Mantle thermal structure and active upwelling during continental breakup in the North Atlantic

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 s 30 km in the near-hotspot zone (6 500 km from the hotspot track) to V18 km in the distal zone (500^1100 km). Magmatic productivity on summed conjugate margins of the North Atlantic decreases through time from 1800 þ 300 to 600 þ 50 km3/km/Ma in the near-hotspot zone and from 700 þ 200 to 300 þ 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 (vT) at breakup was modest (V100^125‡C), varied little along the margin, and transient. Data along the GIR indicate that the potential temperature anomaly (125 þ 50‡C) and upwelling ratio (V4 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 radiusV300 km and vT ofV125‡C impacted the margin around 61 Ma and delivered warm material t