LAUSR

Talc is present in several large-scale fault zones world-wide, and is mineralogically stable at temperature of the upper crust. It is therefore necessary to gain a better understanding of the frictional behavior of talc under a wide range of slip velocity conditions occurring during the seismic cycle. We analyzed the frictional and structural characteristics of room-dry and water-saturated talc gouge by shear experiments on a confined gouge layer at slip-velocity range of 0.002 – 0.66 m/s and normal stress up to 4.1 MPa. Room-dry talc showed a distinct slip-strengthening with the initial friction coefficient of μ ~ 0.4 increased systematically to μ ~ 1 at slip distance D > 1 m. Room-dry talc also displayed velocity-strengthening at slip-distances shorter than 1 m. The water-saturated talc gouge displayed systematic low frictional strength of μ = 0.1 - 0.3 for the entire experimental range, with clear velocity-strengthening behavior with positive (a-b) values (rate dependence parameter of rate- state- friction) of 0.01-0.04. The microstructural analyses revealed distributed shear and systematic dilation (up to 50%) for the room-dry talc, in contrast to the extreme slip localization and strong shear compaction for water-saturated talc. We propose that talc frictional strength is controlled by lubrication along cleavage surfaces that is facilitated by adsorbed water (room-dry) and surplus water (water-saturated). This mechanism can explain our experimental observations of slip-strengthening and velocity-strengthening for both types of talc gouge, as well as other clay minerals. It is thus expected that talc presence in fault-zones would enhance creep and inhibit unstable slip.