Diachronous sub‐volcanic intrusion along deep‐water margins: insights from the Irish Rockall Basin

Abstract

The movement of magma in sedimentary basins often occurs through an extensive and interconnected complex of sills. Field-, modelling-, and seismic reflection-based studies indicate that the emplacement of shallow-level sills is commonly accommodated by the formation of forced folds, which may be expressed at the free surface and onlapped by younger strata. If the age of these onlapping strata can be constrained, important insights can be gained into the timing of magma emplacement and associated regional, tectono-magmatic events. Previous studies have focused on isolated intrusions that are overlain by an individual forced fold formed during a single tectono-magmatic event. However, the structure and evolution of 'compound' folds developed above stacked, interconnected sills, and what they may reveal about polyphase intrusive events has not been investigated. In this study, we use 3D seismic reflection data from the Irish Rockall Basin, offshore western Ireland, to constrain the structural style and emplacement history of a sill complex that contains 82 seismically resolved intrusions. Individual forced folds, <41 km 2 in plan view and with mean fold amplitudes of 111 m, are developed above single intrusions. However, where sills are stacked, broader (100-244 km 2 ), larger amplitude (mean of 296 m) compound folds occur following the coalescence of individual folds. Stratigraphic onlap and truncation observed within the folds throughout the Palaeocene-to-Middle Eocene succession, indicates that emplacement and forced folding initiated at the end Maastrichtian and lasted for ca. 15 Ma, before ceasing near the end of the Ypresian. We demonstrate (i) intrusion-induced forced folds evolve dynamically and can form broad areas of sustained local uplift, and (ii) that the formation of sill complexes within the upper crust may occur over prolonged time periods. This study also highlights the importance of seismic reflection data to understanding the structural style and age relationships between igneous systems.

Unravelling intrusion-induced forced fold kinematics and ground deformation using 3D seismic reflection data

“…The occurrence of subtle onlap and truncation observed within folded strata deposited between these principal phases of magmatism implies that sill emplacement occurred intermittently over~31 Myr (Figure 3, 6, and 7), consistent with previous observations that sills and sill-complexes can assemble incrementally via the accumulation of relatively small-volume magma pulses intruded across protracted periods of time [e.g. Annen 2011;Magee et al 2014;Annen et al 2015;Magee et al 2016;Magee et al 2017a]. We cannot constrain the precise volumes and timing of individual sill emplacement events because: (i) we cannot seismically image presumably thin sills fed by discrete magma pulses; and (ii) we lack detailed biostratigraphic data to constrain the precise ages of the key onlap surfaces and strata deposited during periods of forced folding.…”

“…It is thus difficult to reconcile insights into the processes controlling ground deformation obtained from seismic reflection data, which only provide a snapshot of the cumulative strain accommodating ancient intrusions, and the dynamic uplift and subsidence recorded at active volcanoes. We show that mapping of intra-fold strata and identification of seismic-stratigraphic relationships can be used to unravel the incremental development of sill intrusions and overlying forced folds [see also Magee et al 2014]. Furthermore, our results provide the first evidence from seismic reflection data that the dynamic interplay between uplift and subsidence can control forced fold geometries.…”

“…Trude et al 2003], although we demonstrate that we should not solely rely on defining strata onlapping onto the top of forced folds to constrain emplacement age [e.g. Magee et al 2014]. Identifying seismic-stratigraphic relationships throughout folded sequences allows forced fold kinematics to be unravelled and we show that intermittent subsidence can play an important role in intrusioninduced forced folding.…”

“…from flat-topped to dome-shaped), and occurrence of additional material solely within the folded sequence contrasts with our conceptual model of intrusion-induced forced folding ( Figure 12A) [cf. Pollard and Johnson 1973;Hansen and Cartwright 2006;Galland 2012;Magee et al 2014]. For example, because the geometry and growth of forced folds are controlled by a directly underlying forcing member, it is expected that whatever happens to the upper layers within a forced fold must also happen to the lower layers (Figure 12A) [Stearns 1978].…”

3D structure and formation of hydrothermal vent complexes at the Paleocene-Eocene transition, the Møre Basin, mid-Norwegian margin

“…Evidence for volcanic activity in sedimentary basins is found along the entire European northeast Atlantic margin (Doré et al, 1999). Volcanic processes and deposits may have significant impact on the structural and geodynamic development of the rifted margin and associated sedimentary basins, i.e., the Karoo Basin, the Rockall Basin, the Faroe-Shetland Basin, and the Neuquén Basin (Smallwood and Maresh, 2002;Svensen et al, 2012;Magee et al, 2014;Schofield et al, 2017). The Møre Basin study area is located beneath the outer shelf and slope region offshore mid-Norway, and it shows classic examples of how volcanic activity and igneous intrusions within sedimentary strata may impact the basin structure (Skogseid et al, 1992;Brekke, 2000).…”