Fracture‐Assisted Pressure Solution Creep of Granite: An Example From the Mont Blanc Massif, Western Alps

Abstract The Mont Blanc granite, as other deformed granites, has splitting properties attributed to two perpendicular planes: one formed by aligned fractures and the other by aligned micas in foliation planes. It is shown that this microstructure results from ductile deformation with stress‐driven dissolution of quartz and feldspar that passively concentrates insoluble micas, the dissolved minerals being reprecipitated in perpendicular fractures. Here we perform quantitative analyses of finite strain to show that this deformation involved horizontal shortening (17%–24%) and vertical stretching (21%–32%) that developed in a closed mass transfer system at centimeter to decimeter scale. A viscous creep model is proposed that relates strain rate, stress, and other parameters of deformation. While precise time evolution cannot be modeled due to the lack of the detailed openings and sealing of the fractures, a global steady‐state deformation is proposed that leads to the building of creep law deformation mechanism maps. The measured characteristic fracture spacing (∼600 μm) is compatible with low diffusive mass transfer rate (1.5 10 −20 m 3 /s) and low differential stress values (2–3 MPa), which are also compatible with subcritical fracture propagation (2.6–3.7 MPa for characteristic lengths of 1–0.5 cm, respectively). This ductile deformation likely developed in the Mont Blanc massif from its uplift (∼22 Ma) up to main shear zones development (∼18–14 Ma), possibly influencing the shear zones geometry. Fracture‐assisted pressure solution creep may be the main mechanism of ductile deformation of granitoids or gneisses with splitting properties, associating dissolution‐foliation bands perpendicular to sealed fractures.