear features (Mariani et al., 2006). At 700°C and strain rates lower than 1.4 × 10−5 s−1, the shear strength drops dramatically. Deformation of the muscovite gouge under these conditions has linear viscous characteristics, possibly due to viscous flow._ Ключевые слова: fault, fault zone, mica, sheet surface, experimental evidence, internal piston, steady, deionised water, coarser muscovite, min, band, chemical composition, sample deformed, grain, friction, sample, water, kanagawa, journal structural geology, frictional behaviour, grain size, compaction, early stage, development, showing, holdsworth, reaction weakening, earth, geological society, data, condition, slip, frictional strength, shear strength, miller, material, shear plane, non-stepping experiment, high temperature, deformed, friction coef?cient, ?uid pressure, good agreement, andreas, san, mm, chemical alteration, hydrothermal condition, sample compacted, phase, deformation, reaction chatterjee, sem image, coef?cient, letters, niemeijer spiers, piston surface, hickman, stepping, piston, frictional behavior, constant, rock, strain rate, rst portion, increasing strain, velocity dependence, core, bos, van diggelen, safod, normal, shear direction, band anastomose, sliding velocity, mpa, velocity-stepping experiment, long-term weakness, adsorbed water, rate, mineral surface, wear, kronenberg, tectonophysics, velocity-stepping, lens, muscovite grain, apparent compaction, abstract, ?ow, mariani, velocity, san andreas, foliation, friction coefcient, deformation mechanism, result, spiers, townend, systematic relationship, measured externally, coefcient, structural, diagram illustrating, shear displacement, normal stress, sibson, structural geology, grained, japan journal, plastic deformation, deformed sample, viscous glide, experimental deformation, shear strain, pressure, friction coefcients, shear band, low-crustal condition, roughly horizontal, partial melting, furnace element, surface temperature, microstructure, elsevier, byerlees rule, uid pressure, fault gouge, mineral, sliding, chester, compaction behaviour, ring, collettini, niemeijer, zoback, frictional sliding, reaction, tectonic signicance, chemical breakdown, effective, doi, journal, forcing block, gabbro gouge, gouge, pressure solution, moore, ?ne, experimental, geology, geothermal gradient, shear, remaining, hydrothermal, rutter, reached, surface, water lms, quartz, effective normal, data presented, melting temperature, localized cataclasis, lockner, journal geophysical, study, elevated temperature, scholz, journal structural, velocity-stepping test, observed, size, stick-slip behaviour, median tectonic, experiment, continuous network, room temperature, increase, science, high, behaviour, strength, geophysical, basal dislocation, pore uid, agglomerated grain, ?uid, seff mpa, conning ring, piston-sample assembly, temperature, dislocation glide, shear stress, byerlee, oblique foliation, tullis, london, performed, starting material, zone, hardening, measured, shimamoto, stress, displacement, seff, strain, wa, applied, mechanical behaviour, rate-dependent behaviour, quartz porphyroclasts, evans, mechanical, diggelen, muscovite, reaction product, frictional, experimental study, shear experiment, presence, van