DISPROPORTIONATE WEAKENING OF SERPENTINITE FAULT GOUGE BY TALC AND BRUCITE

Serpentinized ultramafic rocks are associated with faults in plate-tectonic settings that include subduction zone thrust faults, oceanic transform and detachment faults, and some continental plate-boundary transform faults such as the San Andreas. Knowledge of the mechanical behavior of serpentinite fault gouge is important in understanding these fault systems. The low-T serpentine mineral lizardite and the high-T variety antigorite have coefficients of friction µ = 0.5-0.65 in the T range 25°-400°C. Talc and brucite are common components of serpentinite, and both minerals are weak: µ = 0.2-0.3 for brucite and µ = 0.1-0.2 for talc between 25° and 400°C. We are conducting frictional strength experiments to determine the effect of the weaker minerals on the strength of serpentinite fault gouge. These “mixing law” experiments use gouge samples prepared by mixing different proportions by weight of talc or brucite with lizardite- or antigorite-serpentinite. Experiments were run in the temperature range 25°–300°C, at constant effective normal stress of 100 MPa and pore pressure of deionized water of 30-50 MPa. The weaker minerals affect frictional strength by amounts that substantially exceed their abundance in the gouge. For example, 25 wt% of talc or brucite lowers the shear strength of serpentinite gouge by approximately 75% of the difference in strength between the end-member materials. As little as 5% of the weaker mineral reduces µ by at least 0.1, in contrast to data reported for mixtures of clay and quartz in which clay concentrations <15-20 wt% have little effect on gouge strength. The difference in behavior may be a reflection of gouge textures. In mixtures containing subspherical grains such as quartz, minor amounts of clay minerals may be preferentially sequestered in the pore spaces between grains and not involved in shear. Lizardite, antigorite, talc, and brucite have platy habits, and shear of serpentinite gouge is localized to shear planes along which the platy grains have rotated into alignment with the shear surfaces. These shears may develop so as to maximize the proportion of weaker minerals within them. If small amounts of these minerals can have a large effect on the frictional behavior of serpentinite gouge, it might help to explain the mechanics of fault creep and the apparently low coefficients of friction inferred for some major faults such as the San Andreas.