The Effect of Hydrothermally Generated Talc upon Fault Strength

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Abstract

At present there is no theory which describes fully observations of weakness and anomalous slip behaviour on many faults. Recent field studies upon such faults indicate that reactions which generate frictionally weak phyllosilicate minerals, including talc, may be significant. A series of experiments was carried out on a deionised water fluid medium triaxial deformation apparatus to investigate the effect of the syntectonic generation of talc upon fault strength and slip characteristics, where talc is produced by the reaction: lizardite + quartz → talc + H2O. Experiments to investigate reaction kinetics were performed on lizardite and Brazilian quartz powder samples. Talc is generated by this reaction within 72 hours under hydrothermal conditions between 350°C and 500°C and effective pressures of 5 to 50 MPa. Microstructural study shows porous talc overgrowths surrounding lizardite and quartz grains suggesting an armouring effect with progressive reaction.Constant displacement rate tests and subsequent stress relaxation tests were performed upon cylindrical samples of lizardite and Hodge quartzite saw-cut at 35° juxtaposed across the shear zone. Some samples were assembled with pure talc or lizardite gouge between the forcing blocks. Tests were carried out under hydrothermal conditions: 450 °C, 50 to 150 MPa effective pressure and 4.64 x 10-4 mm s-1 shear surface displacement rate. Some samples were deformed at once to assess frictional properties of the starting materials. Other samples were held at hydrothermal conditions for 72 hours prior to deformation, to allow the blocks and gouge to react to talc. Continued reaction to talc was expected during stress relaxation. All samples displayed stable sliding behaviour, with little strain hardening. Friction coefficients averaged from varied effective pressure tests were largely in line with previous studies. A strength contrast is shown between talc gouge (average µ=0.11) and talc grown as a thin veneer between lizardite and quartz forcing blocks, (average µ=0.22) which is likely to be due to asperity effects including the variable contiguity of the talc veneer. Lizardite gouge gives a value of µ=0.45, but when partially reacted to talc gives µ=0.23. This is significantly weaker despite representing perhaps less than 15% talc.Stress relaxation data initially shows similar behaviour for all sample geometries, with a temperature-controlled dominant deformation mechanism. Lizardite gouge with ongoing reaction to talc shows enhanced shear stress reduction at low strain rates (stress exponent falls from n=12 to n=5.5). This is ascribed to the effect of reaction via production of a weaker phase, leading to reduction of frictional strength and grain size, compaction and pressure solution effects. Pure lizardite gouge also shows a reduction in stress exponent to n=7.5, attributed to dissolution-precipitation of lizardite. Microstructural study shows that talc grows as a thin veneer along the quartzite forcing block indicative of localisation of deformation with foliated talc and recrystallised lizardite present within Riedel shear structures in the lizardite gouge. The talc veneers are deformed and may be subject to mechanical smearing, enhancing their contiguity. Results of this study highlight the significance of both mechanical smearing and incongruent pressure solution creep as potential mechanisms for weakening and aseismic creep-of particular interest is the rate of strength reduction versus the rate of reaction and whether this can be extrapolated fully to creep rates on the San Andreas Fault. Similar weak phases of hydrothermal origin have been identified by other studies and the contiguity of these phases is thought crucial to their impact upon weakness, and may be enhanced greatly by the joint effects of syntectonic reaction and mechanical smearing.

Keyword(s)

Creep; Fault; Hydrothermal; Pressure Solution; Talc; Weakness

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