Syn- and post-rift igneous activity in the Porcupine Seabight Basin and adjacent continental margin W of Ireland

Abstract

Summary An igneous province in the Porcupine Seabight Basin and adjacent shelf region W of Ireland records repeated episodes of intrusive and extrusive igneous activity, from the mid-Jurassic to late Oligocene. The ‘Province’ maintains a regionally distinctive NE-SW and NNW-SSE curvilinear trend that is inherent to the basement. Mid-Jurassic pyroclastic airfall deposits were probably contemporaneous with mid-Kimmerian uplift. Voluminous early Cretaceous volcanism was coeval with an episode of rifting during the Valanginian to Barremian interval in the Porcupine and Goban Spur Basins. In particular, the 150 km long ‘Porcupine Median Volcanic Ridge’ (PMVR) is interpreted to be extrusive in the S, but progressively intrusive northwards; from an inferred fissure volcano to a laccolith. Aptian-Albian volcaniclastics are of unknown association, but coincide with the onset of oceanic spreading along the adjacent proto-Atlantic margin. Documented late Cretaceous intrusive and extrusive igneous activity was minor in scale although a period of marked inversion is recorded for the NE Porcupine Basin, which may relate to the emplacement of the areally extensive Brendan Igneous Centre. Intermittent intrusive and extrusive activity throughout the Palaeogene was broadly coincident with the full duration of the ‘Thulean’ Palaeocene igneous event. Further activity, variously dated from Eocene to late Oligocene, is partly attributed to a series of intrabasinal, parallel-aligned plugs of intrusive and extrusive origin situated in the upper Porcupine Basin and termed the ‘Slyne Fissures’. These fissures, whose basement-controlled lineation prolongates onshore as an extended group of sheet intrusions, appear to emanate from the Brendan Centre. Further dykes in County Kerry are also believed to extend towards the same centre. Within the Province a distinction is made between magmatism that is essentially syn-rift and linked to basin development (notably Valanginian-Barremian), and later post-rift activity (Aptian-Tertiary). The size, emplacement character and location of the PMVR may represent a sudden increase in axial volcanism that occurred during advanced stages of basinal rifting, and could be viewed as a prelude to, or transition from rifting to spreading prior to an abrupt failure. Temporal relationships can be proposed between active faulting associated with initial lithospheric stretching (late Kimmerian), late syn-rift igneous activity (Valanginian-early Barremian) and the initiation of subsequent lithospheric stretching (earliest Barremian) and, finally, the onset of seafloor spreading (Albian) on the continental margin W of Goban Spur. The later post-rift activity is described as ‘opportunistic’ and triggered by exterior tectonic events that utilized existing tensile trends within the Basin. Emplacement of the Brendan Centre was probably responsible for a phase of inversion with its concomitant influence on sedimentation patterns. Magmatism in the Eocene and Oligocene was associated with minor faulting, an unconformity and pronounced post-Eocene subsidence, together indicative of a final regional thermal episode.

Crustal strain-dependent serpentinisation in the Porcupine Basin, offshore Ireland

“…4a at rifting rates appropriate for the Porcupine Basin [Reston et al, 2004], especially in the absence of 247 voluminous syn-rift magmatism [Tate & Dobson, 1988] to advect heat. 248…”

An overview of the Upper Palaeozoic–Mesozoic stratigraphy of the NE Atlantic region

“…This was succeeded by a tectonically quiescent, post-rift period interrupted locally in the Porcupine, Slyne and Erris basins by an AptianAlbian rift phase. Early Cretaceous volcanism is suggested in the central part of the Porcupine Basin (Croker & Shannon 1987;Tate & Dobson 1988;Naylor et al 1999) (Fig. 15b, c).…”

“…Lower Cretaceous extrusives and tuffs occur through the Barremian-Albian succession (Tate & Dobson 1988;Naylor et al 2002). The Cenomanian -Danian chalk sequence in the Porcupine Basin ranges from 400 m near the margins of the basin to more than 1000 m in the basin centre (Moore & Shannon 1995).…”

Three-dimensional gravity and magnetic modelling of the Irish sector of the NE Atlantic margin

“…The axial region of the central and northern parts of the Porcupine Basin is transected by a combination of the arcuate, deeply buried Porcupine Volcanic Ridge System and the N-trending Porcupine Arch , enclosure 1). The ridge system is considered by Tate and Dobson (1988) to represent an igneous complex of mainly Cretaceous age, although Reston et al (2001) prefer a serpentinite diapiric origin. The Porcupine Arch is a high-amplitude reflector which may mark the top of the crystalline basement or partially serpentinized mantle (Reston et al, 2004).…”

“…The ridges are considered to be extrusive volcanic edifices which were subsequently draped with sedimentary rocks until finally being overstepped by sediment in Eocene time (Scrutton and Bentley, 1988). The stratigraphic control does not allow them to be dated accurately but they may be of Early Cretaceous age, when there was also significant igneous activity in the Porcupine Basin and on the Labrador Margin (Scrutton and Bentley, 1988;Tate and Dobson, 1988;Balkwill, 1987;DeSilva, 1999). Although the seismically-imaged ridges appear extrusive, modelling by Scrutton and Bentley (1988) indicated that the magnetic anomalies are best explained by a combination of the extrusive units and underlying intrusions, presumably lying along the fissures through which the igneous rocks were emplaced.…”