Strengths of serpentinite gouges at elevated temperatures

Abstract

Abstract. Serpentinite has been proposed as a cause of both low strength and aseismic creep of fault zones. To test these hypotheses, we have measured the strength of chrysotile-, lizardite-, and antigorite-rich serpentinite gouges under hydrothermal conditions, with emphasis on chrysotile, which has thus far received little attention. At 25øC, the coefficient of friction, g, of chrysotile gouge is roughly 0.2, whereas the lizardite-and antigorite-rich gouges are at least twice as strong. The very low room temperature strength of chrysotile is a consequence of its unusually high adsorbed water content. When the adsorbed water is removed, chrysotile is as strong as pure antigorite gouge at room temperature. Heating to -200øC causes the frictional strengths of all three gouges to increase. Limited data suggest that different polytypes of a given serpentine mineral have similar strengths; thus deformation-induced changes in polytype should not affect fault strength. At 25øC, the chrysotile gouge has a transition from velocity strengthening at low velocities to velocity weakening at high velocities, consistent with previous studies. At temperatures up to -200øC, however, chrysotile strength is essentially independent of velocity at low velocities. Overall, chrysotile has a restricted range of velocity-strengthening behavior that migrates to higher velocities with increasing temperature. Less information on velocity dependence is available for the lizardite and antigorite gouges, but their behavior is consistent with that outlined for chrysotile. The marked changes in velocity dependence and strength of chrysotile with heating underscore the hazards of using room temperature data to predict fault behavior at depth. The velocity behavior at elevated temperatures does not rule out serpentinite as a cause of aseismic slip, but in the presence of a hydrostatic fluid pressure gradient, all varieties of serpentine are too strong to explain the apparent weakness of faults such as the San Andreas.

The effect of mineral bond strength and adsorbed water on fault gouge frictional strength

“…Again focusing on the weakest gouges, Morrow et al, (1992) found that montmorillonite was strengthened by around 50% when interlayer water was squeezed out under several hundred MPa pressure at room temperature. Moore et al (1997) found that when the serpentine variety chrysotile was sheared at increasingly higher temperatures and pressures simulating depths of up to 9 km, adsorbed water was removed and the anomalously low frictional strength increased to levels more comparable to the antigorite and lizardite serpentine varieties.…”

“…Adsorbed water, resulting from the attraction of the water dipole to electrically charged mineral surfaces, can also substantially reduce frictional strength of gouge materials by providing a low-resistance slip interface, much like the interlayer water of montmorillonite. For instance, Moore et al (1997)found that the serpentine variety chrysotile, which readily adsorbs water onto its surface, also had a coefficient of friction of 0.2 and was only half as strong as the varieties lizardite and antigorite at room temperature. Israelachvili et al (1988) separated by a thin film of water, the first few layers of water are organized at an atomic level and the shear strength is inversely related to the number of water layers present.…”

Experimental and seismological constraints on the rheology, evolution, and alteration of the lithosphere at oceanic spreading centers

“…The influx of volatiles into mantle peridotite along faults and subsequent diffuse flow of the fluid into the rock along thermal fractures at low temperatures results in serpentinization of mantle peridotite [O'Hanley, 1996]. Both the frictional strength [Moore et al, 1997] and the fracture strength [Escartin et al, 1997a] of serpentinized mantle peridotite are less than that of unaltered peridotite. Thus, the serpentinization process probably has a substantial influence on the tectonic evolution of the oceanic lithosphere [Escartin et al, 1997b].…”

“…Fluids 163 present in the mantle alter ultramafic rocks to serpentine, which can strongly influence the strength of the oceanic lithosphere [e. g., Escartin et al, 1997a;Moore et al, 1997].…”

Scientific drilling into the San Andreas fault and site characterization research: Planning and coordination efforts. Final technical report

“…In contrast, experiments on natural clay-rich fault gouges collected from the San Andreas at depths of less than 0.4 km (Morrow et al, 1982), on synthetic clay-rich fault gouges (Morrow et al, 1992) and on synthetic serpentinite gouges (Moore et al, 1997) at high temperatures and/or confining pressures and hydrostatic fluid pressures indicate coefficients of friction at in-situ conditions that are too high to be reconciled with either the heat-flow or directional constraints. In addition, both natural and synthetic fault gouges deformed in the laboratory generally fail to exhibit the slip-weakening or velocityweakening behavior required for the generation of earthquakes (e.g., Byerlee and Summers, 1976;Logan and Rauenzahn, 1987;Marone et al, 1990;Morrow et al, 1992;Reinen et al, 1994).…”