Seismic Characteristics and Morphology of an Igneous Intrusion in the Vøring Basin, Offshore Norway: New Insights on the Lateral Evolution of Elongated Intrusions

Abstract Igneous sill intrusions are common features in volcanic basins worldwide, constituting important components of basin‐scale magma plumbing systems. While sills exhibiting simple geometries, such as saucer‐shaped sills, are commonly linked to distinct mechanical processes, emplacement mechanisms for sills exhibiting more complex geometries are debated. To better understand the emplacement of complex sills, this study aims to constrain the formation mechanisms associated with the Infinity Sill, a 237 km 2 elongated sill located in the SW Vøring Basin. Detailed seismic interpretation and attribute analysis of an industry‐standard 15,000 km 2 3D seismic data set reveal that the 36 km long and 4–8 km wide Infinity Sill intruded mud‐dominated Late Cretaceous strata. The thickness of the sill has been estimated to be dominantly 50–150 m, with a volume of ∼20 km 3 (ranging between 7.9 and 25.5 km 3 ). The shape of the sill and geometry of the sill elements suggest that the sill propagated as a large, magma‐filled fracture exploiting a pre‐existing polygonal fault network during propagation. The sill originated at a source in the SW, and the asymmetric propagation away from the source facilitated a 36 km lateral elongation of the sill, contrasting along‐axis dike models for elongated intrusion geometries. Local deformation around the sill initially facilitated continuous transgression, but with increasing sill length, forced folding of the overburden triggered abrupt transgression of the sill margins, resulting in a lateral change in geometry. The Infinity Sill, with its distinct geometrical features and changing cross‐sectional geometry, demonstrates that complex sill geometries are formed by a combination of emplacement mechanisms.