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) pro vide new constraints on mantle thermal structure and melting processes duri ng continental breakup in the North Atlantic. Maximum igneous crustal thick ness varies along the margin from > 30 km in the near-hotspot zone ( < 500 km from the hotspot track) to similar to 18 km in the distal zone (500-1100 km). Magmatic productivity on summed conjugate margins of the North Atlant ic decreases through time from 1800 +/- 300 to 600 +/- 50 km(3)/km/Ma in th e near-hotspot zone and from 700 +/- 200 to 300 +/- 50 km(3)/km/Ma in the d istal zone. Comparison of our data with the British/Faeroe margins shows th at both symmetric and asymmetric conjugate volcanic rifted margins exist. J oint consideration of crustal thickness and mean crustal seismic velocity s uggests that along-margin changes in magmatism are principally controlled b y variations in active upwelling rather than mantle temperature. The therma l anomaly (AT) at breakup was modest (similar to 100-125 degreesC), varied little along the margin, and transient. Data along the GIR indicate that th e potential temperature anomaly (125 +/- 50 degreesC) and upwelling ratio ( similar to 4 times passive) of the Iceland hotspot have remained roughly co nstant since 56 Ma. Our results are consistent with a plume-impact model, i n which (1) a plume of radius similar to 300 km and AT of similar to 125 de greesC impacted the margin around 61 Ma and delivered warm material to dist al portions of the margin; (2) at breakup (56 Ma), the lower half of the pl ume head continued to feed actively upwelling mantle into the proximal port ion 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 magm atism in the distal portion of the margin may have been a pre-existing ther mal anomaly unrelated to the plume. <(c)> 2001 Elsevier Science B.V. All ri ghts reserved.