The velocity - dependence of frictional strength of serpentine gouges has been measured at constant normal stress of 110 MPa, pore pressure of 10 MPa, temperature 25, 100 and 200��C , and at sliding rate ranging from 0. 001 to 10��m/s. At 25��C, the coefficient of friction of chrysotile gouge is very low (�̡�0. 2-0. 25), while lizardite and antigorite gouge are much stronger, with �̡�0. 39 and 0. 45, respectively. The frictional strengths of chrysotile and antigorite gouges change little with a temperature increase to 200��C, whereas the strength of lizardite gouge increases substantially with increasing temperature. At 25��C, all three gouges show a transition from weak velocity weakening at high slip rates to velocity strengthening at low slip rates. With increasing temperature, the velocity dependence of each gouge shifts towards more positive values, especially at high slip rates. Based on this study and previous results, we suggest that the presence of serpentine in the fault zone may contribute to the occurrence of stable creep rather than earthquakes, but this effect may be limited to shallow depths. Although chrysotile is one of the weakest rock - forming minerals, it is still too strong to explain the weakness of the San Andreas fault deduced from heat flow data.

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 25 degrees C, the coefficient of friction, mu, 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 similar to 200 degrees C 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 25 degrees C, 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 similar to 200 degrees C, 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.

In this report, we present supporting data relevant to our continuing investigations of the frictional strength of serpentinite.We have measured the strength of chrysotile-, lizardite-, and antigorite-rich serpentinite gouges under hydrothermal conditions.Identification of serpentine minerals was based on X-ray powder diffraction analysis, and petrographic, scanning electron microscope, and electron microprobe techniques were used to fully characterize the samples.Experimental methods and the determination of correction factors, particularly for jacket strength, are outlined in detail; the corrected roomtemperature data are consistent with previous studies.The coefficient of friction, n, of chrysotile gouge is roughly 0.2 at 25 °C, whereas the lizardite-and antigorite-rich gouges are at least twice as strong.The very low roomtemperature strength of chrysotile appears to be a consequence of its high adsorbed water content; when the adsorbed water is removed, chrysotile is as strong as pure antigorite gouge at room temperature.Heating to nearly 200 °C leads to slight to substantial increases in the frictional strengths of all three gouges.The strength increases of chrysotile and, to a lesser extent, of lizardite, are probably caused by the partial loss of adsorbed water upon heating.In addition, all of the heated gouges may have become slightly lithified at the highest temperatures tested.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 25 °C, the chrysotile gouge shows a transition from velocity strengthening at velocities $0.32 nm/s to velocity weakening at velocities £1 \imfs.At elevated temperatures, however, chrysotile-gouge strength is essentially independent of velocity at low velocities.Overall, at a given temperature chrysotile may show a restricted range of velocities over which velocity-strengthening behavior occurs, and this range may shift to progressively higher velocities as temperature increases.Although fewer data are available for the heated lizardite and antigorite gouges, their behavior is consistent with that outlined for chrysotile.* Volume percent; ** Elements identified in SEM; Tr = trace amount.* total Fe as FeO; f antigorite formulae based on end-member composition Mg5.626 §UO10(0^)7.353.

Chrysotile-bearing serpentinite is a constituent of the San Andreas fault zone in central and northern California. At room temperature, chrysotile gouge has a very low coefficient of friction (mu approximate to 0.2), raising the possibility that under hydrothermal conditions mu might be reduced sufficiently (to less than or equal to 0.1) to explain the apparent weakness of the fault. To test this hypothesis, we measured the frictional strength of a pure chrysotile gouge at temperatures to 290 degrees C and axial-shortening velocities as low as 0.001 mu m/s. As temperature increases to approximate to 100 degrees C, the strength of the chrysotile gouge decreases slightly at low velocities, but at temperatures greater than or equal to 200 degrees C, it is substantially stronger and essentially independent of velocity at the lowest velocities tested. We estimate that pure chrysotile gouge at hydrostatic fluid pressure and appropriate temperatures would have shear strength averaged over a depth of 14 km of 50 MPa. Thus, on the sole basis of its strength, chrysotile cannot be the cause of a weak San Andreas fault. However, chrysotile may also contribute to low fault strength by forming mineral seals that promote the development of high fluid pressures.

Maps are presented of the fractures and faults that developed in originally intact cylinders of granite during triaxial experiments at room temperature.Additional deformation features are described graphically.With increasing axial compression prior to sample failure, the samples show increases in the numbers of microfractures, kinked micas, and flame-perthite-bearing feldspars; the latter feature is an exsolution/replacement texture in K-feldspar whose development is commonly attributed to the application of a differential stress.The overall pattern of fracturing following sample failure varies from sample to sample, but the orientation of the main fault in a given sample can be correlated with the confining pressure of the experiment.Interpretive maps of the paths of concentrated shear show that shearing is much more localized during stick-slip motion than during stable sliding.Localization of shear accompanying stick slip has also been observed following friction experiments in which the samples contained a layer of artificial or natural gouge placed along a sawcut.These similar results suggest that the deformation textures reflect the processes controlling the sliding behavior.

The frictional properties of a crushed granite gouge and of gouges rich in montmorillonite, illite, and serpentine minerals have been investigated at temperatures as high as 600°C, confining pressures as high as 2.5 kbar, a pore pressure of 30 bar, and sliding velocities of 4.8 and 4.8×10−2 μm/sec. The gouges showed nearly identical strength behaviors at the two sliding velocities; all four gouges, however, showed a greater tendency to stick-slip movement and somewhat higher stress drops in the experiments at 4.8×10−2 μm/sec. Varying the sliding velocity also had an effect on the mineral assemblages and deformation textures developed in the heated gouges. The principal mineralogical difference was that at 400°C and 1 kbar confining pressure a serpentine breakdown reaction occurred in the experiments at 4.8×10−2 μm/sec but not in those at 4.8 μm/sec. The textures developed in the gouge layers were in part functions of the gouge type and the temperature, but changes in the sliding velocity affected, among other features, the degree of mineral deformation and the orientation of some fractures.

Layers of artificial granite gouge have been deformed on saw-cut granite surfaces inclined 30° to the sample axes. Samples were deformed at a constant confining pressure of 250 MPa and temperatures of 22 to 845°C. The velocity dependence of the steady-state coefficient of friction (μss) was determined by comparing sliding strengths at different sliding rates. The results of these measurements are consistent with those reported bySolberg andByerlee (1984) at room temperature andStesky (1975) between 300 and 400°C. Stesky found that the slip-rate dependence of (μss) increased above 400°C. In the present study, however, the velocity dependence of (μss) was nearly independent of temperature.

Rock samples taken from two outcrops, as well as rare cores from three well bores at the Geysers geothermal field, California, were tested at temperatures and pressures similar to those found in the geothermal field. Both intact and 30° sawcut cylinders were deformed at confining pressures of 200–1000 bars, pore pressure of 30 bars and temperatures of 150° and 240°C. Thin-section and X-ray analysis revealed that some borehole samples had undergone extensive alteration and recrystallization. Constant strain rate tests of 10−4 and 10−6 per sec gave a coefficient of friction of 0.68. Due to the highly fractured nature of the rocks taken from the production zone, intact samples were rarely 50% stronger than the frictional strength. This result suggests that the Geysers reservoir can support shear stresses only as large as its frictional shear strength.