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 (V100^1253C), varied little along the margin, and transient. Data along the GIR indicate that the potential temperature anomaly (125 þ 503C) 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 radius V300 km and vT of V1253C 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 (6 200 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. ß 2001 Elsevier Science B.V. All rights reserved.