The Rockall–Porcupine margin or Irish-Scottish margin is part of the continental margin of Northwestern Europe, lying to the west of Ireland and Scotland. The margin is unusually extensive, ranging from 600 km to 1,000 km away from the nearest land. It consists of a series of relative bathymetric highs, known as banks, with intervening deep water troughs containing several seamounts. It formed as a result of multiple periods of rifting, continuing intermittently through the whole of the Mesozoic era, culminating in break-up at the end of the Paleocene epoch, forming the northern Atlantic Ocean.
Topography The topography of the Rockall–Porcupine margin consists of a series of relatively shallow water banks, with intervening deeper water areas. To the west of the Scottish and Irish landmasses, the continental shelf is formed by the Hebrides Shelf in the north, the Malin Shelf, which straddles the UK–Ireland boundary, and the Irish Shelf. To the southwest of Ireland, beyond the Irish Shelf, lies the deepwater Porcupine Seabight, with the Goban Spur to the south and the Porcupine Bank to the north and west, dividing the seabight from the Rockall Trough. The deepwater Rockall Trough extends for about 1,000 km from the Porcupine Abyssal Plain in the south, to the Wyville-Thomson Ridge in the north. There are a group of seamounts in the northern Rockall Trough, including the Hebrides Terrace, the Anton Dohrn and Rosemary Bank seamounts. To the northwest of the Rockall Trough a series of highs is developed between the trough and the Iceland Basin of the northern North Atlantic. In the northeastern part the intervening high is narrow and segmented into the George Bligh, Lousy and Bill Bailey banks. To the southwest, the high is much broader and is known as the Rockall Plateau. On the southeastern side of the plateau is the Rockall Bank, on the northwestern side are the smaller Edoras Bank and the larger Hatton Bank. In the centre of the plateau is the Hatton–Rockall Trough (or Basin).
Geology
The structure of the Rockall-Porcupine region is mostly the result of Mesozoic rifting. However, before the Triassic, the margin was formed of several contrasting crustal blocks. To the north of the Great Glen Fault system, the crust is Paleoproterozoic or older in age, although two distinct blocks have been identified, the Lewisian complex to the northeast and rocks correlated with the Rhinns complex to the southwest. The boundary between these two blocks has been correlated with the northwest–southeast trending Anton Dohrn Lineament. The Great Glen Fault system is interpreted to extend west of Ireland, crossing the Porcupine Basin and Porcupine Bank, continuing into the eastern Rockall Basin. This is the most northerly of a set of major sub-parallel fault zones of Caledonian age, including the Fair Head–Clew Bay lineament, the Southern Uplands Fault and the Iapetus Suture. Further south, the eastern flank of the Porcupine Basin is assumed to be crossed by the Variscan front.
The oldest preserved unmetamorphosed sedimentary rocks are Carboniferous in age, interpreted to be deposited as two sequences, before and after the Variscan orogeny. They are known from the northern Porcupine Basin and the Slyne and Erris Basins to the north. Late Permian syn-rift deposits are recorded from the Slyne Basin. Sequences of Triassic age are known from drilling results or interpreted from seismic data along the eastern part of the margin from the Porcupine Basin, both flanks of the Irish sector of the Rockall Basin, through the Slyne and Erris basins up to the West Lewis Basin on the eastern flank of the northern UK Rockall Basin. Most of the faults that bound the Triassic sequences are thought to be of Jurassic age. The current rift basin geometries are interpreted to have developed during the middle to late Jurassic in the Porcupine Basin, Slyne and Erris basins and probably in the Rockall Basin. The late Jurassic period saw the greatest degree of crustal stretching, with hyperextension developed in the southern half of both the Rockall and Porcupine basins, although the amount of true oceanic crust that formed is thought to be limited. This rifting event is thought to have stopped in the earliest Cretaceous in the Porcupine Basin, although the lack of well penetrations and the general poor imaging in the Rockall Basin due to later intrusions mean that models that include significant rifting in the early Cretaceous there cannot be ruled out.
Volcanic ridge systems have been interpreted in both the southern Rockall and Porcupine basins. In the Rockall basin there are at least four ridges, known as the Barra Volcanic Ridge System, originally described in 1988, when they were dated as early Cretaceous in age. More recently, these ridges have been interpreted as intrusions that were formed as part of the North Atlantic Igneous Province and therefore of Paleocene to Eocene age. In the Porcupine Basin, the Porcupine Median Volcanic Ridge has been interpreted as an extensive (>85 km in length) set of coalesced volcanic features formed during the early Cretaceous. The Porcupine Basin is also interpreted to have been uplifted during the early Cretaceous by 200–700 m. This transient uplift and the contemporaneous volcanic ridges may be related to the presence of anomalously hot mantle. Scientific drilling on the Porcupine High shows that most of the Lower Cretaceous sequence is missing with only a thin Aptian to Upper Cretaceous sequence preserved. Albian age sediments were also penetrated on the Hatton High in a window through Paleogene lavas, although these were faulted, folded and subsequently eroded (with removal of an estimated ~1,500 m of younger beds) before deposition of a thin sequence of Plio-Pleistocene age. From the analysis of seismic velocities, it is thought likely that rocks of similar age are present in the Hatton-Rockall Basin, although the possibility of an older Mesozoic section cannot be ruled out. Thick Upper Cretaceous sequences are developed in the Porcupine and Rockall basins, while rocks of this age are thin on the Porcupine High and missing on the Hatton and Rockall highs.
