Evidence of thermal pressurization in high-velocity friction experiments on smectite-rich gouges

Thermal pressurization of pore fluid is one of the possible mechanisms responsible for dynamic weakening in landslides and earthquakes, but, to date, has not been reproduced in the laboratory. Here, we report high-velocity experiments performed in a rotary shear friction apparatus on smectite-rich gouges from the 1963 Vaiont landslide (Italy). The gouges were slid under 1 MPa normal stress, for displacements up to 30 m and a slip rate of 1.31 m s-1 under room-humidity and water-saturated conditions. Sample dilatancy was observed in room-humidity runs after similar to 3-4 m of slip, concomitant with an increase in normal stress and a decrease in shear stress. Mineralogical and microstructural investigations suggest that dilatancy resulted from expansion of the H(2)O released by the collapse of the smectite structure due to frictional heating of the slipping zone at T > 200 degrees C. We conclude that sample dilatancy is due to thermal pressurization of the clay-rich gouge.