5.1 Weakening Mechanisms of Serpentinite Faulted Against Quartzofeldspathic Rocks
 Our experiments demonstrate that antigorite- and lizardite-rich serpentinite gouges are dramatically weakened (by as much as 40%) at hydrothermal conditions (T ≥ 200°C), when sheared against quartz-bearing rocks compared to ultramafic rocks. Measurable weakening occurred within ~2 days at 200°C, the lowest temperature tested in this study, suggesting that the weakening mechanism may be effective at even lower temperatures over longer time intervals. In a quartzofeldspathic environment, serpentinite gouge strength decreases markedly with increasing temperature, the opposite of its behavior in an ultramafic system. In addition, only velocity-strengthening behavior was observed, and the measured values of ∆μss/∆lnV are highest at the highest temperature and slowest velocity tested. The frictional strengths of antigorite, lizardite, and chrysotile gouges all reach similar values during shear between granite blocks (Figure 4). Magnesium-rich phyllosilicates crystallized during the experiments, the identity of the minerals changing with time and with wall-rock chemistry. The critical role of a fluid phase is illustrated by tests on oven-dried samples that yielded μ ~ 0.75 for antigorite serpentinite independent of wall-rock chemistry at 250–300°C [Moore and Lockner, 2007; Moore et al., 2010].
 The operation of a fluid-activated, temperature- and rate-dependent weakening mechanism in the serpentinite in these experiments is consistent with a solution-transfer creep process [e.g., Rutter and Mainprice, 1978, 1979; Chester and Higgs, 1992; Chester, 1995; Blanpied et al., 1995; Bos and Spiers, 2001]. The process involves mineral dissolution at one site, followed by diffusion of the dissolved material to a new location where mineral precipitation occurs. Both cataclastic (i.e., grain sliding, rolling, and crushing) and solution-transfer processes are operative to varying degrees in rocks throughout the upper crust. The hydrothermal experiments and modeling of Chester and Higgs  and Chester  on quartz and Blanpied et al.  on granite indicate that cataclasis is the dominant deformation mechanism in quartzose crustal rocks at temperatures below 300–350°C, whereas both solution-precipitation creep and cataclasis are important at higher temperatures (mid-crustal depths). This transition in frictional behavior marks the base of the seismogenic zone in continental crust. The effect of the quartz-bearing wall rocks on the frictional behavior of serpentinite is such that the aseismic slip and weakening become important at T-P conditions that normally would correspond to depths within the seismogenic zone.
 We propose that modifications to the chemistry of the pore fluids resulting from interaction with the granite and quartzite wall rocks significantly enhance the solubility and/or the rate of dissolution of serpentine compared to a purely ultramafic chemical system, thereby promoting the solution-transfer process. Because of the lack of information about fluid chemistry in our samples, we use the chemistry of natural groundwaters to speculate on possible causes. Barnes et al.  analyzed the waters issuing from surface springs associated with ultramafic bodies in California and found that water chemistry varied with the direction of flow of the fluids relative to the ultramafic rock. Fluids sampled from the interiors of large ultramafic bodies are “ultrabasic waters” (pH = 11.8–12.1) whose thermodynamic properties are entirely controlled by reaction with the ultramafic host rocks, even though their isotopic signatures indicate diverse origins from meteoric or connate sources. These waters also have low dissolved silica contents (<25 mg/L SiO2), and they are highly undersaturated with respect to quartz [see also Barnes and O'Neil, 1969]. Groundwaters issuing from springs sited along the contacts between antigorite serpentinite and metadacite bodies are somewhat less basic, with pH = 10.8–10.9, and they are supersaturated with respect to quartz, having dissolved silica contents as high as 4000 mg/L SiO2. Barnes et al.  concluded that these fluids must have flowed along the contact and interacted with the rocks on both sides.
 Dissolution experiments conducted on lizardite [Luce et al., 1972], antigorite [Lin and Clemency, 1981], and chrysotile [Bales and Morgan, 1985] all demonstrate that both the solubility and rate of dissolution of serpentine, and especially the rate of release of Mg to solution, increase with decreasing pH. Bales and Morgan  suggested that dissolution at higher-energy surface sites, in particular, may be pH dependent. The two groups of groundwaters analyzed by Barnes et al.  differ by only 1 pH unit; nevertheless, the lower pH of the waters in contact with both metadacite and serpentinite will favor at least some increase in serpentine solubility, even if it is not the sole factor. Dissolution of quartz combined with ionization of dissolved silica species in basic pore fluids [Krauskopf, 1967]
will maintain pH at a lower value than in a quartz-absent chemical system.
