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