[1] Indenter experiments have been performed on quartz crystals in order to establish a pressure solution creep law relevant at upper to middle crustal conditions. This deformation mechanism contributes to Earth's crust geodynamics, controlling processes as different as fault permeability, strength, and stress evolution during interseismic periods or mechanochemical differentiation during diagenesis and metamorphism. Indenter experiments have been performed at 350°C and 20-120 MPa during months with differential stress varying from 25 to 350 MPa. Several experimental parameters were varied: nature of quartz (synthetic or natural), nature of fluid, manner in which the solid/ solution/solid interface was filled, and orientation of the indented surfaces versus quartz crystallographic c axis. Significant strain rates could only be obtained when using high-solubility solutions (NaOH 1 mol L À1 ). Displacement rates of the indenter were found activated by differential stress, with exponential dependence, as theoretically predicted. The mean thickness of the trapped fluid phase below the indenter was estimated in the range 2-10 nm. Moreover, the development of this trapped fluid phase was relatively fast and allowed fluid penetration into previously dry contact regions by marginal dissolution. The indenter displacement rate was driven by differential stress, and its kinetics was controlled by diffusion along the trapped fluid and the development of a morphological roughness along the interface. Conversely, marginal strain energy driven dissolution was observed around the indenter, and its kinetics was controlled by freesurface reaction. These experimental results are applied to model the interactions between pressure solution and brittle processes in fault zones, providing characteristic time scales for postseismic transitory creep and sealing processes in quartz-rich rocks.

International audienceThe Ubaye valley, one of the most active seismic zones in the French Alps, was visited in 2003–2004 by a prolific and protracted earthquake swarm with a maximum magnitude M L = 2.7. The seismic activity clustered along a 9-km-long, 3- to 8-km-deep rupture zone which trends NW-SE across the valley and dips 80°SW. Focal mechanisms for the larger shocks show either normal faulting with a SW-NE trending extension direction or NW-SE strike slip with right lateral displacement. The activity initiated in the central part of the rupture zone, diffused to its periphery, and eventually concentrated in its southeastern deeper part. A permanent station situated above the swarm allowed us to monitor the entire phenomenon from its inception to its conclusion. The complete time series includes more than 16,000 events, with shocks down to magnitude M L = −1.3. It shows bursts of activity, separated by quiescent periods, with no well-defined subswarms as observed in other similar studies. The Gutenberg-Richter b value significantly varied between 1.0 and 1.5 in the course of the phenomeno