Precise ⁴⁰Ar-³⁹Ar dating of igneous rocks recovered during Leg 163 shows that synbreakup volcanism at Sites 989 and 990 of the Southeast Greenland volcanic rifted margin (63ºN drilling transect) occurred at 57.1 ± 1.3 and 55.7 ± 0.5 Ma, respectively, correlating, most likely, with magnetochron C25 (57.6-55.9 Ma). Fully normally polarized lavas with oceanic-type composition at the tops of these two sites are inferred to correlate with C25n (56.4-55.9 Ma) and imply that plate separation between Greenland and northwest Europe took place at ~56 Ma. Dating of one lava flow at the 66ºN drilling transect to 49.6 ± 0.2 Ma (Hole 988A) confirms the occurrence of postbreakup basaltic volcanism along this volcanic rifted margin.

A virtually complete time frame for ~12 m.y. of pre-, syn-, and postbreakup basaltic magmatism at the Southeast Greenland volcanic rifted margin can now be assembled from the combined results of Legs 152 and 163. Tholeiitic magmatism at the rifted margin falls grossly into three time periods, ~61-60, ~57-54, and ~49 Ma. This time frame correlates with periods of extensive tholeiitic magmatism recognized in onshore flood basalts, sills, dikes, and layered gabbros of the East Greenland Tertiary Igneous Province and suggests that distinct mantle-melting events were triggered, respectively, by plume impact under central Greenland, continental breakup, and later crossing of the plume axis beneath the East Greenland volcanic rifted margin in consequence of north-westward continental drift.