Résumé
We performed approximately 40 friction experiments on glassy basalts (GB) and glass-free basalts (GFB) at slip rates up to 6.5 m/s and normal stress up to 40 MPa (seismic deformation conditions). Once slip was accelerated, friction decayed abruptly with a shorter weakening distance (Dw) in GB than in GFB but with a steady-state friction higher in GB than in GFB. The shorter Dw in GB may be explained by the thermal softening occurring at the glass transition temperature (Tg approximately 500 degrees C), which is lower than the bulk melting temperature (Tm approximately 1200 degrees C) of the GFB. Post experiment microanalyses suggest that the larger crystal fraction measured in GB melts resulted in a higher steady-state friction value compared to the GFB melts. We conclude that the presence of interstitial glass in GB renders the basalt more prone to frictional instabilities and rupture propagation with respect to GFB, but increases the steady-state friction.