ACCOMMODATING SILL-COMPLEX EMPLACEMENT

Field-, modelling- and seismic-based studies have revealed that space generated by forced folding of the overburden and free surface commonly accommodates igneous sill emplacement at shallow crustal levels. Depending on the growth style and geometry of the forced folds, sub-horizontal sills may evolve into laccoliths or obtain a saucer-shaped morphology. These analyses highlight that the behaviour of the host rock during emplacement strongly influences forced fold evolution and may also determine whether additional space-making mechanisms occur (e.g., fluidisation). Studying intrusion-induced forced folds can therefore provide important insights into: (i) the mechanisms controlling emplacement; (ii) how host rock lithology influences the architecture and distribution of volcanic systems; (iii) hydrocarbon trap formation; and (iv) intrusion-related ground deformation.

Over the past decade, the application of seismic reflection data analysis has revolutionised our understanding of the 3D geometrical relationships between igneous intrusions and forced folds. However, the majority of previous studies have primarily focused on forced folds developed above spatially isolated sills, although it is apparent from field and seismic reflection data that the development of laterally extensive complexes of interconnected sills facilitates magma transport in many sedimentary basins.

We use 3D seismic reflection data from the Irish Rockall Basin, offshore western Ireland, to examine the spatial accommodation of a sill-complex that contains 82 seismically resolved intrusions. Individual forced folds (<41 km² with mean fold amplitudes of 111 m) are observed above single intrusions, whereas broader (100–244 km²), larger amplitude (mean of 296 m) ‘compound’ folds overlie stacked sills. Seismic-stratigraphic relationships observed within the folded Palaeocene-to-Middle Eocene succession indicate that emplacement initiated at the end of the Maastrichtian and lasted for c. 15 Ma, before ceasing near the end of the Ypresian.

We demonstrate that: (i) intrusion-induced forced folds evolve dynamically and may coalesce to form broad areas of sustained uplift; and (ii) that the formation of sill-complexes within the upper crust may occur over prolonged time periods.