The role of exhumation in metamorphic dehydration and fluid production

When mountain belts form, crustal rocks undergo metamorphism, resulting in the breakdown of volatile-bearing minerals and the release of water-rich fluids. As these fluids move towards the Earths surface, they can cause generation of ore deposits, enhance deformation of the crust and change rock composition. Generation of such fluids has long been considered to occur dominantly during heating associated with burial of rocks. In contrast, the exhumation of rocks that follows heating has not been expected to generate large amounts of fluid. Here we use mineral-equilibria modelling to show that the erosion-induced exhumation of greywacke a common rock type in mountain-forming regions generates a continual supply of new fluid. Fluid formation is particularly pronounced at temperatures below about 500 C. Such fluids can explain the pairing of seismic and electrical conductivity anomalies observed in the Southern Alps in New Zealand, as well as the formation of vein-infilled backshears there.