Download Accommodating sill-complex emplacement

Accommodating sill-complex emplacement
Craig Magee1, Christopher A-L Jackson1 & Nick Schofield2
1
Department of Earth Science and Engineering, Imperial College, Prince Consort Road, London,
SW7 2BP, England, UK
2
School of Geography, Earth and Environmental Sciences, University of Birmingham, Edgbaston,
Birmingham, B15 2TT, England, UK
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.