The frictional strength of talc gouge in high‐velocity shear experiments

Talc is present in several large‐scale fault zones worldwide 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 and 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. Key Points Experimental shear analysis at seismic slip velocity of confined talc gouge that was either room‐dry or water‐saturated Talc gouge displayed systematic slip‐strengthening and velocity‐strengthening that enhance creep and suppress unstable slip The lack of free water led to distributed shear and strengthening in dry talc; wet talc showed slip localization and no strengthening