Friction of hornblende and tremolite at hydrothermal conditions with low effective normal stress and high pore pressure

In order to understand the friction properties of hornblende and tremolite as common minerals in deep fault zones, we used natural hornblende and tremolite as the simulated gouge material to perform shearing experiments at temperatures from 303 to 607 degrees C under an effective normal stress of 20 MPa, a pore pressure of 100 MPa, and displacement rates of 0.0488-1.22 mu m/s. The steady-state friction coefficient of hornblende ranges from similar to 0.76 to similar to 0.88. The corresponding steady-state rate dependence is velocity-weakening (negative a-b) at temperatures from 303 to 403 degrees C, which transitions to velocity-strengthening (positive a-b) at temperatures above 500 degrees C. Steady-state friction coefficients of tremolite exhibited values between 0.70 and 0.76. The corresponding steady-state rate dependence is velocity-strengthening (positive a-b) for all the tested temperatures. The direct rate effect (a value) for hornblende roughly increases with temperature, while the corresponding evolution effect (b value) essentially declines with temperature. The direct rate effect (a value) of tremolite increases with increasing temperature for temperatures above 400 degrees C, with a quite minor evolution effect (b value) that indicates viscous-like slip behavior at all the tested temperatures. There are a large number of remnants of large particles distributed pervasively over sheared gouges, and significant particle size reduction is only observed in localized shear zones. Pervasive cataclastic flow seems to dominate the shear deformation of hornblende for temperatures above 400 degrees C and that of tremolite for all the tested temperatures. The velocity-weakening behavior of hornblende at low effective normal stress may apply to amphibole-bearing fault rocks in subduction zones where unstable slip events can occur at a narrow temperature range of 300-400 degrees C. In contrast, the very strong velocity-strengthening observed for tremolite provides a favorable condition for blocking the nucleation of unstable slip events.