Accelerated hydration of the Earth's deep crust induced by stress perturbations

The metamorphic cycle associated with the formation of mountain belts produ ces a lower crust containing little or no free fluid(1,2). The introduction of external fluids to dry and impermeable volumes of the Earth's crust is thus a prerequisite for the retrogressive metamorphism later observed in su ch regimes. Such metamorphism can cause significant changes in the crust's physical properties, including its density, rheology and elastic properties (3,4). On a large scale, the introduction of fluids requires the presence o f high-permeability channels, such as faults or fractures, which are the re sult of external tectonic stresses. But extensive interaction between exter nally derived fluids and the fractured rock requires efficient mass transpo rt away from the initial fractures into the rock itself, and this transport often occurs over distances much longer than expected from grain-boundary diffusion. Here we present both field observations and a simple network mod el that demonstrate how the transport of fluids into initially dry rock can be accelerated by perturbations in the local stress field caused by reacti ons with fluids. We also show that the morphology of reaction fronts separa ting `dry' from `wet' rocks depends on the anisotropy of the external stres s field.