Geological and geophysical evidence for a mafic igneous origin of the Porcupine Arch, offshore Ireland

Abstract

Sedimentary basins west of Ireland contain a number of deep seismic structures that have been variously interpreted as fault blocks, serpentinite extrusions or igneous complexes. The Porcupine Arch (PA) is a deep-level (>11 km) domal 50 km wide seismic feature associated with a prominent free-air gravity anomaly high and high P-wave velocities. Detailed seismic mapping of igneous sill complexes in the Porcupine Basin suggests a possible connection with the PA. The sills form a thick (>5 km) interconnected network extending from the PA into the flanking post-rift Cretaceous stratigraphy, suggesting that the PA may be the top of a large (ultra)mafic intrusion that fed the sills. An intrusive origin for the PA is in agreement with geophysical modelling (gravity and V p ), the seismic character of the Porcupine Arch structure and the primitive bulk composition of the Porcupine sills, and is consistent with documented regional Cenozoic uplift in the basin with the development of shallow water and the occurrence of Paleocene–Eocene deltaic depositional systems. Similar mafic–ultramafic intrusive complexes have been documented elsewhere on the northeastern Atlantic margin, including the Rockall Trough. These findings emphasize that higher heat flow in the early Cenozoic may have prevailed over the northern part of the Porcupine Basin.

Multiphase deformation history of the Porcupine Basin, offshore west Ireland

“…The K‐Ar ages for sills intruding Albian–Aptian strata range from 60.6 to 26 Ma (Tate & Dobson, 1988). This Palaeogene magmatism is probably related to an early manifestation of the British Tertiary Igneous Province (Archer et al., 2005; Croker & Klemperer, 1989; Gagnevin et al., 2018). A recent study shows that sills emanate from the Porcupine Arch suggesting an igneous origin for the arch (Gagnevin et al., 2018).…”

“…The Porcupine Basin is also characterized by a number of igneous features including: (a) The Porcupine Arch — a deeply buried high (Figure 1), observable on gravity and seismic data, that has recently been described as an (ultra)mafic intrusion (Gagnevin et al., 2017), as opposed to the exhumed and serpentinized mantle prescribed by earlier work (O’Reilly et al., 2006; Prada et al., 2017; Readman et al., 2005; Reston et al., 2001), (b) The Porcupine Median Volcanic Ridge (PMVR) — an elongate, probably tholeiitic, extrusive volcanic ridge system (Figure 1) occurring within the Early Cretaceous succession (Calvès et al., 2012; Tate, 1993; Tate & Dobson, 1988), and topped by carbonate platforms (Calvès et al., 2012), and (c) sills and dykes emplaced within Jurassic–Eocene strata (Naylor et al., 2002; Tate & Dobson, 1988).…”

From rifting to hyperextension: Upper Jurassic–Lower Cretaceous tectono‐stratigraphy of the Porcupine Basin, Irish Atlantic Margin

“…The shallowing coincides with a prominent deep-crustal, high-amplitude, arch-shaped seismic reflection marking the top of the Porcupine Arch, a feature which is coincident with a free-air positive gravity anomaly (Naylor et al, 2002;O'Reilly et al, 2006;Tate et al, 1993). This feature, which underpins a subtle mid-basin high that was present in the Early Cretaceous (Figures 2b and 3) (Moore & Shannon, 1995), has been variously interpreted as the top of either basement (Naylor et al, 2002), a dense, intruded or underplated, high-velocity igneous body (Gagnevin et al, 2017;Tate et al, 1993), or partially serpentinized mantle (O'Reilly et al, 2006). Seismic P-wave velocity models (Chen et al, 2018;Prada et al, 2017;Watremez et al, 2016) derived from travel time tomography of several wide-angle seismic profiles, acquired using ocean-bottom seismometers and ocean-bottom hydrophones, strongly support the presence of serpentinized mantle in the central parts of the Porcupine Basin and suggests the Porcupine Arch represents the Moho (Figure 3a).…”

From Continental Hyperextension to Seafloor Spreading: New Insights on the Porcupine Basin From Wide‐Angle Seismic Data

“…Based on seismic reflection data, this feature was formally named the Porcupine Arch (Naylor et al, ) and has been interpreted as the top of basement underlying a package of prerift sediments (Johnson et al, ) or as a detachment fault separating the crust and serpentinized mantle (Reston et al, ). Alternatively, Gagnevin et al () suggest, on the basis of mapping widespread sill complexes in seismic reflection data, that the Porcupine Arch is the top of a large mafic intrusion that fed these sills. Gravity and wide‐angle seismic data have been interpreted to suggest that the Arch is the tectonic expression of mantle serpentinization and the consequent formation of detachment faults (O'Reilly et al, ; Readman et al, ) and that the rifting of this part of the Porcupine Basin is magma‐poor.…”