Seismic reflection and refraction data acquired on four transects spanning the Southeast Greenland rifted margin and Greenland-Iceland Ridge (GIR) provide new constraints on mantle thermal structure and melting processes during continental breakup in the North Atlantic. Maximum igneous crustal thickness varies along the margin from > 30 km in the near-hotspot zone (< 500 km from the hotspot track) to ∼ 18 km in the distal zone (500-1100 km). Magmatic productivity on summed conjugate margins of the North Atlantic decreases through time from 1800 ± 300 to 600 ± 50 km³/km/Ma in the near-hotspot zone and from 700 ± 200 to 300 ± 50 km³/km/Ma in the distal zone. Comparison of our data with the British/Faeroe margins shows that both symmetric and asymmetric conjugate volcanic rifted margins exist. Joint consideration of crustal thickness and mean crustal seismic velocity suggests that along-margin changes in magmatism are principally controlled by variations in active upwelling rather than mantle temperature. The thermal anomaly (ΔT) at breakup was modest (∼ 100-125°C), varied little along the margin, and transient. Data along the GIR indicate that the potential temperature anomaly (125 ± 50°C) and upwelling ratio (∼ 4 times passive) of the Iceland hotspot have remained roughly constant since 56 Ma. Our results are consistent with a plume-impact model, in which (1) a plume of radius ∼ 300 km and ΔT of ∼ 125°C impacted the margin around 61 Ma and delivered warm material to distal portions of the margin; (2) at breakup (56 Ma), the lower half of the plume head continued to feed actively upwelling mantle into the proximal portion of the margin; and (3) by 45 Ma, both the remaining plume head and the distal warm layer were exhausted, with excess magmatism thereafter largely confined to a narrow (< 200 km radius) zone immediately above the Iceland plume stem. Alternatively, the warm upper mantle layer that fed excess magmatism in the distal portion of the margin may have been a pre-existing thermal anomaly unrelated to the plume.