TALC FRICTION AT ELEVATED TEMPERATURES AND PRESSURES: A VERY WEAK MINERAL THROUGHOUT THE SEISMOGENIC ZONE

Talc is a constituent of oceanic transform faults, and it has been postulated to form in the mantle wedge above subducting slabs, where it may influence faulting at the slab/mantle interface. In order to better understand the role of talc at depth in fault zones, we are conducting triaxial friction experiments on talc gouge at temperatures to 400 degrees C and effective normal stresses to 150 MPa. Our preliminary results show that talc is one of the weakest minerals at elevated temperatures that has been identified to date, and it is characterized by inherently stable, velocity-strengthening behavior (strength increases with increasing shear rate) at all conditions tested. A talc-rich fault should therefore be extremely weak and slip aseismically at depth. At 100 MPa effective normal stress, the coefficient of friction (= shear stress/effective normal stress) of water-saturated talc decreases from 0.2 at room temperature to a minimum of approximately 0.1 at 300 degrees C, then increases slightly to about 0.12 at 400 degrees C. The coefficient of friction increases with increasing effective normal stress at a given temperature, but it remains below 0.15 at 150 MPa. The strength of thoroughly dry talc varies with temperature to 300 degrees C in the same way as that of water-saturated talc, although dry talc is always stronger than water-saturated talc at equivalent temperature-pressure-strain rate conditions. We have shown previously that the frictional strengths of dry and water-saturated sheet-silicate minerals such as talc are functions of their relatively weak interlayer bonds. Dry shear occurs by cleaving through the interlayer bonds, whereas water-saturated shear is concentrated in thin, structured water films between plate surfaces. The shear strength of the water films is controlled by the degree of attraction of the polar water molecules to the plate surfaces, which in turn is directly correlated with factors that contribute to the interlayer bond strength. The fact that both the dry and water-saturated strengths of talc decrease with increasing temperature to 300 degrees C suggests that the cause may be weakening of the interlayer bonds.