JOHNSTON, SARAH, Statoil Exploration (Ireland) Ltd.,ANTHONY G. DORÉ, Statoil (UK) Ltd. and ANTHONY M. SPENCER, Statoil, Stavanger, Norway

Abstract: Megatectonic Controls on Basin Development West of Ireland

The local tectonic events that determined sedimentation and basin development west of Ireland can be related to megatectonic controls - changes in plate motion - in the Southern North Atlantic region. The Porcupine Basin, at the southernmost limit of the North East Atlantic margin, lay close to the intersection of three plate boundaries that separated Eurasia from North America and controlled opening of the Bay of Biscay. It is possible to show how the changes in tectonic regime through time can be related to the development of these plate boundaries. The tectonic events are demonstrated in this paper using a series of palaeogeographic maps (Fig. 1) based on computer-generated plate reconstructions (PLATES). This approach provides particular insight into the structural evolution of the southern Porcupine Basin, which is complex and little described to date. Our seismic evaluation of this area shows that it was subjected to multiple rifting episodes between the Permian and the mid-Cretaceous, and these observed events can thus be assessed in the context of the broader plate tectonic controls.

Permo-Triassic rifting of the southern Porcupine area is largely conjectural on seismic interpretation, as the superposition of successive rift events usually masks the geometry of the original structures. However, based on the general megaregional picture of the southern North Atlantic, such structures are highly probable (see Late Triassic map, Fig. 1a). The Permo-Triassic rift system developed in several phases in response to the extensional collapse of over-thickened crust following the consolidation of Pangaea. A strongly asymmetric half graben basin style was largely controlled by NE-SW-trending Caledonoid structures along most of the western seaboard of Ireland and the UK. The general northeasterly grain was complicated by reactivated Proterozoic shear zones in the Bay of Biscay Rift Zone, and by Hercynian reactivation in the North Celtic Sea Basin.

In the Early Jurassic minor extensional tectonics took place, largely along NE-SW trends inherited from the Triassic. However, the key event was the marine flooding of the old Permo-Triassic rift systems, associated with the incipient ocean-floor spreading in the Central Atlantic (see Sinemurian-Toarcian map, Fig 1b).

The pattern of marine transgression was interrupted in the Middle Jurassic by further rejuvenation of NE-SW Triassic faults, forming local provenance areas, and by regional uplift or restriction north of the Porcupine Basin. Uplift of the southern Irish landmass took place at this time, giving rise to an unconformity of late Early Bajocian age in the northern Porcupine Basin. Forced regression associated with the uplift caused the southwards migration of a shallow clastic shelf regime, bringing marine sands to the North Celtic Sea Basin and probably much of southern Porcupine.

By Late Jurassic times (see Portlandian map, Fig. 1c) ocean floor spreading was well established as far north as the Azores-Gibraltar Fracture Zone and a marine connection existed from this area to the northern Tethyan margin, where spreading was also occurring. An increased intensity of extensional tectonic activity took place over much of NW Europe. In the southern Porcupine Basin this activity was manifested as significant normal displacement on the N-S "basin bounding" faults and on a major NE-SW en echelon fault set. A return to throughgoing marine conditions took place such that by the Portlandian the predominant depositional mode in the southern Porcupine Basin was that of deep marine shale. From the local well evidence and from the general regional picture, local marine sands derived from uplifted footwalls are expected in syntectonic expansions alongside the major activated fault sets. Source-quality shales could also have been deposited in these local depocentres.

In the Early Cretaceous (Neocomian) rifting propagated northeastward from the active spreading centres south of the Newfoundland Fracture Zone, giving rise to extensional faulting west of Iberia and on the Grand Banks and uplift of the Avalon Platform (Welsink el al., 1989). This rifting may have established the broad outlines of the Rockall Trough, although evidence for the age of this basin is still equivocal. In the southern Porcupine Basin reactivation (or continued activity) took place on earlier fault sets, especially those of NE-SW trend. Significant subsidence along NE-SW lines also occurred in the North Celtic Sea Basin. Sedimentation here was non-marine, probably as a result of uplift of the incipient plate margin north of the proto-Bay of Biscay. The Porcupine Basin, however, was in a thermal subsidence regime following the Jurassic faulting and remained fully marine. Mudrocks were the predominant lithology, but sporadic marine sands were shed into the basin from the uplifted margins.

In the Mid-Cretaceous (see Barremian to Albian map, Fig. 1d) there were significant changes to the plate tectonic configuration. A triple spreading junction developed in the North Atlantic such that spreading was established between Iberia and Grand Banks in the Aptian, in the Bay of Biscay in the late Aptian - late Albian, and northwards to the Charlie Gibbs Fracture Zone in the Albian (Masson & Miles, 1994). In the southern Porcupine Basin, seismic observations suggest general NE-SW extension associated with the development of the Biscay - Charlie Gibbs spreading regime. This activity is manifested as NW-SE normal faults and by minor reactivation of the N-S basin-margin faults. A significant subsidence of the basin centre took place at this time, suggesting considerably more crustal extension than that implied by the brittle faulting observed on seismic. This pattern of basin subsidence continued to take place until recent times (Tate, 1993), giving rise to the present day bathymetric expression of the Porcupine Basin.

AAPG Search and Discovery Article #90923@1999 International Conference and Exhibition, Birmingham, England