The three-dimensional geometry and growth of forced folds above saucer-shaped igneous sills

Forced folds formed at the seabed immediately overlying shallow (<1 km) saucer-shaped sills along the NE Atlantic Margin during the early Paleogene. Examples of this sill-fold relationship are exceptionally well imaged by high-resolution 3D seismic datasets from the NE Rockall Basin. The forced folds are domal in shape, 2–4 km in diameter, exhibit a structural relief of up to 350 m, and comprise sediment volumes of ca. 1 km3. A comparison of the thickness distribution across and volume of a saucer-shaped sill with a high intrusion diameter to depth ratio and the structural relief and volume of its associated forced fold shows a remarkable equivalence. This has the important implication that the structural relief on intrusion-related forced folds can be used as an estimate of the thickness of the underlying sill. The analysed forced folds are interpreted to have formed through three continual growth stages that are directly linked to the mechanical emplacement of the underlying saucer-shaped sills. Their growth was associated with an increase in faulting of the overlying strata and influenced coeval or subsequent development of polygonal fault systems within the overburden. These structures represent a new type of four-way dip closed hydrocarbon trap.