Controls on transgressive sill growth

Igneous sills represent an important contribution to upper crustal magma transport and storage. This study focuses on an exemplary 20–50-m-thick transgressive sill in the Faroe Islands on the European Atlantic passive margin, which is hosted in layered lavas (1–20 m thick) and basaltic volcaniclastic units (1–30 m thick). Preserved steps in the sill, and offset intrusive segments, are consistent with initial propagation via segmented fractures followed by inflation to create a through-going sheet. Although steps correspond to the position of some host rock interfaces and volcaniclastic horizons, most interfaces are bypassed. Transgressive sill contacts are subparallel to thrust faults that record ENE-WSW shortening, which are observed within the surrounding country rock and within the sill. Remnant sill segments are elongate along a NNW-SSE axis, parallel to the derived intermediate stress axis for thrust faults. The overall transgressive geometry is consistent with regional horizontal shortening, with steps indicating transitions between transgressive and lateral sill propagation are controlled locally by layering. This work emphasizes the importance of scale of observation in considering the controls on sill emplacement, and in particular, that layering is not the primary control on geometry.