Magma fingers and host rock fluidization in the emplacement of sills

The Golden Valley Sill in South Africa, like many similar structures imaged in three-dimensional seismic data, is saucer shaped with a transgressive rim. This rim exhibits magma fingers tens to hundreds of meters wide radiating from a central axis and forming the outer dipping rim. Localized fluidization of the host rock is observed close to the margins where the fingers occur, suggesting a causal link between fluidization of the country rocks and finger formation. We relate the large-scale formation of fingers to host rock fluidization caused by flash boiling of pore fluids following the tensional failure of the roof of the inner dish at a maximum radius. Once fluidization has occurred by this method, the mechanical heterogeneity controlling the propagation of a concordant sill is broken and the intrusion can transgress. This work emphasizes the importance of understanding the nature of magma intrusion on host rocks, depending on their lithology, and in particular the ability of host rocks to behave in either a brittle or nonbrittle manner during magma intrusion.