The shapes of dikes: Evidence for the influence of cooling and inelastic deformation

Abstract : We document the shape of dikes from well exposed field locations in the Isle of Rum, Scotland, 14 and Helam Mine, South Africa. The basaltic Rum dikes crop out on a smaller scale than the 15 Helam kimberlite dikes and have a smaller length to thickness ratio (~100:1 Isle of Rum, 16 ~1000:1 Helam Mine). We compare dike thickness field measurements with the geometry 17 predicted by elastic theory, finding best-fit models to estimate magma overpressure and regional 18 stress gradients at the time of dike emplacement. Most of the dike shapes fit poorly with elastic 19 theory, being too thick at the dike ends and too narrow in the middle. Our calculated 20 overpressures and stress gradients are much larger than independent estimates based on rock 21 strength. Dike shape can be explained by a combination of host rock inelastic deformation and 22 magma chilling at the dike’s tapering edges preventing its closure as magma pressure declines 23 during emplacement. The permanent wedging of the dike edges due to chilling has implications 24 for crustal magma transport and strain response in the crust due to dike emplacement