Magma transport through the crust via interconnected sill complexes

Three-dimensional seismic data from the volcanic margin offshore Norway have been used to map the complex three-dimensional geometry of two major sill complexes. These data show that the complexes consist of bowl-shaped or inclined, sheetlike discordant sills that intersect with one another over a vertical distance of 8-12 km and transect basement and sedimentary rocks of the middle to upper crust. Using a simple graphical technique, we demonstrate the interconnectedness of the sill complex, and hence the maximum time for sill emplacement. We show that sills are interconnected by junctions occurring systematically at the lowest points of the overlying sills, and, hence, that the sill complex could have acted as a single throughgoing magmatic plumbing system, transporting mafic melts from mid-lower crustal depths to near surface. Based on this imaging of the magmatic plumbing system, we conclude that sills can form viable and efficient conduits for crustal magma transport in volcanic margins, and may play a role in feeding large extrusive complexes.