 The modifications to fluid chemistry that markedly enhance dissolution of the serpentine minerals are just one aspect of the chemical processes operative during the experiments, whose effects also include changes in the Mg-rich minerals that precipitate over time. Serpentine minerals crystallized during experiments of ≤6 days duration. The two longest experiments (>10 days) showed incipient crystallization of new Mg-rich phyllosilicates, whose compositions varied with the mineralogy of the wall rocks.
 The reaction zones characteristically developed at the contacts between ultramafic and crustal rocks have long been studied as classic examples of metasomatism [e.g., Jahns, 1967; Sanford, 1982; Coleman, 1967; Mori et al., 2007]. Mass transfer across the contact is driven by chemical potential gradients between the rocks on either side. Frantz and Mao [1976, 1979] modeled the development of ultramafic reaction zones as a metasomatic solution-transfer process. Material transfer occurs via intergranular water films by one or both of two mechanisms: (1) diffusion through a stationary pore fluid and (2) convective transport (infiltration) due to motion of the pore fluid relative to the solid framework. A progressive mineral zonation develops on either side of the contact, as illustrated in the experimental investigation conducted by Koons . He placed antigorite serpentinite next to a quartzofeldspathic rock in gold tubes, filled them with water and sealed them, and then held them at 450°C and 200 MPa confining pressure for periods of 15–40 days. New minerals, including talc, tremolite, and chlorite, formed in specific zones at the ultramafic-schist contact, mimicking natural occurrences. He identified different stages of reaction over time—an initial stage of volatile metasomatism (e.g., CO2), followed by migration of SiO2 and subsequently other non-volatile elements. The studied natural reaction zones typically are on the scale of meters, and many meter-scale pods of one rock type embedded in the other have lost all traces of the original mineral assemblage [e.g., Coleman, 1961; Sanford, 1982].
 The textural and mineralogical changes that occurred over time in the shearing experiments are readily explained in terms of such diffusive mass-transfer processes. Reprecipitation of serpentine minerals in the gouge dominated in the short-term experiments, whereas different Mg-rich phyllosilicates crystallized during the 11 day quartzite experiment (talc + smectite (stevensite?)) and the 15 day granite experiment (saponite). Saponitic clays of similar chemistry grew in the serpentinite gouge and on the surface of the granite wall rocks, indicating migration of Si, Al, Na, and K (provided by dissolution of quartz and feldspars; Figures 10d and 10e) into the gouge and of Mg (released by dissolution of serpentine) toward the wall rocks. It should be noted that the smectite clays in the 250°C experiments are likely metastable minerals that crystallized at temperatures above their normal thermal stability range due to favorable growth kinetics. Observed mineralogical changes in the gouge layers were limited to shears located close to the contact with the driving blocks, putting the scale of diffusion at tens of micrometers for those experiments.
 The Mg-smectite clay saponite is extremely weak, μ ~ 0.05 at room temperature and 100 MPa effective normal stress [Lockner et al., 2011]. Talc is similarly weak at temperatures in the approximate range 200–300°C [Moore and Lockner, 2008]. The presence of small amounts of these neocrystallized minerals did not noticeably affect strength during the two long experiments run largely at 0.001 µm/s (Figure 7). Eventually, however, sufficient amounts of these weak minerals would crystallize such that they would influence the frictional behavior of the gouge during an experiment.
5.2 Application to Fault Zones
5.2.1 Faults Juxtaposing Serpentinite and Crustal Rocks
 Our results show that, in the absence of geometric or other impediments, any active crustal fault that shears serpentinite against quartzofeldspathic rocks has the potential for aseismic slip (creep) at seismogenic depths. In particular, these results offer a possible explanation for the long-noted geographic association of serpentinite with creeping faults of the San Andreas system in central and northern California [e.g., Allen, 1968; Hanna et al., 1972; Irwin and Barnes, 1975; Irwin, 1990; Titus et al., 2011]. This inferred correlation of serpentinite with fault creep needs to be tested in considerably more detail to assess its validity. As a result of long-term monitoring efforts [e.g., Galehouse and Lienkaemper, 2003; McFarland et al., 2009], the occurrence and rates of creep have been documented along several faults in the San Francisco Bay area. Detailed field mapping and geophysical investigations are now needed to determine whether or not the occurrence and distribution of serpentinite at depth can be correlated to the patterns of creep along those faults.
