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Three-dimensional seismic data from the NW European Atlantic margin provide detailed data on the relationships between sills, dykes, laccoliths and pre-existing basin structure. The data show that sills are predominantly concave-upwards in shape, being complete or partial versions of radially or bilaterally symmetrical forms that possess flat inner saucers connected to a flat outer rim by a steeply inclined sheet. Sills can be sourced from the steep climbing portions of deeper sills, and can be shown to exploit pre-existing faults to feed shallower sills and laccoliths. Magma flow patterns, as revealed by opacity rendering, suggest that sills propagate upwards and outwards away from the magma feeder. As an individual sill can consist of several leaves emplaced at different stratigraphic levels, and as a sill or dyke can provide magma to other sills and laccoliths, the data suggest that neutral buoyancy concepts may not provide a complete explanation for the mechanism and level of sill emplacement. Instead, the data suggest that the presence of ductile horizons such as overpressured shales may permit sill formation below the neutrally buoyant level. Emplacement below the level of neutral buoyancy would permit sill inflation, and the associated fracturing of the country rock would permit magma to exploit these pathways in order to ascend to shallower levels and feed shallower intrusions.

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