The Paleocene period saw the onset of magmatism in the North Atlantic Igneous Province at about 62 million years ago, continuing up to the start of the Eocene. On the Rockall-Porcupine margin this magmatism is expressed as extensive lava flows on the highs associated with numerous igneous centres. On the line of eventual break-up between NW Europe and East Greenland, seismic reflection data show well-developed Seaward Dipping Reflectors developed on the northwestern edge of much of the Edoras and Hatton highs, showing that this was a volcanic passive margin. On the Hatton-Rockall High, there are several igneous centres, including the Rockall, Lyonesse, Sandaro, Sandastre, Mammal and George Bligh complexes. In the northern Rockall igneous centres include the Anton Dohrn, Rosemary Bank and Hebrides Terrace seamounts and the Darwin and Geikie igneous centres. Sill complexes are developed in many parts of the Rockall Basin and quite extensively in the Porcupine Basin.
Cenozoic sedimentary rocks reach up to 2 km thick in the Rockall Basin and 4 km thick in the Porcupine Basin. Paleocene sediments, beneath the North Atlantic Igneous Province lavas, are known from the northern Rockall Basin and the West Lewis Basin, passing from non-marine on the basin flanks to fully marine within the basin. Within the Porcupine Basin, the Lower Paleocene interval shows continuation of the Chalk deposition that is characteristic of the underlying Upper Cretaceous. In contrast the Upper Paleocene basin margin sequences are dominated by deltaic sandstones, that continue up into the Eocene. During the late Eocene, water depths increased over the whole margin, leading to the onset of deepwater current circulation. This is marked by a major submarine unconformity, close to the Eocene–Oligocene boundary, which is known as C30. Contourite deposition became widespread in the deepwater areas from the Oligocene onwards. During the Oligocene to Miocene the northern part of the margin was affected by pulses of compression. One of the largest structures formed at this time was the Wyville Thomson Ridge, a 180 km long anticline, trending WNW–ESE, which marks the northern boundary of the Rockall Basin.
Oceanography
The complex topography of the Rockall-Porcupine margin has a strong influence on the distribution of water masses in the region. Several water masses are known from the margin, including: Antarctic Bottom Water (AABW), North East Atlantic Deep Water (NEADW), Labrador Sea Water (LSW), Mediterranean Overflow Water (MOW), Norwegian Sea Deep Water (NSDW), Subarctic Intermediate Water (SAIW), Eastern North Atlantic Water (ENAW), North Atlantic Central Water (NACW), which are all distinguished by their characteristic temperatures, chemistries and salinities. The AABW water mass is found only in the deepest areas below about 4,000 m such as in the Porcupine Abyssal Plain. It partly mixes with the overlying NEADW, which is found in the deeper parts of the Porcupine Seabight and the Rockall Trough over a depth range of about 4,000 m to 2,000 m. Above the NEADW the LSW is observed up to a depth of about 1,200 m. To the eastern side the deep water areas the MOW is identified over a depth range of about 1200 m to 700 m. To the west of the Rockall Trough, the same depth interval is identified as SAIW. The uppermost water mass is ENAW or NACW and has a depth range of 100–600 m. There is a permanent thermocline over a depth range of 600–1,000 m, between the warmer shallower waters that interact with the atmosphere and the colder deeper waters.
The AABW is northward moving and enters the southernmost part of the Rockall Trough. The NADW is confined to the Rockall Trough and Porcupine Seabight. The LSW and SAIW both enter the area from the west along the southern edge of the Rockall Plateau, before entering the Rockall Trough and the Porcupine Seabight where they become confined. The MOW moves northwards along the eastern edge of the Atlantic entering the same deeper water areas, although it remains unclear how far north the MOW extends into the Rockall Trough. In the Porcupine Seabight, a strong northward current is observed at about 1,000 m depth. At the northern end of the seabight, the current weakens and turns south along the western margin, forming an anticlockwise circulation. In the Rockall Trough strong northward currents are observed along the eastern margin, which also form an overall clockwise circulation pattern. The northward moving contour currents on the eastern side of the trough involve NADW and LSW, with fast velocities recorded in the 2,000–3,000 m depth range. The southward current on the western margin may involve water from the NSDW that has overflowed the Wyville-Thomson Ridge. At shallower levels the eastern edges of the deeper water areas are affected by the Shelf Edge Current (SEC), which moves northwards in the top 400–500 m, carrying ENAW water. The North Atlantic Current (NAC) carries WNAW water through the Iceland Basin, The Hatton-Rockall Trough and the western Rockall Trough. The ENAW and NACW water masses, carried by the SEC and NAC respectively, increasingly mix as they move northwards through the Rockall Trough, forming a single mass as they enter the Norwegian Sea.