The Neogene stratigraphy of the glaciated European margin from Lofoten to Porcupine

WP3 - ROCKALL-PORCUPINE

Tectonic setting and evolution

The continental margin in the Rockall–Porcupine region can be divided into three main platform areas of the Hebrides-Irish shelf (incorporating structural elements such as the Outer Hebrides Platform and Porcupine High), Rockall High and Hatton High, separated by the deep-water Rockall, Porcupine and Hatton basins (Fig. 17a). The platform areas include a number of structurally linked pre-Cenozoic basins that have no major bathymetric expression, but record a history of episodic crustal extension during Permo-Triassic to Early Cretaceous times, e.g. West Lewis and West Flannan basins, Flannan and Barra troughs, Donegal, Slyne, Erris and Rónán basins (Shannon & Naylor 1998). The deep-water basins represent the most significant phase of extension across the margin, which is thought to have occurred during Late Jurassic to 'mid' Cretaceous time (Musgrove & Mitchener 1996; Johnston et al. 2001). These basins typically contain a Cretaceous and Cenozoic succession, and have an underfilled character. The development of the deep-water basins was accommodated by extreme crustal attenuation. Whereas the thickness of the continental crust beneath the platform areas is normally 25–30km, it thins to as little as 58km beneath the Rockall and Porcupine basins (cf. Stoker et al. 1993 & Corfield et al. 1999, and references therein; Johnson et al. 2001). This phase of extension ultimately led to the separation of Greenland and Rockall during latest Paleocene–earliest Eocene (anomaly 24) time (Ritchie et al. 1999).

The NE–SW trend of these basins is very evident, and clearly parallels the long-lived Caledonian fracture system that includes the Minch and Great Glen faults (Fig. 17a). Such NE–SW-trending faults commonly define the basin-bounding faults. It is also apparent that NW–SE trending lineaments are pervasive throughout the region (Doré et al. 1999). Whilst these lineaments do not generally act as basin-bounding faults, they do form important transfer zones that act to compartmentalise the basins. One of the most important of these lineaments, the Anton Dohrn Lineament, transects the Rockall–Porcupine area and marks a major sinistral offset of the Rockall Basin (Fig. 17a). Isotopic analysis of samples of crystalline basement and Palaeogene lavas to the north and south of this lineament indicates that it is a major crustal lineament juxtaposing Archaean (to the NE) and Lower Proterozoic terranes (Morton & Taylor 1991; Hitchen et al. 1997; Morton 1997). However, definition of this boundary in profile data remains unclear (Fig. 17b). Farther north, the Wyville-Thomson Ridge may similarly be controlled by a NW–SE lineament that has acted to offset the NE Rockall Basin from the Faroe-Shetland Basin. This ridge probably developed to its present form through compressional deformation associated with intra-Cenozoic inversion, which occurred at times of plate reorganisation in the adjacent ocean basin (Doré et al. 1999; Roberts et al. 1999). 

Volcanism

Extensive volcanism accompanied late Mesozoic–early Cenozoic extension across the continental margin. Lower Palaeogene lava fields are widespread in the area off NW Britain, but are less extensive in the Irish Rockall region, although sills are abundant throughout the region. (Fig. 17b, Fig. 17c, and Fig. 17d). Seaward–dipping reflectors characterise the continent–ocean boundary. On Figure 17a, the effects of the volcanism are manifest by the abundance of igneous centres that are marked by the elliptical gravity highs. The Rosemary Bank and Anton Dohrn seamounts and the Barra Volcanic Ridge system represent Cretaceous volcanism within the Rockall Basin. However, the bulk of the known igneous centres, including those of the British Tertiary Igneous Province, including St. Kilda, the Geikie Igneous Centre, Hebrides Terrace Seamount, Darwin and the Rockall Igneous Centre are mostly of Paleocene to early Eocene in age. Others, such as Sigmundur, and the numerous centres in the area of the Hatton Basin and Hatton High are probably also of early Palaeogene age (Ritchie et al. 1999).

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Mid-Cenozoic subsidence and change in sedimentation style

The post-breakup history of the Rockall and Porcupine basins has mainly been dominated by subsidence outpacing sedimentation, resulting in the present-day, starved, deep-water basins (Fig. 17b, Fig. 17c, and Fig. 17d). The Rockall Basin may have subsided by at least 600–700m from late Eocene time (Stoker 1997). Part of this subsidence may have occurred in the late Eocene interval linked to regional submergence of the continental margin. Regional subsidence is implied by the shut-down of shelf-derived source areas, such as the Rockall High and George Bligh Bank and the Hebrides–Malin Shelf which preserve Eocene prograding wedges, and the change to a predominantly bottom-current-influenced deep-marine sedimentary environment in the Rockall Basin (Stoker 1997, 1998). This event is manifest as the C30 Unconformity (Fig. 17b, Fig. 17c, and Fig. 17d). In the Porcupine Basin, the post-Eocene succession is similarly represented by a rapidly deepening marine succession. As in the Rockall Basin, the C30 Unconformity (Fig. 17d) marks a change in depositional style in the Porcupine Basin. Eocene delta progradation driven by uplift of the Porcupine High was replaced by sediment reworking and redeposition in response to the onset of bottom-current activity (McDonnell & Shannon 2001). 

The enhanced subsidence rates indicated by both basins at this time might be related to a change in the spreading rates in the North Atlantic as the opening of the Atlantic continued northwards (Doré et al. 1999). Additionally, a major climatic cooling took place at the Eocene–Oligocene boundary, probably related to the formation of the Antarctic ice sheet, which resulted in a change in oceanic circulation pattern (Kennett et al. 1975). Developing connections between the North Atlantic and the Norwegian-Greenland Sea (Davies et al. 2001) may also have enhanced the influence of bottom currents at this time. 

According to Vanneste et al. (1995), post-Eocene subsidence in the NW Rockall Basin became non-uniform with increased differentiation between the flank and floor of the basin. This differential subsidence may have been driven, at least in part, by a series of Oligocene–Miocene compressional events that have been reported from around the northern margin of the Rockall Basin, including the Wyville-Thomson Ridge (e.g. Roberts 1989; Boldreel & Andersen 1993, 1995; Knott et al. 1993; Doré & Lundin 1996). 

References

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Hitchen, K., Morton, A.C., Mearns, E.W., Whitehouse, M. & Stoker, M.S. 1997. Geological implications from geochemical and isotopic studies of Upper Cretaceous and Lower Tertiary igneous rocks around the northern Rockall Trough. Journal of the Geological Society, London, 154, 517-521.

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