C. M. Binde

The emplacement of igneous sill intrusions induces rapid heating of host rocks, generating fluids that migrate through structures known as hydrothermal vent complexes (HTVCs). While sill geometry and permeability are considered important factors controlling fluid migration around igneous sill intrusions, the precise mechanisms controlling vent distribution remain unclear. To improve the understanding of the primary controls on the final distribution of HTVCs, we are using a 3D seismic data set from the Mid‐Norwegian margin to analyze HTVCs associated with the ∼56 Ma Infinity Sill. The sill covers 237 km ² and is associated with 79 HTVCs, 25 of which are classified as mini‐vents. The HTVCs have outer diameters ranging between ∼0.1–2.7 km, are separated by spacings between ∼1.2–4.8 km, have conduit heights ranging between ∼0.6–1.1 km, and variable upper terminations. Our analysis reveals a large‐scale fluid migration pattern dominated by venting along the sill margins and above the sill interior, controlled by factors such as the fluid generation potential around the sill, the density of local highs in the sill interior as well as the mode and heights of sill transgression. Sill geometry is thus found to affect the formation, size and spacing of HTVCs, and is interpreted to constitute a primary control on the final distribution and morphological characteristics of HTVCs. Our findings contribute to a better understanding of subsurface fluid dynamics associated with sill intrusions emplaced in organic‐rich strata and have broader implications for understanding the climatic effects of hydrothermal venting at the Paleocene‐Eocene transition.

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.