Figure 2 shows the 63ºN transect across the Southeast Greenland margin. Site 917, drilled during Leg 152, comprises three volcanic series. The lower series consists of tholeiitic basalts to picrites at various stages of fractionation, whereas the middle series consists of more fractionated rocks: siliceous basalts, tholeiitic andesites, and dacites. The magmas that gave rise to both these lava series are interpreted as having fractionated in magma chambers in the continental crust (Larsen, Saunders, Clift, et al., 1994; Fitton et al., 1995, 1998a, 1998b). The chemical and isotopic composition of these magmas, notably their high Ba and Sr contents and low ¹⁴³Nd/¹⁴⁴Nd and sometimes high ⁸⁷Sr/⁸⁶Sr isotopic values, suggest that they have been contaminated first by continental crust in granulite facies and later by continental crust in amphibolite facies (Fitton et al., 1998a). The most evolved rocks are also the most contaminated, as expected for fractionation and assimilation in crustal magma chambers.

The upper series at Site 917 is separated from the middle series by a fluvial sandstone that may represent a significant time interval (from ~60 Ma to ~57 Ma) (Sinton and Duncan, 1998; Tegner and Duncan, Chap. 6 this volume). The upper series consists of olivine basalts and picrites showing variable degrees of fractionation, interpreted as erupted during final continental rupture when magma chambers were transient or nonexistent, allowing access of primitive Mg-rich magma to the surface (Fitton et al., 1995, 1998b). Some of the upper series lavas still show evidence of slight crustal contamination (Fitton et al., 1998a).

Sites 915 and 918, situated 70 km apart in the postbreakup part of the SDRS, contain basaltic lavas with a restricted compositional range. These lavas are interpreted as having been filtered through magma chambers in the crust at the spreading oceanic ridge (Larsen, Saunders, Clift, et al., 1994; Fitton et al., 1995, 1998a, 1998b); however, the single lava flow recovered from Site 915 shows slight contamination by continental crust (Fitton et al., 1998a). The overall similarity between the basalts at the two sites suggests that, after continental breakup, the composition of the lavas forming the SDRS was relatively constant.

Sites 915 and 917 are situated only 3 km apart, and the stratigraphic distance between the drilled successions is considered to be no more than a few hundred meters (Fig. 2). Yet this interval between the drilled successions must contain the transition from the variable synbreakup volcanics at Site 917 to the compositionally limited postbreakup volcanics. Site 990, drilled during Leg 163, was designed to penetrate this transition zone (Duncan, Larsen, Allan, et al., 1996).

At the continental end of the 63ºN transect, the character of the most landward and possibly oldest lavas in the SDRS wedge, as well as the nature of the basal unconformity and the underlying basement, was not determined from the Leg 152 work. Site 989 was aimed at penetrating both volcanics and basement, with the expectation that the volcanic succession recovered would either be a lateral equivalent of the Site 917 lower series or older (Duncan, Larsen, Allan, et al., 1996).

In the following, the prebreakup volcanics composing the Site 917 lower and middle series are designated the continental succession; the synbreakup volcanics in the Site 917 upper series form a transition zone, and the postbreakup volcanics comprising Sites 915, 990, and 918 are designated the oceanic succession, even though some of the oceanic rocks, as will be discussed, are contaminated with continental crustal material. For reasons discussed below, the Site 989 volcanics are included in the oceanic series.