 Core recovered as part of the San Andreas Fault Observatory at Depth (SAFOD) deep drilling program [e.g., Zoback et al., 2011] does link serpentinite with creep along the central San Andreas fault. The SAFOD drill hole, located 14 km northwest of Parkfield where the measured creep rate is 25 mm/yr [Titus et al., 2006], crossed the active trace of the San Andreas fault at –2.7 km depth (~112°C). Two creeping strands were identified based on deformation of the steel casing lining the main drill hole and both were successfully sampled during subsequent coring operations. In marked contrast to the quartzofeldspathic sedimentary rock units adjoining them, the creeping traces are 1.6 and 2.6 m wide zones of saponite ± corrensite-rich clayey gouge [Moore and Rymer, 2012] with porphyroclasts of serpentinite and sedimentary rock. The gouge of the main creeping strand contains ~27 wt% MgO, obtained by whole-rock XRF analysis [Bradbury et al., 2011]. At the surface, a narrow tectonic shear zone of serpentinite extends for several kilometers within the fault zone northwest of the drill site. Where visible in outcrop, the sheared contact of serpentinite with the sedimentary wall rocks is marked by saponite-rich gouge identical to that found at depth [Moore and Rymer, 2012]. Moore and Rymer  concluded that the surface outcrops of serpentinite connect with one or both of the creeping strands, which consist of more highly altered, sheared serpentinite. In effect, the tectonically entrained serpentinite at the SAFOD locality corresponds to the sample configuration used in this study (Figure 1b), and the gouges represent the experiment in Figure 10 run for a geologic time span at temperatures within the stability range of saponite. Based on the timing of major changes in the nature and orientations of off-fault geologic structures, Titus et al.  estimated that the fault creep initiated 2–2.5 Myr ago along the central San Andreas fault, and they linked the change to the tectonic incorporation of serpentinite into the fault.
 The forceful injection of serpentinite into overlying rocks, proposed to occur at SAFOD, is a common mode of emplacement of serpentinite [Lockwood, 1971, 1972]. Both active [e.g., 2010Ohlin et al., 2010] and inactive [e.g., Dickinson, 1966; Page et al., 1999] faults have provided pathways facilitating the upward migration of the serpentinite. Despite its common occurrence, the cause of the emplacement has been disputed. The relatively low density of serpentinite compared to that of the overlying rock units is typically cited as a factor promoting this process. However, Phipps  argued that the density contrast between serpentinite and typical crustal rocks is too small to be effective. Furthermore, based on their strength experiments on serpentinized ultramafic rock, Raleigh and Paterson  concluded that tectonic emplacement of serpentinite was unlikely to occur at temperatures below ~300°C, because of its high strength. Cowan and Mansfield  and Saleeby , among others, noted that in many cases tectonically emplaced serpentinites are intensively sheared along the margins, while the interiors have a more massive appearance. They inferred that the sheared-serpentinite margins somehow act as “lubricants,” facilitating low-temperature tectonic transport of the entire mass. Our results suggest that weakening of serpentinite along its contact with crustal rocks may be the source of that lubricating effect.
5.2.2 Faults Within Serpentinite
 The behavior of crustal faults within serpentinite bodies may vary, depending on the chemistry of the pore fluids in the fault zone. In one end-member case, the fault is an open system for fluid flow, and fluid chemistry largely reflects equilibration with an external, crustal rock reservoir. Retention of the crustal signature in groundwater chemistry will be favored by high groundwater flow rates and by passage through relatively small ultramafic masses. Such faults are likely to be characterized by weakening and aseismic slip. They would also provide the best environment for serpentine minerals rather than a metasomatic mineral assemblage to precipitate as the end product of a solution-transfer mechanism.
 Possible examples have been described by Andreani et al.  and Bellot . Andreani et al.  studied a shear zone developed within a kilometer-scale serpentinite lens along a segment of the Santa Ynez fault in California. They concluded that elongate chrysotile fibers in the serpentinite gouge within the shear formed by continuous syntectonic crystallization. Their model involved dissolution of serpentine minerals along R1 and Y subsidiary shears and crystallization of the chrysotile in the gouge in between the shears, resulting in aseismic slip. Bellot  examined a serpentinized ultramafic body, approximately 170 m × 250 m in horizontal dimensions, enclosed within amphibolites in a continental shear zone in the Maures Massif, France. He described crack-seal chrysotile veins that he considered to be the product of solution-transfer processes.
 As the size of the faulted ultramafic body increases, so does the potential for fluid chemistry within the fault to be controlled by the enclosing ultramafic rocks. Candidates would be the ultramafic bodies whose groundwaters, discharged from springs and seeps, have extremely high pH and ultrabasic chemistry [Barnes and O'Neil, 1969; Barnes et al., 1972]. In the end-member case, the frictional behavior of serpentinite documented in previous studies for ultramafic chemical systems [e.g., Reinen et al., 1991, 1992, 1994; Moore et al., 1997, 2004] should be applicable.