Hybrid flow sills: A new mode of igneous sheet intrusion

Three-dimensional (3-D) seismic mapping of the Vigra sill complex has revealed a series of shallow level lobate sills. This complex was emplaced ca. 55 Ma as part of the igneous activity associated with breakup of the northeast Atlantic margin. The complex consists of discordant sheets, some of which are remarkable in that they have an axial seismic amplitude anomaly along their lengths. The anomalies are interpreted as evidence for the development of magma tubes that formed in the subsurface at depths between 200 and 400 m, and that fed the lobate sill segments. We argue that this sill complex represents a hybrid between expansion of sheets by flow and fracture propagation. These hybrid flow sills are suggested to result from magma emplacement into high-porosity and water-saturated sediments at low confining pressures, where the coalescence of flow lobes results in the formation of magma tubes capable of propagating >8 km and terminating in the formation of new flow lobes.