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Volcanism in Basins to the N and W of the British Isles |
Department of Geology, University College of Wales, Aberystwyth, SY23 3DB UK
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.
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