Only a few wells in the northeast Atlantic have penetrated extrusive breakup volcanics imaged by seismic reflection data. Of the 15 relevant ODP/DSDP basement sites, only Sites 642 and 917 have drilled more than 250 m into the volcanic basement (Fig. 1). Voluminous extrusive complexes are exposed in the British Tertiary Volcanic Province, on central east Greenland, and on the Faeroe Islands (Saunders et al., 1997) and provide an important reference for the interpretation of the offshore volcanics. Breakup-related intrusive complexes and sills have been penetrated by more than 15 industry wells west of Shetland and one in the V°ring Basin (Skogly, 1998; K. Hitchen, pers. comm., 1997) but have not been considered in this study.

Only key well-tie and conjugate margin transects are presented and discussed in this study. On the Southeast Greenland Margin, five regional multichannel seismic (MCS) profiles from the GGU-81 and -82 surveys, in addition to a regional grid of high-resolution seismic data along the EG63 and EG66 transects, were reinterpreted (Fig. 1) (Duncan, Larsen, Allan, et al., 1996). On the M°re Margin 12 regional, high-quality MCS reflection profiles were interpreted within a 150-km-wide margin corridor (Alvestad, 1997). Published reflection data and line drawings were assessed for the Jan Mayen Ridge (┼kermoen, 1989), the V°ring Margin (Eldholm, Thiede, Taylor, et al., 1987, 1989; Planke and Eldholm, 1994), and the Rockall Margin (Roberts, Schnitker, et al., 1984; White et al., 1987; Neish, 1993; Barton and White, 1997a, 1997b).

Various methods have previously been used for interpreting the top- and intrabasement seismic reflection patterns on volcanic margins. Initially, characteristic basement reflection configurations were used to map four basement provinces on the V°ring Margin, representing I = Inner Flows; II = irregular basement between the SDR and the V°ring Escarpment; III = SDR; and IV = normal oceanic basement (e.g., Hinz et al., 1982; Skogseid and Eldholm, 1987, 1989). Elsewhere, aspects of seismic stratigraphic have been used, for example, on the Southeast Greenland Margin and around the Faeroe Islands (Larsen and Jakobsdˇttir, 1988; Andersen, 1988). Here, we have used the seismic volcanostratigraphic approach (Fig. 2; Table 1) (Planke et al., in press). The entire breakup-related volcanic complex landward of normal oceanic crust has been defined as one seismic sequence. However, the interpretation focus has been on the characterization and mapping of volcanic seismic facies units, which are calibrated by borehole data. This approach provides for a more detailed interpretation of the structure and morphology of the volcanic constructions, being particularly useful because improved seismic data quality of the volcanics has revealed a more complex internal seismic pattern than previously imaged.