Sample Characterizations and Strength Measurements of Serpentinite Gouges

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.